How Cells Acquire Energy

Chapter 6

Fig. 6.3a, p. 94

leaf’s upper surface photosynthetic cells

two outer layers of membrane

inner membrane system (thylakoids, all interconnecting bychannels) stroma

(see next slide)

Organelles of photosynthesis

Chloroplasts

Photosynthesis Equation

12H2O + 6CO2 6O2 + C2H12O6 + 6H2Owater carbon

dioxideoxygen glucose water

LIGHT ENERGY

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

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

Pigments

• Light-absorbing molecules

• Absorb some wavelengths and transmit others

• Color you see are the wavelengths NOT absorbed

Wavelength (nanometers)

chlorophyll b

chlorophyll a

• 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

Pigment Structure

Excitation of Electrons

• Excitation occurs only when the quantity of energy in an incoming photon matches the amount of energy necessary to boost the electrons of that specific pigment

• Amount of energy needed varies among pigment molecules

Pigments in Photosynthesis

• Bacteria– Pigments in plasma membranes

• Plants– Pigments embedded in thylakoid membrane

system– Pigments and proteins organized into

photosystems– Photosystems located next to electron

transport systems

Photosystems and Electron Transporters

water-splitting complex thylakoidcompartment

H2O 2H + 1/2O2

P680

acceptor

P700

acceptor

pool of electron

transporters

stromaPHOTOSYSTEM II

PHOTOSYSTEM I

• Pigments absorb light energy, give up e- which enter electron transport systems

• Water molecules are split, ATP and NADH are formed, and oxygen is released

• Pigments that gave up electrons get replacements from splitting water

Light-Dependent Reactions

Electron Transport System

• Adjacent to photosystem • Acceptor molecule donates electrons

from reaction center

• As electrons flow through system, energy they release is used to produce ATP and, in some cases, NADPH

Cyclic Electron Flow

electron acceptor electron transport system

e–

e–

e–

e–ATP

Machinery of Noncyclic Electron Flow

photolysis

H2O

NADP+ NADPH

e–

ATP

ATP SYNTHASE

PHOTOSYSTEM IPHOTOSYSTEM II ADP + Pi

e–

Energy ChangesP

ote

nti

al

to t

ran

sfer

en

erg

y (v

oid

s)

H2O 1/2 O2 + 2H+

(PHOTOSYSTEM II)

(PHOTOSYSTEM I)

e–

e–

e–e–

secondtransport

system

NADPHfirst

transport

system

• Synthesis part of

photosynthesis

• Can proceed in the dark

• Take place in the stroma

• Calvin-Benson cycle

Light-Independent Reactions

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

12NADP+

12

4 Pi

PGAL

animation

Summary of Photosynthesislight

6O212H2O

CALVIN-BENSON CYCLE

C6H12O6

(phosphorylated glucose)

NADPHNADP+ATPADP + Pi

PGA PGAL

RuBP

P

6CO2

end product (e.g. sucrose, starch, cellulose)

LIGHT-DEPENDENT REACTIONS