Notes

4.4 Bioenergetics

In this section we will explore how plants harness the Sun’s energy in photosynthesis in order to
make food. This process liberates oxygen which has built up over millions of years in the Earth’s
atmosphere. Both animals and plants use this oxygen to oxidise food in a process called aerobic
respiration which transfers the energy that the organism needs to perform its functions.

Oppositely, anaerobic respiration does not require oxygen to transfer energy. During vigorous
exercise the human body is unable to supply the cells with sufficient oxygen and it switches to
anaerobic respiration. This process will supply energy but also causes the build-up of lactic acid in
muscles which causes fatigue.

4.4.1 Photosynthesis
4.4.1.1 Photosynthetic reaction

Photosynthesis is represented by the equation:

carbon dioxide + water -->light--> glucose + oxygen

Students should recognise the chemical symbols: CO2, H2O, O2
and C6H12O6.

Students should be able to describe photosynthesis as an
endothermic reaction in which energy is transferred from the
environment to the chloroplasts by light.

There are links with this
content to Plant tissues
(page 33), the leaf.

4.4.1.2 Rate of photosynthesis

Students should be able to explain the effects of temperature, light
intensity, carbon dioxide concentration, and the amount of
chlorophyll on the rate of photosynthesis.

Students should be able to:
• measure and calculate rates of photosynthesis
• extract and interpret graphs of photosynthesis rate involving
one limiting factor
• plot and draw appropriate graphs selecting appropriate scale
for axes
• translate information between graphical and numeric form.

Solve simple algebraic
equations.

(HT only) These factors interact and any one of them may be the
factor that limits photosynthesis.

(HT only) Students should be able to explain graphs of
photosynthesis rate involving two or three factors and decide which
is the limiting factor.

(HT only) Students should understand and use inverse proportion –
the inverse square law and light intensity in the context of
photosynthesis.

(HT only) Limiting factors are important in the economics of
enhancing the conditions in greenhouses to gain the maximum rate
of photosynthesis while still maintaining profit.

(HT only)
Use data to relate limiting
factors to the cost
effectiveness of adding
heat, light or carbon dioxide
to greenhouses.

4.4.1.3 Uses of glucose from photosynthesis
Content Key opportunities for
skills development

The glucose produced in photosynthesis may be:
• used for respiration
• converted into insoluble starch for storage
• used to produce fat or oil for storage
• used to produce cellulose, which strengthens the cell wall
• used to produce amino acids for protein synthesis.

To produce proteins, plants also use nitrate ions that are absorbed
from the soil.

4.4.2 Respiration
4.4.2.1 Aerobic and anaerobic respiration

Students should be able to describe cellular respiration as an
exothermic reaction which is continuously occurring in living cells.
The energy transferred supplies all the energy needed for living
processes.

Respiration in cells can take place aerobically (using oxygen) or
anaerobically (without oxygen), to transfer energy.

Students should be able to compare the processes of aerobic and
anaerobic respiration with regard to the need for oxygen, the
differing products and the relative amounts of energy transferred.

Organisms need energy for:
• chemical reactions to build larger molecules
• movement
• keeping warm.

Aerobic respiration is represented by the equation:
glucose + oxygen carbon dioxide + water

Students should recognise the chemical symbols: C6H12O6, O2,
CO2 and H2O.

Anaerobic respiration in muscles is represented by the equation:
glucose lactic acid

As the oxidation of glucose is incomplete in anaerobic respiration
much less energy is transferred than in aerobic respiration.
Anaerobic respiration in plant and yeast cells is represented by the
equation:

glucose ethanol + carbon dioxide

Anaerobic respiration in yeast cells is called fermentation and has
economic importance in the manufacture of bread and alcoholic
drinks.

4.4.2.2 Response to exercise
Content Key opportunities for
skills development

During exercise the human body reacts to the increased demand
for energy.

The heart rate, breathing rate and breath volume increase during
exercise to supply the muscles with more oxygenated blood.
If insufficient oxygen is supplied anaerobic respiration takes place in
muscles.

