Structure and functions in living organisms
Cell structure
All living things are made of cells. Animal cells have a cell membrane (controls what enters and leaves), cytoplasm (where reactions happen), a nucleus (contains DNA, controls the cell), mitochondria (aerobic respiration) and ribosomes (protein synthesis). Plant cells also have a cellulose cell wall (support), chloroplasts (photosynthesis) and a large permanent vacuole (cell sap, keeps cells firm). Bacterial cells have no nucleus: their DNA is a single loop, often with plasmids.
≈ 35 minDiffusion and osmosis
Diffusion is the net movement of particles from a higher to a lower concentration, down a concentration gradient, because of random movement. It's faster with a steeper gradient, larger surface area, shorter distance and higher temperature. Osmosis is the net movement of water molecules from a dilute solution (high water potential) to a more concentrated solution (low water potential) through a partially permeable membrane. Plant cells in water become turgid; in strong sugar solution they become flaccid then plasmolysed. Animal cells in water may burst; in strong solutions they shrink.
≈ 40 minBiological molecules and food tests
Carbohydrates (C, H, O) are built from simple sugars: glucose joins to form starch (plant store), glycogen (animal store) and cellulose (cell walls). Proteins (C, H, O, N, sometimes S) are chains of amino acids. Fats and oils (C, H, O) are made from fatty acids and glycerol. Food tests: iodine turns blue-black with starch; Benedict's (heated) turns brick-red with reducing sugars; biuret turns purple with protein; ethanol emulsion test gives a cloudy white emulsion with fat; DCPIP decolourises with vitamin C. DNA is a double helix with base pairs A–T and C–G.
≈ 40 minEnzyme action
Enzymes are proteins that act as biological catalysts: they speed up reactions without being used up. Each enzyme has an active site with a shape complementary to its substrate (lock and key), forming an enzyme–substrate complex. Activity rises with temperature up to an optimum (about 37 °C for human enzymes) as molecules collide more often; above it the enzyme denatures: the active site changes shape and the substrate no longer fits. Each enzyme also has an optimum pH.
≈ 40 minPhotosynthesis
In photosynthesis, plants use light energy, absorbed by chlorophyll in chloroplasts, to make glucose from carbon dioxide and water, releasing oxygen: . Glucose is used for respiration or converted to starch (storage), cellulose (cell walls), sucrose (transport) and, with nitrate ions, amino acids. The rate is controlled by limiting factors: light intensity, carbon dioxide concentration and temperature; whichever is in shortest supply limits the rate.
≈ 45 minHeart and circulation
Mammals have a double circulation: blood passes through the heart twice per circuit, once to the lungs (pulmonary) and once to the body (systemic), keeping body pressure high. Deoxygenated blood enters the right atrium from the vena cava, passes to the right ventricle, and is pumped through the pulmonary artery to the lungs. Oxygenated blood returns in the pulmonary vein to the left atrium, then the left ventricle, which pumps it out through the aorta. Valves stop backflow. The left ventricle wall is thickest. Coronary heart disease happens when fatty deposits block the coronary arteries.
≈ 45 minDisease and immunity
Pathogens cause disease and spread by direct contact (blood, body fluids) or indirectly (air, water, food, vectors). The body's defences: skin, mucus and cilia, stomach acid, then white blood cells. Each pathogen carries antigens; lymphocytes make antibodies with complementary shapes that bind to them, marking pathogens for destruction. Memory cells remain, giving active immunity: a second infection is fought off faster and stronger. Vaccines contain harmless antigens to trigger this. Passive immunity (antibodies from breast milk or injection) is short-lived.
≈ 40 minAerobic and anaerobic respiration
Respiration releases energy from glucose in every living cell. Aerobic respiration uses oxygen, happens in mitochondria and releases lots of energy: . Anaerobic respiration happens without oxygen and releases much less energy. In muscles: glucose → lactic acid; this builds up during vigorous exercise, causing fatigue and an oxygen debt repaid afterwards by fast breathing. In yeast: glucose → ethanol + carbon dioxide (used in brewing and baking). Energy is used for muscle contraction, protein synthesis, cell division, active transport, growth, nerve impulses and keeping warm.
≈ 40 minKidneys and excretion
Excretion is removing the waste products of metabolism. The lungs excrete carbon dioxide; the kidneys excrete urea, excess water and excess ions as urine. Urea is made in the liver by deamination of excess amino acids. In each kidney nephron, blood is filtered under high pressure in the glomerulus (ultrafiltration): small molecules pass into the tubule. Then all glucose, most water and some ions are selectively reabsorbed. What's left flows to the bladder. Kidney failure is treated by dialysis or a transplant.
≈ 40 minNervous system and the eye
The central nervous system (brain and spinal cord) coordinates responses. Impulses travel: stimulus → receptor → sensory neurone → relay neurone (CNS) → motor neurone → effector → response. Reflexes are fast, automatic and protective. Neurones meet at synapses, where neurotransmitters diffuse across the gap. In the eye, the cornea and lens focus light on the retina (rods for dim light, cones for colour). The pupil reflex: in bright light, circular muscles of the iris contract, making the pupil smaller. Accommodation: for near objects, ciliary muscles contract, suspensory ligaments slacken, and the lens becomes fatter.
