Photosynthesis
Photosynthesis is a series of anabolic reactions that form carbohydrates. Photosynthesis takes in carbon dioxide and water and produces glucose:
6 CO2 + 12 H2O ----------> C6H12O6 + 6 H2O + 6 O2
Autotrophs - make organic molecules themselves by photosynthesis eg green plants, algae.
Heterotrophs - take in organic molecules ready made eg animals (feeding), fungi (decomposing).
Leaf structure
Water comes into the leaf through veins from the roots. Leaf anatomy. CO2 enters the leaf through small holes called stomata.
Chloroplasts are mainly on the top surface of the leaf. Chloroplasts
There are two stages of photosynthesis :
1) Light reactions : use light energy to form ATP and also split water into hydrogen (added to NADP to make NADPH) and oxygen (waste).
2) Calvin cycle : uses NADPH and ATP to form carbohydrate from carbon dioxide.
Light
Different colors of light are different wavelengths : red light has a longer wavelength than blue light.
Chlorophyll reflects ( does not use ) green light; it absorbs ( uses ) red and blue light. Electron acceptor molecules hold high energy electrons, to stop them from wasting the energy as heat.
A photosystem is a chlorophyll molecule, electron acceptor and other light gathering molecules like carotene. One idea to help the search for life on other planets is to look for the characteristic wavelengths of light that are reflected by photosynthetic organisms.
Light reactions
The light reactions take place in the grana of the chloroplast.
A) Cyclic electron flow - uses photosystem I
- produces ATP, using a proton pump.
B) Non-cyclic electron flow - uses photosystems I and II
- produces ATP and NADPH molecules.
- splits water to get electrons from the hydrogen.
Calvin cycle
This takes place in the stroma of the chloroplast.
It takes in CO2 and produces carbohydrate ( glyceraldehyde phosphate ).
The Calvin cycle uses the ATP and NADPH that were produced in the light reactions.
In artificial photosynthesis light is used to split water and produce hydrogen. Hydrogen is an excellent fuel. Hydrogen powered cars have a range of over 300 miles and can be refueled in a few minutes. 2015 Hydrogen-powered Hyundai
Production of organic molecules
Glyceraldehyde phosphate is used to make glucose ( C6H12O6 ). Glucose can be stored in the plant as starch.
Other enzymes convert carbohydrates into lipids.
Plants can make proteins if they have nitrate ( absorbed into roots from fertilizer ) and nucleic acids if they have phosphate.
Three different types of plants:
1) C3 plants : typical plants that open their stomata during the day, and close their stomata at night.
Common plants in cool areas eg Canada.
2) C4 plants : open their stomata only briefly during the day.
They store CO2 as the 4 carbon sugar : oxaloacetate.
Mainly tropical plants eg sugarcane.
3) CAM plants : only open their stomata at night, to conserve water.
These are desert plants like cactus.
Last edited August 2014 by David Byres, David.Byres@fscj.edu
