Topic 1 Cell Biology
1.1 Introduction to Cells
1.1
U1
According to the cell The Cell Theory
theory, living
 All living things are composed of cells (or cell products)
organisms are
 The cell is the smallest unit of life
composed of cells.
 Cells only arise from pre-existing cells
1.1
U2
Organisms
Unicellular organisms must carry out at least seven functions of life
consisting of only
 Metabolism - The web of all the enzyme-catalyzed reactions in a cell e.g.
one cell carry out all
respiration
functions of life in
 Response – the ability to react to changes in the environment
that cell.
 Homeostasis - The maintenance and regulation of internal cell conditions, e.g.
Students are expected
water and pH
to be able to name
 Growth – Irreversible increase in size
and briefly explain
 Excretion - The removal of metabolic waste
these functions of life:
 Reproduction - Producing offspring, either sexually or asexually
nutrition, metabolism,
 Nutrition - Feeding by synthesis of organic molecules (e.g. photosynthesis) or
growth, response,
the absorption of organic matter
excretion,
homeostasis and
reproduction.
1.1
U3
Surface area to
For metabolism to continue, reactants used in metabolism must be absorbed and the
volume ratio is
waste products must be removed
important in the
 Material exchange is a function of the plasma membrane – larger surface area to
limitation of cell size.
volume ratio means that the cell can act more effectively as there is more
membrane to serve every unit of volume that requires change
 Hence, the rate of metabolic reactions are proportional to the surface area to
volume ratio of the cell
Benefits of larger SA:Vol ratio
 Diffusion pathways are shorter – molecules travel less to get in/out of cell, less
time and energy (if it is active transport) is consumed
 Concentration gradients are easier to generate – makes diffusion more efficient
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
Cells maintain their SA:Volume ratio by:
 Dividing and remaining small to maintain a high SA:Volume ratio
 Cells and tissues specialised for gas and material exchange will increase their
surface area to optimise material transfer
o Intestinal tissue of the digestive tract may form a ruffled structure (villi) to
increase the surface area of the inner lining
o Alveoli within the lungs have membranous extensions called microvilli, which
function to increase the total membrane surface
1.1
U4
Multicellular
Multicellular organisms are capable of completing functions that unicellular organisms
organisms have
could not undertake – this is due to the collective actions of individual cells combining to
properties that
create new synergistic effects (emergent properties)
emerge from the
 Cells → Tissues → Organs → Organ systems
interaction of their
cellular components.
1.1
U5
Specialized tissues
can develop by cell
differentiation in
multicellular
organisms.
All specialized cells and the organs constructed from them have developed as a result of
differentiation
1.1
U6
Differentiation
Differentiation is the process during development whereby newly formed cells become
involves the
more specialised and distinct from one another as they mature
expression of some
 All (diploid) cells of an organism share an identical genome – each cell contains
genes and not others
the entire set of genetic instructions for that organism
in a cell’s genome.
 The activation of different instructions (genes) within a cell by chemical signals
will cause it to differentiate
 As a result of gene expression the cell’s metabolism and shape changes to carry
out a specialized function (cell differentiation)
1.1
U7
The capacity of stem
cells to divide and
differentiate along
different pathways is
necessary in
embryonic
development and
also makes stem
cells suitable for
therapeutic uses.
When a cell differentiates and becomes specialised, it loses its capacity to form
alternative cell types
Stem cells are unspecialised cells that have two key qualities:
1. Self Renewal – They can continuously divide and replicate
2. Potency – They have the capacity to differentiate into specialised cell types
There are four main types of stem cells present at various stages of human development:
 Totipotent – Can form any cell type, as well as extra-embryonic (placental) tissue
(e.g. zygote)
 Pluripotent – Can form many cell type (e.g. embryonic stem cells)
 Multipotent – Can differentiate into a number of closely related cell types
(e.g. haematopoietic adult stem cells)
 Unipotent – Cannot differentiate, but are capable of self-renewal (e.g. progenitor
cells, muscle stem cells
1.1
A1
Questioning the cell Striated muscle fibers
theory using atypical
 Muscle cells fuse to form fibers that may be very long (>300mm)
examples, including
 Consequently, they have multiple nuclei despite being surrounded by a single,
striated muscle,
continuous plasma membrane
giant algae and
 Challenges the idea that cells always function as autonomous units
aseptate fungal
Aseptate fungal hyphae
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
1.1
A2
Investigation of
Paramecium:
functions of life in
 Metabolism – most metabolic pathways happen in the cytoplasm
Paramecium and one
 Response – the wave action of the cilia moves the paramecium in response to
named
changes in the environment, e.g. towards food.
photosynthetic
 Homeostasis – contractile vacuole fill up with water and expel through the
unicellular organism.
plasma membrane to manage the water content
 Growth – after consuming and assimilating biomass from food the paramecium
Chlorella or
will get larger until it divides.
