Chapter 10 Photosynthesis NotesPhotosynthesis Overview• The energy used by most* living cells ultimately comes from the sun, and is captured by

plants, algae, or bacteria via photosynthesis.

• Two Reactions

– light dependent reactions

capture energy from sunlight

use energy to produce ATP and NADPH

– Calvin cycle (or light independent reactions)

formation of organic molecules

• Anabolic (small molecules combined)

• Endergonic (stores energy)

• Carbon dioxide (CO2) requiring process that uses light energy (photons) and water (H2O) to produce organic macromolecules (glucose).

– 6CO2 + 6H2O C6H12O6 + 6O2

• Plants have thick layer of cells (mesophyll) rich in chloroplasts.

• Flattened thylakoids in the chloroplasts are stacked into columns, grana.

• Light dependent reactions take place on the thylakoid membranes and generate ATP and NADPH for Calvin cycle,

• The stroma consists of enzymes that carry out Calvin cycle (light independent reactions)

Chloroplasts Internal membranes of chloroplasts, thylakoids, are organized into grana.

– Thylakoid membranes house _________ for capturing light and the machinery to produce ______________.

clustered together to form a _____________________

acts as an antenna, gathering light energy harvested by ______________

Light and Reducing Power• Light-dependent reactions of photosynthesis use the energy of light to reduce NADP to

NADPH and to manufacture ATP.

– Reducing power generated by splitting water is used to convert CO2 into organic matter during carbon fixation.

What does this graph show?

What are the ideal conditions for photosynthesis?

How is the information on this graph relevant to global warming?

How could this information be used to help farmers with crops?

Energy in Photons• Energy content of a photon is inversely proportional to the wavelength of light.

– Highest intensity photons, at the short-wavelength end of the electromagnetic spectrum, are gamma rays.

– Ultraviolet light possesses considerably more energy than visible light.

potent force in disrupting DNA

Electromagnetic Spectrum

Absorption Spectra• Photon absorption depends on its wavelength, and the chemical nature of the molecule it hits.

– Each molecule has a characteristic absorption spectrum.

range and efficiency of photons the molecule is capable of absorbing

Pigments• Pigments are molecules that absorb light in the visible range.

– green plant photosynthesis

carotenoids

chlorophyll

chlorophyll a - main pigment

chlorophyll b - accessory pigment

Absorption Spectra• Fall colors are produced by carotenoids and other accessory pigments. During the spring and

summer, chlorophyll in leaves masks the presence of the carotenoids and the other pigments.

• In the fall when fall arrives, leaves stop making chlorophyll and it no longer present to reflect the green light and leaves the carotenoids to reflect the yellow and orange that the carotenoids and other pigments do not absorb.

• Carotenoids consist of carbon rings linked to chains of alternating single and double bonds.

• They can absorb photons with a wide range of energies, however they are not as efficient as transferring energy as chlorophyll.

• They assist in photosynthesis by capturing energy from light composed of wavelengths that are not efficiently absorbed by chlorophyll.

• A typical carotenoid, β-carotene, has 2 carbon rings connected by a chain of 18 carbon atoms with alternating single and double bonds.

• Splitting a molecule of β-carotene into equal halves produces 2 molecules of vitamin A. Oxidation of vitamin A produces 2 molecules of retinal, the pigment used in vertebrate vision.

• This explain why carrots enhance vision.

Chlorophyll• Chlorophylls absorb photons by means of an excitation process.

– Photons excite electrons in the pigment’s ring structure, and are channeled away through alternating carbon-bond system.

Wavelengths absorbed depend on the available energy levels to which excited electrons can be boosted.

Absorption Spectra

Chlorophyll

Light Dependent Reaction Stages• Primary photoevent

– A photon of light is captured

– An electron within the pigment is excited

• Charge separation

– Excitation energy is transferred to reaction center (specialized chlorophyll pigment)

– Reaction center transfer energy to electron receptor

• Electron transport

– Excited electron is shuttled along a series of electron carrier molecules.

– Protons are transported across the membrane

• Chemiosmosis

– Protons flow back through ATP synthase where chemiosmotic synthesis of ATP takes place (just like in aerobic respiration)

Photosystems• Photosynthesis output increases linearly at low light intensities but lessens at higher

intensities.

– saturation point

• Photosystem - network of pigments that channels excitation energy gathered by any of the molecules to the reaction center

– reaction center allows photon excitation to move away from chlorophylls and is the key conversion of light to chemical energy

Light to Chemical Energy

Figure 10.11

Photosystem Function• Bacteria use a single photosystem.

– electron is joined with a proton to make hydrogen

– electron is recycled to chlorophyll

– Plants use two photosystems photosystem I and II

• generate power to reduce NADP+ to NADPH with enough left over to make ATP

• two stage process: photosystem ______________________________.

• ________________________________ejected electrons end up in NADPH

Cyclic and Noncyclic Photophosphorylation (Video) In bacterial photosynthesis, a _________________________________ is involved. When an

electron is energized by ____________________________, it is ejected from the photosystem reaction center.

The electron the passes to ferredoxin, and then down through the cytochrome b6-f complex, plastocyanin, and finally back to the __________________________.

