Photosynthesis

Photosynthesis

Chapter 7

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Photosynthesis Overview

Energy for all life on Earth ultimately comes from photosynthesis.

6CO2 + 12H2O C6H12O6 + 6H2O + 6O2

Oxygenic photosynthesis is carried out by:cyanobacteria, 7 groups of algae, all land plants


• Photosynthesis takes place in 3 stages:– Capturing energy from sunlight– Using the energy to make ATP and reduce

NADP+ to NADPH• (nicotinamide adenine dinucleotide phosphate)

– Using the ATP and NADPH to synthesize organic molecules (glucose) from CO2

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Photosynthesis is divided into:

light-dependent reactions

-capture energy from sunlight

-make ATP and reduce NADP+ to NADPH

carbon fixation reactions (light-independent reactions)

-use ATP and NADPH to synthesize organic molecules from CO2

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• Photosynthesis takes place in chloroplasts.

• thylakoid membrane – internal membrane arranged in flattened sacs– contain chlorophyll and other pigments– Organized into photosystems

• Capture light and transfer energy (to pigment molecules)

• grana – stacks of thylakoid membranes• stroma – semiliquid substance surrounding thylakoid

membranes (houses the enzymes to make organic molecules)


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• Photosynthesis takes place in the green portions of plants

– Leaf of flowering plant contains mesophyll tissue

– Cells containing chloroplasts

– Specialized to carry on photosynthesis

• CO2 enters leaf through stomata

– Diffuses into chloroplasts in mesophyll cells

– In stroma, CO2 fixed to C6H12O6 (sugar)

– Energy supplied by light

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Discovery of Photosynthesis

The work of many scientists led to the discovery of how photosynthesis works.

Jan Baptista van Helmont (1580-1644)

Joseph Priestly (1733-1804)

Jan Ingen-Housz (1730-1799)

F. F. Blackman (1866-1947)

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Discovery of Photosynthesis

C. B. van Niel, 1930’s

-proposed a general formula:

CO2+H2A + light energy CH2O + H2O + 2A

where H2A is the electron donor

-van Niel identified water as the source of the O2 released from photosynthesis

-Robin Hill (in the 1950s) confirmed van Niel’s proposal that energy from the light reactions fuels carbon fixation (making glucose from CO2)

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Pigments

photon: a particle of light

-acts as a discrete bundle of energy

-energy content of a photon is inversely proportional to the wavelength of the light

photoelectric effect: removal of an electron from a molecule by light

-occurs when photons transfer energy to electrons

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Electromagnetic Spectrum

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Pigments

Pigments: molecules that absorb visible light

Each pigment has a characteristic absorption spectrum, the range and efficiency of photons it is capable of absorbing.

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Pigments

chlorophyll a – primary pigment in plants and cyanobacteria

-absorbs violet-blue and red light

chlorophyll b – secondary pigment absorbing light wavelengths that chlorophyll a does not absorb

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•A graph of percent of light absorbed at each wavelength is a compound’s absorption spectrum.•Action spectrum

•Oxygen production and therefore photosynthetic activity is measured for plants under each specific wavelength; when plotted on a graph, this gives an action spectrum for a compound. •The action spectrum for chlorophyll resembles its absorption spectrum, thus indicating that chlorophyll contributes to photosynthesis.

Pigments

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Pigments

accessory pigments: secondary pigments absorbing light wavelengths other than those absorbed by chlorophyll a

-increase the range of light wavelengths that can be used in photosynthesis

-include: chlorophyll b, carotenoids, phycobiloproteins

-carotenoids also act as antioxidants

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Photosystem OrganizationA photosystem consists of1. an antenna complex (light harvesting

complex) of hundreds of accessory pigment molecules that gather photons and feeds energy to reaaction center

2. a reaction center of one or more chlorophyll a molecules pass electrons out of photosystem (photochemical reactions)

In summary, energy of electrons is transferred through the antenna complex to the reaction center.

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Photosystem OrganizationAt the reaction center

(transmembrane protein complex), the energy from the antenna complex is transferred to chlorophyll a.

This energy causes an electron from chlorophyll to become excited.

The excited electron is transferred from chlorophyll a to an electron acceptor.

Water donates an electron to chlorophyll a to replace the excited electron.


•Light Reactions•Two electron pathways operate in the thylakoid membrane: the noncyclic pathway and the cyclic pathway.•Both pathways produce ATP; only the noncyclic pathway also produces NADPH. •ATP production during photosynthesis is called photophosphorylation; therefore these pathways are also known as cyclic and noncyclic photophosphorylation.