Scenedesmus are
 Reproduction – The nucleus can divide to support cell division by mitosis,
suitable photosynthetic
reproduction is often asexual
unicells, but Euglena
 Excretion – the plasma membrane control the entry and exit of substances
should be avoided as
including expulsion of metabolic waste
it can feed
 Nutrition – food vacuoles contain organisms the paramecium has consumed
heterotrophically.
Chlorella:
 Metabolism – most metabolic pathways happen in the cytoplasm
 Response – the wave action of the cilia moves the algae in response to changes
in the environment, e.g. towards light.
 Homeostasis – contractile vacuole fill up with water and expel through the
plasma membrane to manage the water content




1.1
A3
Use of stem cells to
treat Stargardt’s
disease and one
other named
condition.
Growth – after consuming and assimilating biomass from food the algae will get
larger until it divides.
Reproduction – The nucleus can divide to support cell division, by mitosis (these
cells are undergoing cytokinesis)
Excretion – the plasma membrane control the entry and exit of substances
including the diffusion out of waste oxygen
Nutrition – photosynthesis happens inside the chloroplasts to provide the algae
with food
Stem cells can be used to replace damaged or diseased cells with healthy, functioning
ones
This process requires:
 The use of biochemical solutions to trigger the differentiation of stem cells into the
desired cell type
 Surgical implantation of cells into the patient’s own tissue
 Suppression of host immune system to prevent rejection of cells (if stem cells are
from foreign source)
 Careful monitoring of new cells to ensure they do not become cancerous
Stargardt’s disease
 A genetic disease due to a recessive mutation of a gene that causes a membrane
protein used for active transport in retina cells to malfunction
 Photoreceptor cells (light detectors) in the retina degenerate, ultimately resulting
in progressive vision loss, to the point of blindness
 Treated by replacing dead cells in the retina with functioning ones derived from
stem cells
Leukemia
 Type of cancer that produces abnormally large numbers of white cells
 To remain healthy in the long run, the patient must be able to produce the white
blood cells needed to fight the disease
 Fluid is removed from the bone marrow, to extract adult stem cells – only have
the potential of producing blood cells
 A high dose of chemotherapy drugs is given to the patient, to kill all the cancer
cells in the bone marrow, which disables the bone marrow’s ability to produce
blood cells
 The stem cells are then returned to the patient’s body, re-establishes themselves
in the patient’s bone marrow, multiply and start to produce red and white blood
cells
1.1
A4
Ethics of the
therapeutic use of
stem cells from
specially created
embryos, from the
umbilical cord blood
of a new-born baby
and from an adult’s
own tissues.
Differentiation
Ease of
extraction
Ethics of
extraction
Tumour risk
Genetic damage
Compatibility
Embryonic stem
cells
Almost unlimited
growth potential, can
differentiate into any
type in the body
Can be obtained from
excess embryos
generated by IVF
programs
Removal of cells from
the embryo kills it
Cord blood stem cells
Limited capacity to differentiate into different cell
types – less growth potential than embryonic stem
cells
Easily obtained and
stored, however limited
quantities of stem cells
Umbilical cord is
discarded at birth
whether or not stem
cells are harvested
Lower risk of development
More risk of
becoming tumour
cells
Less chance of genetic damage than adult cells
Likely to be
genetically different to
the patient
Adult stem cells
Difficult to obtain as
there are very few, and
are buried deep in
tissues
Removal of stem cells
does not kill the adult
form which the cells are
taken
Due to accumulation of
mutations through the
life of the adult, genetic
damage can occur
Fully compatible with the patient as the stem cells
are genetically identical – no rejection problems
occur
The ethical considerations associated with the therapeutic use of stem cells will depend
on the source
 Using multipotent adult tissue may be effective for certain conditions, but is limited
in its scope of application
 Stem cells derived from umbilical cord blood need to be stored and preserved at
cost, raising issues of availability and access
 The greatest yield of pluripotent stem cells comes from embryos, but requires the
destruction of a potential living organism
1.1
S1
Use of a light
microscope to
investigate the
structure of cells and 1mm = 1000 𝜇m
tissues, with drawing
of cells. Calculation
of the magnification
of drawings and the
actual size of
structures and
ultrastructures
shown in drawings
or micrographs.
Scale bars are useful
as a way of indicating
actual sizes in
drawings and
micrographs.
magnification =
size of image
actual size of specimen
1.2 Ultrastructure of Cells
1.2
U1
Prokaryotes have a
Prokaryotes are organisms whose cells lack a nucleus ('pro' = before; 'karyon' =
simple cell structure
nucleus)
without
compartmentalization. Prokaryotic Features
Prokaryotic cells will typically contain the following cellular components:
 Cytoplasm – internal fluid component of the cell
 Nucleoid – region of the cytoplasm where the DNA is located (DNA strand is
circular and called a genophore)
 Plasmids – autonomous circular DNA molecules that may be transferred
between bacteria (horizontal gene transfer)
 Ribosomes – complexes of RNA and protein that are responsible for
polypeptide synthesis (prokaryote ribosome = 70S)
 Cell membrane – Semi-permeable and selective barrier surrounding the cell
 Cell wall – rigid outer covering made of peptidoglycan; maintains shape and
prevents bursting (lysis)
 Capsule – a thick polysaccharide layer used for protection against desiccation
(drying out) and phagocytosis
 Flagella – Long, slender projections containing a motor protein that enables
movement (singular: flagellum)
 Pili – Hair-like extensions that enable adherence to surfaces (attachment pili) or
mediate bacterial conjugation (sex pili)
1.2
U2
Eukaryotes have a
compartmentalized
cell structure.
