Chapter 3: Photosynthesis

• Photosynthesis is an endergonic process

powered by visible light

• Visible light is the portion of the

electromagnetic spectrum with wavelengths between 400 and 750 nm.

The Electromagnetic Spectrum

• Plant chloroplasts contain numerous types of pigments that each absorb specific wavelengths of light (and therefore reflect others).

• Multiple pigments maximizes absorption of visible light

• As chlorophyll is usually the most concentrated pigment in plants, they appear green

Absorption Spectra of Photosynthesis Pigments

• Figures 11 & 14, p. 152, 153• Graphs which summarize the wavelengths of

light best absorbed by photosynthetic pigments• Chlorophylls A & B absorb violet, blue and red

light and reflect/appear green (main photosynthetic pigments)

Antenna Pigments• carotenoids absorb blue and green light and

therefore reflect and appear orange• anthocyanins absorb green light and therefore

appear red to purple

Absorption Spectra of Photosynthesis Pigments

Action Spectrum of Photosynthesis

• indicates the rate of photosynthesis at

each wavelength of light in isolation

• photosynthesis generally occurs best at

blue and red wavelengths (due to chlorophyll), and less well at other

wavelengths (since there are fewer antenna pigments absorbing these, but

this varies by species)

Action Spectrum of Photosynthesis

Chloroplast Structure

• Figure 13, p. 144

• double membrane structure (similar to

mitochondrion)

• The inner membrane forms hollow disk-

like sacs called thylakoids, that form stacks called grana

• the fluid surrounding thylakoids is called

stroma

Chloroplast Structure

Photosynthesis Overview

• Photosynthesis consists of 2 distinct

processes: the "light-dependent" and

"light-independent" reactions

• The light dependent reactions (AKA “Light

Reactions”) require light.

• The light-independent reactions (AKA “Dark Reactions”) can happen in the light

or dark.

Light-Dependent Reactions

• Figure 3 & 5,p. 158 & 160

Overview

• 2 types of photosystems are embedded in thylakoid membranes • these contain pigment molecules that absorb light of various

wavelengths

• antenna pigments absorb light and transmit its energy to a specialized chlorophyll A reaction center

• when the reaction center chlorophyll absorbs energy from the antenna pigments, a pair of its electrons become excited and are transferred to an electron transport chain

• Photosystem II (PS II) generates ATP by chemiosmosis as electrons pass down its electron transport chain (analogous to mitochondrion).

• Photosystem I (PS I) generates NADPH when electrons reach the end of its electron transport chain.

Light-Dependent Reactions (Free Energy Diagram)

Light-Dependent Reactions (Thylakoid Membrane View)

Details

Photoactivation of Photosystem II • light rays of various wavelengths strike PS II• Various antenna pigments absorb useful

wavelengths and relay this to the reaction center,

• on average, PSII absorbs 680 nm wavelengths and thus is also called P680

• a pair of the reaction center's electrons are excited, and therefore more easily removed by an electron transport system in the thylakoidmembrane

Photolysis of Water

• the PSII Chlorophyll A reaction center molecule has been oxidized and its electron pair must be replaced

• A PSII enzyme (“Z-protein”) oxidizes water into an oxygen atom, 2 hydrogen ions, and a pair of electrons which reduce the PSII reaction centre

- the oxygen atoms combine as O2, which is a waste product of photosynthesis

Electron Transport

• as the electrons pass from carrier to carrier, some of the energy is used to pump protons into

the thylakoids from the stroma

• the protons accumulate (the pH drops) and

eventually they diffuse out to the stroma again through channels in ATP synthetase complexes, in the process forming ATP from ADP and Pi

• this is called non-cyclic photophosphorylation

Photoactivation of Photosystem I

• Simultaneously• light rays of various wavelengths strike photosystem I• the appropriate antenna pigments absorb useful

wavelengths • the antenna pigments transfer this energy to the

Chlorophyll A reaction centre molecule• On average, PSI absorbs photons with an average

wavelength of 700 nm• one pair of the reaction center's electrons is excited, and

therefore more easily removed by its electron transport system

• this pair of electrons is replaced by the pair from the PS II electron transport chain

