Where It Starts – Photosynthesis

Chapter 7 Part 2

7.6 Light-Independent Reactions:

The Sugar Factory

The cyclic, light-independent reactions of the

Calvin-Benson cycle are the “synthesis” part of

photosynthesis

Calvin-Benson cycle

• Enzyme-mediated reactions that build sugars in

the stroma of chloroplasts

Carbon Fixation

Carbon fixation

• Extraction of carbon atoms from inorganic

sources (atmosphere) and incorporating them

into an organic molecule

• Builds glucose from CO2

• Uses bond energy of molecules formed in light-

dependent reactions (ATP, NADPH)

The Calvin-Benson Cycle

Enzyme rubisco attaches CO2 to RuBP

• Forms two 3-carbon PGA molecules

PGAL is formed

• PGAs receive a phosphate group from ATP, and

hydrogen and electrons from NADPH

• Two PGAL combine to form a 6-carbon sugar

Rubisco is regenerated

Inputs and Outputs of

the Calvin-Benson Cycle

The Calvin-Benson Cycle

Fig. 7-11, p. 115

A Six CO2 in air spaces inside of

a leaf diffuse into a photosynthetic

cell. Rubisco attaches each to a

RuBP molecule. The resulting

intermediates split, so twelve

molecules of PGA form.

A 6CO2

12 ATP 12 PGA 6 RuBPB

6 ADP

12 ADP + 12 Pi

B Each PGA molecule gets a

phosphate group from ATP,

plus hydrogen and electrons

from NADPH. Twelve

intermediate molecules

(PGAL) form.

D The remaining ten

PGAL get phosphate

groups from ATP.

The transfer primes

them for endergonic

reactions that

regenerate the 6

RuBP.

Calvin–Benson

Cycle 6 ATP

12 NADPH

4 Pi12 NADP+

D

C Two of the PGAL

combine and form one

molecule of glucose.

The glucose may enter

reactions that form

other carbohydrates,

such as sucrose and

starch.

12 PGAL 10 PGALC

other moleculesglucose

Fig. 7-11, p. 115

B

12 ATP 12 PGA

12 ADP + 12 Pi

B Each PGA molecule gets a

phosphate group from ATP,

plus hydrogen and electrons

from NADPH. Twelve

intermediate molecules

(PGAL) form.

12 NADPH

12 NADP+

12 PGAL

6 ADPD The

remaining ten

PGAL get

phosphate

groups from

ATP. The

transfer primes

them for

endergonic

reactions that

regenerate the

6 RuBP.

6 ATP

4 Pi

D

10 PGAL

C Two of the PGAL

combine and form one

molecule of glucose.

The glucose may enter

reactions that form

other carbohydrates,

such as sucrose and

starch.

other

moleculesglucose

C

A 6CO2

6 RuBP

A Six CO2 in air spaces inside of

a leaf diffuse into a photosynthetic

cell. Rubisco attaches each to a

RuBP molecule. The resulting

intermediates split, so twelve

molecules of PGA form.

Calvin–Benson

Cycle

Stepped Art

Animation: Calvin-Benson cycle

7.7 Adaptations:

Different Carbon-Fixing Pathways

Environments differ, and so do details of

photosynthesis

• C3 plants

• C4 plants

• CAM plants

Stomata

Stomata

• Small openings through the waxy cuticle covering

epidermal surfaces of leaves and green stems

• Allow CO2 in and O2 out

• Close on dry days to minimize water loss

C3 Plants

C3 plants

• Plants that use only the Calvin–Benson cycle to

fix carbon

• Forms 3-carbon PGA in mesophyll cells

• Used by most plants, but inefficient in dry weather

when stomata are closed

Photorespiration

When stomata are closed, CO2 needed for light-

independent reactions can’t enter, O2 produced

by light-dependent reactions can’t leave

Photorespiration

• At high O2 levels, rubisco attaches to oxygen

instead of carbon

• CO2 is produced rather than fixed

C4 Plants

C4 plants

• Plants that have an additional set of reactions for

sugar production on dry days when stomata are

closed; compensates for inefficiency of rubisco

• Forms 4-carbon oxaloacetate in mesophyll cells,

then bundle-sheath cells make sugar

• Examples: Corn, switchgrass, bamboo

C3 and C4 Plant Leaves

Fig. 7-12a, p. 116

Fig. 7-12a, p. 116

palisade mesophyll

cell

spongy mesophyll

cell

A C3 plant leaves. Chloroplasts are distributed evenly among two

kinds of mesophyll cells in leaves of C3 plants such as basswood

(Tilia americana). The light-dependent and light-independent

reactions occur in both cell types.

