Question

Be sure to include the following terms in your description: Glucose, NADPH, ATP, Calvin cycle, RUBISCO, CO2.

Trace the flow of carbon within the process of photosynthesis

Answer 1

Photosynthesis is the process by which plants, some bacteria and some protistans use the energy from sunlight to produce glucose from carbon dioxide and water. This glucose can be converted into pyruvate which releases adenosine triphosphate (ATP) by cellular respiration. Oxygen is also formed.

Photosynthesis may be summarised by the word equation:

carbon dioxide + water

glucose + oxygen

The conversion of usable sunlight energy into chemical energy is associated with the action of the green pigment chlorophyll.

When chlorophyll a absorbs light energy, an electron gains energy and is 'excited'. The excited electron is transferred to another molecule (called a primary electron acceptor). The chlorophyll molecule is oxidized (loss of electron) and has a positive charge. Photoactivation of chlorophyll a results in the splitting of water molecules and the transfer of energy to ATP and reduced nicotinamide adenine dinucleotide phosphate (NADP).

The chemical reactions involved include:

• condensation reactions - responsible for water molecules splitting out, including phosphorylation (the addition of a phosphate group to an organic compound)
• oxidation/reduction (redox) reactions involving electron transfer

Photosynthesis is a two stage process.

The Light dependent reactions, a light-dependent series of reactions which occur in the grana, and require the direct energy of light to make energy-carrier molecules that are used in the second process:

• light energy is trapped by chlorophyll to make ATP (photophosphorylation)

• at the same time water is split into oxygen, hydrogen ions and free electrons:

  2H₂O    4H⁺ + O₂ + 4e^- (photolysis)

• the electrons then react with a carrier molecule nicotinamide adenine dinucleotide phosphate (NADP), changing it from its oxidised state (NADP⁺) to its reduced state (NADPH):

  NADP⁺ + 2e^- + 2H⁺    NADPH + H⁺

The light-independent reactions, a light-independent series of reactions which occur in the stroma of the chloroplasts, when the products of the light reaction, ATP and NADPH, are used to make carbohydrates from carbon dioxide (reduction); initially glyceraldehyde 3-phosphate (a 3-carbon atom molecule) is formed.

In the Light-Independent Process (the Dark reaction) carbon dioxide from the atmosphere (or water for aquatic/marine organisms) is captured and modified by the addition of hydrogen to form carbohydrates. The incorporation of carbon dioxide into organic compounds is known as carbon fixation. The energy for this comes from the first phase of the photosynthetic process. Living systems cannot directly utilize light energy, but can, through a complicated series of reactions, convert it into C-C bond energy that can be released by glycolysis and other metabolic processes.

Carbon dioxide combines with a five-carbon sugar, ribulose 1,5-biphosphate (RuBP). A six-carbon sugar forms but is unstable. Each molecule breaks down to form two glycerate 3-phosphate (GP) molecules.

These glycerate 3-phosphate (GP) molecules are phosphorylated by ATP into glycerate diphosphate molecules.

These are reduced by NADPH to two molecules of glyceraldehyde 3-phosphate (GALP).

Of each pair of GALP molecules produced:

• one molecule is the initial end product of photosynthesis; it is quickly converted to glucose and other carbohydrates, lipids or amino acids
• one molecule forms RuBP through a series of chemical reactions

The first steps in the Calvin cycle

The first stable product of the Calvin Cycle is phosphoglycerate (PGA), a 3-C chemical. The energy from ATP and NADPH energy carriers generated by the photosystems is used to phosphorylate the PGA. Eventually there are 12 molecules of glyceraldehyde phosphate (also known as phosphoglyceraldehyde or PGAL, a 3-C), two of which are removed from the cycle to make a glucose. The remaining PGAL molecules are converted by ATP energy to reform six RuBP molecules, and thus start the cycle again.