Photosynthesis

Chapter 7

Photo means light; synthesis means “to put together”. Plants make glucose from CO2, H2O and some other elements, mainly N, P, and K. The chemical formula is opposite the one for respiration:6CO2 + 6H2O + light C6H12O6 + 6O2.

Biochemists use “heavy water” with an isotope of oxygen, 18O rather than 16O, and traced the radioactive elements passing through the plant and determined that plants split heavy H2O producing heavy O2.

Photosynthesis takes place in 2 parts. The first part powers ADP ATP using energy from light units called photons. This is called phosphorylation.

The first reaction also reduces an electron carrier, NADP+ to NADPH. Phosphorylation and reduction of NADP+ are called the light dependent reaction.

Light in photons travels in a straight line . They hit a surface and bounce off at an angle. Photons have different amounts of energy, called wavelengths. Wavelengths are measured in nanometers (millionths of millimeters). The ones we can see are called the “visible spectrum”.

The visible spectrum goes from u high energy wavelengths which are very short between crests, 390 nm. At the low energy end are infrared wavelengths at 800 nm.

Plants have pigments that use all most wavelengths except green and yellow which are reflected rather than being absorbed.

A photon is pure energy and has no mass. Blue wavelengths have about 2x as much energy as red. Light intensity depends on the number of photons received. A molecule that absorbs a photon boosts an electron to a higher energy level.

High energy ultraviolet wavelengths of light are absorbed by the ozone layer in the atmosphere. Red wavelengths are absorbed by H2O vapor and CO2. When molecules absorb photons and electrons kicked to higher energy levels the electrons are passed to other molecules..

The light absorbing pigments of plants are mainly chlorophyll a and to a lesser extent chlorophyll b. Yellow and orange carotenoids also absorb some light. Pigment molecules form a complex that transfers light to chemical energy.

Some wavelengths do not promote photosynthesis, those longer than 680 nm. But a combination of 680 and 700 nm. at the same time will. Photosynthesis is more efficient using light of 2 or more wavelengths because it consists of 2 sets reactions called Photosystem I and Photosystem II.

The chlorophyll at Photosystem I absorbs light at 700 nm., deep red and is called P700. The chlorophyll at photosystem II absorbs light at at 680 nm., red, and is called P680. The photosystems are within the chloroplasts inside the thylakoids (stacks of disks).

Excited electrons of photosystem I can take either of 2 paths. The first is called cyclic photophosphorylation and produces ATP. The second is noncyclic photophosphorylation and makes NADPH that provides electrons to make sugar.

Noncyclic photophosphorylation occurs when photosystem I receives electrons from photosystem II. Excited electrons from photosystem II all take one path to the electron hungry photosystem I.

Cyclic Photophosphorylation

When P700 absorbs a photon its free energy increases and it becomes a powerful electron donor. Excited P700 gives up an electron and loses most of the energy it got from the photon. P700 can gain another photon. The electron acceptor is a cytochrome complex that passes the electron on to another acceptor.

Passage of electrons through the cytochrome complex result in a proton gradient. Protons move from a space outside the thylakoid (stroma) into the thylakoid space. An electron transport chain pumps protons out of the stroma and into the thylakoid space.

Inside the thylakoid membrane is ATP synthase that produces ATP’s. Protons flow down the gradient ATP is made in the stroma and later will be used to make glucose.

Noncyclic Electron Flow

Energy of the excited electrons of P700 is used to make NADPH an electron carrying coenzyme that chloroplasts use to build glucose. It provides most of the reducing power for glucose synthesis.

After 2 P700 have given up 2 excited electrons to form NADPH it is oxidized and needs an electron. Photosystem II provides electrons to P700 Chlorophyll of photosystem II, absorbs available light at 680nm and is called P680. Photo-excited P680 is a more powerful electron donor than excited P700.

Electrons move through photosystem II’s electron transport chain from excited p680 to oxidized P700. By regenerating P700 photosystem II supplies electrons to photosystem I. Photosystem II produces ATP as protons flow down the gradient produced by the electron transport chain.

If an electron from excited P680 is transferred to oxidized P700, P680 must be regenerated. The electron that regenerates P680 comes from water. H2O is split by light in the light dependent reaction. The splitting of water by light is photolysis.

Photosystem II takes electrons from water, separates H’s from O. Oxygen is released as a waste product.

Photosynthetic prokaryotes evolved about 3 billion years ago producing O2 in the atmosphere. Today 50-70% of atmospheric O2 comes from marine algae.

The 2 photosystems

1.Split water to Hydrogen and oxygen

2.Produce ATP

3.Produce NADPH

ATP and NADPH remain in the chloroplasts where they contribute to the synthesis of sugar (light independent reaction).

Reducing NADP+ to NADPH requires 2 electrons therefore photosystem I and II must each absorb 2 photons . To move electrons along the paths requires 2 electrons so it takes 4 electrons to make NADPH.

The electons flow from photosystem II to photosystem I and supply energy to pump protons across the thylakoid membrane . The protons flow back through ATP synthase to produce of ATP. Energy stored in NADPH and ATP make glucose (or other carbohydrate).

Plants transport sugar from cell to cell. Hydrogen for glucose comes from water. Carbon and oxygen come from CO2. To synthesize glucose from ATP and NADPH also takes many small steps.

Discovery of the isotope carbon – 14 allowed Melvin Calvin to determine what compounds were involved in synthesis. Calvin used paper chromatography where pigments were dotted onto paper and put in a solvent. The solvent separated the pigments by molecular weight into beta carotene, xanthophyll, chlorophyll a and chlorophyll b.

Calvin identified the radioactive C14 compounds by placing the paper on photographic film which it developed.

Part I – Capturing carbon.

CO2 and a 5-carbon sugar, ribulose biphosphate catalyzed by an enzyme, Rubisco produce a 6-Carbon compound that immediately breaks into 2, 3-carbon molecules called phosphoglycerate. It is unstable and becomes 2, 3-carbon molecules of glyceraldehyde phosphate.

These may become glucose, fat, or amino acids but some will replenish RuBP in 7 steps and requiring ATP. Rubisco is a slow enzyme so it makes up over half of the protein in the chloroplasts.

Photosynthesis in is subject to drought and other environmental problems. The carbohydrates, etc. produced by plants form the trunk, leaves, stems, etc. 3 major factors that affect photosynthesis are available water, CO2, and photons of appropriate wavelength. Intensity of light (number of photons striking a leaf per day) is important.

In hot, dry weather plants close their stomata to conserve water but it also limits their CO2 uptake. If O2 in a plant builds up, CO2 decreases initiating photorespiration where plants use their sugar. This occurs because Rubisco requires a lot of CO2.

When CO2 levels are low Rubisco converts ribulose biphosphate into 1 phosphoglycerate molecule and a 2 Carbon compound which is a waste product. During photorespiration plants waste half of the carbohydrates they have made. Photorespiration produces no ATP or NADPH.

Some plants have a biochemical pathway that allows them to maintain high CO2 even when stomata are closed. The first reaction does not make phosphoglycerate , rather it makes a 4 carbon compound –oxaloacetic acid (OAA), which cells convert to carbohydrate and CO2 which enters the Calvin cycle.

Corn 4-5 leaf stage.

Credit: © Inga Spence/Visuals Unlimited 304561

Ethanol in gasoline comes fro corn and sugar cane. Cacti use a modified C4 pathway , CAM (crassulacean acid metabolism). They open stomata at night and store CO2 until the next day. This requires a lot of ATP so they grow very slowly.