Fig. 7-12b, p. 116

Fig. 7-12b, p. 116

bundle-sheath cell

mesophyll cell

B C4 plant leaves. In C4 plants such as corn (Zea mays), carbon is fixed

the first time in mesophyll cells, which are near the air spaces in the leaf,

but have few chloroplasts. Specialized bundle-sheath cells ringing the

leaf veins closely associate with mesophyll cells. Carbon fixation occurs

for the second time in bundle-sheath cells, which are stuffed with

rubisco-containing chloroplasts.

CAM Plants

CAM plants (Crassulacean Acid Metabolism)

• Plants with an alternative carbon-fixing pathway

that allows them to conserve water in climates

where days are hot

• Forms 4-carbon oxaloacetate at night, which is

later broken down to CO2 for sugar production

• Example: succulents, cactuses

A CAM Plant

Jade plant (Crassula argentea)

C3, C4, and CAM Reactions

Fig. 7-13a, p. 117

Fig. 7-13a, p. 117

mesophyll cell

O2CO2

RuBPglycolate

Calvin–

Benson

CyclePGA

sugarATP NADPH

A C3 plants. On dry days, stomata close and oxygen

accumulates to high concentration inside leaves. The

excess causes rubisco to attach oxygen instead of

carbon to RuBP. Cells lose carbon and energy as they

make sugars.

Fig. 7-13b, p. 117

Fig. 7-13b, p. 117

mesophyll cell

CO2

from

inside

plant

oxaloacetateC4

Cycle

CO2bundle-sheath cell

RuBPCalvin–

Benson

Cycle PGA

sugar

B C4 plants. Oxygen also builds up inside

leaves when stomata close during

photosynthesis. An additional pathway in

these plants keeps the CO2 concentration high

enough to prevent rubisco from using oxygen.

Fig. 7-13c, p. 117

Fig. 7-13c, p. 117

mesophyll cell

CO2

from

outsid

e plant

C4

Cycle

oxaloacetate

night

day

CO2

RuBPCalvin–

Benson

CyclePGA

sugar

C CAM plants open stomata and fix carbon

using a C4 pathway at night. When stomata

are closed during the day, the organic

compounds made during the night are

converted to CO2 that enters the Calvin–

Benson cycle.

7.6-7.7 Key Concepts:

Making Sugars

The second stage is the “synthesis” part of

photosynthesis, in which sugars are assembled

from CO2

The reactions use ATP and NADPH that form in

the first stage of photosynthesis

Details of the reactions vary among organisms

7.8 Photosynthesis and the Atmosphere

The evolution of photosynthesis dramatically

and permanently changed Earth’s atmosphere

Different Food Sources

Autotrophs

• Organisms that make their own food using energy

from the environment and inorganic carbon

Heterotrophs

• Organisms that get energy and carbon from

organic molecules assembled by other organisms

Two Kinds of Autotrophs

Chemoautotrophs

• Extract energy and carbon from simple molecules

in the environment (hydrogen sulfide, methane)

• Used before the atmosphere contained oxygen

Photoautotrophs

• Use photosynthesis to make food from CO2 and

water, releasing O2

• Allowed oxygen to accumulate in the atmosphere

Earth With and Without

Oxygen Atmosphere

Fig. 7-15a, p. 118

Fig. 7-15b, p. 118

Effects of Atmospheric Oxygen

Selection pressure on evolution of life

• Oxygen radicals

Development of ATP-forming reactions

• Aerobic respiration

Formation of ozone (O3) layer

• Protection from UV radiation

7.8 Key Concepts:

Evolution and Photosynthesis

The evolution of photosynthesis changed the

composition of Earth’s atmosphere

New pathways that detoxified the oxygen by-

product of photosynthesis evolved

7.9 A Burning Concern

Earth’s natural atmospheric cycle of carbon

dioxide is out of balance, mainly as a result of

human activity

The Carbon Cycle

Photosynthesis locks CO2 from the atmosphere

in organic molecules; aerobic respiration returns

CO2 to the atmosphere

• A balanced cycle of the biosphere

Humans burn wood and fossil fuels for energy,

releasing locked carbon into the atmosphere

• Contributes to global warming, disrupting

biological systems

Fossil Fuel Emissions

7.9 Key Concepts:

Photosynthesis, CO2 & Global Warming

Photosynthesis by autotrophs removes CO2

from the atmosphere; metabolism by all

organisms puts it back in

Human activities have disrupted this balance,

and contribute to global warming

Animation: C3-C4 comparison

Animation: Harvesting photo energy

Animation: Light-dependent reactions

Animation: Photosynthesis overview

Animation: Structure of a chloroplast

Animation: Wavelengths of light

ABC video: Solar Power

Video: Biofuels