Lecture 9: Photosynthesis

I. Characteristics of Light

A. Light is composed of particles that travel as waves

1. Comprises a small part of the electromagnetic spectrum

B. Radiation varies in wavelength

1. Visible spectrum

a. Red has longer wavelengths

b. Violet has shorter wavelengths

C. Light is comprised of photons

1. Photons are carriers of electromagnetic radiation

2. The energy of a photon is inversely related to its wavelength

a. Short wavelengths have high energy photons

D. Photon’s energy can be transferred to matter

1. Energize electrons

2. Potential outcomes of being energized

a. Energized electron may return to the ground state

i. Energy will dissipate as heat

ii. Energy be emitted as light—fluoresce

b. Electron may leave the atom and be accepted by an electron acceptor

i. Electron acceptors are the basis of photosynthesis

II. Chloroplasts

A. Photosynthesis in eukaryotes takes place in chloroplasts

1. Chloroplasts typically contain chlorophyll

B. Photosynthesis is primarily associated with mesophyll cells

1. Mesophyll cells contain numerous chloroplasts

C. Structure of the chloroplast

1. Double membrane

a. Inner membrane encloses the stroma

2. Thylakoid

a. Third set of membranes enclosing the thylakoid interior space

3. Stacks of thylakoids are known as grana

4. Chlorophyll and other pigments are embedded in the thylakoid membranes

D. Prokaryotes lack chloroplasts

1. Thylakoid membranes are formed from in-folding of the plasma membrane

E. Chlorophyll

1. Contained in the thylakoid membrane

2. Photopigment that absorb light with particular wavelengths

a. Chlorophyll absorbs light in the red and blue regions of the spectrum

i. Appears green to our visual system

3. Structure of chlorophyll molecules

a. Porphyrin ring

i. Absorbs light energy

b. Tail

i. Embeds the molecule in the thylakoid membrane

c. At the center of the porphyrin ring is a magnesium atom

4. Types of chlorophyll

a. Chlorophyll a contributes most to light-dependent photosynthetic reactions

b. Chlorophyll b is an accessory pigment

i. Similar to chlorophyll a

ii. Differs in functional groups of the porphyrin ring

5. Carotenoids absorb different wavelengths than the chlorophylls

a. Act as accessory pigments

b. Appear yellow and orange

c. Widen the action spectrum for photosynthesis

III. Overview of Photosynthetic Reaction

A. 6 CO2 + 12 H2O → C6H12O6 + 6 O2 + 6 H2O

1. Water appears as both a reactant and a product

a. No net yield of water

2. Photosynthesis involves light dependent reactions and the carbon fixation reactions

a. Carbon fixation does not require light

B. Light-dependent reactions

1. Produce ATP and NADPH

2. Occur in the thylakoid membranes

3. Overview

a. Energy from light causes chlorophyll to expel a high-energy electron

b. This electron is transferred to an acceptor molecule

i. Free energy state is systematically decreased in electron transport chains

ii. Free energy drives the formation of ATP and NADPH

c. Electron lost is replaced by an electron from water

C. Carbon fixation reactions

1. Produce carbohydrates

2. Products of the light-dependent reactions drive the carbon fixation reactions

3. Fixation

a. Process that converts an inorganic atom to an organic compound

i. Carbon atom to glucose

IV. Light-Dependent Reactions

A. Convert light energy to chemical energy

1. 12 H2O + 12 NADP+ + 18 ADP + 18 Pi → 6 O2 + 12 NADPH + 18 ATP

B. Require light and chlorophyll

C. Photosystems I and II

1. Include antenna complexes that trap light

a. Antenna complexes are aggregations of pigment molecules and electron acceptors

2. The reaction center is made of a complex of chlorophyll molecules and proteins

a. The reaction centers are characterized by chlorophyll a molecules with slightly different

absorption spectra

i. In photosystem I absorption peaks at 700 nm—P700

ii. In photosystem II absorption peaks at 680 nm—P680

D. Production of ATP and NADPH

1. Requires both photosystems (I and II)

2. 2 ATP molecules and 1 NADPH molecule are produced for every 2 electrons

3. NADPH is formed by transfer of high energy electrons to NADP+

a. Pigments in photosystem I absorb energy and transfer it to the reaction center

b. A molecule of P700 is excited and emits an electron

c. The electron is transferred to the primary acceptor

d. The electron is then transferred to ferredoxin

i. Sequentially through an electron transport chain

e. NADP+ is the terminal electron acceptor

f. NADP+ is reduced to form NADPH

i. Released into the stroma

4. Water is the electron source for non-cyclic photophosphorylation

a. Photosystem II is also activated by light

i. Emits an electron to a primary electron acceptor

b. The electron is transferred to a series of acceptors, and ultimately to photosystem I

