TOPIC 2: CELLS
2.1 Cell Theory
2.1.1: Outline the cell theory
The cell theory states that:1) Cells are the smallest unit of life.
2) All organisms are composed of one or more cells.
3) All cells come from pre-existing cells.
2.1.2: Discuss the evidence for the cell theory
 The cell theory states that:1) Cells are the smallest unit of life.
2) All organisms are composed of one or more cells.
3) All cells come from pre-existing cells.

The cell theory can’t be ‘proven true’ because that would require us to examine every single
cell, which would be an impossible task. However the theory would be ‘proven false’ if a
scientist made a discovery that violated the existing cell theory.

Evidence for the cell theory comes from observation and experimentation. When observed
with a light microscope, every kind of cell - from every kind of organism – has so far upheld
the central tenets of the cell theory.

Whenever scientists attempt to separate cells into their component parts, none of those
parts can sustain the characteristics of life. Therefore life is an emergent property, which
means that it arises from the interaction of the component parts: “the whole is greater than
the sum of the parts”

The statement that all living cells come from pre-existing cells does not mean that life has
always existed; biologists believe that self-replicating molecules gradually evolved into the
earliest cells.

Some cell types (e.g. certain muscle, fungal and algal cells) are relatively large with multiple
nuclei. This is not really evidence against the cell theory; it just shows that cells can come in
a variety of forms.
Multi-nucleated muscle cell
cell
Multi-nucleated algae cell
1
Multi-nucleated fungus
2.1.3: State that all unicellular organisms carry out all the functions of life
2.1.4: Compare the relative sizes of molecules/cell membrane
thickness/viruses/bacteria/organelles and cells using the appropriate SI unit
Object
Size
Eukaryotic cell
Nucleus
Prokaryotic cells & Eukaryotic organelles
Virus
Membrane
Molecules
20-100 µm
10-20 µm
1-10 µm
100 nm
10 nm
1 nm
2.1.5: Calculate the linear magnification of drawings and the actual size of specimens in
images of known magnification
Measuring a cell
It is possible to estimate the diameter of a cell in the following way: 1) Measure the diameter of the field of view (FOV) using a ruler (let’s say it is 0.5mm).
2) Judge the diameter of the cell compared to the diameter of the FOV (let’s say it is 1/10)
3) Do a little math to approximate the cell’s diameter: 1/10 of 0.5mm = 0.05mm = 50µm.
Drawing a cell
When you make a drawing of a cell it is important to state the magnification (i.e. how much
larger the drawing is than the actual specimen).
Magnification = size of drawing / size of specimen
1) Measure the diameter of your cell drawing (let’s say it is 15cm)
2) Measure diameter of the actual specimen as shown above (50µm) Convert the units of one
so both are the same (50µm = 0.005cm)
3) Divide the diameter of the drawing by the diameter of the specimen (15cm/0.005cm = 3000X
)
Now you need to add a 1cm scale bar beneath your drawing to indicate the cell’s size. You need
to do a little algebra to calculate the length (in µm) that the scale bar represents.
In our example; 1cm / 15cm = x / 0.005cm x = 0.005cm / 15cm = 0.00033cm = 3.33µm
2.1.6: Explain the importance of the surface area to volume ratio as a factor limiting cell size
 Nutrients and wastes are moved across a cell’s surface by the process of diffusion. Diffusion
works well for small cells but not for large cells. The reason for this is explained by
geometry.
2

When a cell grows its rate of metabolism increases at the same rate as its volume. This
means that the rate of diffusion – which provides the materials for metabolism -must
increase proportionally with a cell’s growth. However, this can’t happen because a cell’s
surface area increases at a much slower rate than its volume.

Therefore, as a cell grows – and its surface-to-volume ratio decreases - it becomes
increasingly difficult for the cell to obtain nutrients and expel wastes by diffusion. And at
some point it simply becomes impossible. When cells grow too large they can divide in two
by the process of mitosis.

Thus cells are small because they can’t be large. A large cell – if one existed – would have a
big volume requiring lots of nutrients, and producing lots of wastes – but its surface area
would be too small for diffusion to be able to meet the cell’s needs.
2.1.7: State that multi-cellular organisms show emergent properties
2.1.7: Explain that the cells of multi-cellular organisms differentiate to carry out specialized
functions by expressing some genes but not others
 The different types of cells in a multi-cellular organism result from 'cellular differentiation',
which can be defined as “the divergence in structure and function of different types of cells
as they become specialized during an organism’s development”.

Each cell contains the entire genome (set of genes) of an organism. So how is it that some
cells become a certain type and others become another type? The answer is that different
kinds of cells express different genes. In other words, cells ‘differentiate’ by expression of
some genes and suppression of others.
3

Cells know which genes to ‘turn on’ and which to keep ‘turned off’ by receiving chemical
information from their surroundings.