Eukaryotes are organisms whose cells contain a nucleus (‘eu’ = good /
true; ‘karyon’ = nucleus)
 They have a more complex structure and are believed to have evolved from
prokaryotic cells (via endosymbiosis)
 Their chromosomes are bund by a nuclear envelope consisting of a double
layer of membrane
 Eukaryotic cells are compartmentalised by membrane-bound structures
(organelles) that perform specific roles
Animal Cell
Plant Cell
The advantages of being compartmentalized:
 Efficiency in metabolism - enzymes and substrates are localized and much
more concentrated
 Localized conditions - pH and other such factors can be kept at optimal
levels. The optimal pH level for one process in one part of the cell
 Toxic/ damaging substances can be isolated - e.g. digestive enzymes (that
could digest itself) are stored in lysozymes
 Numbers and locations of organelles can be changed - dependent on the
cell's requirements
1.2
U3
Electron microscopes Resolution is the shortest distance between two points that can be distinguished
have a much higher
resolution than light
Beams of electrons have a much shorter wavelength than light, so electron microscopes
microscopes.
have a much higher resolution
 Electron microspores have a resolution that is 200 times greater than light
microscopes
 Light microscopes reveal the structure of cells, but electron microscopes reveal
the ultrastructure
1.2
A1
Structure and
function of organelles
within exocrine gland
cells of the pancreas
and within palisade
mesophyll cells of the
leaf.
Structure
Phospholipid bilayer embedded
with proteins
Function
Semi-permeable and selective
barrier surrounding the cell
Double membrane structure with
folded inner membrane (cristae),
fluid inside (matrix)
Two subunits made of RNA and
protein; larger in eukaryotes
(80S) than prokaryotes (70S)
Double membrane structure with
pores; contains DNA associated
with histone proteins
Site of aerobic respiration (ATP
production)
Rough
endoplasmic
reticulum
A network of flattened membrane
sacs (cisternae) that are studded
with ribosomes (rough ER)
Golgi apparatus
Consists of flattened membrane
sacs called cisternae
Vesicles
Lysosomes
Single membrane with fluid inside
Single membrane structures
formed by the golgi apparatus,
Protein synthesized by the
ribosomes of the rER passes into its
cisternae and is then carried by
vesicles
Involved in the sorting, storing,
modification and export of secretory
products
Transport material inside the cell
Break down ingested food in vesicles
using digestive enzymes
Plasma
membrane
Mitochondrion
Ribosomes
Nucleus
Site of polypeptide synthesis (this
process is called translation)
Stores genetic material (DNA) as
chromatin; nucleolus is site of
ribosome assembly
Microtubules
and centrioles
(animals only)
Cilia and flagella
Chloroplast
(plants only)
Vacuoles
Cell wall (plants
only)
1.2
A2
Prokaryotes divide by
binary fission.
1.2
S1
Drawing of the
ultrastructure of
prokaryotic cells
based on electron
micrographs.
Drawings of prokaryotic
cells should show the
cell wall, pili and
flagella, and plasma
membrane enclosing
cytoplasm that contains
70S ribosomes and a
nucleoid with naked
DNA.
containing high concentration of
digestive enzymes
Microtubules are cylindrical
fibres. Centrioles form an anchor
point for microtubules during cell
division. Also present inside cilia
and flagella
Whip like structures projecting
from the cell surface. Contain a
ring of nine double microtubules
and two central ones
Double membrane structure with
internal stacks of membranous
discs (thylakoids)
Fluid-filled internal cavity
surrounded by a membrane
External outer covering made
of cellulose
Radiating microtubules form spindle
fibres and move chromosomes
during cell division
Cilia and flagella can be used for
locomotion
Site of photosynthesis –
manufactured organic molecules are
stored in various plastids
Maintains hydrostatic pressure
Provides support and mechanical
strength; prevents excess water
uptake
Prokaryotes reproduce asexually using the process of binary fission
1. The DNA is replicated semi conservatively
2. The two DNA loops attach to the membrane
3. The membrane elongates and pinches off (cytokinesis) forming two separate
cells
4. The two daughter cells are genetically identically (clones)
1.2
S2
Drawing of the
ultrastructure of
eukaryotic cells
based on electron
micrographs.
Drawings of eukaryotic
cells should show a
plasma membrane
enclosing cytoplasm
that contains 80S
ribosomes and a
nucleus, mitochondria
and other membranebound organelles are
present in the
cytoplasm. Some
eukaryotic cells have a
cell wall.
1.2
S3
Interpretation of
electron micrographs
to identify organelles
and deduce the
function of
specialized cells.