The incomplete oxidation of glucose causes a build up of
lactic acid and creates an oxygen debt. During long periods of
vigorous activity muscles become fatigued and stop contracting
efficiently.

Investigations into the effect
of exercise on the body.

(HT only) Blood flowing through the muscles transports the lactic
acid to the liver where it is converted back into glucose.

Oxygen
debt is the amount of extra oxygen the body needs after exercise to
react with the accumulated lactic acid and remove it from the cells.

4.4.2.3 Metabolism

Students should be able to explain the importance of sugars, amino
acids, fatty acids and glycerol in the synthesis and breakdown of
carbohydrates, proteins and lipids.

Metabolism is the sum of all the reactions in a cell or the body.
The energy transferred by respiration in cells is used by the
organism for the continual enzyme controlled processes of
metabolism that synthesise new molecules.

Metabolism includes:
• conversion of glucose to starch, glycogen and cellulose

• the formation of lipid molecules from a molecule of glycerol
and three molecules of fatty acids

• the use of glucose and nitrate ions to form amino acids which
in turn are used to synthesise proteins

• respiration

• breakdown of excess proteins to form urea for excretion.

All of these aspects are covered in more detail in the relevant
specification section but are linked together here.

4.5 Homeostasis and response

Cells in the body can only survive within narrow physical and chemical limits. They require a
constant temperature and pH as well as a constant supply of dissolved food and water.

In order todo this the body requires control systems that constantly monitor and adjust the composition of the
blood and tissues. These control systems include receptors which sense changes and effectors
that bring about changes.

Hormonal coordination is particularly important in
reproduction since it controls the menstrual cycle. An understanding of the role of hormones in
reproduction has allowed scientists to develop not only contraceptive drugs but also drugs which
can increase fertility.

4.5.1 Homeostasis

Students should be able to explain that homeostasis is the
regulation of the internal conditions of a cell or organism to maintain
optimum conditions for function in response to internal and external
changes.

Homeostasis maintains optimal conditions for enzyme action and all
cell functions.

In the human body, these include control of:
• blood glucose concentration
• body temperature
• water levels.

These automatic control systems may involve nervous responses or
chemical responses.

All control systems include:
• cells called receptors, which detect stimuli (changes in the
environment)

• coordination centres (such as the brain, spinal cord and
pancreas) that receive and process information from
receptors

• effectors, muscles or glands, which bring about responses
which restore optimum levels.

4.5.2 The human nervous system

Students should be able to explain how the structure of the nervous
system is adapted to its functions.

The nervous system enables humans to react to their surroundings
and to coordinate their behaviour.

Information from receptors passes along cells (neurones) as
electrical impulses to the central nervous system (CNS). The CNS
is the brain and spinal cord.

The CNS coordinates the response of
effectors which may be muscles contracting or glands secreting
hormones.

stimulus receptor coordinator effector response
Students should be able to explain how the various structures in a
reflex arc – including the sensory neurone, synapse relay neurone
and motor neurone – relate to their function. Students should
understand why reflex actions are important
.
Reflex actions are automatic and rapid; they do not involve the
conscious part of the brain.

Students should be able to extract and interpret data from graphs,
charts and tables, about the functioning of the nervous system.

Students should be able to translate information about reaction
times between numerical and graphical forms.

4.5.3 Hormonal coordination in humans

4.5.3.1 Human endocrine system

Students should be able to describe the principles of hormonal
coordination and control by the human endocrine system.

The endocrine system is composed of glands which secrete
chemicals called hormones directly into the bloodstream.

The blood
carries the hormone to a target organ where it produces an effect.
Compared to the nervous system the effects are slower but act for
longer.

The pituitary gland in the brain is a ‘master gland’ which secretes
several hormones into the blood in response to body conditions.
These hormones in turn act on other glands to stimulate other
hormones to be released to bring about effects.

Students should be able to identify the position of the following on a
diagram of the human body:
• pituitary gland
• pancreas
• thyroid
• adrenal gland
• ovary
• testes.

4.5.3.2 Control of blood glucose concentration

Blood glucose concentration is monitored and controlled by the
pancreas.

