Photosynthesis (PS)

Chapter 10

“Self” Feeders

Autotrophs (producers)– Self-produce organic materials such as glucose using

inorganic compounds like CO2 , H2O

– Ultimate source of organic compounds for heterotrophs

– Classes• Photoautotrophs• chemoautotrophs

Autotroph Classification

Photoautotrophs – use light as the energy

source (photosynthesis)

– in plants, algae, some protists, and some prokaryotes

Chemoautotrophs

- energy source from oxidizing inorganic substances (sulfur and ammonia)

– unique to bacteria

“Other” Feeders

Heterotrophs (Consumers)– Uses organic compounds created by others

• 1° Consumers (vegetarians)• 2° Consumers (carnivores and omnivores)• Decomposers (detritivores)

Chloroplast Structure

Chlorophyll pigment is embedded in the thylakoid membrane

Where within this organelle is the chlorophyll pigment located?

Chloroplasts = Sites for PS • Found mainly in mesophyll cells

(interior leaf tissue) • O2 exits and CO2 enters the leaf

through stomata underneath the leaf.• Veins deliver water

from the roots and carry off sugar from mesophyll cells to other plant areas.

PS Equation

Net equation of photosynthesis:6CO2 + 6H2O + light energy -> C6H12O6 + 6O2

In reality, photosynthesis adds one CO2 at a time:

CO2 + H2O + light energy -> CH2O + O2

(CH2O)n = generic formula for a sugar

2 Stages of PS

• Light Reactions– In Thylakoid

Membrane

• Requires Light– chlorophyll

pigment absorbs light energy

• Calvin Cycle– In Stroma

• AKA Light Independent or Dark Reactions

• ** Be careful** occurs only during the day because NADPH and ATP made in the light reactions drive the Calvin Cycle

Absorption Spectra

• In the thylakoid, several pigments differ in their absorption spectrum.

• Chlorophyll a (dominant pigment) absorbs best in the red and blue wavelengths, and least in the green.

• Other pigments (chll b, carotenoids)have different absorption spectra and can transfer energy to chll a

Actions Spectrum

Collectively, these pigments determine an overall action spectrum for photosynthesis.

Photosystems

• Photosystem = light gathering complex

• A few hundred chll a, chll b, and carotenoids combined = photosystem

• Only chll a participates directly in the light reactions

Thylakoid Membrane Populated by Two Photosystems

• P700 (Photosystem I) has a reaction center with an absorption peak at 700nm.

• P 680 (Photosystem II) has a reaction center with a peak at 680nm.

Photosystem Function

1) Sun E (photons) hit Photosystems

2) 2 e- excited from chll a (stores PE)

3) Energy transferred from one pigment to another until E tranferred to chll a

4) Position of chll a is near the “reaction center” where 1st light driven chemical rxn of PS occurs

Light Reactions

• Non-cyclic Electron Flow

• Cyclic Electron Flow

Steps to Non-cyclic Electron Flow

1. P680 absorbs light2. Light excites 2e- and

passes e- to primary electron acceptor

3. Water is split into

½ O2 oxygen gasH+ to thylakoid space2e- resupplies P680

Steps to Non-cyclic Electron Flow

4. e- pass through ETC, where they “fall” in E

5. E from e- is captured to produce ~ 1.5 ATP’s through chemiosmotic phosphorylation

Steps to Non-cyclic Electron Flow

6. e- from P680 replenish e- in P700

7. e- in P700 are excited again by light and passed to a primary electron acceptor

Steps to Non-cyclic Electron Flow

8. e- cascades down short ETC, where e- combine with and reduce NADP+ and H+ to form NADPH (E rich molecule)

Non-cyclic Electron Flow

Non-Cyclic Electron Flow

ATP and NADPH created in the Light Reactions are used in the Calvin Cycle

Cyclic Electron Flow

• Calvin Cycle – uses up more ATP than NADPH so cyclic electron flow helps to generate more ATP’s

• Short circuit – e- fall back from P700 primary electron acceptor to the 1st ETC to generate more ATP’s via chemiosmotic phosphorylation

Cyclic Electron Flow

Calvin Cycle (in stroma)

• ATP drives the cycle• NADPH = reducing power for adding

high energy e- to make sugar• 3 Basic parts

1. CO2 fixation = Carboxylation2. Reduction

3. Regeneration of CO2 acceptor (RuBP) – Ribulose bi-phosphate

CO2 Fixation

Rubisco catalyzes the fixation of CO2 to a C5 compound, RuBP (Ribulose biphosphate)

Initial C6 compound= unstable

Splits into PGA (3-phosphoglycerate)

Reduction

Hydrolysis of 6 ATP

NADPH is oxidized to NADP+

PGAL sugar created = G3P (glyceraldehyde-3-phosphate)

1/6 PGAL made into glucose and other organic compounds

Regeneration of RuBP

Hydrolysis of 3 ATP’s to regenerate RuBP

Evolution of Photorespiration

• C3 plants = plants that make a C3 compound like PGA as the 1st product in the Calvin Cycle

• In arid/dry climates, plants close stomata to prevent dehydration limits CO2 intake

• Result Rubisco accepts O2 in place of CO2

• C5 intermediate results which splits into a C3 and C2 compound. C2 compound is exported as waste to peroxisomes and mitochondria to regenerate CO2 = photorespiration

Photorespiration – Detriment or not?

• C2 waste and C3 compound created means no C5 RuBP regeneration for Calvin Cycle. This means reduced PS rate!!

• no sugar made

• C4 Photosynthesis and CAM Photosynthesis evolved to minimize photorespiration

C4 Photosynthesis

• C4 plants = plants that have alternate CO2 fixation route that makes a C4 compound as the 1st compound (corn, grasses, sugar cane)

C3

Leaf Anatomy Differs b/t C3 and C4 plants•Bundle Sheath Cells @ veins of leaf•Mesophyll Cells @ outside of bundle sheaths loosely arranged

C4 Photosynthesis

• CO2 fixed by PEP carboxylase (not Rubisco) in mesophyll cells

• CO2 and combines with C3 PEP (phosphoenol pyruvate) to form OAA (oxaloacetate)

• OAA converts to C4 malate

• Malate enters bundle sheath cells via plasmodesmata

• CO2 released from malate and captured by Rubisco to enter Calvin Cycle

CAM Photosynthesis

• CAM = Crassulacean Acid Metabolism

• In succulent plants like cactus

• Succulents open stomata at night and close them during the day, thus needing a way to fix CO2 at night

CAM Photosynthesis

• At night, CO2 fixed by PEP carboxylase (not Rubisco)

• Same chemical pathway as C4 PS to create malate except…

• Malic Acid (malate) temporarily stored in Vacuoles at night

• During the day, CO2 is released from the C4 malate and fixed by Rubisco to be used in the Calvin Cycle