Biology — Core Revision Notes

1. Cells & Organisation

All living organisms are made of cells — the fundamental unit of life. Animal cells contain a nucleus, cytoplasm, cell membrane, mitochondria, and ribosomes. Plant cells have all of these PLUS a cellulose cell wall, a permanent vacuole filled with cell sap, and chloroplasts. The hierarchy of organisation is: cells → tissues → organs → organ systems → organism. A tissue is a group of similar cells working together (e.g. muscle tissue). An organ is a structure made of different tissues working together (e.g. the heart). An organ system is a group of organs working together (e.g. the circulatory system).

Organelle functions — learn all of these

• Nucleus: contains DNA (chromosomes), controls cell activities and protein synthesis via mRNA
• Cell membrane: phospholipid bilayer, controls what enters and leaves by diffusion, osmosis, and active transport
• Cytoplasm: jelly-like medium where chemical reactions of metabolism take place
• Mitochondria: double-membraned organelle; site of aerobic respiration — releases ATP energy
• Ribosomes: tiny structures in cytoplasm and on rough ER; site of protein synthesis (translation)
• Chloroplasts (plant only): contain chlorophyll; absorb light energy for photosynthesis
• Cell wall (plant only): rigid cellulose layer outside membrane; gives structural support and prevents over-expansion
• Vacuole (plant only): large central vacuole filled with cell sap; maintains turgor pressure
• Rough ER: membrane network studded with ribosomes; synthesises and transports proteins
• Golgi body: modifies, packages, and dispatches proteins and lipids in vesicles

Labeled animal cell diagram — note the absence of cell wall, chloroplasts, and large central vacuole compared to plant cells.

2. Transport in Cells

There are three key transport mechanisms. Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down the concentration gradient. It is passive (no ATP required) and depends on concentration gradient, surface area, temperature, and distance. Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential across a partially permeable membrane. It is a special case of diffusion. Active transport is the movement of molecules or ions across a membrane against their concentration gradient, using energy from ATP and carrier proteins. It is used in root hair cells to absorb mineral ions, and in the small intestine to absorb glucose.

Factors affecting rate of diffusion

• Concentration gradient: steeper gradient = faster diffusion (most marks)
• Surface area: larger area = faster diffusion (e.g. alveoli, villi, root hairs)
• Distance / thickness of membrane: shorter distance = faster diffusion
• Temperature: higher temperature = more kinetic energy = faster movement
• Size of molecules: smaller molecules diffuse faster

3. Nutrition & the Digestive System

Digestion is the breakdown of large insoluble food molecules into small soluble molecules that can be absorbed into the blood. Physical (mechanical) digestion breaks food into smaller pieces to increase surface area (e.g. chewing, stomach churning). Chemical digestion uses enzymes to hydrolyse molecules. Amylase breaks starch → maltose (produced in salivary glands and pancreas, acts in mouth and small intestine). Protease breaks proteins → amino acids (pepsin in stomach, trypsin in small intestine). Lipase breaks lipids → fatty acids + glycerol (produced in pancreas, acts in small intestine). Bile is produced in the liver, stored in the gall bladder, and emulsifies fats — breaking large fat droplets into smaller ones to increase surface area for lipase.

The digestive organs and their roles

• Mouth: mechanical digestion (teeth) and chemical — amylase in saliva starts starch digestion
• Oesophagus: muscular tube, peristalsis moves food bolus to stomach
• Stomach: muscular walls churn food; hydrochloric acid (pH 2) kills bacteria and activates pepsin; pepsin digests protein
• Pancreas: produces amylase, protease (trypsin), and lipase; also secretes sodium bicarbonate to neutralise stomach acid
• Small intestine (ileum): final digestion + absorption; villi and microvilli massively increase surface area; absorbed into blood capillaries (glucose, amino acids) and lacteals (fatty acids, glycerol)
• Large intestine (colon): absorbs water; compacts waste into faeces
• Liver: produces bile, detoxifies substances, stores glycogen, makes plasma proteins

4. Photosynthesis

Photosynthesis is the process by which green plants use light energy to convert carbon dioxide and water into glucose and oxygen. Word equation: carbon dioxide + water → glucose + oxygen. Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Chlorophyll in the chloroplasts absorbs mainly red and blue light (reflects green light, which is why leaves look green). The glucose produced is used for: respiration (energy), making cellulose (cell walls), making starch (storage), making amino acids (with nitrogen), and making lipids.

