GCSE AQA Biology Revision Notes

Eukaryotes and prokaryotes

All living things are made of cells, but cells fall into two broad groups. Eukaryotic cells have their genetic material enclosed in a nucleus. Animal cells, plant cells and fungal cells are all eukaryotic. Prokaryotic cells have no nucleus — their genetic material sits free in the cytoplasm as a single loop of DNA, often with small extra rings called plasmids. Bacteria are prokaryotes.

Prokaryotic cells are much smaller, typically 0.2–2 micrometres across, while eukaryotic cells are usually 10–100 micrometres. A common exam question asks you to compare the two, so learn the distinguishing features rather than just the definitions.

Animal and plant cells

Both animal and plant cells contain a nucleus, cytoplasm, cell membrane, mitochondria and ribosomes.

• Nucleus — contains the chromosomes and controls the cell's activities.
• Cytoplasm — where most chemical reactions happen.
• Cell membrane — controls what enters and leaves.
• Mitochondria — the site of aerobic respiration, releasing energy.
• Ribosomes — where proteins are synthesised.

Plant cells additionally have a cell wall made of cellulose for strength, a permanent vacuole containing sap, and often chloroplasts containing chlorophyll for photosynthesis. Note that not every plant cell has chloroplasts — root hair cells do not, because they receive no light.

Cell specialisation

Cells become specialised so they can carry out a particular function efficiently. Their structure reflects that function:

• Sperm cell — a tail for swimming and many mitochondria to release the energy for it.
• Nerve cell — long and thin to carry impulses over distance, with branched endings to connect to other cells.
• Muscle cell — packed with protein fibres that shorten, and many mitochondria.
• Root hair cell — a long projection giving a large surface area for absorbing water and mineral ions.
• Xylem and phloem — xylem cells lose their end walls to form hollow tubes for water transport; phloem cells form tubes with sieve plates for transporting dissolved sugars.

When a question asks you to explain an adaptation, always link the structural feature back to the function. "It has many mitochondria" earns little; "many mitochondria release the energy needed for the tail to move" earns the mark.

Cell differentiation

Differentiation is the process by which a cell becomes specialised. In animals, most cells differentiate early in development and thereafter cell division is mainly for repair and replacement. In plants, many cells retain the ability to differentiate throughout the plant's life.

Microscopy

Light microscopes use light and glass lenses and have a maximum useful magnification of around ×2000. Electron microscopes use a beam of electrons, giving both much higher magnification and much higher resolution — the ability to distinguish two points that are close together. Electron microscopes let us see sub-cellular structures such as ribosomes and internal mitochondrial membranes.

The magnification equation is one you must be able to rearrange:

magnification = size of image ÷ size of real object

Watch your units. Convert everything to the same unit before calculating. There are 1000 micrometres (µm) in a millimetre, and 1000 nanometres (nm) in a micrometre.

Required practical: microscopy

You need to be able to use a light microscope to observe and draw cells. The method in outline:

1. Place the slide on the stage and select the lowest-power objective lens.
2. Use the coarse focus to bring the stage and lens close, then focus until the image is sharp.
3. Switch to a higher-power objective and refine with the fine focus.
4. Draw with a sharp pencil, using clear unbroken lines, no shading, and label with straight ruled lines. Record the total magnification.

Total magnification = eyepiece lens magnification × objective lens magnification.

Culturing microorganisms

Bacteria are grown on an agar gel plate containing nutrients. To keep cultures uncontaminated:

• Sterilise petri dishes and culture media before use.
• Pass inoculating loops through a flame before transferring bacteria.
• Secure the lid with tape, but do not seal it completely — this prevents anaerobic bacteria growing.
• Store plates upside down so condensation does not drip onto the agar.

In school laboratories, cultures are incubated at a maximum of 25 °C. This reduces the likelihood of growing pathogens that are harmful to humans.

To calculate the area of an inhibition zone around an antibiotic disc, use the area of a circle, πr². Remember that the radius is half the measured diameter.

Chromosomes

The nucleus of a cell contains chromosomes, which are made of DNA molecules. Each chromosome carries a large number of genes. In body cells, chromosomes are normally found in pairs — humans have 23 pairs, so 46 chromosomes in total.

Mitosis and the cell cycle

Cells divide by mitosis in a series of stages known as the cell cycle. Before a cell can divide it has to grow and copy its contents.

