1
DISTANCE EDUCATION CENTRE
BIOLOGY Module 3 Cells
BIOLOGY REVISION BOOKLET
Key Words
Organism:
a living plant,
animal, fungus,
bacterium or
virus
Biology:
The study of
living organisms
This booklet is specifically prepared for students who have not covered
these topics in Biology Unit 1 or Biology Unit 2 in Year 11. It will also be
useful for new students who have been out of school for a while and are
returning to study; it will help to prepare you for the study of Biology
Unit 3 in year 12.
There are countless millions of individual living things, or organisms, on
Earth. Some kinds are familiar to us, such as the various animals and
plants which surround us; others such as fungi and bacteria are less
familiar and may even be invisible to the naked eye.
Biology - the scientific study of life – is concerned with the ways in
which biologists investigate the living world, with the knowledge that has
come from those investigations, and with the ways that knowledge can be
applied to help solve human problems.
How is a city similar to a cell?
We all live in a civilised society - we either live in or have visited a large
town or city. Did you know that the organisation within a city can be
compared to the organisation within a single plant or animal cell?
Cities have a clearly defined edge, whether it is a strong, defensive wall as
in Medieval times, or a more modern ring-road. Cells, too, have an outer
edge called the plasma membrane, which controls what enters and leaves
the cell. Many energy-rich activities occur within the city and hence the
city needs a constant supply of energy to be distributed to where it is
needed. In the cell the mitochondria perform this function. During this
process, however, waste is produced, and like the waste-disposal service
available to a city, the cell has to recycle, break down or remove any
waste that is generated.
A city is organised so that different activities occur in different buildings
or areas - you would not want a hospital in the same building as an
abattoir! So, too, a cell, which is capable of carrying out thousands of
chemical reactions at once, has different membrane-bound compartments
where these reactions occur - they are the organelles. These compartments
represent the 'factories" of the cell. New goods and products which are
needed by the cell, such as proteins and glucose are continually being
made from raw materials. This production usually takes place on a
cellular production line until the final product is achieved.
The sites of various activities are not randomly distributed within either a
city or a cell. In a city there are industrial zones, residential zones and
central business districts. These are all linked by an effective
communication system. It is the same in a cell; a cell has a complex set of
structures that define the centre, distinguish one end from the other and
provide routes for transportation.
Like cities, cells are highly organised, so at the city centre lies the city
council who are in charge of the city, and at the centre of the cell lies the
2
nucleus. Inside the nucleus are the chromosomes, which carry in chemical
code on the DNA all the instructions on how the cell is to develop and
function.
In this booklet we will investigate the structure and functioning of cells
and how they are organised to achieve the amazing array of events that
occur within them.
Life at the molecular level
Living things share a suite of characteristics: movement, respiration,
sensitivity, growth, reproduction, excretion and nutrition.
Key Word:
Bacteria
(singular:
bacterium)
Microscopic living
organisms that
live all around you
and inside you.
Many are
harmless and
useful, but some
can cause disease.
Living organisms are amazingly diverse in appearance - from tiny insects
to huge trees, from worms to elephants, from bacteria to mushrooms. But
the closer you look at all of them, the more similar they become.
Just like a city has buildings (and streets and drains), a tree has leaves
(and branches and a trunk); just like the buildings are made of smaller
units (bricks), so the tree is made of smaller units (cells).
Like the tree, all of these organisms are composed of cells.
So, what is a cell?
Cell sel (plural cells) / noun
basic unit of living thing: the smallest independently functioning unit in
the structure of an organism, usually consisting of one or more nuclei
surrounded by cytoplasm and enclosed by a membrane. Cells also contain
organelles such as mitochondria, lysosomes, and ribosomes.
(Source: Encarta http://encarta.msn.com/dictionary_/cell )
Key Word
Cell:
The cell is the
smallest unit in a
living organism
that can work on
its own. A cell can
make its own food
and reproduce
without help from
other cells.
The cell is the site of life; it is the functioning unit structure from
which living organisms are made. This is one of the fundamental
principles of biology known as The Cell Theory. While cells share many
common features, there are also differences between cells that are related
to their particular roles in organisms.
If we are to understand life we need to understand how cells work. In the
17th Century, microscopes capable of viewing cells were made for the first
time. The English scientist Robert Hooke (1635 – 1703) was the first
person to observe cells - he viewed thin pieces of cork through a
microscope. What he saw had no name then; he called them “cells”
because they reminded him of little boxes, like the cells of a honeycomb.
Since then, studies of cells from all types of organisms by various
scientists have led to the formulation of the cell theory which states that:
• All organisms are composed of cells (and the products of cells).
• All cells come from pre-existing cells.
• The cell is the smallest living organisational unit.
When Robert Hooke viewed the cork cells under his microscope he was
only looking at a very thin slice and so was not looking at whole cells.
3
Cells are three-dimensional. However, like Hooke, when looking under
the microscope we are only seeing a two dimensional slice through a cell
or one side of a whole cell.
Go to the following website to see cells in 3-D:

