How Cells Acquire Energy

Chapter 7

Photoautotrophs

• Capture sunlight energy and use it to carry out photosynthesis

– Plants

– Some bacteria

– Many protistans

Fig. 10-2

(a) Plants

(c) Unicellular protist10 µm

1.5 µm

40 µm(d) Cyanobacteria

(e) Purple sulfur bacteria

(b) Multicellular alga

Linked Processes

Photosynthesis

• Energy-storing pathway

• Releases oxygen

• Requires carbon dioxide

Aerobic Respiration

• Energy-releasing pathway

• Requires oxygen

• Releases carbon dioxide

Fig. 10-3Leaf cross section

Vein

Mesophyll

StomataCO2 O2

ChloroplastMesophyll cell

Outermembrane

Intermembranespace

5 µm

Innermembrane

Thylakoidspace

Thylakoid

GranumStroma

1 µm

Chloroplast Structure

two outer membranes

inner membrane system(thylakoids connected by channels)

stroma

Figure 7.3d, Page 116

Photosynthesis Equation

12H2O + 6CO2 6O2 + C2H12O6 + 6H2O

Water Carbon Dioxide

Oxygen Glucose Water

LIGHT ENERGY

In-text figurePage 115

Where Atoms End Up

Products 6O2 C6H12O6 6H2O

Reactants 12H2O 6CO2

In-text figurePage 116

Two Stages of Photosynthesis

sunlight water uptake carbon dioxide uptake

ATP

ADP + Pi

NADPH

NADP+

glucoseP

oxygen release

LIGHT-INDEPENDENT

REACTIONS

LIGHT-DEPENDENT REACTIONS

new water

In-text figurePage 117

Electromagnetic Spectrum

Shortest Gamma rays

wavelength X-rays

UV radiation

Visible light

Infrared radiation

Microwaves

Longest Radio waves

wavelength

Visible Light

• Wavelengths humans perceive as different colors

• Violet (380 nm) to red (750 nm)

• Longer wavelengths, lower energy

Figure 7.5aPage 118

Photons

• Packets of light energy

• Each type of photon has fixed amount of energy

• Photons having most energy travel as shortest wavelength (blue-violet light)

Pigments

• Color you see is the wavelengths not absorbed

• Light-catching part of molecule often has alternating single and double bonds

• These bonds contain electrons that are capable of being moved to higher energy levels by absorbing light

Fig. 10-7

Reflectedlight

Absorbedlight

Light

Chloroplast

Transmittedlight

Granum

Variety of Pigments

Chlorophylls a and b

Carotenoids

Xanthophils

ChlorophyllsW

avel

eng

th a

bso

rpti

on

(%

)

Wavelength (nanometers)

chlorophyll b

chlorophyll a

Main pigments in most photoautotrophs

Figure 7.6a Page 119 Figure 7.7Page 120

Accessory Pigmentsp

erce

nt

of

wav

elen

gth

s ab

sorb

ed

wavelengths (nanometers)

beta-carotenephycoerythrin (a phycobilin)

Carotenoids, Phycobilins, Anthocyanins

Pigments in Photosynthesis

• Bacteria– Pigments in plasma membranes

• Plants– Pigments and proteins organized into

photosystems that are embedded in thylakoid membrane system

Arrangement of Photosystems

water-splitting complex thylakoidcompartment

H2O 2H + 1/2O2

P680

acceptor

P700

acceptor

pool of electron carriers stromaPHOTOSYSTEM II

PHOTOSYSTEM I

Figure 7.10Page 121

• Pigments absorb light energy, give up e-, which enter electron transfer chains

• Water molecules split, ATP and NADH form, and oxygen is released

• Pigments that gave up electrons get replacements

Light-Dependent Reactions

Photosystem Function: Harvester Pigments

• Most pigments in photosystem are harvester pigments

• When excited by light energy, these pigments transfer energy to adjacent pigment molecules

• Each transfer involves energy loss

Photosystem Function: Reaction Center

• Energy is reduced to level that can be captured by molecule of chlorophyll a

• This molecule (P700 or P680) is the reaction center of a photosystem

• Reaction center accepts energy and donates electron to acceptor molecule

Fig. 10-12

THYLAKOID SPACE(INTERIOR OF THYLAKOID)