1. Growth and DNA replication — the cell grows, sub-cellular structures such as ribosomes and mitochondria increase in number, and the DNA replicates so each chromosome forms an identical copy of itself.
2. Mitosis — one set of chromosomes is pulled to each end of the cell and the nucleus divides.
3. Division — the cytoplasm and cell membrane divide, forming two genetically identical daughter cells.

Mitosis matters because it produces genetically identical cells. It is used for growth, for development, and for repairing damaged tissue.

Stem cells

A stem cell is an undifferentiated cell that can give rise to more cells of the same type, and to other cell types by differentiating.

• Embryonic stem cells can differentiate into most types of human cell.
• Adult stem cells, such as those in bone marrow, form a more limited range — bone marrow stem cells form blood cells.
• In plants, the meristem tissue found at root and shoot tips can differentiate into any plant cell throughout the plant's life.

Stem cells in medicine

Stem cells from human embryos can be cloned and made to differentiate into specialised cells for treating conditions such as diabetes and paralysis.

Therapeutic cloning produces an embryo with the same genes as the patient, so stem cells from it are not rejected by the patient's immune system.

Balanced arguments you should be able to give:

• In favour — the potential to cure currently untreatable conditions; embryos used are often unwanted from fertility clinics.
• Against — some people object on religious or ethical grounds because an embryo is a potential human life; there are risks of transferring viral infection; procedures are expensive and the technology is still developing.

Plant meristem stem cells can be used to produce clones of rare species to protect them from extinction, or to produce large numbers of identical crop plants with desirable characteristics such as disease resistance.

Diffusion

Diffusion is the spreading out of the particles of any substance in solution, or particles of a gas, resulting in a net movement from an area of higher concentration to an area of lower concentration. It is a passive process — it requires no energy from the cell.

Examples you should know: oxygen and carbon dioxide moving in gas exchange, and urea diffusing from cells into the blood plasma for excretion by the kidney.

The rate of diffusion is increased by:

• a steeper concentration gradient,
• a higher temperature,
• a larger surface area of the membrane.

Osmosis

Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane. The membrane lets water molecules through but not the larger solute molecules.

A precise definition earns marks. Saying "water moves from high to low concentration" is ambiguous — be clear you mean water moving from a dilute solution (high water potential) to a concentrated solution (low water potential).

Required practical: osmosis

The standard investigation uses potato cylinders in sugar solutions of different concentration.

1. Cut potato cylinders of equal size and blot them dry.
2. Measure and record the mass of each.
3. Place each in a different concentration of sugar solution for a set time.
4. Remove, blot dry, and re-measure the mass.
5. Calculate the percentage change in mass for each concentration.

Percentage change is used rather than raw change because the cylinders are not all of exactly equal starting mass, so it allows fair comparison.

Interpreting results: a gain in mass means water entered the cells, so the solution was more dilute than the cell contents. A loss means water left the cells. The concentration at which there is no change in mass is equal to the concentration inside the potato cells.

Active transport

Active transport moves substances from a more dilute solution to a more concentrated solution — against a concentration gradient. This requires energy from respiration.

Two examples to learn:

• Root hair cells absorb mineral ions from very dilute soil solutions, allowing plants to take up the ions they need for healthy growth.
• The small intestine absorbs glucose from the gut into the blood, even when the glucose concentration in the gut is lower than in the blood.

Exchange surfaces

A single-celled organism has a large surface area to volume ratio, so diffusion across its surface is sufficient. As organisms get larger, the ratio falls and diffusion alone becomes too slow.

Effective exchange surfaces are adapted by having:

• a large surface area,
• a thin membrane, giving a short diffusion path,
• in animals, an efficient blood supply to maintain the concentration gradient,
• in animals for gas exchange, ventilation to move the medium across the surface.

You should be able to calculate and compare surface area to volume ratios for simple shapes such as cubes — a frequent source of marks.

Cells, tissues, organs and organ systems

Large multicellular organisms are organised in a hierarchy. Each level is built from the one below it:

• Cells are the basic building blocks of all living organisms.
• A tissue is a group of similar cells working together to carry out a particular function.
• An organ is an aggregation of several tissues performing a specific function.
• An organ system is a group of organs that work together to form organisms.

The stomach is a useful worked example of an organ containing several tissues: muscular tissue to churn the contents, glandular tissue to produce digestive juices, and epithelial tissue to cover the inside and outside of the organ.

Learn the order — cells, tissues, organs, organ systems, organism — and be ready to place an unfamiliar example at the right level. Examiners often use a structure you have not studied to test whether you understand the principle rather than a memorised list.

