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Biology Year 11 Module 1
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Cells as the Basis of Life
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Biology Module 1 “Cells as the Basis of Life”
Format: OnScreen
copyright © 2005-17 KEEP IT SIMPLE SCIENCE www.keepitsimplescience.com.au
Slide 1
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Topic Outline
What is this topic about?
Cells as
the Basis
of Life
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To keep it as simple as possible, (K.I.S.S.
Principle) this topic covers:
1. DIFFERENT TYPES of CELLS
Eukaryotic & prokaryotic cells. How we know...
light microscopes, electron microscopes, x-ray
crystallography, isotopic “tracers”.
1. Different
Types of Cells
Eukaryotic & Prokaryotic
2. Cell
Structures
Technologies to understand cells
2. CELL STRUCTURES
Main features of plant & animal cells.
Organelles... the nucleus, mitochondria, E.R.,
ribosomes, golgi body, lysosomes, chloroplasts.
Structure of membranes.
Plant v. animal
3. Cell
Functions
Major organelles visible with light &
electron microscopes.
Membrane structure
3. CELL FUNCTIONS
a) STUFF GETS IN & OUT
Diffusion & osmosis. Active v. passive transport.
Endocytosis & exocytosis.
Importance of the SA/Vol. ratio.
b) FOOD & ENERGY for CELLS
Photosynthesis & cellular respiration.
What cells need, and need to get rid of.
c) BIOCHEMICAL CONTROL... ENZYMES
Properties & importance of enzymes.
Effects of temperature & pH on enzyme activity.
c) Biochemical
Control... Enzymes
a) Stuff Gets
In & Out
Diffusion & osmosis
Active v. passive
transport
Endocytosis
& Exocytosis
Importance of
SA/Vol. ratio
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Properties of enzymes
b) Food &
Energy for
Cells
Effects of temperature & pH
on enzyme activity
Photosynthesis
Cellular respiration
What cells need
Slide 2
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Introduction
Comparison:
Light & Electron ‘Scopes
The Cell Theory
Light (Optical) Electron
Microscope
Microscope
The “Cell Theory” is one of the fundamental concepts in Biology.
It simply states:
• All living organisms are composed of cells or are the product of
cells. (e.g. viruses)
• All cells are produced from pre-existing cells.
The evidence supporting the Cell Theory has come mainly from the use
of microscopes to examine living things.
Our knowledge of cell structure and function has developed as the
technology of microscopes advanced over the last 300 years or so.
Initially only light (optical) microscopes were available, but since the
1930’s, electron microscopes have revealed more detail of cell structure
and function.
beam of light
focused by
glass lenses
Magnification generally about
500 X.
Maximum
about 2,000 X
Resolution
(ability to see
fine details)
about 0.2 μm
max.
μm)
micrometres (μ
How Big Are Cells Anyway?
beam of electrons
focused by magnetic
fields
up to 10,000,000 X
(5000 times more
powerful)
about 0.0002 μm
(1,000 times better
detail)
1 μm = 0.000001(10-6)metre.
1 micrometre is 1/1000 of a millimetre.
Typical Plant Cell
20-100 μm
University
students using a
“Scanning Electron
Microscope” (SEM).
Typical Animal Cell
5 - 20 μm
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Photo by
Daniel Schwen
Bacterial Cells
0.1 - 5 μm
SCALE: 100 μm
(0.1 mm)
How the
image
is formed
(used under
Creative Commons
Attribution-Share Alike
2.5 Generic Licence)
Pathologist using a “Light”
(or “Optical”) Microscope to
view blood cells.
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Slide 3
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1. Different Types of Cells
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Hooke’s
microscope
The first scientific observation of cells was made
with a newly-invented (and very primitive)
microscope by Robert Hooke (English, 1665). He saw
a lot of boxy, identical cells, but soon it was realised
that cells occur in many sizes, shapes and types.
However, the full details of the different cell types
had to wait about 300 years until modern
technologies could unravel all the scientific facts.
Some of these technologies will be described
shortly, but meanwhile, what are the different cell
types?
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Eukaryotic Cells
Familiar living things, including all
plants & animals, are composed of cells
described as “eukaryotic”. (“eu” = true, “karyo” =
“kernel” (Greek). Here karyotic refers to the nucleus of a
cell)
All eukaryotic cells have a distinct cell nucleus
containing thread-like structures called
EUKARYOTIC
Animal Cell
chromosomes. The chromosomes hold the
with lots of
genetic information in the form of DNA. Essentially, the
membrane-based
nucleus is the “control-centre” of the cell.
organelles.
Prokaryotic Cells
(“pro” = before)
In contrast to a plant
or animal cell, a
bacterial cell is very
different.
There is NO nucleus
with chromosomes.
Certain structures
are present within
the cell, but none are
membrane-based.
PROCARYOTIC
Bacterial Cell
has no organelles bound
by membranes.
Prokaryotic cells are generally much smaller
than any eukaryotic cell for reasons that will be
covered later.
In an evolutionary sense, the prokaryotes are
the more ancient & primitive, while eukaryotes
are more advanced & more recent.
Archaea & Eubacteria
A relatively recent discovery has complicated
the simple division between prokaryotes &
eukaryotes: it is now known that there are 2
distinct types of prokaryotes.
As well as the nucleus, every eukaryotic cell also contains a variety of
other structures built from, or surrounded by, membranes. Collectively,
these are called “organelles”. In this topic you will study some details
of the important organelles.
The “Eubacteria” (true-bacteria) have been
known for 150 years and were thought to be
the full story of prokaryotic cells. In the 1980’s
new technology revealed another totally
different type: the “Archaea” (means “ancient”)
Any living thing composed of eukaryotic cells may be described as a
“eukaryote”. This includes all plants & animals, the fungi and a variety
of single-celled creatures such as protozoa & diatoms.
Archaean cells are prokaryotic, but very
different chemically to “normal” bacteria. Their
lineage dates back perhaps 3.5 billion years!
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Technologies to Understand Cells
The Light Microscope
Our understanding of cells and their structure & function was
initially due entirely to the optical microscope. Here is a brief
history:
Robert Hooke 1665
Over the next 150 years, microscopes improved,
and it was suspected that cells were present in all
living things.
Robert Brown, 1827
Brown was the first to discover structures inside
cells. He discovered and described the nucleus
inside plant cells.
Hooke is credited with being the
first person to see cells and name
them.
What
Hooke
saw
Using a primitive microscope, he
looked at a piece of cork (dead
tree bark) and saw tiny “boxes”
like the rooms and compartments
of a gaol or monastery. (hence
“cells”)
These are Hooke’s drawings of
Anton van
what he saw in the cork.
Leeuwenhoek
In 1676, van Leeuwenhoek used a very simple microscope, but it
was equipped with an excellent lens, through which he saw living
micro-organisms swimming around in a drop of water.
Van Leeuwenhoek’s
sketches of the
“animalcules”
(microscopic living
things) which he
discovered.
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By about 1840, the
“Cell Theory” was
becoming accepted
by most biologists,
because cells were
observed in every
organism studied.
Louis Pasteur’s
discoveries showed
that infectious
diseases were caused
by “germs”, which
were microscopic,
cellular organisms.
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Portrait of
Louis Pasteur
in his laboratory
Rudolf Virchow, 1859
and Walther Flemming, 1879
Between them, these two German scientists
clarified the process of cell division, by which cells
produce more cells. This established the principle
that all cells come from pre-existing cells.
Slide 5
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Technologies to Understand Cells
The Electron Microscope
(cont.)
Transmission Elect. Micro. (TEM)
The electron microscope was invented between about 1926
to 1933. A number of scientists, engineers & companies
were involved. For full details you should search a reliable
website such as Wikipedia.
TEM image
of a single
bacterial
cell.
Photo by
Peter
Highton
To form a biological image with a
TEM the sample has to be dried &
fixed into a special resin, then
sliced extremely thinly.
The first commercial equipment became available about
(used under
The electrons pass through the
1938, but because of WWII this technology did not have
Creative
sample so the image is flat and
Commons
much scientific impact until the 1950’s.
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2-D and shows the fine details of
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the structures within.
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permitted.
Electron microscopes use beams
Licence)
of electrons, focused by electric &
TEM images can achieve
magnetic fields, to form images at
extremely high magifications & resolution, but the
magnifications & resolutions far
preparation of the specimens is difficult & highly technical.
superior to a light microscope.
