Understanding Biology: Structure and functions in living organisms: Level of organisation to Movement of substances into and out of cells: Edexcel IGCSE Science Single Award Explained

Cells are the building blocks of every living organism. This part of the Edexcel IGCSE (4SS0) Science Single Award specification takes you from the smallest structures inside a cell all the way to how substances move in and out of them.

If the previous section on the nature and variety of organisms was the vocabulary, this is where the grammar starts. You are learning how cells are built, what the molecules inside them do, and how substances cross cell membranes. These four topics form the opening stretch of the largest section in the biology component, and they underpin almost everything you will study afterwards.

This article covers the edexcel igcse science single award biology: structure and functions in living organisms: level of organisation to movement of substances into and out of cells content. It is written as a set of edexcel igcse science single award revision notes you can work through topic by topic, with common mistakes flagged and self-check questions at the end.

Level of organisation

The specification asks you to describe the levels of organisation in organisms. It is a simple hierarchy, but getting the order right matters.

1. Organelles are structures within a cell that carry out specific functions (for example, mitochondria release energy through respiration).
2. Cells are the basic unit of life. Each cell contains organelles.
3. Tissues are groups of similar cells working together to perform a shared function (for example, muscle tissue contracts to produce movement).
4. Organs are structures made from several different tissues working together (for example, the heart contains muscle tissue, nerve tissue and connective tissue).
5. Systems are groups of organs working together to carry out a major function (for example, the circulatory system moves blood around the body).

Think of it like building a house: bricks (organelles) make walls (cells), walls make rooms (tissues), rooms make floors (organs), and the whole building is the system. The analogy is rough, but it helps lock the order into memory.

Cell structure

You need to know the structures present in cells and what each one does. The table below covers every structure the specification lists.

The most common exam mistake here is forgetting that both plant and animal cells have mitochondria. Students often associate mitochondria only with animal cells because plants "make their own food." But plants respire too. Every living cell that carries out aerobic respiration needs mitochondria.

Biological molecules

This topic introduces the large molecules that make up living organisms and the enzymes that control reactions involving them.

The three main groups

• Carbohydrates contain carbon, hydrogen and oxygen. Large carbohydrates (starch, glycogen) are built from smaller units called simple sugars (like glucose). Starch is the storage carbohydrate in plants; glycogen is the storage carbohydrate in animals.
• Proteins contain carbon, hydrogen, oxygen and nitrogen (and sometimes sulfur). They are built from smaller units called amino acids. Proteins have many roles: enzymes, structural components, antibodies.
• Lipids (fats and oils) contain carbon, hydrogen and oxygen. They are built from fatty acids and glycerol. Lipids store energy, insulate the body and form part of cell membranes.

Food tests

The specification requires you to investigate food samples for glucose, starch, protein and fat. Examiners regularly test whether you know the correct reagent and the expected colour change.

Enzymes

Enzymes are biological catalysts. They speed up metabolic reactions without being used up. Each enzyme has an active site with a specific shape that only fits particular substrate molecules, like a lock fits a key.

The lock-and-key model is the standard way to explain enzyme specificity at this level. The substrate fits into the active site the way a key fits a particular lock. If the shape of the active site changes, the substrate cannot bind, and the reaction does not happen. Two factors the specification focuses on are temperature and pH.

• Temperature: As temperature rises, enzyme activity increases because molecules move faster and collide more often with the active site. Above a certain temperature (the optimum), the enzyme begins to denature. The active site changes shape, the substrate no longer fits, and the rate of reaction drops. This is not the same as the enzyme being "killed" (enzymes are not alive).
• pH: Each enzyme works best at a specific pH. Moving away from this optimum alters the shape of the active site and reduces the rate of reaction. Pepsin, for example, works best in acidic conditions (around pH 2), while pancreatic enzymes work best in slightly alkaline conditions.

Movement of substances into and out of cells

Three transport mechanisms appear in the specification: diffusion, osmosis and active transport. Being able to define each one precisely and give an example is essential.

• Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration. It does not require energy. Example: oxygen diffusing from the alveoli into the blood in the lungs.
• Osmosis is the net movement of water molecules from a dilute solution (high water concentration) to a more concentrated solution (lower water concentration) through a partially permeable membrane. It does not require energy. Example: water moving into root hair cells from the soil.
• Active transport is the movement of substances against the concentration gradient (from low to high concentration). It requires energy from respiration. Example: mineral ions being absorbed into root hair cells from the soil, where the concentration inside the cell is already higher than in the soil water.

The factors that affect the rate of these processes are also examinable. A larger surface area to volume ratio increases the rate of diffusion. A shorter distance speeds it up. A higher temperature increases the kinetic energy of particles, so they move faster. A steeper concentration gradient drives faster net movement. Understanding these factors and being able to apply them to unfamiliar scenarios is a skill the exam rewards heavily.

Self-check questions

1. Put the following in order from smallest to largest: organ, organelle, tissue, cell, system.
2. Name one structure found in plant cells but not in animal cells, and state its function.
3. Which food test uses Benedict's solution, and what does a positive result look like?
4. Explain why an enzyme stops working at very high temperatures.
5. A student places a piece of potato into a concentrated sugar solution. After 30 minutes, the potato feels soft and floppy. Explain why.

Answers:

1. Organelle, cell, tissue, organ, system.
2. Cell wall, made of cellulose, which provides structural support and prevents the cell from bursting when water enters by osmosis. (Chloroplasts or vacuole are also acceptable.)
3. Benedict's solution tests for reducing sugars such as glucose. When heated with a reducing sugar, the solution changes from blue to orange-red.
4. At very high temperatures, the bonds holding the enzyme's three-dimensional shape break. The active site changes shape (the enzyme is denatured), so the substrate can no longer fit into it. The rate of reaction falls because fewer enzyme-substrate complexes can form.
5. Water has moved out of the potato cells by osmosis. The sugar solution outside the cells has a lower water concentration than the cytoplasm inside the cells, so water moves out through the partially permeable cell membrane. The cells lose turgor pressure and the potato becomes flaccid.

The biology: structure and functions in living organisms: level of organisation to movement of substances into and out of cells edexcel igcse content is foundational. In the igcse 4ss0 biology: structure and functions in living organisms: level of organisation to movement of substances into and out of cells section, every concept you master here, from cell structure to osmosis, will reappear when you study nutrition, gas exchange, transport and ecology. The edexcel igcse science single award notes on the Green Bridge CBT platform walk through each learning objective individually, and the edexcel igcse science single award practice questions let you test yourself under exam conditions. The edexcel igcse science single award explained at this level is about building secure understanding one concept at a time.