Cambridge IGCSE Marine Science 4: Nutrients and energy

Topic 4 follows matter and energy through marine organisms and ecosystems. Cambridge 0697 requires nutrient functions and cycling, respiration, photosynthesis, productivity, feeding relationships and ecological pyramids. Food tests, combustion and light-intensity methods remain in the separate practical hub.

Nutrients and essential elements

Nutrients are substances required for growth, repair or chemical energy. They include gases such as carbon dioxide, dissolved salts and organic compounds.

• carbohydrates, including glucose and starch, supply energy;
• lipids provide energy, insulation and lipid-soluble vitamins;
• proteins support growth and tissue repair and can supply energy;
• vitamins and mineral salts are micronutrients needed in smaller amounts.

Nutrients also provide essential elements. Nitrogen is required to make proteins, carbon to make organic compounds, magnesium to make chlorophyll, calcium to make bones, shells and coral skeletons, and iron to make haemoglobin that carries oxygen in vertebrates and some invertebrates.

Nutrient cycling

Producers take up dissolved substances and incorporate elements into organic material. Feeding passes matter through food chains. Death, excretion and waste return organic material to the environment. Decomposer bacteria break it down, releasing inorganic nutrients that producers can reuse.

Some dead material sinks through the water column as marine snow. Nutrients may therefore leave surface waters temporarily. Upwelling can return cold, nutrient-rich water to the surface, supporting producer growth. Matter cycles, but energy does not cycle in the same way.

Respiration

Respiration is a set of chemical reactions in cells that breaks down nutrient molecules and releases usable energy. Every living cell respires. Aerobic respiration is summarised by:

$$\text{oxygen} + \text{glucose} \rightarrow \text{carbon dioxide} + \text{water}$$