http://www.ibiblio.org/virtualcell
Now try these questions:
Question 1
(a)
Why were cells unknown until the 17th century?
(b)
Biologists often cut several sections of a cell to find out
about its structure.
Give two reasons why having several sections from
different areas of the same cell is important.
Please note: Answers to all of the questions in this booklet are
provided at the end of the booklet, so you don’t have to do the questions you can refer to the response sheet if you like.
Exceptions to the cell theory
Viruses present some problems to the cell theory.
What are viruses? Are they living or non-living organisms?
Did you know?
All viruses can
gradually change
to form new
versions or strains
of that type of
virus. That is why
scientists discover
a new strain of flu
every year.
Viruses are the smallest living things known – they can be over a million
times smaller than bacteria. Viruses are so small that they can only be
seen under a special type of microscope called an electron microscope.
Electron microscopes are used to study fine details and very small things
– they can magnify things by up to one million times. (You will learn
more about microscopes later in this chapter).
Some viruses are harmless, but many cause illnesses, ranging from the
common cold (Influenza virus) to AIDS (HIV).
Viruses are not regarded as cells because they do not have the basic
structures of cells (i.e. the nucleus, cytoplasm and cell membrane). They
not only lack the complex structures found in cells but only show a few
characteristics of living things. They cannot become active outside a
living host cell, which means that they cannot live on their own. Instead
they force their way into the cells of living creatures and use these cells to
make more viruses. After a time these infected cells die and the viruses
set off to find new cells to attack. They simply exist as inert virus particles
called virions.
The main parts of a typical virus are shown below. After you have read
about cells, you will see how different viruses really are.
4
Figure 1: A typical Virus
Looking at Cells
There are many different types of cells. The shell of an emu egg has a
single cell inside it (which is made up of the egg yolk and white). On the
other hand, about a hundred thousand bacterial cells could fit on an area
the size of a full stop! So, you see, cells have a large range of sizes.
Did you know?
Your body is made
out of about 100
million, million
cells.
Did you know?
A drop of blood
the size of a
pinhead contains
about five million
red blood cells,
7,500 white blood
cells and over
250,000 platelets!
Most living things start life as just one cell. In fact, you started life as only
one cell. So how did you become so big?
How does a single cell become millions of cells?
A single cell can divide into two, these two then grow to full size and
divide into four, and so on. They do this by a process known as Mitosis
(you will study this process later in the course).
Not only can cells replicate, they can also differentiate (meaning: to
become different) to become many different types of cells.
Your body now has a variety of tissues - these are groups of the same
types of cells. Examples of tissues are muscle tissue and connective
tissue. Groups of tissues in turn make up organs such as your heart and
lungs and groups of organs form organ systems, such as the respiratory
system.
As various organisms have adapted to their environment, the degree of
cellular organisation has altered. All cells need a constant input of
substances like oxygen, glucose and a removal of wastes such as carbon
dioxide and nitrogenous wastes.
There is really no such thing as a typical cell. Cells are specialised for
many different purposes and their structures reflect those purposes.
However, there are some features that are shared by all cells. Examination
of cells using various microscopes reveals much about their internal
organisation. Each living cell is a small compartment with an outer
boundary known as the plasma membrane (also referred to as the cell
membrane or plasmalemma). Inside each living cell is a fluid, known as
the cytosol which consists mainly of water containing many dissolved
substances.
Another feature of all living cells is that they contain genetic material in
the form of DNA, which carries hereditary information, directs the cell’s
activities, and is passed accurately from generation to generation. Living
things can be classified into two different kinds on the basis of their
internal structure - prokaryotes and eukaryotes.
5
 Prokaryotic cells
FACT
1 micrometre abbreviated as 1
µm - is one
millionth of a
metre, i.e. 10-6m
(or one
thousandth of a
millimeter).
1 µm. =
0.000001 metres
(or)
Prokaryotic cells are relatively simple and very little internal structures
can be seen, even with an electron microscope. They lack membranebound organelles and in particular, they lack a clearly defined structure to
house their DNA - the nucleus. They contain a single circular DNA
chromosome in the middle of the cell. They are small cells which range in
size from 0.5 to 1.0 micrometre (or micron).
Organisms that are made of prokaryotic cells are called Prokaryotes and
include bacteria and cyanobacteria. They are
unicellular organisms which means that they are made up of just a single
cell (uni: meaning “one”) and are so small that they are invisible to the
naked eye.
Key Word
Prokaryote
Latin: pro, before
+ Greek: karyon, a
nut, kernel or
nucleus.
Figure 2: Prokaryotic Cell Structure
Courtesy: http://micro.magnet.fsu.edu/cells/procaryotes/images/procaryote.jpg
 Eukaryotic cells
Key Words
Unicellular:
one-celled.
Multicellular:
many-celled.
Eukaryotic cells have a much more complex structure than prokaryotic
cells. They contain many different kinds of membrane-bound structures
called organelles suspended in the cytosol. These organelles carry out
specific functions within the cell. One of these organelles is a nucleus with
a clearly defined membrane called a nuclear membrane or nuclear envelope.
The DNA of a eukaryotic cell is located inside the nucleus.
Eukaryotic cells are relatively large cells which range in size from 30 to 150
micrometres.
Organisms that are made up of eukaryotic cells are called Eukaryotes.
They are multicellular organisms (multi: made up of
many cells) and include all animals, plants, fungi and protists. So the
majority of organisms typically contain eukaryotic cells.
6
Key Word
Eukaryote:
from the Greek
- eu, well
+ karyon,
nucleus
Figure 3: Anatomy of a eukaryotic cell
Courtesy: http://micro.magnet.fsu.edu/cells/bacteriacell.html
Key properties of Prokaryotes and Eukaryotes
Characteristics
Prokaryotes
Eukaryotes
Size of cell
Small cell size (0.2 – 2 µm.)
Larger cell size(10 -200 µm.)
Nucleus
No nuclear membrane or
nucleoli
Membrane enclosed
organelles
Cell wall
Absent
True nucleus consisting of
nuclear membrane and
nucleoli
Present
Chromosomes(DNA)
Usually present, chemically
complex.
Rigid cell walls
Single circular chromosome
Cell division
Binary fission (cell splitting)
When present, chemically
