Pearson Edexcel International GCSE Science Single Award 2: Biology: Structures and functions in living organisms

Topic 2 explains how specialised structures make life processes possible, from organelles inside cells to organ systems in the human body. These notes follow only the Science Single Award points in Pearson 4SS0 Issue 4. They do not import the additional plant transport, diet, excretion, coordination or homeostasis content found in Double Award or separate Biology.

Practical methods named in the specification are introduced here through their biological purpose. Planning, measurement, graphing and evaluation are developed in the separate Single Award practical-skills hub.

Organisation and cell structure

Living systems form a hierarchy. Organelles are specialised structures inside a cell. Similar cells can form a tissue, several tissues can form an organ, and organs can cooperate in an organ system. For example, cardiac muscle cells contribute to muscle tissue, several tissues form the heart, and the heart belongs to the circulatory system.

Animal and plant cells share a nucleus, cytoplasm, cell membrane and mitochondria. The nucleus contains genetic material and controls cell activities. Cytoplasm is where many chemical reactions occur. The cell membrane controls movement into and out of the cell. Mitochondria are the site of aerobic respiration.

Plant cells also have a cellulose cell wall, chloroplasts and a permanent vacuole. The wall supports the cell, chloroplasts contain chlorophyll for photosynthesis, and the vacuole contains cell sap and helps keep the cell firm. A plant cell does not necessarily contain many chloroplasts: root cells, for example, normally receive no light.

Biological molecules and enzymes

Carbohydrates and lipids contain carbon, hydrogen and oxygen. Proteins contain these elements and nitrogen. Starch and glycogen are large carbohydrate molecules assembled from simple sugars. Proteins are assembled from amino acids. Lipids are formed from fatty acids and glycerol.

Food tests connect these molecule groups to observable evidence:

• iodine solution changes from orange-brown to blue-black when starch is present;
• Benedict's solution changes from blue through green, yellow or orange to brick-red when a reducing sugar is heated with it;
• the biuret test changes from blue to lilac or purple when protein is present;
• the ethanol emulsion test produces a cloudy white emulsion when lipid is present.

An enzyme is a biological catalyst. Its active site has a shape that allows a particular substrate to bind. At low temperatures, molecules move slowly and successful collisions are less frequent. Increasing temperature raises collision frequency until the optimum is reached. Above the optimum, bonds maintaining the enzyme's shape are disrupted, the active site changes and the enzyme denatures. An unsuitable pH can also change the active site. A fair investigation changes temperature or pH while controlling enzyme concentration, substrate concentration, volume and reaction time.