AQA GCSE Biology

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AQA GCSE Biology

4.1 Cell biology

4.1.1 Cell structure
 4.1.1.1 Eukaryotes and prokaryotes
o Plant and animal cells (eukaryotic cells) have cell membrane, cytoplasm and genetic
material in a nucleus.
o Bacterial cells (prokaryotic cells) - smaller with cytoplasm and cell membrane
surrounded by cell wall. Genetic material not in nucleus - single DNA loop and
sometimes small DNA rings – ‘plasmids’.
 4.1.1.2 Animal and plant cells
o plant cells often have:
 chloroplasts
 permanent vacuole filled with cell sap.
o Plant and algal cells also have a cell wall made of cellulose to strengthen the cell.
 4.1.1.3 Cell specialisation
o Cells may be specialised to carry out a particular function:
 sperm cells, nerve cells and muscle cells in animals
 root hair cells, xylem and phloem cells in plants.
 4.1.1.4 Cell differentiation
o Explain the importance of cell differentiation.
o As an organism develops, cells differentiate to form different types of cells.
 Most types of animal cell differentiate at an early stage.
 Many types of plant cells retain the ability to differentiate throughout life.
o In mature animals, cell division is mainly restricted to repair and replacement. As a
cell differentiates it acquires sub-cellular structures to enable it to carry out a certain
function. It has become a specialised cell.
 4.1.1.5 Microscopy
o Understand how microscopy techniques have developed over time
o Explain how electron microscopy has increased understanding of sub-cellular
structures.
o Differences in magnification and resolution.
o An electron microscope has much higher magnification and resolving power than a
light microscope (can be used to study cells in much finer detail)..
o Calculations involving magnification, real size and image size using the formula:

magnication = size-of-image
size-of-real-object

4.1.2 Cell division
 4.1.2.1 Chromosomes
o Nucleus of cell contains chromosomes, normally found in pairs, made of DNA. Each
chromosome has a large number of genes.
 4.1.2.2 Mitosis and the cell cycle
o Cells divide in ‘cell cycle’.
o During the cell cycle genetic material is doubled and then divided into two identical
cells.
o Before division it grows and increases numbers of sub-cellular structures (ribosomes
and mitochondria).
o DNA replicates - two copies of each chromosome.
o Mitosis - one set of chromosomes pulled to each end of cell and nucleus divides.
o Finally cytoplasm and cell membranes divide - two identical cells.
o Mitosis - important in growth and development of multicellular organisms.
 4.1.2.3 Stem cells
o Stem cell = undifferentiated cell capable of creating more cells (same type), and
other cells from differentiation.
o Human embryo stem cells can be cloned differentiate into most types of human
cells.
o Adult bone marrow stem cells can form blood cells.
o Plant meristem tissue can differentiate into any type of plant cell, throughout life of
plant.
o Treatment with stem cells --- diabetes and paralysis.
o Therapeutic cloning-embryo produced with same genes as patient. Stem cells from
embryo not rejected by patient’s body - used for medical treatment.
o Stem cells - potential risks eg. transfer of viral infection, ethical or religious
objections.
o Stem cells from plant meristems used to clones plants quickly and economically.
o Rare species cloned to protect from extinction.
o Crop plants - disease resistance cloned - large numbers of identical plants.

4.1.3 Transport in cells
 4.1.3.1 Diffusion
o Substances move into/out of cells across cell membranes via diffusion.
o Diffusion = spreading out of particles (in solution or gas), resulting in net movement
from area of higher concentration to area of lower concentration.

o Eg. oxygen and carbon dioxide in gas exchange,
o Waste product urea from cells into blood plasma for excretion in kidney.
o Factors affecting rate of diffusion
 difference in concentrations (concentration gradient)
 temperature
 surface area of the membrane.
o Single-celled organism large surface area to volume ratio. Sufficient transport of
molecules in and out organism for it to survive.
o Multicellular organisms - surfaces and organ systems specialised for exchanging
materials, allowing sufficient molecules to be transported into and out of cells for
the organism’s needs.
o Effectiveness of exchange surface increased by:
 large surface area
 thin membrane (short diffusion path)
 (animals) efficient blood supply
 (animals) gaseous exchange being ventilated.
 4.1.3.2 Osmosis
o Water may move across cell membranes via osmosis. Osmosis - diffusion of water
from a dilute solution to a concentrated solution through a partially permeable
membrane.
 4.1.3.3 Active transport
o Moves substances from dilute solution to concentrated solution (against a
concentration gradient).
o Requires energy from respiration.
o Allows mineral ions into plant root hairs from very dilute solutions in soil (ions for
healthy growth), sugar molecules absorbed from lower concentrations in gut into
blood (higher sugar concentration)to be used for cell respiration.