simple.
Flexible cell walls
Multiple linear chromosomes,
enclosed in nucleus
Mitosis
Note: The pictures you see of cells here, in the textbook and other sources
are only flat two-dimensional images - just like photographs of you are flat
two-dimensional images. Cells, also like you, are three dimensional in
reality.
7
Eukaryote organelles
Key Word
Organelles:
“little organs”
or tiny
structures
inside cells,
each of which
has a specific
function.
Just as a human body contains many specialised organs essential for
survival, such as the heart, kidneys, liver, lungs and brain, a cell contains
many organelles.
Organelles are subcellular structures involved in specific functions of the
cell. In other words, each organelle has a specific job to carry out within
the cell to keep the cell working. Many organelles are found in most cells.
Each organelle is surrounded by a membrane and they are found inside the
cytosol of the cell.
Organelles within a cell do not act in isolation, but interact with each other.
The normal functioning of each kind of cell depends on the combined
actions of its various organelles. All types of cells perform similar basic
processes and many also carry out highly specialised functions. The
activities of cells require considerable energy, and require the production of
a variety of biological molecules that are assembled into new organelles,
used for repair or exported from the cell. All these processes are catalysed
by enzymes and are precisely regulated. Some biochemical processes
involve hundreds of enzymes operating sequentially along a complex
integrated chemical pathway; each step is tightly controlled.
A brief summary of the structure and functions of the different parts of cells
and organelles follows (in alphabetical order):
Cell wall:
Found in bacterial (prokaryotic), fungal and plant cells only, a non-living,
cellulose structure outside the plasma membrane. The cell wall provides
support, prevents expansion of the cell, and allows water and dissolved
substances to pass freely through it. The cell wall varies in composition
between plants, fungi and bacteria.
Centrioles:
A pair of small cylindrical structures composed of micro tubules. They are
involved in the separation of chromosomes during cell division in animal
cells and protists.
Chloroplast:
Found in the photosynthetic cells of green plants and algae; a green
organelle (due to the abundant presence of the pigment chlorophyll) in
which photosynthesis takes place. It is composed of many folded layers of
membrane. Chloroplasts are often called the "sunlight trappers" because
they trap the radiant energy of sunlight and transform it to chemical energy
present in organic molecules such as sugars.
Cytoplasm:
(From the Greek: kytos, a hollow vessel + plasma, fluid). The contents of a
cell, other than the nucleus. It is jelly-like, more than 90% water and
contains ions, salts, enzymes, food molecules and organelles.
Cytosol:
The fluid component of cytoplasm in which organelles are located.
Endoplasmic reticulum: A network of intracellular membranes, which links with the plasma
membrane and other membranous organelles. It may be rough (associated
with ribosomes attached) or smooth (lacking ribosomes). Transport of
substances within cells occurs through this system of channels. Present in
both prokaryotes and eukaryotes.
8
Golgi apparatus
(also called a Golgi complex or Golgi bodies):
This is the “warehouse” of the cell. The cell does not use all of the proteins
that it makes, so the spare proteins are stored here until they are needed. It
consists of a stack of flat membrane sacs where the final synthesis and
packaging of proteins into membrane-bound vesicles occurs before they are
secreted from the cell. It is linked to the endoplasmic reticulum. Only found
in eukaryotes.
Lysosomes:
Found in most animal cells; membrane-bound vesicles that contain
powerful enzymes which break down debris and foreign material that is
brought into their sacs. They also destroy old and diseased parts of the cell.
Mitochondria:
Found only in eukaryotes, they are organelles composed of many folded
layers of membrane. Mitochondria are involved in the energy
transformations that release energy for use by the cell. Cellular respiration
occurs in mitochondria, so it is known as the "energy-supplying" organelle.
The more active a cell is the more mitochondria it needs. For example, liver
cells work hard at changing food into energy so the liver has plenty of
mitochondria to enable it to do this.
Nucleus:
(From the Latin: nucleus, a kernel or nut). This is the “headquarters” or the
control centre of the cells of animals, plants, algae and fungi (the
eukaryotes).
It is a large organelle, surrounded by a double-layered nuclear membrane
containing pores that allow movement between the nucleus and the
cytoplasm. It stains differently from cytoplasm and so often looks darker in
prepared slides. The nucleus contains genetic material (DNA) and controls
cellular activities. Some cells such as the human liver cells contain more
than one nucleus.
Plasma membrane: Forms a protective layer around the cell. It encloses the cytoplasm in all
cells (holding in the contents of the cell) and controls the movement of
substances into and out of the cell. It is responsible for recognition,
adhesion and chemical communication between cells.
Plastids:
A group of organelles found only in plant cells, all of which develop from
simple organelles called proplasts.
Chloroplasts and amyloplasts are plastids. Amyloplasts store starch in roots
or storage tissue, such as in potato tubers, and may be involved in
geotropism and chromoplasts (which contain colour pigments and are found
in petals and fruit).
Ribosomes:
Present in both prokaryotes and eukaryotes, they are tiny organelles located
in the cytosol. They are not enclosed by a membrane. Although they are
free within prokaryotic cells, in eukaryotes many are attached to
membranous internal channels called endoplasmic reticulum, within the
cell. They are the organelle where protein production occurs and so are the
“chemical factory” of the cell.
Tonoplast:
Vacuole membrane in plant cells, which regulates the movement of
substances into the vacuole.
9
Vacuoles:
Membrane-bound liquid-filled spaces found in most cells in variable
numbers. Plant cells typically have large fluid-filled vacuoles, containing
cell sap, that provide physical support (turgidity) and storage. In other cells,
vacuoles may be involved in intracellular digestion (food vacuoles) or water
balance (contractile vacuoles).
Vesicles:
Membrane-bound organelles often associated with transport within
cell.
the
Please go to the following website to see some animations and 3-D
diagrams of the different types of cells as well as the structure and functions
of their organelles. Once you are in the site, select a topic and press "GO".
The site contains a lot of very useful and interesting information.
http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa/cells/cellsre
v3.shtml
Try out these other websites as well:


http://www.ibiblio.org/virtualcell
www.cellsalive.com
What do cells look like when viewed under the electron microscope?
Refer to the following diagrams which show the composition of plant and
animal cells.
Animal cells
The cells in your body are not very different from the cells of a frog or an
elephant. In fact all animal cells have the parts shown
below:
10
Plant cells
Almost all plant cells have the parts shown below:
Courtesy: Nash B. &Wilkinson J., Biology 12,Macmillan, 199l.
Using the above diagrams of the animal and plant cells, please answer the following questions:
Question 2
What are fungi?
Fungi (singular:
fungus). Types of
organisms that has
no leaves, roots or
flowers. Moulds,
mushrooms and
toadstools are all
fungi.
(a)
Identify 3 features that ALL cells have in common.
(b)
State 3 differences between a plant cell and an animal cell.
(c)
State 3 ways in which they are similar.
(d)
Name a structure which is unique to plant cells.
Question 3
Scientists group cells into four main types: bacterial cells, fungal cells,
animal cells and plant cells.
Look at the generalised diagrams below showing the four main types of
cells.
11
animal cell (>10 µm)
fungal cell (>10 µm)
plant cell (>10 µm)
bacterial cell (<10 µm)
Complete the following table to show the presence () or absence (x) of
the following features - distinct nucleus, mitochondria, size <10 µm,
chloroplasts and cell wall in the different cell types. You may like to do
some additional research.
Animal
Plant
Fungi
Distinct nucleus
Mitochondria
Size < 10 µm
Chloroplasts
Cell wall
Question 4
The following diagram shows a cross section of an ovary cell.
Examine the above diagram and complete the following table:
Bacteria
12
Structure
A
Endoplasmic Reticulum
B
Ribosome
Function
Packages and distributes material for
C
transport out of cell
D
Nucleus
E
Major site of ATP production
F
Contains fluid mixture of digestive
enzymes.
(Answers are provided at the end of the booklet)
The composition of living organisms
The similarities between different organisms become even greater when
you look more closely at cells and the atoms and molecules they are
composed of. All life is composed of the same few elements. There are
92 naturally occurring elements. Only 11 of these are found in organisms
in more than trace amounts, and four of these - carbon (C), hydrogen (H),
oxygen (O) and nitrogen (N) - make up 99% of organisms by weight. The
similarities of all organisms at the molecular level points to their common
origin. Understanding the structure and properties of these molecules and
the ways they interact is fundamental to developing an understanding of
biological processes and the functions of organisms.
THE CELL MEMBRANE
What is the cell membrane?
The boundary of all living cells is a plasma membrane which controls
entry of dissolved substances into and out of the cell.
Most organelles, including the nucleus, endoplasmic reticulum,
mitochondria, plastids (chloroplasts), lysosomes, and vacuoles are also
surrounded by membranes. These membranes form discrete
compartments and control the intracellular movement of substances.
The composition of the plasma membrane
Key Word
The cell
membrane
forms a
protective layer
around the cell.
It also holds the
contents of the
cell together.
The plasma membrane is an ultra thin and pliable layer with an average
thickness of less than 0.000 01mm. and can be seen using an electron
microscope.
The outer cell membrane is thicker than the membranes of the
intracellular organelles. Otherwise, the basic structure of all cell
membranes is the same.
What are the components of the membranes?
13
They have a basically similar phospholipid bilayer structure. The central
region of the membrane consists of two layers of phospholipid molecules
in the fluid state and the molecules are arranged with their hydrophobic
(water repelling) tails aligned towards each other, and their water
attracting or hydrophilic (polar) heads towards the outside. Associated
with the membrane are other molecules including proteins, carbohydrates,
and cholesterol as represented by the fluid mosaic model shown above
The lipid molecules and some proteins are free to move about within the
layers.
Proteins
Proteins are composed of the elements carbon, nitrogen, oxygen and
hydrogen (and some also contain sulphur).These are located somewhat
randomly throughout the membrane. Some are present only on the
surface, others are wholly or partially embedded in the lipid layers; some
of these may penetrate all the way through the membrane. Proteins
provide pores or channels which allow polar (having negative and
positive charges) molecules and ions below certain sizes to go through.
They also form channels or gates which permit or enhance the passage of
specific ions and molecules. Sometimes the passage of these specific
molecules requires the expenditure of energy (this is known as "active
transport").
Membrane proteins may also be enzymes that carry out membraneassociated reactions e.g. final digestion of some food molecules as they
pass through the membrane of the gut.
Other proteins are receptors for hormones or other specific compounds.
Carbohydrates
Those associated with the plasma membranes are usually found on the
outer surface of the membrane, linked to protruding proteins. They are
thought to play a role in the adhesion of cells to one another, in the
'recognition' of molecules that interact with cells such as hormones
antibodies and viruses. Carbohydrates are composed of carbon, oxygen
and hydrogen.
Cholesterol
Cholesterol is so much in the news these days in terms of heart attack
risk.Membranes in higher organisms contain large numbers of cholesterol
molecules, located in between the phospholipid molecules. They make the
membrane less fluid and more stable. Without these cholesterol
molecules, the cell membrane rapidly breaks down, and releases its
contents. Cholesterol also decreases the permeability of the membrane to
small water-soluble molecules, making it an important component of cell
membranes.
So health conscious people, beware of restricting cholesterol too much in
your diet!
14
Movement in and out of cells
All cells must be able to take in and expel various substances across their
membranes in order to survive, grow and reproduce. Generally, these
substances are in solution, but in some cases, may be tiny solid particles.
Because a plasma membrane allows only some dissolved materials to
cross it, the membrane is said to be a partially-permeable (partiallypermeable may also be called semi or selectively or differentiallypermeable) boundary.
KEY WORDS
Permeable:
allows substances
to pass through.
Impermeable:
does not allow
substances to pass
through.
Semipermeable:
allows only some
substances to pass
through.
What structures can pass through the membrane?
Not all dissolved substances can cross the membrane equally well. Factors
affecting the passage of substances are:
•
•
•
•
molecular size
electrical charge
number of attached water molecules
solubility in fats.
Generally, substances which dissolve in alcohol or oil penetrate
particularly rapidly through the membrane, so the membrane is said to be
permeable to these substances - it allows them to pass through easily.
Due to its phospholipid bilayer, the membrane is impermeable to watersoluble molecules - this means that it does not allow them to pass through.
The special functions of membranes
Cell membranes play an important role in cells and some of the functions
are listed below:
• prevent dilution of cell cytoplasm.
• permit selective control of molecules that enter and leave cells.
• establish intracellular compartments (separating hereditary material,
lysosome enzymes, secretory products of cells, cytoplasm).
• restrict movement of substances between one part of a cell and
another.
• prevent uniform mixing of cellular contents.
• permit the regulation of many enzymatic processes that take place
within the cell.
• produce electrical activity of excitable cells, and
• have receptors involved in intercellular communication (directly
between cells and by hormones and nerves).
Because life is 'watery', one of the most important properties of
membranes is that they are impermeable to most water-soluble molecules.
Small-uncharged molecules such as water, oxygen and carbon dioxide,
can go through but large molecules are prevented from passing in and out.