Reduction of NADP+

• the excited electron pair from PSI passes

from carrier to carrier and reduces a molecule of NADP+ from the stroma,

forming NADPH

Cyclic Photophosphorylation

(Figure 6, p. 160)

• in primitive versions of photosynthesis, PS I works independently of PS II in a process called cyclic photophosphorylation

• electrons from the reaction center of PS I are excited by antenna pigments, and boosted to the first electron carrier of the PS I electron transport chain

• they are then shunted to the PS II electron transport chain

• the electrons return to PS I, and are used to only generate ATP but not NADPH

• This is why the free energy diagram has PS II before PS I; PS II was added later

• this process is cyclic since the electrons start and end at the same place

Cyclic Photophosphorylation

Videos

• Khan Academy Light Reactions 1

http://youtu.be/GR2GA7chA_c

• Khan Academy Light Reactions and Photophosphorylation

http://youtu.be/yfR36PMWegg

Light-Independent Reactions

Figure 9, p. 161

• the ATP and NADPH from the light-

dependent reactions provide energy and

electrons to reduce carbon dioxide to glucose

Melvin Calvin Experiment • Calvin determined the light dependent reactions while a professor at Berkeley

• He got the 1961 Nobel prize in chemistry

for this experiment

• Algae were grown in “lollypop” flasks

• They were provided with radioactive CO2

and illuminated for a few seconds.

• Using the stopcock on the bottom of the flasks, the algae were immediately

dumped into boiling alcohol

• This immediately stopped the light-independent reactions so that Calvin could

determine the first radioactive compound formed

• Calvin separated the molecules in the

algae using 2-dimensional

chromatography.

Sample Radiograph

• The spots on the photographic plates

showed which spots on the chromatography paper were radioactive

• These spots were cut out of the paper and the radioactive molecules were re-

dissolved and identified.

• Calvin repeated this process with slightly

longer illumination of the algae each time.

• He did this as many times as was required

to find all the radioactive organic carbon molecules

The C3 Cycle (Calvin Cycle)

• occurs in the chloroplast stroma

• The first molecule Calvin could identify

had 3 carbons, so this cycle is called the

C3 cycle or the Calvin cycle.

• begins and ends with the 5 carbon sugar ribulose bisphosphate (RuBP)

• consists of 3 basic steps:

Step i) Carbon Fixation

• ribulose bisphosphate carboxylase (AKA

Rubisco) combines 3CO2 with 3RuBP (5 C) (3C + 15 C), forming 3 unstable 6

carbon compounds (18 C), which spontaneously split into six 3-carbon

molecules of 3-phosphoglyerate (18 C); an

intermediate of glycolysis

Step ii) Reduction to glyceraldehyde phosphate

• the free energy of ATP and the electrons

from NADPH are transferred to 3-phosphoglycerate, forming

phosphoglyceraldehyde (AKA G3P, PGAL or glyceraldehyde phosphate)

Step iii) Regeneration of RuBP

• 5 out of 6 of the produced molecules of

phosphoglyceraldehyde (3C x 5) are used to regenerate 3 molecules of RuBP (5C x

3)

• this process also requires ATP

Synthesis of Carbohydrate and other Products

Figure 14, p. 165

• 2 G3P out of each 12 generated in the C3 cycle enter the metabolic pool

• G3P is a molecule in glycolysis and can readily be converted to glucose by reversing the glycolysis pathway

• G3P can also be converted to glycerol, fatty acids, amino acids and other metabolites

Bozeman Photosynthesis

http://youtu.be/g78utcLQrJ4

Photorespiration

Section 3.4

• Problem: Rubisco can accept O2 in place of CO2 (kind of like competitive inhibition)

• In hot dry conditions, plants close their stomata to conserve water

• CO2 can’t get into the leaf and O2 can’t get out, so O2 outcompetesCO2 for the Rubisco active site

• Rubisco adds O2 to RuBP, and the product splits into a 3C molecule that remains in the C3 cycle and a 2C molecule that exits the cycle

• The 2-C molecule is broken down with no benefit to the plant.• photorespiration reduces photosynthetic output by siphoning away

as much as 50% of the carbon fixed by the C3 cycle (depending onthe environmental conditions the plant is facing).

