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Chapter 10Physiology: Photosynthesis

Topics• Photoautotropism

• Photosysnthesis as redox anabolism

• Light absorption

• Light-dependent reactions

• Production of phosphorylation and reducing power

• Light-independent reactions – C3 cycle

• Use of phosphorylation and reducing power

• C4 and CAM plants

Photoautotrophs

• Gather energy directly from light• Use sun’s energy to assimilate small inorganic molecules

• Green plants, algae and some bacteria

• Build all of their own molecules using just carbon dioxide, water, and various cations and anions

• Use these self-made organic molecules for energy

• Heterotrophs cannot do this• Take in organic molecules and respire them for energy

• Animals, all completely parasitic plants, all fungi, protozoa and fungi-like protists, nonphotosynthetic prokaryotes

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Plant Photoautotrophism• Plants have photoautotrophic and heterotrophic tissues

• Chlorophyllous leaves and stems – photoautotrophic

• Roots, wood, and flowers are heterotrophic

• Survive on carbohydrates imported through phloem

• A tissue often change their type of trophism – discuss sink and source –agricultural implications

• Younger leaves to mature leaves

• Immature fruits to older fruits

• Tubers

• Photosynthesis converts carbon dioxide to carbohydrate – write basic equation here – mention need for energy and reducing power

Photosynthesis – main reactants and products

• Water and CO2 - abundant - diffuse into the plant automatically• They are also very stable and contain little chemical energy,

so large amounts of energy can be put into them

• Carbohydrates – energy-rich but stable and chemically unreactive

• Main reactants and products of photosynthesis are nontoxic

Discuss redox reaction – simultaneous reduction and oxidation

Light energy

• Visible light - one small segment of the electromagnetic radiation spectrum

• Light can be treated as sets of photons (or quanta) • Short wavelengths (λ)

have high energy

• Longer wavelengths have little.

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Light absorption

• Absorption spectrum = wavelengths of light a pigment absorbs• Chlorophyll-a absorbs red

light and blue light very well

• Action spectrum shows which wavelengths are most effective at powering photosynthesis

Light absorption

• Accessory pigments are molecules that strongly absorb wavelengths not absorbed by chlorophyll-a• Most common accessory pigments in land plants are

chlorophyll-b and the carotenoids

• Overcomes the narrow absorbance of chlorophyll-a and widens the action spectrum of photosynthesis

• Absorbed energy passes to chlorophyll-a via resonance• chlorophyll-b is very efficient at passing energy

• Carotenoids are poor (10% transfer rate) and seem more important in absorbing excess light the protecting the chlorophylls from light and oxidative damage

Light energy – chl excitation

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Figure 8-4b,cPage 159

10µm(b)

Thylakoids Outer membrane

Innermembrane

Intermembranespace

Thylakoidmembrane

Stroma Granum(stack ofthylakoids)

Thylakoidlumen(c)

Structure for function

Energy source

Electron source

Energy and reducing power

Oxidized H2O

Reduced CO2

Light-dependent and independent reactions happen concurrently

Light-Dependent Reactions – on thylakoid membrane make ATP and NADPH

• To control the very reactive electron of an excited chlorophyll-a, all the working components are packed into a granule, a photosystem - PS

• Units with little chlorophyll-b are photosystem I (PSI)

• Units where chlorophyll-b is present at levels almost equal to chlorophyll-a are photosystem II (PSII)

Primaryelectronacceptor

Photon

Photosystem

Chloroplast

Thylakoid

Light harvestingantennacomplexes

Reactioncenter

e–

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PSI is reduced by PSII• Light strikes pigments in PSI

array, energy is transferred to the reaction center

• At the reaction center is a pair of special molecules of chlorophyll a called P700

PSI e carriers

• Energy excites an electron of P700, e passed to Fe4S4, to ferredoxin, then to the enzyme ferredoxin-NADP+ reductase (FNR), which then reduces NADP+ to NADPH

• Pheophytin, (Q) QA and QB on D proteins

• Plastoquinones, like cytochromes, transport electrons over short distances within thylakoid membrane

• Cytochromes are intrinsic membrane proteins • contain a cofactor, heme, which holds an iron atom

• carry electrons only between sites that are extremely close together within a membrane

• Plastocyanin is a small protein that carries electrons on a copper atom• It is loosely associated with thylakoid membranes

PS II e carriers

• Both photosystems work together for chemiosmotic

phosphorylation (thylakoid membrane H+ impermeable) – ATP energy production, and NADPH reducing power production reducing

Light-Dependent Reactions – PSI and PSII

Pheophytin and Q

Fe4S4

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Light-Dependent Reactions - PSI and PSII• First, P680 (PSII) chlorophyll a absorbs light and is

excited and donates e to pheophytin – PSII gets oxidized

• Reduced phaeophytin transfers e to QA QB

• QB donates e to Plastaquinone pool

• Plastoquinone donates e to cytochrome b6/f complex, which in turn donates e to plastocyanine