15
Having studied the structure and properties of the membrane, answer the
following questions:
Question 5
(a)
(b)
(c)
(d)
List two functions of membranes in cells.
What is meant by the label partially permeable in reference to the
plasma membrane?
Why do volatile anaesthetics such as chloroform and ether work
quickly? In other words what properties could they have which
would enable them to easily cross over cell membranes?
Why is alcohol absorbed more quickly from the gut than food?
(Answers are provided at the end of the booklet)
How do substances pass in and out of membranes?
Dissolved substances that are able to cross a plasma membrane - from
outside a cell to the inside or from the inside to the outside - mainly do so
by the following processes:
•
•
•
•
diffusion
osmosis
active transport
endocytosis
Diffusion
Key Word
Diffusion:
The movement of
particles from a
region of high
concentration to a
region of lower
concentration,
resulting in an
even distribution.
What happens if you put some coloured solution at one end of a water
filled container with some water in it? In this case, the coloured solution
id added to the left-hand side of the container (see diagram below).
After a few hours, the colour has spread evenly throughout the solution.
How does this happen?
There is a movement of particles from the region of high concentration of
colour molecules (on the left-hand side) to a region of low concentration
of colour molecules (the water). Such a process is known as diffusion.
Diffusion is the result of the constant motion of particles
• it can occur through the pores of cell membranes
• no energy is expended by a cell when diffusion occurs – so this is a
passive process, and is called “passive diffusion”
16
• gas particles diffuse more quickly than other molecules across
membranes
• diffusion occurs in both directions across a membrane
• the net or overall direction of flow is from the area of the higher
concentration of a substance to an area of lower concentration of a
substance.
Once diffusion is complete, the end result is a uniform or even
concentration on each side of the membrane.
For an animation of simple diffusion, go to:
http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa_pre_2011
/cells/cells3.shtml
Why is diffusion important to the cell?
Diffusion is one way cells take in materials from the environment; it is
also one way they lose unwanted materials (wastes) when they are
produced in the cell.
Water, oxygen, carbon dioxide and small ions and molecules can diffuse
freely through the cell membrane, in both directions, because the
membrane is permeable to them.
Do you think all substances can pass through the cell membrane?
The answer is no. Remember that the membrane acts as a barrier to some
substances while allowing others to pass through - such a membrane is
said to be selectively permeable or partially-permeable because it only
allows certain substances to pass through it, but not others.
Osmosis
Key Words
Solution:
A mixture of one
substance (the
solute) dissolved
in another (the
solvent).
Osmosis is a special type of diffusion.
It is the movement of water molecules through a semi-permeable
membrane and down a concentration gradient.
The water diffuses from a region of high water concentration to a region
of lower water concentration.
Another way of saying this is that the water diffuses from a region of low
solute concentration to a region of high solute concentration (the solute is
the substance that is dissolved in the water eg. salt or sugar).
To see an animation of osmosis go to:
http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa_pr
e_2011/cells/cells4.shtml
Now answer the following questions :
17
Question 6
It is possible to dissolve the outer hard shell of a hen's egg. This leaves
just the cell membrane undamaged and the cell contents intact. Shells
were removed in this way from several eggs and each egg was placed in
one of the three different solutions below:
Solution l
Distilled water
Solution 2
0.5M sucrose (M = molar concentration)
Solution 3
l .5M sucrose
The weight of the eggs was recorded at regular time intervals over an
hour. Using your knowledge of diffusion:
a. What difference would you expect between the weights of the eggs
placed in distilled water and that placed in the 0.5M sucrose? Explain.
b. Under what conditions would an egg show neither an increase nor a
decrease in weight?
1) Distilled Water
2) 0.5M sucrose solution
3) 1.5M sucrose solution
18
Question 7
19
(Answers are provided at the end of the booklet)
20
Facilitated Diffusion
The movement of some substances across the plasma membrane is
assisted or facilitated by carrier protein molecules. This form of diffusion,
involving a specific carrier molecule, is known as facilitated diffusion
(meaning “assisted” diffusion). The net direction of movement is from a
region of higher concentration of a substance to a region of lower
concentration, and so the process does not require energy. Movement of
substances by facilitated diffusion mainly involves substances that cannot
diffuse across the plasma membrane by dissolving in the lipid layer of
themembrane. For example, the movement of glucose molecules across
the plasma membrane of red blood cells involves a specific carrier
molecule.
See the following websites for animations of facilitated diffusion:
http://highered.mheducation.com/sites/0072495855/student_view0/chapte
r2/animation__how_facilitated_diffusion_works.html
http://www.d.umn.edu/~sdowning/Membranes/diffusionanimation.html
Active transport
This involves the movement of dissolved substances into or out of cells
against the concentration gradient.
Because the net or overall movement is against a concentration gradient,
active transport uses energy to pump materials or carry them across the
membrane. It is an energy requiring process. They otherwise might have
been blocked by the diffusion gradient or their own mineral salts entering
the cells in this way.
Active transport plays an important role in some animals that live in fresh
water, for example frogs. Frogs tend to lose salts by diffusion across their
skin-cell plasma membranes into the surrounding fresh water. So to
balance the loss of salt that occurs, energy is used to drive active transport
of salts from a region of low concentration in the surrounding water,
across plasma membranes, into the frog skin cells that have a high
concentration of salts.
See the website below for an animation of active transport.
http://www.bbc.co.uk/schools/gcsebitesize/science/add_ocr_pre_2
011/homeostasis/importancerev6.shtml
21
Endocytosis
An entirely different type of active uptake is endocytosis.
This is a process used when solid or large particles, which cannot
pass across the plasma membrane passively, need to be taken into a
cell. In this process the cell membrane folds around the particles
and 'pulls' them into the cytosol of the cell as shown in the diagram
below.
The particles enclosed in a vesicle are then acted upon and broken
down by enzymes. (If enzyme action does not occur, the vesicle is
disintegrated by lysosomes).
When the material to be transported is a solid food particle, the
type of endocytosis is called phagocytosis. This type of food
transport is used by the single-celled organisms called Amoeba.
Although some cells are capable of phagocytosis, most cells are
not.
Most eukaryotic cells rely on pinocytosis, a form of endocytosis
that involves material that is in solution being transported into
cells.
When large molecules and particles need to be taken out of the
cell, a small membrane-bound vesicle moves through the
cytoplasm to the plasma membrane, where it joins with it and then
releases its contents out of the cell - this is called exocytosis (as in
exit). Note the summary below.
22
Endocytosis
Bulk transport of material into a
cell
If material to be transported is
solid
If material to be transported is
liquid
The process is called:
Phagocytosis
From the Greek:phagos=eating
and cyto=cell
The process is called:
Pinocytosis
From the Greek: pinus=drink
and cyto=cell
Figure 6: A summary of Endocytosis
Go to the following website for a visual representation of
phagocytosis:
http://www.stolaf.edu/people/giannini/flashanimat/cellstructure
s/phagocitosis.swf
To sum up:
In general, substances pass in and out of cells by four main
processes:

Diffusion- simple and facilitated
Passive processes
(no energy is required)
(

Osmosis- diffusion of water

Active transport

Bulk transport
Energy requiring processes
23
Now, try the question below:
Question 8
Substances may move across the cell membrane by a variety of
mechanisms. Some of these require respiratory energy while others do
not. Draw and complete the table below, which details how various
substances MOST TYPICALLY move across the cell membrane.
An example has been completed.
Substance Mechanism of Main part of
Is respiratory
Movement
membrane involved
energy
required?
Carbon
Diffusion
Phospholipid bilayer
dioxide
Water
Glucose
Alcohol
(Answers are provided at the end of the booklet)
No
24
Surface Area and Volume
When an object (e.g. a cell) is small it has a large surface area in
comparison to its volume. In this case diffusion will be an effective
way to transport materials (e.g. gases) into the cell. As an object
becomes larger, its surface area compared to its volume is smaller.
Diffusion is no longer an effective way to transport materials to the
inside. For this reason, there is a physical limit for the size of a
cell, with the effectiveness of diffusion being the controlling
factor.
Volume: = 8 cm3
Surface area: = 24 cm2
Volume: = 8cm3 for 8 cubes
Surface area: = 6 cm2 for 1 cube
= 48 cm2 for 8
cubes
The eight small cells and the single large cells have the same total
volume, but their surface areas are different. The small cells
together have twice the total surface area of the large cell, because
there are more exposed (inner) surfaces. Real organisms have
complex shapes, but the same principles apply.
The surface-area volume relationship has important implications
for processes involving transport into and out of cells across
membranes. For activities such as gas exchange, the surface area
available for diffusion is a major factor limiting the rate at which
oxygen can be supplied to tissues.
The diagrams below shows four imaginary cells of different sizes
(cells do not actually grow to this size, their large size is for the
sake of the exercise). They range from a small 2 cm cube to a large
5 cm cube. This exercise investigates the effect of cell size on the
efficiency of diffusion.
25
Now try the next question:
Question 9
Calculate the volume, surface area and the ratio of surface area to volume for
each of the four cubes above (the first has been done for you).
When completing the table below show your calculations.
Cube size
Surface area
Volume
2 cm cube
2 x 2 x 6 = 24 cm2
2 x 2 x 2 = 8 cm3
(2 cm x 2 cm x 6 sides)
(height x width x depth)
Surface area to volume
ratio
24 to 8 = 3:1
3 cm cube
4 cm cube
5 cm cube
Courtesy Year 12 Biology Student Resource and Activity Manual 2003, Biozone
26
Microscopes and Microscopy
Types of Microscopes
Since cells are so tiny, a microscope must be used to study them. There
are two main types of microscopes – optical and electron. Optical
microscopes are the kind usually found in homes or schools. Electron
microscopes are very complex and expensive machines and are mainly
used in medicine and industry.
Optical microscopes
Did you know?
Mono: refers to
one or single.
Bi: refers to two
(for example
bicycle)
Ocular: (Latin:
oculus) Of or
relating to the
eye
The light or optical microscope is one of the most important instruments
used in biology practicals, and its correct use is a basic and essential
skill of biology. Most optical microscopes can magnify objects from
about 50 up to 1000 times their real size. The most powerful ones can
magnify objects up to 2000 times. You will find that optical microscopes
are easy to use, especially once you know a little about them.
High power light microscopes use a combination of lenses to magnify
objects up to several hundred times. They are called compound
microscopes because there are two or more separate lenses involved.
Some microscopes have only one piece and are called monocular
microscopes; some have two eyepieces and are therefore called
binocular microscopes.
Some microscopes such as the one in Figure 7 below have a mirror
rather than a built-in internal light source. The specimens viewed under
these types of microscopes must be thin and mostly transparent. They
are usually mounted onto a microscope slide and may be stained for
easier observation of internal structure.
Bacteria and red blood cells can be seen in this way as they can be
magnified up to 1500 times ( this can be written as:1500 x ). Light is
focused up through the condenser and if the specimen is thick or opaque,
little or no detail will be visible.
27
Did you know?
Most bacteria
are so small that
you could fit
1,000 middlesized ones on
this full stop.
That is why they
can only be seen
with a
microscope.
Figure 7 : A Monocular microscope
Courtesy: http://www.eduplace.com/kids/sla/6/microscope.html
What about focusing? How do you do that?
FACT
Scientists use
microscopes to
test samples of
river and lake
water for
pollution. The
number and type
of plants they can
see, especially
blue-green algae,
help them to
measure how
polluted the water
is.
a. Turn to the lowest magnification-the lenses must click
firmly into place.
b. Point a bench lamp at the mirror, direct as much as light as
possible up to the eyepiece.
c. Put a slide on the stage. The part you wish to see must be
right in the middle of the hole in the stage.
d. Looking from the side, turn the focusing knob until the
stage is almost touching the objective. Make sure that it
does not touch the slide.
e. Look through the eyepiece. Turn the focusing knob slowly
to separate stage and objective until you have a sharp, clear
picture. It is now in focus.
f. Move the slide around until you have the appropriate part
of the specimen in the centre of your field of view.
g. Change to a higher magnification for a closer look.
28
FIELD OF VIEW
The area of the slide that you see when you look through a
microscope is called the field of view. If you know how wide your
field of view is, you can estimate the size of things you see in the
field of view.
By carefully placing a thin metric ruler on the stage and focusing
under low power, we can measure the field of view in millimetres.
But when we use a microscope we should measure in micrometers
or microns (µm).
A micron is one millionth of a meter.
1 000 000 µm = 1000 mm = 100 cm = 1 m
It is handy to remember that 1000 µm = 1 mm
MICROSCOPIC MEASUREMENTS
Let’s look at these examples:
Example 1:
If the diameter of the low power field is
1.6 mm = 1600 µm
For example, if it takes four cells to stretch
across a field of view 1600 µm long then
each cell will be approximately 400 µm
long.
eyepiece
X10
How can you calculate the magnification?
objective
lens:
X40
The magnification can be calculated by multiplying the
magnification written on the eye piece (in this case x 10)
by the magnification of the objective lens you are using
(here it is x 40).
For this microscope this is:
10 x 40 = 400 x (the x means 'times' ).
So, the object you are looking at appears 400 times larger
than it is in real life.
29
Example 2:
x 100 field of view
The diagram shows the edge of a
millimetre ruler viewed under the
microscope with the lenses listed below.
The field shown is the low power field of
view.
Magnification of the eye piece = 10 x
Low power objective
= 10 x
High power objective
= 40 x
a. What is the highest magnification you could get using this
microscope?
10 x 40 = 400 x
b. What is the approximate width of the field of view in micrometers?
Each white space is 1mm.We can see approximately 3 ½ white
spaces. That is equivalent to 3.5mm. So, the answer is 3500 µm.
c. What would be the width of the field of view under high power?
The ratio of low to high power for this microscope is 10/40 or
¼. So, under high power we will see ¼ of the low power field of
view.
Therefore, the width of the field of view under high power is
¼ x 3500 = 875 µm.
d. If 5 cells fit across the high power field of view, what is the
approximate size of each cell?
The high power field of view = 875 µm, so the size of one cell
= 875 /5 = 175 µm.
Although this looks confusing right now, don’t worry!
It will all make sense once you actually get to use a microscope in
the laboratory.
30
Electron microscopes
Electron microscopes (EMs) use a beam of electrons, instead of
light, to produce an image. The higher resolution of EMs is due to
the shorter wavelengths of electrons. There are two basic types of
electron microscopes: scanning electron microscopes (SEM) and
transmission electron microscopes (TEM).
In SEMs, the electrons are bounced off the surface of an object to
produce detailed images of the external appearance or the outside
surface of the specimen only. A microscope of this power can
easily obtain clear pictures of organisms as small as bacteria and
viruses. It can magnify specimens 100 000 times (100 000 X).
TEMs produce very clear images of specially prepared very thin
sections of material. They can magnify objects several hundred
thousand times (up to a million times or 1 000 000 X). Therefore,
the internal structure or organelles of cells can clearly be seen
using a TEM.
Figure 11 below shows an electron microscope image of a black
ant. Note the 3-D image which reveals details that would be
impossible to see with visible light.
All electron microscope images are black and white, but scientists
can add colour to them using a computer. This makes it easier to
see the details in the picture. These pictures are called “falsecolour images”.
Figure 11: Electron microscope image of a black ant.
Courtesy: http://www.pbrc.hawaii.edu/bemf/microangela/qant.htm
31
For more interesting images go to the following websites

