9 Photosynthesis: Capturing Energy

Lecture Outline

I. Light is composed of particles that travel as waves

A. Light is a small part of the electromagnetic spectrum

B. Radiation varies in wavelength

1. In the visible spectrum, red has longer wavelengths, and violet has shorter wavelengths

C. Light is composed of packets of energy called photons

1. The energy of a photon is inversely related to its wavelength

2. Short wavelengths have high energy photons

D. When a molecule absorbs a photon of light energy, one electron is energized

1. This energized electron may return to the ground state and dissipate the energy as heat or it will fluoresce (emit light)

2. Alternatively, the electron may leave the atom and be accepted by an electron acceptor

a) Electron acceptors are the basis of photosynthesis

II. Photosynthesis in eukaryotes takes place in chloroplasts

A. Chloroplasts typically contain chlorophyll

B. Mesophyll cells contain numerous chloroplasts

C. The chloroplast has a double membrane

1. The inner membrane encloses the stroma

2. Thylakoids are a third set of membranes enclosing the thylakoid interior space

3. Stacks of thylakoids are known as grana

4. Chlorophyll and other pigments are embedded in the thylakoid membranes

D. Prokaryotes have no chloroplasts, but thylakoid membranes are formed from in-foldings of the plasma membrane

E. Chlorophyll is found in the thylakoid membrane

1. Pigments absorb light of particular wavelengths

2. Chlorophyll absorbs light in the red and blue regions of the spectrum; therefore it A looks@ greenish to our eyes

3. Chlorophyll molecules are composed of a porphyrin ring, which absorbs energy, and a long tail, which embeds the molecule in the thylakoid membrane

4. At the center of the porphyrin ring is a magnesium atom

5. Chlorophyll a is the most important in the light-dependent reactions

6. Chlorophyll b is an accessory pigment that is very similar to chlorophyll a (differs in functional groups of the porphyrin ring)

7. Carotenoids absorb different wavelengths than the chlorophylls

8. Carotenoids appear yellow and orange to our eyes (think carrots!)



- 1 -

III. Chlorophyll is the main photosynthetic pigment

A. Absorption spectra plot absorption of light against varying wavelengths

B. Action spectra plot the effectiveness of the varying wavelengths of light

1. Engelmann in 1883 demonstrated the spectra with Spirogyra

2. Since the action spectra matched the absorption spectra of chlorophyll, he deduced that chlorophyll was responsible for photosynthesis

3. Carotenoids widen the action spectrum for photosynthesis

IV. Photosynthesis is the conversion of light energy to chemical bond energy

A. 6 CO2 + 12 H2 O ± C6H12O6 + 6 O2 + 6 H2O

1. Water appears as both a reactant and a product

2. However, there is no net yield of water

3. Photosynthesis can be divided into the light dependent and the carbon fixation reactions (= light independent reactions)

4. This is a redox reaction

B. ATP and NADPH are the products of the light-dependent reactions

1. Occurs in the thylakoid membranes

2. Energy from light causes chlorophyll to expel a high-energy electron to an acceptor molecule

3. The electron is replaced by an electron from water

4. ATP and NADPH are produced

C. Carbohydrates are produced during the carbon fixation reactions

1. These reactions are not driven by light; rather, they are driven by the products of the light-dependent reactions

2. Fixation refers to the conversion of an inorganic atom, in this case carbon in the form of carbon dioxide, to an organic compound, in this case glucose (ultimately)

V. The light-dependent reactions convert light energy to chemical energy

A. 12 H2O + 12 NADP+ + 18 ADP + 18 Pi ± 6 O2 + 12 NADPH + 18 ATP

B. This reaction is dependent on light and chlorophyll

C. Photosystems I and II include antenna complexes that trap light

1. Antenna complexes are aggregations of pigment molecules and electron acceptors

2. The reaction center is made of a complex of chlorophyll molecules and proteins

a) The reaction centers are characterized by chlorophyll a molecules with slightly different absorption spectra

3. The chlorophyll a molecules of photosystem I have an absorption peak at 700 nm, called P700

4. The chlorophyll a molecules of photosystem II have an absorption peak at 680 nm, called P680

D. Noncyclic photophosphorylation produces ATP and NADPH

1. Both photosystem I and II are involved in this process

2. For every 2 electrons passed to the electron transport chain, 2 ATP molecules and 1 NADPH molecule are produced in the A Z scheme@

3. NADPH is formed by transfer of high energy electrons to NADP+

- 2 -

a) Pigments in photosystem I absorb energy and transfer it to the reaction center

b) A molecule of P700 is excited and emits an electron

c) The electron is transferred to the primary acceptor, then to ferredoxin, then to NADP+

d) NADP+ is reduced to form NADPH, which is released into the stroma

4. Water is the electron source for noncyclic photophosphorylation

a) Photosystem II is also activated and emits an electron to a primary electron acceptor

b) The electron is transferred to a series of acceptors, and ultimately to

photosystem I

c) Electrons in photosystem II are replaced by electrons from water

(1) Photolysis involves splitting of water into oxygen, 2 protons and 2 electrons

