Grove Academy
Biology Department
National 5
Summary notes
Cell Biology Unit
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National 5 Biology summary notes
Cell ultrastructure
All living things are made from cells. Cells can not usually be viewed with the naked
eye, you must use a microscope to view cells and to see the detail in the structures
which make up cells.
There are lots of different types of cell, the structure of animal, plant, bacteria
and fungal cells are considered in this unit. Each of these cell types has a different
structure.
Animal cells have a nucleus, cytoplasm, cell membrane, ribosomes and mitochondria.
Plant cells have all of these structures (nucleus, cytoplasm, cell membrane,
ribosomes and mitochondria) and they also have a vacuole, cell wall and chloroplasts.
Both bacteria and fungi are types of micro-organism. Their structures are also
different; Fungi cells are similar to plant cells but they do not have chloroplasts.
Bacterial cells are different because they have no organelles.
Plant cells walls are made of a structural carbohydrate called Cellulose. The cell
wall helps to maintain the cells rigid structure. The cell walls of fungus and bacteria
are not made of cellulose.
Cell
Function
Found in…
Nucleus
Controls cell activities
Animal, plant and fungal cells
Cell membrane
Controls the entry and exit of
molecules in and out of a cell
Animal, plant, fungal and
The site of chemical reactions
Animal, plant, fungal and
Ultrastructure
Cytoplasm
bacterial cells
bacterial cells
Cell wall
Ribosome
Supports the cell structure and
Plants, fungal and bacterial
shape
cells
The site of protein synthesis
Animal, plant, fungal and
bacterial cells
Mitochondrion
The site of aerobic respiration
Animal, plant and fungal cells
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Chloroplast
Site of photosynthesis
Green plant cells
Vacuole
Contains water and solutes as cell
Green plants and fungal cells
sap
Plasmid
Small ring of genetic material
Bacterial cells
Transport across cell membranes
All cells have a cell membrane. The purpose of
the cell membrane is to control which
substances enter and exit the cell. The cell
membrane is composed of phospoholipids and
proteins and is described as being selectively
permeable.(i.e. it allows some substances to pass
across it but not all substances)
Molecules move across the cell membrane by
diffusion, osmosis and active transport.
Diffusion
Diffusion is the movement of molecules from an area of high concentration to an
area of low concentration (down a concentration gradient)
Diffusion is a passive process, this means that no energy is needed for it to take
place
Diffusion is important to cells as it is an important strategy to move materials in
and out of cells. To get raw materials in and to get rid of waste materials.
The following substances move into cells by diffusion – glucose, oxygen and amino
acids.
Carbon dioxide moves out of cells by diffusion.
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Osmosis
Osmosis is the movement of water molecules from an area of high water
concentration to an area of low water concentration, through a semi-permeable
membrane.
Osmosis is a passive process, it does not require energy.
A solution is often made up by dissolving a substance in water. e.g. a 5% glucose
solution will contain 5% Glucose and 95% water.
It is important to remember that a 5% sucrose solution will contain more WATER
than a 10% sucrose solution which will contain only 90% water.
Cells respond differently to treatment in solutions. Pure water (100%) has a higher
water concentration than most cells. Animal cells will burst when placed in a solution
with higher water concentration than their cell contents. Plant cells become turgid
if left in a solution with higher water concentration.
If placed in a solution with a lower water concentration that the cell contents;
animal cells shrink and plant cells become plasmolysed.
Stronger water
solution outside cell
Water concentration
is the same inside and
outside cell
Stronger water
solution inside cell
Cell Bursts
Cell is normal
Cell shrinks
Cell is turgid
Cell is normal
Cell plasmolysed
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Active transport
Active transport it the movement of molecules from an area of low concentration
to an area of high concentration (against the concentration gradient)
This is an active process, it requires energy from ATP.
Molecules are often pumped across the membrane during active transport by the
proteins in the membrane.
Because active transport is dependent on ATP, the factors which affect the rate
of respiration will also affect the rate of active transport e.g. temperature and
oxygen availability.
