Chapter 10—Photosynthesis

From photons to food…

I. Photosynthesis in Nature

• Autotrophs—producers

(plants, algae, some protists, some

bacteria)

• Heterotrophs—consumers

(animals, fungi, bacteria)

Locating Photosynthesis

in a Plant

Leaves are the major organs of

photosynthesis

Chloroplasts are the organelle

responsible

Stomata—pores that allow gas

exchange

Mesophyll—interior tissue of the

leaf

Chlorophyll—green pigment in

thylakoid membrane, absorbs

light energy

II. The Pathways of Photosynthesis

• Tracking Atoms through Photosynthesis

6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2

Respiration Photosynthesis

Energy is released from

sugar

Electrons from hydrogen in

oxidized food lose potential

energy through ETC,

forming water at end

Energy is used to make ATP

Energy is stored in sugar

Electrons from water gain

potential energy as they

reduce CO2 to sugar

Energy is provided by light

Overview of Photosynthesis

Overview of Photosynthesis

Overview of Photosynthesis

Light reactions = “photo” Calvin Cycle = “synthesis”

Electromagnetic Spectrum

Visible light is the radiation (energy) that drives photosynthesis

Light meets the chloroplast—why leaves are

green

Pigment—molecule

that absorbs visible

light

Ex. Chlorophyll

(absorbs red and

blue, reflects green)

Spectrophotometer--

Measures the

proportions of light of

different wavelengths

absorbed or transmitted

by a pigment solution

Using a spectrophotometer to find an

absorption spectrum

Which colors work best for

photosynthesis?

Absorption spectrum vs. Action

spectrum

Structure of Chlorophyll

Where does the light energy

absorbed by chlorophyll go?

Excitation of Chlorophyll by light

The energy of an absorbed photon is converted to the potential energy

of an e- raised to an excited state, (higher orbital = unstable)

Returning to ground state releases heat and light (fluorescence)

Chlorophyll

solution

excited

with UV

light

Photosystems—Light Harvesting Complexes

The electron acceptor traps the high-energy e- before it can return to

ground state in the chlorophyll

• Photosystem =

– complex of proteins & other molecules

– includes an ―antenna‖ of 100s of pigment molecules

– has a specific reaction center (primary e-

acceptor next to reaction center chlorophyll) • Photosystem I

– Reaction center—P 700 (absorbs best at λ of 700 nm)

• Photosystem II – Reaction center—P 680 (absorbs best at λ of 680 nm)

Electromagnetic Spectrum

Noncyclic electron flow during the light

rxns generates ATP & NADPH

ATP synthesis = noncyclic photophosphorylation

Uses both Photosystem I & II, & two ETCs

Solar powered light reactions → make ATP (chemical energy) & NADPH

(reducing power) → used in Calvin Cycle (sugar-making reactions)

Cyclic Electron Flow—alternative path for e-’s

Uses Photosystem I, but not Photosystem II

Generates ATP = cyclic photophosphorylation

but not NADPH or O2

ATP produced is used

in Calvin Cycle

Comparing chemiosmosis in chloroplasts &

mitochondria

ETCs in both organelles

pump H+ across a

membrane against the

[gradient]

H+ diffuses back across

lending the energy for

ATP synthesis

Light Reactions & Chemiosmosis

Calvin Cycle—converts CO2 to sugar using

ATP & NADPH

Phase 1—Carbon Fixation

Incorporating CO2 into an organic compound

Enzyme = Rubisco (most abundant enzyme on earth)

Phase 2—Reduction

Phosphate is added from

ATP

Electrons are added from

NADPH

1 3-carbon sugar (G3P)

leaves the cycle for every 3

CO2 added

Phase 3—Regeneration

of CO2 acceptor (RuBP)

ATP is used to

rearrange carbon

compounds back into

RuBP and cycle begins

again

Net Input:

9 ATP & 6 NADPH

Net Output:

1 G3P (3-carbon sugar)

(becomes glucose and

other carbs)

It’s gettin’ hot (& dry) in here!

• On hot, dry days—stomata close to conserve water (therefore O2,↑, CO2 ↓, photosyn. rate goes down)

• Photorespiration—metabolic pathway that decreases photosynthetic output by pulling organic compounds out of the Calvin cycle

• Rubisco adds O2 to the Calvin Cycle instead of CO2

• consumes O2, releases CO2, and makes no ATP or food

– Done by C3 plants (1st product of Calvin Cycle is a

3-C compound)

• Examples: rice, wheat, soybeans

– Can drain as much as 50% of the carbon fixed by the Calvin cycle

C4 plants are adapted to hot, dry

climates

C4 plants—(first product of Calvin cycle

is a 4-C compound)

Examples: sugarcane, corn

C4 & CAM photosynthesis—minimize

photorespiration (in hot, dry climates)

Both involve:

1. CO2 incorporated into

4-C organic acids

(carbon fixation)

2. Organic acids release

CO2 to the Calvin cycle

C4 plants—uses spatial

separation (different

types of cells)

CAM plants—uses

temporal separation

(same cells, different

times)

&

cacti

Light Reactions Calvin Cycle

Reactions

Carried out by

molecules in the

thylakoid membrane

Convert light energy to

the chemical energy of

ATP & NADPH

Split H2O and release O2

to the atmosphere

Take place in the

stroma

Use ATP & NADPH to

convert CO2 to the

sugar G3P

Return ADP, inorganic

phosphate, and NADP+

to the light reactions