Understanding Biology: Structure and functions in living organisms: Respiration to Co-ordination and response: Edexcel IGCSE Science Double Award Explained

These five topics form the second half of the largest biology section, covering how organisms obtain energy, exchange gases, transport materials, remove waste and respond to their environment.

The Edexcel IGCSE Science Double Award (4SD0) biology paper devotes considerable space to structure and function. This article addresses the second group of topics within that section: respiration, gas exchange, transport, excretion, and co-ordination and response. These are examined with considerable regularity and carry substantial marks. The edexcel igcse science double award revision notes below cover every specification point with the precision required for full marks.

Respiration

Respiration is the process by which organisms release energy from food molecules, producing ATP. ATP provides energy for cells to carry out their functions. There are two types of respiration, and the igcse specification requires knowledge of both.

Aerobic respiration

Word equation: glucose + oxygen -> carbon dioxide + water (+ energy)

Balanced symbol equation: C₆H₁₂O₆ + 6O₂ -> 6CO₂ + 6H₂O (+ energy released)

Aerobic respiration takes place in mitochondria and requires oxygen. It releases more energy per glucose molecule than anaerobic respiration.

Anaerobic respiration

In animals: glucose -> lactic acid (+ small amount of energy)

In plants and yeast: glucose -> ethanol + carbon dioxide (+ small amount of energy)

Anaerobic respiration occurs when oxygen is insufficient. It releases less energy because glucose is only partially broken down.

Feature	Aerobic	Anaerobic
Oxygen required	Yes	No
Energy released	Large amount	Small amount
Products	CO₂ and water	Lactic acid (animals) or ethanol + CO₂ (plants/yeast)
Glucose breakdown	Complete	Incomplete

Gas exchange

The structure of the thorax and the mechanism of ventilation are core examination topics. The specification requires detailed knowledge of the following structures: ribs, intercostal muscles, diaphragm, trachea, bronchi, bronchioles, alveoli and pleural membranes.

Ventilation mechanism

Inhalation: The intercostal muscles contract, pulling the ribs upward and outward. The diaphragm contracts and flattens. The volume of the thorax increases, pressure decreases, and air is drawn into the lungs.
Exhalation: The intercostal muscles relax, the ribs move downward and inward. The diaphragm relaxes and domes upward. The volume decreases, pressure increases, and air is forced out.

Alveoli and gas exchange

Alveoli are adapted for efficient gas exchange by diffusion. They have a large surface area, thin walls (one cell thick for a short diffusion distance), a rich blood supply to maintain a steep concentration gradient, and a moist lining to dissolve gases.

Smoking and health

The specification requires understanding of the biological consequences of smoking. Tar damages cilia in the airways, leading to mucus build-up, chronic bronchitis and increased risk of lung cancer. Carbon monoxide in smoke binds to haemoglobin, reducing the blood's capacity to carry oxygen. Nicotine raises blood pressure, increasing the risk of coronary heart disease.

Transport

This topic covers transport in both plants and animals. In the edexcel igcse science double award notes for the biology paper, transport is one of the most detail-intensive areas.

Why multicellular organisms need a transport system

Simple unicellular organisms can rely on diffusion because they have a large surface area to volume ratio and short diffusion distances. Multicellular organisms are too large for diffusion alone, so they require specialised transport systems.

Plant transport

Xylem transports water and mineral ions from the roots to the rest of the plant. Xylem vessels are dead, hollow tubes reinforced with lignin.
Phloem transports sucrose and amino acids from the leaves (where they are made) to other parts of the plant. This is called translocation.

Blood composition

Component	Function
Red blood cells	Contain haemoglobin; transport oxygen. Biconcave shape increases surface area. No nucleus (more room for haemoglobin).
White blood cells	Part of the immune system; defend against pathogens by phagocytosis and antibody production.
Platelets	Cell fragments involved in blood clotting at wound sites.
Plasma	Liquid component; transports CO₂, digested food, urea, hormones and heat energy.

The heart

The heart has four chambers: two atria (receive blood) and two ventricles (pump blood out). The right side pumps deoxygenated blood to the lungs; the left side pumps oxygenated blood to the body. The left ventricle has a thicker muscular wall because it must pump blood at higher pressure around the entire body. Heart rate increases during exercise and under the influence of adrenaline.

Blood vessels

Arteries carry blood away from the heart. They have thick, muscular, elastic walls to withstand high pressure.
Veins carry blood back to the heart. They have thinner walls, larger lumens, and valves to prevent backflow.
Capillaries connect arteries to veins. Their walls are one cell thick, allowing exchange of substances between blood and tissues.

Specific factors that increase the risk of coronary heart disease include smoking, high-fat diet, lack of exercise and stress.

Excretion

Excretion is the removal of metabolic waste products from the body. The edexcel igcse science double award explained at this level focuses on three organs of excretion:

Lungs: excrete carbon dioxide (from respiration) and water vapour.
Kidneys: excrete urea (from the breakdown of excess amino acids in the liver) in urine, along with excess water and salts.
Skin: excretes water and salts in sweat.