If the blood glucose concentration is too high, the pancreas
produces the hormone insulin that causes glucose to move from the
blood into the cells. In liver and muscle cells excess glucose is
converted to glycogen for storage.

Students should be able to explain how insulin controls blood
glucose (sugar) levels in the body.

Type 1 diabetes is a disorder in which the pancreas fails to produce
sufficient insulin. It is characterised by uncontrolled high blood
glucose levels and is normally treated with insulin injections.

In Type 2 diabetes the body cells no longer respond to insulin
produced by the pancreas. A carbohydrate controlled diet and an
exercise regime are common treatments. Obesity is a risk factor for
Type 2 diabetes.

Students should be able to compare Type 1 and Type 2 diabetes
and explain how they can be treated.

Evaluate information around
the relationship between
obesity and diabetes, and
make recommendations
taking into account social
and ethical issues.

Students should be able to extract information and interpret data
from graphs that show the effect of insulin in blood glucose levels in
both people with diabetes and people without diabetes.


(HT only) If the blood glucose concentration is too low, the pancreas
produces the hormone glucagon that causes glycogen to be
converted into glucose and released into the blood.

(HT only) Students should be able to explain how glucagon
interacts with insulin in a negative feedback cycle to control blood
glucose (sugar) levels in the body.

4.5.3.3 Hormones in human reproduction

Students should be able to describe the roles of hormones in
human reproduction, including the menstrual cycle.

During puberty reproductive hormones cause secondary sex
characteristics to develop.

Oestrogen is the main female reproductive hormone produced in
the ovary. At puberty eggs begin to mature and one is released
approximately every 28 days. This is called ovulation.

Testosterone is the main male reproductive hormone produced by
the testes and it stimulates sperm production.

Several hormones are involved in the menstrual cycle of a woman.
• Follicle stimulating hormone (FSH) causes maturation of an
egg in the ovary.

• Luteinising hormone (LH) stimulates the release of the egg.
• Oestrogen and progesterone are involved in maintaining the
uterus lining.

(HT only) Students should be able to explain the interactions of
FSH, oestrogen, LH and progesterone, in the control of the
menstrual cycle.

(HT only) Students should be able to extract and interpret data from
graphs showing hormone levels during the menstrual cycle.

4.5.3.4 Contraception

Students should be able to evaluate the different hormonal and
non-hormonal methods of contraception.

Fertility can be controlled by a variety of hormonal and nonhormonal methods of contraception.

These include:
• oral contraceptives that contain hormones to inhibit FSH
production so that no eggs mature

• injection, implant or skin patch of slow release progesterone
to inhibit the maturation and release of eggs for a number of
months or years

• barrier methods such as condoms and diaphragms which
prevent the sperm reaching an egg

• intrauterine devices which prevent the implantation of an
embryo or release a hormone

• spermicidal agents which kill or disable sperm

• abstaining from intercourse when an egg may be in the
oviduct

• surgical methods of male and female sterilisation.

Show why issues around
contraception cannot be
answered by science alone.

Explain everyday and
technological applications of
science; evaluate
associated personal, social,
economic and
environmental implications;
and make decisions based
on the evaluation of
evidence and arguments.

The use of hormones to treat infertility (HT only)

Students should be able to explain the use of hormones in modern
reproductive technologies to treat infertility.

This includes giving FSH and LH in a 'fertility drug' to a woman. She
may then become pregnant in the normal way.

In Vitro Fertilisation (IVF) treatment.
• IVF involves giving a mother FSH and LH to stimulate the
maturation of several eggs.

• The eggs are collected from the mother and fertilised by
sperm from the father in the laboratory.

• The fertilised eggs develop into embryos.

• At the stage when they are tiny balls of cells, one or two
embryos are inserted into the mother's uterus (womb).

Developments of
microscopy techniques
have enabled IVF
treatments to develop.


Understand social and
ethical issues associated
with IVF treatments.


Although fertility treatment gives a woman the chance to have a
baby of her own:

• it is very emotionally and physically stressful
• the success rates are not high
• it can lead to multiple births which are a risk to both the
babies and the mother.


