Biology

Cell structure

• Eukaryotic cells (animal, plant, fungi) have a nucleus. Prokaryotic cells (bacteria) don't: their DNA is a single loop free in the cytoplasm, plus plasmids.
• Prokaryotes are much smaller (around 1 µm vs 10–100 µm).
• Units: 1 mm = 1000 µm, 1 µm = 1000 nm. Standard form: 0.000 025 m = 2.5 × 10⁻⁵ m.

Specialised cells

Differentiation: a cell becomes specialised. Most animal cells differentiate early; many plant cells can differentiate all their life.

Microscopy

• Convert units first so both sizes match (usually to µm).
• Electron microscopes have much higher magnification and resolution (can tell two close points apart), so you can see sub-cellular structures like ribosomes and the detail of mitochondria.
• Light microscopes: cheap, portable, can view living cells, colour.

Cell division

• Body cells have 23 pairs of chromosomes (46).
• Cell cycle: (1) cell grows, makes more ribosomes and mitochondria, and DNA replicates; (2) mitosis: one set of chromosomes is pulled to each end, the nucleus divides; (3) cytoplasm and membrane divide → two identical cells.
• Mitosis is for growth, repair and asexual reproduction.

Stem cells

• Undifferentiated cells that can become many types of cell.
• Embryonic: can become any cell type. Adult bone marrow: can become blood cells. Meristems in plant tips: any plant cell, all their life (used to clone plants quickly and save rare species).
• Therapeutic cloning: an embryo made with the patient's own genes, so the stem cells aren't rejected.
• Could treat diabetes and paralysis. Issues: ethical/religious objections to using embryos, risk of viral infection, cost.

Transport in cells

• Diffusion: net movement of particles from a higher to a lower concentration. Faster with a steeper concentration gradient, higher temperature, bigger surface area. Examples: oxygen and CO₂ in the lungs, urea from cells into blood.
• Osmosis: diffusion of water from a dilute solution to a more concentrated one through a partially permeable membrane.
• Active transport: moves substances from a lower to a higher concentration (against the gradient), needs energy from respiration. Examples: mineral ions into root hairs, glucose from gut into blood.

Surface area to volume ratio

Bigger organisms have a smaller SA:V ratio, so diffusion through their surface is too slow. They need exchange surfaces adapted with: large surface area, thin membrane (short diffusion path), good blood supply and ventilation (keep the gradient steep). E.g. alveoli, villi, fish gills, leaves.

Culturing microorganisms Triple

• Bacteria divide by binary fission, as often as every 20 minutes.
• Number of bacteria = start number × 2^{number of divisions}.
• Aseptic technique: sterilise Petri dishes and agar, flame the inoculating loop, lid taped (not sealed all round), stored upside down, incubate at 25 °C in school (not 37 °C, to avoid growing human pathogens).
• Area of clear zone = πr².

Cells → tissues → organs → organ systems → organism.

Enzymes

• Biological catalysts made of protein. Each has an active site that fits one substrate (lock and key).
• Too hot or wrong pH → the active site changes shape (denatured), the substrate no longer fits.
• Rate of reaction = 1000 ÷ time taken (or amount ÷ time).

Bile: made in the liver, stored in the gall bladder. It's alkaline (neutralises stomach acid, giving the best pH for enzymes in the small intestine) and emulsifies fats into small droplets (bigger surface area for lipase).

The heart and blood

• Double circulatory system: right side pumps blood to the lungs, left side pumps blood to the body.
• Order: vena cava → right atrium → right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium → left ventricle → aorta → body.
• Left ventricle wall is thicker: it pumps blood all round the body at higher pressure.
• Valves stop blood flowing backwards. Coronary arteries supply the heart muscle with oxygen.
• Natural pacemaker: cells in the right atrium set the resting heart rate. Artificial pacemakers fix irregular rhythms.

• Red blood cells: haemoglobin carries oxygen; no nucleus (more room); biconcave (large surface area).
• White blood cells: fight infection. Platelets: help blood clot. Plasma: carries cells, CO₂, urea, glucose, hormones, antibodies.

Lungs

Air → trachea → bronchi → bronchioles → alveoli. Alveoli: huge surface area, thin walls, rich blood supply. Oxygen diffuses into the blood, CO₂ out.

Coronary heart disease (CHD)

• Fatty material builds up in coronary arteries → less oxygen to heart muscle.
• Stents: mesh tube keeps the artery open. Quick, effective, but surgery risk and clots.
• Statins: lower blood cholesterol, so less fatty build-up. No surgery, but taken for life, side effects, slow to work.
• Faulty valves → biological or mechanical replacements. Heart failure → transplant or artificial heart (while waiting).