1.2 Membrane Cell Structure
1.3
U1
Phospholipids form Structure of Phospholipids:
bilayers in water due
 Consist of a polar head (hydrophilic) composed of a glycerol and a phosphate
to the amphipathic
molecule
properties of
 Consist of two non-polar tails (hydrophobic) composed of fatty acid
phospholipid
(hydrocarbon) chains
molecules.
 Because phospholipids contain both hydrophilic (water-loving) and lipophilic (fatloving) regions, they are classed as amphipathic
Amphipathic
phospholipids have
The phosphate heads are attracted to water but the hydrocarbon tails are attracted to
hydrophilic and
each other, but not to water
hydrophobic
 Because of this, the phospholipids become arranged into double layers, with the
properties.
hydrophobic tails facing inwards towards each other and the hydrophilic heads
facing the water on either side
 This double layer is called the phospholipid bilayer
Properties of the Phospholipid Bilayer:
 The bilayer is held together by weak hydrophobic interactions between the tails
 Hydrophilic / hydrophobic layers restrict the passage of many substances
 Individual phospholipids can move within the bilayer, allowing for membrane
fluidity and flexibility
 This fluidity allows for the spontaneous breaking and reforming of membranes
(endocytosis / exocytosis)
1.3
U2
Membrane proteins
Proteins
are diverse in terms
 Integral proteins are permanently embedded, many go all the way through and
of structure, position
are polytopic (many surface), integral proteins penetrating just one surface are
in the membrane and
monotopic
function.
 Peripheral proteins usually have a temporary association with the membrane,
they can be monotopic or attach to the surface
Glycoproteins
 Are proteins with an oligosaccharide (few sugar) chain attached
 They are important for cell recognition by the immune system and as hormone
receptors
Functions of Membrane Proteins
Transport: Protein channels (facilitated) and protein pumps (active)
Receptors: Peptide-based hormones (insulin, glucagon, etc.)
Anchorage: Cytoskeleton attachments and extracellular matrix
Cell recognition: MHC proteins and antigens
Intercellular joinings: Tight junctions and plasmodesmata
Enzymatic activity: Metabolic pathways (e.g. electron transport chain)
1.3
U3
Cholesterol is a
Cholesterol is a component of animal cell membranes, where it functions to maintain
component of animal integrity and mechanical stability
cell membranes.
 Cholesterol is a type of lipid, but not a fat or oil
 Most of a cholesterol molecule is hydrophobic and therefore is attracted to the
hydrophobic hydrocarbon tails in the centre of the membrane
 One end of the cholesterol molecule has a hydroxyl (-OH) group which is
hydrophilic and attracted to the phosphate heads of the periphery of the
membrane
 They are therefore amphipathic and are positioned between phospholipids in the
membrane
1.3
A1
Cholesterol in
mammalian
membranes reduces
membrane fluidity
and permeability to
some solutes.
The hydrophobic hydrocarbon tails usually behave as a liquid, but the hydrophilic
phosphate heads act more like a solid
 Overall the membrane is fluid as the components of the membrane are free to
move
It is important to regulate the degree of fluidity:
 Membranes need to be fluid enough so that the cell can move and the required
substances can move across the membrane
 If too fluid however the membrane could not effectively restrict the movement of
substances across itself
Cholesterol’s role in membrane fluidity
 Cholesterol disrupts the regular packing of the hydrocarbon tails of phospholipid
molecules, so prevents them crystallising and behaving as a solid
 It restricts the molecular motion and therefore the fluidity of the membrane
 It reduces the permeability to hydrophilic particles such as sodium ion and
hydrogen ions
 Due to its shape cholesterol can help membranes to curve into a concave
shape, which helps the formation of vesicles during endocytosis
1.3
S1
Drawing of the fluid
mosaic model.
Drawings of the fluid
mosaic model of
membrane structure
can be two
dimensional rather
than three
dimensional. Individual
phospholipid
molecules should be
shown using the
symbol of a circle with
two parallel lines
attached. A range of
membrane proteins
should be shown
including
glycoproteins.
1.3
S2
Analysis of evidence When viewed under a transmission electron microscope, membranes exhibit a
from electron
characteristic 'trilaminar’ appearance (3 layers – two dark outer layers and a lighter inner
microscopy that led region)
to the proposal of
the Davson-Danielli
Electron Micrograph of Plasma Membrane
model.
Danielli and Davson proposed a model whereby two layers of protein flanked a central
phospholipid bilayer
 The model was described as a 'lipo-protein sandwich’, as the lipid layer was
sandwiched between two protein layers
 The dark segments seen under electron microscope were identified (wrongly) as
representing the two protein layers
1.3
S3
Analysis of the
falsification of the
Davson-Danielli
model that led to the
Singer-Nicolson
model.
