Cell Theory
The main principles of cell theory state:
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Cells are the smallest functional units of life
All organisms are composed of one or more cells
All cells come from pre-existing cells
Cell theory history (maybe extra)
1. Zacharias Janssen (1590) – invented the first compound microscope
2. Robert Hooke (1665) – used light microscope to look at cork (dead plant tissue)
a. Coined the term ‘cell’
3. Anton van Leeuwenhoek (1673) – first to see living microscopic organisms (microorganisms
animalcules) in pond water
4. Matthias Schleiden and Theodore Schwann (1839) – both credited for 1st two cell theory
principles
5. Rudolph Virchow (1855) – credited for developed for 3rd principle of cell theory
Cell Theory Discrepancies
Striated muscles fibres
Muscle cells fuse to form fibres that may be very long (>300mm)
Consequently, they have multiple nuclei despite being surrounded by a single, continuous plasma
membrane
Challenges the idea that cells always function as single units.
Aseptate fungal hyphae
Fungi may have filamentous structures called hyphae, which are separated into cells by internal
walls called septa
Some fungi are not partitioned by septa and hence have a continuous cytoplasm along the length of
the hyphae
Challenges the idea that living structures are composed of discrete cells.
Giant Algae
Certain species of unicellular algae may grow to very large sizes (e.g. Acetabularia may exceed 7 cm
in length)
Challenges the idea that larger organisms are always made of many microscopic cells.
Functions of Life
MR SHENG
Metabolism - the chemical reactions that occur in organisms in order for them to maintain life, such
as the synthesis of ATP during cellular respiration
Reproduction – process of producing new individuals resembling parent organism in all essential
features
Sensitivity – Responsiveness to external and internal stimuli
Homeostasis – Maintenance of a constant internal environment
Excretion – Removal of toxic waste products which were produced by the body’s own metabolism
Nutrition – exchanging materials and gases with the environment
Growth – movement and change in shape or size
FUNCTION
OF LIFE
Metabolism
Reproduction
Sensitivity
Homeostasis
PARAMECIUM
CHLAMYDOMONAS
Most metabolic reactions are catalysed by enzymes and take place in the
cytoplasm.
It can carry out both sexual and asexual
reproduction, though the latter is more
common. The cell divides into two
daughter cells in a process called binary
fission (asexual reproduction).
It can carry out both sexual and
asexual reproduction. When
Chlamydomonas reaches a certain
size, each cell reproduces, either by
binary fission or sexual reproduction.
The wave action of the beating cilia
helps to propel Paramecium in
response to changes in the
environment, e.g. towards warmer
water and away from cool
temperatures.
A light sensitive “eyespot” can sense
bright light and the cell will respond
by swimming towards it.
Contractile vacuoles remove water from
the cell to keep the water content in the
cell within a tolerable limit.
Contractile vacuoles are found at the
base of the flagella and expel water
through the plasma membrane of the
cell to keep the cell’s water within
tolerable limits.
Excretion
Digested nutrients from the food
vacuoles pass into the cytoplasm, and
the vacuole shrinks. When the
vacuole, with its fully digested
contents, reaches the Paramecium's
anal pore, it ruptures, expelling its
waste contents to the environment.
Nutrition
Paramecium is a heterotroph. It engulfs
food particles in vacuoles where
digestion takes place. The soluble
products are then absorbed into the
cytoplasm of the cell. It feeds on
microorganisms, such as bacteria, algae
and yeasts.
Chlamydomonas is an autotroph;
it uses its large chloroplast to carry
out photosynthesis to produce its
own food.
As it consumes food, the Paramecium
enlarges. Once it reaches a certain size it
will divide into two daughter cells.
Production of organic molecules
during photosynthesis and absorption
of minerals causes the organism to
increase in size. Once it reaches a
certain size it will divide into two
daughter cells.
Growth
It uses the whole surface of its
plasma membrane to excrete its
waste products.
Cells need to produce chemical energy (via metabolism) to survive and this requires the exchange of
materials with the environment
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The rate of metabolism of a cell is a function of its mass / volume (larger cells need more
energy to sustain essential functions)
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The rate of material exchange is a function of its surface area (large membrane surface
equates to more material movement)
As a cell grows, volume (units3) increases faster than surface area (units2), leading to a decreased
SA:Vol ratio

If metabolic rate exceeds the rate of exchange of vital materials and wastes (low SA:Vol
ratio), the cell will eventually die
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Hence growing cells tend to divide and remain small in order to maintain a high SA:Vol ratio
suitable for survival
Cells and tissues that are specialised for gas or material exchanges will increase their surface area to
optimise material transfer
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Intestinal tissue of the digestive tract may form a ruffled structure (villi) to increase the
surface area of the inner lining
Alveoli within the lungs have membranous extensions called microvilli, which function to
increase the total membrane surface
Explain the importance of surface area to volume ratio as a factor limiting cell size. (5)
As volume of a cell increases, ratio of SA:vol decreases
Food/oxygen enters through the surface of cells
Wastes leaves through the surface of cells
Rate of substance crossing the membrane depends on SA
More metabolic activity in a larger cell means more food and oxygen required
Large volume means longer diffusion time, more wastes produced
Excess heat generated will not be lost efficiently (with low SA:Vol ratio)
Eventually SA can no longer serve the requirements of cell
This critical ratio stimulates mitosis
Thus the size of the cell is reduced and kept within size limits
Cell Differentiation
Differentiation is the process during development whereby newly formed cells become more
specialised and distinct from one another as they mature
All the cells in an organism such as a human, are derived by mitotic divisions of the fertilised egg
(zygote). The cells therefore contain all the same genes (same instructions for every protein
needed).
