AP Biology

Photosynthesis: Reaction is the opposite of glycolysis

6CO2 + 6H2O → C6H12O6 + 6O2

Photosynthesis in nature Autotrophs:

biotic producers; photoautotrophs; chemoautotrophs; obtains organic food without eating other organisms

Heterotrophs: biotic consumers; obtains organic food by eating other organisms or their by-products (includes decomposers)

The chloroplastSites of photosynthesis

Occurs in specialized plant cell (mesophyll)

Gas is exchanged in openings called stomata

Chloroplast is organelle responsible for photosynthessDouble membraneContains thylakoids, grana, stromaLight is absorbed in pigments called

chlorophyll

Photosynthesis2 Reactions

Light ReactionsCalvin Cycle

PhotosynthesisLight Reactions – Light energy is

converted to chemical energy to split hydrogen from water. As the name implies, you MUST have

light for this reactionLight is first absorbed by the

chloroplastsTakes place in the grana of the chloroplasts (the coin-like stacks of sacs . . . thylakoids).

Light absorption Chloroplasts only absorb a portion of the sun Sunlight is composed of white light which

contains all the colors (example: prism) The chloroplasts contain pigments that only absorb

certain “colors” of light Chlorophyll a: absorbs mostly “red” light

Chlorophyll does not really absorb much “green” light so this light is reflected (why many plants appear green)

Carotenoids: absorb mostly “yellow”, “orange” and “brown” light

In the fall, many leaves lose their carotenoids and that is why they take on the fall colors because that light is reflected.

Purpose of the light reactionThe purpose of the light reaction is to capture light energy and then convert it to usable chemical energyChemical energy is stored in the ATP molecule and in NADPH molecules

As a result of the light reaction, O2 is released

PURPOSE OF CALVIN CYCLE

(C3 PATHWAY) Calvin cycle is the process by which atmospheric

CO2 is taken in by the plant and utilized to make the high energy glucose molecule

In order to produce glucose, CO2 must be incorporated into an organic compound, carbon fixation

In the first step, CO2 diffuses into the stroma (again, inside the chloroplast) from the surrounding cytosol of the cell

The carbon dioxide then goes through a series of “cycles” to create organic compounds

Photosynthesis: an overview Redox process H2O is split, e- (along w/

H+) are transferred to CO2, reducing it to sugar

Light reactions: Uses 2 photosystems Creates ATP and NADPHCalvin cycle: Fixes CO2 to create larger

(energy rich) organic compounds

Uses ATP and NADPH from light reactions

Light energy to chemical energy

To convert the sunlight to chemical energy (usable by the plant and other organisms), a redox reaction must occur in two:

photosystems : a cluster of pigment molecules and proteins

Photosystem I – where electrons from photosystem II continue to move electrons to reduce a molecule

Photosystem II – where the light energy is initially used to oxidize a molecule (release an electron)

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

Photosystem II Photosystem II (P680):

photons excite chlorophyll, e- to an acceptor

e- are replaced by splitting of H2O (release of O2)

e-’s travel to Photosystem II down an electron transport chain (cytochromes)

as e- fall, ADP ---> ATP (photophosphorylation)

START WITH PHOTOSYSTEM II

Accessory pigments absorb light energy and transfer it to chlorophyll a molecules

As a result of the light energy, electrons are released from the chlorophyll a molecule in an oxidation reaction

The free electrons from the chlorophyll a molecule are then “accepted” by a protein called a primary electron acceptor which reduces the molecule

The electrons then move from one molecule to another in a series of events called the electron transport chain Think of this as a water wheel with the electrons

being the water and it fuels a pump

Photosystem IIThe electron transport chain uses its

energy to pump H+ into the thylakoid membrane

This creates a high concentration of H+ inside the thylakoidThink of adding more and more air to a

balloon. The more you add, the more pressure is inside the balloon. This “pressure” created by a high concentration of H+ is used to create ATP

Photosystem II The high pressure created by the H+ pump is

“released” through a process called chemiosmosis

An enzyme, called ATP synthase, is coupled to the release of the H+ ions

This enzyme uses the energy from the H+ pump to add a phosphate molecule to ADP to form the energy rich molecule ATP

ADP + Phosphate + Energy ATP

Photosystem II Electrons from photosystem II can add to electrons

used in photosystem I (kind of a reserve supply) If the electrons from photosystem II run out, the

light reaction stops NOTE: another source of e-: there is an enzyme in

the thylakoid membrane that splits water molecules to provide electrons for the electron transport chain and H+ for chemiosmosis

2H2O 4H+ + 4e- + O2

Note: This is how a plant releases O2 so we can breathe

Photosystem I Photosystem I (P700):

‘fallen’ e- replace excited e- to primary e- acceptor

2nd ETC (NADP+ reductase) transfers e- to NADP+ NADPH (...to Calvin cycle…)

Photosystems produce equal amounts of ATP and NADPH

Photosystem IAccessory pigments absorb light energy and

transfer it to chlorophyll a molecule (just like in photosystem II) simultaneously with photosystem II

