Living Systems

MOE syllabus alignment

• Lower Secondary theme: Systems.
• Core Ideas / Practices lens: Structure / Function / Balance.
• Study focus: How body systems work together, including transport, respiration, nutrition, coordination and health.

Alignment note: Independently written and mapped to public MOE/SEAB syllabus structures. Not affiliated with MOE, SEAB or Cambridge.

Key terms

Organ systemDigestionAbsorptionEnzymeSubstrateActive siteDenaturationOptimum temperaturepHArteryVeinCapillaryRespirationGas exchangeHomeostasisStimulusResponseReceptorEffectorNeuronHormoneInsulinGlucagon

The digestive system

Overview of digestion

Digestion breaks large, insoluble food molecules into small, soluble molecules that can be absorbed into the bloodstream.

• Mouth — mechanical digestion (chewing); salivary amylase begins starch digestion.
• Stomach — pepsin (protease) digests proteins; hydrochloric acid provides acidic pH (~2) and kills bacteria.
• Small intestine — main site of chemical digestion and absorption. Lipase, protease and amylase from pancreas; bile from liver emulsifies fats.
• Large intestine — water absorption; faeces formed.

Adaptations for absorption in small intestine

• Very long (6–7 m) — increases surface area.
• Villi and microvilli on inner wall — further increase surface area enormously.
• Thin walls — short diffusion distance.
• Good blood supply — maintains concentration gradient for diffusion.

Enzymes

What enzymes are

Enzymes are biological catalysts — they speed up chemical reactions without being used up. They are made of protein and have a specific three-dimensional shape.

The active site and specificity

Each enzyme has an active site with a shape complementary to its specific substrate. Only the correct substrate fits (like a lock and key). This is why amylase digests starch but not protein.

Effect of temperature

• As temperature increases, enzyme activity increases — more kinetic energy means more enzyme-substrate collisions.
• At the optimum temperature (~37 °C in humans), activity is at maximum.
• Above optimum, the enzyme begins to denature — heat breaks bonds in the protein, changing the shape of the active site. The substrate no longer fits. This is permanent.
• At low temperatures, activity slows but enzymes are NOT denatured — warming up restores activity.

Effect of pH

• Each enzyme has an optimum pH. Extreme pH (too acidic or too alkaline) also denatures enzymes.
• Pepsin (stomach) works best at ~pH 2; amylase (saliva) at ~pH 7.

The circulatory system

Heart and blood vessels

• Arteries — carry blood AWAY from the heart; thick, muscular, elastic walls to withstand high pressure; carry oxygenated blood (except pulmonary artery).
• Veins — carry blood TOWARDS the heart; thinner walls, larger lumen; have valves to prevent backflow; carry deoxygenated blood (except pulmonary vein).
• Capillaries — one cell thick; site of gas, nutrient and waste exchange between blood and tissues.

Double circulation

Humans have two circulatory loops: pulmonary circulation (heart → lungs → heart) for oxygenation, and systemic circulation (heart → body → heart) for oxygen and nutrient delivery.

The respiratory system and gas exchange

Breathing vs respiration

• Breathing (ventilation) — the physical movement of air in and out of the lungs.
• Gas exchange — oxygen diffuses from air in alveoli into blood; CO₂ diffuses from blood into alveoli.
• Cellular respiration — chemical process in cells that releases energy from glucose.

Alveoli adaptations for gas exchange

• Large number → large total surface area.
• Walls one cell thick → short diffusion distance.
• Moist lining → gases dissolve for diffusion.
• Rich blood supply → maintains steep concentration gradient.

Word equations

Coordination — nervous system

Neuron types

• Sensory neuron — carries impulses from receptors to CNS (central nervous system).
• Motor neuron — carries impulses from CNS to effectors (muscles or glands).
• Relay neuron — connects sensory and motor neurons within the CNS.

The reflex arc

Reflex responses are rapid and involuntary — the signal does not need to travel to the brain for processing before the response occurs (though the brain becomes aware afterwards).

Coordination — endocrine (hormonal) system

• Hormones are chemical messengers produced by glands, released into the blood, and act on target organs.
• Slower than nervous system, but effects last longer.
• Insulin (produced by pancreas) — lowers blood glucose; stimulates cells to take up glucose and liver to store glucose as glycogen.
• Glucagon (produced by pancreas) — raises blood glucose; stimulates liver to break down glycogen into glucose.
• Adrenaline (produced by adrenal glands) — prepares body for "fight or flight"; increases heart rate, breathing rate, blood glucose.

Homeostasis — maintaining a stable internal environment

The body maintains stable conditions despite external changes. Key examples:

• Blood glucose — regulated by insulin and glucagon (negative feedback: rise → insulin released → glucose falls back to normal).
• Body temperature — sweating and vasodilation (cooling); shivering and vasoconstriction (warming).
• Water balance — kidneys adjust urine concentration; ADH hormone controls water reabsorption.

Health — diet, exercise and disease

• Balanced diet — carbohydrates (energy), proteins (growth and repair), fats (energy store, insulation), vitamins and minerals (metabolic functions), water, fibre.
• Exercise — increases heart rate and breathing rate to supply more O₂ to muscles; builds muscle; improves cardiovascular health.
• Pathogens — bacteria and viruses cause infectious disease. Immune system responds with white blood cells (phagocytes engulf pathogens; lymphocytes produce antibodies).
• Antibiotics — effective against bacterial infections only; do not work on viruses.

Past-year style question set

1. During exercise, breathing rate and heart rate increase. Explain why, referring to both the respiratory and circulatory systems.
2. A student places amylase solution in a water bath at 70 °C for 5 minutes, then tests it with starch solution. The starch is not digested. Explain why.
3. Name the blood vessel that: (a) carries blood from the heart to the lungs; (b) carries oxygenated blood from the lungs to the heart.
4. Explain why a reflex response is faster than a voluntary action.
5. Blood glucose rises after a meal. Describe the hormonal response that returns it to normal.

Answer points

1. Muscles respire faster, using more O₂ and producing more CO₂. Breathing rate increases to bring in more O₂ and remove CO₂. Heart rate increases so blood transports O₂ and glucose to muscles faster.
2. 70 °C is above the optimum temperature for amylase. The high temperature denatures the enzyme, permanently changing the shape of its active site. Starch can no longer bind and be digested.
3. (a) Pulmonary artery. (b) Pulmonary vein.
4. In a reflex arc, the signal travels through the spinal cord directly to the effector without waiting for processing by the brain, reducing response time.
5. The pancreas detects high blood glucose and secretes insulin. Insulin causes body cells to take up glucose and the liver to convert glucose to glycogen (glycogenesis). Blood glucose returns to the set point.

Must-know checklist

• Can describe the role of each digestive organ and its enzymes.
• Can explain enzyme specificity, optimum temperature, and denaturation.
• Can distinguish arteries, veins and capillaries by structure and function.
• Can list alveolar adaptations for gas exchange.
• Can trace a reflex arc from stimulus to response.
• Can compare nervous and hormonal coordination (speed, duration, specificity).
• Can explain how insulin and glucagon regulate blood glucose (negative feedback).
• Can explain why antibiotics are ineffective against viruses.