How Cells Acquire Energy

Chapter 7

Overview:I. PlacesA. Chloroplasts : photosynthesis organelleB. Thylakoid: discs

i. light-dependent: sunlight to chemical energyii. ETC and water split = forming H+, e- , ATP: oxygen is by-product

C. Stroma: fliud interiori. light-independent: makes glucoseii. carbon dioxide reduced to form glucose sunlight

12H2O + 6CO2 ————> 6O2 + C6H12O6 + 6H2O

• Photoautotrophs – “self-nourishing”

– Carbon source is carbon dioxide

– Energy source is sunlight– Capture sunlight energy and use it to carry out

photosynthesis

– Plants, Some bacteria, Many protistans

• Heterotrophs

– Get carbon and energy by eating autotrophs or one

another

II. SunlightA. photoautotrophs intercept only about

1% of solar energy B. wavelengthC. Electromagnetic spectrum

Short-----------------------------------------------LongGamma, x-ray, UV Visible infrared, microwaves, radioToo much energy just right energy not enough energy

VIBGYOR

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)

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

E. Pigments: molecules that absorb photonsi. the color you see is the color reflectedii. absorption spectrum

-chlorophylls: absorb all except green-carotenoids: accessory pigments: absorb blue-violet and blue-green

-xanthophylls and beta-carotene-abundant in fruits, flowers and vegetables-can be seen in leaves during autumn

-anthocyanins: blue or red accessory pigment -phycobilins: blue accessory pigment

iii. photosystems: organization of pigments-proteins and 200-300 pigment molecules-PSII (p680)-PSI (p700)

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

Light-Dependent ReactionsI. General

A. ThylakoidB. 3 Basic steps

i. photosystems harvest sunlightii. solar energy converted to chemical energy ATPiii. NADP+ (coenzyme) picks up e- and H+

II. “Random Walk” of EnergyA. sunlight is trapped and randomly passed

from chlorophyll b and carotenoids to reaction center (chlorophyll a)

B. Low energy electrons become “excited” and is passed to e- acceptor

C. e- can follow 2 pathways

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

Figure 7.11Page 122

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

Cyclic Electron Flow

electron acceptor

electron transfer chain

e–

e–

e–

e–

ATP

Electron flow through transfer chain sets up

conditions for ATP formation at other membrane sites.

Figure 7.12Page 122

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

III. Cyclic (1 word) Pathway A. PSI (700) – one photosystem

B. ATP – one productC. oldest pathway

IV. Non-Cyclic (2 words) PathwayA. PSII to PSI (2 photosystems)B. ATP and NADPH (2 products)C. uses water (photolysis)D. Evolution of oxygen

V. Chemiosmotic model for ATP formationA. hydrogen ions released by photolysis

accumulate inside the thylakoidi. oxygen is given off as waste

B. more H+ accumulates from the ETC C. the build up of H+ ions inside the thylakoid results in 2 gradients: concentration and

electrici. these 2 gradients force H+ into the stroma through ATP synthase forming 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

• Synthesis part of photosynthesis

• Can proceed in the dark

• Take place in the stroma

• Calvin-Benson cycle• need ATP (energy) and NADPH (H+ and e-)• Uses carbon dioxide

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

II. Calvin-Benson CycleA. Capture of carbon dioxide (Carbon Fixation – 1st step)

i. stromaii. enzyme (RuBP Carboxylase – Rubisco) + RuBP (ribulosebisphosphate)

+ CO2 yields 6-C intermediate that splits into 2 , 3-C PGA (phosphoglycerate) molecules

B. Reduction of PGAi. PGA accepts one phosphate group from ATP plus hydrogen and electrons from the NADPH, resulting in the formation of the intermediate PGAL (phosphorglyceraldehyde)

C. Regeneration of RuBPi. to form 1 six carbon sugar requires 6 CO2

molecules and the formation of 12 PGAL’sii. most of the PGAL get recycled back to RuBP, but 2 of the PGAL combine forming glucose with a phosphate groupiii. when glucose is phosphorylated like this it is primed to enter reactions to become sucrose,

cellulose and starch (the main plant carbohydrates)-the ADP, NADP+ and released phosphate diffuse through the stroma back to the light-dependent reactions

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

C4, C3 and CAM plantsI. General

A. since environments differ, photosynthesis is not the same in all plants

B. We will compare the carbon-fixing adaptations in plants

i. compared to C3 plants (most plants), C4 and CAM plants have modified ways of fixing carbon for photosynthesis during hot, dry conditions

C. Stomata: tiny openings in leaf surface through which gases diffuse

i. closes on hot dry days to conserve water; when this happens CO2 gets used up and O2 builds upD. a high O2 concentration triggers photorespiration, a process that lowers a plant’s sugar making capacity

i. rubisco (the enzyme in the calvin-benson cycle that attaches to CO2 in carbon fixation) also attaches to oxygen

ii. competition between CO2 and O2 for rubiscoiii. sugars are still formed, but at a lower rate

• 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

• C3 plants

– bluegrass, beans, wheat, rice, oats (most plants)

• In hot, dry conditions photorespiration occurs

The C3 Pathway

III. C4 plantsA. corn, bermuda grass, sugar caneB. oxaloacetate (4 carbons) is the first

intermediate in these plantsC. these plants fix carbon twice, in 2 types of photosynthetic cells, so CO2 levels do not decline as much

D. 2 types of photosynthetic cellsi. mesophyll uses PEP (phenolpyruvate, 3-C) to fix CO2 ii. oxaloacetate forms, then malate which diffuses into adjoining bundle sheathsiii. in the bundle sheaths, pyruvate forms. CO2 is released and enters the C-B cycleiv. the pyruvate regenerates PEP for use in the C4 cycle in the mesophyll cells

E. Advantages: small stomata, lose less water, make more glucose than C3 plants during hot, bright, dry days

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

IV. CAM (crassulacean acid metabolism) plantsA. desert plants (cacti)B. Type of C4

i. closes stomates during day, open at night (fix CO2 at night)ii. at night, mesophyll cells use C4 cycle and store malate and other products until the next dayiii. day time, stomata close: malate releases CO2 which enters the C-B cycle in the same cells

-this way photosynthesis continues without water loss

Summary of Photosynthesis

Figure 7.21Page 129

light6O2

12H2O

CALVIN-BENSON CYCLE

C6H12O6

(phosphorylated glucose)

NADPHNADP+ATPADP + Pi

PGA PGAL

RuBP

P

6CO2

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

LIGHT-DEPENDENT REACTIONS

6H2O

LIGHT-INDEPENDENT REACTIONS