Biology IGCSE Revision Guide

For the 2026/2027 Syllabus

What is movement in living organisms?

Movement is an action by an organism or part of an organism that causes a change of position or place.

What is respiration?

Respiration is the chemical reactions in cells that break down nutrient molecules and release energy for metabolism.

What is sensitivity?

Sensitivity is the ability of an organism to detect and respond to changes in the internal or external environment.

What is growth?

Growth is a permanent increase in size and dry mass of an organism.

What is reproduction?

Reproduction is the process by which organisms produce more of the same kind.

What is excretion?

Excretion is the removal of waste products of metabolism and substances in excess of requirements.

What is nutrition?

Nutrition is the intake of materials for energy, growth, and development.

What is a species?

A species is a group of organisms that can reproduce to produce fertile offspring.

What is the binomial system of naming species?

It is an internationally agreed system where each species is given a two-part scientific name showing its genus and species.

What is the purpose of classification systems?

Classification systems aim to reflect evolutionary relationships between organisms.

How are DNA base sequences used in classification?

Organisms with more similar DNA base sequences are more closely related and share a more recent common ancestor.

What are the five kingdoms of organisms?

Animal, Plant, Fungus, Prokaryote, and Protoctist.

What are the main groups of vertebrates?

Mammals, birds, reptiles, amphibians, and fish.

What are the main groups of arthropods?

Myriapods, insects, arachnids, and crustaceans.

What are the features of viruses?

Viruses have a protein coat and genetic material (DNA or RNA).

What are the key features of insects?

Insects have 3 body parts (head, thorax, abdomen), 6 legs, 1 pair of antennae, and usually wings.

What are the key features of arachnids?

Arachnids have 2 body parts (cephalothorax and abdomen), 8 legs, and no antennae or wings.

What are the key features of crustaceans?

Crustaceans have a hard exoskeleton, 2 pairs of antennae, and usually more than 4 pairs of legs.

What are the key features of myriapods?

Myriapods have many body segments, each with 1 or 2 pairs of legs, and 1 pair of antennae.

How do insects differ from arachnids?

Insects have 6 legs and 3 body parts; arachnids have 8 legs and 2 body parts.

How do crustaceans differ from insects?

Crustaceans have more legs and 2 pairs of antennae; insects have 6 legs and 1 pair of antennae. Crustaceans also have an exosceleton

What are examples of each arthropod group?

Insects: ants, butterflies

Arachnids: spiders, scorpions

Crustaceans: crabs, lobsters

Myriapods: centipedes, millipedes

What structures are found in both plant and animal cells?

Cell membrane, nucleus, cytoplasm, ribosomes, mitochondria.

What structures are found only in plant cells?

Cell wall, chloroplasts, large central vacuole.

What is the function of the nucleus?

The nucleus controls cell activities and contains genetic material (DNA).

What is the function of chloroplasts?

Chloroplasts carry out photosynthesis and contain chlorophyll.

What is the function of mitochondria?

Mitochondria are the site of aerobic respiration, releasing energy for the cell.

What structures are found in a bacterial cell?

Cell wall, cell membrane, cytoplasm, ribosomes, circular DNA, plasmids.

What is the function of plasmids in bacterial cells?

Plasmids are small loops of DNA that carry extra genetic information, often including antibiotic resistance.

What is the function of ciliated cells?

Ciliated cells move mucus out of the trachea and bronchi using tiny hair-like structures.

What is the function of root hair cells?

Root hair cells absorb water and minerals from the soil.

What is the function of palisade mesophyll cells?

Palisade cells carry out photosynthesis in leaves.

What is the function of neurones?

Neurones conduct electrical impulses throughout the body.

What is the function of red blood cells?

Red blood cells transport oxygen using haemoglobin.

What is the function of sperm and egg cells?

Sperm and egg cells are gametes used in reproduction.

What is the definition of a tissue?

A tissue is a group of similar cells working together to perform a specific function.

What is the formula for magnification?

Magnification = Image size ÷ Actual size

(I AM Formula)

How many micrometres (μm) are in 1 millimetre (mm)?

1 mm = 1000 μm

What is diffusion?

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, due to their random movement.

What provides the energy for diffusion?

The kinetic energy of randomly moving molecules and ions.

How do substances move in and out of cells?

Some substances move by diffusion through the cell membrane.

Why is diffusion important in living organisms?

It allows gases like oxygen and carbon dioxide, and solutes like glucose, to move where needed for respiration, photosynthesis, and other processes.

What factors affect the rate of diffusion?

Surface area, temperature, concentration gradient, and distance.

What is the role of water as a solvent in organisms?

Water dissolves substances for digestion, excretion, and transport in the body.