Evaluate from the
perspective of patients and
doctors the methods of
treating infertility.

4.5.3.6 Feedback systems (HT only)
Content Key opportunities for
skills development

Students should be able to explain the roles of thyroxine and
adrenaline in the body.

Adrenaline is produced by the adrenal glands in times of fear or
stress. It increases the heart rate and boosts the delivery of oxygen
and glucose to the brain and muscles, preparing the body for ‘flight
or fight’.

Thyroxine from the thyroid gland stimulates the basal metabolic
rate. It plays an important role in growth and development.

Thyroxine levels are controlled by negative feedback.
Interpret and explain simple diagrams of negative
feedback control.

4.6 Inheritance, variation and evolution
In this section we will discover how the number of chromosomes are halved during meiosis and
then combined with new genes from the sexual partner to produce unique offspring. Gene
mutations occur continuously and on rare occasions can affect the functioning of the animal or
plant. These mutations may be damaging and lead to a number of genetic disorders or death.

Very
rarely a new mutation can be beneficial and consequently, lead to increased fitness in the
individual. Variation generated by mutations and sexual reproduction is the basis for natural
selection; this is how species evolve. An understanding of these processes has allowed scientists
to intervene through selective breeding to produce livestock with favoured characteristics.

Once
new varieties of plants or animals have been produced it is possible to clone individuals to produce
larger numbers of identical individuals all carrying the favourable characteristic. Scientists have
now discovered how to take genes from one species and introduce them in to the genome of
another by a process called genetic engineering.

In spite of the huge potential benefits that this
technology can offer, genetic modification still remains highly controversial.

4.6.1 Reproduction
4.6.1.1 Sexual and asexual reproduction


Students should understand that meiosis leads to non-identical cells
being formed while mitosis leads to identical cells being formed.
Sexual reproduction involves the joining (fusion) of male and female
gametes:

• sperm and egg cells in animals

• pollen and egg cells in flowering plants.

In sexual reproduction there is mixing of genetic information which
leads to variety in the offspring. The formation of gametes involves
meiosis.

Asexual reproduction involves only one parent and no fusion of
gametes. There is no mixing of genetic information. This leads to
genetically identical offspring (clones). Only mitosis is involved.

4.6.1.2 Meiosis

Students should be able to explain how meiosis halves the number
of chromosomes in gametes and fertilisation restores the full
number of chromosomes.

Cells in reproductive organs divide by meiosis to form gametes.
When a cell divides to form gametes:

• copies of the genetic information are made

• the cell divides twice to form four gametes, each with a single
set of chromosomes

• all gametes are genetically different from each other.

Gametes join at fertilisation to restore the normal number of
chromosomes. The new cell divides by mitosis. The number of cells
increases. As the embryo develops cells differentiate.

Knowledge of the stages of meiosis is not required.

Modelling behaviour of
chromosomes during
meiosis.

DNA and the genome


Students should be able to describe the structure of DNA and
define genome.

The genetic material in the nucleus of a cell is composed of a
chemical called DNA. DNA is a polymer made up of two strands
forming a double helix. The DNA is contained in structures called
chromosomes.

A gene is a small section of DNA on a chromosome. Each gene
codes for a particular sequence of amino acids, to make a specific
protein.

The genome of an organism is the entire genetic material of that
organism.

The whole human genome has now been studied and
this will have great importance for medicine in the future.
Students should be able to discuss the importance of
understanding the human genome.

This is limited to the:
• search for genes linked to different types of disease
• understanding and treatment of inherited disorders
• use in tracing human migration patterns from the past.

4.6.1.4 Genetic inheritance
Content Key opportunities for
skills development

Students should be able to explain the terms:
• gamete
• chromosome
• gene
• allele
• dominant
• recessive
• homozygous
• heterozygous
• genotype
• phenotype.

Some characteristics are controlled by a single gene, such as: fur
colour in mice; and red-green colour blindness in humans. Each
gene may have different forms called alleles.