http://education.denniskunkel.com/ZoomIn.php

http://www.microscopy-uk.org.uk/full_menu.html
then click onto: “Micro-organisms in Ponds”
Now try the next question:
Question 10
The following table shows the units of length used in Science:
Units of Length (international system)
Units
1 meter (m)
1 millimeter (mm)
1 micrometer (µm)
1 nanometer (nm)
Equivalent
Meters
1m
10-3m
10-6m
10-9m
=1000 millimeters
=1000 micrometers
=1000 nanometers
=1000picometeres
Using the information provided above, calculate the equivalent length in millimeters
(mm) of the following measurements:
1. 0.25 µm: ______________
2. 450 µm: ________________
3. 200 nm: ________________
(Remember to check all of your answers at the end of the booklet).
32
Levels of organisation
Unicellular organisms such as bacteria are made up of only one cell ("uni" meaning one
as in unicycle - a bicycle with one wheel); they must carry out all the metabolic processes
necessary for life. They are complex cells capable of independent existence. In contrast,
multicellular ("multi" meaning many) organisms have millions of cells that depend on
each other for survival. During development of a multicellular organism, groups of cells
become specialised to perform particular functions that serve the whole organism.
Specialised cells have fewer functions than those found in unicellular organisms but the
functions they have are very highly developed. In addition, each group of specialised
cells must coordinate with other specialised cells. Let us look at the different levels of
organisation that interact to ensure proper functioning of the whole organism.
Tissues
When cells that are specialised in the same way aggregate to perform a common function,
they are called a tissue. Different kinds of tissue serve different functions in an organism.
For example, cardiac muscle is a special tissue found only in the heart. Epidermal tissue
is any tissue that forms a thin layer around a structure - it may be a layer of plant cells
forming the outermost layer of leaves or it may be the outer layer of human skin, the
epidermis.
Organs
In multicellular organisms, groups of different tissues often work together to ensure a
particular function is successfully performed. A collection of such tissues is called an
organ. Your heart, lungs, kidneys, brain and stomach are all different organs. Tissues of
the stomach include epithelium, smooth muscle and blood. In plants, organs are the root,
stem, flower and leaf. Tissues of a leaf include the epithelium and vascular tissue.
Organ systems
Your digestive system comprises various organs that work together to ensure that the
food you eat is digested and that the nutrients it contains are absorbed and transported to
all cells of your body. This organisation is called an organ system. Your digestive system
commences with your mouth and includes organs such as your teeth, oesophagus,
stomach, intestines and liver. Once digested food has been absorbed by cells lining the
intestine, it is transported by the blood circulatory system throughout your body. This
system links with the respiratory system where it picks up oxygen, also for delivery. As
blood delivers nutrients and oxygen to all tissues, it collects wastes for delivery to the
excretory systems of the body.
Because plants do not move from place to place, their energy needs are far less than
mobile animals. Hence, plants do not have the equivalent of complex organ systems that
animals have. Green plants produce their own food through the process known as
photosynthesis and this process also delivers oxygen directly to some cells. Other cells
rely on diffusion to get their oxygen. The extensive root system of a plant ensures that it
absorbs sufficient water for survival. An extensive vascular system then delivers that
water to all parts of the plant.
33
REVISION BOOKLET - ANSWER SHEET
Question 1
(a) Why were cells unknown until the 17th century?
The microscope was not invented until the 17th century, enabling people to see
cells for the first time.
(b) Biologists often cut several sections of a cell to find out about its structure.
Give two reasons why having several sections from different areas of the same
cell is important.
Observing several sections of a cell ensures that all structures within a cell are
seen and it enables one to find out about the three-dimensional structure of a cell.
Looking at only one section only enables us to look at a single thin layer through
the cell.
Question 2
(a) Identify 3 features that ALL cells have in common.
All cells contain DNA, all cells have a plasma membrane and all cells have a
cytoplasm.
(b) State 3 differences between a plant cell and an animal cell.
An animal cell has centrioles and small vacuoles. Plant cells have chloroplasts, a
cellulose cell wall and large vacuoles.
(c) State 3 ways in which they are similar.
Both plant and animal cells have a cell membrane, nucleus and cytoplasm.
(d) Name a structure which is unique to plant cells.
Chloroplasts are unique to plant cells.
Question 3
Distinct nucleus
Mitochondria
Size <10m
Chloroplast
Cell wall
Animal


