Chapter 7: Photosynthesis (Outline)

Photosynthetic Organisms Flowering Plants as Photosynthesizers

Photosynthesis Light Reactions

Noncyclic Pathway Calvin Cycle Reactions

Carbon Dioxide Fixation Reduction of Carbon Dioxide Regeneration of RuBP C4 CAM

Photosynthetic Organisms

Organisms cannot live without energy

Almost all the energy that organisms used comes from the solar energy

Photosynthesis:

A process that captures solar energy and transforms it into chemical energy (carbohydrate)

Occurrs in higher plants, algae, mosses, photosynthetic protists and some bacteria (cyanobacteria)

Photosynthetic Organisms

Autotrophs – organisms having the ability to synthesize carbohydrates

Heterotrophs – are consumers that use carbohydrates produced by photosynthesis

Flowering Plants & Photosynthesis

Photosynthesis takes place in the green portions of plants Leaf of flowering plant contains mesophyll tissue

with chloroplasts Specialized to carry on photosynthesis

CO2 enters leaf through stomata Diffuses into chloroplasts in mesophyll cells

In the stroma, CO2 is combined with H2O to form C6H12O6 (sugar)

Chlorophyll is capable of absorbing solar energy, which drives photosynthesis

Photosynthesis

The process of photosynthesis is an example of a redox reaction (i.e. movement of electrons from one molecule to another) In living things, electrons are accompanied by

hydrogen ions (H+ + e)

Oxidation is the loss of hydrogen atoms; Reduction is the gain of hydrogen atoms

Photosynthesis

Carbon dioxide reduction requires energy and hydrogen atoms Solar energy will be used to generate ATP needed

to reduce CO2 to sugar

Electrons needed to reduce CO2 will be carried by a coenzyme (NADP+)

NADP+ is the active redox coenzyme of photosynthesis When NADP+ is reduced, it accepts 2 e- and H+

When NADP+ is oxidized, it gives up its electrons

Photosynthetic Reactions

In the 1930’s, C. B. van Niel studying photosynthesis in bacteria discovered that CO2 was not split in the process of carbohydrate synthesis

Researchers later confirmed using the isotope oxygen (18O) that O2 given off from photosynthesis comes from water not CO2

When water splits, O2 is released and the hydrogen atoms (2 e- + H+) are taken up by NADPH

Photosynthetic Reactions

Light Reactions: Chlorophyll present in thylakoid membranes

absorbs solar energy This energizes electrons Electrons move down electron transport chain

Pumps H+ into thylakoids Used to make ATP out of ADP and NADPH out of

NADP+

Calvin Cycle Reactions: In the stroma, CO2 is reduced to a carbohydrate Reduction requires the ATP and NADPH produced

in the light reactions

Photosynthesis Overview

Photosynthetic Pigments

Pigments: Molecules that absorb some colors in rainbow

(visible light) more than others, why? Colors least absorbed reflected/transmitted most

Absorption Spectra Graph showing relative absorption of the various

colors of the rainbow Chlorophylls a and b (main pigments) are green

because they absorb much of the reds and blues of white light

Carotenoids are accessory pigments

Phosynthetic Pigments and Photosynthesis

Spectrophotometer An instrument which measures the amount of

light of a specified wavelength which passes through a medium

Amount of light absorbed at each wavelength is plotted on a graph, resulting in an absorption spectra

Light Reactions

The light reactions utilize two photosystems:

Photosystem I (PSI)

Photosystem II (PSII)

A photosystem consists of

Pigment complex (molecules of chlorophylls a & b the carotenoids) and electron acceptor molecules that help collect solar energy like an antenna

Occur in the thylakoid membranes

The Noncyclic Electron Pathway

Uses two photosystems, PS I and PS II Energy enters the system when PS II becomes

excited by light Causes an electron to be ejected from the

reaction center (chlorophyll a) Electron travels down electron transport chain to

PS I PS II takes replacement electron from H2O, which

splits, releasing O2 and H+ ions The H+ ions concentrate in thylakoid chambers Which causes ATP production, how?