Structure of membrane proteins
 Protein extracted from the membranes were very varied in size and globular in
shape – unlike the type of structural protein that would form uniform and
continuous layers on periphery of the membrane
 The phospholipid bilayer was insoluble (indicating hydrophobic surfaces)
Fluorescent antibody tagging
 Membrane proteins from two different cells were tagged with red and green
fluorescent markers respectively
 When the two cells were fused, the markers became mixed throughout the
membrane of the fused cell
 This demonstrated that the membrane proteins could move and did not form a
static layer (as per Davson-Danielli)
Freeze-etched electron micrographs
 Rapid freezing of cells and then fracturing them – fracture occurs along lines of
weakness
 Revealed irregular rough surfaces within the membrane
 These rough surfaces were interpreted as being transmembrane proteins,
demonstrating that proteins were not solely localised to the outside of the
membrane structure
1.3 Membrane Transport
1.4
U1
Particles move
across membranes
by simple diffusion,
facilitated diffusion,
osmosis and active
transport.
Cellular membranes possess two key qualities:
 They are semi-permeable (only certain materials may freely cross
– large and charged substances are typically blocked)
 They are selective (membrane proteins may regulate the passage of material
that cannot freely cross)
The polar heads are attracted to other polar molecules and the non-polar tails repel any
polar molecules, preventing passage of ions through the membrane.
Passive Transport


Passive transport involves the movement of material along a concentration
gradient (high concentration ⇒ low concentration)
Because materials are moving down a concentration gradient, it
does not require the expenditure of energy (ATP hydrolysis)
Diffusion is the net movement of molecules from a region of high concentration to a
region of low concentration
 This directional movement along a gradient is passive and will continue until
molecules become evenly dispersed (equilibrium)
 Small and non-polar (lipophilic) molecules will be able to freely diffuse across
cell membranes (e.g. O2, CO2, glycerol)
The rate of diffusion can be influenced by a number of factors, including:
 Temperature (affects kinetic energy of particles in solution)
 Molecular size (larger particles are subjected to greater resistance within a fluid
medium)
 Steepness of gradient (rate of diffusion will be greater with a higher
concentration gradient)
Osmosis is the net movement of water molecules across a semi-permeable
membrane from a region of low solute concentration to a region of high
solute concentration (until equilibrium is reached)
 Water is considered the universal solvent – it will associate with, and dissolve,
polar or charged molecules (solutes)
 Because solutes cannot cross a cell membrane unaided, water will move to
equalise the two solutions
 At a higher solute concentration, there are less free water molecules in solution
as water is associated with the solute
 Osmosis is essentially the diffusion of free water molecules and hence occurs
from regions of low solute concentration
Solutions may be loosely categorised as hypertonic, hypotonic or isotonic according to
their relative osmolarity
 Solutions with a relatively higher osmolarity are categorised as hypertonic (high
solute concentration ⇒ gains water)
 Solutions with a relatively lower osmolarity are categorised as hypotonic (low
solute concentration ⇒ loses water)
 Solutions that have the same osmolarity are categorised as isotonic (same
solute concentration ⇒ no net water flow)
Aquaporin is an integral protein that acts as a pore in the membrane that speeds the
movement of water molecules
Facilitated diffusion is the passive movement of molecules across the cell membrane
via the aid of a membrane protein
 It is utilised by molecules that are unable to freely cross the phospholipid bilayer
(e.g. large, polar molecules and ions)
 This process is mediated by channel proteins
Active Transport
Active transport uses energy to move molecules against a concentration gradient
 This type of movement across a membrane is not diffusion and energy is
needed to carry it out (ATP)
Active transport involves the use of carrier proteins (called protein pumps due to their
use of energy)
 A specific solute will bind to the protein pump on one side of the membrane
 The hydrolysis of ATP (to ADP + Pi) causes a conformational change in the
protein pump
 The solute molecule is consequently translocated across the membrane
(against the gradient) and released
1.4
U2
The fluidity of
Endocytosis – the taking in of
membranes allows
external substances by
materials to be taken forming a vesicle
into cells by
 Inward pouching of
endocytosis or
plasma membrane,
released by
pinched off to form a
exocytosis.
vesicle containing the
external material
 Phagocytosis (solid
substance ingestion)
Pinocytosis (liquid
substances ingested)
the
and
Exocytosis – the release of
substances from a cell
(secretion)
 Vesicle fuses with the
plasma membrane,
the
contents are then
outside the membrane
are
 Digestive enzymes
released from gland
cells
by exocytosis
 Exocytosis can also be used to expel waste products or unwanted materials
1.4
U3
1.4
A1
Vesicles move
materials within
cells.
Vesicles can be used to move materials around inside cells
Structure and
function of sodium–
potassium pumps
for active transport
and potassium
channels for
facilitated diffusion
in axons.
