Photosynthesis

• the process by which light energy is converted to chemical bond energy and carbon is fixed into organic compounds. The general formula is:

Two main processes of photosynthesis

• light dependent or light reactions-use light energy directly to produce ATP that powers the light-independent reactions

• light independent reactions-consist of the Calvin cycle, which produces sugar. To power the production of sugar, the Calvin cycle uses the ATP formed during the light reactions.

Both reactions occur only when light is present.

PHOTOSYNTHETIC PIGMENTS-absorb light energy and use it to provide energy to carry out photosynthesis

• Plants contain two major groups of pigments, the chlorophylls and carotenoids.

• Chlorophyll a (Its double bonds play a critical role in the light reactions & are source of the electrons that flow through the electron transport chains during photosynthesis.)and chlorophyll b are green and absorb all wave lengths of light in the red, blue, and violet range.

• Carotenoids are yellow, orange, and red. They absorb light in the blue, green, and violet range.

• Xantho phyll, another photosynthetic pigment, is a carotenoid with a slight chemical variation.

Chloroplasts• contain grana, where the

light reactions occur• stroma, where the light-

independent reactions occur. –

• The grana consist of layers of membranes called thylakoids- the site of photosystems I and II.

• The chloroplast is enclosed by a double membrane.

PHOTOSYSTEMS• Light-harvesting complexes in the thylakoid membranes of chloroplasts.

(a few hundred in each thylakoid) • Consists of a reaction center containing chlorophyll a and a region

containing several hundred antenna pigment molecules that funnel energy into chlorophyll a.

• Two types cooperate in the light reactions of photosynthesis, PS I and PS II. (named in the order that they were discovered)

• PS II operates first, followed by PS I.• PS I absorbs light best in the 700 nm range; also called P700. • PS II absorbs light best in the 680 nm range; also called P680.

LIGHT-DEPENDENT REACTIONS—THE LIGHT REACTIONS

• Light is absorbed by the photosystems (PS II & I) in the thylakoid membranes and electrons flow through electron transport chains.

• there are two possible routes for electron flow: noncyclic and cyclic photophosphorylation.

Noncyclic Photophosphorylation• During N.P.- electrons enter two electron transport chains,

and ATP and NADPH are formed. The process begins in PS II and proceeds through the following steps:

• Photosystem II—P680. Energy is absorbed by P680. Electrons from the double bonds in the head of chlorophyll a become energized and move to a higher energy level. They are captured by a primary electron acceptor.

• Photolysis. Water gets split apart, providing electrons to replace those lost from chlorophyll a in P680. Photolysis splits water into two electrons, two protons, and one oxygen atom. Two oxygen atoms combine to form one 02 molecule, which is released into the air as a waste product of photosynthesis.

Photosystem II- P680 (cont.)

• Electron transport chain. Electrons from P680 pass along an electron transport chain consisting of plastoquinone (PQ), a complex of two cytochromes and several other proteins, and ultimately end up in P700 (PSI). This flow of electrons is exergonic and provides energy to produce ATP by chemiosmosis. Because this ATP synthesis is powered by light, it is called photophosphorylation.

Chemiosmosis• The process by which ATP is formed during the light

reactions. Protons that were released from water during pho tolysis are pumped by the thylakoid membrane from the stroma into the thyla koid space (lumen). ATP is formed as these protons diffuse down the gradient from the thylakoid space, through the ATP-synthase channels, and into the stroma. The ATP produced here provides the energy that powers the Calvin cycle.

• NADP becomes reduced when it picks up the two protons that were released from water in P680. Newly formed NADPH carries hydrogen to the Calvin cycle.

• photosystem II

Photosystem I—P700• Energy is absorbed by P700. Electrons from the

head of chlorophyll a become energized and are captured by a primary electron receptor. This is similar to the way it happens in P680.

• Two differences are:1. the electrons that escape from chlorophyll a are

replaced with electrons from photosystem II, instead of from water.

2. This electron transport chain contains ferrodoxin and ends with the production of NADPH, not ATP.

Cyclic Photophosphorylation• The purpose of cyclic photophosphorylation is to produce

ATP. No NADPH is pro duced, and no oxygen is released.• The production of sugar during the Calvin cycle consumes

enormous amounts of ATP, periodically, the chloroplast runs low on ATP. When it does, the chloroplast carries out cyclic photophosphorylation to replenish the ATP levels.

