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Cell Biology
By Lauren Somers

Contents
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Introduction to cells
Ultrastructure of cells
Membrane structure
Membrane transport

The origin of cells
Cell division

Introduction to cells

Cell theory
• The cell is the fundamental unit of all life forms
• All organisms are composed of one of more cell and cells are the smallest unit of life
...


• Cells can only come from pre-existing cells
...
Stained yeast cells can be used to observe
this
• If they are placed in water of different salinities the contractile vacuole can be seen forming
and emptying
• Chlorella is a photosynthetic organism with a rapid growth rate, although its cells are small it
can quickly reproduce, turning water green and opaque, this happens in water that is rich in
nitrates and phosphates
...

Living organisms can be
examined

Very thin sections of material
supported on metal grids
Living organisms cannot be
examined

Very thin sections of material
supported on metal grids
...


• This allows it to perform more complicated functions
• One cell can function on its own, but with other cells in a group it can produce tissues and organs that can carry out
a range of roles in the organism

• Cells form tissues, tissues form organs, organs for organ systems and organ systems work in synergy so that the
whole organism can carry out a complex range of tasks

Becoming multicellular
Differentiation

• In some cells particular genes are expressed that are not expressed in others
• Differentiation involves the expression of some genes from the genome in
the cell but not others

Stem cells
• The fertilised egg from and organism contains all the information needed to develop it into a complex organism
with many different types of cells
...

• A fertilised egg cell divides quickly and forms a blastocyst in which all cells are alike
...
Once they have done this it cant
be reversed
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• Stem cells differ from other cells because: they are unspecialised; they can divide repeatedly to make large numbers
of new cells; they can differentiate into several types of cells; they have a large nucleus relative to the volume of the
cytoplasm
...

• Some of the most recent research aims to grow stem cells to replace damaged or diseased tissue
in patients suffering from degenerative diseases such as Alzheimer's disease or multiple sclerosis

• Early work concentrated on embryonic stem cells, but these can only be obtained from discarded
embryos from in vitro fertilisation clinics and there is much debate about the ethics of doing this
...
These cells can be harvested, grown and used

Stem cells
Therapeutic use of stem cells
• Another source of stem cells is the blood in the umbilical cord of a newborn baby
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Cord blood can be used to treat certain types of
leukaemia
...
After
chemotherapy to destroy the patients own bone marrow cells, the stem cells which are a
correct match are given by transfusion
...

• This treatment works well in young children but there are not enough cells in one cord to
meet the needs of an adult patient
...
Allowing them to
divide in the lab produces more blood cells but not more stem cells
...


Stem cells
• Therapeutic use of stem cells 2
• Recent stem cell research has been into the treatment of Stargardt’s disease using retinal

pigment epithelium (RPE)
...
Retinal cells
can be made from embryonic stem cells
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Not only did the cells survive but the number of cells
increased, they began to develop important visual pigment and the patients noticed
improvement in their vision
...


Stem cells
Therapeutic use of stem cells 3

• Stem cell therapy has also been used in treating type 1 diabetes and research
is continuing to treat a range of neurological conditions such as Alzheimer’s
disease

Nature of science
Looking for trends and discrepancies – developing theories

• A theory is a well-established and widely accepted principle that arises from extensive observation
of trends and discrepancies and incorporates facts, laws, predictions and tested hypotheses
...


• If any evidence is found that contradicts a hypothesis, it must be rejected and a new hypothesis
formulated
...


• An example of discrepancy is the cells that do not conform to cell theory
...


• Prokaryotic cells are usually much smaller in volume than more complex cells
as they have no nucleus
...
It differs slightly
from mitosis
...


•

The chromosome is found in a region of the cytoplasm called the nucleoid
...
Bacteria also contain additional small circles of DNA called plasmids
...


•

Ribosomes are found in all prokaryotic cells
...

Prokaryotes have 70S ribosomes, which are smaller than those in eukaryotes

•

A flagellum is present in some prokaryotic cells, it projects from the cell wall and allows the cell to move

•

Some bacteria have pili, these structures, found on the cell wall, can connect to other bacterial cells, drawing them closer so that genetic
material can be exchanged

Eukaryotic cells
• Eukaryotic organisms have cells that contain a nucleus
...


• Eukaryotic cells contain structures called organelles each of which forms a
compartment where a specific function takes place (compartmentalisation)
...
Chromosomes are composed of DNA
combined with histone protein, forming chromatin
...
Small gaps in the envelope called nuclear pores are visible, and it is through these that material passes
between the nucleus and the rest of the cell
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• Associated with the nuclear envelope is a series of membranes known as endoplasmic reticulum (ER)
...
As proteins are
produced they collect in the spaces between the membranes known as the cisternae
...
ER with no ribosomes attached is
known as smooth endoplasmic reticulum (sER)
...
It also produces phospholipids for the construction of membranes and lipids in the cell
...
It

processes proteins from the rER, collecting, packing and modifying them and then releasing them for transport to
various parts of the cell or secretion from the cell
...


• Eukaryotic cells also contain mitochondria which are elongated structures surrounded by a double membrane that

are found throughout the cytoplasm
...
The inner membrane is folded to form cristae,
which greatly increase the surface area for the production of ATP
...


