GCSE Science/Photosynthesis

Photosynthesis

Like all living things, plants need food to live. This food is used for energy and to make new materials when plants grow. Plants are able to take two inorganic chemicals, carbon dioxide gas and water, to make an organic chemical, glucose. This simple food can be used as an energy source or converted into other useful organic molecules.

The process requires an input of energy. Plants have found a way to capture the energy from sunlight using a pigment called chlorophyll. Once this light energy has been captured it can be used to create glucose, converting the light energy into chemical energy. Oxygen gas is released as a waste chemical. As light energy is used to create organic materials the process is named Photosynthesis.

The formula for photosynthesis is:

$6{\text{CO}}_2+6{\text{H}}_2\text{O}\stackrel{\text{<mrow data-mjx-texclass="ORD"><mi mathvariant="normal">S</mi><mi mathvariant="normal">u</mi><mi mathvariant="normal">n</mi><mi mathvariant="normal">l</mi><mi mathvariant="normal">i</mi><mi mathvariant="normal">g</mi><mi mathvariant="normal">h</mi><mi mathvariant="normal">t</mi></mrow>}}{\to }{\text{C}}_6{\text{H}}_{12}{\text{O}}_6+6{\text{O}}_2$

The Structure of a leaf

Photosynthesis takes place in the leaves of plants. The leaves have many adaptations to make sure that as much photosynthesis goes on as possible. The more a plant can photosynthesize, the more food it can make and the faster it can grow.

1. The leaf has a waxy cuticle to stop it losing water and drying out.
2. The epidermis is a protective layer of cells and contains no chloroplasts.
3. The palisade layer contains the most chloroplasts as it is near the top of the leaf. The chloroplasts contain the pigment chlorophyll. It is here that photosynthesis takes place.
4. The palisade cells are arranged upright. This means the light has to pass through the cell lengthways and so increases the chance of light hitting a chloroplast and being absorbed.
5. The spongy layer contains fewer chloroplasts, enough to catch what the palisade layer cannot absorb.
6. The spongy layer has air spaces to make it easier for gases to circulate in the leaf.
7. The vascular bundle provides the leaf with water via the xylem vessels. Food, such as sugar, made in the leaf is transported in the phloem vessels to the rest of the leaf.
8. The stomata (stoma - singular) are tiny pores that allow carbon dioxide to enter the leaf while oxygen leaves the leaf.
9. Guard cells can open or close the stomatal pores to regulate how much gas can enter or leave the leaf. At night the pores close, opening in the daytime.

Limiting Factors

Photosynthesis requires the following 3 factors:

1. Carbon dioxide
2. Water
3. Light

There are a 3 factors which affect the rate at which a plant can make glucose by photosynthesis.

1. The Concentration of Carbon Dioxide
2. The Light Intensity
3. The Temperature

Respiration in plants

Respiration is the production of energy from glucose and oxygen with the release of carbon dioxide and water as waste products. This is the opposite of photosynthesis which is the production of glucose and oxygen fron the energy in sunlight, carbon dioxide and water.

Respiration:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy released

Glucose + Oxygen → Carbon dioxide + water + Energy released

Photosynthesis:

6CO₂ + 6H₂O + Energy → C₆H₁₂O₆ + 6O₂

Carbon dioxide + water + Energy → Glucose + Oxygen

Plants use the energy from respiration to power the processes involved in growth.

Uses of glucose

The energy obtained from respiration is used to turn glucose into many other substances. Typical uses of glucose are:

• Storage products

- glucose is used to make starch which can be converted back to glucose as required. Potatoes and rice are examples of parts of plants that contain starch.

- glucose is converted into lipids, especially in seeds. Sunflower oil and rapeseed oil, which are used in cooking, come from sunflower and oilseed rape seeds.

• Structural products

- glucose is converted to cellulose to make cell walls.

• Other products

- glucose and nitrates are used to make amino acids which are used to make proteins.

- glucose is also a basic raw material for making chlorophyll.