As a result of the light energy, electrons are released from the chlorophyll a molecule in an oxidation reaction

The free electrons from the chlorophyll a molecule are then “accepted” by a protein called a primary electron acceptor which reduces the moleculeNOTE: So far this is just like photosystem II (it’s the same process)

Photosystem I The electrons then move from one molecule to

another in a second electron transport chain This electron transport chain ends on the side

of the thylakoid membrane that faces the stroma (a solution that surrounds the grana)

At this point the electron is used to combine with H+ and NADP+ to form NADPH (a high energy molecule) NADP+ + H+ + 2e- + Energy NADPH (reduction) NADPH is then used in the Calvin cycle

Summary1. Light is absorbed by pigments in the grana of the

thylakoid (inside a chloroplast)2. The pigment chlorophyll releases electrons to move

through an electron transport chain3. Photosystem II creates a H+ pump that creates an

uneven distribution of H+ on each side of the thylakoid membrane

ATP synthase uses the “pressure” create from this H+ pump to create ATP

4. Photosystem I uses the electron transport to combine with H+ and NADP+ to form NADPH

Calvin cycleCalvin Cycle – ATP and NADPH from the light reactions are used along with CO2 to form a simple organic compound (made of carbon)Takes place in the stroma of the chloroplasts (the liquid filling).

Byproducts are C6H12O6 (glucose, sort of), ADP, and NADP+ (which return to the light reactions).

The Calvin cycle 3 molecules of CO2 are

‘fixed’ into glyceraldehyde 3-phosphate (G3P)

Phases: 1- Carbon fixation~ each

CO2 is attached to RuBP (rubisco enzyme)

2- Reduction~ electrons from NADPH reduces to G3P; ATP used up

3- Regeneration~ G3P rearranged to RuBP; ATP used; cycle continues

The Calvin cycleWith the aid of an enzyme, CO2 is combined with

a 5-C molecule called ribulose-bisphosphate (RuBP) to form 2 molecules called 3-phosphoglycerate (3-PGA) . . . 3-C organic compoundCO2 + RuBP 2 3-PGA

The 3-PGA is converted to another molecule called glyceraldehyde 3-phospate (G3P)2 3-PGA 2 G3P (3 carbon organic compound)This process uses up one ATP and one NADPH

created in the light cycle

The Calvin cycleOne molecule of G3P leaves the Calvin

cycle and is used to make organic compounds (carbohydrates)

The other molecule of G3P remains in the Calvin cycle to make more RuBP

Summary: Now the CO2 is fixed into an organic compound that can be used by the cell to make either energy or structural molecules

Calvin Cycle IMPORTANT NOTE: Glucose is not directly created by

photosynthesis The G3P molecule is the one created This acts as a precursor to MANY organic molecules

Glucose is emphasized because it is most important to us

Breakdown of photosynthesis:

6CO2 + 6H2O + energy C6H12O6 + 6O2

H2O is used in the light cycle to provide electrons for electron transport chain

CO2 is absorbed from atmosphere for Calvin cycle O2 is produced as a by-product in light cycle Glucose is eventually created from G3P molecules

Calvin Cycle, net synthesisFor each G3P (and for 3 CO2)

Consumption of 9 ATP’s & 6 NADPH (light reactions regenerate these molecules)

G3P can then be used by the plant to make glucose and other organic compounds

NADP+

ADP NADPH

CO2

H2O

O2

CHLOROPLAST

ATP

Sunlight

Cyclic electron flow Alternative cycle

when ATP is deficient Photosystem II used

but not I; produces ATP but no NADPH

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

Cyclic photophosphorylation

Alternative carbon fixation methods, I Photorespiration: hot/dry

days; stomata close; CO2 decrease, O2 increase in leaves; O2 added to rubisco; no ATP or food generated

Two Solutions….. 1- C4 plants: 2

photosynthetic cells, bundle-sheath & mesophyll; PEP carboxylase (instead of rubisco) fixes CO2 in mesophyll; new 4C molecule releases CO2 (grasses)

Alternative carbon fixation methods, II

2- CAM plants: open stomata during night, close during day (crassulacean acid metabolism); cacti, pineapples, etc.

Factors that affect photosynthesis

Light intensity In general, the higher the light intensity, the faster the

light cycle Eventually all electrons are being used and you hit a

maximum rate for photosynthesis Carbon dioxide levels

Works like light intensity: increased CO2 levels increase the rate of photosynthesis

When the maximum level of CO2 is used, the rate does not increase further

Temperature Increasing temperature increases the rate of reactions

and photosynthesis At a point, the heat denatures (or breaks apart) the

enzymes needed for photosynthesis and the rate decreases

A review of photosynthesis

Photo-SystemI

Photo-systemII

NADP+

ADPNADPHATP

CalvinCO2

H2O

O2

ATP

ATP

NAD+ NADH

ElectronTransportSystem

Cycle

CitricAcid

Heat

CHLOROPLAST MITOCHONDRIONATP

Glycolysis

Glucose Pyruvate

Cycle

Sunlight