The alleles present, or genotype, operate at a molecular level to
develop characteristics that can be expressed as a phenotype.
A dominant allele is always expressed, even if only one copy is
present. A recessive allele is only expressed if two copies are
present (therefore no dominant allele present).

If the two alleles present are the same the organism is homozygous
for that trait, but if the alleles are different they are heterozygous.
Most characteristics are a result of multiple genes interacting, rather
than a single gene.

Students should be able to understand the concept of probability in
predicting the results of a single gene cross, but recall that most
phenotype features are the result of multiple genes rather than
single gene inheritance.


Students should be able to use direct proportion and simple ratios
to express the outcome of a genetic cross.

Students should be able to complete a Punnett square diagram and
extract and interpret information from genetic crosses and family
trees.


(HT only) Students should be able to construct a genetic cross by
Punnett square diagram and use it to make predictions using the
theory of probability.

4.6.1.6 Sex determination

Ordinary human body cells contain 23 pairs of chromosomes.
22 pairs control characteristics only, but one of the pairs carries the
genes that determine sex.

• In females the sex chromosomes are the same (XX).
• In males the chromosomes are different (XY).

Students should to be able to carry out a genetic cross to show sex
inheritance.

Students should understand and use direct proportion and simple
ratios in genetic crosses.


4.6.2 Variation and evolution
4.6.2.1 Variation


Students should be able to describe simply how the genome and its
interaction with the environment influence the development of the
phenotype of an organism.

Differences in the characteristics of individuals in a population is
called variation and may be due to differences in:
• the genes they have inherited (genetic causes)
• the conditions in which they have developed (environmental
causes)
• a combination of genes and the environment.

Students should be able to:

• state that there is usually extensive genetic variation within a
population of a species

• recall that all variants arise from mutations and that: most
have no effect on the phenotype; some influence phenotype;
very few determine phenotype.

Mutations occur continuously. Very rarely a mutation will lead to a
new phenotype. If the new phenotype is suited to an environmental
change it can lead to a relatively rapid change in the species.

4.6.2.2 Evolution

Students should be able to describe evolution as a change in the
inherited characteristics of a population over time through a process
of natural selection which may result in the formation of a new
species.

The theory of evolution by natural selection states that all species of
living things have evolved from simple life forms that first developed
more than three billion years ago.

Students should be able to explain how evolution occurs through
natural selection of variants that give rise to phenotypes best suited
to their environment.

If two populations of one species become so different in phenotype
that they can no longer interbreed to produce fertile offspring they
have formed two new species.

4.6.2.3 Selective breeding

Students should be able to explain the impact of selective breeding
of food plants and domesticated animals.

Selective breeding (artificial selection) is the process by which
humans breed plants and animals for particular genetic
characteristics.

Humans have been doing this for thousands of
years since they first bred food crops from wild plants and
domesticated animals.

Selective breeding involves choosing parents with the desired
characteristic from a mixed population.

They are bred together.
From the offspring those with the desired characteristic are bred
together. This continues over many generations until all the
offspring show the desired characteristic.

The characteristic can be chosen for usefulness or appearance:

• Disease resistance in food crops.
• Animals which produce more meat or milk.
• Domestic dogs with a gentle nature.
• Large or unusual flowers.
Selective breeding can lead to ‘inbreeding’ where some breeds are
particularly prone to disease or inherited defects.


4 Genetic engineering

Students should be able to describe genetic engineering as a
process which involves modifying the genome of an organism by
introducing a gene from another organism to give a desired
characteristic.

Plant crops have been genetically engineered to be resistant to
diseases or to produce bigger better fruits.
Bacterial cells have been genetically engineered to produce useful
substances such as human insulin to treat diabetes.

Students should be able to explain the potential benefits and risks
of genetic engineering in agriculture and in medicine and that some
people have objections.

In genetic engineering, genes from the chromosomes of humans
and other organisms can be ‘cut out’ and transferred to cells of
other organisms.

Crops that have had their genes modified in this way are called
genetically modified (GM) crops. GM crops include ones that are
resistant to insect attack or to herbicides. GM crops generally show
increased yields.

Concerns about GM crops include the effect on populations of wild
flowers and insects. Some people feel the effects of eating GM
crops on human health have not been fully explored.