DNA and production of proteins
Deoxyribonucleic acid (DNA) is stored in the nucleus of every cell. It is a code
made up of 4 letters (A, T, C and G) which contains instructions for the cell to
produce all proteins needed by cells.
DNA is a double-stranded helix (looks like a twisted ladder). The 2 strands are held
together by complementary base pairing between the bases (A=T and C=G). The
order of the bases on the DNA determines the order of amino acids in a protein.
A gene is a section of DNA which codes for a particular protein.
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Protein Synthesis.
DNA is the copy of the genetic code for making proteins which the cells need on a
day to day basis. DNA must stay in the nucleus where it is unlikely to be damaged.
Proteins are made (synthesised) at ribosomes which are outside the nucleus, in the
cytoplasm. In order for the instructions to make the protein to reach the
ribosomes a copy of the DNA has to be made in the nucleus and move to the
ribosome. This copy is called mRNA, m standing for messenger. mRNA copies the
sequence of bases from the DNA and travels to the ribosome where the amino
acids are arranged into the correct order for the protein. This is how protein
synthesis (the production of protein) occurs.
Proteins and enzymes
Proteins
Proteins are very important biological molecules. They control a lot of the things
which keep cells functioning. Proteins are built up from chains of amino acids.
There are about 20 naturally occurring amino acids, when they are arranged in
different combinations they make a very large variety of molecules which are
different shapes, the shape of a protein is important in determining it’s function.
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Enzymes, hormones, antibodies, receptors and structural proteins are all examples
of different proteins. Hormones are chemical messengers which allow different
parts of our bodies to communicate with each other, they travel around the body
through the blood. Antibodies are part of the body’s defence against disease, they
are produced by white blood cells to stop infection. Receptor proteins allow
messengers like hormones to bind to a target cell so that the message is passed on.
More detail about enzymes given below.
Enzymes
Enzymes are biological catalysts, they speed up chemical reactions without being
used up or changed in the reaction. They are made by all living cells and have a role
to play in most cellular processes. Each enzyme has a different shape which is very
important to its function. Enzymes work by binding to the molecule needing changed
(this is called the substrate). Each enzyme only binds to one substrate, because of
the shape only 1 type molecule will fit into the enzymes active site (like a lock and
key). Enzymes are described as being specific because of this property.
Enzymes can carry out either a degradation (breakdown) or a synthesis (build up)
reaction. The substance they act on is called the Substrate and the substance they
produce is called the product.
The activity of an enzyme is affected by the conditions in the environment e.g. pH
and temperature are very important. The conditions an enzyme works best at are
described as the optimum conditions for that enzyme; the best temperature is the
optimum temperature and the best pH is the optimum pH which allows for maximum
enzyme acativity.
Other proteins are also affected by changing the pH and the temperature, the
reasons that temperature and pH affect proteins (including enzymes) is they cause
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the shape of the protein to change so it can not carry out its job as efficiently. If
the shape is changed so that the protein does not function anymore the protein is
described as being denatured.
Enzymes can be classified into two groups;
Synthesis enzymes, which build up small substrates to produce a larger
product
Degradation enzymes, which break down a large substrate to produce
smaller products.
Enzyme examples
The table below contains some examples of enzymes and their substrates and
products
SUBSTRATE
ENZYME
WHERE IS
ENZYME
FOUND?
Starch
Amylase
Saliva and
small intestine
Fat
Lipase
Stomach
Protein
Hydrogen
Peroxide
Glucose-1Phosphate
Pepsin
Small intestine
Catalase
All living cells
Phosphorylase
Plant cells
PRODUCT
SYNTHESIS
OR
DEGRADATIVE
Maltose
Degradative
Fatty acids
and Glycerol
Peptides
Oxygen and
water
Starch
Degradative
Degradative
Degradative
Synthesis
Enzyme Experiments
The rate of enzyme activity can be measured in different ways.
If one of the products is a gas (e.g. the production of oxygen in the breakdown of
hydrogen peroxide by catalase) then washing up liquid can be added to the
reactants and the height of bubbles produced over a certain period of time can be
measured as the rate of reaction.