Limiting factors of photosynthesis

• Light intensity: more light = more photosynthesis (up to the saturation point)
• CO₂ concentration: more CO₂ = faster photosynthesis (major limiting factor in greenhouses)
• Temperature: enzyme-controlled; rate rises with temperature until enzymes denature (~40–45°C)
• Water availability: water is a reactant; severe shortage reduces rate significantly
• At any moment, only ONE factor is the limiting factor — the one in shortest supply

5. Respiration

Aerobic respiration releases energy from glucose using oxygen. It occurs in the mitochondria. Word equation: glucose + oxygen → carbon dioxide + water (+ energy). Symbol equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. Anaerobic respiration occurs without oxygen and releases much less energy. In animals: glucose → lactic acid (causes muscle fatigue). In yeast/plants: glucose → ethanol + carbon dioxide (fermentation). The oxygen debt after exercise is repaid when lactic acid is oxidised to CO₂ and water during recovery. Respiration is NOT the same as breathing — respiration is a cellular chemical process.

6. The Nervous System

The nervous system consists of the central nervous system (CNS: brain + spinal cord) and the peripheral nervous system (nerves that connect CNS to the body). Sensory neurones carry impulses FROM receptors TO the CNS. Motor neurones carry impulses FROM the CNS TO effectors (muscles or glands). Relay neurones connect sensory and motor neurones within the CNS. A reflex arc is a rapid, automatic response that bypasses the brain for speed: receptor → sensory neurone → relay neurone (in spinal cord) → motor neurone → effector. Synapses are junctions between neurones — a chemical neurotransmitter is released and diffuses across the synaptic cleft to bind to receptors on the next neurone.

Hormones vs Nerves — comparison

• Nervous: electrical impulse, fast (milliseconds), short-lived, precise (specific muscle/gland), via neurones
• Hormonal: chemical in blood, slow (seconds to hours), long-lasting, widespread effect, via bloodstream
• Adrenaline: released in stress/danger — increases heart rate, dilates pupils, redirects blood to muscles
• Insulin: released by pancreas when blood glucose is HIGH → converts glucose to glycogen (stored in liver/muscle)
• Glucagon: released when blood glucose is LOW → converts glycogen back to glucose
• Oestrogen and progesterone: control the menstrual cycle and female secondary sexual characteristics

7. Genetics & Inheritance

DNA is a double helix polymer made of nucleotide monomers. Each nucleotide contains a phosphate group, deoxyribose sugar, and one of four bases: Adenine (A), Thymine (T), Cytosine (C), Guanine (G). Complementary base pairing: A–T and C–G. A gene is a section of DNA that codes for a specific protein (via mRNA and ribosomes). A chromosome is a coiled strand of DNA. Human body cells contain 46 chromosomes (23 pairs). Gametes contain 23 chromosomes (haploid). Dominant alleles are always expressed; recessive alleles are only expressed when homozygous. Genotype = the alleles present; Phenotype = the observable characteristic. Homozygous = two identical alleles (AA or aa). Heterozygous = two different alleles (Aa).

Genetic cross essentials

• Always draw a Punnett square — it is the safest method in exams
• Monohybrid cross: one trait — track dominant (capital) and recessive (lowercase) alleles
• Cystic fibrosis: recessive — both parents must carry 'f' for a child to be affected
• Huntington's disease: dominant — only ONE copy of the allele causes the disease
• Sex determination: females are XX, males are XY — all eggs carry X, sperm carry X or Y
• Codominance: both alleles are expressed — e.g. blood type AB (I^A and I^B both expressed)
• State the ratio of offspring (e.g. 3:1 or 1:2:1) and always express genotypes AND phenotypes

8. Ecology & the Environment

A habitat is the place where an organism lives. A population is all the organisms of one species in an area. A community is all populations of different species living in an area. An ecosystem is the community plus the non-living (abiotic) environment. A food chain shows feeding relationships and the direction of energy transfer (arrow means 'eaten by'). Producers are plants that make food via photosynthesis. Primary consumers eat producers. Secondary consumers eat primary consumers. Tertiary consumers are at the top. A food web is multiple interconnected food chains. The biomass pyramid shows that energy is lost at each trophic level — only about 10% is transferred. Energy is lost through: respiration, excretion, movement, heat loss.

Human impacts on ecosystems

• Deforestation: destroys habitats, reduces biodiversity, increases CO₂, causes soil erosion and flooding
• Global warming (greenhouse effect): CO₂, methane, water vapour trap heat; causes rising sea levels, habitat loss, extreme weather
• Eutrophication: excess fertilisers run into water → algal bloom → blocks light → aquatic plants die → decomposers multiply → use up O₂ → fish die (deoxygenation)
• Acid rain: SO₂ and NOₓ from burning fossil fuels dissolve in rain → damages forests, acidifies lakes
• Pesticides and bioaccumulation: toxins build up in higher trophic levels (e.g. DDT in fish → birds of prey)
• Extinction: habitat loss, hunting, and pollution reduce biodiversity permanently