Organisation
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Levels of organisation
Living organisms are organised into a hierarchy of structures, from smallest to largest.
Example: muscle cell → muscle tissue → heart → circulatory system → human.
Levels of organisation — Key Knowledge
- Cell basic building block
- Tissue group of similar cells working together
- Organ group of different tissues working together
- Organ system group of organs working together
- Organism made up of organ systems
The digestive system
The digestive system is an organ system that breaks down food into small soluble molecules that can be absorbed into the blood.
Bile is not an enzyme — it emulsifies fats (breaks them into smaller droplets to increase surface area for lipase).
The digestive system — Key Knowledge
- Mouth mechanical breakdown + amylase breaks down starch
- Stomach protease breaks down proteins + hydrochloric acid kills bacteria and provides acidic pH
- Small intestine further digestion + absorption of nutrients into blood
- Large intestine absorbs water
- Liver produces bile — emulsifies fats, neutralises stomach acid
- Pancreas produces digestive enzymes — protease, lipase, amylase
Enzymes
Enzymes are biological catalysts — they speed up chemical reactions without being used up. Each enzyme has a specifically shaped active site that matches its substrate.
Enzymes are proteins — their shape (and therefore function) depends on the precise order of amino acids.
Enzymes — Key Knowledge
- Biological catalyst speeds up reactions, not used up
- Active site specifically shaped region where the substrate binds
- Lock and key model substrate fits into active site like a key fits a lock
- Specificity each enzyme only works on one type of substrate
Factors affecting enzymes
Enzyme activity is affected by temperature and pH. At the optimum, the rate of reaction is highest. Beyond the optimum, the enzyme denatures.
Denaturation is permanent — cooling a denatured enzyme does not restore its function. Enzymes are not alive, so they don't "die".
Factors affecting enzymes — Key Knowledge
- Optimum temperature ~37°C for human enzymes
- Denaturation active site changes shape permanently at high temperatures — substrate can no longer fit
- Optimum pH varies by enzyme — e.g. stomach protease works best at pH 2, small intestine enzymes at pH 7–8
The heart
The heart is a muscular organ that pumps blood around the body. It has four chambers and is part of a double circulatory system.
The left ventricle wall is thicker because it pumps blood to the whole body, not just the lungs.
The heart — Key Knowledge
- Right atrium receives deoxygenated blood from body
- Right ventricle pumps blood to lungs
- Left atrium receives oxygenated blood from lungs
- Left ventricle pumps blood to body — thickest wall, highest pressure
- Double circulatory system blood passes through the heart twice per circuit — once to lungs, once to body
Blood vessels
Three types of blood vessel carry blood around the body, each adapted for its function.
Arteries = Away from heart. Veins = to heart (Valves). The pulmonary artery is an exception — it carries deoxygenated blood.
Blood vessels — Key Knowledge
- Arteries thick muscular walls, small lumen, carry blood away from the heart at high pressure
- Veins thinner walls, larger lumen, valves to prevent backflow, carry blood to the heart at low pressure
- Capillaries one cell thick walls, very narrow, allow exchange of substances with tissues
Blood components
Blood is a tissue made up of plasma, red blood cells, white blood cells, and platelets.
Red blood cells have no nucleus so there is more room for haemoglobin — maximising oxygen transport.
Blood components — Key Knowledge
- Plasma liquid that carries dissolved substances — glucose, amino acids, CO₂, urea, hormones
- Red blood cells contain haemoglobin to carry oxygen, no nucleus, biconcave disc shape for large surface area
- White blood cells fight infection — phagocytosis, antibody production, antitoxin production
- Platelets cell fragments that help blood clot at wound sites
Coronary heart disease and risk factors
Coronary heart disease occurs when fatty deposits build up in the coronary arteries, reducing blood flow to the heart muscle. Non-communicable diseases are linked to lifestyle risk factors.
Stents are a physical treatment; statins are a chemical treatment. Both have advantages and disadvantages.
Coronary heart disease and risk factors — Key Knowledge
- Coronary arteries supply heart muscle with oxygenated blood
- Fatty deposits narrow the arteries, reducing blood flow
- Stents wire mesh tubes inserted to hold arteries open
- Statins drugs that reduce blood cholesterol
- Risk factors for non-communicable diseases: diet high fat/sugar
- smoking, alcohol, lack of exercise
Cancer
Cancer is the result of uncontrolled cell division, forming a mass of cells called a tumour.
Benign tumours are not cancer — they can still be dangerous if they compress nearby organs, but they don't spread.
Cancer — Key Knowledge
- Benign tumour grows in one place, does not invade other tissues, not cancerous
- Malignant tumour can spread to other parts of the body through the blood — metastasis, cancerous
- Risk factors: smoking, obesity, UV exposure, viral infections, genetic factors
Plant tissues
Plants are made of specialised tissues, each adapted for a particular function.
The leaf is an organ — it contains epidermal, mesophyll, xylem, and phloem tissues all working together.
Plant tissues — Key Knowledge
- Epidermal tissue covers the outer surfaces of the plant
- Palisade mesophyll near the top of the leaf, packed with chloroplasts for photosynthesis
- Spongy mesophyll air spaces allow gas exchange within the leaf
- Xylem tissue transports water and mineral ions
- Phloem tissue transports dissolved sugars
Xylem
Xylem tissue transports water and mineral ions from the roots to the leaves and stems. The flow is one-way — upwards only.
Xylem also provides structural support — the lignin makes stems rigid.
Xylem — Key Knowledge
- Dead cells no living contents
- Hollow tubes continuous column of water
- Lignin strengthens the cell walls
- Passive transport driven by transpiration pull — no energy required from the plant
Phloem and translocation
Phloem tissue transports dissolved sugars (mainly sucrose) from the leaves to the rest of the plant. This process is called translocation.
Sugars move both up and down — from leaves (where they're made) to roots, fruits, and growing tips (where they're needed).
Phloem and translocation — Key Knowledge
- Translocation movement of dissolved sugars through phloem
- Living cells with sieve plates and companion cells
- Active process requires energy from respiration
Transpiration
Transpiration is the loss of water vapour from the surface of leaves through stomata. It creates a continuous transpiration stream that pulls water up through the xylem.
Transpiration is a consequence of gas exchange — stomata must open for CO₂ to enter for photosynthesis, but water is lost as a result.
Transpiration — Key Knowledge
- Stomata tiny pores on the leaf surface, controlled by guard cells
- Transpiration stream continuous movement of water from roots through xylem to leaves
- Factors affecting rate: temperature higher = faster
- humidity lower = faster
- wind speed higher = faster
- light intensity higher = faster — stomata open wider