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Cell Biology – Facts to Learn for National 5 Biology
A. Cell Structure
1. Tiny structures within cells make up the ultrastructure of the cell and are referred to as
organelles. Plant cells are surrounded by a tough cell wall made of cellulose which
supports the cell. The nucleus controls cell activities while the vacuole stores water.
Chloroplasts are where photosynthesis occurs while mitochondria are where aerobic
respiration occurs. Ribosomes are tiny particles in the cytoplasm where protein synthesis
happens. Animal cells also contain a nucleus, mitochondria and ribosomes.
2. Fungal cells have most of the structures found in plant cells. However, they do not contain
chloroplasts so fungi cannot photosynthesise. In addition, their cell wall is not made of
cellulose. Bacterial cells do not have distinct organelles and their cell wall is different is
structure to that of plant and fungal cells. They contain small rings of genetic material
(DNA) called plasmids.
B. Transport Across Cell Membranes
3. The fluid mosaic model is used to describe the structure of cell membranes. A membrane
contains protein and lipid molecules. There is a double layer of lipid molecules which are
very flexible (fluid) and repel water. Protein molecules randomly occur (mosaic
arrangement) in the membrane. Many of these proteins are simply embedded into the surface
of the membrane while others stretch right across it. Some of these proteins contain
pores in them through which molecules can diffuse into and out of the cell.
4. The cell membrane controls the entry and exit of substances. Small molecules such as
glucose and oxygen enter cells through the pores in the membrane, while carbon dioxide
leaves the cell through these pores. These substances all diffuse from an area of high
concentration to an area of low concentration. Diffusion is a form of passive transport as
molecules naturally move down the concentration gradient, without the use of energy.
Large molecules such as starch are too big to pass through the pores in the cell membrane.
As the membrane only allows some molecules to diffuse through it is said to be selectively
permeable.
5. Osmosis is the name given to the movement of water across the cell membrane. Water
always diffuses down the concentration gradient. This means that animal cells can burst
when placed in pure water and shrink when placed in a strong solution.
The vacuole of plant cells fills up with water when the cells are placed in pure water.
However, the cell cannot burst because of the rigid cell wall. A plant cell full of water is
said to be turgid. When plant cells are placed in a strong solution, water leaves the vacuole
which shrinks. This draws the membrane away from the cell wall and the cell is said to be
plasmolysed.
6. Active transport is the movement of molecules across the cell membrane from low to high
concentrations. This movement against the concentration gradient is brought about by
membrane proteins which carry the molecules across the membrane. Active transport
requires energy to move molecules in the opposite direction to passive transport or
diffusion.
C. Producing New Cells
7. Most cells, apart from sex cells, are diploid cells which have two matching sets of
chromosomes. Just before a cell divides these chromosomes are replicated during a
process called mitosis. This maintains the diploid chromosome compliment in all new
cells, so all these cells have a complete set of genes.
8. Mitosis describes the division of the nucleus. Firstly, chromosomes become visible inside
the nucleus. Each chromosome has copied itself exactly, forming two chromatids held
together by a centromere. Next the nuclear membrane disappears and the chromosomes
line up along the equator of the cell. Spindle fibres then form and these pull the
chromatids apart, towards each pole of the cell. Finally, nuclear membranes reform
around each set of chromosomes and the cytoplasm divides, creating two cells.
9. Cells can be produced in a laboratory using cell culture techniques. An appropriate growth
medium containing nutrients is required. This can be a solid agar gel or liquid broth.
Aseptic techniques, which create a sterile environment, must be used to prevent
contamination of cultures. So surfaces must be cleaned with disinfectant and all equipment
heated in an autoclave. The culture should be provided with optimum growing conditions
including a warm temperature, the ideal pH and plenty of oxygen.
D. DNA And The Production Of Proteins
10. Chromosomes are made of DNA. A molecule of DNA consists of two strands twisted into
a double helix shape. Each strand contains a chain of base molecules. The two strands are
held together by weak chemical bonds between the bases. There are four types of bases in
DNA and they join together forming complementary base pairs, between the strands. The
base ‘A’ always pairs with the base ‘T’ while the base ‘G’ always pairs with the base ‘C’.
11. DNA carries the genetic information for making proteins. The sequence of bases, on one
strand of the DNA, forms genes. The base sequence in a gene acts as a code which
determines the amino acid sequence in the protein made.
12. Messenger RNA (mRNA) is a molecule which carries a copy of the code from the DNA
inside the nucleus to a ribosome in the cytoplasm. It is on the ribosome that the protein is
synthesised, being assembled from amino acids.
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E. Proteins and Enzymes
13. Proteins fold up into different shapes depending on their sequence of amino acids. The
shape that they form determines their functions. Some structural proteins are found in
membranes where they can create pores. Other proteins form hormones which are released
into the blood and have an effect on a part of the body. Antibodies are protein molecules
which are made by white blood cells to defend the body against disease. Many proteins are
enzymes which speed up chemical reactions in the body.