Cellular differentiation is important for multi-cellular organisms because the presence of
many cooperating cell types enables organisms to perform functions that are beyond the
reach of a single cell type. In other words, multi-cellular organisms show emergent
properties.
2.1.8: State that stem cells retain the capacity to divide and have the ability to differentiate
along different pathways
2.1.9: Outline one therapeutic use of stem cells





A number of stem cell treatments already exist, although most are still experimental and/or
costly, with the notable exception of bone marrow transplantation.
Bone marrow transplants use stem cells derived from the bone marrow or blood.
The stem cells are infused intravenously to repopulate the bone marrow and produce new
blood cells.
Collecting stem cells provides a bigger graft, and does not require that the donor be
subjected to general anaesthesia to collect the graft.
Bone marrow transplant remains a risky procedure and is reserved for patients with life
threatening diseases of the blood, bone marrow, or certain types of cancer.
2.2 Prokaryotic Cells
2.2.1: Draw and label a diagram to show the structure and function of Escherichia coli (E. coli)
2.2.2: Annotate the diagram from 2.2.1 with the functions of each named structure
1) All relevant structures are present
2) Structures are correct relative sizes
3) Structures drawn in correct locations
4) labels are straight, precise & nonoverlapping
5) Drawing is neat and without shading
4
2.2.3: Identify structures from 2.2.1 in electron micrographs of E.coli.
2.2.4:
2.2.4:State that prokaryotic cells divide by binary fission
2.3 Eukaryotic Cells
2.3.1:Draw and label a diagram of the ultra structure of a liver cell as an example of an animal
cell
5
2.3.2:Annotate the liver cell diagram with the functions of each named structure
2.3.2: Identify the structures of a liver cell from an electron micrograph
6
2.3.4: Compare prokaryotic and eukaryotic cells
Prokaryotes
Eukaryotes
Naked DNA
DNA associated with proteins
DNA is in the cytoplasm
DNA is in the nucleus
Mitochondria absent
Mitochondria present
Smaller than 5 µm
Larger than 10 µm
70S ribosomes
80S ribosomes
Internal membranes lacking
Internal membranes compartmentalize
different cellular functions
2.3.5: State three differences between plant and animal cells
 Plant cells have cell walls, which make them appear rectangular-shaped. These structures
are composed of cellulose, hemicellulose, and a variety of other materials.
 Plant cells have chlorophyll, the light-absorbing pigment required for photosynthesis. This
pigment is contained in structures called chloroplasts, which makes plants appear green.
 Plants cells have a large, central vacuole. While animal cells may have one or more small
vacuoles, they do not take up the volume that the central vacuole does (up to 90% of the
entire cell volume!). The vacuole stores water and ions, and may be used for storage of
toxins
Water Vacuole
Chloroplast
Cell Wall
Plants have a large sac in their cytoplasm that stores and breaks down waste
products. It fills with water to enlarge plant cells providing turgor. Animal
cells lack a large water vacuole.
Plants have a medium-sized organelle called a chloroplast that performs
photosynthesis to make glucose. Animal cells do not have chloroplasts.
Plant cells have an outer layer called a cell wall that is made of cellulose.
The cell wall protects plant cells from mechanical damage. Animal cells do
not have a cell wall.
2.3.6: Outline two roles of extracellular components
The Cell Wall
 The plant cell wall is a rigid structure that surrounds cells.
 It protects and maintains the shape of plant cells as well as providing a filtering mechanism
to keep the cells from over-expanding in water.
 Cell walls also provide support to plants, holding them upright against gravity.
Glycoproteins
 Animal cells secrete glycoproteins, which are composed of a protein and a carbohydrate.
 Glycoproteins are used in proteins that are located in the extracellular matrix (the space
outside the cell).
7

One example of glycoproteins found in the body are mucins, which are secreted in the
digestive tracts. The sugars attached to mucins make them resistant to proteolysis by
digestive enzymes.
 Glycoproteins on the surface of lymphocytes allow them to stick to other types of cells and
move across their surfaces.
2.4 Membranes
2.4.1: Draw and label a diagram to show the structure of membranes
2.4.2:Explain how the hydrophobic and hydrophilic properties of phospholipids help to
maintain the structure of cell membranes
 Hydrophilic molecules are attracted to water and hydrophobic molecules are attracted to
one another in the presence of water.
 The structure of cell membranes is
maintained by the hydrophilic and
hydrophobic properties of phospholipids. A
phospholipid consists of a polar hydrophilic
head and a pair of nonpolar hydrophobic
tails.
 Cells and organelles are surrounded by
water and they contain a watery
cytoplasm, which causes the phospholipids
to spontaneously arrange themselves in a
double layer that is very stable.
8
Phospolipid bilayer

