General Biology/Cells/Photosynthesis

General Biology | Getting Started | Cells | Genetics | Classification | Evolution | Tissues & Systems | Additional Material

• One of most important reactions in history of life:
  • source of atmospheric O₂
  • ultimately led to aerobic respiration and eukaryotes
• Responsible for bulk of glucose production
• Early experiments showed that mass of plant must be derived from substances in the air, not the soil
• Experiments with isotopes showed that liberated oxygen comes from water
• Experiments also showed that light is essential but that some reactions (e.g., reduction of CO₂) continue in the dark
• Plants do two big, important things during photosynthesis: gain energy (absorb light) and build sugar (glucose).
• Photosynthesis can be divided into two series of chemical reactions: the light (light-dependent) reactions and the dark (light-independent) reactions. In light reactions, light is absorbed; in dark reactions, sugar is built.
• Occurs when plants, algae, and autotrophic bacteria absorb light energy and build glucose.

Light Reactions

• Part of the electromagnetic spectrum
• Consists of units of energy called photons
• Photons at UV end of spectrum have more energy than those at the red end
• Occur on the surface of thylakoid disks
• Chlorophyll and other plant pigments differentially absorb photons
  • Chlorophyll a: light to chemical energy
  • Chlorophyll b: accessory chlorophyll
  • Chlorophylls absorb primarily blue and red (green reflected back, hence the green color of plants)

Accessory pigments

• Chlorophyll is a major light gathering pigment
  • Absorbs light with considerable efficiency (i.e., retaining energy)
• Accessory pigments
  • Chlorophyll b
  • Carotenoinds
• capture light of wavelengths not captured by chlorophylls
• Confer other colors to plant leaves (autumn colors too)

Photosynthetic steps

• Primary photoevent: light photon captured by photosystem and energy transferred to electron donated by water
• Electron transport: excited electron is shuttled along imbedded series of electron carriers to proton pump and electron is transferred to acceptor
• Chemiosmosis: transport of protons back into chloroplast drives synthesis of ATP

The Even More Detailed Light Reactions

What the Light Reactions Do:

The light reactions of photosynthesis occur in chloroplasts in and on the thylakoid disks. During the light reactions, light energy charges up ATP molecules. More specifically, light turns the chloroplast into an acid battery, and this battery charges up ATP.

How the "Chloroplast-Battery" Charges ATP:

The stroma is the fluid inside of the chloroplasts, and it carries a negative charge. This means that it contains about a "gazillion" extra electrons. The solvent of stroma is water.

The fluid inside the thylakoid disks is positively charged because it contains a lot of hydrogen (H+) ions. The pH here is low, making the fluid very acidic. The solvent of thylakoid disk fluid is water.

A chloroplast acts like a battery, because it has separated a strong positive charge and a strong negative charge in two different compartments. Energy is released when H+ ions (free protons) flow from the inside of a thylakoid disk to the stroma. This is electrical energy, since it is a flow of charged particles.

The protons pass through special channels (made of protein) in the thylakoid membrane; this reaction is 'exothermic.' The energy that is given off is used to fuel this reaction (Pi is the phosphate ion):

                    ADP + Pi --> ATP

The proton can go to the negative stroma, but only if it uses its energy to charge up ATP. Since one reaction wants to go, and the other one doesn't, and since the first reaction releases energy and the second one absorbs energy, the two reactions are known to be 'coupled' together so that the first fuels the second. Of course, a special enzyme must be involved for this to happen.

Chlorophyll Molecules on a Thylakoid Disk:

Hundreds of chlorophyll molecules cover the surface of a thylakoid disk, making the disk green. The nonpolar "tails" of the chlorophyll molecule are embedded in the membrane of the thylakoid.

“Dark” reactions

• ATP drives endergonic reactions
• NADPH provides hydrogens for reduction of CO₂ to carbohydrate (C-H bonds)
• Occur in the stroma
• First step in carbon fixation

The Detailed Dark Reactions

What the Dark Reactions Do:

The dark reactions build sugar from carbon dioxide gas (CO2), water (H2O), and energy from ATP molecules that were charged up during the light reactions. The dark reactions occur in the stroma of a chloroplast. Dark reactions usually occur in the light, but they don't have to. They'll occur in the dark until the chloroplast's supply of ATP runs out (usually about 30 seconds).

The Calvin Cycle:

The Calvin Cycle is the fancy name for the metabolic pathway that builds sugar. This means that it involves a whole lot of chemical reactions, and it uses a lot of different enzymes to catalyze the reactions.

Carbon dioxide gas is stable, therefore the bonds that hold the carbon and oxygen atoms are strong. Therefore it takes a lot of energy to break the bonds and separate the carbon atoms from the oxygen atoms. The energy needed to do this comes from ATP molecules.

When inorganic carbon (like from CO2) is being added to an organic molecule (such as sugar), this is called carbon fixation.

It takes 2 complete turns of the Calvin Cycle to make a glucose molecule.

Reference

Some portions of this text is based on notes very generously donated by Paul Doerder, Ph.D., of the Cleveland State University. The detailed portions are not provided by Dr. Doerder.