However, not all cells carry out the same function. To carry out specific functions, cell must
differentiate
Cell differentiation is the process by which different genes are switched on or off within the nucleus
in response to a stimulus.
The active gene (switched on) is transcribed to make mRNA. This in turn is used to translated into
proteins at the ribosome. This protein will control specific processes or form structures in cell,
making the cell differentiated. This will affect the shape of cell and number of organelles in it.
Emergent properties arise from when the interaction of individual components produce new
functions.
E.g. multicellular organisms are capable of completing functions that individual cells (/unicellular)
couldn’t undertake
Stem Cells
Stem Cells are undifferentiated cells that have the potential to develop many different types of
specialised cells from the instructions in their DNA.
1. Totipotent: undifferentiated cells that can give rise to all cell types, including extraembryonic (placental) tissue
 Main source = after IVF- unused embryos
2. Pluripotent: undifferentiated cells that can give rise to most cells e.g. embryonic
stem cells
 Source = Blood that drains from placenta and umbilical cord after birth
 Can be frozen and stored and used if required for that individual
3. Multipotent: cells that can give rise to a limited range of cell types e.g.
hematopoietic adult stem cells
 Difficult to extract – somatic cells in adults
4. Unipotent: cannot differentiate, but are capable of self renewal e.g. progenitor cells,
muscle stem cells
Source of stem
cell
advantages
disadvantages
embryonic
Can differentiate into any cell
type
Can research into diseases like
Parkinson’s
Totipotent and pluripotent
Easy to extract
Lots of potential
Ethical concerns
Long term impact unknown
Higher risk of rejection genetically different
Higher risk of cells turning into
cancer cells
Removal of cells kills embryo
Umbilical cord
Low risk of rejection if stored
for yourself
Pluripotent
Stored for later use
Few ethical concern
Easily obtained and stored
Finite source of cord blood stem
cells
Takes longer to graft than bone
marrow
Higher risk of cells turning into
cancer cells
Expensive storing
Risk of infection
adult
Can treat organ specific diseases High rejection rates
Fewer ethical issues
Not for all cells - limited purpose
Aware of the long term effects
Dangerous extraction process
Multipotent
Risk of stem cells becoming
cancerous
Light Microscopy
Conventions to follow for microscopic structures:
1. 3-4 cells
2. x40 magnification
3. Label
4. No shading
5. Title
6. Single Pencil Line
7. Only draw what you can see
8. Take care of shapes and proportions of the cells correct
Visible features: nucleus, cell wall, flagella, pseudopodia, food vacuoles, chloroplasts,
mitochondria
Molecules → plasma membrane → virus → organelle/bacteria → animal cell → plant cell
(smallest)
(largest)
Magnification is the process of enlarging something only in appearance, not in physical size
Magnification=
image size
actual size
Microscope resolution is the shortest distance between two separate points in a microscope’s
field of view that can still be distinguished as distinct objects.
The resolution of a light microscope is 200 nm compared to 0.1 nm for an electron microscope.
Light Microscopes:
Specimens can be living or dead but need to be thin, strained with a coloured dye to make them
visible.
All of today = compound microscopes - use multiple cells
Light microscopy has a resolution of about 200 nm, which is good enough to see
cells, but not the details of cell organelles.
Electron Microscopes:
Beam of electrons to illuminate the specimen.
Resolution is less than 1nm - observe sub-cellular ultrastructure
Specimens must be fixed in plastic and viewed in a vacuum, and must therefore be dead
Specimens can be damaged by the electron beam and they must be stained with an electrondense chemical
Two kinds of electron microscope = transmission electron microscope (TEM) and scanning
electron microscope (SEM)
TEM: most common, best magnification and resolution; gives cross-section
SEM: poorer magnification and resolution but excellent 3d images
Light Microscope
Electron Microscope
Easy and fast sample prep
Sample prep is complex and time consuming
Live cell imaging possible
Fixed samples only (high vacuum)
Large field of view
Limited field of view
High throughout
Low throughout
User-friendly
Requires expertise
Diffraction limited resolution (0.25micro)
Sub-nanometer resolution (0.25)
Information limited to labels
Comprehensive