Outside the cell, the hydrophilic heads face the water and their hydrophobic tails are
directed inward. Inside the cell, the hydrophilic heads face the cytoplasm and their
hydrophobic tails are directed outward.
2.4.3: List the functions of membrane proteins
Protein Type
Function
Enzymes
There are many enzymes in membranes. ATP synthase, for example,
catalyzes the formation of ATP.
Electron carriers
Electron carrier proteins are arranged in chains in the membrane. They
are
needed for photosynthesis and cellular respiration.
Channels for passive
Membranes need channels to allow certain molecules to diffuse in or
transport
out of the cell. A channel is formed by 2 adjacent integral proteins.
Pumps for active
ATP energy is needed to transport certain molecules across a
transport
membrane.
Protein pumps are needed to accomplish this task.
Hormone binding
Cells communicate with one another using hormones. To receive a
sites
hormone
message a cell requires an integral protein that has a hormone binding
site on
its outer surface.
2.4.4: Define diffusion and osmosis
Diffusion
The passive movement of particles from a region of high concentration to a region of low
concentration.
Osmosis
The passive movement of water molecules, across a partially permeable membrane, from a
region of lower solute concentration to an area of higher solute concentration.
2.4.5: Explain passive transport across membranes by simple diffusion and facilitated
diffusion
 During passive transport, substances diffuse across cellular membranes with no expenditure
of cellular energy; instead they are powered by the potential energy that is released as they
flow down a concentration gradient.
 Diffusion across a cell membrane is caused by the random movement of particles from the
side of the membrane where they are more concentrated to the side of the membrane
where they are less concentrated.
 Osmosis is the diffusion of water molecules, across the cell membrane, from a region of
lower solute concentration to an area of higher solute concentration.
 Most water soluble molecules, such as amino acids, monosacharides and ions (K+, Na+,
Ca2+), can not move directly through the phospholipid bilayer. These molecules can only
9

diffuse through special pores formed by integral proteins, by a process called facilitated
diffusion.
The rate of facilitated diffusion is dependent on the number of appropriate transport
proteins as well as the magnitude of the gradient.
2.4.6: Explain the role of protein pumps and ATP in active transport across membranes
 Some integral proteins move molecules against a concentration gradient by the process of
active transport.
 Active transport allows cells to control the movement of particles into and out of the cell.
 Active transport requires: 1) a transport protein in the cell membrane; 2) a change in the
shape of the transport protein; and 3) hydrolysis of ATP
 An example of active transport is the sodium-potassium pump which functions in most
animal cells to: 1) maintain a steep concentration gradient of Na+ and K+ ions across the cell
surface membrane, 2) maintain cell volume and 3) maintain osmotic balance. The pump is
run by a transport protein that alternates between two shapes.
 The process begins when three sodium ions from the cytoplasm attach to the transport
protein. Then an ATP molecule attaches a phosphate group to the protein causing it to
change its shape and release the sodium ions to the outside. Next a single potassium ion
from the extracellular fluid attaches to the transport protein causing the protein to resume
its original configuration and deliver the potassium ion to the cytoplasm. The protein is now
available to repeat the process.
Sodium Potassium Pump
10
2.4.7; Explain how vesicles are used to transport materials within a cell between the rough
ER, Golgi apparatus and plasma membrane
 Large molecules such as proteins and
polysacharides cannot diffuse (or be pumped)
across cell membranes. Instead they are
transported by means of vesicles.
 Vesicle mediated transport is dependent on: 1)
the fluidity of biological membranes, which
enables them to change shape and form
vesicles and 2) the fact that all cellular
membranes are made of the same basic
materials, which means that portions of
membrane can bud off from one cell surface
and then fuse with another.
 Materials such as wastes and hormones are
moved out of cells by exocytosis.
 During exocytosis, a vesicle arrives from the
Golgi complex and fuses its membrane with the
plasma membrane of the cell. Then the vesicle
opens to the extracellular fluid and its contents
diffuse out.
 Fluids and large molecules enter cells by
endocytosis.
 In pinocytosis (the endocytosis of fluid) a small
portion of the cell surface membrane pinches
inward. Membrane then surrounds some of the extracellular fluid and buds off in the
cytoplasm as a tiny vesicle.
 Endocytosis of large molecules requires receptors on the outer surface of the membrane.
2.4.8: Describe how the fluidity of the membrane allows it to change shape, break and reform during endocytosis and exocytosis
 X-ray diffraction is a technique used to study cellular membranes. The technique has
revealed how cellular membranes behave.
 Cellular membranes are fluid (flexible) because phospholipids and proteins can change
location in the horizontal plane of the membrane.
 The behaviour of cellular membranes is influenced by the proportion of lipids, proteins,
cholesterol and other molecules in the membrane.
 For example, lipids can initiate the fusion of two membranes making exocytosis possible:
exocytosis is when a vesicle membrane fuses with the plasma membrane. And cholesterol
can initiate membrane breakage, making endocytosis possible: endocytosis is when a
vesicle is formed by the infolding of the plasma membrane.
 The illustration below shows how membranes behave when they approach one another:
11
2.5 Cell Division
2.5.1: Outline the stages in the cell cycle including interphase (G1, S1, G2), mitosis and
cytokinesis
 During its life, a cell passes through a well-ordered series of events known as the cell
division cycle.
 The cell division cycle involves three basic stages: interphase, mitosis and cytokinesis.
 Interphase is much longer than mitosis and it begins as soon as a daughter cell is formed. It
is an active period during which: the
cell grows; DNA transcription and
DNA replication occur; and
biochemical reactions are
performed.
 Mitosis is a continuous process but it
is useful to divide it into four stages:
prophase, metaphase, anaphase, and
telophase. During mitosis, the cell
separates its cytoplasm, organelles
and DNA equally. Mitosis is
necessary for: embryonic
development, the normal growth of
tissues; the repair of damaged
tissues; replacement of aging cells;
and asexual reproduction in prokaryotes.
12

