Biological Molecules and Biomass

From small molecules to living material

A plant’s leaves, an animal’s muscles and the food we eat all contain biological molecules. Three important groups are carbohydrates, lipids and proteins. Organisms need these molecules both as sources of fuel and as materials for building cells.

Synthesis means building larger molecules from smaller ones. The reverse process breaks larger molecules down into their smaller components. The relationships are:

Sugars, fatty acids and glycerol contain carbon, hydrogen and oxygen. Amino acids also contain nitrogen, so an organism needs a source of nitrogen to make proteins.

These relationships explain why digestion matters. Large food molecules must be broken down into smaller molecules before they can be absorbed. Once absorbed, the smaller molecules can be used to build the consumer’s own carbohydrates, lipids and proteins. For example, amino acids obtained by digesting food proteins can be joined together to make proteins needed for growth and repair. The food protein is not simply moved intact into a new muscle.

Producers provide the starting material

Photosynthetic organisms, especially plants and algae, are the main producers of food and therefore biomass for life on Earth. They use light energy to make glucose from substances in their environment:

$$\text{carbon dioxide} + \text{water} \xrightarrow{\text{light}} \text{glucose} + \text{oxygen}$$

Chlorophyll absorbs the light energy needed for photosynthesis. That energy becomes chemical energy stored in the products. Light supplies energy; carbon dioxide and water supply the material from which glucose is made.

Glucose is a starting point for making other biological molecules. Plants can join sugars together to make starch, and use the products of photosynthesis to make fatty acids, glycerol and other substances. They also take in nitrogen compounds, including nitrate ions, to make amino acids and then proteins. In this way, substances from the environment become plant biomass.

Consumers obtain carbon and nitrogen compounds by feeding on plants or other organisms and digesting their biomass. These compounds provide both fuel for respiration and materials for building the consumer’s own cells. Even an animal that eats other animals depends on the food originally produced by organisms such as plants and algae.

Identifying molecules with food tests

A qualitative test tells you whether a particular substance is present. It does not give an exact mass or concentration. Food tests use characteristic changes in colour or appearance to identify biological molecules.

Use separate portions of a sample for each test so that reagents from one test do not interfere with another. Where a solution is needed, a solid food can be crushed with distilled water and filtered to obtain a liquid sample. For the lipid test, use a separate portion with ethanol.

A precipitate is an insoluble solid formed during a reaction. An emulsion contains tiny droplets of one liquid dispersed through another: in the lipid test, these droplets make the mixture look cloudy.

For Benedict’s test, read the result after heating. For the other tests, observe the result after adding and mixing the reagents. If the original sample is cloudy, separate solid material before assessing the lipid test so that suspended food particles are not mistaken for a positive result.

Run a negative control alongside each test, replacing the food sample with distilled water but using the same reagents and procedure. This shows the appearance of a negative result and provides a comparison.

Wear eye protection throughout. Iodine and copper(II) sulfate can harm the eyes, and sodium hydroxide can damage skin and eyes; rinse any splashes promptly with plenty of water and tell the teacher. Ethanol is highly flammable, so keep it away from flames. Use a water bath for Benedict’s test, handle hot test tubes with a holder, and keep their openings pointed away from people.

Turning observations into conclusions

A blue-black iodine result supports the conclusion that starch is present. It does not show that all carbohydrates are present, or tell you how much starch there is.

Similarly, a purple Biuret result identifies protein, while a cloudy white emulsion identifies lipid. A Benedict’s result that stays blue means that reducing sugar was not detected under the test conditions; it does not mean that the sample contains no food molecules.

A food can give more than one positive result because it may contain several types of biological molecule. Interpret each test separately, then combine the conclusions about the sample.

Substances organisms need to take in

Cells continually use substances in their reactions and to maintain their structures. Organisms therefore need to exchange substances with their surroundings and transport them to the cells that require them.

In animals, digestion supplies small, dissolved food molecules that can be absorbed and transported to cells. In plants, carbon dioxide enters leaves by diffusion, while roots take in water and dissolved mineral ions. Water enters root cells by osmosis, and mineral ions can be absorbed by active transport against a concentration gradient. Sugars made in leaves are transported in the phloem to other parts of the plant.

Plants also respire: photosynthesis does not replace their need to release energy from food. The important distinction is that producers make organic food from environmental substances, whereas consumers obtain it by feeding.

Building and breaking down molecules

• Sugars ⇄ complex carbohydrates.
• Fatty acids + glycerol ⇄ lipids.
• Amino acids ⇄ proteins.
• Synthesis builds larger molecules; digestion produces smaller molecules for absorption.
• Amino acids contain nitrogen as well as carbon, hydrogen and oxygen.

Producers and biomass

Plants and algae are the main producers of food and biomass. Light energy enables them to make glucose:

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

Producers use glucose and mineral ions to build biological molecules. Consumers obtain carbon and nitrogen compounds by feeding and digestion.

Food-test recall

Use separate samples and distilled-water negative controls. Qualitative tests identify presence, not exact quantity.

Required substances

• Oxygen: aerobic respiration.
• Water: cellular reactions and transport; photosynthesis in producers.
• Dissolved food molecules: respiration, growth and repair.
• Mineral ions: essential molecules and cell structures; nitrates supply nitrogen for amino acids.
• Carbon dioxide: photosynthesis.

Plants respire as well as photosynthesise.