The human digestive system

The digestive system is an organ system in which several organs work together to digest and absorb food. Food passes from the mouth, down the oesophagus, into the stomach, then into the small intestine, the large intestine and out through the rectum and anus. The liver, gall bladder and pancreas are accessory organs that release substances into the gut.

Digestion breaks large insoluble molecules into small soluble ones that can be absorbed into the bloodstream. Absorption happens mainly in the small intestine; water is absorbed in the large intestine.

Enzymes

Enzymes are biological catalysts — they speed up reactions without being used up. They are large protein molecules, and each has an active site whose shape is complementary to its substrate. Because a particular enzyme only fits a particular substrate, enzymes are specific. This is often called the lock-and-key model.

Digestive enzymes

• Carbohydrase — breaks down carbohydrates to simple sugars. The main one, amylase, breaks starch down into sugars and is produced in the salivary glands, the pancreas and the small intestine.
• Protease — breaks down proteins into amino acids. Produced in the stomach (as pepsin), the pancreas and the small intestine.
• Lipase — breaks down lipids into fatty acids and glycerol. Produced in the pancreas and the small intestine.

The products of digestion are used to build new carbohydrates, lipids and proteins, and some glucose is used in respiration.

Bile is made in the liver and stored in the gall bladder. It is alkaline, so it neutralises the hydrochloric acid from the stomach, and it emulsifies fat into small droplets, increasing the surface area for lipase to work on. Both effects speed up fat digestion. Bile is not an enzyme — a common and costly error.

Required practical: food tests

• Starch — add iodine solution. A blue-black colour shows starch is present.
• Sugars — add Benedict's solution and heat in a water bath. Blue means none; green, yellow, then brick red indicate increasing amounts of sugar.
• Protein — add Biuret solution. A purple or mauve colour shows protein.
• Lipids — shake with ethanol, then add water. A cloudy white emulsion shows lipid.

Factors affecting enzyme action

Enzyme activity rises with temperature up to an optimum, then falls sharply. Beyond the optimum, the active site changes shape and the enzyme is denatured — the substrate no longer fits and the reaction stops. The same happens at an unsuitable pH.

Be precise in the wording. Enzymes are denatured, not "killed" — they are molecules, not living things. Different enzymes have different optimum pH values: stomach protease works at about pH 2, while enzymes in the small intestine work in alkaline conditions.

The heart and blood vessels

The heart is an organ that pumps blood around the body in a double circulatory system. One circuit carries blood to the lungs to pick up oxygen; the other carries oxygenated blood to the body tissues. The advantage of two circuits is that blood can be pumped to the body at high pressure, giving a faster flow rate.

Blood enters the atria, which contract to push it into the ventricles. The ventricles contract to push blood out of the heart. Valves prevent backflow. Follow the route carefully:

body → vena cava → right atrium → right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium → left ventricle → aorta → body

The left ventricle has a thicker muscular wall than the right, because it pumps blood all the way round the body rather than just to the lungs. The heart's own muscle is supplied by the coronary arteries. Resting heart rate is controlled by a group of cells in the right atrium that act as a natural pacemaker; an artificial pacemaker can be fitted to correct an irregular heartbeat.

Three vessel types, each adapted to its job:

• Arteries — carry blood away from the heart. Thick, muscular, elastic walls withstand high pressure.
• Veins — carry blood towards the heart. Thinner walls, wider lumen, and valves to prevent backflow at low pressure.
• Capillaries — walls one cell thick and permeable, so substances diffuse into and out of cells over a very short distance.

Blood

Blood is a tissue consisting of a fluid called plasma in which three things are suspended:

• Red blood cells — contain haemoglobin, which binds oxygen. They have no nucleus, leaving more room for haemoglobin, and a biconcave shape giving a large surface area.
• White blood cells — part of the immune response; some engulf pathogens, others produce antibodies.
• Platelets — small fragments of cells involved in clotting.

Plasma transports dissolved substances including carbon dioxide, glucose, urea and hormones.

Coronary heart disease

In coronary heart disease, layers of fatty material build up inside the coronary arteries. This narrows them, reducing blood flow, so the heart muscle receives less oxygen.

• Stents are mesh tubes inserted to keep the arteries open. They work for a long time and recovery is quick, but there is a risk of infection from surgery and of a clot forming near the stent.
• Statins are drugs that reduce blood cholesterol, slowing the rate of fatty deposit formation. They must be taken long term and can have side effects.

Faulty heart valves can become stiff or leaky and may be replaced with biological or mechanical valves. In heart failure, a donor heart may be transplanted, or an artificial heart used to keep a patient alive while waiting or to allow the heart to rest and recover.

