Catabolic and

Anabolic Pathways depend on ATP / ADP

shuttle

Figure 10.0 Sunbeams

Question:•Where does photosynthesis take place?

http://www.sumanasinc.com/webcontent/anisamples/majorsbiology/harvestinglight.swf

Plants• Autotrophs: self-producers.• Location:

1. Leaves

a. stoma

b. mesophyll cells

StomaMesophyllCell

Chloroplast

Stomata (stoma)• Pores in a plant’s cuticle through which water

and gases are exchanged between the plant and the atmosphere.

Guard Cell

Guard Cell

Carbon Dioxide (CO2)

Oxygen (O2)

Mesophyll Cell

Cell Wall

Nucleus

Chloroplast

Central Vacuole

Chloroplast

• Organelle where photosynthesis takes place.

GranumThylakoid

Stroma

Outer Membrane

Inner Membrane

Thylakoid

Thylakoid Membrane

Thylakoid SpaceGranum

Chloroplast• Chlorophyll – green pigment located

inside chloroplasts• Found in mesophyll (interior of leaf)• Double membrane• Enclose stroma (dense liquid)

containing grana made up of thylakoid membranes which contain chlorophyll

PhotosynthesisH2O CO2

O2 C6H12O6

Light Reaction

Dark Reaction

Light is AdsorbedBy

Chlorophyll

Which splitswater

Chloroplast

ATP andNADPH2

ADPNADP

Calvin Cycle

Energy

Used Energy and is recycled.

+

+

Photosynthesis

• An anabolic, endergonic, carbon dioxide (CO2) requiring process that uses light energy (photons) and water (H2O) to produce organic macromolecules (glucose).

6CO2 + 6H2O C6H12O6 + 6O2

glucose

SUN

photons

Question:•Why are plants green?

Chlorophyll Molecules• Located in the thylakoid membranes.

• Chlorophyll have Mg+ in the center.

• Chlorophyll pigments harvest energy (photons) by absorbing certain wavelengths (blue-420 nm and red-660 nm are most important).

• Plants are green because the green wavelength is reflected, not absorbed.

Light Absorption Spectrum

Absorption of Chlorophyll

wavelength

Absorption

violet blue green yellow orange red

Question:•During the fall, what causes the leaves to change colors?

Fall Colors• In addition to the chlorophyll pigments, there

are other accessory pigments present.

• During the fall, the green chlorophyll pigments are greatly reduced revealing the other pigments.

• Carotenoids are pigments that are either red or yellow.

Chromatography• Pigments can be separated out using the

process of CHROMATOGRAPHY.

Redox Reaction• The transfer of one or more electrons from

one reactant to another.

• Two types:

1. Oxidation

2. Reduction

Oxidation Reaction• The loss of electrons from a substance.• Or the gain of oxygen.

glucose

6CO2 + 6H2O C6H12O6 + 6O2

Oxidation

Reduction Reaction• The gain of electrons to a substance.• Or the loss of oxygen.

glucose

6CO2 + 6H2O C6H12O6 + 6O2

Reduction

Breakdown of Photosynthesis

• Two main parts (reactions).

1. Light Reaction or

Light Dependent Reaction

Produces energy from solar power (photons) in the form of ATP and NADPH.

Breakdown of Photosynthesis

2. Calvin Cycle or

Light Independent Reaction or

Carbon Fixation or

C3 Fixation

Uses energy (ATP and NADPH) from light rxn to make sugar (glucose).

Chlorophyll• Two different types : Chlorophyll a and b• “a” – main pigment in photosynthesis• “b” – accessory pigment

– Both have similar structure but absorb different wavelengths of light

• Chlorophyll absorbs light of a particular wavelength, electrons are excited and jump to higher energy level, drops back down and gives off heat

• This energy is passed along until it finds Chlorophyll a, which, when excited, passes electron to primary electron acceptor…powering light dependent reactions.

How a photosystem harvests light..

1. Light Dependent Reaction

• Occurs in the Thylakoid membranes

• During the light reaction, there are two possible routes for electron flow.

A. Photosystem II

B. Photosystem I

*Pigments of the thylakoid space organize themselves into groups called photosystems

Chemiosmosis• Powers ATP synthesis.

• Located in the thylakoid membranes.

• Uses ETC and ATP synthase (enzyme) to make ATP.

• Photophosphorylation: addition of phosphate to ADP to make ATP.

Chemiosmosis…Making ATPH+ H+

ATP Synthase

H+ H+ H+ H+

H+ H+high H+

concentration

H+ADP + P ATP

PS II PS IE

TC

low H+

concentration

H+ThylakoidSpace

Thylakoid

SUN (Proton Pumping)

Things to Remember….

Light Reactions…• Occur in the Thylakoid Membrane• Inputs are water and light• Products : ATP, NADPH and O2

• Oxygen produced comes from the water, not Carbon Dioxide!

