• Chapter 10~ Photosynthesis

A. Photosynthesis in nature

• Autotrophs: (SELF-FEEDERS) 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)

2. The chloroplast

• Sites of photosynthesis• Pigment: chlorophyll• Plant cell: mesophyll• Gas exchange: stomata• Double membrane• Thylakoids, grana,

stroma

2. The chloroplast

Thylakoid

Stroma

Grana

Photosynthesis: An Overview

The equation describing the net process of photosynthesis is:6CO2 + 6H2O + light energy -> C6H12O6 + 6O2

In reality, photosynthesis adds one CO2 at a time:CO2 + H2O + light energy -> CH2O + O2

( CH2O represents the general formula for a sugar.)

1. THE EQUATION:

Photosynthesis: An Overview FATE OF THE ATOMS OF REACTANTS:

Photosynthesis: An Overview

• Scientists confirmed that Oxygen given off by plants is from WATER not CARBON DIOXIDE

• Hydrogen extracted from water is incorporated into sugar and the oxygen is released to the atmosphere ~ WHERE DOES IT GO THEN?

Originally it was thought that O came from CO2…..

Photosynthesis: An Overview• Photosynthesis is a redox reaction. • It reverses the direction of

electron flow in respiration.• Water is split and electrons

transferred with H+ from water to Carbon Dioxide, reducing it to SUGAR..

• Polar covalent bonds (unequal sharing) are converted to nonpolar covalent bonds (equal sharing).

• Light boosts the potential energy of electrons as they move from water to sugar.

Photosynthesis: an overview2 major steps:

1. Light reactions (“photo”)

• Occurs in the THYLAKOID:

• NADP+ (electron acceptor) to NADPH

• Photophosphorylation: ADP ---> ATP

Photosynthesis: an overview2 major steps:

2. Calvin cycle (“synthesis”)• Occurs in the stroma • CO2 into an organic molecule

via carbon fixation.• Carbon backbone is reduced

with electrons provided by NADPH.

• ATP from the light reaction also powers parts of the Calvin cycle.

Light:The most important segment for life is a narrow band between 380 to 750 nm, visible light.

Photon: A discrete amount of light energy.

• The amount of energy packaged in a photon is inversely related to its wavelength. (Short wavelength= high energy).• Different pigments absorb photons of different wavelengths.• A leaf looks green because chlorophyll, absorbs red and blue light, transmitting/reflecting green light.

Chlorophyll a• Chlorophyll a: absorbs best in the red and blue

wavelengths, and least in the green.• Action spectrum for photosynthesis: wavelength

plotted against some sort of photosynthetic rate.• Action spectrum does not match exactly the

absorption spectrum of any one photosynthetic pigment

WHY?• Only Chlorophyll a participates directly in the

light reactions • BUT: accessory photosynthetic pigments absorb

light and transfer energy to chlorophyll a.

Accessory Pigments

• Chlorophyll b, with a slightly different structure than chlorophyll a, has a slightly different absorption spectrum and funnels the energy from these wavelengths to chlorophyll a.

Accessory Pigments• Carotenoids funnel the energy from other

wavelengths to chlorophyll a, participate in photoprotection

• Anthocyanins: give color to such familiar things as cranberries, red apples, concord grapes, blueberries, cherries, strawberries, and plums.

WHY DO LEAVES CHANGE COLOR?

Chlorophyll and Light

• When a molecule absorbs a photon, electrons is elevated to an orbital with more potential energy.

• ELECTRON: Ground state Excited State• Each pigment has a unique absorption spectrum.• In chlorophyll a and b, it is an electron from

magnesium in the porphyrin ring that is excited and therefore unstable.

• Generally, they drop to their ground state in a billionth of a second, releasing heat energy (Unless what???)

Check yourself:

• What are the two major steps in photosynthesis and where do they occur?

• What makes chlorophyll a so unique?

Two Types of Photosystems

Light Rxts: 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

Two Types of Photosystems• Photosystem I : the P700 center,

absorption peak at 700nm.• Photosystem II has a reaction center

with a peak at 680nm.• Differences between these reaction

centers are because of proteins associated with each reaction center.

• These two photosystems work together to use light energy to generate ATP and NADPH.

Electron Flow: Noncyclic1) Photosystem II absorbs light,

an excited e-captured by the primary electron acceptor, leaves reaction center oxidized.

2) Enzyme extracts electrons from water, supplies them to the oxidized reaction center.

3) Photoexcited electrons pass along an electron transport chain before ending up at an oxidized photosystem I reaction center.

Electron Flow: Noncyclic4) As these electrons pass

along the transport chain, their energy is harnessed to produce ATP.

5) At the bottom of this electron transport chain, the electrons fill an electron “hole” in an oxidized P700 center.( This hole is created when photons excite electrons on the photosystem I complex.)

Electron Flow: Noncyclic • Excited e- are captured by a

2nd primary electron acceptor, which transmits them to a 2nd electron transport chain.

• These electrons are passed from the transport chain to NADP+, creating NADPH.

• NADPH will carry the reducing power of these high-energy electrons to the Calvin cycle.

• Noncyclic electron flow produces ATP and NADPH in roughly equal quantities.

