For about 1,400 years, doctors followed the Greek physician Galen, who taught that blood was made in the liver and used up by the body. In 1628 the English doctor William Harvey published a different idea. He worked out how much blood the heart pumps with each beat and realised the body couldn't possibly make that much new blood every hour. The same blood must go round and round. His one gap: he couldn't see how blood got from arteries to veins. In 1661 Marcello Malpighi found the answer under a microscope: tiny vessels we now call capillaries.
Levels of organisation
Cells → tissues → organs → organ systems → organism. Topic 2 follows that idea through digestion, circulation and plants.
Digestion and enzymes
Enzymes work like a lock and key: the active site fits one substrate. Heat or the wrong pH changes its shape and denatures it.
- Amylase: starch → sugars
- Proteases: proteins → amino acids
- Lipases: lipids → fatty acids and glycerol
Bile, made in the liver and stored in the gall bladder, isn't an enzyme. It neutralises stomach acid and emulsifies fat into droplets so lipase works faster.
Required practicals: food tests (Benedict's, iodine, Biuret, Sudan III), and the effect of pH on amylase.
Heart and blood
Humans have a double circulatory system. The right side of the heart sends blood to the lungs; the left side sends oxygenated blood to the body. A pacemaker in the right atrium sets the rhythm.
- Arteries: away from the heart, high pressure, thick walls.
- Veins: back to the heart, low pressure, valves.
- Capillaries: one cell thick, for exchange.
In coronary heart disease, fatty deposits narrow the arteries feeding the heart muscle. Stents hold them open; statins lower cholesterol.
Plants
Leaves are organs too. The palisade mesophyll does most photosynthesis; the spongy mesophyll has air spaces; stomata let gases in and out. Xylem carries water and minerals up (transpiration); phloem carries sugars both ways (translocation).
Common exam mistakes
- "Bile digests fat." It emulsifies it; lipase digests it.
- "Arteries carry oxygenated blood." Arteries carry blood away from the heart. The pulmonary artery carries deoxygenated blood.
- Swapping xylem and phloem. Xylem: water, up only. Phloem: sugars, both ways.
- "Humidity speeds up transpiration." It slows it down.
Try it
Say each answer before you flip. Then put yourself in Harvey's place in 1628: blood must be getting from arteries to veins somehow. What would you have tested next?
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