(see slide 3) Objects cannot be
Scanning Elect. Micro. (SEM)
viewed by eye, but are displayed on
A Scanning Electron Microscope
screens, as photos, or captured as
image often appears 3-D and can
digital images in computers.
show amazing surface details. This
is because the specimen has been
In the sections which follow, you
coated with a layer of heavy metal
will see examples of images of
(eg gold) only one or 2 atoms thick.
cells seen by both light microscope Modern Electron Microscope
and by electron ‘scope.
Photo by David J Morgan
(used under Creative Commons
The electron beam does not pass
Attribution-Share Alike 2.0 Licence)
through it, but is scattered from it.
The electron microscope revealed
Computer analysis of the scattering
cellular details which hugely increased our understanding
effects generate an image of the
of the structure & function of living cells.
surface topography.
SEM image of bacterial cells
You need to be aware that there are 2 main types of
being attacked by a human
Any colours are artificial and
electron microscope. Each has its own advantages &
immune cell. Photo: NIAID
(used under Creative Commons
computer-generated.
disadvantages.
Attribution-Share Alike 2.0 Licence)
Biology Module 1 “Cells as the Basis of Life”
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Technologies to Understand Cells
(cont.)
X-Ray Crystallography
Unravelling of DNA
We now know that all of the thousands of chemical reactions in a
living cell are dependant on, or controlled by, huge biological
molecules, especially the proteins & the nucleic acids (of which
DNA is the most famous). Furthermore, we know that it is the
precise 3-dimentional shape of these “macro-molecules” which
is critical to their functioning.
(More on this later in this topic.)
By the 1950’s it was known that a
substance known as DNA was the
basis of heredity. Its chemical
composition was known, but noone could figure out how it could
function as a gene.
How can we study the shape of a molecule?
James Watson (USA) & Francis
Crick (UK) (and others) used x-ray
scattering patterns from
crystallised DNA to discover the
now famous double-helix shape.
Just over 100 years ago, x-rays were discovered and immediately
scientists began using x-rays for all sorts of reasons, including
medical imaging of broken bones, etc.
An Australian
Photographic film
father & son team,
sensitive to x-rays
William &
Lawrence Bragg,
used x-rays to
probe the
structure of
Crystal
x-ray
matter. They
beam
beamed x-rays
X-rays diffracted by the crystal through pure
lattice & form Interference
crystals &
patterns which are captured on
captured on film
the film.
the patterns of the
scattered rays. They figured out how the diffraction patterns
related to the arrangement of atoms within the crystal. They were
awarded the Nobel Prize for Physics in 1915. At the time, no-one
could predict how important this would be for Biology!
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Microscopes, both optical & electron, have allowed detailed images of cells and cell parts.
However, understanding exactly what is going inside a cell is largely a matter of molecular structures and chemical reactions.
The technologies described here will give you a simple over-view of how we have discovered the functioning of cells.
Armed with the chemical analysis
AND the shape, Watson & Crick
were able to develop a theory for
the functioning of DNA.
Part of an x-ray
diffraction image of a
large protein.
Mathematical analysis
of this pattern by
computer can determine
the 3-D shape of the
molecule.
Photo: Jeff Dahl
(used under Creative
Commons Attribution-Share
Alike 3.0 Licence)
Slide 7
This led to understanding the
“genetic code” and later to the
“Human Genome Project”. The
knowledge gained is now a
cornerstone for modern Biology
& Medicine.
Meanwhile, X-Ray Crystallography
continues to quietly contribute
more & more knowledge of the
shapes of biological molecules,
helping us understand how it all
works.
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More Technologies to Understand Cells
Isotopic Tracers
Within each microscopic living cell, thousands of chemical
reactions are constantly occurring. Many processes
involve a sequence or chain of reactions which need to
occur in strict order, each one controlled by huge macromolecules with a precise shape to “grab” chemicals and
either ram them together, or tear them apart, then “hand
them on” to the next step.
How have we been able to unravel such complexity
occurring within a pin-point-sized bag of life? Traditional,
test-tube chemical analysis does NOT get you very far.
Isotopes
You should already be aware that all chemical elements
occur in 2 or more variant forms called isotopes. The
difference is the number of neutrons in the nucleus of each
atom. Some isotopes are unstable & may spontaneously
emit various radiations... they are “radioactive”.
One of the best known examples concerns 2 of the
isotopes of carbon:
“Carbon-12” “Carbon-14”
6p+
6n0
12
6
C
6p+
8n0
14
6
C
These atoms have the
same number of
electronss so they are
chemically identical and
react the same way.
However, carbon-14 is
radio-active and can be
identified by the radiation
it emits.
Example of the “Tracer” Method
You should be familiar with the overall chemistry of
photosynthesis in plants:
carbon
dioxide
+ water
CO2 + H2O
glucose + oxygen
glucose + O2
Now, here is a simple question about this process:
Where does the oxygen (O2) come from? Is it the oxygen
originally in the CO2 or is it from the H2O?
If a plant is exposed to CO2 containing some atoms of a
different isotope of oxygen, that isotope will be later
detected entirely in the glucose.
However, if a plant is exposed to H2O containing some
atoms of the different isotope of oxygen, the isotope will
be later detected entirely in the oxygen gas released from
the plant.
Therefore, all the oxygen gas in our atmosphere (which
has been released from photosynthesising plants) was
originally in water molecules. This experiment has “traced”
the pathway of oxygen atoms through the process.
This is an extremely simple example of how the “tracer
method” can be used to study chemical pathways in living
cells.
This is just a “taste” of some important technologies. If interested, you might research the “Ultra-Centrifuge” (allows
parts of cells to be separated), Gas Chromatography (allows separation & identification of the dozens of chemicals in a
chain of reactions) and Automatic Sequencing equipment to study DNA and/or proteins.
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Slide 8
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Discusssion / Activity 1
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The following activity might be for class discussion, or there may be paper copies for you to complete.
If studying independently, please use these questions to check your comprehension before moving on.
Cell Types & Technologies
Student Name .................................
1. a) Outline the major differences between eukaryotic & prokaryotic cells.
b) For each named living thing, identify it as either eukaryote or prokaryote. (E or P)
palm tree ...........
mouse ...........
anthrax bacteria ............ mushroom .........
2. What accident of history led to our use of the word “cell” for these “units of life”?
3. a) In general terms, (no numbers required) how does the magnification & resolution of an electron
microscope compare to that of an optical (light) microscope?
b) What is the meaning of the 2 words underlined in part (a)?
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c) There are 2 types of electron microscope. Name them, and outline the differences in terms of the
pathway of electrons and how you would recognise an image formed from each type.
4. a) Outline what x-ray crystallography is, and what it can tell us about cell structure or function.
b) Outline what “isotopic tracing” is, and what it can tell us about cell structure or function.
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KISS Resources for NSW Syllabuses & Australian Curriculum.
2. Cell Structures
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Examining Cells
The syllabus requires that you examine a variety of cells; prokaryotic and eukaryotic. Hopefully, you will do some prac.work using a microscope to
examine fresh, living cells as well as prepared slides of eukaryotic cells. With a typical school light microscope you might not be able to examine
prokaryotes at all. You will probably study TEM & SEM images to become familiar with prokaryotic cells.
Sketching Cells Through the Microscope
Photos Taken Through a Microscope
You will probably learn how to use a microscope, look at some cells
through it and sketch them. You probably will NOT view bacteria (too
small), but might see the following examples.
Try to identify all the visible cell parts that you see and label them.
Human Blood
cytoplasm
magnified 400X
nucleus
cell
membrane
Even at maximum
magnification you will
probably not see any
detail
A simple water plant which grows
in hair-like filaments.
Low magnification, natural colour.
Paramecium
Always label
your sketches
These round
cells are in human
blood. The rod-shaped cells are
bacteria which cause a disease
called Anthrax.
Colours are caused by using a dye
to stain cells for easier viewing.
(unicellular organism)
magnified 100X
cytoplasm
cytoplasm
nucleus
cell wall
Onion Skin
magnified 100X
nucleus
cell
membrane
Human Cheek Cells
magnified 400X
Learn to sketch inside a circle which represents the “field of
view” of the microscope.
Sketch only a few of the cells, to scale.
