Bio 360, Fall 2001

Bio 360, Fall 2001 September 18, 2001

Photosynthesis I

I. The chemical reactions of photosynthesis

A. Overall

6CO₂+ 12H₂O* + light energy -> C₆H₁₂O₆ + 6H₂O + 6O₂*

This reaction can be divided into two basic parts

B. Two major events

1. Light reactions - require light

2 H₂O + n ADP + 2 NADP + light -> n ATP + 2 NADPH +2 H⁺ + O₂

These reactions are unique to photosynthetic organisms

Depend on light

Products

ATP = chemical energy

NADPH = electron donor for reductions

O₂

2. Dark reactions (Light-independent reactions)- can, but do not necessarily, occur in the dark

CO₂+ 2 NADPH + 2 H⁺ + n ATP -> (CH₂O) + 2 NADP + n ADP + H₂O

many of the individual steps are possible in nonphotosynthetic organisms

requires presence of and absorption of CO₂

II. Where does photosynthesis occur?

A. Leaf anatomy

1. Upper epidermis

compact cells

resistant to water loss

waxy cuticle

no chloroplasts

2. Lower epidermis

similar to upper epidermis

contains stomata

stomata - pores surrounded by 2 guard cells; regulate gas exchange

3. Mesophyll

pallisade layer - long cells with many chloroplasts; site of most photosynthesis

spongy layer - irregularly shaped cells; fewer cholorplasts

site of much gas exchange

4. Vascular system

veins - xylem, water transport; phloem, transport of metabolites

bundle sheath cells

B. Chloroplasts

1. Membranes (grana, thylakoids)

site of photosynthetic pigments - light harvesting

2. Stroma - space outside of grana, etc.

site of dark reactions

3. Spaces inside grana

III. How is light absorbed? Chlorophyll

A. Structure of the chlorophyll molecule

1. porphyrin ring - light absorption

every other bond is a double bond

electrons are shared around this ring of conjugated double bonds

any modification of the ring changes the color of light absorbed

2. Phytol tail - positions the molecule in the membrane

B. Light absorption

1. Chlorophyll in solution absorbs light primarily at two wavelengths 420nm and 675nm.

2. Light absorption

a. Light energy comes in discrete units called photons or quanta. Light of shorter wavelengths has more energy

b. Energy of electrons also comes in discrete units as represented by the orbital or energy level of the electron.

c. Each photon of light interacts with one electron

d. The photon may raise the electron to a new energy level.

3. The kinds of bonds in a molecule determine the energy levels available for electrons and hence the wavelength of light that can raise an electron to the next energy level.

4. In a single bond between two carbons, UV light is required to raise electrons to the next level.

5. The ring structure and delocalized electrons in chlorophyll provide lower energy levels for electrons. Light in the visible spectrum can be used to raise electrons to these energy levels.

C. Light absorption by Chlorophyll

d. Green light raises the electron to 6 where there is no orbital and the electron falls back to 1.

e. Blue light raises the electron to 7, two orbitals, but the electron quickly falls back to 2.

f. So all electrons end up at 2 or 1. Electrons at 2 have energy that can be used to drive the reactions of photosynthesis.

D. Fates of excited electrons

1. Fall back to the ground state and emit heat

2. Fall back to the ground state and emit light and heat -this produces fluorescence and is typical of damaged plants

3. Photosynthesis - the energy of the electron gets transferred to other compounds

E. The actual absorbance spectrum of chlorophyll is much broader than that of the pure compound.

The wavelength of light absorbed varies slightly based on the orientation of the chlorophyll in the membrane and the presence of other molecules.

IV. What happens to the energy contained in the electrons?

A. Most chlorophyll molecules pass the energy to other chlorophyll molecules that have slightly lower energy orbitals.

B. Two special kinds of chlorophyll, P680 and P700 trap electrons and can pass the electrons to other molecules.

C. P680, called photosystem II. passes its electron to a series of electron transport molecules. As the electron passes from molecule to molecule, its energy is lost and ATP is produced. P680 gets a replacement electron from water. In consequence, O_2,ATP, and hydrogen ions are produced.

