Bio-Chapt6-Photosynthesis

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Photosynthesis

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Transformation of solar energy into the chemical energy of a carbohydrate

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Stomata

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Small openings in the leaves of a plant that allow carbon dioxide to enter

Sing- stoma

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Mesophyll cells

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Conductors of photosynthesis. Co2 and water diffuse into these cells and then into the chloroplasts

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Chloroplast

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Organelle of a mesophyll cell that carries out photosynthesis. Double membrane surrounds fluid filled stroma, membrane system within stroma contain thylakoids, stacked to form grana

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Stroma

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Fluid filled area in a chloroplast surrounded by a double membrane

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Thylakoids

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Flattened sacs within the stroma formed by a membrane system

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Grana

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Stacked thylakoids

Sing- granum

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Thylakoid space

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Inner compartment within chloroplasts, formed because space within each thylakoid is connected to each other

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Chlorophyll

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Pigment within the membrane of the thylakoid capable of absolving solar energy

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Reduction

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Molecule gains electrons and hydrogen ions

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Oxidation

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Molecule gives up electrons and hydrogen ions

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Redox reactions

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Coupled reactions of reduction and oxidation

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Light reactions -“photo”

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Occurring in the thylakoids:

-chlorophyll absorbs solar energy and energizes electrons

-water is oxidized, releasing electrons, H+, and oxygen

-ATP is produced from ADP+P with help of an electron transport chain

-NADP+, an enzyme helper, accepts electrons and becomes NAPDH

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Calvin cycle- “synthesis”

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Occurring in the stroma:

-CO2 is taken up by one of the molecules in the cycle

-ATP and NADPH from the light reactions reduction CO2 to a carbohydrate (G3P)

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Photosynthetic pigments

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Chlorophylls-absorb violet, blue and red wavelength best

Carotenoids- absorb violet blue green range

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Photosystems

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Process by which light reactions capture solar energy and store in hydrogen ion gradient & generate ATP/NADPH. Consist of a pigment complex and electron acceptor

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PSII

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Splits water. Due to absorption of solar energy, electrons in reaction center of PSII are energize and escape to nearby electron acceptor molecule. Replacement electrons are removed from water, split by photolysis, releasing oxygen and two hydrogen ions.

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Electron transport chain

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A series of carriers pass electrons from one to another, releasing energy stored in the form of a hydrogen ion gradient.

“Establishes an energy gradient”

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PSI

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Produces NADPH. Absorbs solar energy, its electrons escaping to an electron acceptor. Replaced by low energy electrons from the electron transport chain. Electron acceptor passes its electrons to NADP+ molecule, this accepts electrons and a hydrogen ion and is reduced to NADPH

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ATP production

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Thylakoid space acts as reservoir for hydrogen items. Each time water is split two hydrogen ions remain in the thylakoid space. As electrons move down the electron transport chain they give up energy. This energy is used to pump H+ from the stroma into the thylakoid space, establishing a H+ gradient, which contains potential energy. H+ then flows down its concentration gradient across the thylakoid membrane at the ATP synthase complex, and energy is released, causing coenzyme ATP synthase to change shape and produce ATP from ADP + P. The production of ATP captures the released energy

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NADPH production

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NADPH+ coenzyme (non protein helper) that accepts electrons and then hydrogen ion to become NADPH.

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ATP synthase

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Coenzyme involved in producing ATP from ADP +P

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ATP synthase complex

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Space in the thylakoid membrane where hydrogen ions flow down concentration gradient

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Calvin Cycle Stages

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1) CO2 fixation

2) CO2 reduction

3) regeneration of the first substrate, RuBP

These reactions produce molecules of G3P which are used to produce glucose and other organic molecules. Processes are powered by ATP/NADPH from light reactions

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CO2 fixation

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CO2 from atmosphere attached to RuBP, a 5 carbon acceptor molecule. Results in a 6 carbon molecule that splits into 2 3 carbon molecules

(Fixed-incorporated into organic molecule)

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RuBP carboxylase (rubisco)

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Enzyme for reactions in co2 fixation

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Reduction of CO2

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Sequence of reactions powered by NADPH and some ATP from light reactions. Electrons are added to NADPH and carbon dioxide is reduced to G3P, a carb

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Regeneration of RuBP

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Uses ATP from the light reactions to regenerate RuBP so that it can return to the beginning of the cycle. For every three turns of the cycle 5 molecules of G3P are used to re-form 3 molecules of RuBP needed to begin the next cycle. One G3P leaves the cycle to become sugar

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RuBP

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5 carbon acceptor molecule- utilizes enzyme RuBisCo in reaction to attach to CO2

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G3P

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Building block for all molecules plant/algae needs. Fatty acids, glycerol, which are combined into plant oils. Amino acids formed from nitrogen added to the hydrocarbon skeleton of G3P. Glucose phosphate most often the result of G3P metabolism. Can combine with fructose to form sucrose (carb transport molecule in plants). Starting point for the synthesis of starch (storage form of glucose) and cellulose (structural component of plant cell walls-fiber in diets cannot be digested)

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C3 photosynthesis

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Characterized by above light reactions/Calvin cycle. Moderate temp/rainfall. First detectable molecule after co2 fixation a molecule made of three carbon atoms. Calvin cycle occurs in chloroplasts. Majority of plants

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Photorespiration

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Competition between trapped o2 within leaf spaces and c02 for the active site of rubisco (first enzyme of the Calvin cycle), thus less c3 is produced. Reduced efficiency of c3 plants in hot, dry conditions.

(Hot dry conditions lead to the closing of the stomata, which prevents the loss of water but also prevents co2 from entering and traps o2)

Wheat, rice, oats

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C4 photosynthesis

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Partitioning in space. Adaptation for hot, dry climates. First detectable molecule following co2 fixation is a 4 carbon atom molecule. Chloroplasts located in mesophyll cells but also bundle sheath cells which surround the leaf vein. Mesophyll cells arranged concentrically around bundle sheath cells, shielding them from o2 in leaf spaces. Calvin cycle occurs only in the bundle sheath cells. Co2 needed for Calvin cycle taken not from air but from the mesophyll cells where it has been fixed by a c3 molecule, forming a c4 molecule. C4 molecule is modified and pumped into bundle sheath cells. In hot climates net photosynthesis is 2-3 times that of a c3 plant

Sugarcane, corn, Bermuda grass, crabgrass

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CAM photosynthesis

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Crassulacean-acid metabolism. Succulent plants grow in desert (cacti). Partitioning in time. At night c3 molecules used to fix co2 molecules forming c4 molecules. Stored in vacuoles in mesophyll cells. During day c4 molecules release co2 to the Calvin cycle when NADPH/ATP are available from light reactions. Primary advantage conservation of water. Stomata only open at night, therefore only then is atmospheric co2 available. Photosynthesis is limited in these plants, but allows them to live in stressful conditions

Cacti, stonecrops, orchids, bromeliads. Some ferns and cone-bearing trees