Cross-Section of part of a Plant Stem.
Colours are due to staining.
Lots more images
in the following
section
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Slide 10
SEM photo. Colours are
computer enhanced.
The red cells are bacteria
infecting human tissue.
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Generalised
ANIMAL CELL
Cell Organelles Visible with a Light Microscope
Nucleus
Cell Wall
Generalised
PLANT CELL
on the outside of the
cell membrane
Cell
Membrane
Large
VACUOLE
Cytoplasm
Small
Vacuoles
(if any at all)
Chloroplasts
There are probably no actual cells
which looks just like these.
Real shapes vary greatly.
Differences Between Plant & Animal Cells
Plant cells have a tough CELL WALL on the outside of their cell
membrane. Animal cells never have a cell wall.
which absorb light
and make food for
the plant
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Many plant cells contain a large VACUOLE. Animal cells rarely have
vacuoles, and if present they are small.
Many plant cells contain CHLOROPLASTS. These are green in colour
because they contain the pigment chlorophyll. Chloroplasts are the sites
of PHOTOSYNTHESIS, where plants make food.
Note: not all plant cells have chloroplasts...
for example, cells in the underground roots cannot photosynthesise,
so do not contain any chloroplasts.
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Photo (through a microscope) of a
mass of plant cells. The dark blobs
are vacuoles of stored food.
What else can you identify?
The Electron Microscope reveals much more
detail than this... next slide.
Slide 11
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What the Electron Microscope Reveals
The superior magnifying power and resolution of the electron microscope has given us a much more
detailed knowledge of the cell and its organelles.
The diagram below left shows a plant cell with the added details that the electron microscope has revealed.
The extra organelles (labelled in blue) shown are generally NOT visible with a light microscope.
Vacuole
Cell Wall
Cell Membrane
Electron Microscope (TEM)
view of an animal cell
Chloroplast
internal structure
Stacks of flat
membranes (grana)
contain the chlorophyll.
Golgi
The tiny
Mitochondrion.
Site of cellular
respiration.
Ribosomes
Nucleolus
are often
attached
to the E.R.
Lysosomes
Nucleus
Nucleus
Lysosome
Extra detail
revealed.
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Endoplasmic Reticulum
(E.R.)
Golgi apparatus
Cell
membrane
A network of membrane
structures connected to the
nucleus & extending
throughout the cytoplasm.
Mitochondria
2 μm
Photo by Itayba
(used under Creative Commons Attribution-Share Alike3.0 Unported Licence)
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Cell Organelles... Structure & Function
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Note that the organelles detailed in the following slides are typically found
ONLY in eukaryotic cells. In fact, the presence of these structures defines a eukaryote.
The Nucleus
Nuclear membrane
with pores, for RNA exit
This is the control centre of the cell.
Nucleolus
for RNA
manufacture
Inside the nucleus are the chromosomes containing DNA,
the genetic material. There is often a nucleolus present.
This is the site for production of RNA, a “messenger”
chemical which leaves the nucleus carrying instructions to
other organelles. The nuclear membrane has holes or
“pores” to allow RNA to exit.
Mitochondria
Nuclear material
“chromatin”.
(Chromosomes unwound
and spread out)
(singular: mitochondrion)
Inner membrane
folded into “cristae”
with respiration
enzymes
attached.
This is where cellular respiration occurs
Glucose + Oxygen
(sugar)
Carbon + Water + ATP
Dioxide
The ATP produced by respiration carries chemical energy
all over the cell to power all the processes of life. The
mitochondria are therefore, the “power stations” of the cell,
converting the energy of food into the readily usable form
of ATP.
Inside a mitochondrion is a folded membrane with many
projections (“cristae”). This structure provides a greater
surface area, where the enzymes (control chemicals) for
respiration are attached in correct sequence for the steps
of the process.
Image of actual
MITOCHONDRIA using an
Electron Microscope (TEM)
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Sketch
of a
Mitochondrion
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This structure helps the organelle
do its job more efficiently.
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More Cell Organelles
Endoplasmic Reticulum (E.R.)
E.R. is a network of membranes which form channels and
compartments throughout the cytoplasm of the cell. Its function can be
compared to the internal walls of an office building which divide the
building into “rooms” where different operations can be kept separate
so that each does not interfere with others.
ENDOPLASMIC
RETICULUM
Membranes
Mitochondrion
E.R.
membranes
coated with
ribosomes
Membranes enclose
channels and “rooms”
RIBOSOMES
attached to membranes
The Golgi Apparatus is a semi-circular
arrangement of membranes which are concerned with
packaging chemicals into small membrane sacs
(“vesicles”) for storage or secretion.
GOLGI BODY
Curved
membrane sacs
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Lysosomes form
this way
Vesicles pinch-off for
storage or secretion
Nucleus
The E.R. structure provides channels for chemicals and
“messengers” to travel accurately to the correct locations, and
for chemical production to occur in isolation from other
operations.
This structure helps cells function
Often found attached to the E.R. are the tiny Ribosomes.
These are the sites of production of proteins, the main
structural and functional chemicals of living cells. RNA
“messengers” from the nucleus attach to a ribosome to make
the specific proteins that the cell needs.
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One type of “vesicle” produced
by a Golgi Body is the
Lysosome. These membrane
sacs contain digestive enzymes
which can destroy any foreign
proteins which enter the cell.
Lysosome enzymes also
rapidly digest the contents of a
cell which has died, so that
your body can clean up the remains and replace the dead
cell.
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Yet More Cell Organelles
Chloroplasts
“Stroma”
zone
CHLOROPLAST
Chloroplasts are found only in
photosynthetic plant cells.
The electron microscope has
revealed that the chloroplast is not
just a bag of chlorophyll, but has an
organised internal structure
which makes its functioning
more efficient.
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Stroma
1 μm
Grana
Membrane stacks
(“grana”)
containing chlorophyll
Double
membrane
envelope
TEM Photo by and3k & caper437
(used under Creative Commons Attribution-Share
Alike3.0 Unported Licence)
The “grana” are stacked membrane sacs containing chlorophyll, which absorbs the light energy for photosynthesis. This
light-capturing step is kept separate from the “stroma” zone, where the chemical reactions to make food are completed.
The Importance of Membranes
Except for the tiny ribosomes, all the cell organelles
are built from, and surrounded by, membranes.
The membranes provide:-
• the infrastructure of the cell.
• channels for chemicals to move through.
• packaging for chemicals which need to be stored.
• points of attachment for chemicals (“enzymes”).
• control over what moves in or out of each organelle,
and in or out of the entire cell.
The “membrane-bound” organelles help the cell’s various
functions to be carried out with greater efficiency.
Having these membrane-based organelles is the defining
characteristic of the “Eukaryotic” group of organisms, which
includes all plants & animals.
Prokaryotic cells (such as bacteria) do have lots of tiny
structures inside, but do NOT have any membrane-type
organelles, and can only operate efficiently by being very small.
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Although not specified by the syllabus,
it will help greatly if you have some basic knowledge about...
Chemicals in Cells
The Chemicals
That Cells Are Made From
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INORGANIC CHEMICALS
These include small simple molecules like water (H2O)
and carbon dioxide (CO2), as well as mineral ions such
as calcium, nitrate, phosphate, chloride, etc.
Although these are often considered of lesser
importance, you should remember that all living things
are 75%- 95% water.
Carbohydrates
ORGANIC CHEMICALS
“Organic” chemicals are based on the element carbon,
which can form chains, rings and networks and so build
the very complex molecules needed to make a living cell.
Many are “polymers” made by joining together many
smaller molecules to form huge “macro-molecules”.
Proteins
Lipids
Nucleic Acids
There are four main categories to know about... Next slide.
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ORGANIC CHEMICALS
®
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LIPIDS are the fats and oils.
All cell membranes are built from lipid & protein.
Lipids are used as a way to store excess energy.
Carbohydrates can be converted to fat for storage.
PROTEINS
are the main structural chemicals of organelles, cells,
bone, skin & hair. Life is built from protein.
Proteins are polymers, made from amino acid molecules
joined in chains.
Amino acid
molecules
NUCLEIC ACIDS
Part of a protein molecule...
a chain of amino acids
(DNA & RNA)
are the most complex of all.
DNA is the genetic information of
every cell. RNA is the “messenger”
sent out from the nucleus to control all
cell activities.