An axon is part of a neuron with the function of conveying messages rapidly from one
part of the body to another in an electrical form called a nerve impulse
 A nerve impulse involves rapid movements of sodium and then potassium
across the axon membrane
An example of moving the vesicle contents occurs in the secretory cells
 Protein is synthesized by ribosomes on the rough endoplasmic reticulum and
accumulates inside the rER
 Vesicles containing the protein bud off the rER and carry them to the Golgi
apparatus
 The vesicles fuse with the Golgi apparatus, which processes the protein into its
final form
 When this has been done, vesicles bud off the Golgi apparatus and move to the
plasma membrane, where the protein is secreted
The sodium-potassium pump follows a repeating cycle of steps that result in three
sodium ions being pumped out of the axon and two potassium ions being pumped in
 Each time the pump goes around this cycle it uses one ATP
1. The interior of the pump is open to the inside of the axon; three sodium ions
enter the pmp and attach to their binding sites
2. ATP transfers a phosphate group from itself to the pump; this causes the pump
to change shape and the interior is then closed
3. The interior of the pump opens to the outside of the axon and three sodium ions
are released
4. Two potassium ions from outside can then enter and attach to their binding
sites
5. Binding of potassium causes release of the phosphate group; this causes the
pump to change shape again so that it is again only open to the inside of the
axon
6. The interior of the pump opens to the inside of the axon and the two potassium
ions are released; sodium ions can then enter and bind to the pump again
1.4
A2
Tissues or organs to In solutions with higher osmolarity (hypertonic solution), water leaves the cells by
be used in medical
osmosis so their cytoplasm shrinks in volume, causing indentations
procedures must be
 Conversely, in a solution with lower osmolarity (hypotonic) the cells take in
bathed in a solution
water by osmosis and swell up, and may eventually burst
with the same
osmolarity as the
Both hypertonic and hypotonic solutions therefore damage human cells
cytoplasm to
 In a solution with same osmolarity as the cells (isotonic) , water molecules enter
prevent osmosis.
and leave the cell at the same rate so they remain healthy
 It is therefore important for any human tissues and organs to be bathed in an
isotonic solution during medical procedures
 Usually an isotonic sodium chloride solution is used, which is called normal
saline
1.4
S1
Estimation of
osmolarity in tissues
by bathing samples
in hypotonic and
hypertonic
solutions.
Osmosis experiments
are a useful
opportunity to stress
the need for accurate
mass and volume
measurements in
scientific experiments
1.5 Origin of Cells
1.5
U1
Cells can only be
formed by division
of pre-existing cells.
Students should be
aware that the 64
codons in the genetic
code have the same
meanings in nearly all
organisms, but that
there are some minor
variations that are
likely to have accrued
since the common
origin of life on Earth.
Cells only arise from the division of preexisting cells
 Mitosis – duplication of the DNA and the nucleus, results in genetically identical
diploid daughter cells
 Meiosis – cell division in parents, generates haploid gametes (sex cells)
Other evidences of the cell theory
1. Cells are highly complex structure and no mechanism has been found for
producing cells form simpler subunits
2. All known examples of growth, be it of a tissue, an organism population, are all
a result of cell division
3. Viruses are produced from simpler subunits, but they do not consist of cells,
and they can only be produced inside the host cells that they have infected
4. Genetic code is universal each of the 64 codons (a codon is a combination of 3
DNA bases) produces the same amino acid in translation, regardless of the
organism
Base pairs (ATGC) → codon (3 base pairs) → genes (many codons) → Chromosome
(many genes)
1.5
U2
The first cells must
have arisen from
non-living material.
There is evidence that suggest complex structures arising in a series of stages over
logn periods of time. There are hypotheses of how some of the main stages could have
occurred:
1. Non-living synthesis of simple organic molecules:
 Miller and Urey recreated the conditions of pre-biotic Earth in a closed system
 They passed steam through a mixture of methane, hydrogen and ammonia
(representative of the atmosphere of the early Earth)
 Electrical discharges were used to stimulate lightning
 They found that amino acids and other carbon compounds needed for life were
produced
2. Assembly of these organic molecules into polymers:
 A possible site for the origin of the first carbon compounds is around deep-sea
vents (cracks in the Earth’s surface)
 They are characterized by gushing hot water carrying reduced inorganic
chemicals
 These chemicals represent readily accessible supplies of energy, a source of
energy for the assembly of carbon compounds into polymers
3. Formation of membranes to package the organic molecules
 Experiments have shown that phospholipids natural assemble into bilayers, if
conditions are correct.
 Formation of the bilayer creates an isolated internal environment which allows
optimal conditions, e.g. for replication or catalysis to be maintained.
4. Formation of polymers that can self-replicate (enabling inheritance)
 DNA though very stable and effective at storing information is not able to selfreplicate – enzymes are required


1.5
U3
The origin of
eukaryotic cells can
be explained by the
endosymbiotic
theory.
Evidence for the
endosymbiotic theory
is expected. The
origin of eukaryote
cilia and flagella does
not need to be
included.
However, RNA can both store information and self-replicate - it can catalyze the
formation of copies of itself and hence are speculated to be the genetic material
used in early Earth
In ribosomes RNA is found in the catalytic site and plays a role in peptide bond
formation
Endosymbiotic theory explains the existence of several organelles of eukaryotes. The
theory states that the organelles (e.g. mitochondria and chloroplasts) originated as
symbioses between separate single-celled organisms.