Carbon dioxide and oxygen are also lost from the stomata of leaves as waste products of metabolism (respiration produces CO₂; photosynthesis produces O₂).

Co-ordination and response

This topic covers how organisms detect and respond to changes in their environment, using both the nervous system and hormones.

Homeostasis

Homeostasis is the maintenance of a constant internal environment. A co-ordinated response requires a stimulus, a receptor (detects the stimulus) and an effector (carries out the response).

The nervous system

The central nervous system (CNS) consists of the brain and spinal cord. It is linked to sense organs by nerves. Stimulation of receptors sends electrical impulses along nerves to and from the CNS. Neurotransmitters carry signals across synapses (gaps between neurones).

A reflex arc is a rapid, automatic response: receptor -> sensory neurone -> relay neurone (in the CNS) -> motor neurone -> effector (muscle or gland).

The eye

The eye focuses light on the retina. The ciliary muscles and suspensory ligaments adjust the shape of the lens for near and distant objects. The iris controls the amount of light entering the eye by adjusting the size of the pupil.

Temperature regulation

The skin plays a role in temperature regulation through sweating (evaporation cools the body), vasoconstriction (blood vessels near the surface narrow to reduce heat loss) and vasodilation (blood vessels near the surface widen to increase heat loss).

Hormones

The specification requires knowledge of five hormones:

Adrenaline: prepares the body for "fight or flight," increases heart rate.
Insulin: lowers blood glucose levels by stimulating cells to absorb glucose.
Testosterone: controls development of male secondary sexual characteristics.
Progesterone: maintains the uterus lining during pregnancy and the menstrual cycle.
Oestrogen: controls development of female secondary sexual characteristics; involved in the menstrual cycle.

Plant responses

Plants respond to stimuli through tropisms. Roots show positive geotropism (grow towards gravity) and negative phototropism. Stems show negative geotropism and positive phototropism (grow towards light). Auxin is a plant hormone that controls the phototropic response by accumulating on the shaded side of the stem, causing cells there to elongate more, bending the stem towards light.

Self-check questions

Write the word equation for anaerobic respiration in yeast.
Describe two adaptations of alveoli for efficient gas exchange.
State the function of xylem and phloem in plant transport.
Explain why the left ventricle wall is thicker than the right ventricle wall.
Name the three organs of excretion and state one waste product each removes.
Describe the pathway of a reflex arc from stimulus to response.
Explain how auxin causes a stem to bend towards light.
State the roles of insulin and adrenaline.

Self-check questions

State two differences between the structure of a plant cell and an animal cell.
Describe the process of osmosis and explain why it is important in living organisms.
Explain the difference between an exothermic and an endothermic reaction, giving one example of each.
A car accelerates from rest to 20 m/s in 5 seconds. Calculate its acceleration and the resultant force if the car has a mass of 1200 kg.
Describe the structure of an atom, naming each subatomic particle and stating its relative charge and mass.

Linking the three sciences

One of the distinctive features of the Edexcel IGCSE Science Double Award is the way the three disciplines connect. Energy, for example, runs through all three: chemical energy in bond-breaking and bond-making, kinetic energy in moving objects, and metabolic energy in respiration. When you revise a topic in one science, look for the thread that connects it to the others. A question on photosynthesis might ask about the light energy absorbed, which links to the physics of the electromagnetic spectrum. A question on rates of reaction might reference enzymes, which links to biological catalysts. These cross-disciplinary connections are not accidental; the specification is designed to reward students who can think across boundaries.

Examiners value precise scientific language. "The particles move faster" is weaker than "the particles have greater kinetic energy, so a higher proportion exceed the activation energy on collision." Each additional technical term, used correctly, signals understanding rather than guesswork, and that distinction is what separates a grade 5 answer from a grade 7.

Revision tip: After finishing a topic in one science, write a single sentence connecting it to each of the other two. If you cannot, that gap is worth revisiting. The strongest Double Award candidates are the ones who see the three sciences as one interconnected system rather than three separate subjects that happen to share an exam.

Self-check questions

Describe the structure of an animal cell and name three organelles found in both plant and animal cells.
Balance this equation and identify the type of reaction: Mg + HCl → MgCl₂ + H₂
A force of 50 N is applied to an object of mass 10 kg. Calculate the acceleration and state the unit.
Explain why the rate of photosynthesis increases with light intensity up to a certain point, and what happens beyond that point.
Describe one similarity and one difference between the nervous system and the endocrine system as methods of coordination in humans.

This material forms the second half of the most content-heavy section in the biology paper. For edexcel igcse science double award practice questions, past papers consistently include extended-answer questions on the heart, gas exchange and the nervous system. The biology: structure and functions in living organisms: respiration to co-ordination and response edexcel igcse section is substantial, but methodical revision of each topic in sequence will ensure comprehensive coverage. The igcse 4sd0 biology: structure and functions in living organisms: respiration to co-ordination and response content demands thorough engagement rather than surface-level familiarity.