DNA is a huge polymer of sugars,
phosphate and “bases” coiled in a
double helix shape.
CARBOHYDRATES
include the sugars, starch and others.
monosaccharides
(mono = one, saccharide = sugar)
are simple sugars such as glucose C6H12O6
disaccharides (di = two)
are sugars made from TWO monosaccharides joined together,
such as “table sugar” (sucrose).
polysaccharides
(poly = many)
are huge molecules made from thousands of sugar molecules
joined in chains or networks.
Examples are:
Starch... made by plants, to store excess sugar.
Glycogen... made by animals, to store sugar.
Cellulose... made by plants as a structural chemical.
The CELL WALL of a plant cell is made from cellulose.
Disaccharide
Monosaccharide
sugar
sugar
molecules
Polysaccharide.
Small part of a
Starch molecule
Uses of Carbohydrates
Sugars are energy chemicals. Glucose is made by plants in
photosynthesis, and is the “fuel” for cellular respiration to make ATP
to power all cells.
Starch & Glycogen are polymer molecules used to store sugars as a
food reserve. Starch is the main nutrient chemical in the plant foods
we eat.
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Cellulose & Lignin are polymers of sugar used by plants structurally.
Cellulose makes the tough cell wall of all plant cells. Lignin is a strong
material used to reinforce the walls of “veins” in plants.
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The Structure of the Cell Membrane
The electron microscope and other modern analysis methods have revealed the structure of the
membranes which surround a cell and form most of the cell organelles.
The membrane is extremely thin; just two molecules thick. The basic
chemical unit is a “phospholipid” molecule; a lipid (fat) with phosphate
groups attached. Each molecule has two distinct ends; one which is
attracted to water molecules (“hydrophilic”) and the other is repelled
by water (“hydrophobic”).
“Hydro” = water. “philic” = to like. “phobic” = hate / fear.
Other molecules are embedded in the phospholipid bilayer. They are
mostly proteins, many with carbohydrates attached.
Membrane proteins
MEMBRANE STRUCTURE
Outside of cell
One
phospholipid
-philic
-phobic
Inside of cell
Double layer of
phospholipid molecules
Two layers of phospholipids form each membrane. The molecules cling
to each other, and line up with their hydrophilic ends outwards. The
water-loving ends are attracted to the watery environment both inside
and outside the cell.
The hydrophobic ends are repelled from the watery surroundings, and
cling together inside the membrane itself. A membrane is like a thin
layer of oil floating on water. It is fluid and flexible, but clings together
forming an unbroken “skin” on the surface of a cell.
The membrane is NOT solid: it is in fact a liquid or “fluid” structure. It
is held together by the mutual attractions of the phospholipid
molecules. At the microscopic level, these attractive forces are strong
enough for the fluid layer to form a barrier between the inside &
outside of the cell.
These other molecules have various functions:
• “receptors” for messenger chemicals.
• identification markers, so your body knows its own cells from any
foreign invaders.
• to help chemicals get through the membrane.
This concept of the membrane is called the “Fluid-Mosaic Model”:
this refers to a liquid structure and the different molecules embedded
within it are like the different shapes & colours in a “mosaic” tile
pattern.
Electron Microscope
(TEM)
view of part of a cell.
Golgi
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The double-layer
structure of the cell
membrane is clearly
visible.
Nucleus
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Modelling the Cell Membrane
®
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A simple model may help you understand the cell membrane a little better.
You might make a more sophisticated model in class. If not, you can easily do this at home.
You need about 100 “cotton buds”, plastic drinking straw, sticky tape, rubber band.
How the Model Relates to a Cell Membrane
Step 1
Line up about a dozen cotton buds in a
neat row. Use sticky tape to hold them
together in a flat “panel”.
1. Chemical Structure
Each cotton bud represents two phospholipid molecules joined tail-totail. In the top photo a texta line has been drawn to emphasise this.
Repeat, until you have at least 5-6
“panels” of cotton buds.
The cotton wool represents the hydrophilic “head” of the molecule. The
shaft represents the two hydrophobic tails clinging together.
Step 2
2. Flexibility
If you gently manipulate your model, you can see that the entire
structure is flexible. Real membranes are thought to be even more
flexible and in fact are a liquid structure: the phospholipid molecules
cling together, but the wall of molecules can warp & bend without
rupturing.
Stack your panels neatly on top of each
other so the model becomes more & more
3-dimensional. Between two of the middle
panels, place one or two cut pieces of
plastic drinking straw.
Membrane
Outside
of cell
Intside
of cell
Step 3
Gently wrap a
rubber band loosely around your stack of
panels. This holds the entire model together
so it can be placed upright & gently
manipulated. Be gentle & careful or else the
cotton buds will tend to begin pointing in all
directions & lose their nice parallel pattern.
Schematic
diagram
of a cell
membrane,
according to the
“Fluid Mosaic
Model”.
3. Acts as a Barrier, but Some Things Can Get Through
Stand your model upright on the bench. Now gently sprinkle a few
grains of rice (or similar) onto the cotton wool heads. Notice that the
rice sits on top & cannot penetrate your “membrane”.
Now sprinkle a few grains above the drinking straws.
Your membrane can also let things through!
(Be aware that this is a very simplistic model of what really happens!)
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Image by
Lady of Hats
(Mariana Ruiz)
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Discusssion / Activity 2
The following activity might be for class discussion, or there may be paper copies for you to complete.
If studying independently, please use these questions to check your comprehension before moving on.
Cell Structure
Student Name .................................
1. Name the parts labelled a,b,c,etc. in this plant cell.
All are visible with a light ‘scope.
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2. Match the lists. (connect matching items with an arrow)
Cell Organelle
Mitochondria
Endoplasmic Reticulum
Nucleus
Golgi Apparatus
Ribosome
Chloroplast
c
a
e
d
b
f
Function or Description
Makes proteins
Makes food using light energy
Control centre of cell
Cellular respiration site
Packaging of chemicals
Network of membranes, internal compartments
3. Underline any organelle in Q2 which is usually only visible using an electron microscope.
4. a) The basic chemical unit in a membrane is a “phospholipid”. What is this?
b) In what important way are the 2 ends of each molecule different?
(In your answer use “hydrophilic” & “hydrophobic”, and define these words)
c) The structure of the cell membrane is described by the “Fluid-Mosaic Model”.
In what way is it “fluid”?
In what way is it “mosaic”?
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3. Cell Functions
®
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a) How Stuff Gets In & Out
How Chemicals Pass Through Membranes
keep it simple science
The cell membrane as the boundary of a cell is a bit like growing a plant hedge as the boundary of a field.
It stops the cows and horses getting out, but a mouse, or a lizard, can easily crawl through it.
Similarly, a membrane is “semi-permeable”; it prevents most (especially large) molecules getting through, but
allows others to pass through easily. Small molecules like water (H2O), oxygen (O2) and carbon dioxide
(CO2) pass freely through the membrane like a lizard through a hedge.
Many chemical substances
constantly move in and out of
a living cell.
To understand how this happens, you must learn about the processes of DIFFUSION & OSMOSIS.
Diffusion
Diffusion occurs in every liquid or gas because the atoms
and molecules are constantly moving. The particles
“jiggle” about at random in what is called “Brownian
motion”. (Named for its discoverer Robert Brown, the same
man who discovered the cell nucleus.)
Imagine a water solution containing a dissolved chemical,
but it is NOT evenly distributed... it is more concentrated in
one place than elsewhere. As the molecules jiggle about at
random, they will automatically spread out to make the
concentration even out. This process is called DIFFUSION.
To start
with, the
dissolved
material is
not evenly
distributed.
In a living cell, there is often a “concentration gradient”
from the outside to the inside of the cell.
For example, because a cell keeps consuming oxygen for
cellular respiration, the inside of the cell usually has a low
concentration of O2 dissolved in the water of the cytoplasm.
On the outside, there may be a lot of O2.
DIFFUSION of SMALL MOLECULES into a CELL
If the molecules can cross the membrane, diffusion will cause them to
move from higher to lower concentration.
Higher
concentration
outside cell
High
concentration
Lower
concentration
inside
Later
Lower
concentration
Diffusion
causes the
dissolved
solute to
spread out
uniformly.
Equal concentration
throughout
DIFFUSION DRIVES MOLECULES THROUGH THE
MEMBRANES along the concentration gradient.