C4 and CAM Plants

C3 Leaf Cross Section

• In “normal” C3 photosynthesis the light-

dependent and light-independent reactions both take place in the same chloroplasts in

the same mesophyll cells.

• Bundle-sheath cells just form a layer

around the leaf veins and don’t photosynthesize

C4 Leaf Cross-Section • In C4 photosynthesis, mesophyll cells and

bundle sheath cells form concentric layers around the leaf veins

• The mesophyll cells and bundle sheath

cells have different types of chloroplasts

• The light-dependent reactions happen in the mesophyll cells

• The light-independent reactions happen in the bundle sheath cells

Minimizing Photorespiration

Figure 2, p. 169

• C4 mesophyll cells have the very picky enzyme “PEPCo” that carboxylates phosphoenolpyruvate (3C), making oxaloacetate (4C)

• PEP carboxylase (“PepCo”) has no attraction for O2, thus CO2 can be fixed even when it is hot and dry and the stomata are partially closed

• Oxaloacetate is then converted to malate and exported to adjacent bundle sheath cells through plasmodesmata (pores connecting plant cells)

• The malate is decarboxylated in the bundle sheath cells• By releasing CO2 right next to Rubisco, photorespiration just doesn’t

occur

• This gives C4 plants such as corn an advantage over C3 plants indrought-ridden areas

Crassulacean Acid Metabolism (CAM)

Figure 3, 4, p. 170, 171

• Occurs in succulent plants such as cacti

• Stomata are closed during the day to conserve water

• Stomata open at night and CO2 is fixed by PEP carboxylase, forming various organic acids

• the organic acids formed at night are decarboxylated the next day, releasing CO2 for the C3 cycle, preventing photorespiration

Special Notes:

1) All C3, C4 and CAM plants use the C3

cycle

2) In C4 plants the initial carbon fixation is in

a different place than the C3 cycle

3) In CAM plants the initial carbon fixation occurs at a different time than the C3 cycle

Factors Affecting Photosynthesis

Section 3.5

1) Light Intensity (Figure 2, p. 173)

• As light intensity increases, the rate of photosynthesis also increases, up to the

point where something else becomes a limiting factor. If plants are illuminated

excessively, then they might overheat, which is described in ii), below.

1) Light Intensity

1) Light Intensity 2) Wavelength (Light Colour)

• Photosynthesis works better with red or

blue wavelengths

• This is because the primary photosynthetic

pigment is chlorophyll, and it absorbs light at these wavelengths

2) Wavelength (Light Colour) 2) Wavelength (Light Colour)

3) Temperature

Figure 5, p. 175

• As temperature increases, the rate of enzyme-catalyzed reactions also increases (due to faster diffusion of substrates and products into/away from the active site and increased flexibility of the enzyme). After a certain point, however, the enzymes become denatured and photosynthesis stops.

3) Temperature

3) Temperature 4) Carbon Dioxide Concentration

Figure 4, p. 174

• As the concentration of carbon dioxide

increases, the rate of photosynthesis

increases, up to the point where something else becomes the limiting

factor.

4) CO2 Concentration 4) CO2 Concentration

5) Oxygen Concentration

Figure 6, p. 175

• As the concentration of oxygen goes up,

the rate of photosynthesis goes down (due

to photorespiration).

• The following graph illustrates the trend for a C3 plant.

5) Oxygen Concentration