Biology - Photosynthesis

Green plants make their own food (glucose and starch), using sunlight. This process is called photosynthesis.

Photosynthesis produces glucose for biomass and energy. Oxygen is released as a by-product. The equation for photosynthesis is as follows:

                                                       light energy

carbon dioxide    +       water                                glucose    +    oxygen 

                                                       chlorophyll 

       6CO2        +    6H10                              C6H1206   +       602                            

How we understand photosynthesis has changed and evolved over time. The Greek scientists through that plants gained mass only by taking in minerals from the soil. Van Halmont carried out many experiments and concluded that plant growth cannot be due to the nutrients in the soil.

Joseph Priestley put a plant in a jar of air, and a plant with in a jar with a mouse inside. He changed the combinations of plants and mouse and concluded that oxygen is produced by plants.

The glucose produced in photosynthesis can be immediately to produce energy through respiration, or it can be converted into other substances that the plant needs. Glucose and starch can be converted into:

• Energy (during respiration).
• Proteins for growth and repair.
• Starch, fats or oils that can be stored in seeds.
• Cellulose, which is needed for plant cell walls.

Glucose is soluble. It can be transported around the plant as soluble sugar, but it must be converted into starch, which is insoluble, in order to be stored.

Starch is a very useful storage molecule. It is insoluble so it doesn't affect the water concentration inside the cells where it's stored. It also does not move away in solution from storage areas. If the cell stored soluble glucose, the inside of the cells would become very concentrated and water would constantly move in through osmosis, which would make the cell swell.

Using radioactive oxygen-18, scientists discovered that the oxygen produced as a by-product in photosynthesis come from the water and not in the carbon dioxide. Only when oxygen-18 is introduced via the water do you get a radioactive waste product of oxygen. This shows that photosynthesis is a two stage process. Firstly, light energy is used to split water, releasing oxygen gas and hydrogen ions. Secondly, the carbon dioxide gas combines with the hydrogen (ions) to make glucose.

Plants need light and warmth to grow. This is why they grow faster in the summer. 

Photosynthesis can be increased  by increasing:

• The temperature - using heaters in a greenhouse.
• The light intensity - using lamps in a greenhouse.
• The carbon dioxide (CO2) concentration - using chemicals, or as a by-product of using gas heaters in a greenhouse.

As the temperature rises, so does the rate of photosynthesis. This means temperature is the limiting factor on the rate of photosynthesis. As the temperature approaches 45°C, the enzymes controlling photosynthesis start to denature and the rate of photo synthesis declines to zero.

As the carbon dioxide concentration rises, so does the rate of photosynthesis. So carbon dioxide is limiting the rate of photosynthesis, up to a certain point. After this point, a rise in carbon dioxide is no longer the limiting factor; light or temperature must be.

As the light intensity increases, so does the rate of photosynthesis. This means light intensity is limiting the rate of photosynthesis, up to a certain point. After this point, a rise in light tensity has no effect. Light intensity is no longer limiting the factor; carbon dioxide and temperature must be.

Plants respire to break down glucose to release energy. They respire all the time, i.e. day and night.

During the day, light is available from the Sun so plants photosynthesise; taking oxygen as a by product.

During the day and night, they respire, absorbing oxygen and giving out carbon dioxide. Respiration is the reverse of photosynthesis. Plants photosynthesise much faster than they respire when light is avaliable. That is why they give out oxygen during the day.