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Osmosis
®
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Osmosis is a special case of diffusion which occurs when the concentration gradient involves
dissolved molecules or ions which CANNOT get through the membrane.
For example, consider a cell which is surrounded by a
solution containing a lot of dissolved sugar. The sugar cannot
diffuse through the membrane to equalise the concentrations.
In such a situation, water (which can go through the
membrane) will diffuse toward the high sugar concentration,
as if attempting to equalise by diluting the sugar.
OSMOSIS
High
concentration
of sugar
outside cell
KISS Resources for NSW Syllabuses & Australian Curriculum.
Water diffuses
OUT of cell
H 2O
The opposite situation can also happen. A cell’s cytoplasm
contains many dissolved chemicals. If the outside
environment around the cell is more watery (less
concentrated in dissolved substances) then osmosis will
cause water to diffuse inwards.
This can cause cells to “pump up” with water and helps
maintain their shape. It can also cause problems for
organisms living in fresh water environments.
Dissolved chemicals
cannot diffuse out...
H2O
H 2O
H 2O
H 2O
This is how plants
absorb water into their
roots, even when the
soil seems almost dry.
Sugar cannot get
in through the
membrane
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In this case, the cell will lose water and might shrink and
shrivel up.
Loss of water by osmosis can be a problem for living
things in water environments with high levels of dissolved
chemicals such as salt.
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...so water diffuses into
the cell.
Slide 22
H 2O
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Observing Diffusion & Osmosis
®
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Osmosis
Diffusion
Your teacher may have a more sophisticated experiment
for you to do. If not, this is a simple activity you could do
in class or even at home.
You might do one of these activities yourself,
or see it demonstrated.
one drop
of food
colour
dye
The food colour
spreads out
through the water
by itself.
Water
Without any
stirring, it automixes through the
water.
Cut 2 pieces of fresh celery from the same stick. Trim
them to be exactly the same length.
If possible, pat dry with a tissue, then weigh each to the
nearest 0.1g and record.
Fluids (liquids and gases) seem to be able to mix
themselves together automatically... “Diffusion”.
Gas Jar
of air
glass
separator
Gas Jar of
brown gas
When the
separator is
removed, the
two gases mix
themselves
together.
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The explanation is in the Moving-Particle Model of
matter. In liquids and gases, the particles are moving
around. If 2 different gases or liquids are side-by-side,
then the moving particles will automatically mix.
Any dissolved molecules will spread out evenly.
Is diffusion faster in liquid or gas?
What effect would temperature have?
Place each into a small beaker of water or salt solution, as
shown and leave overnight.
Identical pieces of
celery soaked in
different liquids
for 24 hours.
Pure
Water
One loses water
due to osmosis.
conc.
Salt
soln.
Next day, pat each piece dry with a tissue and re-weigh.
One piece of celery may have lost a small amount of mass.
Compare their lengths.
They might not be exactly the same any more.
Bend or cut each piece and note the texture and
“crispness”. One will be hard and crisp, the other softer
and “rubbery”.
Try to explain these results on the basis of movement of
water (NOT salt!!) in/out of the living celery cells.
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Other Ways Substances Get Through Membranes
Passive & Active Transport
Diffusion and Osmosis are vitally important for many
chemicals (especially water) to get in and out of cells.
Diffusion and osmosis happen automatically and without
the cell having to use any energy. We say these are
“passive transport” processes.
What about all the other important chemicals which cannot
get through the membrane? Many proteins, carbohydrates
and other molecules regularly move into or out of cells.
How do they get in or out?
Cells have other ways to deliberately move substances
across the membrane apart from diffusion and osmosis.
The membrane contains special protein channels &
mechanisms which can “carry” chemicals through the
membrane.
Some Active Transport Mechanisms
Sodium-Potassium Pump
One notable example of an active transport mechanism is the “Na-K pump” which
is present in every animal cell.
Background info: Animal cells are the only cell type with NO cell wall. This means that if
they swell up tightly with water (become “turgid”) they are in danger of bursting open. In
contrast, a plant cell has a tough, rigid cell wall. If a plant cell becomes turgid there is little
danger of bursting... plants habitually keep their cells turgid to support their leaves, etc.
The Na-K pump is like an air-lock system with 2 doors, but only one door can ever
be open at any one time. By oscillating between these 2 “doors” the mechanism
actively pumps sodium ions (Na+) OUT of the cell and potassium ions (K+) INTO
the cell. This allows the cell to maintain an “osmotic pressure” which prevents it
absorbing excess water and bursting.
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Concentration
maintained
These ways to transport materials across membranes
require the cell to use energy (ATP from cellular
respiration) to move substances. We say these are “active
transport” processes.
Not only can “active transport” move substances which
cannot normally penetrate the membrane, but it can even
do so against the concentration gradient.
An analogy to this might help: a passive process, such as
diffusion, is like water running downhill in a pipe. It
happens naturally without any energy expenditure.
However, active transport is like using a pump to force
water uphill through the pipe. Energy will be required to
run the pump to push water against gravity.
Schematic of the Na-K Pump
Inside of Cell
Image by Lady of Hats (Mariana Ruiz)
Note how the “2-door system” requires ATP (the energy chemical made in the
mitrochondria) to power it... it is “active” transport.
As well as maintaining osmotic pressure in every animal cell, the Na-K pump is
essential for the sending of nerve signals, for kidney function & many other body
processes.
As well as the Na-K pump, there are other “channel-based pumps” which move
specific chemicals through the membrane.
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Endocytosis
(“endo” = into / inside, “cyto” = cell, “-sis” implies a process)
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Another, quite different
mechanism involves a
number of related
processes collectively
known as “endocytosis”.
To keep it as simple as
possible (KISS Principle)
this involves the
membrane pinching
outwards to surround the
desired substance and
envelop it.
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The membrane then
rejoins to itself to seal
the cell, leaving the target
substance inside, sealed
in a small vacuole or
Image by
“vesicle”.
Lady of Hats
Phagocytosis
Pinocytosis (“Pino” = drink)
is a version of endocytosis by which cells take in a small parcel of fluid, including any
dissolved chemicals.
(“Phago” = eating, “cyto” = cell)
is a version of endocytosis which takes
solid particles into a cell. The best known
occurrence involves a type of white blood
cell called a “phagocyte” (literally an “eating
cell”) which absorbs infectious germs, dead
cells & fragments. Once inside the
phagocyte the encapsulated solids are
destroyed & digested by a cocktail of
enzymes.
is another variation which can target specific chemicals which the cell needs to absorb, such as
hormones or specific types of protein. The “targetting” is achieved by receptor molecules
embedded on the outer surface of the membrane in a shallow “coated pit”. A receptor can
recognise (by shape) the target molecules & “lock-on” by forming a loose chemical bond. Once
the receptors are “loaded”, the membrane is stimulated to encapsulate the pit into a vesicle
taken inside the cell.
This is also the mechanism which many
single-cell organism eat food particles.
Later, the vesicle membrane is dissolved to release the absorbed chemical, possibly after being
transported to the appropriate cell location which needs the target substance.
Receptor-Mediated Endocytosis
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Exocytosis
(“exo” = outside)
As well as moving substances into the cell, there are many substances which need to be moved out of the cell.
“Secretion” refers to chemicals being released by a cell for some useful purpose. For example, the cells of your salivary glands secrete
saliva into the food while you chew it. This moistens the food for easier swallowing, but also begins digesting the food with a digestive enzyme in
the saliva. Nerve cells secrete a “neurotransmitter” chemical across the nerve synapse to make the nerve signal carry on into the next neuron cell.
Chemicals destined for secretion are often packaged inside small vacuoles by the golgi body organelles. The actual secretion process occurs
rather like endocytosis running in reverse. (However, in full technical detail there are significant differences.)
“Excretion” refers to the removal of unwanted, possibly toxic, waste materials. These may be encapsulated in small vesicles to protect the
inside of the cell from possibly dangerous substances.
The actual removal of these wastes follows the same pathway as for secretion... the process of exocytosis.
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Both Endocytosis & Exocytosis, in all their variations, require the cell to use energy...
they are ACTIVE TRANSPORT processes.
The energy is supplied in the form of ATP, manufactured in the mitochondria by cellular respiration.
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Importance of the Surface Area to Volume Ratio
Why are cells so small? The answer requires a mathematical study...
