Cells, Nutrition, and Transport in Living Things
Cells and organisation
All living things are made of cells. Plant and animal cells share a cell membrane, cytoplasm, and nucleus; plant cells also have a cellulose cell wall, chloroplasts, and a large permanent vacuole. Know the function of each part: the nucleus controls activities and holds genetic material; mitochondria release energy in respiration; chloroplasts carry out photosynthesis; the membrane controls what enters and leaves. Cells group into tissues, organs, and organ systems, and organisation increases efficiency.
Nutrition
Plants make their own food by photosynthesis, using carbon dioxide + water + light energy to produce glucose and oxygen, catalysed by chlorophyll. Limiting factors include light intensity, carbon dioxide concentration, and temperature. Animals are heterotrophs; a balanced diet needs carbohydrates, proteins, fats, vitamins, minerals, fibre, and water. Enzymes are biological catalysts with specific active sites; temperature and pH affect their activity, and high temperatures denature them. Traps include confusing photosynthesis with respiration (both occur in plants, day and night) and forgetting that respiration releases energy in all living cells.
Transport
In mammals, the heart pumps blood through arteries, veins, and capillaries. Arteries carry blood away from the heart at high pressure; veins return blood and contain valves; capillaries are one cell thick for exchange. Red blood cells carry oxygen using haemoglobin. Plants transport water and minerals in xylem (upward, transpiration pull) and sugars in phloem (both directions).
Worked example
A plant is given more light and its photosynthesis rate rises until it plateaus - light is the limiting factor until another factor (CO₂ or temperature) takes over. This explains why adding more light beyond the plateau has no effect.