c. Electrons in photosystem II are replaced by electrons from water

i. Photolysis involves splitting of water into oxygen, 2 protons and 2 electrons

d. Non-cyclic photophosphorylation is a continuous linear process

i. Requires continuous supply of new reactants

E. Cyclic photophosphorylation produces ATP

1. Process only occurs in photosystem I

2. Electrons from P700 are returned to P700

3. No oxygen is released

i. Photolysis does not occur

4. ATP is formed but not NADPH

5. Not a predominant mechanism for ATP formation

a. May occur when NADP+ is not available

F. ATP synthesis occurs by chemiosmosis

1. Process requires both photosystem II and photosystem I

a. Pigments in photosystem II absorb energy and transfer it to the reaction center

b. A molecule of P680 is excited and emits an electron

c. The electron is transferred to the primary acceptor

d. Electron moves sequentially through an electron transport chain

i. Energy is used to pump protons into the thylakoid space

e. Chemiosmosis couples ATP synthesis and electron transport

i. As electrons are transferred between carriers, protons are pumped into the

thylakoid interior space

ii. Proton gradient causes a difference of about 3 pH units across the thylakoid

membrane

f. Protons diffuse through channels formed by ATP synthase

i. Catalyze the phosphorylation of ADP to form ATP

g. ATP is ultimately released into the stroma

V. Carbon Fixation Reactions

A. Energy captured during light reaction used to synthesize glucose

1. 12 NADPH + 18 ATP + 6 CO2 → C6H12O6 + 12 NADP+ + 18 ADP + 18 Pi + 6 H2O

B. Most plants use the Calvin (C3) cycle to fix carbon

1. CO2 with ribulose bisphosphate (RuBP)

a. RuBP is a highly reactive 5-carbon molecule

b. Reaction is catalyzed by rubisco

2. Forms an unstable 6-carbon molecule

i. Immediately breaks down into 2 molecules of the 3-carbon phosphoglycerate (PGA)

4. PGA is phosphorylated by ATP and reduced by NADPH to form glyceraldehyde-3-phosphate

(G3P)

5. G3P is rearranged into RuBP or glucose

a. 10 of 12 molecules of G3P are converted into RuBP

b. 2 of 12 molecules of G3P are converted into glucose or fructose

C. Photorespiration reduces photosynthetic efficiency

1. In C4 plants, during hot, dry periods, oxygen molecules in the chloroplast bind with Rubisco,

preventing carbon fixation

2. RuBP and oxygen combine, and the intermediates in the Calvin cycle degrade to form carbon

dioxide and water

3. Like anaerobic respiration, photorespiration produces carbon dioxide and water and requires

oxygen

4. Unlike aerobic respiration, photorespiration does not produce ATP

5. Photorespiration is negligible in C4 plants because the concentration of carbon dioxide is

always high in the bundle sheath cells

D. The initial carbon fixation step differs in C4 plants

1. Carbon dioxide is relatively sparse in the atmosphere

a. To obtain carbon dioxide as a carbon source, plants must keep their stomata open but

risk water loss

2. The C4 pathway efficiently fixes CO2 at low concentrations

a. This pathway precedes the C3 pathway

b. Carbon dioxide is first fixed into a 4-carbon molecule

i. Pxaloacetate

ii. Occurs in the mesophyll cells

c. PEP carboxylase catalyzes the fixation of carbon dioxide

i. High affinity for carbon dioxide

d. Bundle sheath cells surround the vascular bundles in a leaf

e. Oxaloacetate is converted into malate

i. Malate is passed to the bundle sheath cells

ii. Requires NADPH

f. Malate is decarboxylated to form carbon dioxide and pyruvate

i. NADPH is regenerated

g. Pyruvate returns to the mesophyll cell

i. Regenerates phosphoenol pyruvate

ii. Requires ATP

3. C4 pathway requires 30 ATPs per sugar molecule

a. More efficient at high light intensities

b. C3 pathway is optimal at lower light intensities

E. Carbon fixation in CAM plants

1. Crassulacean acid metabolism

a. Succulent plants (family Crassulaceae)

2. CAM plants fix CO2 at night

3. CAM plants fix carbon dioxide at night

a. Form malate

i. Stored in vacuoles

4. CAM plants close their stomata during the daytime to reduce water loss

a. Allows plants to live in highly xeric conditions