• Lysosomes are spherical organelles with little internal structure, made by the golg apparatus
...
They are important in cell death, breaking down old organelles
and, in white blood cells, digesting bacteria which have been engulfed by phagocytosis
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• Ribosomes are the site of protein synthesis in cells
...
They
are made of RNA and protein but don’t have a membrane around them
...


Eukaryotic cell structure 3
• The plasma membrane controls the movement of materials into and out of the cell and the cytoplasm, which fills
up much of the volume, provides a medium for many metabolic reactions
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All plant cells have a cellulose cell wall and most have a large central vacuole
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These are found in cells exposed to the light as they are the sites of
photosynthesis
...
Both chloroplasts and
mitochondria have their own DNA and are able to reproduce independently of the cell
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The outer
cell call is composed to cellulose and other carbohydrates such as lignin and pectin giving plant cells further support
and a more rigid structure than animal cells
...


Membrane structure

The structure of membranes
• Membranes not only provide shape for a cell and enclose its contents, a considerable amount of activity occurs at
membrane surfaces, especially at the plasma membrane in contact with the extracellular space
...


• All membranes have the same basic structure, they are 7-10 nm thick and composed of two layers of phospholipids,
forming a bilayer
...
The lipophilic (hydrophobic) parts all point towards each other and the
hydrophilic areas point outwards
...
The hydrophilic ‘heads’ appear on the outside of the membrane in water while the hydrophobic
‘tails’ orient themselves inside the double layer away from the water
...
It
is called fluid because they can float into any position in the membrane
...


The structure of membranes 2
• Embedded in the bilayer are different molecules that contribute to the function of membranes
...
One end of the cholesterol molecule associates with
the polar heads while other parts are embedded in the membrane next to the fatty acids chains
...


• There are also different proteins in the bilayer
...
Many of the proteins on the outer surface are glycoproteins that have
carbohydrate groups attached
...
Others are important in cell-to-cell
communication and adhesion
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Some proteins span the bilayer acting as channels for ions and molecules to pass by
passive transport or forming pumps for active transport
...

• The existence of a lipid bilayer was originally proposed and outlined by Gorter and Grendel in

1925
...
This attempted to explain surface tension
...
Fresh observations
using freeze-etching were also important
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Nature of science
Falsification of theories – developing a model of membrane structure

• Any scientific theory or model can only exist until it is disproved
• The Danson-Danielli model was accepted until new evidence called the
model into question
...
This was disproved
when it was found that different organisms transport different substances
across their membranes using different proteins
...
This
is a passive process which requires no energy, movement occurs via a simple concentration gradient
...


• An important example of simple diffusion is during respiration
...
As the cell respires the oxygen concentration inside becomes less than the concentration outside
so oxygen molecules diffuse in
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• Simple diffusion occurs when the membrane is fully permeable to the substance or where channel proteins are large
enough for the substance to pass through

• Large molecules and charged particles cannot pass through the membrane by simple diffusion so certain proteins
form channels through which they can pass
...

Some of these are gated and only open to allow certain ions to pass through when they are stimulated to do so
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Substances like these are transported by facilitated diffusion
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This allows a faster diffusion rate for
molecules that particular cells need
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Diffusion, facilitated diffusion and osmosis
Osmosis

• This is the passive movement of water across a partially permeable membrane from an area of lower solute
concentration to an area of higher solute concentration
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• An animal cell placed in pure water will take in water by osmosis until eventually it may burst
...
In either situation the
cell will not function properly and its metabolism will be affected
...


• Plant cells are also affected by osmosis but their cell wall prevents them from being damaged or bursting
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A plant cell that is full becomes firm and rigid, known as turgor
...

• To move these substances into the cell against the concentration gradient, the cell must use
metabolic energy released from the breakdown of ATP into ADP + Pi
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Specific proteins in the plasma membrane act as transporters to move substances
through
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• The sodium potassium pump maintains the concentration of sodium and potassium ions in
the cells and extracellular fluid
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Sodium ions are pumped out and
potassium ions are pumped in
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Neither diffusion or active transport will work here
...

Uptake is called endocytosis and export is exocytosis
...


• During endocytosis, part of the plasma membrane is pulled inward and surrounds the liquid or solid that is
to me moved into the cell
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This is how white blood cells take care of bacteria
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From here they are enclosed within a membrane bound package known as a
vesicle and moved to the plasma membrane along microtubules
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As it approaches the plasma membrane, it is able to fuse with
it and in doing so release its contents to the outside
...


Endocytosis and exocytosis 2
• Vesicles also help to transfer and organise substances in the cell
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• Nerve impulses are able to pass across synapses due to this process
...

