Photosynthesis overviewCh 10

What is the equation for photosynthesis?

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

What does each part do for the plant? Epidermis – protects leaf tissues   Cuticle – waxy covering, resists water,

dirt   Stoma- pore used for gaseous exchange

(water, CO2, O2) (stomata – plural)   guard cell – open & close stomata

Vein – vascular tissue   Xylem – moves water & minerals from roots to leaf   Phloem – moves sap from photosynthesis to other

parts of plant   palisade layer (mesophyll) – dense upper middle

layer of dicot leaf, where photosynthesis takes place 

spongy mesophyll – lower middle layer of dicot leaf, has lots of air spaces, where photosynthesis takes place

ChloroplastInner membrane

Stroma

Thylakoid(thylakoid space inside)

Outer membraneGranum (stack of thylakoids)

2 stages of photosynthesisStage Where it

occursReactants Products

Light dependent reactions

Thylakoid membrane in chloroplast

H2O, light

ADP,NADP+

ATP, NADPHO2 (waste)

Calvin cycle(light independent reactions)

Stroma CO2ATPNADPH

2 G3P to make C6H12O6

ADPNADP+

Tracking atoms in photosynthesis

- Redox reaction: H2O is oxidized, CO2 is reduced- Endergonic reaction – requires energy- Experiments done with heavy water to determine where oxygen comes from – it comes from water

Capturing energy from light

Plant PigmentsPigments – a substance that

absorbs lightPlant pigments absorb light in the

blue/violet and red region of the spectrum

Plant pigments reflect light in the yellow/green region of the spectrum

Absorbance vs. Action spectrum

Absorbance spectra = range of wavelengths absorbed by a particular pigment

Chlorophyll A & B

Chlorophyll a – main photosynthetic pigment

Accessory pigments: help broaden spectrumChlorophyll bCarotenoids

Action spectra = range of wavelengths capable of driving a particular biological process

Chlorophyll excited by light

When pigment gets excited by light it goes from ground state to excited state

When electron goes back to ground state, it gives off energy

Englemann’s experiment- first action spectrum- 1883- Used filamentous

algae- Put on flat surface

w/water & CO2- Solution of aerobic

bacteriaLooked to see where

bacteria built up

Photosystem Light gathering complex –

various pigment molecules bound to proteins

Reaction center – has special chlorophyll A associated with a primary electron acceptor

Photon excites pigments – they transfer electrons until they get to the chlorophyll a in reaction center

Non cyclic photophosphorylation

Cyclic Phosphorylation

Cyclic photophosphorylation Alternate electron path Photosystem 1 transfers electrons back

to the ETC from PS II. This generates ATP through photophosphorylation, rather than NADPH.

- some bacteria only have PS 1, some plants have cells that only have PS1

Calvin Cycle(equations for 3 turns of cycle)

1) Fixation of carbon dioxide: CO2 reacts with RuBP, produces 2 molecules of 3 PGA. (catalyzed by rubisco)  3 RuBP + 3 CO2 6 3PGA2) Reduction of 3PG to form glyceraldehyde-3-phosphate, or G3P  6 3PGA + 6 ATP + 6 NADPH (5 G3P to step 3, 1 G3P yield)

3) Regeneration of the CO2 acceptor, RuBP, ribulose biphosphate  5 G3P + 3 ATP 3 RuBP

Rubisco is the key enzyme

One complete turn of the Calvin Cycle (with one CO2): 1 CO2 + 2 NADPH + 3 ATP (CH2O) + 2 NADP+ + 3 ADP + 3 Pi

For each turn, one CO2 is converted into one (CH2O) unit

It takes 3 turns to produce one net G3P It takes 6 turns to produce one 6 carbon

carbohydrate, such as glucose

Calvin cycle animation http://www.science.smith.edu/departme

nts/Biology/Bio231/calvin.html

Rubiscoslow: Catalyzes 3 reactions per second vs.

thousands per second for other enzymes   Inefficient: catalyzes addition of either

CO2 or oxygen.

CO2 and oxygen compete at the enzyme’s active sites.  

•In most plants (C3 plants), • initial fixation of CO2 (via rubisco), •forms a three-carbon compound (3-phosphoglycerate)

•In photorespiration, •rubisco adds O2 instead of CO2 in the Calvin cycle, •producing a two-carbon compound

Photorespiration

The reaction of oxygen and RuBP results in photorespiration, which consumes ATP. 

Photorespiration consumes energy and releases fixed CO2 , so it “undoes”

photosynthesis. 

Photorespiration consumes O2 and organic fuel, and releases CO2 without producing ATP or sugar

Photorespiration

Alternate pathways for photosynthesis• Problem - Dehydration is a problem• This can result in trade-offs with

photosynthesis

• On hot, dry days, plants close stomata, - conserves H2O

• - limits photosynthesis• - reduces access to CO2 • - causes O2 to build up

• These conditions favor an apparently wasteful process called photorespiration

• Photorespiration • may be an evolutionary relic because rubisco first

evolved at a time when the atmosphere had far less O2 and more CO2

• limits damaging products of light reactions that build up in the absence of the Calvin cycle

• - a problem because on a hot, dry day it can drain as much as 50% of the carbon fixed by the Calvin cycle

C4 plantsC4 plants minimize the cost of

photorespiration by incorporating CO2 into four-carbon compounds in mesophyll cells

This step requires the enzyme PEP carboxylase

PEP carboxylase has a higher affinity for CO2 than rubisco does; it can fix CO2 even when CO2 concentrations are low

These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycle

CAM plantsOther plants also use PEP carboxylase to fix and

accumulate CO2.Some plants, including succulents, use

crassulacean acid metabolism (CAM) to fix carbon

CAM plants open their stomata at night, incorporating CO2 into organic acids

(oxaloacetate – 4 C, then converted to malic acid)Stomata close during the day, and CO2 is released

from organic acids and used in the Calvin cycle (malic acid goes to chloroplasts)

Plants can make everything they need from CO2, H2O, sulfate, phosphate & ammonium.- Some G3P can enter glycolysis cycle & be converted to pyruvate- Some G3P can enter gluconeogenesis pathway and form 6 carbon sugars, and then sucrose

Chemiosmosis in Mitochondria vs. chloroplast