≈ 45 minBreathing and gas exchange
Air passes through the trachea, bronchi and bronchioles to millions of alveoli, where oxygen diffuses into the blood and CO₂ diffuses out. Alveoli have a large surface area, thin walls (one cell thick), a rich capillary network and are ventilated, keeping concentration gradients steep. To inhale, the external intercostal muscles contract (ribs up and out) and the diaphragm contracts and flattens: volume increases, pressure decreases, air flows in. Exhaling reverses this. Expired air has less oxygen, more CO₂ and more water vapour. Smoking damages cilia and alveoli and causes cancer.
≈ 40 minXylem, phloem and water uptake
Plants have two transport tissues in vascular bundles. Xylem carries water and mineral ions up from the roots; its vessels are dead, hollow tubes with no end walls and thick walls strengthened with lignin, which also support the plant. Phloem carries sucrose and amino acids, both up and down, from where they're made or stored (sources) to where they're needed (sinks). Water enters root hair cells by osmosis, crosses the root cortex to the xylem, and travels up to the leaves.
≈ 30 minDiet
A balanced diet contains the right amounts of carbohydrates (energy), fats (energy store, insulation), proteins (growth and repair), vitamins (C for healthy skin and gums; D for absorbing calcium), minerals (calcium for bones and teeth; iron for haemoglobin), fibre (helps food move through the gut) and water. Lack of a nutrient causes deficiency diseases: scurvy (vitamin C), rickets (vitamin D or calcium), anaemia (iron). Energy needs depend on age, sex, activity and pregnancy.
≈ 35 minHormones and homeostasis
Hormones are chemicals made by endocrine glands and carried in the blood to target organs. They act more slowly than nerves but for longer. Adrenaline (adrenal glands) prepares the body for action; insulin (pancreas) lowers blood glucose by making the liver and muscles take up glucose and store it as glycogen; glucagon raises it. Homeostasis keeps internal conditions constant using negative feedback. Body temperature is controlled by the brain: when too hot, sweating and vasodilation; when too cold, shivering and vasoconstriction. In Type 1 diabetes, the pancreas makes no insulin.
≈ 45 minSpecialised cells and magnification
Cells specialise to do particular jobs: root hair cells have a long extension for absorbing water; red blood cells have no nucleus and a biconcave shape to carry oxygen; sperm cells have a tail and many mitochondria. Similar cells form tissues, tissues form organs, organs form organ systems. Magnification , using the same units; .
≈ 40 minActive transport and surface area
Active transport moves particles against a concentration gradient (from low to high concentration) using energy from respiration and carrier proteins in the membrane. Root hair cells use it to absorb mineral ions from dilute soil water; gut cells use it to absorb glucose. Larger organisms have a smaller surface area to volume ratio, so they need specialised exchange surfaces and transport systems.
≈ 30 minBlood vessels and blood
Arteries carry blood at high pressure away from the heart: thick muscular, elastic walls, narrow lumen. Veins carry blood at low pressure back to the heart: thin walls, wide lumen, valves. Capillaries are one cell thick, allowing exchange with tissues. Blood contains red blood cells (haemoglobin carries oxygen), white blood cells (phagocytes engulf pathogens; lymphocytes make antibodies), platelets (clotting) and plasma (carries CO₂, glucose, urea, hormones, heat).
≈ 40 minDigestion and absorption
Digestion breaks large insoluble molecules into small soluble ones that can be absorbed. Physical digestion (teeth chewing, stomach churning) increases surface area. Chemical digestion uses enzymes: amylase (mouth, pancreas) turns starch into maltose; protease (pepsin in stomach, trypsin from pancreas) turns proteins into amino acids; lipase (pancreas) turns fats into fatty acids and glycerol. Bile from the liver neutralises stomach acid and emulsifies fats. Products are absorbed in the small intestine, whose villi give a huge surface area; water is absorbed mainly in the small intestine and colon.
≈ 45 minLeaf structure and mineral ions
Leaves are adapted for photosynthesis: broad and thin (large surface area, short diffusion distance), with a waxy cuticle, a transparent upper epidermis, a palisade mesophyll layer packed with chloroplasts, a spongy mesophyll with air spaces for gas exchange, stomata (opened and closed by guard cells) and vascular bundles (xylem brings water, phloem removes sugars). Plants also need nitrate ions to make amino acids (deficiency: stunted growth, yellow older leaves) and magnesium ions to make chlorophyll (deficiency: yellowing between veins).
≈ 30 minTranspiration and translocation
Transpiration is the loss of water vapour from leaves: water evaporates from mesophyll cell surfaces into air spaces, then diffuses out through the stomata. This pulls a continuous column of water up the xylem (the transpiration stream), because water molecules stick together (cohesion). Transpiration is faster when it is hotter, drier (low humidity), windier and brighter (more stomata open). It's measured with a potometer. Translocation moves sucrose and amino acids in the phloem from sources (leaves, storage organs) to sinks (growing points, roots, fruits).
≈ 40 minTropisms
A tropism is a growth response of a plant to a directional stimulus. Phototropism: shoots grow towards light (positive), helping leaves absorb light. Gravitropism: roots grow down with gravity (positive), shoots grow up (negative). These are controlled by auxin, a hormone made in the shoot tip. In a shoot, auxin moves to the shaded side, where it makes cells elongate more, so the shoot bends towards the light.
≈ 30 min