X
X
X
Plant


X


Fungi


X
X

Bacteria
X
X

X

Eukaryotic cells must possess: a distinct nucleus, membrane bound organelles (such
as chloroplast or Golgi bodies) and mitochondria and have a size > 10 m. This is not
the case for prokaryotes. Bacteria are prokaryotes, while plants, animals and fungi are
eukaryotes.
34
Question 4
Structure
Function
A
Endoplasmic reticulum
Processes and transports proteins. Contains enzymes
important for metabolism.
B
Ribosomes
Site of protein production
C
Golgi Apparatus
D
Nucleus
Packages and distributes material for transport out
of the cell
Contains the DNA, genetic material of the cell.
E
Mitochondria
Major site of ATP production
F
Lysosome
Contains fluid mixture of digestive enzymes
Question 5
(a) List two functions of membranes in cells.
Any of the following:
 To compartmentalise regions of different function within the cell.
 For controlling the entry and exit of substances.
 Fulfilling a role in recognition and communication between cells.
(b) What is meant by the label partially permeable in reference to the plasma
membrane?
A partially permeable membrane only allows some dissolved substances to pass
through in either direction, into or out of the cell, but not others. It is therefore
selective.
(c) Why do volatile anesthetics such as chloroform and ether work quickly? In order
words what properties could they have which would enable them to easily cross over
cell membranes?
Chloroform and ether are lipid soluble organic substances and therefore easily
dissolve into the phospholipid bilayer of a cell membrane.
(d) Why is alcohol absorbed more quickly from the gut than food?
Alcohol is organic substance, which is soluble in oil and therefore penetrates
rapidly through the membrane as oils also move through the phospholipid bilayer
with ease.
35
Question 6
(a) What difference would you expect between the weights of the eggs placed in
distilled water and that placed in the 0.5 M sucrose? Explain.
 The egg placed in distilled water gained more weight than that place in the 0.5
sucrose solutions. This is because when the egg is placed in distilled water,
the water concentration on the outside is much higher than in the inside,
causing the water to diffuse into the cell, resulting in a gain in weight.
 The concentration of the water in the 0.5M sucrose is not as concentrated as
the distilled water. Therefore the amount of diffusion is not as great, resulting
in less weight gain.
(b) Under what conditions would an egg show neither an increase nor a decrease in
weight?
The egg will have water in it but will also have some ions in it. So the water is
less concentrated (fewer water molecules) than in the distilled water. When the
external water concentration is the same as that of the egg, there is no net
movement of water. So an egg would show neither an increase nor a decrease in
weight.
Question 7
36
Question 8
Substance
Mechanism of
Movement
Carbon dioxide
Diffusion
Water
Osmosis
Glucose
Facilitated
diffusion
Diffusion
alcohol
Main part of
membrane
involved
Phospholipid
bilayer
Phospholipid
bilayer
Protein
Phospholipid
bilayer
Is Respiratory
Energy required?
No
No
No
No
Question 9
Cube
size
2 cm
cube
3 cm
cube
4 cm
cube
5 cm
cube
Surface area
Volume
Surface area to
volume ratio
2 x 2 x 6 = 24 cm2
2 x 2 x 2 = 8 cm3
(2 cm x 2 cm x 6 sides)
(height x width x depth)
24 to 8 = 3:1
3 x 3 x 6 = 54 cm²
3 x 3 x 3 = 27 cm³
54 to 27 = 2:1
4 x 4 x 6 = 96 cm²
4 x 4 x 4 = 64 cm³
96 to 64 = 3:2
5 x 5 x 6 = 150 cm²
5 x 5 x 5 = 125 cm³
150 to 125 = 6:5
37
Question 10
Units of Length (international system)
Units
1 meter (m)
1 millimeter (mm)
1 micrometer (µm)
1 nanometer (nm)
Equivalent
Meters
1m
10-3m
10-6m
10-9m
=1000 millimeters
=1000 micrometers
=1000 nanometers
=1000picometeres
Using the information provided above, calculate the equivalent length in millimeters
(mm) of the following measurements:
1. 0.25 µm = 0.00025 mm ( to convert µm to mm, you have to divide by 1,000
because there are 1,000 µm in one mm.)
2. 450 µm = 0.450 mm ( again, divide by 1,000)
3. 200 nm = 0.000200 mm (to convert nm to mm, you have to divide by
1, 000, 000, because there are 1, 000, 000 µm in one mm.)
We hope that this revision booklet has been useful and will help to prepare
you for Biology.
We are looking forward to teaching you Biology .
38
BIBLIOGRAPHY
Allan, R., Year 11 Biozone Student Resource and Activity Manual,
Biozone International Ltd., 2002.
Allan, R., Year 11 Biozone Student Resource and Activity Manual,
Biozone International Ltd., 2005.
Cells, the Units of Life, ASEP, Victoria, 1974
Heinemann Biology Two, 4th edition, Harcourt Education, 2005
Kinnear, J and Martin, M, Nature of Biology Book 1, Jacaranda
Publishing, 2005
Rogers, Kirsteen. The Usborne Internet-Linked Complete Book of the
Microscope. London : Usborne Publishing Ltd., 2001.
Semple, A et al, Nelson Biology 2nd edition, 2005