4.2 Organisation

 4.2.1 Principles of organisation
o Cells - basic building blocks of all living organisms.
o Tissue - group of cells with similar structure and function.
o Organs - aggregations of tissues performing specific functions.
o Organs organised into organ systems, working together to form organisms.

4.2.2 Animal tissues, organs and organ systems
 4.2.2.1 The human digestive system
o Assumes knowledge of digestive system from Key Stage 3.
o The digestive system example of organ system - several organs work together to
digest and absorb food.
o Describe enzyme molecules and relate activity to temperature and pH changes.
o Enzymes catalyse specific reactions in living organisms due to shape of active site.
o Use the ‘lock and key theory’ as model of enzyme action.
o Recall the sites of production and the action of amylase, proteases and lipases.
o Digestive enzymes convert food into small soluble molecules that are absorbed into
bloodstream.
o Carbohydrases break down carbohydrates to simple sugars. Amylase breaks down
starch.
o Proteases break down proteins to amino acids.
o Lipases break down lipids (fats) to glycerol and fatty acids.
o Products of digestion used to build new carbohydrates, lipids and proteins and some
glucose in respiration.
o Bile made in liver, stored in gall bladder - alkaline and neutralises hydrochloric acid
from stomach and emulsifies fat increasing the surface area. The alkaline conditions
and large surface area increase rate of fat breakdown by lipase.
 4.2.2.2 The heart and blood vessels
o Know the structure and functioning of the human heart and lungs (adaption for
gaseous exchange).
o The heart (organ) pumps blood around body in double circulatory system. Right
ventricle pumps blood to lungs (gas exchange). The left ventricle pumps blood
around rest of body.
o Blood vessels limited to aorta, vena cava, pulmonary artery, pulmonary vein and
coronary arteries. Heart valves not required.
o The body contains three different types of blood vessel:
 arteries
 veins
 capillaries.
o Natural resting heart rate controlled by cells in right atrium (act as a pacemaker).
Artificial pacemakers - electrical devices to correct irregularities in heart rate.
o Lungs restricted to the trachea, bronchi, alveoli and the capillary network
surrounding alveoli.
 4.2.2.3 Blood
o Blood - tissue consisting of plasma, in which red blood cells, white blood cells and
platelets are suspended.
 4.2.2.4 Coronary heart disease: a non-communicable disease
o Evaluate the advantages and disadvantages of treating cardiovascular diseases by
drugs, mechanical devices or transplant.
o Coronary heart disease - fatty material builds up inside the coronary arteries,
narrowing them. Reduces flow of blood through coronary arteries, resulting in lack
of oxygen for heart muscle. Stents keep coronary arteries open.
o Statins widely used to reduce blood cholesterol levels which slows down the rate of
fatty material deposit.
o In some heart valves become faulty, preventing the valve opening fully, or it might
develop a leak. Understand the consequences of faulty valves- replaced using
biological or mechanical valves.
o In the case of heart failure a donor heart, or heart and lungs can be transplanted.
Artificial hearts occasionally used to keep patients alive whilst waiting for a heart
transplant, or allow heart to rest to help recovery.
 4.2.2.5 Health issues
o Describe relationship between health and disease and the interactions between
types of disease.
o Health is the state of physical and mental well-being.
o Diseases, both communicable and non-communicable, are major causes of ill health.
Other factors - diet, stress and life situations affect physical and mental health.
o Different types of disease may interact.
 Defects in immune system mean individual is more likely to suffer from
infectious diseases.
 Viruses living in cells can trigger cancers.
 Immune reactions initially caused by a pathogen can trigger allergies (skin
rashes and asthma).
 Severe physical ill health can lead to depression and other mental illness.
 4.2.2.6 The effect of lifestyle on some non-communicable diseases
o Discuss the human and financial cost of these non-communicable diseases to an
individual, a local community, a nation or globally
o Explain lifestyle factors including diet, alcohol and smoking on non-communicable
diseases at local, national and global levels.
o Risk factors increase rate of a disease.
 aspects of lifestyle
 substances in body or environment.
 Diet, smoking and exercise on cardiovascular disease.
 Obesity as risk factor for Type 2 diabetes.
 Alcohol on the liver and brain function.
 The effect of smoking on lung disease and lung cancer.
 The effects of smoking and alcohol on unborn babies.
 Carcinogens, including ionising radiation, as risk factors in cancer.
o Many diseases are caused by the interaction of a number of factors.
 4.2.2.7 Cancer
o Cancer - the result of changes in cells that lead to uncontrolled growth and division.
o Benign tumours - growths of abnormal cells contained in one area, usually within
membrane (do not invade other parts of body).
o Malignant tumour cells are cancers invading neighbouring tissues and spreading to
different parts of the body in blood forming secondary tumours.
o Scientists identified lifestyle risk factors and genetic risk factors for some cancers.