Electron Flow: Noncyclic

• Noncyclic Electron Flow

Electron Flow: Cyclic• Photoexcited electrons from photosystem I,

but not photosystem II, can take this alternative pathway

• Cyclic electron flow converts light energy to chemical energy in the form of ATP.

• Excited electrons cycle from their reaction center to a primary acceptor, along an electron transport chain, and return to the oxidized P700 chlorophyll.

• As electrons flow along the electron transport chain, they generate ATP by cyclic photophosphorylation.

• The Calvin cycle consumes more ATP than NADPH

•Cyclic electron flow allows the chloroplast to generate enough surplus ATP to satisfy the higher demand for ATP in the Calvin cycle.

Light Reactions

Animation of Light Reactions

Chemiosmosis: A ComparisonMitochondria Chloroplast

How it works: Electron Transport Chain pumps protons from low conc. to high conc. Protons diffuse back through membrane, driving synthesis of ATP

Structures: Protons pumped from matrix to intermembrane space

Protons pumped from stroma to thylakoid space

Energy Source: FOOD LIGHTImportant Term: Oxidative

phosphorylationPhotophosphorylation

PHOTOSYNTHESIS

• http://www.sumanasinc.com/webcontent/animations/content/harvestinglight.html

PROVIDE A SUMMARY ON YOUR PAPER FOR THE LIGHT REACTIONS

Calvin Cycle• The Calvin cycle regenerates its starting material after

molecules enter and leave the cycle.• CO 2 enters the cycle and leaves as sugar by the power of

ATP and electrons carried on NADPH• The actual sugar product of the Calvin cycle is not

glucose, but a three-carbon sugar, glyceraldehyde-3-phosphate (G3P).

• Each turn of the Calvin cycle fixes one carbon.• For the net synthesis of one G3P molecule, the cycle must

take place three times, fixing three molecules of CO2.

The Calvin Cycle• Phases:

1- Carbon fixation~ • CO2 molecule is attached to a

five-carbon sugar, ribulose bisphosphate (RuBP).

• catalyzed by RuBP carboxylase or rubisco.

• 6 intermediate splits in half: 2 molecules of 3-phosphoglycerate per CO2.

•3 molecules of CO2 are ‘fixed’ into glyceraldehyde 3-phosphate (G3P)

Calvin Cycle

2: Reduction~• 3-phosphoglycerate + phosphate

group from ATP =1,3-bisphosphoglycerate.

• e- from NADPH reduces 1,3-bisphosphoglycerate to G3P (PGAL)

• If our goal was to produce one G3P net, we would start with 3CO2 (3C) and three RuBP (15C).

• This molecule can exit the cycle to be used by the plant cell.

The Calvin Cycle3. Regeneration:• 5 G3P (PGAL) (15C) must remain

in the cycle: regenerate three RuBP, complex rearrangement of molecules.

• Net synthesis of one G3P molecule, the Calvin recycle consumes 9ATP and 6 NAPDH.

• It “costs” three ATP and two NADPH per CO2.

• G3P from the Calvin cycle: starting material for metabolic

pathways synthesis of other organic compounds

Calvin Cycle, net synthesis G3P= (PGAL)

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

Problem: Dehydration• Solutions to the

dehydration problem conflict with photosynthesis.

• If stomata are the only route for gas exchange, what is a problem they create when they are open?

• On hot, dry days plants close the stomata to conserve water, but this causes problems for photosynthesis….WHY?

Photorespiration• If O2 is greater than CO2 in leaves…• Rubisco fixes O2 instead of CO2• 5-C compound produced• 5-C -> 1 PGA enters Calvin Cycle

1 glycolate (2-C) exits choloroplasts and enters peroxisomes

Decreases productivityFostered by Hot, Dry, Bright Days.

C4 Plants•Examples: sugercane, corn

•Mesophyll cells incorporate CO2 into organic molecules

•Enzyme: phosphoenolpyruvate carboxylase, adds CO2 to phosphoenolpyruvate (PEP) to form oxaloacetetate.

•PEP carboxylase has a very high affinity for CO2and can fix CO2 efficiently when rubisco cannot, i.e. on hot, dry days when the stomata are closed.

The C4 plants fix CO2 first in a four-carbon compound, in the mesophyll cells

A Review of Photosynthesis:

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C4 Plants• Fix CO2 as a 4-C

compound• Segregate CO2

fixation from Calvin cycle

• Acts as CO2 pump• PEP carboxylase has

lower affinity for O2 than rubisco

• Adaptation in hot regions with intense sunlight

CAM Plants• CAM plants: crassulacean

acid metabolism (CAM), open stomata during the night and close them during the day

• Examples: succulent plants, cacti, pineapples, and several other plant families.

• Night: these plants fix CO2 into a variety of organic acids in mesophyll cells.

• Day: the light reactions supply ATP and NADPH to the Calvin cycle and CO2 is released from the organic acids.

Alternative carbon fixation methods:• Both C4 and CAM

plants add CO2 into organic intermediates before it enters the Calvin cycle.

In C4 plants, carbon fixation and the Calvin cycle are spatially separated.

In CAM plants, carbon fixation and the Calvin cycle are temporally separated

A Review of Photosynthesis