Development of the Nucleus
 A prokaryote grows in size and develops folds in its membrane to maintain an
efficient SA:Vol
 The infoldings are pinched off forming an internal membrane
 The nucleoid region is enclosed in the internal membrane and hence becomes
the nucleus
Development of Mitochondria
 An aerobic proteobacterium enters a larger anaerobic prokaryote (possibly as
prey or a parasite)
 It survives digestion to become a valuable endosymbiont (An endosymbiont is a
cell which lives inside another cell with mutual benefit)
 The aerobic proteobacterium provides a rich source of ATP to its host enabling
it to out-compete other anaerobic prokaryotes
 As the host cell grows and divides so does the aerobic proteobacterium
therefore subsequent generations automatically contain aerobic
proteobacterium
 The aerobic proteobacterium evolves and is assimilated and to become a
mitochondrion
Evidence for Endosymbiosis
Mitochondria and chloroplasts are both organelles suggested to have arisen via
endosymbiosis
 Evidence that supports the extracellular origins of these organelles can be seen
by looking at certain key features:
Component
Membranes
Antibiotics
Division
DNA
Ribosomes
1.5
A1
Evidence from
Pasteur’s
experiments that
spontaneous
generation of cells
and organisms does
not now occur on
Earth.
Evidence
Some organelles have double membranes
Susceptible to antibiotics
Reproduction occurs via a fission-like process
Has own DNA which is naked and circular (like
prokaryotic DNA structure)
Have ribosomes which are 70S in size
Louis Pasteur experimented whether sterile nutrient broth (everything that is needed for
a living organism) to determine if life can be generated spontaneously
Method:
 Two experiments were set up
 In both, Pasteur added nutrient broth to flasks and bent the necks of the flasks
into S- shapes
 Each flask was then heated to boil the broth in order to kill all existing microbes
 After the broth had been sterilized, Pasteur broke off the swan necks from the
flasks in Experiment 1, exposing the nutrient broth within them to air from above

The flasks in Experiment 2 were left alone
Results
 The broth in experiment 1 turned cloudy whilst the broth in experiment 2
remained clear
 Microbe growth only occurred in experiment 1
This suggested that spontaneous generation of life no longer occurs on Earth, and that
life only arises from pre-existing life forms
1.6 Cell Division
1.6
U1
Mitosis is division of Mitosis is the nuclear division by which replicated copies of a cell’s DNA are organized
the nucleus into two into chromosomes.
genetically identical
daughter nuclei.
Reasons for Cell Division
 Growth - cells can only get to a certain SA:Vol ratio
The sequence of
 Asexual reproduction - certain eukaryotic organisms ay reproduce asexually by
events in the four
mitosis and for single celled organisms, cell division
phases of mitosis
 Tissue repair - damaged tissue can recover by replacing dead or damaged cells
should be known. To
 Embryonic development - a fertilised egg will undergo mitosis and
avoid confusion in
differentiation in order to develop into an embryo
terminology, teachers
are encouraged to
The cell cycle is the series of events through which cells pass to divide and create
refer to the two parts
two identical daughter cells.
of a chromosome as
sister chromatids,
while they are
attached to each other
by a centromere in the
early stages of
mitosis. From
anaphase onwards,
when sister
chromatids have
separated to form
individual structures,
they should be
Chromosome and Chromatid
referred to as
chromosomes.
 A chromosome is the condensed form of DNA which is visible during mitosis (via
microscopy)
 As the DNA is replicated during the S phase of interphase, the chromosome will
initially contain two identical DNA strands
 These genetically identical strands are called sister chromatids and are held
together by a central region called the centromere
Interphase
DNA is present as uncondensed chromatin (not visible under microscope)
 DNA is contained within a clearly defined nucleus
 Centrosomes and other organelles have been duplicated
 Cell is enlarged in preparation for division
Prophase
 DNA supercoils and chromosomes condense (becoming visible under
microscope)
 Chromosomes are comprised of genetically identical sister chromatids (joined at
a centromere)
 Paired centrosomes move to the opposite poles of the cell and form microtubule
spindle fibres
 The nuclear membrane breaks down and the nucleus dissolves
Metaphase
 Microtubule spindle fibres from both centrosomes connect to the centromere of
each chromosome


Microtubule depolymerisation causes spindle fibres to shorten in length and
contract
This causes chromosomes to align along the centre of the cell (equatorial plane
or metaphase plate)
Anaphase
 Continued contraction of the spindle fibres causes genetically identical sister
chromatids to separate
 Once the chromatids separate, they are each considered an individual
chromosome in their own right
 The genetically identical chromosomes move to the opposite poles of the cell
Telophase
 Once the two chromosome sets arrive at the poles, spindle fibres dissolve
 Chromosomes decondense (no longer visible under light microscope)
 Nuclear membranes reform around each chromosome set
 Cytokinesis occurs concurrently, splitting the cell into two
1.6
U2
Chromosomes
condense by
supercoiling during
mitosis.



Human cells are on average less than 5 μm in diameter.
