Chapter 10:

Photosynthesis

PCA:

• Define: • Photosynthesis• Autotrophs vs. Heterotrophs• Producers vs. Consumers

Photosynthesis in nature

• Autotrophs: self feeders/ producers• photoautotrophs/ chemoautotrophs• obtains organic food without eating

other organisms

• Heterotrophs: consumers• obtains organic food by eating other

organisms or their by-products (includes decomposers)

The chloroplast

• Sites of photosynthesis

• Pigment: chlorophyll

• Plant cell: mesophyll

• Gas exchange: stomata

• Chloroplast/Double membrane• Thylakoids, grana, stroma

Photosynthesis: an overview

• Redox process • Compared to cellular respiration, it

reverses the flow of electrons• Photons split water, excite electrons

• Electrons are then transferred to CO2• Reducing it to a sugar• Electrons increase in potential

energy

Two stage process:

•1. Light reactions• Converts solar energy to

potential energy• Produces ATP

(photophosphorylation)• Reduces NADP+ to NADPH

Two Stage Process:

•2. Calvin Cycle• Incorporates CO2 from the air to

organic molecules• Requires ATP• Occurs during the day time

because needs light to reduce NADP+

• Light independent reaction(dark reaction)

Stage One: Light Reactions

•Converts solar energy to chemical energy (ATP and NADPH)

•Uses photosynthetic pigments• Chlorophyll a and b• Carotenoids

Photosystems• Light harvesting units of the

thylakoid membrane

• Composed mainly of protein and pigment antenna complexes

• Antenna pigment molecules are struck by photons

• Energy is passed to reaction centers (redox location)

• Excited e- from chlorophyll is trapped by a primary e- acceptor

Noncyclic electron flow

•Photosystem II (P680):

• photons excite electrons of chlorophyll to an acceptor

• electrons are replaced by splitting of H2O (release of O2)

• electrons travel to Photosystem I down an electron transport chain (PqcytochromesPc)

• as electrons fall, ADP ATP (noncyclic photophosphorylation)

•Photosystem I (P700): • Fallen electrons are excited by a photon

• Electrons enter second electron transport chain• Ferrodoxin (Fp)• Electrons from Fp reduce NADP+ to

NADPH

Cyclic electron flow

• Alternative cycle when ATP is deficient

• Photosystem I used but not II; produces ATP but no NADPH

• Why? The Calvin cycle consumes more ATP than NADPH…….

• Cyclic photophosphorylation

The Calvin Cycle:

• Uses ATP and NADPH from light reactions to convert CO2 to sugar• Anabolic• Endergonic

The Calvin Cycle:

•Glucose is not produced directly•3 carbon sugar molecule

(G3P)• Glyceraldehyde 3- phosphate

Three phases of the Calvin Cycle:

•1. Carbon Fixation:•Each molecule of CO2 is fixed

to ribose biphosphate(RuBP)• 5 carbon sugar• Catalyzed by Rubisco

• RuBP carboxylase

Three phases of the Calvin Cycle:

•1. The product of step one is a short lived 6 carbon intermediate molecule• Forms two molecules of 3-

phosphoglycerate

Three phases of the Calvin Cycle:

•2. Reduction: • Each 3- phosphoglycerate is

phosphorylate by ATP• Produces 1,3 bisphophoglycerate

• Each 1,3 biphosphoglycerate gets reduced to G3P by NADPH• 1 of the 6 G3P produced will go on to

produce glucose

Three phases of the Calvin Cycle:

•3. Regeneration of RuBP: •The other 5 G3P molecules will

be rearranged into 3 molecules of RuBP

Input vs. Output

•3 CO2 molecules enter one at a time•3 short lived 6 carbon

molecules are produced•6, 3-phosphoglycerate

molecules result

Input vs. Output

•6, 3 –phosphoglycerate are phosphorylated into 6, 1,3 biphosphoglycerate•6 ATP

Input vs. Output• 6, 1,3 biphosphoglycerate are

reduced to 6 G3P by NADPH

• 1 of the 6 G3P will go onto form glucose

• The other 5 of the G3P will go onto regenerate RuBP•Phosphorylated by 3 ATP

Input vs. Output

•9 ATP consumed

•6 NADPH oxidized

How do plants fix carbon in hot/arid

climates?• Plants close their stomata to

prevent dehydration during hot and dry days.• How does this affect

photosynthesis?

Alternative carbon fixation

methods:

•Photorespiration: • Plant adds O2 to the Calvin cycle

when CO2 is unavailable• No benefit to the plant

• No ATP• Not sugar produced

C4 Pathway

• C4 plants: alternative mode of carbon fixation• 2 photosynthetic cells, bundle-

sheath & mesophyll• PEP carboxylase (instead of rubisco)

fixes CO2 in mesophyll• new 4C molecule releases CO2 to

Calvin cycle (grasses, corn, sugar cane)

Alternative carbon fixation

methods, II

• CAM plants: (crassulacean acid metabolism)• open stomata during night, close during day• Cacti and other succulents(Jade plant)

pineapples• CO2 is taken in at night and fixed into

different organic acids• During the day, when stomata are close but

light reaction is active the organic acids deliver CO2 to the Calvin cycle

A review of photosynthesis