The lungs and gas exchange

Air travels down the trachea, which divides into two bronchi, then into bronchioles, and finally into millions of alveoli where gas exchange happens. Oxygen diffuses into the blood and carbon dioxide diffuses out.

The alveoli are effective because they provide a very large surface area, have walls only one cell thick giving a short diffusion path, are surrounded by a dense capillary network maintaining the concentration gradient, and are moist so gases dissolve.

Health issues

Health is the state of physical and mental wellbeing. Disease is a major cause of ill health, but other factors matter too — diet, stress and life situation all affect both physical and mental health.

Diseases fall into two groups. Communicable diseases are caused by pathogens and can be passed between people. Non-communicable diseases cannot be transmitted; they include cancer, coronary heart disease and type 2 diabetes.

Different types of disease can interact:

• Defects in the immune system make a person more likely to suffer from infectious diseases.
• Viruses living in cells can trigger cancers.
• Immune reactions initially caused by a pathogen can trigger allergies such as skin rashes and asthma.
• Severe physical ill health can lead to depression and other mental illness.

Risk factors for non-communicable disease

A risk factor is anything linked to an increased rate of a disease. Risk factors may be aspects of a person's lifestyle or substances in their body or environment.

• Diet, smoking and exercise affect the risk of cardiovascular disease.
• Obesity is a risk factor for type 2 diabetes.
• Alcohol affects the liver and brain function, and drinking in pregnancy affects the unborn baby.
• Smoking affects lung disease and lung cancer, and smoking in pregnancy affects the unborn baby.
• Carcinogens, including ionising radiation, are risk factors for cancer.

Non-communicable diseases carry human and financial costs — to the individual, to their family, and to the health service and wider economy.

Be careful with the language of causation. A correlation between a factor and a disease is not the same as proof that the factor causes it. To show a causal mechanism you need evidence of how the factor produces the effect. Questions frequently reward candidates who spot this distinction and penalise those who over-claim from a graph.

Cancer

Cancer results from changes in cells that lead to uncontrolled growth and division. This forms a tumour.

• Benign tumours are growths of abnormal cells contained in one area, usually within a membrane. They do not invade other parts of the body.
• Malignant tumour cells are cancers. They invade neighbouring tissues and spread to different parts of the body in the blood, where they form secondary tumours.

Both genetic risk factors and lifestyle risk factors contribute to cancer.

Plant tissues

A leaf is a plant organ, and it contains several tissues, each with a role:

• Epidermal tissue — covers the leaf surfaces. The upper epidermis is often covered by a waxy cuticle that reduces water loss.
• Palisade mesophyll — packed with chloroplasts, and positioned near the top of the leaf to absorb the most light for photosynthesis.
• Spongy mesophyll — contains large air spaces that allow gases to diffuse to and from the cells.
• Xylem and phloem — transport water, mineral ions and dissolved sugars.
• Meristem tissue — found at growing tips of shoots and roots, able to differentiate into any plant cell type.

Plant organ system

The roots, stem and leaves form an organ system that transports substances around the plant.

Stomata are tiny openings, mostly on the lower surface of the leaf, that control gas exchange and water loss. Each is surrounded by two guard cells, which change shape to open or close the pore. When water is plentiful the guard cells swell and the stomata open; when the plant is short of water they lose water, become flaccid and the stomata close, limiting further loss.

Being on the underside reduces water loss, because that surface is cooler and shaded.

Transpiration and translocation

Transpiration is the loss of water vapour from the leaves. Water evaporates from the surfaces of cells inside the leaf and diffuses out through the stomata. This creates a pull that draws water up through the xylem from the roots — the transpiration stream. Xylem consists of hollow tubes strengthened by lignin, carrying water and mineral ions in one direction only: upwards.

Translocation is the movement of dissolved sugars made in the leaves to the rest of the plant, for immediate use or storage. This happens in the phloem, which is made of columns of elongated living cells with small pores in their end walls, and can move substances in both directions.

A clean way to keep them apart: xylem carries water up, phloem carries food both ways.

Factors affecting transpiration

The rate of transpiration increases with:

• higher temperature — water molecules evaporate and diffuse faster,
• lower humidity — a steeper concentration gradient of water vapour between leaf and air,
• more air movement — wind removes water vapour from around the stomata, maintaining the gradient,
• higher light intensity — stomata open wider for photosynthesis, so more water escapes.

Transpiration rate can be measured with a potometer, which tracks the movement of an air bubble along a capillary tube to show the rate of water uptake.