What is osmosis?

Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane.

How does water move into and out of cells?

Water moves by osmosis through the PARTIALLY PERMEABLE cell membrane.

What supports plant cells?

The pressure of water inside the cells pressing outward on the cell wall (turgor pressure).

What happens to plant cells in different solutions?

In dilute solution: cells become turgid

In concentrated solution: cells become flaccid or undergo plasmolysis

What is active transport?

Active transport is the movement of particles from a region of lower concentration to a region of higher concentration, against the concentration gradient, using energy from respiration.

Why is active transport important?

It allows cells to absorb ions and molecules even when they are in lower concentration outside the cell, such as ion uptake by root hair cells.

What helps active transport happen?

Protein carriers in the cell membrane move molecules or ions across using energy.

Where can we see diffusion in animal cells?

Small intestine: digested food products are diffused from the lumen to the blood

Lungs: gas exchange (CO2 and O2)

Where can we see diffusion in plant cells?

Leaves:

• O2 is diffused from the air spaces between mesophyll cells to mitochondria

• CO2 is diffused from the air spaces between mesophyll cells to the chloroplasts

• Water vapour is diffused from the alveolar spaces to the blood

What do you call an animal cell that has lost water due to osmosis and has shrunk?

It became crenated

What do you call an animal cell that has gained water due to osmosis and has burst?

It became lysed

Hyptonic solution

A solution that is dilute (has a large water potential). Water enters the cell by osmosis -> cell swells (plants), cell bursts/becomes lysed (animals)

Hypertonic solution

A solution that is concentrated (has a low water potential). Water leaves cells by osmosis -> cell shrinks/plasmolyses (plants)

Isotonic solution

Same solution concentration, same water potential. No movement of water. Flaccid: cell stays the same

Suggest why eating too much salt can lead to high blood pressure

Eating too much salt increases the amount of sodium in the blood.

This causes the body to retain more water to dilute the sodium.

The extra water increases the volume of blood in the circulatory system.

Higher blood volume puts more pressure on the walls of the arteries.

This leads to an increase in blood pressure.

Why do we use fresh fruits/vegetables while experimenting osmosis?

This is because osmosis only happens across a PARTIALLY PERMEABLE membrane. When boiling the plant, the membrane will be destroyed, thus, making it unable to experiment osmosis

Important note: is ink counted as water or as non-water while experimenting osmosi?

Ink is counted as non-water in osmosis experiments.

How to calculate breathing rate

Breathing rate = 60÷time taken for one breath

Where in the blood is most carbon dioxide transported?

Blood plasma

Describe the effect of increased carbon dioxide concentration on blood pH

pH decreases: becomes more acidid

Because:

CO2 reacts with H2O to form H+ ions (which are acidic)

Why does carbon dioxide diffusion increase during exercise?

During exercise, you will have more aerobic respiration occurring. This means that more carbon dioxide will be made. The huge amount of carbon dioxide in your body will cause a steeper concentration gradient (which means a faster rate of diffusion)

What are the three main chemicals of life?

Carbohydrates, Lipids, and Proteins

What elements make up carbohydrates?

Carbon, hydrogen, and oxygen.

What elements make up fats and oils? (lipids)

Carbon, hydrogen, and oxygen.

What elements make up proteins?

Carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur.

What are starch, glycogen, and cellulose made from?

They are made from glucose molecules.

What are proteins made from?

Proteins are made from amino acids.

What are fats and oils made from?

Fats and oils are made from fatty acids and glycerol. (1 glycerol and 3 fatty acid chains)

What is the iodine test used for?

To test for starch; a positive result turns iodine solution from brown to blue-black.

What is the Benedict's test used for?

To test for reducing sugars; a positive result turns Benedict's solution from blue to orange/red after heating.

What is the biuret test used for?

To test for proteins; a positive result turns biuret solution from blue to purple.

What is the ethanol emulsion test used for?

To test for fats and oils; a positive result forms a white emulsion.

What is the DCPIP test used for?

To test for vitamin C; a positive result causes DCPIP to lose its blue color.

What is the structure of a DNA molecule?

DNA has two strands coiled into a double helix, with base pairs holding the strands together.

How do DNA bases pair up?

Adenine pairs with thymine (A-T), and cytosine pairs with guanine (C-G).

Name the steps needed for testing for glucose

Add Benedict's solution into sample solution in test tube

Heat at 60 - 70 ° in water bath for 5 minutes

Take test tube out of water bath and observe the colour

A positive test will show a colour change from blue to orange or brick red

Name the steps needed for testing for starch

Add drops of iodine solution to the food sample

A positive test will show a colour change from orange-brown to blue-black