Photosynthesis

Factors affecting photosynthesis 1. Temperature Increases the rate of photosynthesis Most plants have an optimal range Higher temperatures denature the enzymes http://www.ont.co.za/Focus200307.jpg

Factors affecting Photosynthesis 2. Light intensity increasing intensity increases the rate of photosynthesis

Factors affecting Photosynthesis 3. Carbon dioxide Increasing carbon dioxide increases photosynthesis

Factors affecting Photosynthesis 4. Water If water is not available, stomata close and there is decreased CO2

The Chloroplast Stroma: The fluid in the chloroplast surrounding the thylakoid membrane, used in making sugar from carbon dioxide. Thylakoids: A flattened membrane sac, used to convert light energy to a useable form, where the pigment is. Granum: Stacked portion of the thylakoid membrane, used in the light reaction.

Excited Chlorophyll When photons of light are absorbed by the pigments, the energy cannot disappear. Instead, the electrons in the pigments are excited and move to a higher orbital. This excitement cannot last for long, and this energy needs to be stored in energy storing molecules.

Chlorophyll Required! Porphyrin ring with central Mg atom, attached to a hydrocarbon chain Different types have different functional groups Hydrocarbon chain holds the molecule in place in the membrane

The Overview

The Overview Photosynthesis is two processes, each with multiple stages. light dependent reactions convert solar energy to chemical energy. Includes exciting electrons and splitting water molecules 2) The light independent reactions. Includes the Calvin cycle which incorporates CO2 from the atmosphere into an organic molecule and uses energy from the light reaction to reduce the new carbon piece to sugar.

Light-Dependent phase Occurs in grana Sunlight energy hits chlorophyll exciting electrons. The return of electrons to ground state provides energy for 2 processes: A) Photolysis "light splitting" B) Phosphorylation "formation of ATP"

A) Photolysis Splitting of water into oxygen and hydrogen H2O  OH- + H+ + Energy Hydroxides combine to form water Oxygen is a waste product and diffuses out of chloroplast Provides electrons that will be excited when light is absorbed and fuel ATP formation. Hydrogen excess drives the rest of the reaction. Extra hydrogen picked up by H carrier: NADP NADP  NADPH2 NADPH2 diffuses out into stroma for use in the dark phase (Calvin Cycle)

B) Phosphorylation Formation of ATP ADP + Pi  ATP used in dark phase

Electron Transport Chain Excited electrons are passed along the thylakoid membrane ( much like in cellular respiration) through the use of special membrane proteins The energy from the electrons is used to pump Hydrogen ions H+ into the Thylakoid inner membrane creating a large concentration gradient. When Hydrogen ions flow back across the membrane into the stroma, ATP is formed from ADP, Pi and ATP Synthase. This ATP will be used for energy to convert CO2 and H20 into GLUCOSE!!!!

There are two types of photosystems, Photosystem II and Photosystem I There are two types of photosystems, Photosystem II and Photosystem I. (PSII goes 1st, weird, I know!).

Light Dependent Phase-thylakoid Groups of chlorophyll and carotenoid pigment molecules are found in the thylakoid membrane Photosystem I Photosystem II pigment molecules in both photosystems absorb light energy causing electrons to become excited fueling the formation of ATP and NADPH ( energy carrying molecules).

The End Results of the Light Reaction The light-reaction “machinery” produces ATP and NADPH on the stroma side of the thylakoid. electrons are pushed from water, where they are at low potential energy, to NADPH, where they have high potential energy. This process also produces ATP. Oxygen is a byproduct.

The End Results of the Light Reaction

Light-independent "dark" phase Calvin cycle or "carbon fixation phase" Occurs in stroma hydrogen combines with carbon dioxide to form glucose Hydrogen and CO2 diffuse into the leaf Combines in a series of cyclic enzyme reactions to form glucose ATP from light phase provides energy

The Calvin Cycle Each turn of the Calvin cycle fixes one carbon. For the net synthesis of one G3P molecule, the cycle must take place three times, fixing three molecules of CO2. To make one glucose molecules would require six cycles and the fixation of six CO2 molecules.

Part 1 of the Calvin Cycle In the carbon fixation phase, each CO2 molecule is attached to a five-carbon sugar, ribulose bisphosphate (RuBP). This is catalyzed by RuBP carboxylase or rubisco (an enzyme). The six-carbon intermediate splits in half to form two molecules of 3-phosphoglycerate per CO2.

Part 1

Part 2 of the Calvin Cycle During reduction (the addition of electrons to a substance), each 3-phosphoglycerate receives another phosphate group from ATP to form 1,3 bisphosphoglycerate. A pair of electrons from NADPH reduces each 1,3 bisphosphoglycerate to G3P.

Part 2

Part 3 of the Calvin Cycle In the last phase, regeneration of the CO2 acceptor (RuBP), these five G3P molecules are rearranged to form 3 RuBP molecules. To do this, the cycle must spend three more molecules of ATP (one per RuBP) to complete the cycle and prepare for the next.

A Big Issue… One of the major problems facing terrestrial plants is dehydration. At times, solutions to this problem conflict with other metabolic processes, especially photosynthesis. The stomata are not only the major route for gas exchange (CO2 in and O2 out), but also for the evaporative loss of water. On hot, dry days plants close the stomata to conserve water, but this causes problems for photosynthesis.

Photorespiration In most plants initial fixation of CO2 occurs via rubisco and results in a three-carbon compound, 3-phosphoglycerate. When their stomata are closed on a hot, dry day, CO2 levels drop as CO2 is consumed in the Calvin cycle. At the same time, O2 levels rise as the light reaction converts light to chemical energy. While rubisco normally accepts CO2, when the O2/CO2 ratio increases (on a hot, dry day with closed stomata), rubisco can add O2 to RuBP.

When rubisco adds O2 to RuBP, RuBP splits into a three-carbon piece and a two-carbon piece in a process called photorespiration. The two-carbon fragment is exported from the chloroplast and degraded to CO2 by mitochondria and peroxisomes. Unlike normal respiration, this process produces no ATP, nor additional organic molecules. Photorespiration decreases photosynthetic output by taking organic material from the Calvin cycle. Photorespiration can drain away as much as 50% of the carbon fixed by the Calvin cycle on a hot, dry day. Certain plant species have evolved alternate modes of carbon fixation to minimize photorespiration.

Chloroplasts vs. Mitochondria Chloroplasts and mitochondria generate ATP by the same mechanism: chemiosmosis. An electron transport chain pumps protons across a membrane as electrons are passed along a series of more electronegative carriers. This builds the proton-motive force in the form of an H+ gradient across the membrane. ATP synthase molecules harness the proton-motive force to generate ATP as H+ diffuses back across the membrane. Mitochondria transfer chemical energy from food molecules to ATP and chloroplasts transform light energy into the chemical energy of ATP.

Chloroplasts vs. Mitochondria