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

The second half of Section 2 in the Pearson Edexcel IGCSE Biology specification covers five interconnected topics that together explain how organisms obtain energy, exchange gases, transport substances, remove waste and coordinate their responses to the environment.

These five topics form some of the most commonly examined material in the 4BI1 course. If you are looking for edexcel igcse biology structure and functions in living organisms: respiration to co-ordination and response content, this article provides edexcel igcse biology revision notes on respiration, gas exchange, transport, excretion and coordination, this article is structured to match the specification point by point. Every concept connects to the others: respiration produces the ATP that fuels active transport, gas exchange delivers the oxygen that respiration needs, transport carries substances to where they are required, excretion removes the waste that metabolism produces, and coordination ties all of these systems together through nervous and hormonal control.

Respiration

Respiration is a chemical process that occurs in every living cell. It is not breathing. Breathing (ventilation) is the mechanical process that moves air in and out of the lungs. Respiration is the set of chemical reactions that break down glucose to release energy in the form of ATP. ATP provides energy for cellular processes such as muscle contraction, active transport, protein synthesis and cell division.

Aerobic respiration

Aerobic respiration requires oxygen. The word equation is:

glucose + oxygen → carbon dioxide + water (+ energy released)

The balanced chemical symbol equation is:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Anaerobic respiration

Anaerobic respiration occurs without oxygen and releases far less energy per glucose molecule.

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

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

Anaerobic respiration in yeast is the basis of bread-making and brewing, topics that connect to Section 5 (Use of biological resources).

Gas exchange

Gas exchange relies on diffusion: oxygen moves from regions of higher concentration to regions of lower concentration, and carbon dioxide moves in the opposite direction. This occurs both in leaves (plants) and in the lungs (humans).

Gas exchange in leaves

Stomata (small pores on the leaf surface, mainly the lower epidermis) allow carbon dioxide to diffuse in and oxygen to diffuse out during photosynthesis. Guard cells control the opening and closing of stomata. During the day, when photosynthesis exceeds respiration, there is a net uptake of CO₂ and a net release of O₂. At night, only respiration occurs, so there is a net uptake of O₂ and a net release of CO₂.

Gas exchange in the lungs

The human thorax contains the structures responsible for ventilation and gas exchange:

• Trachea: the windpipe, supported by C-shaped cartilage rings
• Bronchi: two branches of the trachea, one leading to each lung
• Bronchioles: smaller tubes that branch from the bronchi
• Alveoli: tiny air sacs at the ends of bronchioles where gas exchange occurs
• Intercostal muscles and diaphragm: work together to ventilate the lungs
• Pleural membranes: surround the lungs and reduce friction during breathing

Alveoli are adapted for gas exchange: they have a very large surface area (millions of alveoli in each lung), thin walls (one cell thick for a short diffusion distance), a moist lining to dissolve gases, and a rich blood supply to maintain a steep concentration gradient.

Ventilation works as follows. During inhalation, the intercostal muscles contract, pulling the ribs up and out, while the diaphragm contracts and flattens. This increases the volume of the thorax, decreases the pressure inside, and air rushes in. During exhalation, the reverse happens: intercostal muscles relax, ribs move down and in, the diaphragm relaxes and domes upward, volume decreases, pressure increases, and air is pushed out.

Smoking and health

The specification requires you to understand the biological consequences of smoking. Tar in cigarette smoke damages the cilia lining the airways, leading to a build-up of mucus, chronic coughing and increased risk of infection. Tar also contains carcinogens that can cause lung cancer. Carbon monoxide binds irreversibly to haemoglobin, reducing the blood's oxygen-carrying capacity. Nicotine increases heart rate and blood pressure. Smoking is a major risk factor for coronary heart disease, as it damages the lining of blood vessels and promotes the build-up of fatty deposits (atherosclerosis).

Transport

Simple unicellular organisms rely on diffusion alone because they have a large surface area to volume ratio and short diffusion distances. Multicellular organisms need a dedicated transport system because diffusion alone is too slow over larger distances.

Transport in plants

Plants have two types of transport tissue:

Root hair cells absorb water from the soil by osmosis. The root hair cell has a lower water concentration than the surrounding soil water, so water moves in across the partially permeable membrane.

Transpiration is the evaporation of water from the surface of a plant, mainly through the stomata. It creates a continuous pull that draws water up through the xylem. The rate of transpiration is affected by humidity (lower humidity increases the rate), wind speed (more wind increases evaporation), temperature (higher temperature increases evaporation) and light intensity (light causes stomata to open, increasing transpiration).

Transport in humans: blood

Blood has four components, each with a distinct function:

• Red blood cells: Transport oxygen. Biconcave disc shape maximises surface area. No nucleus to carry more haemoglobin. Haemoglobin binds oxygen in the lungs and releases it in the tissues.
• White blood cells: Part of the immune system. Phagocytes engulf and digest pathogens. Lymphocytes produce antibodies specific to particular pathogens.
• Platelets: Cell fragments involved in blood clotting, which seals wounds to prevent blood loss and block entry of microorganisms.
• Plasma: The liquid component. Transports carbon dioxide (as hydrogen carbonate ions), digested food (glucose, amino acids), urea, hormones and heat energy.

Vaccination works by introducing a dead or weakened form of a pathogen, stimulating lymphocytes to produce antibodies and, crucially, memory cells. If the real pathogen enters the body later, memory cells enable a faster, larger antibody response, destroying the pathogen before it causes disease.

