Photosynthesis

Chapter 8

Adapted by G. Cornwall, Ph.D. From Raven’s Biology, McGraw Hill Publishing

2

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 – chloroplasts

Chloroplast• Thylakoid membrane –

internal membrane– Contains chlorophyll and other

photosynthetic pigments– Pigments clustered into

photosystems• Grana – stacks of flattened

sacs of thylakoid membrane• Stroma lamella – connect

grana• Stroma – semiliquid

surrounding thylakoid membranes

3

4

Stages• Light-dependent reactions

– Require light1.Capture energy from sunlight2.Make ATP and reduce NADP+ to NADPH

• Carbon fixation reactions or light-independent reactions– Does not require light3.Use ATP and NADPH to synthesize organic

molecules from CO2

5

6

Pigments• Molecules that absorb light energy in the visible

range• Light is a form of energy• Photon – 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

7

Absorption spectrum• When a photon strikes a

molecule, its energy is either – Lost as heat– Absorbed by the electrons

of the molecule• Boosts electrons into higher

energy level

• Absorption spectrum – range and efficiency of photons molecule is capable of absorbing

• Organisms have evolved a variety of different pigments

• Only two general types are used in green plant photosynthesis– Chlorophylls– Carotenoids

• In some organisms, other molecules also absorb light energy

8

Chlorophylls

• Chlorophyll a– Main pigment in plants and cyanobacteria– Only pigment that can act directly to convert

light energy to chemical energy– Absorbs violet-blue and red light

• Chlorophyll b– Accessory pigment or secondary pigment

absorbing light wavelengths that chlorophyll a does not absorb

9

• Structure of chlorophyll

• porphyrin ring– Complex ring structure

with alternating double and single bonds

– Magnesium ion at the center of the ring

• Photons excite electrons in the ring

• Electrons are shuttled away from the ring

10

• Action spectrum– Relative effectiveness of different

wavelengths of light in promoting photosynthesis

– Corresponds to the absorption spectrum for chlorophylls

11

• Carotenoids– Carbon rings linked to

chains with alternating single and double bonds

– Can absorb photons with a wide range of energies

– Also scavenge free radicals – antioxidant• Protective role

• Phycobiloproteins– Important in low-light

ocean areas12

13

Photosystem Organization• Antenna complex

– Hundreds of accessory pigment molecules

– Gather photons and feed the captured light energy to the reaction center

• Reaction center– 1 or more chlorophyll a

molecules– Passes excited electrons

out of the photosystem

Antenna complex• Also called light-harvesting

complex• Captures photons from

sunlight and channels them to the reaction center chlorophylls

• In chloroplasts, light-harvesting complexes consist of a web of chlorophyll molecules linked together and held tightly in the thylakoid membrane by a matrix of proteins

14

Reaction center• Transmembrane protein–

pigment complex• When a chlorophyll in the

reaction center absorbs a photon of light, an electron is excited to a higher energy level

• Light-energized electron can be transferred to the primary electron acceptor, reducing it

• Oxidized chlorophyll then fills its electron “hole” by oxidizing a donor molecule 15

16

Light-Dependent Reactions1. Primary photoevent

– Photon of light is captured by a pigment molecule2. Charge separation

– Energy is transferred to the reaction center; an excited electron is transferred to an acceptor molecule

3. Electron transport– Electrons move through carriers to reduce NADP+

4. Chemiosmosis– Produces ATP

Cap

ture

of l

ight

ene

rgy

17

• In sulfur bacteria, only one photosystem is used

• Generates ATP via electron transport

• Anoxygenic photosynthesis

• Excited electron passed to electron transport chain

• Generates a proton gradient for ATP synthesis

Cyclic photophosphorylation

18

Chloroplasts have two connected photosystems

• Oxygenic photosynthesis• Photosystem I (P700)

– Functions like sulfur bacteria• Photosystem II (P680)

– Can generate an oxidation potential high enough to oxidize water

• Working together, the two photosystems carry out a noncyclic transfer of electrons that is used to generate both ATP and NADPH

19

20

• The Two Photosystems in Plants Work Together– Photosystem II drives ATP production

• Electron is passed to the reaction center of PS I (Replaces electron lost through excitation)

• the energy released is used to synthesize ATP– Photosystem I drives NADPH production.

• Electron is passed to NADP+ NADPH

21

Noncyclic photophosphorylation

• Plants use photosystems II and I in series to produce both ATP and NADPH

• Path of electrons not a circle• Photosystems replenished

with electrons obtained by splitting water

• Z diagram

Chemiosmosis• Electrochemical gradient used to synthesize ATP• Chloroplast has ATP synthase enzymes in the

thylakoid membrane– Allows protons back into stroma

• Stroma also contains enzymes that catalyze the reactions of carbon fixation – the Calvin cycle reactions

22

• Making more ATP– It takes more energy to fix Carbon (1 ½ ATP /

NADPH)– PS I can short circuit to make extra ATP via Cyclic

phosphorylation.