STROMA

e–

Pigmentmolecules

Photon

Transferof energy

Special pair ofchlorophyll amolecules

Th

yla

koid

me

mb

ran

e

Photosystem

Primaryelectronacceptor

Reaction-centercomplex

Light-harvestingcomplexes

Electron Transfer Chain

• Adjacent to photosystem • Acceptor molecule donates electrons

from reaction center

• As electrons pass along chain, energy they release is used to produce ATP

Cyclic Electron Flow

• Electrons – are donated by P700 in photosystem I to

acceptor molecule

– flow through electron transfer chain and back to P700

• Electron flow drives ATP formation

• No NADPH is formed

Fig. 10-15

ATPPhotosystem II

Photosystem I

Primary acceptor

Pq

Cytochromecomplex

Fd

Pc

Primaryacceptor

Fd

NADP+

reductaseNADPH

NADP+

+ H+

Noncyclic Electron Flow

• Two-step pathway for light absorption

and electron excitation

• Uses two photosystems: type I and

type II

• Produces ATP and NADPH

• Involves photolysis - splitting of water

Machinery of Noncyclic Electron Flow

photolysis

H2O

NADP+ NADPH

e–

ATP

ATP SYNTHASE

PHOTOSYSTEM IPHOTOSYSTEM II ADP + Pi

e–

first electron transfer chain

second electron transfer chain

Figure 7.13aPage 123

Pigmentmolecules

Light

P680

e–

Primaryacceptor

2

1

e–

e–

2 H+

O2

+3

H2O

1/2

4

Pq

Pc

Cytochromecomplex

Electron transport chain

5

ATP

Photosystem I(PS I)

Light

Primaryacceptor

e–

P700

6

Fd

Electron transport chain

NADP+

reductase

NADP+

+ H+

NADPH

8

7

e–

e–

6

Fig. 10-13-5

Photosystem II(PS II)

Energy Changes

Figure 7.13bPage 123

Po

ten

tial

to

tra

nsf

er e

ner

gy

(vo

lts)

H2O 1/2O2 + 2H+

(Photosystem II)

(Photosystem I)

e– e–

e–e–

secondtransfer

chain

NADPHfirst

transferchain

Chemiosmotic Model of ATP Formation

• Electrical and H+ concentration gradients are created between thylakoid compartment and stroma

• H+ flows down gradients into stroma through ATP synthesis

• Flow of ions drives formation of ATP

Chemiosmotic Model for ATP Formation

ADP + Pi

ATP SYNTHASE

Gradients propel H+ through ATP synthases;ATP forms by phosphate-group transfer

ATP

H+ is shunted across membrane by some components of the first electron transfer chain

PHOTOSYSTEM II

H2Oe–

acceptor

Photolysis in the thylakoid compartment splits water

Figure 7.15Page 124

Fig. 10-17

Light

Fd

Cytochromecomplex

ADP +

i H+

ATPP

ATPsynthase

ToCalvinCycle

STROMA(low H+ concentration)

Thylakoidmembrane

THYLAKOID SPACE(high H+ concentration)

STROMA(low H+ concentration)

Photosystem II Photosystem I

4 H+

4 H+

Pq

Pc

LightNADP+

reductase

NADP+ + H+

NADPH

+2 H+

H2OO2

e–

e–

1/21

2

3

• Synthesis part of

photosynthesis

• Can proceed in the dark

• Take place in the stroma

• Calvin-Benson cycle

Light-Independent Reactions

Calvin-Benson Cycle

• Overall reactants

– Carbon dioxide

– ATP

– NADPH

• Overall products

– Glucose

– ADP

– NADP+

Reaction pathway is cyclic and RuBP (ribulose bisphosphate) is regenerated

Calvin- Benson Cycle

CARBON FIXATION

6 CO2 (from the air)

6 6RuBP

PGA

unstable intermediate

6 ADP

6

12

12ATP

ATP

NADPH

10

12PGAL

glucoseP

PGAL2

Pi

12 ADP12 Pi

12 NADP+

12

4 Pi

PGAL

Figure 7.16Page 125

• In Calvin-Benson cycle, the first stable intermediate is a three-carbon PGA

• Because the first intermediate has three carbons, the pathway is called the C3 pathway

The C3 Pathway

Photorespiration in C3 Plants

• On hot, dry days stomata close

• Inside leaf – Oxygen levels rise– Carbon dioxide levels drop

• Rubisco attaches RuBP to oxygen instead of carbon dioxide

• Only one PGAL forms instead of two

C3 and C4 plant structure compared

C4 Plants

• Carbon dioxide is fixed twice

– In mesophyll cells, carbon dioxide is fixed to

form four-carbon oxaloacetate

– Oxaloacetate is transferred to bundle-sheath

cells

– Carbon dioxide is released and fixed again

in Calvin-Benson cycle

CAM Plants

• Carbon is fixed twice (in same cells)

• Night – Carbon dioxide is fixed to form organic

acids

• Day– Carbon dioxide is released and fixed in

Calvin-Benson cycle

Light

Fig. 10-5-4

H2O

Chloroplast

LightReactions

NADP+

P

ADP

i+

ATP

NADPH

O2

CalvinCycle

CO2

[CH2O]

(sugar)

Satellite Images Show Photosynthesis

 Photosynthetic activity in spring

Atlantic Ocean

Figure 7.20Page 128