The Importance of Photosynthesis
Photosynthesis transforms light energy from the sun into chemical energy, which is used to synthsise large organic molecules. It releases oxygen, from water, into the atmosphere -all aerobes depend on photosynthesis in order to respire.
Photoautotrophs can photosynthesise.
Chemoautotrophs use energy from exergonic reactions to synthesise organic molecules.
Heterotrops ingest and digest complex organic molecules, releasing their chemical potential energy.
Photosynthesis takes place on the chloroplasts.
Chloroplasts and Photosystems
- Surrounded by a double membrane (the envelope). The inner membrane has transport proteins and can control entry and exit of substances between the cytoplasm and the stroma.
- There are many grana (stacks of thykaloid membranes), providing a large surface area for photosynthetic pigments, electron carriers and ATP synthase.
- Proteins embedded in the grana hold the photosystems in place.
- The fluid-filled stroma contains the enzymes needed for the light-independent stage.
- The grana are surrounded by stroma so that products of the light-dependent reaction can pass into the stroma.
Photosynthetic pigments --> molecules that absorb light energy. Each pigment absorbs a range of wavelengths in the visible region. Other wavelengths are reflected.
Photosystem --> a funnel-shaped light-harvesting cluster of photosynthetic pigments. The primary pigment reaction centre is a molecule of chlorophyll a. P680 is found in photosystem 2 and its peak of absorption is 680nm. P700 is found in photosystem 1 and its peak is 700nm.
The accessory pigments are caratenoid pigments. They absorb blue light and pass the energy to the primary pigment reaction centre
The Light-Dependent Stage
Water is photolysed in photosystem 2. It provides hydrogen ions which undergo chemiosmosis to make ATP, and are then accepted by NADP to be used during the light independent stage. Electrons are also produced. Oxygen is a by-product, which diffuses through the stoma.
--> the making of ATP from ADP and Pi in the presence of light.
Electrons in photosystem 1 are excited, passed through an electron carrier chain, and back to the chlorophyll from which they were lost. Small amounts of ATP are made. This is used by guard cells to open the stomata.
Light strikes photosystem 2, exciting a pair of electrons. They pass along an electron carrier chain. Light strikes photosystem 1, exciting a pair of electrons. They reduce NADP. The electrons from PSII replace those lost from PS1. The electrons from photolysis of water replace those lost from PSII. Protons from photolysis of water take part in chemiosmosis and reduce NADP in the stroma.
The Light-Independent Stage
1. Carbon dioxide (that has diffused in through the stomata) and RuBP combine (in the presence of rubisco), to produce 2x GP (3C).
2. GP is reduced (using NADPH) and phosphorylated (using ATP) to TP.
3. 5 of every 6 molecules of TP are phosphorylated (using ATP) to 3 RuBP.
How the products of the Calvin cycle are used
GP is used to make amino acids and fatty acids.
2xTP can combine to make hexose sugars, or glycerol. Glycerol can combine with fatty acids (from GP) to make lipids.
The limiting factor is the factor that is present at the lowest (or least favourable) value.
Carbon dioxide is required for the Calvin cycle. Growers can increase carbon dioxide concentration by burning methane- or oil-fired heaters.
Light causes stomata to open, so carbon dioxide can enter. It is trapped by chlorophyll where it excites electrons. it also undergoes photolysis, producing electrons and protons, and oxygen as a byproduct. The light intensity varies throughout the day.
The enzymes used in the Calvin cycle require a relatively high temperature for photosynthesis. Above 25 degrees some of the enzymes are denatured, and oxygen more successfully competes for the active sites on rubisco. High temperatures also cause more water loss, which will close stomata, limiting carbon dioxide concentration.