Photosynthesis

The Electromagnetic Spectrum

 Light is a form of energy known as electromagnetic radiation

 The segment of the electromagnetic spectrum most important to life is the

narrow band between about 380 and 750 nanometres (nm)

 This radiation is known as visible light because it is detected as colours by the

human eye and drives photosynthesis

Photosynthetic Pigments

 Substances that absorb visible light are called biological pigments

 The photosynthetic pigments of plants fall into two categories:

 Chlorophylls (A and B), which absorb red and blue-violet light

 They are the main photosynthetic pigment in plants and give leaves their green colour (as

the green wavelengths are not absorbed)

 Carotenoids, which absorb strongly in the blue-violet, appear orange, yellow, or

red

 They are considered to be accessory pigments

Note: some of the other pigments become visible during autumn when green

chlorophyl is broken down. Some of these pigments aid photosynthesis, such

as carotenoids, some are related to other functions.

Absorption Spectrum

 The absorption

spectrum of different

photosynthetic

pigments provides

clues to their role in

photosynthesis, since

light can only

perform work if it is

absorbed

Action Spectrum

 An action spectrum

profiles the effectiveness

of different wavelength

light in fueling

photosynthesis

 It is obtained by plotting

wavelength against a

measure of

photosynthetic rate (e.g.

CO2 consumption)

Factors Affecting Photosynthetic Rate

 The rate at which plants can produce carbohydrate

(the photosynthetic rate) is dependent on several

environmental factors

 This is due to many steps requiring catalysis by

enzymes

 Some factors have a greater effect than others, these

include:

 The amount of light available

 The level of carbon dioxide (CO2)

 The temperature

Carbon Fixation

 The Calvin cycle begins with the carboxylation of ribulose bisphosphate

(RuBP)

 The enzyme Rubisco catalyses the attachment of carbon dioxide (1C) to RuBP (5C)

 The 6C compound that is formed is unstable and breaks down into two 3C

molecules

 These two 3C molecules are called glycerate-3-phosphate (GP)

Reduction of GP

 The next stage of the Calvin cycle involves the reduction of glycerate-3-

phosphate (GP)

 GP is converted into an alternative 3C compound called triose phosphate (TP)

 This restructuring of the molecule is mediated by ATP and NADPH (reduction)

 ATP and NADPH are produced by the light dependent reactions (non-cyclic)

Regeneration of RuBP

 The final stage of the Calvin cycle involves the formation of carbon

compounds

 One of the TP molecules is used to produce carbon compounds (carbohydrates)

 The remaining TP molecules are used to regenerate RuBP stocks (requires ATP)

 Two cycles of the Calvin cycle would be required to produce a typical hexose

sugar (e.g. glucose)

Light Independent Reactions

Conversion of Triose Phosphate

 Triose phosphate, produced during photosynthesis, is

the base product leading to the formation of many

other molecules. It is converted to:

 Glucose, the fuel for cellular respiration

 Cellulose, a component of plant cell walls is formed using

glucose

 Starch granules act as a reserve supply of energy, to be

converted back into glucose when required

 Glucose is converted to other sugars such as fructose,

found in ripe fruit, and sucrose, found in sugar cane

 Lipids and amino acids