5. Noncyclic photophosphorylation is a continuous linear process

E. Cyclic photophosphorylation produces ATP but no NADPH

1. Cyclic photophosphorylation is the simplest type of photosynthetic pathway and involves photosystem I only

2. Electrons from P700 are returned to P700

3. No oxygen is released because photolysis does not occur

4. No NADPH is formed, only ATP

5. The significance of this process is somewhat unclear: It may occur in plant cells when NADP+ is depleted, and therefore there is no compound to receive electrons from ferrodoxin

6. Ancient bacteria use this process to produce ATP from the sun

F. ATP synthesis occurs by chemiosmosis

1. As electrons are transferred along the electron transport chain between photosystem II and photosystem I, the energy is used to pump protons into the thylakoid space

2. Because light is coupled to this process, it is called photophosphorylation

3. The chemiosmotic model explains the coupling of ATP synthesis and electron transport

a) Electron transport chains are embedded in the thylakoid membranes

b) As electrons are transferred between carriers, protons are pumped into the thylakoid interior space

c) This proton gradient causes a difference of about 3 pH units across the thylakoid membrane

d) Protons diffuse through channels formed by ATP synthase and catalyze the phosphorylation of ADP to form ATP by chemiosmosis

e) ATP is ultimately released into the stroma

VI. The carbon fixation reactions require ATP and NADPH

A. 12 NADPH + 18 ATP + 6 CO2 ± C6H12O6 + 12 NADP+ + 18 ADP + 18 Pi + 6 H2O

B Most plants use the Calvin (C3) cycle to fix carbon

1. Calvin and Benson in the 1950s described the details

2. Reaction begins when one molecule of carbon dioxide reacts with ribulose bisphosphate (RuBP)

- 3 -

a) RuBP is a highly reactive 5-carbon molecule

b) Reaction is catalyzed by Rubisco

3. Forms an unstable 6-carbon molecule that immediately breaks down into 2 molecules of the 3-carbon phosphoglycerate (PGA)

4. PGA is phosphorylated by ATP and reduced by NADPH to form glyceraldehyde-3-phosphate (G3P)

5. G3P is rearranged into RuBP or glucose

a) 10 of 12 molecules of G3P are converted into RuBP

b) 2 of 12 molecules of G3P are converted into glucose or fructose

C. Photorespiration reduces photosynthetic efficiency

1. In C4 plants, during hot, dry periods, oxygen molecules in the chloroplast bind with Rubisco, preventing carbon fixation

2. RuBP and oxygen combine, and the intermediates in the Calvin cycle degrade to form carbon dioxide and water

3. Like anaerobic respiration, photorespiration produces carbon dioxide and water and requires oxygen

4. Unlike aerobic respiration, photorespiration does not produce ATP

5. Photorespiration is negligible in C4 plants because the concentration of carbon dioxide is always high in the bundle sheath cells

6. Genetic engineering aims to produce plants with Rubisco with a much lower affinity for oxygen to increase the yields of valuable crop plants

D. The initial carbon fixation step differs in C4 plants and in CAM plants

1. Carbon dioxide is relatively sparse in the atmosphere

a) To obtain carbon dioxide as a carbon source, plants must keep their stomata open but risk water loss

2. The C4 pathway efficiently fixes CO2 at low concentrations

a) This pathway precedes the C3 pathway

b) Carbon dioxide is first fixed into a 4-carbon molecule, oxaloacetate, in the mesophyll cells

c) PEP carboxylase catalyzes the fixation of carbon dioxide

(1) Has a very high affinity for carbon dioxide

d) Bundle sheath cells surround the vascular bundles in a leaf

e) Oxaloacetate is converted into malate, which is passed to the bundle sheath cells

(1) The reaction requires NADPH

f) Malate is decarboxylated to form carbon dioxide and pyruvate

(1) NADPH is regenerated

g) Pyruvate returns to the mesophyll cell where it regenerates phosphoenol pyruvate with ATP

h) The C4 pathway requires 30 ATPs per sugar molecule produced but is more efficient at high light intensities

i) At lower light and temperatures, the C3 pathway is optimal

j) The C4 pathway evolved independently several times and is seen in sugar cane, corn, and crabgrass

3. CAM plants fix CO2 at night

- 4 -

a) Succulent plants (family Crassulaceae) and many others in 25 families, including Cactaceae, Liliaceae, and Orchidaceae, exhibit crassulacean acid metabolism (CAM)

b) CAM plants fix carbon dioxide at night, forming malate, which is stored in vacuoles

c) C4 pathways separate photosynthesis in different cells; CAM separates pathways temporally

d) CAM plants close their stomata during the daytime to reduce water loss

e) This photosynthetic mechanism allows plants to live in highly xeric conditions