Plant systems B:
Photosynthesis, nutrition and respiration
Photosynthesis Nutrition Respiration

All living things require energy. The energy source for all life on Earth is ultimately the Sun. Without the ability of plant pigments to capture the Sun's light energy, life as we know it on Earth would not exist.

Photosynthesis is the process by which plant cells make sugar (glucose) using sunlight energy, water and carbon dioxide. The sugar is then transported to all parts of the plant and used as an energy source for growth and survival, or stored as starch for later use. Animals eating these plants obtain this ready-made energy source.

A green pigment, chlorophyll, is stored within small organelles called chloroplasts within many plant cells. It is mainly found in leaves and stems, thereby giving these parts a green appearance.

Click for larger image Chlorophyll, an iron-based molecule, can absorb light and become energised, and then pass on this energy to combine the water and carbon dioxide molecules to form sugar.
Water travels up from the soil through roots and xylem tubes (see Plant systems A: Transport and support) and the carbon dioxide moves into the leaf cells through small holes in the leaf surface called stomata. Whole plant showing water passing from soil up roots and xylem, and carbon dioxide entering leaves.

The chemical equation for photosynthesis is:

6 CO2 + 12H2 O C6 H12 O6 + 6 H2 O + 6 O2

Click for larger image Six molecules of carbon dioxide combine with twelve molecules of water in the presence of light to form one molecule of glucose sugar, six molecules of water and six molecules of oxygen.

Oxygen is a by-product of this process. Any excess oxygen not required by the plant is released into the atmosphere, and is therefore available for animals to use.

Further uptake of other minerals from the soil provides the necessary elements required by the plant to make proteins and all other substances for growth and functioning of the plant. These too are taken in by animals when they eat the plant as food.

Since all living things need energy, animals are ultimately dependent on plants for food.
In the mitochondria (see Animal cells) of all plant and animal cells, the energy trapped in the sugar molecule is released during the process of cellular respiration. The sugar combines with oxygen to form carbon dioxide (CO2) and water as by-products, and the energy is given off.

6 O2 + C6 H12O6 6CO2 + H2O + ENERGY

Six molecules of oxygen combine with one molecule of sugar to form ENERGY, plus six molecules of carbon dioxide and six molecules of water.

Click for larger image Note that carbon dioxide is a by-product of respiration, which is then available for plants to use in photosynthesis. In other words, plants and animals are interdependent and the balance of oxygen and carbon dioxide in nature is vitally important.

The energy released in respiration is available for movement, growth, and all the life functions ("metabolism").

Cellular respiration is the exact opposite to the chemical reaction of photosynthesis.
A healthy plant in good growing conditions produces more sugar through photosynthesis than it uses up in cellular respiration. It also produces more oxygen than it needs, so a supply of both food and oxygen for animals is maintained. The logging of vast areas of rainforest can greatly upset this balance in nature because photosynthesis is reduced.

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Plant Systems A: Transport and support
Plant Systems C: Reproduction and flowering plants
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