5.3.1

# Biomass

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## Plant Biomass

Plants are central to all ecosystems because they produce biomass through photosynthesis.

### Producing biomass

• Green plants are called producers because they use light energy to produce organic molecules (e.g. carbohydrates).
• Organic molecules produced by the plant are used in two ways:
• As respiratory substrates.
• To make other biological molecules (carbohydrates, proteins and lipids). These biological molecules form new biomass.

### Biomass in the food chain

• Biomass is the chemical energy store for an organism.
• Energy stored within biomass can be passed along the food chain to different trophic levels.

### Food chains

• Food chains show how energy flows within a biological community.
• Producers (plants and algae) are at the base of all food chains. They produce biomass.
• Consumers rely on producers as their source of energy. When they eat plants and algae, the chemical energy stored in their biomass is transferred.

## Measuring Biomass

We can measure biomass in terms of dry mass of tissue per unit area or the mass of carbon in an organism. Calorimeters are a piece of apparatus that are used to measure biomass.

### Dry mass of tissue per unit area

• Dry mass is the mass of an organism after its water has been removed.
• To remove the water, the organism is dried in an oven for several days.
• The dry mass of a sample can be used to estimate the total dry mass of a population or given area.
• Dry mass is usually measured in grams or kilograms per meter squared (g m-2, kg m-2).

### Mass of carbon

• This is a measure of the mass of carbon an organism has.
• The water content of an individual is not taken into account.
• The mass of carbon is usually about half the dry mass.

### Calorimetry

• The energy contained within dry biomass can be measured using a calorimeter.
• A calorimeter burns biomass and calculates the chemical energy released per gram.
• This calculation is made by measuring the change in temperature of a known volume of water that happens as a result of the sample being burned.