ZIMSEC O-Level Combined Science (4003)

Nutrition, Transport and Respiration: notes

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Photosynthesis and leaves

Photosynthesis: plants make glucose from carbon dioxide and water, using light energy and chlorophyll. Oxygen is released.
Word equation: carbon dioxide + water → glucose + oxygen (in light, with chlorophyll).
Leaf structure: palisade cells have many chloroplasts, stomata let gases in and out, and the waxy cuticle reduces water loss.
Limiting factors: light intensity, carbon dioxide concentration and temperature. The factor in shortest supply limits the rate.
Mineral needs: nitrates are used to make proteins, and magnesium is needed to make chlorophyll.
Worked calculation: a pondweed gives off 24 cm³ of oxygen in 8 minutes. Rate = 24 ÷ 8 = 3 cm³ per minute.

Balanced diet: the right mix of carbohydrates, proteins, fats, vitamins, minerals, fibre and water.
Food tests: starch turns iodine blue-black; reducing sugar gives a brick-red colour with heated Benedict's; protein turns biuret violet; fat gives a cloudy white emulsion with ethanol.
Deficiency diseases: kwashiorkor and marasmus (too little protein), rickets (too little vitamin D or calcium), scurvy (too little vitamin C), goitre (too little iodine).

Digestion: breaking large food molecules into small, soluble ones using enzymes.
Amylase: changes starch into maltose (in the mouth and pancreas).
Protease: changes proteins into amino acids (in the stomach and pancreas).
Lipase: changes fats into fatty acids and glycerol (in the pancreas). Bile emulsifies fats.
Absorption: small molecules pass into the blood through villi in the small intestine, which have a large surface area.

Heart: pumps blood through four chambers. Atria receive blood and ventricles pump it out.
Blood vessels: arteries carry blood away from the heart and have thick walls. Veins carry blood to the heart and have valves. Capillaries are one cell thick, for exchange.
Blood: red cells carry oxygen using haemoglobin, white cells fight disease, platelets help clotting, and plasma carries carbon dioxide, glucose and urea.
Xylem: carries water and minerals up from the roots. Phloem carries sugars in both directions.
Transpiration: loss of water vapour from leaves through stomata, which pulls water up the xylem.

Aerobic respiration: glucose + oxygen → carbon dioxide + water + lots of energy.
Anaerobic respiration: glucose → lactic acid (in animals) or ethanol + carbon dioxide (in yeast), releasing little energy.
Gas exchange: humans use alveoli in the lungs, while plants use stomata and lenticels.
Kidneys: filter the blood to remove urea and keep water and salt balanced (homeostasis).

Oxygen: it is a product of photosynthesis, not a reactant.
Night respiration: plants respire all the time, not only at night.
Arteries and veins: arteries carry blood away from the heart, not always oxygen-rich blood.
Xylem and phloem: xylem carries water up only, while phloem carries sugars both ways.
Anaerobic energy: anaerobic respiration releases far less energy than aerobic respiration.