• Two different pathways…..

Cyclic vs. Noncyclic Pathways…

• Noncyclic Light Reaction– Electrons taken from Chlorophyll a are not

recycled back down to the ground state – therefore do not make it back to the chlorophyll molecule when the reaction is complete. Electrons end up on NADPH

• Cyclic Light Reaction– Uses ONLY Photosystem I, sunlight hits P700– Energy given off down ETC, only produces

ATP

…Because of the Calvin Cycle

• Calvin Cycle uses more ATP than NADPH• This is eventually a problem because the

Light Reaction produces equal amounts of ATP and NADPH

• Plants compensate for this disparity by dropping into the cyclic phase when ATP is needed to keep the Light Independent Reaction from coming to a grinding halt!!!

Calvin Cycle

GranumThylakoid

Stroma

Outer Membrane

Inner Membrane

Light-Independent Reaction takes place in Stroma

Begins Synthesis Phase of Photosynthesis

Chloroplast

Meet Mr. Melvin Calvin

Calvin Cycle• The light reactions provide ATP and

NADPH for the Calvin cycle• Process of carbon fixation – CO2 from air

incorporated into organic molecules• Fixed carbon reduced to form a

carbohydrate (powered by NADPH electrons and energy from ATP)

• Enzyme “Rubisco” assists in reduction• Product is actually glyceraldehyde-3-

phosphate (G3P or GAP) – a 3-carbon molecule

Calvin Cycle• For 1 molecule of GAP, the cycle must take

place 3 times• GAP can be used to make carbohydrates

(like glucose), amino acids, lipids, nucleic acids, and other needed molecules.

• The Calvin cycle begins and ends with ribulose-1,5-biphosphate (RuBP)

• At the end of the cycle, 3 CO2 molecules have formed 6 GAP molecules.

• One GAP molecule is used by the plant cell, the other 5 are used to regenerate RuBP

Calvin Cycle

Things to remember about Calvin Cycle (C3 Fixation)…

• Occurs in Stroma of Chloroplast• Inputs: NADPH, ATP and CO2

• Products: NADP+, ADP and a sugar.• More ATP used than NADPH = need for

Cyclic Photophosphorylation• Carbon of sugar comes from CO2

• To produce glucose: it takes 6 turns and uses 18 ATP and 12 NADPH.

• Some plants must make changes to the system in order to successfully use light to produce energy.

• Transpiration = water loss

• Stomata can be closed to prevent water loss, however they experience a shortage of CO2 and the O2 produced in photosynthesis is unable to leave plant.

Guard Cell

Guard Cell

Photorespiration• Occurs on hot, dry, bright days.

• Stomates close.

• Fixation of O2 instead of CO2.

• Produces 2-C molecules instead of 3-C sugar molecules.

• Produces no sugar molecules or no ATP.• Result = plants have lowered capacity for

growth

Photorespiration

• Because of photorespiration: Plants have special adaptations to limit the effect of photorespiration.

1. C4 plants

2. CAM plants

C3 Plants• Examples: rice, wheat, bluegrass, and

soybeans• The cyclic series of reactions whereby CO2

is fixed into a 3-carbon carbohydrate during the Calvin cycle.

• During hot, dry days, these plants must keep their stomata closed to prevent water loss.

• O2 build up and CO2 cannot be replenished.• Uses photorespiration to convert O2 to CO2.

• Produces no ATP nor food – actually decreases photosynthetic output.

C4 Plants• Examples: tropical and desert plants, as well

as sugar cane, and corn. (15% of plants)• The series of reactions that efficiently fixes CO2

into 4- carbon organic acids for later release and incorporation into the C3 (Calvin) cycle.

• The 4-carbon molecule (oxaloacetate) can easily regenerate CO2 that reenters the Calvin cycle.

• The reaction requires lots of ATP and is only advantageous when lots of light and little water are available.

C4 Plants vs. C3 Plants• Has 2 different types of photosynthetic

cells: Mesophyll Cells and Bundle Sheath Cells

• Divides photosynthesis spatially.

–Light rxn - mesophyll cells.–Calvin cycle - bundle sheath cells.

• C4 uses PEP Carboxylase to drive photosynthesis by fixing carbon.

C4 Leaf Anatomy and the C4 Pathway

CAM Plants• Examples: succulents including many cacti and

pineapples that live in very arid climates (5%)• Open stomata at night and close them during the

day.• The mesophyll cells of CAM plants store the CO2 (at

night) in the form of acids until needed the next day.• During the day when stomata are closed, CO2 is

released from the organic acids and enters the Calvin cycle.

• This must occur during the day instead of night because the light reactions must supply ATP and NADPH for the Calvin cycle.

C4 vs. CAM Plants

A Review of Photosynthesis