The heart and circulatory system

The heart has four chambers: two atria (upper) and two ventricles (lower). The right side pumps deoxygenated blood to the lungs (pulmonary circulation), and the left side pumps oxygenated blood to the body (systemic circulation). The left ventricle has a thicker muscular wall because it needs to generate higher pressure to push blood around the entire body.

During exercise, the heart rate increases to deliver more oxygen and glucose to muscles for respiration. Adrenaline also increases heart rate in response to stress or danger.

Risk factors for coronary heart disease include smoking, high blood pressure, high cholesterol levels, lack of exercise and a diet high in saturated fats.

Excretion

Excretion is the removal of metabolic waste products. The main excretory organs are the lungs (remove CO₂ from respiration), the kidneys (remove urea, excess water and ions) and the skin (removes water, ions and small amounts of urea in sweat).

The urinary system consists of two kidneys, two ureters, a bladder and the urethra. Each kidney contains approximately one million nephrons, the functional units responsible for filtering blood and producing urine.

A nephron works through several stages: ultrafiltration in the Bowman's capsule (blood is filtered under high pressure; small molecules such as water, glucose, urea and ions pass into the capsule, while large molecules such as proteins and blood cells remain in the blood), selective reabsorption of glucose at the proximal convoluted tubule (all glucose is reabsorbed back into the blood by active transport), and water reabsorption in the collecting duct (controlled by ADH).

ADH (anti-diuretic hormone) regulates the water content of the blood. When blood water concentration is low, the pituitary gland releases more ADH, making the collecting duct walls more permeable to water. More water is reabsorbed, producing a smaller volume of more concentrated urine. When blood water concentration is high, less ADH is released, and more dilute urine is produced. This is an example of homeostasis, which is the maintenance of a constant internal environment.

Co-ordination and response

Organisms respond to changes in their environment through coordination systems. The edexcel igcse biology notes for this topic span both nervous and hormonal (endocrine) control, along with plant responses.

The nervous system

A coordinated response requires three components: a stimulus (a change in the environment), a receptor (detects the stimulus) and an effector (produces the response, either a muscle or a gland).

The central nervous system (CNS) consists of the brain and spinal cord. It is connected to the rest of the body by nerves. Stimulation of receptors in sense organs sends electrical impulses along sensory neurones to the CNS. The CNS processes the information and sends impulses along motor neurones to effectors.

A reflex arc is a rapid, automatic, protective response. The classic example is withdrawing a finger from a hot object: receptor in the skin detects heat, sensory neurone carries the impulse to the spinal cord, relay neurone in the spinal cord connects to a motor neurone, the motor neurone carries the impulse to a muscle (effector), which contracts to pull the finger away. At synapses (gaps between neurones), neurotransmitters are released to carry the signal across the gap.

The eye

The eye focuses light onto the retina. For distant objects, the ciliary muscles relax, the suspensory ligaments pull tight, and the lens becomes thinner and less curved. For near objects, the ciliary muscles contract, the suspensory ligaments slacken, and the lens becomes thicker and more curved. The iris controls the amount of light entering the eye: in bright light, circular muscles contract and the pupil gets smaller; in dim light, radial muscles contract and the pupil gets larger.

Temperature regulation

The skin plays a central role in thermoregulation. When the body is too hot, sweat glands produce sweat, which evaporates and cools the skin. Blood vessels near the skin surface dilate (vasodilation), increasing blood flow to the surface and allowing more heat to be lost by radiation. When the body is too cold, sweat production decreases, blood vessels constrict (vasoconstriction), reducing blood flow to the surface and conserving heat. Shivering (rapid muscle contractions) also generates heat.

Hormones

The specification lists several hormones and their roles. These are commonly tested as edexcel igcse biology practice questions:

Plant responses

Plants respond to stimuli through tropisms. Phototropism is growth towards or away from light. Geotropism (gravitropism) is growth towards or away from gravity. Stems are positively phototropic (grow towards light) and negatively geotropic (grow away from gravity). Roots are negatively phototropic and positively geotropic.

Auxin is the plant hormone responsible for phototropism in stems. When light shines from one side, auxin accumulates on the shaded side, causing cells there to elongate more. This differential growth bends the stem towards the light.

Self-check questions

Test your understanding of the igcse 4bi1 structure and functions in living organisms: respiration to co-ordination and response material covered in the edexcel specification. These are the kinds of questions that appear on the exam for this section.

1. Write the balanced symbol equation for aerobic respiration.
2. State the word equation for anaerobic respiration in yeast.
3. Name three features of alveoli that make them efficient at gas exchange.
4. Explain why the left ventricle wall is thicker than the right ventricle wall.
5. Describe the role of ADH in regulating blood water content.
6. Outline the pathway of a reflex arc from stimulus to response.
7. Explain how auxin causes a plant stem to grow towards light.
8. Compare nervous and hormonal communication in terms of speed, duration and method of transmission.

The structure and functions in living organisms: respiration to co-ordination and response edexcel igcse topics are tested extensively on both Paper 1 (2 hours, 110 marks) and Paper 2 (1 hour 15 minutes, 70 marks). A thorough understanding of these systems, and how they interconnect, is the difference between a competent answer and one that earns full marks. Study these edexcel igcse biology explained concepts until you can reproduce the key definitions, equations and mechanisms from memory, then test yourself with past paper questions to confirm that the knowledge transfers to exam conditions.