Consider this series of cubes of increasing size:
Length of
one side
= 1 unit
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Length of
one side
= 2 units
Length
of one
side
= 4 units
Length of
one side
= 3 units
Surface Area:
Six squares, each 1x1
SA = 6 sq.units
Surface Area:
Six squares, each 2x2
SA = 24 sq.units
Surface Area:
Six squares, each 3x3
SA = 54 sq.units
Surface Area:
Six squares, each 4x4
SA = 96 sq.units
Volume = 1x1x1 = 1 cu.unit
Volume = 2x2x2 = 8 cu.unit
Volume = 3x3x3 = 27 cu.unit
Volume = 4x4x4 = 64 cu.unit
SA = 6
vol
SA = 3
vol
SA = 2
vol
SA = 1.5
vol
Notice that as the cubes get larger: • Surface Area increases, and...
• Volume increases, but...
• SA / Vol Ratio DECREASES, because the volume grows faster than the surface area.
This pattern is the same for any shape... as any object gets bigger, the ratio between its Surface Area and its Volume gets smaller.
What’s this got to do with cells?
The amount of food, oxygen or other substances a cell needs depends on its volume... the bigger the cell, the more it
needs according to its volume. But, all cells have to get whatever they need in through their cell membrane,
and the size of the membrane is all about surface area.
Cells must feed their Volume, through their Surface Area
As any cell gets bigger, it becomes more and more difficult for it to get enough food, water and oxygen because its
SA/Vol. ratio keeps shrinking. Getting rid of waste products also becomes more difficult.
Large cells are impossible... all single-celled organisms are microscopic, and all larger organisms are multi-cellular.
The only way to be big is to have lots of small cells.
What is true for cells, is also true for membrane-based organelles. It is better to have many, small mitochondria rather than a few larger ones.
A larger mitochondrion has a lower SA/Vol. ratio. It will be less efficient at absorbing glucose & oxygen & getting rid of wastes,
and exporting ATP to where it is needed. In a cell which uses a lot of energy (eg muscle cell) it is always found that
there are a multitude of small mitochondria, never just a few very large ones.
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How Stuff Gets In & Out... a Summary
When you think about substances moving
through a membrane, there are 3 factors to be
considered:
SA / Vol. Ratio
This ratio basically determines whether a cell (or
organelle) is able to transport enough materials in
& out across the membrane to meet its needs. You
now know that the smaller the cell is, the higher
the ratio, so the more likely it is to achieve
sufficient supply of nutrients and removal of
wastes.
Be aware that this is NOT entirely about size...
shape matters as well. Elongated, irregular shapes
with lots of folds & projections have higher ratios
than compact, regular shapes like a sphere.
Concentration Gradient
For substances which can cross a membrane by passive transport (diffusion & osmosis of
water) the difference in concentration of the substance inside the cell compared to its
concentration on the outside is another important factor.
The bigger this difference, or “concentration gradient”, the faster will be the rate of diffusion.
The Nature of the Substance
Finally, the nature of the chemical substance itself can have a big effect.
For example, think about oxygen, a small molecule which can pass through the membrane
easily. If there is a large concentration gradient, its rate of diffusion through the membrane
can be so fast that this can partly compensate for a poor SA/Vol ratio.
Conversely, dissolved ions or large proteins must rely on active transport to cross the
membrane. In this case, not only is the SA/Vol ratio involved, but also the rate at which the
cell can supply energy to drive the “pump”, or endocytosis cycle, or whatever active
process is involved.
Why are Prokaryotes so Small?
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Now we will try to answer a question which arose near the very beginning of this topic.
Typically, a prokaryotic cell (eg bacterium) is only about 1/10 the size of an average animal cell, or even less in
many cases. In this photo we can only see a small part of the human cell... it is 100 times larger than the 2
bacterial cells.
There are 2 things to consider to understand why prokaryotes are so small:
Organelles
The presence of membrane-based organelles in eukaryotes makes all their functions much more efficient.
Organelles allow the cell to carry out specialist functions in an enclosed space with all the control chemicals
(enzymes) in place. The chemicals involved in a process are concentrated together where needed and other
cellular processes cannot interfere with whatever the organelle is doing.
SA / Vol. Ratio
SEM image of bacterial cells being
attacked by a human immune cell.
Photo: NIAID (used under Creative Commons
Attribution-Share Alike 2.0 Licence)
Without any membrane-based organelles, a prokaryotic cell is inherently far less efficient. The only way it can
thrive is to be as efficient as possible by having a high SA/Vol. ratio. This can only be achieved by being very
small. Therefore, all prokaryotic cells are relatively small.
Get it?
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Discusssion / Activity 3
The following activity might be for class discussion, or there may be paper copies for you to complete.
If studying independently, please use these questions to check your comprehension before moving on.
Stuff Gets In & Out
Student Name .................................
1. Membranes are described as “semi-permeable”. What does this mean? Give examples.
2. Explain the difference between Diffusion & Osmosis using the phrase “concentration gradient”.
3.
a) What is the difference between “active” & “passive” transport?
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b) To which category do diffusion & osmosis belong?
c) In general terms, what are “endocytosis” & “exocytosis”? How are they different?
4. As any shape gets larger, what happens to its:
a) surface area?
b) volume?
c) SA/Vol ratio?
b) How does this relate to living cells and why they are always microscopically small?
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3. Cell Functions
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b) Food & Energy for Cells
Autotrophs & Heterotrophs
(auto = “self”, hetero = “other, not self”, troph = “to eat, feeding”)
keep it simple science
An autotroph is an organism that makes its own food. All plants are autotrophic, making their own food by photosynthesis.
Any organism that cannot make its own food must be a heterotroph. All animals are heterotrophic, and so are the fungi
and most bacteria. A heterotrophic animal eats plants or other animals which have eaten plants,
and so on according to the food chain involved.
Photosynthesis in Plants
6H2O
All plants make their own food from the simple, low-energy
raw materials water (H2O) and carbon dioxide (CO2) using
the energy of sunlight, to make the high-energy sugar
glucose (C6H12O6), with oxygen gas (O2) as a by-product.
ligh
t en
erg
y
The energy of light is
absorbed by chlorophyll,
the green pigment in the
leaves of plants.
from
air
Water & CO2
are low-energy
chemicals
high-energy
sugar (food)
released
to air
The energy of the light
is being stored as
chemical energy in the
glucose molecules
6CO2
C6H12O6
+
6O2
Summarising photosynthesis with this brief equation is
very deceptive. Photosynthesis actually occurs as a
complex series of chemical steps inside the chloroplast.
There are 2 main stages, which take place in different parts
of the chloroplast, as summarised below.
PHOTOSYNTHESIS in the CHLOROPLAST
WATER + CARBON chlorophyll GLUCOSE + OXYGEN
DIOXIDE
from
soil
+
ligh
t
Phase 1
In the grana,
chlorophyll absorbs
light energy and uses
it to split water molecules into
hydrogen and oxygen.
The oxygen is released.
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Slide 30
Phase 2
In the stroma,
a cycle of
reactions
builds
glucose from
CO2 and the
hydrogen from
the water.
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Cellular Respiration
®
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How Organisms Use Energy
Everything that an organism does requires energy.
Organisms:Move
Grow new cells & Repair body tissue
Reproduce
Seek, Eat and Assimilate their food
Respond to happenings around them
Keep their bodies warm
ADP+P
is the process which releases the energy stored in food.
It takes place in every living cell on the planet and after photosynthesis
is the next most important biological
to
process on Earth.
energy
s
r
e
f
s
tran
ATP
ocess
The pr
Major energy
compound in
foods
in air
Carbon + Water
Dioxide
Waste
products
Although the process can be written as a simple chemical reaction,
this is very deceptive. Cellular respiration actually takes place as a
sequence of about 50 chemical steps... this equation is merely a
summary of the overall process.
C6H12O6 + 6O2
Cellular Respiration
Glucose + Oxygen
(sugar)
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Energy-carrying
chemical used in
all cells to power
life processes.
More About ATP
ATP stands for “adenosine tri-phosphate”.
The molecule can be represented by this simple diagram:
Adenosine 3 phosphate groups
The bond holding the 3rd phosphate
P
P
group contains a lot of chemical
P
energy.