Human chromosomes are 15mm to 85mm (15,000μm to 85,000 μm) in length
It is therefore essential to package chromosomes into much shorter structures
Condensation occurs by means of repeatedly coiling the DNA molecule to make the
chromosome shorter and wider – this process is called supercoiling
 This process occurs during the first stage of mitosis
1.6
U3
Cytokinesis occurs
Cytokinesis is the division of the cytoplasm and hence the cell
after mitosis and is
 The division of the cell into two daughter cells (cytokinesis) occurs concurrently
different in plant and
with telophase.
animal cells.
Animal cells
 Microfilaments pulls the plasma membrane inward
 Produces cleavage furrow
 When the cleavage furrow reaches the centre of the cell it is pinched apart
Plant cells
 During Telophase, vesicles migrate to the centre of the cell
 Vesicles fuse to form tubular structures
 The tubular structure merge with the plasma membrane (the cell plate)
 Completes the division of the cytoplasm
 Vesicles deposit substances in the lumen between the daughter cells to form the
middle lamella (‘gluing’ the cells together)
 Both daughter cell secrete cellulose to form their new adjoining cell walls
1.6
U4
Interphase is a very
active phase of the
cell cycle with many
processes occurring
in the nucleus and
cytoplasm.
Interphase
The stage in the development of a cell between two successive divisions
This phase of the cell cycle is a continuum of three distinct stages:
 G1 – First intermediate gap stage in which the cell grows and prepares for DNA
replication
 S – Synthesis stage in which DNA is replicated
 G2 – Second intermediate gap stage in which the cell finishes growing and
prepares for cell division
During interphase, the cell carries out its normal functions
 DNA replication – DNA is copied during the S phase of interphase
 Organelle duplication – Organelles must be duplicated for twin daughter cells
 Cell growth – Cytoplasmic volume must increase prior to division
 Transcription / translation – Key proteins and enzymes must be synthesised
 Obtain nutrients – Vital cellular materials must be present before division
 Respiration (cellular) – ATP production is needed to drive the division process
1.6
U5
Cyclins are involved
in the control of the
cell cycle.
Cyclins are a family of proteins that control the progression of cells through the cell cycle
 Cells cannot progress to the next stage of the cell cycle unless the specific cyclin
reaches its threshold
 Cyclins bind to enzymes called cyclin-dependent kinases
 These kinases then become active and attach phosphate groups to other
proteins in the cell
 The attachment of phosphate triggers the other proteins to become active and
carry out tasks (specific to one of the phases of the cell cycle)
1.6
U6
Mutagens,
oncogenes and
metastasis are
involved in the
development of
primary and
secondary tumors.
Tumours are abnormal cell growths resulting from uncontrolled cell division and can
occur in any tissue or organ
 Diseases caused by the growth of tumours are collectively known as cancers
Mutations
A mutation is a change in an organism’s genetic code
 A mutation in the base sequence of certain genes can result in cancer
Mutagens
A mutagen is an agent that changes the genetic material of an organism (either acts on
the DNA or the replicative machinery)
Mutagens may be physical, chemical or biological in origin:
 Physical – Sources of radiation including X-rays (ionising), ultraviolet (UV) light
and radioactive decay
 Chemical – DNA interacting substances including reactive oxygen species (ROS)
and metals (e.g. arsenic)
 Biological – Viruses, certain bacteria and mobile genetic elements (transposons)
Mutagens that lead to the formation of cancer are further classified as carcinogens
Oncogenes
An oncogene is a gene that produces the proteins that controls the cell cycle.
 A mutation in the oncogene can produce proteins that may not function properly,
causing uncontrolled cell cycle
Metastasis
Tumour cells may either remain in their original location (benign) or spread and invade
neighbouring tissue (malignant)
 Metastasis is the spread of cancer from one location (primary tumour) to another,
forming a secondary tumour
 Secondary tumours are made up of the same type of cell as the primary tumour –
this affects the type of treatment required
o For example, if breast cancer spread to the liver, the patient has secondary
breast cancer of the liver (treat with breast cancer drugs)
1.6
A1
The correlation
between smoking
and incidence of
cancers.
1.6
S1
Identification of
phases of mitosis in
cells viewed with a
microscope or in a
micrograph.
Prophase
Preparation of
temporary mounts of
root squashes is
recommended but
phases in mitosis can
also be viewed using
permanent slides.
Metaphase
A correlation in science is a relationship between two variable factors
 There is a positive correlation between cigarette smoking and the death rate due
to cancer
 Many researches show that the more cigarettes smoked per day, the higher the
death rate due to cancer
 More than 20 chemicals found in tobacco have caused cancers in laboratory
animals and humans
 More than 40 other chemicals found in tobacco have been identified as
carcinogens
Anaphase
Telophase
1.6
S2
Determination of a
mitotic index from a
micrograph.
Mitotic index - the ratio between the number of cells in a tissue and the total number of
observed cells
𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑐𝑒𝑙𝑙𝑠 𝑖𝑛 𝑚𝑖𝑡𝑜𝑠𝑖𝑠
𝑡𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑐𝑒𝑙𝑙𝑠