How Cells Acquire Energy

Chapter 6

• Photoautotrophs

– Carbon source is carbon dioxide

– Energy source is sunlight

• Heterotrophs

– Get carbon and energy by eating

autotrophs or one another

Carbon and Energy Sources

Photoautotrophs

• Capture sunlight energy and use it to carry out photosynthesis

– Plants

– Some bacteria

– Many protistans

T.E. Englemann’s Experiment

Background

• Certain bacterial cells will move toward places where oxygen concentration is high

• Photosynthesis produces oxygen

T.E. Englemann’s Experiment

Hypothesis

• Movement of bacteria can be used to determine optimal light wavelengths for photosynthesis

T.E. Englemann’s Experiment

Method

• Algal strand placed on microscope slide and illuminated by light of varying wavelengths

• Oxygen-requiring bacteria placed on same slide

T.E. Englemann’s Experiment

T.E. Englemann’s Experiment

Results Bacteria congregated where red and violet wavelengths illuminated alga

ConclusionBacteria moved to where algal cells released more oxygen--areas illuminated by the most effective light for photosynthesis

Linked Processes

Photosynthesis

• Energy-storing pathway

• Releases oxygen

• Requires carbon dioxide

Aerobic Respiration

• Energy-releasing pathway

• Requires oxygen

• Releases carbon dioxide

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

• Continual input of solar energy into

Earth’s atmosphere

• Almost 1/3 is reflected back into space

• Of the energy that reaches Earth’s

surface, about 1% is intercepted by

photoautotrophs

Sunlight Energy

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

Photons

• Packets of light energy

• Each type of photon has fixed amount of energy

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

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

Variety of Pigments

Chlorophylls a and b

Carotenoids

Anthocyanins

Phycobilins

Chlorophylls

Main pigments in most photoautotrophsW

avel

eng

th a

bso

rpti

on

(%

)

Wavelength (nanometers)

chlorophyll b

chlorophyll a

Carotenoids

• Found in all photoautotrophs

• Absorb blue-violet and blue-green that chlorophylls miss

• Reflect red, yellow, orange wavelengths

• Two types– Carotenes - pure hydrocarbons

– Xanthophylls - contain oxygen

Anthocyanins & Phycobilins

Red to purple pigments

• Anthocyanins– Give many flowers their colors

• Phycobilins– Found in red algae and cyanobacteria

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

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

Pigments in a Photosystem

reaction center (a specialized chlorophyll a molecule)

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

• Electrons – are donated by P700 in photosystem I to

acceptor molecule

– flow through electron transport system and back to P700

• Electron flow drives ATP formation

• No NADPH is formed

Cyclic Electron Flow

electron acceptor electron transport system

e–

e–

e–

e–

ATP

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–

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

Chemiosmotic Model of ATP Formation

• When water is split during photolysis, hydrogen ions are released into thylakoid compartment

• More hydrogen ions are pumped into the thylakoid compartment when the electron transport system operates

Chemiosmotic Model of ATP Formation

• Electrical and H+ concentration gradient exists between thylakoid compartment and stroma

• H+ flows down gradients into stroma through ATP synthesis

• Flow of ions drives formation of ATP

• 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

12NADP+

12

4 Pi

PGAL

Building Glucose

• PGA accepts– phosphate from ATP

– hydrogen and electrons from NADPH

• PGAL (phosphoglyceraldehyde) forms

• When 12 PGAL have formed– 10 are used to regenerate RuBP

– 2 combine to form phosphorylated glucose

Using the Products of Photosynthesis

• Phosphorylated glucose is the building block for:

– sucrose• The most easily transported plant carbohydrate

– starch• The most common storage form

• 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

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

• Fissures in sea-floor where seawater

mixes with molten rock

• Complex ecosystem is based on energy

from these vents

• Bacteria are producers in this system

Hydrothermal Vents

Light and Life at the Vents

• Vents release faint radiation at low end of visible spectrum

• These photons could be used to carry out photosynthesis

• Nisbet and Van Dover hypothesize that the first cells may have arisen at hydrothermal vent systems

Supporting Evidence

• Absorption spectra for ancient photosynthetic bacteria correspond to wavelengths measured at the vents

• Photosynthetic machinery contains iron, sulfur, manganese, and other minerals that are abundant at the vents

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