Chapter 10 Photosynthesis

Photosynthesis Lecture Outline

• Overview

• The Chloroplast

• Discovery of the process.

• Light and pigments

• Light reactions

• Calvin cycle and carbon fixation

• C₄ and CAM metabolism

Fig. 10.2

Fig. 9.2

Photosynthesis

• 6CO₂ +12H₂O + light → C₆H₁₂0₆ +6H₂O + 6 O₂.

– Ultimate source of energy for life on earth.

– Carbon is fixed into organic forms.

– Oxygen is produced.

Figure 10.5 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle

Overview of Photosynthesis

• Light reactions convert light energy to ATP and NADPH.

• Uses electron transport and chemiosmosis.

• ATP and NADPH provide energy for assembly of CO₂ to sugars in the Calvin Cycle.

Figure 10.3 Focusing in on the location of photosynthesis in a plant

Anatomy of a chloroplast

• Enclosed in a double membrane (outer and inner)

• Inside, flattened thylakoids stacked into columns called grana.

• The stroma is a semifluid substance that surrounds thylakoids.

• The thylakoid membranes contain chlorophyll, and the photosystems that harvest light energy.

– “light reactions”

• Stroma contains the enzymes of the Calvin cycle.

– carbon assembly

Discovery of the Process of Photosynthesis

• The mass of the growing plant comes from the air!

• Vegetation can restore air!

• CO₂ + H₂O + light energy → carbohydrate + O₂.

• The oxygen comes from H₂O, not the CO₂ . The water splitting light reactions are separate from the carbon compound building dark reactions.

– Van Niel, Blackman 1930’s -1950’s;

• The steps of carbon dioxide fixation are elucidated.

– Calvin, 1940’s

Figure 10.4 Tracking atoms through photosynthesis

Properties of Light

• Light is a form of electromagnetic energy that has the properties of waves when it travels.

• Light is composed of energy containing photons.

• The energy of a photon depends of the wavelength of light.

• The shorter the wavelength, the higher the photon energy.

In the visible spectrum, each wavelength of light appears as a different color.

Figure 10.6 The electromagnetic spectrum

Pigments

• A specific atom can only absorb certain photons of light corresponding to the atoms available electron energy levels.

• Absorption of a photon of light boosts the energy of an atom.

Fig. 2.7

Pigments

• Pigments are molecules that are especially good at absorbing light.

• The visible color of a pigment is due to light that is reflected, not absorbed.

Fig 10.7

Fig. 10.10 Structure of Chlorophyll.

- Chlorophyll a is the main photosynthetic pigment.

- Chlorophyll b is an accessory pigment.

Absorption Spectra

• Each molecule, and therefore each pigment, has a characteristic absorption spectrum.

Figure 10.8 Determining an absorption spectrum

Fig. 10.9a. Chlorophyll a and b absorb wavelengths of light between 400- 500 nm (blue), and between 600 – 700 nm (red).

What color light do you think would be most effective for photosynthesis?

Figure 10.9 Evidence that chloroplast pigments participate in photosynthesis: absorption and action spectra for photosynthesis in an alga

Excitation of Chlorophyll by Light

• When a pigment absorbs light

– It goes from a ground state to an excited state, which is unstable. Fig. 10.11a

• If an isolated solution of chlorophyll is illuminated

– It will fluoresce, giving off light and heat

A photosystem

– Is composed of a reaction center surrounded by a number of light-harvesting complexes. Fig. 12

Photosystems

• Chlorophyll is organized together with proteins and other kinds organic molecules into photosystems.

• Electrons captured by chlorophyll molecules and passed to a ‘reaction center’ consisting of a chlorophyll a molecule located next to a molecule called the “primary electron acceptor”.

• The ‘primary electron acceptor’ transfers energy from the pigments to protons in an electron transport chain, and is the first step in the ‘light reactions”.

Figure 10.12 How a photosystem harvests light

There are two types of photosystems in the thylakoid membranes.

– called photosystem I and photosystem II in order of their discovery.

– Each has a characteristic reaction center.

– The photosystem I reaction center chlorophyll is known as P700, while the photosystem II reaction center is known as P680.

– Contain identical chlorophyll a molecules, but are associated with different proteins, so have different absorptive properties.

The Light Reactions

• H₂0 + ADP + NADP⁺ → O₂+ ATP + NADPH

• Photosystem I reduces NADP⁺ to NADPH

• Photosystem II uses chemiosmosis and ATP synthase to make ATP.

• This is noncyclic photophosphorylation. Electrons are transferred to NADPH and leave the system.

– (they will be transferred to carbon dioxide to make sugars in the Calvin cycle)

Figure 10.13

Figure 10.14 A mechanical analogy for the light reactions

Cyclic photophosphorylation

• ATP and NADPH are made is roughly equal proportions by non-cyclic photophosphorylation.

• However, the Calvin cycle consumes more ATP than NADPH.

• If the chloroplast runs low on ATP then NADPH builds up and stimulates a shift from noncyclic to cyclic electron flow.

• With cyclic electron flow, only photosystem I is used, but ATP is made instead of NADPH.

Figure 10.15 Cyclic electron flow

Figure 10.16 Comparison of chemiosmosis in mitochondria and chloroplasts

Figure 10.17

Figure 10.x1 Melvin Calvin

The Calvin Cycle

• 3 CO₂ + 9 ATP + 6NADPH + water → Glyceraldehyde 3 phosphate (3C) + 8 P + 9 ADP + 6 NADP⁺.

• Glyceraldehyde 3 phosphate (3C) is converted to glucose and other sugars by subsequent pathways.

• Carbon fixation is controlled by the enzyme Rubisco.

Figure 10.18

Photorespiration

• Rubisco also catalyzes the oxidation of ribulose 1,5 bisphosphate, resulting in the release of CO₂.

• At 25°C, carboxylation is 4x greater than oxidation.

– 20% loss of fixed carbon

• At higher temperatures the rate of oxidation is increased.

Strategies to Reduce CO₂ Loss from Photorespiration

• The C₄ Pathway

• Crassulacean Acid Metabolism (CAM)

The C₄ Pathway

• C₄ pathway occurs in mesophyll cells

• Fixes CO₂ to a 4 carbon compound

• The 4 carbon compound is transported to bundle sheath cells, and CO₂ is released.

• The [CO₂] is high, decreasing photorespiration.

• C₄ plants include corn, sugar cane and grasses.

Figure 10.19 C₄ leaf anatomy and the C₄ pathway

Costs of the C₄ Pathway

• It costs 2 ATP each to transport CO₂ into the bundle sheath cells.

– So, 30 ATP are needed per molecule of glucose, versus 18.

• However, photorespiration would have removed over half the CO₂ fixed.

CAM

• CAM plants open their stomata during the night and close them during the day.

– This is the opposite of C₃ plants.

• The pathway is used at night to fix carbon.

• During the day, the C₄ compounds are decarboxylated to release CO₂.

• The high levels of CO₂ reduce photorespiration.

• Both these steps occur in the mesophyll cells.

Figure 10.20

Figure 10.21 A review of photosynthesis

The Energy Cycle

6CO₂ +12H₂O + light → C₆H₁₂0₆ +6H₂O + 6 O₂.

• Photosynthesis

C₆H₁₂0₆ +6H₂O + 6 O₂ → 6CO₂ +12H₂O + 36 ATP

• Aerobic Respiration

The End.