Card Quiz A

What high energy molecule is the final product of photosynthesis?

Oxygen ATP NADPH Glucose

Card Quiz AWhere is the chlorophyll located in a plant?

Cristae Outer membrane of the chloroplast Thylakoid membrane Stroma

Card Quiz AA particle of light is a ____.

Wave Photon Proton Newton

Card Quiz AWhat happens at the reaction center of a photosystem?

Light is absorbed An electron is energized NADP+ is reduced ATP is formed

Card Quiz AWhich of the following is made during the light reactions.

ADP and NADP+ Glucose and ATP ATP and NADPH NADPH and Carbon Dioxide

Card Quiz Answers

Green Red Blue

Green Green

30

Carbon Fixation – Calvin Cycle

• To build carbohydrates cells use• Energy

– ATP from light-dependent reactions– Cyclic and noncyclic photophosphorylation– Drives endergonic reaction

• Reduction potential– NADPH from photosystem I– Source of protons and energetic electrons

31

Calvin cycle

• Named after Melvin Calvin (1911–1997)• Also called C3 photosynthesis• Key step is attachment of CO2 to RuBP to

form PGA• Uses enzyme ribulose bisphosphate

carboxylase/oxygenase or rubisco

32

3 phases1. Carbon fixation

– RuBP + CO2 → PGA2. Reduction

– PGA is reduced to G3P3. Regeneration of RuBP

– PGA is used to regenerate RuBP

• 3 turns incorporate enough carbon to produce a new G3P

• 6 turns incorporate enough carbon for 1 glucose

33

34

Output of Calvin cycle

• Glucose is not a direct product of the Calvin cycle

• G3P is a 3 carbon sugar– Used to form sucrose

• Major transport sugar in plants• Disaccharide made of fructose and glucose

– Used to make starch• Insoluble glucose polymer• Stored for later use

35

Energy cycle• Photosynthesis uses the products of respiration

as starting substrates• Respiration uses the products of photosynthesis

as starting substrates• Production of glucose from G3P even uses part

of the ancient glycolytic pathway, run in reverse• Principal proteins involved in electron transport

and ATP production in plants are evolutionarily related to those in mitochondria

36

37

Photorespiration• Rubisco has 2 enzymatic activities

– Carboxylation • Addition of CO2 to RuBP• Favored under normal conditions

– Photorespiration• Oxidation of RuBP by the addition of O2• Favored when stoma are closed in hot conditions• Creates low-CO2 and high-O2

• CO2 and O2 compete for the active site on RuBP

38

Types of photosynthesis

• C3

– Plants that fix carbon using only C3 photosynthesis (the Calvin cycle)

• C4 and CAM– Add CO2 to PEP to form 4 carbon molecule– Use PEP carboxylase– Greater affinity for CO2, no oxidase activity– C4 – spatial solution– CAM – temporal solution

39

C4 plants• Corn, sugarcane, sorghum, and a

number of other grasses• Initially fix carbon using PEP

carboxylase in mesophyll cells• Produces oxaloacetate, converted to

malate, transported to bundle-sheath cells

• Within the bundle-sheath cells, malate is decarboxylated to produce pyruvate and CO2

• Carbon fixation then by rubisco and the Calvin cycle

• C4 pathway, although it overcomes the problems of photorespiration, does have a cost

• To produce a single glucose requires 12 additional ATP compared with the Calvin cycle alone

• C4 photosynthesis is advantageous in hot dry climates where photorespiration would remove more than half of the carbon fixed by the usual C3 pathway alone

40

41

CAM plants• Many succulent (water-storing) plants,

such as cacti, pineapples, and some members of about two dozen other plant groups

• Stomata open during the night and close during the day– Reverse of that in most plants

• Fix CO2 using PEP carboxylase during the night and store in vacuole

• When stomata closed during the day, organic acids are decarboxylated to yield high levels of CO2

• High levels of CO2 drive the Calvin cycle and minimize photorespiration

42

Compare C4 and CAM

• Both use both C3 and C4 pathways• C4 – two pathways occur in different cells• CAM – C4 pathway at night and the C3

pathway during the day

43

Card Quiz BWhich structure is responsible for gas exchange ?

Matrix Stroma Stoma Thylakoid

Question 4

Making glucose via photosynthesis costs ____ATPs and ___NADPHs. However, 1 molecule of glucose is enough to make ___ ATPs.

18, 12, 36 12, 18, 36 12, 36, 18 36, 18, 12

Question 9What is the function of Rubisco?

Absorption of photon energy Reduction of NADP+ Chemiosmosis Carbon fixation

Card Quiz BWhere could a botanist expect to find C4 plants?

Canada Costa Rica Tundra Mount Everest

Card Quiz B

The end product of photosynthesis is the starting material of cellular respiration.

a. This is trueb. This is false

Card Quiz B

Which of the following would prevent the formation of phosphoglycerate?

Denaturation of the Rubisco enzyme Non-functional b6-f complex Anaerobic conditions All of the above