GCSE Photosynthesis

Photosynthesis

What is it? Photosynthesis is a chemical process through which plants, some bacteria and algae, produce glucose and oxygen from carbon dioxide and water, using only light as a source of energy.During photosynthesis radiant energy from the sun (sunlight) is absorbed by green plants. The energy is used to convert carbon dioxide, water and minerals the plants take in from their surroundings into sugar and gaseous oxygen.Photosynthesis is critical to life on Earth. Without photosynthesis the food supply would finish and the Earth’s atmosphere would lose its oxygen.The 2 stages of photosynthesis

The photochemical stage involves the radiant energy supplied by sunlight and involves reactions called light dependant reactions. Green plants contain a light absorbing pigment called chlorophyll. When a molecule of chlorophyll absorbs light it uses the energy to boost electrons to a higher energy level and the molecule is said to be excited. The electrons at the higher energy levels are transferred along chains of electron carrier molecules. The energy transfers of the electrons are responsible for the formation of key energy carrying molecules along with the splitting of the water molecule to oxygen and hydrogen. These molecules then undergo the second stage of photosynthesis, the biochemical reaction. Here they react with hydrogen formed from the splitting of water in the photochemical stage and carbon dioxide from the atmosphere with the presence of enzymes to form the organic glucose molecules. The reactions in this stage are referred to as light-independent reactions because they do not require light as they use the energy already provided by the light in the light-dependent reactions. Thus, photosynthesis has transferred the energy from light to chemical energy in the sugar.

Factors limiting photosynthesisPhotosynthesis can be limited by three factors: light intensity, carbon dioxide concentration and temperature.Without enough light, a plant cannot photosynthesise very quickly, even if there is plenty of water and carbon dioxide. Increasing the light intensity will boost the speed of photosynthesis.Sometimes photosynthesis is limited by the concentration of carbon dioxide in the air. Even if there is plenty of light, a plant cannot photosynthesise if there is insufficient carbon dioxide. If it gets too cold, the rate of photosynthesis will decrease. Plants cannot photosynthesise if it gets too hot.If you plot the rate of photosynthesis against the levels of these three limiting factors, you get graphs like the ones above. In practice, any one of these factors could limit the rate of photosynthesis.

Storage and uses of Glucose The glucose produced by photposynthesis may be used by plants and algae in various ways