The Noncyclic Electron Pathway

PS I captures light energy and ejects the energized electron (e-) from the reaction center Transferred permanently by electron

acceptors to a molecule of NADP+

Causes NADPH production NADP+ + 2 e- + H+ NADPH and ATP produced are used by the

Calvin cycle reaction (reduction of CO2) in the stroma

Organization of theThylakoid Membrane PS II:

Pigment complex and electron-acceptors Adjacent to an enzyme that oxidizes water Oxygen is released as a gas (by-product)

Electron transport chain: Consists of Pq (plastoquinone) and cytochrome complexes Carries electrons from PS II to PS I Pumps H+ from the stroma into thylakoid space

PS I: Pigment complex and electron acceptors Adjacent to enzyme that reduces NADP+ to NADPH

ATP synthase complex: Has a channel for H+ flow Which drives ATP synthase to join ADP and Pi

Organization of a Thylakoid

ATP Production Thylakoid space acts as a reservoir for H+ ions Each time water is oxidized, two H+ remain in

the thylakoid space Electrons yield energy

Used to pump H+ across thylakoid membrane Move from stroma into the thylakoid space

Flow of H+ back across thylakoid membrane Energizes ATP synthase Enzymatically produces ATP from ADP + Pi

This method of producing ATP is called chemiosmosis

Calvin Cycle Reactions:Overview of C3 Photosynthesis

A cyclical series of reactions

Utilizes atmospheric carbon dioxide to produce carbohydrates

Known as C3 photosynthesis

Involves three phases:

Carbon dioxide fixation

Carbon dioxide reduction

RuBP Regeneration

Calvin Cycle Reactions:Phase 1: Carbon Dioxide Fixation

CO2 enters the stroma of the chloroplast via the stomata of the leaves

CO2 is attached to 5-carbon RuBP molecule

Result in an intermediate 6-carbon molecule

This splits into two 3-carbon molecules (3PG)

Reaction is accelerated by RuBP Carboxylase (Rubisco)

CO2 now “fixed” because it is part of a carbohydrate

Calvin Cycle Reactions:Phase 2: Carbon Dioxide Reduction

ATP phosphorylates each 3PG molecule and creates 1,3-bisphosphoglycerate (BPG)

BPG is then reduced by NADPH to glyceraldehyde-3-phosphate (G3P)

NADPH and some ATP used for the reduction reactions are produced in light reactions

Calvin Cycle Reactions:Phase 3: Regeneration of RuBP

RuBP used in CO2 fixation must be replaced

Every three turns of Calvin Cycle, Five G3P molecules are used To remake three RuBP molecules 5 X 3 (carbons in G3P) = 3 X 5 (carbons in RuBP)

The Calvin Cycle

C3 Plants

C3 plants include more than 95% percent of the plant species on earth (e.g. trees)

Called C3 because the CO2 is first incorporated into a 3-carbon compound

In hot dry conditions, the photosynthetic efficiency of C3 plants suffers because of photorespiration Stomata must close to avoid wilting CO2 decreases and O2 increases

O2 starts combining with RuBP instead of CO2

CO2 + RUBP Rubisco    2 3PG

C4 Photosynthesis: C4 Plants

Use a supplementary method of CO2 uptake forming a 4-carbon molecule instead of the two 3-carbon molecules

In C4 plants CO2 is inserted into a 3-carbon molecule called

phosphoenolpyruvate (PEP) forming Oxaloacetate, a C4 molecule in the mesophyll cells Oxaloacetate is converted into malate, which is

transported into the bundle sheath cells Here the 4-carbon molecule is broken down into

CO2, which enters the Calvin cycle to form sugars & starch

Chloroplast distribution inC4 vs. C3 Plants

CO2 Fixation in C4 vs. C3 Plants

In hot and dry climates

C4 plants avoid photorespiration as PEP in mesophyll cells does not bind to O2

Net productivity of C4 is about 2-3 times of C3 plants

In cool, moist conditions, CO2 fixation in C3 is three times more efficient than in C4

CAM Photosynthesis

Called CAM after the plant family in which it was first found (Crassulaceae)

CAM plants partition carbon fixation by time

Crassulacean-Acid Metabolism During the night through their open stomata

CAM plants use PEPCase to fix CO2

Forms C4 molecules (malate)

Stored in large vacuoles in mesophyll cells

During daylight NADPH and ATP are available

Stomata closed for water conservation

C4 molecules release CO2 to Calvin (C3) cycle

CO2 Fixation in a CAM Plant

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.)