4.2.3 Plant tissues, organs and systems
 4.2.3.1 Plant tissues
o Explain how the structures of plant tissues relate to function.
o Plant tissues include:
 epidermal tissues
 palisade mesophyll
 spongy mesophyll
 xylem and phloem
 meristem tissue found at the growing tips of shoots and roots.
o The leaf is a plant organ. Knowledge limited to epidermis, palisade and spongy
mesophyll, xylem and phloem, and guard cells surrounding stomata.
 4.2.3.2 Plant organ system
o Explain how structure of root hair cells, xylem and phloem adapted to function.
o Explain the effect of changing temperature, humidity, air movement and light
intensity on the rate of transpiration.
o The roots, stem and leaves form plant organ system for transport of substances
around the plant.
o Describe the process of transpiration and translocation, including structure and
function of stomata.
o Root hair cells adapted for the efficient uptake of water by osmosis, and mineral
ions by active transport.
o Xylem tissue transports water and mineral ions from the roots to the stems and
leaves. Made of hollow tubes strengthened by lignin and adapted for the transport
of water in the transpiration stream.
o Stomata and guard cells control gas exchange and water loss.
o Phloem tissue transports dissolved sugars from leaves to rest of the plant for
immediate use or storage. The movement of food molecules through phloem tissue
is called translocation.
o Phloem is composed of tubes of elongated cells. Cell sap can move from one phloem
cell to the next through pores in the end walls.

4.4 Bioenergetics
4.4.1 Photosynthesis
 4.4.1.1 Photosynthetic reaction
o Photosynthesis is represented by the equation:
o carbon dioxide + water --- light → glucose + oxygen
o Chemical symbols: CO , H O, O and C H O .
2 2 2 6 12 6
o Describe photosynthesis as an endothermic reaction - energy is transferred from
environment to chloroplasts by light.
 4.4.1.2 Rate of photosynthesis
o Explain the effects of temperature, light intensity, carbon dioxide concentration, and
amount of chlorophyll on rate of photosynthesis.
o (HT only) Factors interact and any one may be the limiting factor.
o (HT only) Explain graphs of photosynthesis rate with two or three factors and decide
which is the limiting factor.
o (HT only) Understand and use inverse proportion– the inverse square law and light
intensity in the context of photosynthesis.
o (HT only) Limiting factors important economically to enhance conditions in greenhouses
allowing maximum rate of photosynthesis and profit.
 4.4.1.3 Uses of glucose from photosynthesis
o 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, (strengthens the cell wall)
 used to produce amino acids for protein synthesis.
o To produce proteins, plants also use nitrate ions that are absorbed from soil.

4.4.2 Respiration
 4.4.2.1 Aerobic and anaerobic respiration
o Cellular respiration - an exothermic reaction occurring in living cells. Energy transferred
supplies all the energy needed for living processes.
o Respiration in cells can take place aerobically (using oxygen) or anaerobically (without
oxygen), to transfer energy.
o Compare the processes of aerobic and anaerobic respiration - need for oxygen, products
and energy transferred.
o Organisms need energy for:
 chemical reactions to build larger molecules
 movement
 keeping warm.
o Aerobic respiration is represented by the equation:
o glucose + oxygen → carbon dioxide + water
o (Recognise the chemical symbols: CO , H O, O and C H O ..)
2 2 2 6 12 6
o Anaerobic respiration in muscles is represented by the equation:
o glucose → lactic acid
o Oxidation of glucose incomplete in anaerobic respiration so much less energy is
transferred than in aerobic respiration.
o Anaerobic respiration in plant and yeast cells is represented by the equation:
o glucose → ethanol + carbon dioxide
o Anaerobic respiration in yeast cells is called fermentation and has economic importance
- manufacture of bread and alcoholic drinks.
• 4.4.2.2 Response to exercise
o During exercise the body reacts to the increased demand for energy.
o The heart rate, breathing rate and breath volume increase during exercise to supply
muscles with more oxygenated blood.
o If insufficient oxygen is supplied anaerobic respiration occurs 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.
o (HT only) Blood flowing through muscles transports the lactic acid to liver converts it
back into glucose. Oxygen debt - 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
o Explain the importance of sugars, amino acids, fatty acids and glycerol in the synthesis
and breakdown of carbohydrates, proteins and lipids.
o Metabolism - sum of all the reactions in a cell or the body.
o The energy transferred by respiration in cells is used for (continual enzyme controlled
processes) metabolism that synthesise new molecules.
o Metabolism includes:
 conversion of glucose to starch, glycogen and cellulose
 the formation of lipid molecules from 1 molecule of glycerol and 3 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.