The ____________________ during this electron transport generates a ____________________ which is used to produce _________________.

Since the excited electron __________________ reaction center, this mechanism for making ATP is called ______________________________________.

The reducing power needed for ____________________________ is not generated in the process of cyclic photophosphorylation.

Plants and cyanobacteria utilize _______________________________ which work sequentially to produce both ____________________________. First, a _____________________________ ejects a high-energy electron from photosystem II.

The electron lost from photosystem II does not return to photosystem II, but is replaced by an electron generated from _______________________________________________________ ____________________________.

The electron then travels from the excited reaction center of photosystem II to plastoquinone, to the b6-f complex to plastocyanin and finally to the reaction center of Photosystem I.

This electron transport system generates a ________________________________ that is used to produce ____________.

Since the ___________________________ does not return to photosystem II, this mechanism for making ATP is called _____________________________________________________.

When photosystem I absorbs a photon of light, it ejects a ____________________________. The energy from this light _________________________ is used to generate reducing power in the form of ______________________.

The ejected electron is replaced by an electron from photosystem II.

Photosynthetic Electron Transport and ATP Synthesis (Video) In plants, photosynthesis occurs in specialized organelles called ____________________. The

internal membranes of chloroplasts are organized into sacs called ___________________. Surrounding the thylakoid membrane system is a semiliquid substance called ______________.

Photosynthetic pigments are clustered together to form _____________________________. When a photon of light strikes the reaction center of photosystem II, it excites an ___________.

Two ____________________ bind to an enzyme at the reaction center. This enzyme splits the _____________ and uses the electrons from the water to replace the

electrons removed from the reaction center. ____________________ is produced in this process.

The primary electron acceptor for the __________________________________ leaving the photosystem II is plastoquinone. The reduced plastoquinone passes the excited electrons to a ________________________ embedded in the membrane called the b6-f complex.

Arrival of the ______________________ causes the b6-f complex to pump protons from the ______________ into the ________________________________, thereby generating a proton gradient across the ____________________________.

Because the thylakoid membrane is impermeable to _______________, the protons in the strom must pass through the channels provided by _____________________________.

As protons pass through ADP is ________________________________ to ATP and released into the stroma. This process for making ATP is referred to as __________________________.

When photosystem I absorbs a ______________________, its reaction center passes _______ _______________ to ferrodoxin. The enzyme ____________________________ then transfers electrons to NADP to form NADPH.

Electrons lost from photosystem I are replaced by electrons generated from photosystem II. A small protein called plastocycanin (pC) then carries electrons from the b6-f complex to photosystem I.

Calvin Cycle• Also referred to as C3 photosynthesis

– The key step in the Calvin cycle – the event that makes the reduction of CO2 possible – is the attachment of CO2 to a organic molecule RuBP

– C3 plants - ribulose 1,5-bisphosphate (RuBP) is carboxylated to form a three-carbon compound via rubisco activity

– CO2 binds to RuBP in a key process call carbon fixation forming 2 3-carbon molecules of 3-phosphoglycerate (PGA)

– The enzyme that carries out this reaction, ribose biphosphate carboxylase/oxygenase (rubisco), is a very large, four-subunit enzyme present in the stroma.

Photorespiration• In photorespiration, O2 is incorporated into RuBP, which undergoes additional reactions that

release CO2.

– decreased yields of photosynthesis

– Rubisco has a second enzyme activity that interferes with the Calvin cycle, oxidizing RuBP.

– This essentially undoes the Calvin cycle.

– Temperature effects the rate of carboxylation.

– At 25˚ C the rate of carboxylation is 4 times greater then the rate of oxidation.

– As temperature increass the rate of carboxylation decreases.

– In hot arid conditions stoma close the leaf to conserve water.

– The closing of the stoma cuts off the supply of CO2 and does not allow O2 produced to exit.

– These conditions favor photorespiration

– Plants that use only the C3 Photosynthesis (the Calvin Cycle) are called C3 plants.

– Other plants use C4 photosynthesis in which phosphoenolpyruvate (PEP) is carboxylated to form a 4 carbon compound using the enzyme PEP carboxylase.

– This enzyme has no oxidation activity and thus no photorespiration.

C4 Pathway• Plants adapted to warmer environments deal with the loss of CO2 in two ways:

– C4 conducted in mesophyll cells, Calvin cycle in bundle sheath cells

creates high local levels of CO2 to favor carboxylation reaction of rubisco

isolates CO2 production spatially

Crassulacean Acid Metabolism (CAM)• CAM is another strategy to decrease photorespiration in hot regions.

– Used by succulents (cacti and pineapple)

• CAM plants open stomata during the night, and close them during the day to cut-down the loss of water vapor.

– this opening of stomata is reverse of most plants

– in the day high levels of CO2 drive the Calvin cycle and minimize photorespiration

– like C4 plants, CAM plants utilize both C4 and C3 pathways. they differ from C4 plants because they use the C4 pathway at night and the C3 pathway during the day in the same cells.

– in C4 plants the two pathways occur in different cells

• isolates CO2 production temporarily