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If an indicator can be added that changes colour when the experiment is complete
(e.g. add iodine to glucose-1-phosphate and phosphorylase and it will change
blue/black as starch is produced) then the time taken to observe the colour change
can be used to measure the rate of activity.
Once a protocol has been established to measure how quickly the reaction occurs it
can be repeated at different temperatures, or with different concentrations of
substrate, or at different pH’s to investigate the effect of making changes to the
speed of the enzyme activity. The best pH or temperature is called the optimum.
Genetic engineering
The structure of DNA is
identical in all living
things, as is the process
for synthesising proteins
at the ribosome. This
means that it is possible
from humans to transfer
DNA from one organism
to another, this is called
genetic engineering.
Humans can use genetic
engineering to get
organisms to produce
proteins which would they
would not normally
produce. For example
hormones like human
insulin and human growth
hormone are both
proteins which can be
used to treat human
conditions.
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Scientists can put the gene for either of these products into host bacteria which
grow and multiply quickly so that bacteria produce the proteins and they can be
isolated from the bacteria and used to treat humans. These organisms are called
GM organisms.
Respiration
Respiration is carried out by all cells to make energy. Respiration relies on
diffusion to get the raw materials into cells and to release some of the products.
During respiration chemical energy trapped inside substrates such as glucose is
released by the cell in a series of enzyme-controlled reactions.
Energy is used and moved in cells by a molecule called adenosine triphosphate
(ATP). During respiration the chemical energy trapped in glucose is moved into ATP
molecules so that it can be used for other processes in the cell. In cells energy is
needed for many cellular activities including muscle cell contraction, cell division,
protein synthesis and transmission of nerve impulses.
In the presence of oxygen (Aerobic)
During respiration in the presence of oxygen, glucose is broken down by enzymes to
pyruvate. Pyruvate is subsequently broken down to carbon dioxide and water. This
is complete breakdown of glucose as the products are relatively harmless. Because
the glucose is completely broken down a large number of ATP molecules are
produced for every molecule of glucose.
In the absence of oxygen – Fermentation
Cells still carry out respiration to make ATP if there is no oxygen available, but the
process is less efficient. The breakdown process is different in animal cells and
plant cells.
In animal cells glucose is broken down to pyruvate by enzymes. Pyruvate is then
turned into lactate. This process only produces 2 ATP from every molecule of
glucose.
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National 5 Biology summary notes
In plant cells glucose is broken down to pyruvate by enzymes. Pyruvate is then
turned into alcohol and carbon dioxide. This process also only produces 2 ATP for
every molecule of glucose.
The breakdown of Glucose to pyruvate (in both aerobic and fermentation pathways)
releases enough energy to generate 2 ATP and occurs in the cytoplasm. The
aerobic stages of respiration (pyruvate to carbon dioxide, water and ATP) occur in
the mitochondria of cells.
AEROBIC
RESPIRATION
Cell Type
FERMENTATION
All living cells
Animal cells
Yeast & Plant
cells
Oxygen needed
Yes
No
No
Substrate (source
of chemical
energy)
Glucose
glucose
Glucose
carbon dioxide and
water
lactate
carbon dioxide and
ethanol
Number of ATP
produced (per
glucose)
Many (38)
2
2
Process controlled
by
Enzymes
enzymes
Enzymes
first stage in
cytoplasm and is
completed in
mitochondria
Cytoplasm only
Cytoplasm only
Final products
Location in cell
Respirometer
A respirometer is a device used to measure the rate of respiration for a particular
respiring organism. In the example below the organism is peas, but there could be
an animal in place of the peas. There is an air lock in the experiment, in this case at
the bottom of the tube (X or Y). This means that as the peas respire and use up
the oxygen the liquid in the tube will move towards the peas, the speed of this
movement can be used to measure the rate of respiration. In this example the tube
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with the boiled and sterilised peas is a control as these peas are dead and not
respiring, so the liquid in tube X wouldn’t move at all, showing that the movement of
the liquid in tube Y was caused by the respiring peas using up Oxygen. This
experiment could be repeated at different temperatures to find the effect of
changing the temperature on the rate of respiration.
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