4.6 Inheritance, variation and evolution

4.6.1 Reproduction
 4.6.1.1 Sexual and asexual reproduction
o Understand that meiosis leads to non-identical cells being formed while mitosis leads to
identical cells being formed.
o Sexual reproduction involves the joining (fusion) of male and female gametes:
 sperm and egg cells in animals
 pollen and egg cells in flowering plants

o Sexual reproduction - mixing of genetic information leads to variety in offspring. The
formation of gametes involves meiosis.
o Asexual reproduction involves only one parent and no fusion of gametes. No mixing of
genetic information. Leads to genetically identical offspring (clones). Only mitosis is
involved.
 4.6.1.2 Meiosis
o Explain meiosis halves the number of chromosomes in gametes and fertilisation restores
the full number of chromosomes.
o Cells in reproductive organs divide by meiosis to form gametes.
o 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.
o 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.
 4.6.1.3 DNA and the genome
o Describe the structure of DNA and define genome.
o Genetic material in the nucleus of a cell = chemical called DNA.
o DNA - polymer of two strands forming a double helix. The DNA is contained in structures
called chromosomes.
o Gene - small section of DNA on a chromosome.
o Each gene codes for a particular sequence of amino acids, to make a specific protein.
o Genome - 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.
o Allows us to
 search for genes linked to different types of disease
 understand and treat inherited disorders
 trace human migration patterns from the past.
 4.6.1.4 Genetic inheritance
o Explain the terms:
 Gamete
 Chromosome
 Gene
 Allele
 Dominant
 Recessive
 Homozygous
 Heterozygous
 Genotype
 Phenotype.
o Some characteristics are controlled by a single gene, such as: fur colour in mice; and red-
green colour blindness in humans.
o Each gene may have different forms called alleles.
o The alleles present, or genotype, develop characteristics that are expressed as a
phenotype.
o 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).
o If the two alleles present are the same the organism is homozygous for that trait, but if
the alleles are different they are heterozygous.
o Most characteristics are a result of multiple genes interacting, rather than a single gene.
o Understand the probability of 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.
o (HT only) Construct a genetic cross by Punnett square diagram and use it to make
predictions using the theory of probability.
 4.6.1.5 Inherited disorders
o Some disorders are inherited. These disorders are caused by the inheritance of certain
alleles.
 Polydactyly (having extra fingers or toes) is caused by a dominant allele.
 Cystic fibrosis (a disorder of cell membranes) is caused by a recessive allele.
o Make informed judgements about the economic, social and ethical issues concerning
embryo screening, given appropriate information.
 Polydactyly (having extra fingers or toes) is caused by a dominant allele.
 Cystic fibrosis (a disorder of cell membranes) is caused by a recessive allele.
o Make informed judgements about the economic, social and ethical issues concerning
embryo screening, given appropriate information.
 4.6.1.6 Sex determination
o Ordinary human body cells contain 23 pairs of chromosomes.
o 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).
o Carry out a genetic cross to show sex inheritance.

4.6.2 Variation and evolution
 4.6.2.1 Variation
o Describe simply how the genome and its interaction with the environment influence the
development of the phenotype of an organism.
o Differences in the characteristics of individuals in a population is called variation and
may be due to differences in:
 Genes they have inherited (genetic causes)
 Conditions in which they have developed (environmental causes)
 A combination of genes and the environment.
o There is usually extensive genetic variation within a population of a species
o All variants arise from mutations and that: most have no effect on the phenotype; some
influence phenotype; very few determine phenotype.
o Mutations occur continuously.
o 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
o Evolution - 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.
o 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.
o 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
o Explain the impact of selective breeding of food plants and domesticated animals.
o 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.
o 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.
o 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.
o Selective breeding can lead to ‘inbreeding’ where some breeds are particularly prone to
disease or inherited defects.
 4.6.2.4 Genetic engineering
o Genetic engineering - process which involves modifying the genome of an organism by
introducing a gene from another organism to give a desired characteristic.
o Plant crops have been genetically engineered to be resistant to diseases or to produce
bigger better fruits.
o Bacterial cells have been genetically engineered to produce useful substances such as
human insulin to treat diabetes.
o In genetic engineering, genes from the chromosomes of humans and other organisms
can be ‘cut out’ and transferred to cells of other organisms.
o 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.
o 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.
o Modern medical research is exploring the possibility of genetic modification to
overcome some inherited disorders.
o (HT only) Describe the main steps in the process of genetic engineering.
o (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.

4.6.3 The development of understanding of genetics and evolution
 4.6.3.1 Evidence for evolution
o Evidence for evolution including fossils and antibiotic resistance in bacteria.
o The theory of evolution by natural selection is now widely accepted.
o 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.
 4.6.3.2 Fossils
o Fossils are the ‘remains’ of organisms from millions of years ago, which are found in
rocks.
o 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.