B2
Topic 1 revision
The building blocks of life
Class revision with exam
questions
Animal cells
What is aerobic respiration?
Aerobic respiration is the process of releasing
energy through the oxidation of glucose molecules.
glucose
+
oxygen
carbon
dioxide
C6H12O6
+
6O2
6CO2
+
water ( + energy)
+
6H20
( + ATP)
This reaction releases energy in the form of ATP – a
compound that can readily be used in cellular processes.
Plant cells
What is photosynthesis?
Photosynthesis is a chemical reaction where light
energy is used to convert carbon dioxide and water into
glucose and oxygen.
light energy
carbon
dioxide
+
water
6CO2
+
6H20
glucose
+
oxygen
C6H12O6
+
6O2
chlorophyll
The reaction uses light energy from the Sun and takes
place in chloroplasts of plant cells.
What organelles are present in a
bacterial cell?
Cytoplasm
Slime layer
flagella
Plasmid
DNA
Cell wall
Cell
membrane
Chromosomal
DNA
What organelles are present in a
bacterial cell?
Chromosomal
DNA
Plasmid
DNA
Plasmid DNA
are just loops
of DNA.
Light Microscopes
The magnification can be
worked out by using a simple
formula:
Total magnification = magnifying power x magnifying power
of eyepiece lens
of objective lens
Total magnification =
x10
x
x50
How did Watson
and Crick develop
their model of
DNA?
Started
with ideas
and a
hypothesis
Conducted
various
experiments
Written and
published a
paper on their
model.
Gathered
evidence from
other sources
Extracting DNA
Action
Why?
Finely chopped kiwi fruit
tissue is added to
saline/detergent solution
Membranes of the cell brake
up, releasing chromosomes.
The mixture is kept warm at
50˚C for 15 mins, then
filtered.
Protein-digesting enzymes in
the cells digest the protein
part of the chromosomes,
releasing DNA.
DNA precipitates from the
filtrate.
Cooled methanol is carefully
added to the filtrate in equal
volumes.
Look for a change at the
boundary.
DNA strands are visible in the
methanol layer.
What happens to DNA in cell division?
Before cell division takes place, the chromosomes duplicate.
How does this affect DNA?
Each chromosome is made
of a DNA molecule, so DNA
also needs to replicate.
DNA is a very
interesting molecule
because it is able to
copy itself. It is able to
do this because it is
double stranded.
How does DNA replicate?
The replication of DNA is a continuous
process but can be broken down into
several stages:
 The DNA helix unwinds.
 The two strands separate.
 New bases bond to each
strand, creating two new
molecules of DNA.
 Each molecule of DNA
winds up again, creating
two new helices.
Mitosis
Most cells contain
2 set of
chromosomes (1
from mum and 1
from dad)
Mitosis results in two
daughter cells.
The DNA
replicates
and make
chromatids
The cell splits
apart creating
two identical cells
Meiosis
Mitosis results in four
daughter cells.
Most cells contain 2 set of
chromosomes (1 from mum
and 1 from dad)
The DNA replicates and
make chromatids
The cell splits apart
The cell splits apart again
Mutation!
DNA is able to copy itself very accurately – for every 1 billion
bases replicated, only 1 will be wrong!
Sometimes, however, mistakes do happen. When this
happens, it is called a mutation.
Most mutations are harmful and many
have no effect, but sometimes a mutation
results in a new, beneficial characteristic
for the individual.
How important are mutations in
natural selection and evolution?
Replicating micro-organisms
Genetically-engineered
micro-organisms, such as
bacteria and yeast, can easily
be replicated on a large scale.
Tanks called fermenters or
bioreactors are used. These
enable the micro-organisms to
be grown, or ‘cultured’, at
optimum pH, temperature and
nutrient levels.
The product can be
continuously removed
and purified.
GM crops
Crops can be given extra genes for new and useful
characteristics. They are genetically modified (GM).
What characteristics might be
useful in crops?
 pest resistance
 frost resistance
 disease resistance
 herbicide resistance
 drought resistance
 longer shelf life
Plants with extra vitamins
Rice can be genetically modified to make beta-carotene,
a substance that is converted into vitamin A in the body.
The colour of the rice is
an indication of how
much more betacarotene it contains.
The GM rice is called
‘Golden Rice’ and is
being developed to
help fight vitamin A
deficiency and
blindness in developing
countries.
Taking cuttings
A plant can easily and quickly be cloned by taking a cutting.
This is a fast way of cloning.
stem cut from
parent plant
cutting dipped in rooting
powder and planted
The problem with this method is that it cannot produce many clones at once.
Tissue culture
Embryo transplantation
It is more difficult to clone animals than plants. A technique
used to create clone animals is embryo transplanting. For
example, a cattle farmer would follow this process:
1. Sperm is taken from the best bull and used to fertilize an
egg from the best cow.
2. The fertilized egg divides to form an embryo containing
several cells.
3. The embryo is separated into individual cells, which go on
to form new embryos. Each embryo contains the same
genes.
4. The embryos are implanted into surrogate cows.
5. The cows give birth to calves, which are all clones of each
other.
Human therapeutic cloning
The DNA of an embryo cell can be replaced with the DNA
from a patient’s cell.
patient’s
DNA inserted
embryo cell
nucleus removed
5 day-old
embryo
stem cells removed
– the embryo dies
The embryo produces stem cells containing the patient’s
genes. The cells will not be rejected, so immune-suppressing
drugs are not needed. This process is therapeutic cloning.
Higher only
What are stem cells?
Stem cells are unspecialized cells that can develop into any type of cell. Early
embryos are good sources of stem cells.
stem cells
red blood cells
liver cells
muscle cells
Using stem cells
Stem cells could be injected into damaged organs to rebuild the tissues. This
would reduce the need for organ transplants.
injected into patient
with diseased liver
stem cells
liver is repaired
The stem cells would need to have the same genes as the patient, otherwise
they would be rejected by the patient’s immune system. They would need to
be clones.
How could you make an embryo that only contained
your genes?
What happens at the active site?
In the same way that a key fits into a lock, so a substrate is thought to
fit into an enzyme’s active site. The enzyme is the lock, and the reactant
is the key.
+
enzyme
+
reactant
↔
↔
+
↔
enzyme-reactant
complex
↔
enzyme
+
products
Factors affecting enzymes
The rate of enzyme–catalyzed reactions depends on several factors. What
are some of these?
Factors that affect the rate of a reaction include:
 temperature
 substrate concentration
 pH
 surface area
 enzyme concentration
 pressure.
All enzymes work best at only one particular temperature and pH: this is
called the optimum.
Different enzymes have different optimum temperatures and pH
values.
EXAM TIP:
NEVER say that the
enzyme is destroyed or
has died or is killed. It is
DENATURED as it loses
its shape only, meaning
that it cannot match up
to the substrate
correctly.
denatured