• Takes place in thylakoid membrane• Uses two photosystems, PS-I and PS-II (consists of pigment

complexes)• PS II captures light energy• Causes an electron to be ejected from the reaction center

(chlorophyll a)– Electron travels down electron transport chain to PS I– Replaced with an electron from water– causes H+ to concentrate in thylakoid chambers– causes ATP production

• PS I captures light energy (electrons and H)– Transferred permanently to a molecule of NADP+

– Causes NADPH production

Light Reactions:The Noncyclic Electron Pathway

Light Reactions:Noncyclic Electron Pathway

• Uses only photosystem I (PS-I)

• Begins when PS I complex absorbs solar energy

• Electron ejected from reaction center

– Travels down electron transport chain

– Causes H+ to concentrate in thylakoid chambers

– Which causes ATP production

– Electron returns to PS-I (cyclic)

• Pathway only results in ATP production

Light Reactions:The Cyclic Electron Pathway

Light Reactions:Cyclic Electron Pathway

•The Organization of the Thylakoid Membrane•PS II consists of a pigment complex and electron-acceptor molecules; it oxidizes H2O and produces O2.•The electron transport system consists of cytochrome complexes and transports electrons and pumps H+ ions into the thylakoid space.•PS I has a pigment complex and electron-acceptor molecules; it is associated with an enzyme that reduces NADP+ to NADPH.•ATP synthase complex has an H+ channel and ATP synthase; it produces ATP.

• Thylakoid space acts as a reservoir for hydrogen ions (H+)• Each time water is oxidized, two H+ remain in the thylakoid

space• Electrons yield energy

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

• Flow of H+ back across thylakoid membrane– Energizes ATP synthase– Enzymatically produces ATP from ADP + P

• This method of producing ATP is called chemiosmosis

ATP Production

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Calvin Cycle Reactions:Carbon Dioxide Fixation

• CO2 is attached to 5-carbon RuBP molecule

– Result in a 6-carbon molecule

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

– Reaction accelerated by RuBP Carboxylase (Rubisco)

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

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Calvin Cycle Reactions:Carbon Dioxide Reduction

• 3PG reduced to BPG

• BPG then reduced to G3P

• Utilizes NADPH and some ATP produced in light reactions

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Calvin Cycle Reactions:Regeneration of RuBP

• RuBP used in CO2 fixation must be replaced

• Every three turns of Calvin Cycle,

– Five G3P (a 3-carbon molecule) used To remake three RuBP (a 5-carbon molecule)

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The Calvin Cycle: Fixation of CO2

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Importance of Calvin Cycle• G3P (glyceraldehyde-3-phosphate) can be

converted to many other molecules

• The hydrocarbon skeleton of G3P can form

– Fatty acids and glycerol to make plant oils

– Glucose phosphate (simple sugar)

– Fructose (which with glucose = sucrose)

– Starch and cellulose

– Amino acids

Other Types of PhotosynthesisC4 Photosynthesis and CAM Photosynthesis

•In C3 plants, the Calvin cycle

fixes CO2 directly; the first

molecule following CO2

fixation is 3PG.•In hot weather, stomata close to save water; CO2

concentration decreases in leaves; O2 increases.

•O2 combines with RuBP

instead of CO2 •This is called photorespiration since oxygen is taken up and CO2

is produced; this produces less 3PG.

Most plants are C3 plants

•In a C3 plant, mesophyll cells contain well‑formed chloroplasts,

arranged in parallel layers.•In C4 plants, bundle sheath cells as well as the mesophyll cells

contain chloroplasts.•In C4 leaf, mesophyll cells are arranged concentrically around the

bundle sheath cells.

C4 Photosynthesis

•Remember C3 plants use RuBP carboxylase to fix CO2

to RuBP in mesophyll; the first detected molecule is 3PG.•C4 plants use the enzyme PEP carboxylase

(PEPCase) to fix CO2 to PEP (phosphoenolpyruvate, a

C3 molecule); the end product is oxaloacetate (a C4

molecule).•In C4 plants, CO2 is taken up in mesophyll cells and

malate, a reduced form of oxaloacetate, is pumped into the bundle‑sheath cells; here CO2 enters Calvin cycle.

•In hot, dry climates, net photosynthetic rate of C4

plants (e.g., corn) is 2–3 times that of C3 plants.

•Photorespiration does not occur in C4 leaves because

PEP does not combine with O2; even when stomata are

closed, CO2 is delivered to the Calvin cycle in bundle

sheath cells.•C4 plants have advantage over C3 plants in hot and dry

weather because photorespiration does not occur; e.g., bluegrass (C3) dominates lawns in early summer,

whereas crabgrass (C4) takes over in the hot

midsummer.

C4 Photosynthesis

•CAM (crassulacean‑acid metabolism) plants form a C4 molecule at night when stomata can

open without loss of water; found in many succulent desert plants including the family Crassulaceae. •At night, CAM plants use PEPCase to fix CO2 by

forming C4 molecule stored in large vacuoles in

mesophyll.•C4 formed at night is broken down to CO2 during

the day and enters the Calvin cycle which now has NADPH and ATP available to it from the light‑dependent reactions.•CAM plants open stomata only at night, allowing CO2 to enter photosynthesizing tissues; during

the day, stomata are closed to conserve water but now CO2 cannot enter photosynthesizing

tissues.•Photosynthesis in a CAM plant is minimal, due to limited amount of CO2 fixed at night; but this

does allow CAM plants to live under stressful conditions.

CAM Photosynthesis

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Photosynthesis