• Cyclic electron flow takes photo excited electrons on a short-circuit pathway. Electrons travel from the P680 electron transport chain to P700, to a primary elec tron acceptor, and then back to the cytochrome complex in the P680 electron transport chain

THE CALVIN CYCLE• Is a cyclical process that produces the 3-carbon sugar PGAL

(phosphoglyceraldehyde). Carbon enters the stomates of a leaf in the form of CO2 and becomes fixed or incorporated into PGAL.

Important aspects of the Calvin cycle:• The process that occurs during the Calvin cycle is carbon fixation.• It is a reduction reaction since carbon is gaining hydrogen.• CO2 enters the Calvin cycle and becomes attached to a 5-carbon

sugar, ribulose biphosphate (RuBP), forming a 6-carbon molecule. The 6-carbon molecule is unstable and immediately breaks down into two 3-carbon molecules of 3-phos phoglycerate (3-PGA). The enzyme that catalyzes this first step is ribulose biphosphate carboxylase (rubisco).

• It uses the products of the light reactions: ATP and NADPH.• Occurs only in the light.

PHOTORESPIRATION• In most plants, CO2 enters the Calvin cycle and is fixed into 3-PGA

by rubisco. These plants are- C-3 plants because the first step produces the compound 3-PGA, which contains three carbons.

• This first step is not very efficient because rubisco binds with 02 as well as with CO2. When rubisco binds with 02 instead of CO2, this process, photorespiration, diverts the process of photosynthesis in two ways:

1. Unlike normal respiration, no ATP is produced2. Unlike normal photosynthesis, no sugar is formed• Instead, peroxisomes break down the products of

photorespiration. • Does not produce any useful products• This process is probably a vestige from the ancient earth when the

atmosphere had little or no free oxygen to divert rubisco and sugar production.

C-4 photosynthesis

• is a modification for dry environments. • C-4 plants exhibit modified anatomy and biochemical

pathways that enable them to minimize excess water loss and maximize sugar production.

• As a result, C-4 plants thrive in hot and sunny environments where C-3 plants would wilt and die. Examples of C-4 plants are corn, sugar cane, and crabgrass.

• In C-4 plants, a series of steps precedes the Calvin cycle. The steps pump CO2 that entered the leaf away from the air spaces near the stomates.

The details are described: •CO2 enters the mesophyll cell of the leaf and combines with a 3-carbon molecule, PEP (phosphoenolpyruvate), to form the 4-carbon molecule (C-4 plants) oxaloacetate. •The enzyme that catalyzes this reac tion, PEP carboxylase, does not bind with oxygen and can therefore fix CO2 more efficiently than rubisco. •From oxaloacetate the mesophyll cell produces malic acid or malate, which it pumps through the plasmodesmata into the adjacent bundle sheath cell. Once the malate is in the bundle sheath cell, it releases its CO2, which gets incorporated into PGAL by the Calvin cycle.• Because the bundle-sheath cell is deep within the leaf and little oxygen is present, rubisco can fix CO2 efficiently without being diverted to the dead end of photorespiration. •This biochemical pathway is called the Hatch-Slack pathway. Its purpose is to remove CO2 from the air space near the stomate.•With CO2 inside bundle-sheath cells, there is a steep CO2 gradient between the airspace in the mesophyll of the leaf near the stomates and the atmo sphere around the leaf. •Thus, C-4 plants can maximize the amount of CO2 that diffuses into the air space in the leaf and minimize the time stomates must remain open.

CAM-Photosynthesis Crassulacean Acid Metabolism

• Stomata open at night (when evaporation rates are usually lower) and are usually closed during the day.

• Similar to C4 photosyn. the oxaloacetate is converted to malic acid and stored during the night. During the day, the acid is broken down and the CO2 is released to RUBISCO for photosynthesis

• Adaptive Value: – Better Water Use Efficiency than C3 plants under

arid conditions due to opening stomata at night when transpiration rates are lower (no sunlight, lower temperatures, lower wind speeds, etc.).

• CAM plants include many succulents such as cacti and agaves and also some orchids and bromeliads