D. P700, or photosystem II, passes its electrons to an electron transport chain and NADPH is produced.

E. ATP production depends on the production of a gradient in hydrogen ion concentration from the stroma to the interior of the thylakoids. This is rather like ATP production in mitochondria

V. Dark reactions

A. Fixation of carbon dioxide

1. CO₂is combined with RuBP ( a five carbon compound) to form 2 molecules of PGA (a 3 carbon compound)

3. CO₂diffuses into leaves through the stomata. The rate of diffusion depends on the CO₂concentration and whether the stomata can remain open

B. The remainder of the dark reactions are a series of reactions that get one carbon to a carbohydrate and regenerate RUBP.

This takes 2 NADPH and 3 ATP per carbon dioxide fixed.

C. C₃ reactions

1. The enzyme responsible for CO₂ fixation is RuBP-carboxylase

a. A very abundant protein (25% of total chloroplast protein)

b. RuBP- carboxylase can react with O₂as well as CO₂ because oxygen is so much more abundant in the atmosphere, up to 30% of the action of the enzyme is involved in this reaction (photorespiration)

c. Photorespiration limits the rate of photosynthesis in two ways competition of oxygen for the enzyme resulting release of CO₂

2. All reactions occur in the mesophyll cells

D. C₄ photosynthesis

1. A different enzyme is responsible for the first steps of CO₂fixation PEP carboxylase - has a much higher affinity for CO_2,photorespiration is not a problem

Initially CO₂is bound to a C ₄ compound

RuBP-carboxylase is still present

2. Leaf anatomy is different (Kranz anatomy); the vascular bundle is surrounded by a layer of thick walled, gas impermeable bundle sheath cells and an outer layer of thin walled mesophyll cells.

3. The two parts of CO₂fixation are separated in space.

CO₂ fixation occurs in the mesophyll cells where C₄ compounds are made by PEP carboxylase, an enzyme with high affinity for CO₂

C₄ compounds are transported to the bundle sheath cells where CO₂is released and used by RuBP to make carbohydrates.

4. Benefits

a. There is little oxygen competition for RuBP, so more CO₂can be fixed.

b. Since, CO₂is used efficiently, the stomata do not have to be open as much and so water is conserved.

5. Costs - energy must be expended, 2 additional ATP's per CO₂

6. Occurrence - Found in many grasses, such as corn and wheat.

E. CAM - crassulacean acid metabolism

1. The parts of CO₂fixation are separated in time. Stomata open at night and close during the day, opposite to the usual pattern

a. Night, stomata open

CO₂is fixed by PEP carboxylase to form malic acid.

Starch is broken down to form PEP

b. Day, CO₂is released from malic acid and is used by RuBP to form carbohydrates. Stomata are closed.

2. Benefits - reduced water loss because stomata open at night when the air is cooler and more moist

3. Costs - - energetically expensive, 3.5 ATP/CO₂

carbon is fixed very slowly, so plants grow very slowly

4. Occurrence - Cacti, bromeliads, succulents, plants of very hot, dry climates

VI. Efficiency of photosynthesis - Efficiency = energy stored/energy input

A. Light reactions

input = 4 quanta of light = 680 kj/einstein

output = 1.5 ATP = 45kj energy/mole of ATP

+ 2 high energy electrons = 211.2 kj/mole of electrons

+ 256.5 kj stored energy

efficiency = 256.5/680 x 100 % = 38%

This represents the efficiency of photosynthesis if only red light is present and all quanta are used for photosynthesis.

For white light, the efficiency is 20% if all quanta that can be absorbed are used for photosynthesis.

Typically, the efficiency is <10%

B. Dark reactions

input = 8 quanta of light/CO₂fixed x 6 CO₂hexose

8160 kj of energy from red light to make one mole of hexose

output = 2880 kj/mole of hexose

efficiency = 2880/8160 x 100% = 35%

Note that very little energy is lost in the dark reactions.

Actual, measured efficiencies are much lower, 5-10%

C. In the field, typically about 1% of the available light energy is converted to stored energy.

What limits photosynthesis in the field

1. Light

2. CO₂

3. Water - to absorb CO₂, the stomata must open when the stomata are open, H₂O is lost

4. Temperature