High-energy bond
ATP will readily transfer the 3rd phosphate group to other
chemicals (with help from an enzyme). When this occurs,
energy is transferred which can force other reactions to go.
6CO2 +
energy transfer
6H2O
ATP
Don’t forget that the essential product of respiration is the energycarrier “ATP”. The CO2 and H2O are merely waste products to be
recycled in the ecosystem like all chemicals. Each 1 molecule of
glucose results in the production of up to 38 molecules of ATP.
A common misconception is that plants do PHOTOSYNTHESIS and
make food, while animals do RESPIRATION to use the food.
It’s true that plants do photosynthesis and make (virtually) all the
food on Earth, but respiration is carried out by all living things...
animals AND plants.
Luckily for us animals, the plants carry out enough photosynthesis
to feed themselves AND produce a surplus to feed us as well.
ADP=adenosine di-phosphate
The molecule now has only
2 phosphate groups, so it is
called “ADP”.
P
P
P
Energy transfer when
P-group is detached
ATP is the “energy currency” of a cell. It can transfer
energy to power any process. Then, the ADP goes back
to a mitochondrion and is “re-charged” when energy
from glucose (via cellular respiration) is used to join
another phosphate group on to make ATP again.
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Photosynthesis & Cellular Respiration
You will have noticed that these two vital processes, when written as summary equations, are exact opposites.
This is really not true because the precise chemical pathway of one process is NOT the opposite of the other.
They both follow complex, multi-stage, quite different pathways.
What is really happening is ENERGY FLOW through the food chains of
an ecosystem. Photosynthesis captures the energy of light and stores
it in a high energy food compound like glucose. Cellular respiration
releases that stored energy in the form of ATP which can power all
cellular and life activities... growing, moving, keeping warm etc.
As you have learned previously, in all ecosystems there is a constant
input and flow of energy via the food chains, while the chemicals
such as H2O, O2, and CO2 simply get re-cycled over and over.
The Most Important Process on Earth
Photosynthesis makes virtually all the food on Earth, for all living things.
It also makes all the oxygen in the atmosphere for us animals to breathe.
Light energy
For these two reasons, photosynthesis has to be considered the
most important biological process on the planet.
CARBON
DIOXIDE
+
WATER
ATP
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CHLOROPLAST site of
photosynthesis
GLUCOSE
+
OXYGEN
MITOCHONDRIA - site of
cellular respiration
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What Happens to Glucose in a Plant?
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If photosynthesis only makes glucose, where do all the other biological chemicals in a plant come from?
Glucose is a monosaccharide sugar, a member of the carbohydrate
group. It is easy for a plant to convert glucose into other types of
carbohydrate.
GLUCOSE
molecules
Glucose can also be converted chemically into lipids... fats and oils,
since they contain exactly the same chemical elements (carbon,
hydrogen & oxygen only - CHO).
Other sugars,
such as sucrose
joined in pairs
’s
00 n)
10 tio
in isa
ed er
in m
jo oly
(p
CELLULOSE
for building new cell walls
GLUCOSE
LIPIDS (oils)
Making proteins and nucleic acids is more difficult, since these
contain additional chemical elements, especially nitrogen, phosphorus
and sulfur.
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This is where the “minerals” such as nitrate, phosphate and sulfate
come in. Soil minerals are often called “plant nutrients”, and a
gardener may say he/she is “feeding” the plants when applying
fertiliser, but these minerals are NOT food.
They are the essential ingredients needed so plants can make proteins
and DNA etc, from the real food... glucose.
STARCH
for storage of food
In fact, plants convert glucose to STARCH so rapidly that the cells in
a plant leaf become packed with starch grains when it is
photosynthesising.
THIS IS THE BASIS OF EXPERIMENTS YOU MAY HAVE DONE
(See next slide)
Amino
acids
Soil minerals
nitrate, sulfate, etc
GLUCOSE
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chemical
conversion
Slide 33
Amino
acids
PROTEIN
Polymerisation
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Experiments with Photosynthesis
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The classic experiment you have probably done, is to partly cover a leaf with light-proof aluminium foil,
and then expose it to light for several days. The aim is to prove that light is necessary for photosynthesis.
Lig
ht
No light,
no starch
Result
Alu
m
Experimental
Set-up
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iniu
mf
oil
After several days, the leaf is decolourised (so the test can
be seen more easily) and then tested with IODINE solution.
Why Iodine? It detects STARCH, not glucose.
As explained before, the glucose produced by
photosynthesis is immediately converted to starch. The
iodine test is used because it is the test for starch.
Iodine test shows
lots of starch here
Sure enough, you probably found that any part of the leaf
exposed to light turned black when soaked in iodine,
while parts under the foil did not go black.
This shows that any part of a leaf allowed to
photosynthesise will build up a store of starch from the
glucose it makes. The first product of photosynthesis is
glucose, but it is rapidly converted to other things.
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Cell Requirements
ENERGY
SIMPLE
CHEMICALS
WASTE REMOVAL
NEEDS
Summary: What Cells Need
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Animal Cells
eukaryotic heterotrophic
Bacterial Cells
prokaryotic
Light for photosynthesis
Complex, high energy carbohydrates
(or lipids & proteins which can be
converted) made by other organisms
Varied.
Some are autotrophs requiring light.
Others are “chemotrophs” requiring
certain inorganic chemicals (eg SO2 ) as
energy sources.
Many are heterotrophs which feed on
plant/animal wastes. Somes weirdos can
feed on chemicals such as petrol.
H2O & CO2 (photosynthesis)
H 2O
All need H2O
O2 (cellular resp.)
O2
A range of simple inorganic
“minerals” (ions) including
nitrates, phosphates, sulfates,
calcium, magnesium, etc.
A range of “minerals” & “vitamins”
which are generally supplied in a
“balanced diet”. (What this means
varies from one species to another)
Beyond that, the needs are highly varied.
Many require O2, but others are poisoned
by it. Photosynthetic types need CO2,
while some need SO2 or CO2 & H2 for
chemosynthesis.
All have a need for simple ions like
calcium, potassium or iron,
but precise details vary.
In daylight, surplus O2 is
“excreted” by simple diffusion
from cells, then to the air via
stomates.
Most animals cannot tolerate a
build-up of CO2. Individual cells
excrete it by simple diffusion, but
then a specialist system involving
blood transport, lungs or gills, etc. is
needed to remove it from the body.
Another critical toxic waste is urea
(from protein metabolism) excreted in
urine via the kidneys, or similar.
Plant Cells
eukaryotic autotrophic
Plants such as mangroves
may need to excrete excess
salt by active transport from
specialist cells.
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Slide 35
Waste products can be CO2, methane,
metal sulfides, lactic acid, etc.
depending on exactly how each species
gets its energy.
However, since all prokaryotes are singlecelled, excretion is carried out by simple
diffusion, or by active transport across
the cell membrane.
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Discusssion / Activity 4
®
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The following activity might be for class discussion, or there may be paper copies for you to complete.
If studying independently, please use these questions to check your comprehension before moving on.
Food & Energy for Cells
Student Name .................................
1. a) In plants, photosynthesis occurs in 2 stages, in different parts of a chloroplast.
Outline these 2 stages and precisely where each occurs.
b) How many molecules of water & CO2 are required to produce one molecule of glucose?
2. When you look at the summary chemical equations for photosynthesis & cellular respiration, they
seem to be exactly opposite processes. Comment on this statement.
3. Describe the ATP
living cell.
ADP cycle & explain why ATP can be considered as the “energy currency” of a
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4. Compare & contrast what exactly is needed to supply energy to an autotroph compared to a
heterotroph (assume eukaryotic cells).
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3. Cell Functions
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c) Biochemical Control... Enzymes
Metabolism is Chemistry
The Importance of Shape
Everything that happens inside a living cell is really a matter of
chemistry... “metabolism”.
For example...
Many of the properties of enzymes are related to their precise 3dimensional shape.
• For your body to grow, cells must divide and add more membranes,
cytoplasm and organelles. This involves the chemical construction of
new DNA molecules, new phospholipids for membranes and so on.
The shape of the enzyme fits the
“substrate” molecule(s) as closely
as a key fits a lock.
Enzyme
molecule
• All these chemical reactions require energy. Energy is delivered by the
ATP molecule, itself the product of a series of chemical reactions in the
mitochondria... cellular respiration.