14. All living cells make enzymes which function as biological catalysts. These are chemicals
which speed up cellular reactions but they remain unchanged by the reaction. The
folded shape of an enzyme creates an area known as the active site on the enzyme. This
allows it to react with a substance that is complementary to it and fits into it. This
substance is called the substrate and each enzyme reacts with a specific substrate. This
means that different enzymes catalyse different cellular reactions.
15. Each enzyme works best in its optimum conditions. Enzymes and other proteins can be
affected by temperature and pH. For example, the enzyme amylase found in saliva works
best at a pH of 7 while the enzyme pepsin released in the stomach works best at a pH of
2.5. Both these enzymes work best at an optimum temperature of 37oC, body temperature.
Enzymes are denatured by temperatures greater than 50oC. The high temperatures
permanently change the shape of the enzyme meaning that it no longer functions and the
reaction slows down and stops.
F. Genetic Engineering
16. Genetic information can be transferred from one cell to another naturally by bacteria or
viruses. Bacteria contain small circular plasmids made up of genes. Scientists can
transfer genes from one organism (e.g. a human being) to another organism (e.g. a
bacterium). This process is called genetic engineering.
17. Firstly, during genetic engineering scientists have to identify the section of DNA that
contains the required gene. For example, the gene that causes insulin production was
located at the top of a human chromosome. An enzyme is used to remove the DNA
section containing the gene. A plasmid is removed from a bacterial cell and cut open
using the same enzyme. A different enzyme is then used to insert the gene into the
bacterial plasmid. The plasmid is then inserted into a bacterium which multiplies many
times forming millions of genetically modified bacteria. These bacteria now produce the
desired product e.g. human insulin.
G. Photosynthesis
18. Photosynthesis is a process which occurs in green plants where light energy is converted
into chemical energy (sugar). Photosynthesis is a two-stage process. The first stage
involves two light reactions. Firstly, light energy from the sun is trapped by chlorophyll
and converted into chemical energy in the form of ATP. Secondly, the light energy is
used to split water into hydrogen and oxygen. The hydrogen produced attaches to
hydrogen acceptor molecules while excess oxygen diffuses out of the cell.
19. The second stage of photosynthesis is called carbon fixation. This is a series of enzymecontrolled reactions which use the hydrogen and ATP produced by the light reactions.
The hydrogen reacts with carbon dioxide to produce sugar. The chemical energy in
sugar (glucose) can be used for respiration, to generate energy. It can also be converted to
other substances such as starch for storage or cellulose for constructing cell walls.
Carbohydrates such as sugar can be used by organisms to produce fats and proteins.
20. A limiting factor is a factor which slows down a process because it is in short supply.
Light intensity, temperature and carbon dioxide concentration can all limit the rate of
photosynthesis and so slow down plant growth. For example, in the evenings in Spring the
temperature may be warm enough and there will be enough carbon dioxide in the air for
plants to photosynthesise. However, light intensities will be low and these will limit the
rate of photosynthesis at this time of the year.
H. Respiration
21. The chemical energy stored in glucose is released by all cells through a series of enzymecontrolled reactions called respiration. This released energy is used, by cells, to generate
ATP from ADP and inorganic phosphate (Pi). When a cell requires energy, the
chemical energy stored in ATP can be immediately released by breaking it down to
ADP and inorganic phosphate. This energy can be used for cellular activities including
muscle cell contraction, cell division, protein synthesis and transmission of nerve
impulses. ATP is then regenerated, from ADP and Pi, during respiration.
22. During respiration glucose is firstly broken down to pyruvate, in the cell cytoplasm. If
there is no oxygen available, the pyruvate is converted to lactic acid in animal cells. In
plant and yeast cells the pyruvate is converted into alcohol/ethanol and carbon dioxide.
The breakdown of each glucose molecule during these fermentation pathways yields two
molecules of ATP. Fermentation pathways occur when no oxygen is present inside the
cell.
23. If oxygen is present, glucose is broken down to pyruvate which then enters the
mitochondrion. Here the pyruvate is broken down completely into carbon dioxide and
water. This complete breakdown of a glucose molecule, in the presence of oxygen,
produces 38 molecules of ATP.
24. Respiration begins in the cytoplasm. The process of fermentation is completed in the
cytoplasm. Summary equations for fermentation are shown below.
Glucose
lactic acid (in animal cells)
Glucose
ethanol and carbon dioxide (in plant cells)
Aerobic respiration occurs when oxygen is present. Aerobic respiration starts in the
cytoplasm and is completed in the mitochondria. A summary equation for aerobic
respiration is shown below.
Glucose + oxygen
carbon dioxide and water
Cells such as muscle, companion, sperm and neurones have a high number of
mitochondria as they require a lot of energy.