They stimulate the adjacent nerve and then are reabsorbed by endocytosis to
be recycled and reused
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The origin of cells

How are new cells formed?
• Cells can only come from pre-existing cells
• Louis Pasteur carried out experiments providing evidence to support this
theory
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How did the first cells originate?
• All organisms alive today are made up of cells
...
5 billion years ago and
the structures inside them from chemicals present at the time
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• Molecules must have formed that could replicate themselves and control other chemical
reactions

• The mixture of the molecules must have been enclose within membrane-bound vesicles

How did the first cells originate?
The endosymbiotic theory

• This explains how eukaryotes could have developed from simple cells
...
There is evidence to suggest that some prokaryotes were
engulfed by other cells, and remained within their membranes inside the cell where they provided some
advantages
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They both:
•
•
•

Contain ribosomes smaller than those found in eukaryotes but the same size as those found in bacteria

•

Can replicate themselves by binary fission

Contain small, circular pieces of DNA resembling plasmids

Have their own envelope surrounding them on the inner membrane made of proteins synthesised by the organelle,
suggesting that it may have used this ability when it was independent
...


• This supports the theory that some organelles are modified bacteria that were engulfed by

phagocytosis, early in the evolution of eukaryotic cells
...
The double outer
envelope of chloroplasts and bacteria may have originated from the bacteria membrane and the
membrane of an engulfing vesicle
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• Critics also note that mitochondria and chloroplasts cannot survive independently outside the cell,
which they make be expected to do if they were once independent, but some argue that they have
simply lost this ability over time
...
Chromosomes are
made from nucleic acids such as DNA and built into the chromosomes is a code, used by the cell to assemble all the
molecules it needs to live
...
They are adenine (A), cytosine (C), guanine (G) and

thymine (T)
...
The code is read in sets of 3 bases known as ‘codons’ and 64 codons can be made from the 4 bases
...


• The genetic code is said to be universal because all living organisms share it whith only few variations due to mutations
...


• The study of genetic code, together with the study of molecular processes show that these vital processes are similar in all
living organisms, suggesting that they had a common origin
...


• The cytoplasm and nucleus divide in processes known as mitosis and
cytokinesis, which are phases in the cell cycle
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• Part of this phase is spent in preparation for division (the G2 phase) and part of it is the period

immediately after division ( the G1 phase)
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• If a cell is examined under a light microscope during interphase, not much seems to be

happening, but in reality the cell is very active, it is carrying out its normal activities and preparing
for mitosis
...


• During interphase many proteins need to be synthesised and mitochondria increase so that the

respiratory rate would be rapid enough to provide enough energy for cell division
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Mitosis
• The two new cells formed by mitosis and cytokinesis are genetically identical
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This kind of
reproduction is known as asexual as no gametes are used and the offspring
are genetically identical to the parent
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• During interphase they were drawn out into long threads to allow the cell access to the genes but now
they coil round themselves several times to make a supercoil
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• We can follow this because supercoiled chromosomes are visible with a microscope
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Also visible are centrioles which move to
opposite sides of the cell as microtubules form between them
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• At the end of prophase the nuclear membrane breaks down
...

• As it disappears more space is created so that the chromosomes can move
into position
...


Mitosis
Anaphase

• The centromeres split and the sister chromatids are pulled apart and move
towards the centrioles at opposite poles of the cell as the spindle fibres
shorten
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Mitosis
Telophase
• One the two sets of chromosomes reach their opposite poles, the spindle fibres break down
and a nuclear envelope forms around each set of chromosomes
...

• Following this phase the plasma membrane pinches in and the two new nuclei become
separated
...

• Plant cells divide differently: a cell plate forms along the centre, splitting the cytoplasm into
two regions
...
Gradually this builds up forming two new
cells
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They interact with other proteins

called CDKs (cyclin dependant kinases) to form enzymes that direct the cell through the cell
cycle and control specific events such as microtubule formation and chromatid alignment
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In 2001, Hunt, together with Lee Hartwell and Paul Nurse who also contributed to
the discovery, was awarded the Nobel prize in physiology and medicine
...
Cyclins are divided into four types based on their
behaviour in vertebrae and yeast cells but some have different functions in different types of
cells
...
G1 cyclins coordinate
cell growth and the start of a new cell cycle
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The levels of S cyclins remain high throughout S
phase, G2 and the first part of mitosis to promote early events
...


Cyclin activity graph

Primary and secondary tumours
• In most cases mitosis continues until a tissue has grown sufficiently or repairs have been made
...
Sometimes however, this does not happen
...
These can
be either benign or malignant, where some of the abnormal cells migrate to other tissues or organs and continue to
grow tumours there
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• Cancer occurs when a primary tumour breaks off and migrates to form new secondary tumours in a process known
as metastasis
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These are
chemical, physical and biological agents called mutagens
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The DNA changes caused are called mutations but, because some of them cause cancer, some are said to be
carcinogens
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• Many tumours are activated by oncogenes, which are genes with the potential to cause cancer, they can be either

altered normal genes or genes that are expressed at abnormally high levels
...
Most oncogenes become active due to an additional process
such as mutation, exposure to mutagens or viral infections
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In the UK approximately 85% of lung cancers
in both men and women are a result of smoking
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If they give up, their risk of contracting cancer decreases
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The highest level of male lung cancer was in the early 1970s as a
result of a peak in smoking 20-30 years earlier
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It has come to describe the
role of chance in science and how unexpected discoveries are sometimes made
...


• The discovery of cyclins is an example of a serendipitous discovery
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