Modern medical research is exploring the possibility of genetic
modification to overcome some inherited disorders.

(HT only) Students should be able to describe the main steps in the
process of genetic engineering.

(HT only) In genetic engineering:
• enzymes are used to isolate the required gene; this gene is
inserted into a vector, usually a bacterial plasmid or a virus

• the vector is used to insert the gene into the required cells

• genes are transferred to the cells of animals, plants or
microorganisms at an early stage in their development so that
they develop with desired characteristics.

(HT only)
Interpret information about
genetic engineering
techniques and to make
informed judgements about
issues concerning cloning
and genetic engineering,
including GM crops.

4.6.3 The development of understanding of genetics and evolution
4.6.3.1 Evidence for evolution

Students should be able to describe the evidence for evolution
including fossils and antibiotic resistance in bacteria.

The theory of evolution by natural selection is now widely accepted.
Evidence for Darwin’s theory is now available as it has been shown
that characteristics are passed on to offspring in genes.

There is
further evidence in the fossil record and the knowledge of how
resistance to antibiotics evolves in bacteria.

2 Fossils

Fossils are the ‘remains’ of organisms from millions of years ago,
which are found in rocks.

Fossils may be formed:
• from parts of organisms that have not decayed because one
or more of the conditions needed for decay are absent

• when parts of the organism are replaced by minerals as they
decay

• as preserved traces of organisms, such as footprints, burrows
and rootlet traces.


Extract and interpret
information from charts,
graphs and tables.

Many early forms of life were soft-bodied, which means that they
have left few traces behind. What traces there were have been
mainly destroyed by geological activity.

This is why scientists
cannot be certain about how life began on Earth.

Appreciate why the fossil
record is incomplete.

We can learn from fossils how much or how little different
organisms have changed as life developed on Earth.

Understand how scientific
methods and theories
develop over time.

Students should be able to extract and interpret information from
charts, graphs and tables such as evolutionary trees.

4.6.3.3 Extinction


Extinctions occur when there are no remaining individuals of a
species still alive.
Students should be able to describe factors which may contribute to
the extinction of a species.

4.6.3.4 Resistant bacteria

Bacteria can evolve rapidly because they reproduce at a fast rate.
Mutations of bacterial pathogens produce new strains. Some strains
might be resistant to antibiotics, and so are not killed.

They survive
and reproduce, so the population of the resistant strain rises. The
resistant strain will then spread because people are not immune to
it and there is no effective treatment.

MRSA is resistant to antibiotics.

To reduce the rate of development of antibiotic resistant strains:

• doctors should not prescribe antibiotics inappropriately, such
as treating non-serious or viral infections

• patients should complete their course of antibiotics so all
bacteria are killed and none survive to mutate and form
resistant strains

• the agricultural use of antibiotics should be restricted.
The development of new antibiotics is costly and slow. It is unlikely
to keep up with the emergence of new resistant strains.

There are links with this
content to Antibiotics and
painkillers (page 37).

4.6.4 Classification of living organisms


Traditionally living things have been classified into groups
depending on their structure and characteristics in a system
developed by Carl Linnaeus.

Linnaeus classified living things into kingdom, phylum, class, order,
family, genus and species. Organisms are named by the binomial
system of genus and species.

Students should be able to use information given to show
understanding of the Linnaean system.

Students should be able to describe the impact of developments in
biology on classification systems.

As evidence of internal structures became more developed due to
improvements in microscopes, and the understanding of
biochemical processes progressed, new models of classification
were proposed.

Due to evidence available from chemical analysis there is now a
‘three-domain system’ developed by Carl Woese. In this system
organisms are divided into:

• Archaea (primitive bacteria usually living in extreme
environments)

• Bacteria (true bacteria)

• Eukaryota (which includes protists, fungi, plants and animals).

Understand how scientific
methods and theories
develop over time.

Evolutionary trees are a method used by scientists to show how
they believe organisms are related. They use current classification
data for living organisms and fossil data for extinct organisms.

Interpret evolutionary trees.