Health and disease

• Communicable diseases spread between people (pathogens). Non-communicable don't (CHD, cancer, diabetes).
• Risk factors: smoking (lung disease, cancer, CVD), obesity (type 2 diabetes), alcohol (liver and brain damage), poor diet and lack of exercise (CVD), carcinogens and ionising radiation (cancer).
• Benign tumour: stays in one place, in a membrane, not cancer. Malignant tumour: invades tissue, spreads in the blood to form secondary tumours (cancer).
• A correlation between a factor and a disease doesn't prove one causes the other. You need a mechanism or controlled evidence.

Plant tissues

• Transpiration: water evaporates from leaves and diffuses out of stomata, pulling more water up the xylem.
• Faster when: hotter, brighter (more stomata open), windier (vapour blown away), drier air (lower humidity).
• Translocation: moving dissolved sugars in the phloem.
• Measured with a potometer: the air bubble's movement shows water uptake.

Pathogens are microorganisms that cause disease: viruses, bacteria, protists, fungi. Spread by direct contact, water, or air.

• Bacteria reproduce fast and make toxins that damage tissues.
• Viruses live and reproduce inside cells, then burst out, damaging them.

Body defences

• Non-specific: skin (barrier, antimicrobial secretions), nose hairs and mucus, cilia and mucus in trachea and bronchi, stomach acid kills pathogens.
• White blood cells: phagocytosis (engulf and digest pathogens), produce antibodies (specific to the pathogen's antigens), produce antitoxins (neutralise toxins).

Vaccination

A small amount of dead or inactive pathogen is injected. Its antigens trigger white blood cells to make antibodies. If the real pathogen enters later, white blood cells make the right antibodies quickly and in large amounts, so the pathogen is destroyed before you get ill. If most people are vaccinated, spread is reduced (herd immunity).

Medicines

• Antibiotics (e.g. penicillin) kill bacteria inside the body. They don't work on viruses, because viruses live inside cells. Overuse leads to resistant strains (e.g. MRSA).
• Painkillers treat symptoms, not the cause.
• From plants and microbes: digitalis (foxgloves, heart), aspirin (willow), penicillin (Alexander Fleming, from Penicillium mould).
• Drug testing: preclinical tests on cells, tissues and live animals (toxicity, efficacy) → clinical trials on healthy volunteers at very low dose (safety) → patients (find optimum dose). Double-blind: neither patient nor doctor knows who has the drug or the placebo. Results are peer reviewed.

Monoclonal antibodies Triple

• Identical antibodies from one clone of cells, made by fusing a mouse lymphocyte with a tumour cell (a hybridoma).
• Uses: pregnancy tests (bind to the hormone HCG), diagnosing disease, measuring chemicals in blood, finding or treating cancer cells (carrying drugs straight to them).
• Problem: more side effects than expected.

Plant disease and defences Triple

• Signs: stunted growth, spots, decay, growths, malformed leaves, discolouration, pests.
• Deficiencies: nitrate → stunted growth (needed for protein); magnesium → chlorosis, yellow leaves (needed for chlorophyll).
• Physical defences: cellulose cell walls, waxy cuticle, layer of dead cells (bark). Chemical: antibacterial chemicals, poisons. Mechanical: thorns, hairs, drooping leaves, mimicry.

Photosynthesis

• Happens in chloroplasts, using light energy absorbed by chlorophyll.
• Endothermic: energy is transferred from the environment to the chloroplasts by light.
• Limiting factors: light intensity, CO₂ concentration, temperature, amount of chlorophyll. Whichever is in shortest supply limits the rate.
• On a graph: while the line rises, the factor on the x-axis is limiting. Where it levels off, something else is limiting.
• Too hot → enzymes denature → rate drops sharply.
• HT Light intensity ∝ 1 / distance². Halve the distance → 4 times the light intensity.
• HT Greenhouse farmers balance the cost of extra heat, light and CO₂ against the extra crop they sell.

Uses of glucose

• Respiration (releases energy).
• Converted to starch for storage (insoluble).
• Fats and oils for storage (seeds).
• Cellulose to strengthen cell walls.
• With nitrate ions from the soil → amino acids → proteins.

Respiration

Respiration is an exothermic reaction that happens continuously in every living cell. It releases energy for chemical reactions (building larger molecules), movement and keeping warm.

Exercise

• During exercise heart rate, breathing rate and breath volume increase, delivering more oxygen and glucose to muscles and removing more CO₂.
• If not enough oxygen arrives, muscles respire anaerobically → lactic acid builds up → muscle fatigue.
• Oxygen debt: the extra oxygen needed after exercise to react with the lactic acid and remove it.
• HT Lactic acid is carried in the blood to the liver, where it's converted back to glucose.

Metabolism

The sum of all the reactions in a cell or the body. Examples: glucose → starch, glycogen and cellulose; glycerol + three fatty acids → lipids; glucose + nitrate → amino acids → proteins; respiration; excess proteins → urea (excreted).