Cytokinesis is the pinching in of the plasma membrane to divide the cell into two equal
daughter cells. Cytokinesis follows mitosis immediately.
2.5.2: State that tumours (cancers) are the result of uncontrolled cell division and that these
can occur in any organ or tissue
2.5.3: State that interphase is an active period in the life of a cell when many metabolic
reactions occur
2.5.4: Describe the events that occur in the four phases of mitosis (prophase, metaphase,
anaphase and telophase)
 During mitosis, the cell separates its cytoplasm, organelles and DNA equally. Mitosis is a
continuous process but it is useful to divide it into four stages: prophase, metaphase,
anaphase, and telophase.
 During prophase, the nuclear membrane disappears and a framework of microtubules is
formed. The microtubules form spindle fibres that originate at the poles. Some spindle
fibres span the entire cell and others attach to the chromosomes. The spindle fibres
function to move the chromosomes.
 During metaphase, each chromosome is positioned along the central axis of the cell called
the metaphase plate. The centromeres are situated directly along the metaphase plate with
the each chromatid positioned on opposite sides of the metaphase plate. The cell begins to
elongate.
 During anaphase, each centromere splits into two, causing sister chromatids to separate.
Once separated, each chromatid is considered a chromosome and the once-joined sisters
are pulled to opposite poles of the cell by the microtubules. Also during anaphase, the
entire cell begins to elongate and, therefore, further separates the sister chromosomes.
 During telophase, a daughter nuclei begins to form at each pole, enveloping the gathered
chromosomes. Finally during cytokinesis, the cytoplasm divides resulting in two genetically
identical daughter cells.
2.5.5: Explain how mitosis produces two genetically identical nuclei
 The genetic information (DNA) of a cell is stored in structures called chromosomes.
 When cells divide they must pass on all of their genetic information to each daughter cell.
Therefore, each chromosome must be replicated before cell division so that each daughter
cell receives all the instructions it needs to function.
 When a chromosome is replicated the two identical chromosomes are joined together by a
centromere and they are called sister chromatids.
 During mitosis, the cell separates its cytoplasm, organelles and DNA equally.
 Mitosis is a continuous process but it is useful to divide it into four stages: prophase,
metaphase, anaphase, and telophase.
 During prophase, the nuclear membrane disappears and a framework of microtubules is
formed. The microtubules form spindle fibers that originate at the poles. Some spindle
fibers span the entire cell and others attach to the chromosomes. The spindle fibers
function to move the chromosomes.
13



During metaphase, each chromosome is positioned along the central axis of the cell called
the metaphase plate. The centromeres are situated directly along the metaphase plate with
the each chromatid positioned on opposite sides of the metaphase plate. The cell begins to
elongate.
During anaphase, each centromere splits into two, causing sister chromatids to separate.
Once separated, each chromatid is considered a chromosome and the once-joined sisters
are pulled to opposite poles of the cell by the microtubules. Also during anaphase, the
entire cell begins to elongate and, therefore, further separates the sister chromosomes.
At telophase, a daughter nuclei begins to form at each pole, enveloping the gathered
chromosomes. Finally during cytokinesis, the cytoplasm divides resulting in two genetically
identical
daughter cells.
2.5.6: State that growth, embryonic development, tissue repair and asexual reproduction
involve mitosis
14