All of these reactions are “metabolism”: the sum total of all the
thousands of chemical reactions going on constantly in all the billions of
cells in your body.
Enzymes
Every reaction requires a catalyst... a chemical which speeds the reaction
up and makes it happen, without being changed in the process. In living
cells there is a catalyst for every different reaction.
Various
Only this
one fits
Different
Substrate
Molecules
This is why enzymes are “substrate-specific”...
only one particular enzyme can fit each substrate molecule. Each
chemical reaction requires a different enzyme.
Changes in temperature and pH (acidity) can cause the shape of the
enzyme to change. If it changes its shape even slightly, it might not fit
the substrate properly any more, so the reaction cannot run as
quickly and efficiently. This is why enzymes are found to work best at
particular “optimum” temperature and pH values.
Biological catalysts are called enzymes.
• Enzymes are protein molecules.
• Each has a particular 3-dimensional shape, which fits its “substrate”
perfectly.
• Enzymes are highly “specific”. This means that each enzyme will only
catalyse one particular reaction, and no other.
• Enzymes only work effectively in a relatively narrow range of
temperature and pH (acidity).
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Substrate...
Enzyme shape
at optimum pH
and
temperature
Slide 37
Shape changes
slightly at
different pH or
temp.
...no longer
fits enzyme
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From Amino Acids to Enzyme to Metabolic Control
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This schematic diagram outlines how an enzyme is made & how it can control a metabolic reaction in a cell.
Amino acid
molecules
Protein, with precise 3-D shape...
Polypeptide
chain
Polymerization
...becomes an
ENZYME molecule
d
de yme
l
fo
y enz
l
se n
ci is a
e
Pr in
e
ot
pr
Substrate
molecules are chemically
attracted to
the enzyme’s active site
Enzyme’s
“Active Site”
has a shape to fit the
substrate(s) exactly
Twists & folds
ENZYME
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Product released
from enzyme
ENZYME
Substrate molecules brought
together and react with each other
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Slide 38
ENZYME can react with
more substrate
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Enzyme Activity Graphs
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You may do experimental work to measure the “activity” of an enzyme under different conditions of temperature or pH.
You may have measured the rate of a chemical reaction being catalysed by an
enzyme, such as:
• the rate of milk clotting by junket tablets.
• the rate of digestion of some starch by amylase.
A common way to measure the rate of a reaction is to
measure the time taken for a reaction to reach
completion... the shorter the time taken, the faster the
reaction. This is why the reciprocal of time taken
(1/time) is used as the measure of rate of reaction.
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• the rate of decomposition of hydrogen peroxide by “catalase” enzyme.
The Effect of Temperature
When enzyme activity is measured at different temperatures, the results produce a graph as below.
1/time taken for reaction (rate)
Experimental
Points
Explanations
As temperature rises the rate increases because the molecules move faster and are
more likely to collide and react. All chemical reactions show this response.
However, beyond a certain “peak” temperature, the enzyme’s 3-D shape begins to
change. The substrate no longer fits the active site so well, and the reaction slows. If the
temperature was lowered again, the enzyme shape, and reaction rate could be restored.
If the temperature reaches an extreme level, the distortion of the enzyme’s shape may
result in total shut-down of the reaction. The enzyme may be permanently distorted out
of shape, and its activity cannot be restored. We say the enzyme has been “denatured”.
Temperature
Optimum Temperature of Enzymes
Not all enzymes will “peak” at the same temperature, or have exactly the same shape graph. In
mammals, most enzymes will peak at around the animal’s normal body temperature, and often work
only within a narrow range of temperatures.
An enzyme from a plant may show a much broader graph, indicating that it will work, at least partly,
at a wider range of temperatures.
An enzyme from a thermophilic bacteria from a hot volcanic spring will show a totally different
“peak” temperature, indicating that its metabolism will perform most efficiently at temperatures that
would kill other organisms.
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Slide 39
Reaction Rate
Mammal
Enzyme
Plant
Enzyme
0
20
Thermophilic
bacteria
enzyme
40
60
80
Temperature (oC)
100
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The Effect of pH
When the temperature is kept constant and an enzyme tested at various pH levels, the results will produce a graph as shown.
1/time (rate) Enzyme Activity
Generally, all intra-cellular
enzymes (i.e. those from within a
cell) will show peak activity at
about pH = 7, very close to
neutrality.
The shape of the pH graph is
usually symmetrical on either side
of the “peak”.
The digestive enzyme “pepsin”
from the stomach shows an
optimum pH about 2 or 3, meaning
that it works best in the acidic
environment.
The explanation for the shape
is as follows:
Intra-cellular
enzyme
At the optimum pH the enzyme’s 3-D
shape is ideal for the substrate, so
reaction rate is maximum.
Enzyme Activity
Pepsin.
(Stomach
enzyme)
2 3 4 5 6 7 8 9 10
pH
At any pH higher or lower than
optimum, the enzyme’s shape begins to
change. The substrate no longer fits,
so activity is less.
1
2
3
4
5
At extremes of pH, the enzyme can be
denatured and shows no activity at all.
6 7 8 9 10 11
pH
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The pH Scale
The acidity or alkalinity of any solution is measured on a numerical scale known as “pH”.
On the pH scale, anything which is neutral (neither acid nor alkaline)
has a pH = 7.
increasing
acidity
3
4
5
increasing
alkalinity
Neutral
6
7
8
9
10
11
The inside environment of a cell, and most parts of an organism’s body, is always very close to pH 7... i.e. neutral.
An exception is in the stomach where conditions are strongly acidic. (approx. pH 2)
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The “Bottom Line” for a Cell to Thrive
Now that you know some basics about enzymes, we can end this topic by discussing just why animals (and people) can
die in a “heat wave” (or a blizzard), why pouring vinegar on weeds kills them and why it is so important to excrete
the CO2 your cells are constantly producing while making ATP.
Temperature Impacts on Cells
Impacts of Changing pH
To stay alive, a human’s body temperature must be close to 37oC. If it varies by more than
about 4oC either side of this, it is life-threatening!
Now you can figure out why.
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Now look at the graph:
If your body temperature drops to 32o, or goes
above, say, 40o, this enzyme will STOP
FUNCTIONING. This could stop ATP production, or
stop nerve signals in your brain. Either one could
stop your heart!
Same reason... if the pH goes up or down by
just 0.5 of a pH unit some critical enzyme
molecules will change their 3-D shape & might
not fit their substrate properly. This could slow
down, or stop, some vital biochemical pathway.
This is why it is (for example) very important to
get rid of the CO2 you constantly produce in
your hundreds of billions of cells busily making
ATP by cellular respiration.
Enzyme Activity
Somewhere in your cells there are critical chemical
pathways controlled by enzymes with very narrow
activity curves, as shown by this graph.
These pathways might be in one of the many steps
in cellular respiration in all your mitochondria.
Maybe it’s an enzyme involved with exocytosis of a
neuro-transmitter which passes nerve signals from
one nerve cell in your brain to another.
Whatever it is, it is vital to your survival.
Likewise, the pH of your cellular & body fluids
(eg blood) is also critical for your survival.
The problem with CO2 is not that it is
“poisonous” in some vague, mysterious way.
Specifically, its danger is pH change!
Temperature
32
37
When CO2 dissolves in your blood beyond
certain concentrations, it increases acidity. This
can quickly lead to “acidosis” in your body
fluids which can kill you (by malfunction of vital
enzymes) within minutes.
42
Body temperature & pH are critical to survival because vital enzymes can only perform efficiently
in a narrow range of temperature and/or pH.
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Discusssion / Activity 5
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The following activity might be for class discussion, or there may be paper copies for you to complete.
If studying independently, please use these questions to check your comprehension before moving on.
Enzymes
Student Name .................................
1. What is meant by “metabolism”?
2.
a) Enzymes are said to be “substrate specific”. What does this mean?
School Inspection only.
Copying NOT permitted.
b) Explain how the shape of an enzyme molecule is linked to this specificity.
3. Sketch the shape of the graph of enzyme activity plotted against:
a) temperature.
b) pH.
Act.
Act.
temp
pH
4. How is the shape of these graphs connected to enzyme shape?
Biology Module 1 “Cells as the Basis of Life”
Format: OnScreen
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Slide 42
Inspection Copy for school evaluation
only. Copying NOT permitted.