Chapter 5: Life Processes

Equation / formula : 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (sunlight, chlorophyll)

In the figure : Chloroplasts trap light; the glucose is stored as starch and oxygen leaves through the stomata.

Rule 1 : What the plant needs and gives — CO₂ comes in through stomata, water rises from the roots, chlorophyll traps light, water is split to give oxygen, and CO₂ is changed into glucose.

• Easy : A plant kept in a dark room cannot make food.
• Medium : A leaf partly covered with black paper and kept in sun turns blue-black with iodine only in the uncovered part.
• For exams : Boil the leaf in alcohol first to remove green colour. Potassium hydroxide in a flask absorbs CO₂, so that leaf makes no starch.

Rule 2 : Stomata and guard cells — Each stomatal pore has two guard cells. When they take in water they swell and the pore opens; when they lose water they shrink and the pore closes.

• Easy : Stomata are open in daylight so CO₂ can enter.
• Medium : Water vapour also leaves through open stomata, so the plant has to balance food-making and water-saving.
• For exams : Some desert plants open stomata at night to take in CO₂ and keep them closed by day to lose less water.

Spot the difference

Equation / formula : Starch → sugar (amylase); Protein → amino acids (pepsin, trypsin); Fat → fatty acids + glycerol (bile + lipase).

In the figure : The food pipe pushes food along by peristalsis; the large intestine absorbs water and the rest leaves through the anus.

Rule 1 : Digestive juices — Saliva has amylase for starch. The stomach makes hydrochloric acid, pepsin and mucus. Bile from the liver and pancreatic juice enter the small intestine; intestinal juice finishes digestion.

• Easy : Chewing roti: amylase turns starch into sugar, so it tastes sweet.
• Medium : Acid in the stomach kills germs and helps pepsin; mucus protects the stomach wall from the acid.
• For exams : Bile has no enzyme; it makes the medium alkaline and breaks fat into tiny droplets (emulsification) so lipase can act faster.

Rule 2 : Absorption and egestion — The small intestine is long and its wall has finger-like villi full of blood vessels, so digested food is absorbed quickly. The large intestine absorbs water; undigested waste leaves as faeces.

• Easy : Villi increase the surface for absorption.
• Medium : Absorbed food goes into the blood and is used by cells for energy, growth and repair (assimilation).
• For exams : Herbivores have a longer small intestine than carnivores because cellulose is hard and slow to digest.

Spot the difference

Equation / formula : Aerobic: glucose + oxygen → CO₂ + water + energy. In muscle: glucose → lactic acid + energy. In yeast: glucose → ethanol + CO₂ + energy.

In the figure : Aerobic respiration gives the most energy per glucose; ATP is the energy currency used for muscle work and making proteins.

Rule 1 : Types of respiration — All three paths start with the same step, glycolysis, in the cytoplasm. What happens next depends on whether oxygen is available.

• Easy : Bread dough rises because yeast gives out CO₂.
• Medium : Heavy exercise: oxygen runs short, muscles make lactic acid and we get cramps.
• For exams : Aerobic respiration releases far more ATP per glucose than anaerobic respiration.

Rule 2 : Breathing in humans, animals and plants — Air goes nose → throat → windpipe (kept open by cartilage rings) → bronchi → alveoli. Millions of alveoli give a large surface where oxygen enters the blood and CO₂ leaves. Haemoglobin carries the oxygen.

• Easy : Inhale: diaphragm flattens and ribs move up, so the chest gets bigger and air rushes in.
• Medium : Fish use gills, earthworms use moist skin, insects use air tubes (tracheae), plants use stomata and lenticels.
• For exams : Water holds less oxygen than air, so aquatic animals breathe faster than land animals.

Spot the difference

In the figure : Vena cava brings blood to the right atrium; the pulmonary artery takes it to the lungs; the pulmonary vein brings it back to the left atrium; the aorta carries it out.

Rule 1 : Heart, vessels and blood — Arteries carry blood away from the heart and have thick elastic walls. Veins bring it back and have valves. Capillaries are one cell thick, so exchange with cells happens there. Blood has plasma, red cells (haemoglobin), white cells and platelets.

• Easy : Ventricle walls are thicker than atrium walls because they pump blood out.
• Medium : Pulmonary artery carries oxygen-poor blood and pulmonary vein carries oxygen-rich blood; these are the exceptions.
• For exams : Normal blood pressure is about 120/80 mm Hg (systolic/diastolic); platelets clot blood at a cut; lymph carries digested fat and drains extra fluid.

Rule 2 : Transport in plants — Root hairs absorb water by osmosis. Transpiration (loss of water vapour from stomata) creates a pull that draws water up the xylem. Phloem moves food from leaves to all parts using energy (ATP).

• Easy : Xylem: water up. Phloem: food up and down.
• Medium : Transpiration is fastest on a hot, dry, windy day and also cools the plant.
• For exams : At night stomata are closed, so water movement then depends mainly on root pressure.

Spot the difference

In the figure : Blood is filtered under pressure in the glomerulus; glucose, amino acids, salts and water are taken back in the tubule; the rest is urine.

Rule 1 : Making urine and kidney failure — Steps: filtration, selective reabsorption, then collection. How much water is reabsorbed depends on how much extra water the body has. If kidneys fail, wastes build up in the blood, so dialysis (artificial kidney) cleans the blood.

• Easy : Healthy urine has urea and salts but no glucose.
• Medium : Drinking a lot of water gives large, pale urine; too little water gives small, dark urine.
• For exams : In dialysis, blood passes through tubes with a semi-permeable membrane in dialysing fluid; wastes diffuse out. It does not reabsorb useful substances like a real kidney.

Rule 2 : Excretion in plants — Plants have no kidneys. They lose extra water by transpiration, give out O₂ and CO₂ through stomata, and store wastes as resins, gums and latex in old xylem, bark and falling leaves.

• Easy : Autumn leaves carry wastes away when they fall.
• Medium : Rubber tree gives latex; tannins from bark are used to make leather.
• For exams : Some wastes are also passed out into the soil through roots.

Spot the difference