Chapter 5: Photosynthesis

2. Photolysis

- in photolysis, the splitting of water-provides 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 O2 molecule, which is released into the air as a waste product of photosynthesis H20 ---> 2H+ 2e- + 02

Modification for Dry Environment

-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 -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

Chlorophyll a

-Chlorophyll a is a pigment that participates directly in the light reactions of photosynthesis -it is a large molecule with a single magnesium atom in the head surrounded by alternating double and single bonds -the head of Chlorophyll a, called the porphyrin ring, is attached to a long hydrocarbon tail -the double bonds play a critical role in the light reactions; they are the source of electrons that flow through the electron transport chains during photosynthesis

NADP

-NADP becomes reduced when it picks up the two protons that were released from water in P680 -the newly formed NADPH carries hydrogen to the Calvin cycle to make sugar in the light-independent reactions

PS I

-PS I absorbs light best in the 700 nm range: hence it is also called P700

PS II

-PS II operates before PS I -PS II absorbs light best in the 680 nm range; hence it is also called P680

Antenna pigments

-antenna pigments are pigments such as chlorophyll b that capture light in wavelengths other than those captured by chlorophyll a -antenna pigments absorb photons of light and pass the energy along to chlorophyll a, which is directly involved in the transformation of light energy to sugars

Chemiosmosis

-chemiosmosis is the process by which ATP is formed during the light reactions of photosynthesis -protons that were released from water during photolysis are pumped by the thylakoid membrane from the stroma into the thylakoid space -ATP is formed as these protons diffuse down the gradient from the thylakoid space, through ATP synthase channels, and into the stroma -ATP produced here provides energy that powers Calvin cycle

Noncyclic photophosphorylation

-during noncyclic photophosphorylation, electrons enter two electron transport chains; the products are ATP and NADPH -process begins in PS II and proceeds through following steps: 1. Photosystem II(P680) 2. Photolysis 3. Electron Transport Chain 4. Chemiosmosis 5. NADP 6. Photosystem I(P700)

REMEMBER

-electrons flow from water to P680 to P700 to NADP, which carries them to the Calvin cycle

3. Electron Transport Chain

-electrons from P680 pass along an electron transport chain consisting of several molecules including cytochromes and several other proteins and ultimately end up in P700(PS I) -this flow of electrons is exergonic and provides energy to produce ATP by chemiosmosis, the same way ATP is produced in the mitochondria -because ATP synthesis is powered by light, it is called photophosphorylation

1: Photosystem II

-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 -the electrons are captured by a primary electron acceptor

Photosystem I- P700

-energy is absorbed by P700 -electrons from head of chlorophyll a become energized and are captured by a primary electron acceptor -electrons that escape from chlorophyll a are replaced with electrons from photosystem II -P700 produces NADPH, not ATP

Photorespiration

-in most plants CO2 enters the Calvin cycle and is fixed into 3-PGA by the enzyme rubisco; these plants are called C-3 plants because the first step produces the compound 3-PGA, which contains 3 carbons -first step of Calvin Cycle is not very efficient because rubisco binds with O2 as well as CO2 -when rubisco binds with O2 instead of CO2, this process, photorespiration, diverts the process of photosynthesis in two ways: 1. no ATP is produced 2. no sugar is formed -instead peroxisomes break down products of photorespiration

Light-Dependent Reactions-The Light Reactions

-in the light-dependent reactions, light is absorbed by the photosystems(PS II and PS I) in the thylakoid membranes and electrons flow through electron transport chains -there are two possible routes for electron flow: noncyclic flow and cyclic photophosphorylation

Photosynthesis

-photosynthesis is a process by which light energy is converted to chemical energy and carbon is fixed into organic compounds -photosynthesis provides all the oxygen we breathe, the food we eat, and the fossil fuel we consume -there are two main processes of photosynthesis: light dependent reactions and light independent reactions -both light dependent and light independent reactions occur only when light is present -photosynthesis is a reduction reaction

Photosynthetic Pigments

-photosynthetic pigments absorb light energy and use it to provide energy to carry out photosynthesis -plants contain two major groups of pigment, the chlorophylls and carotenoids -Chlorophyll a and chlorophyll b are green and absorb all wavelengths of light in red, blue, green, and violet range -carotenoids are yellow, orange, and red; they absorb light in blue, green, and violet range

Photosystems

-photosystems are light harvesting complexes in the thylakoid membranes of chloroplasts -there are a few hundred photosystems in each thylakoid -each photosystem consists of a reactions center containing chlorophyll a and a region containing several hundred antenna pigment molecules that funnel energy into chlorophyll a -there are two types of photosystems, PS I and PS II, which cooperate in the light reactions of photosynthesis

Calvin Cycle

-the Calvin Cycle is the main business of the light-independent reactions -the Calvin Cycle is a cyclical process that produces the 3-carbon sugar PGAL -carbon enters the stomates of a leaf in the form of CO2, and becomes fixed or incorporated into PGAL -the process that occurs during the Calvin cycle is carbon fixation -the Calvin Cycle is a reduction reaction since carbon is gaining protons and electrons -CO2 enters the Calvin cycle and becomes attached to a 5-carbon sugar, ribulose biphosphate(RuBP), forming a 6-carbon molecule -6-carbon molecule is unstable and immediately breaks down into 3-carbon molecules of 3-phosphoglycerate(3-PGA) -enzyme that catalyzes this first step is rubisco -calvin cycle does not directly depend on light; uses the products of light dependent reactions: ATP and NADPH -Calvin cycle, like the light-dependent reactions, occurs only in the light

Chloroplast

-the chloroplast contains photosynthetic pigments that, along with enzymes, carry out photosynthesis -the chloroplast contains grana, where light dependent reactions occur, and stroma, where light independent reactions occur -grana consist of layers of membranes called thylakoids, the site of photosystem I and II -chloroplast is enclosed by a double membrane

Light Dependent Reactions

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

Light Independent Reactions

-the light independent reactions consist of the Calvin cycle, which produces sugar -to power the production of sugar, the Calvin cycle uses ATP formed during the light dependent reactions

Cyclic Photophosphorylation

-the sole purpose of cyclic photophosphorylation is to produce ATP -no NADPH is produced, and no oxygen is released -the production of sugar that occurs during Calvin cycle consumes enormous amounts of ATP, so 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 P680 electron transport chain to P700, a primary electron acceptor, and then back to the cytochrome complex in the P680 electron transport chain

overview of noncyclic photophosphorylation

light-----> p680 oxygen released----> ATP produced------> P700--------> NADPH produced(NADPH carries H+ to Calvin cycle)