Photosynthesis

A. Background 1. The conversion of light energy (from the sun)into chemical energy (stored in sugar & organic molecules. 2. Plants, algae (protists), cyanobacteria, phytoplankton 3. Plants are primary producers – produce

organic molecules from CO2 & H2O. They are the bottom of the terrestrial food chain. All complex organisms on land depend on plants (ultimately) for

food & O2.

I. Overview

B. Photosynthetic Structures

Fig 10.3

a. Chloroplast

i. Location

ii. Structure

2 membranes (inner & outer)

Thylakoid membranes – site of the light reactions

Stroma – site of the Calvin cycle, contains rubisco

3. Plants

1. Cyanobacteria

2. Protists

1. Reaction a. 6CO2 + 6H20 C6H12O6 + 6O2

O2 is released as a by-product from the splitting of

water

2. 2 interdependent pathways: a. Light reactions – chlorophyll absorbs light

energy to create high energy molecules: ATP &

NADPH + H+. These are used to drive:

b. Calvin cycle – carbon fixation

C. PhotoSyNthesis (PSN) - overview

Figure 10.4 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle (Layer 3)

Fig 10.5

1. What is light?? = electromagnetic energy(radiation)2. Consists of groups of particles called photons3. Travels in waves4. Wavelength = distance between peaks in wave5. Only visible light drives PSN: 380 to 750nm

D. Characteristics of Light

Figure 10.5 The electromagnetic spectrum

Fig 10.6

1. Molecular substances that absorb visible light.

2. Chlorophyll a – main PSN pigment, absorbs

red & blue light (reflects green) to initiate the

light reactions

3. Accessory pigments – absorb light energy &

transfer it to Chlorophyll a, Chlorophyll b,

Carotenoids, Xanthophylls

E. Plant Pigments

Fig 10.9

A. Light Dependent Reactions 1. Background a. The functional units of chlorophyll &accessory pigments that work together to absorb aphoton of light. “Kicks” electrons to an excited state(high energy) b. 2 main components: i. Light-harvesting complex – accessorypigments absorb light, pass their excited electrons to ii. Reaction center complex – a pair of chlorophyll molecules. Passes the excited electrons to a primary electron acceptor.

II. Photosystems

Fig 10.12

Fig 10.11

i. Photosystem I (P700) – absorbs 700nm

wavelengths best (far-red)

ii. Photosystem II (P680) – absorbs 680nm

wavelengths best (red)

c. Types of photosystems in the thylakoid membranes:

a. Structure and Location2. Non-cyclic Photophosphorylation

b. Steps i. PS II absorbs light, splitting water into H+, electrons, and O2. ii. Further light absorption by PS II kicks the electrons up to anexcited state. iii. Excited electrons are pass from PS II to PS I along an electrontransport chain (PSII Pq cytochrome complex Pc PS I).Energy released at every step used to drive ATP synthesis iv. When electron reaches PS I, light absorption kicks it up toexcited state again. v. Excited electron is passed to Fd to NADP+ - the terminalelectron acceptor. Reduced to NADPH

Fig 10.13

Figure 10.17

i. PathwayNon-cyclic electron flow produces ATP & NADPH + H+.Energy released during electron transfer drives proton pump atcytochrome complex.H+ pumped from stroma into thylakoid lumen, creating anelectrochemical gradientH+ diffusion back out into stroma drives ATP synthesis in Stroma.

c. ATP Production

d. Electron Carrier (NADPH + H+)i. Pathway

a. Structure and Location

b. Process or Steps

i. Produces ATP only

ii. electrons transferred from Fd back to the

cytochrome complex (instead of to NADP+)

iii. thus more H+ pumped into lumen thus more ATP

produced

3. Cyclic Photophosphorylation

i. Non-cyclic flow produces equal amounts of ATP &

NADPH + H+.

ii. But… Calvin cycle requires more ATP than NADPH +

H+. So…. NADPH + H+ builds up in the stroma, triggering

The shift to cyclic phosphorylation

c. Why cyclic flow?

Fig 10.15

Fig 10.5

1. The Calvin Cycle a. Location

The use of ATP & NADPH + H+ to convert CO2 to carbohydrates2. Process and Steps:

a. Carbon fixation: RuBP + CO2 3PGAb. Reduction:

3PGA + 2ATP + NADPH + H+ G3Pc. Regeneration of RuBP from G3P

d. G3P leaves the chloroplast to become ?!?!

B. Light Independent Reactions

Rubisco

Fig 10.18

A. Overall efficiency < 35%

B. Wavelength (λ) of light – shorter λ

have greater energy. PSN use of longer λ

means more photons needed

C. Cyclic phosphorylation – need extra

photons just for extra ATP production

III. Problems and Limitations

1. Stepsa. light-dependent inhibition of C fixation

b. Cause: increased O2 concentration in leaf – How? Stomata are closed

c. Rubisco adds O2 to RuBP to make a compound

(phosphoglycolate) that is converted back to CO2 in themitochondria. d. No carb’s produced, but energy required. e. Wastes about 50% of C compounds in the chloroplast

D. Photorespiration

A. C3 Plants senescentIV. Plant Adaptations

1. Strategy2. Examples

B. C4 plants

1. Strategy

a. 4-C compound instead of a 3-C compound (3PGA)

as first product of Calvin cycle

b. CO2 + PEP Oxaloacetate Malate

c. Malate exported to bundle sheath cells

d. CO2 released from malate to enter Calvin cycle

PEP carboxylase

2. Examples

Fig 10.18

a. PEP carboxylase has much higher affinity for CO2

than Rubisco

b. CO2 is stored as malate in the bundle-sheath cells

a. thus plenty of CO2 even when stomata closed

c. Benefits?

i. Increased WUE (?)

ii. Little photorespiration

3. Why is the C4 pathway more efficient?

1. Strategy a. Stomata open only at night

b. CO2 is stored in organic acids in mesophyll cells.

c. During day, CO2 released from organic acids and

enters Calvin cycle in same mesophyll cells

d. Same benefits as C4 pathway, but even greater

WUE.

C. Crassulacean Acid Metabolism (CAM)

2. Examples

Fig 10.20

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