• By this time P700 (PSI) is oxidized to receive e from plastocyanine

• Oxidized P680 is reduced by e from O2 evolving Mn-Ca-Cl cluster: electrons are stripped off from H2O, protons from water and pumped by PQ + Cyt action used for chemiosmosis, oxygen released, photophosphorylation and making reducing power accomplished by these membrane-dependent (light-dependent) redox reactions

Light-Dependent Reactions

• Electrons flow smoothly from water to NADPH = noncyclic electron transport

• However, too little ATP produced this way relative to the amount of NADPH produced

• After electrons reach ferredoxin in photosystem I, they can be transferred to plastoquinones of photosystem II instead of being used to make NADPH• Plastoquinones carry electrons along just as though they

had gotten them from QB

Cyclic Electron Transport

• They use their energy to pump more protons into the thylakoid lumen

• With this cyclic electron transport, chloroplasts make extra ATP neededthru enhanced chemiosmosis

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Light-Dependent Reactions

• Summary:• Reaction center (RC) chl a in PSII is excited (chl a to chl a*)

by light

• RC-Chl a* donates e to PSII-associated electron transport chain – redox reactions

• ATP is produced by PSII-associated electron transport chain with lumen acidity and ATP synthase

• Oxidized PSI-RC is reduced by e from PSII, and PSI is boosted by light again

• e from PSI chl a* passes e through a short second electron transport chain – redox reactions - to NADP+ to reduce NADP+ to NADPH

Stroma (light-independent) reactions• Conversion of carbon dioxide to carbohydrate

occurs in stroma • also called the Calvin/Benson cycle, or C3 cycle

• First step is carbon fixation:• An acceptor molecule (ribulose-1,5-bisphosphate; RuBP)

reacts with a CO2 molecule

• New C-6 molecule produced promptly breaks into 2 identical molecules each containing 3 carbons -

3-phosphoglycerate (PGA, C-3) = first detectable product

hence called C3 cycle

• The enzyme that carries out this reaction is RuBPcarboxylase/oxygenase (RUBISCO)

Stroma Reactions: RUBISCO

• RUBISCO is one of the largest and most complex enzymes known

• A quaternary enzyme with 8 small and 8 large proteins

• Makes up 30% of the protein in a leaf

• Most abundant protein on Earth

• Critical to food production, without it heterotrophs would starve

• RUBISCO - not ideal for making carbohydrates -has a low specificity for CO2 and frequently adds O2 instead

• However it is highly conserved evolutionarily

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Stroma Reactions: Energization and Reduction of C

Anabolic metabolism• 3-Phosphoglyceraldehyde – a versatile molecule

• 3PGAL plus water, nitrates, sulfates, minerals etc. plants construct everything inside themselves – including starch in chloroplast – in night, starch to sugars for transport – chloroplasts are ready again in morning to make starch

• 3PGAL is rearranged and altered in the cytoplasm to build up larger, more complex molecules – anabolism

• Synthetic pathways of polysaccharides and fats (storage forms of energy and carbon) are important because NADPH and ATP cannot be stored for even a short time

• Glucose and sucrose - stored for weeks or months

• Starch - stored for years, but it is too large to be transported

• Lipids are an even more concentrated storage form of energy that can be synthesized rapidly and stored in large quantities.

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Photosynthesis and Environment: Light

• Light compensation point is the level of light at which photosynthesis matches respiration• Plants grown for a long

time in conditions below the light compensation point respire faster than they photosynthesize and starve to death

• Light can become too intense, and some plants in bright environments have developed adaptations to reflect light, such as thick trichomes or heavy cuticle

• Understory plants of forests are adapted to low light

Leaf Adaptations - Xeric• In hot, dry habitats, plant leaf cells

are packed closely without intercellular space and stomata sunken

Small intercellular surface area and

sunken stomata retard transpiration

• Another method: cylindrical leaves - reduced external surface area

Xeric plants - Transpiration is less but photosynthesis and growth slow because carbon dioxide absorption by cell surfaces and whole leaf slows

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Water Balance

• Balance between water loss and photosynthesis is critical• Most plants keep their

stomata open during the day

• If water stressed, stomata will close

• This prevents entry of CO2, and reduces photosynthetic activity

• WUE = amount of CO2

absorbed for each H2O lost –important for survival

• But under stress - high light and To, low [CO2] -Rubisco binds to O2 more than to CO2 – in C3 plants 25% less photosynthesis than potential

• Lead to photorespiration – C2 Cycle - makes 1 PGA but loses energy and N

Rubisco’s Oxygenation Problem, Photorespiration and Evolution of C4 Cycle

3C

C4 Cycle - Hatch and Slack pathway• CO2 is

absorbed, moved and concentrated away from O2

for Rubisco to work on. Kranz anatomy -Vascular bundle - chlorophyllous cells surrounded by mesophyll cells

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C4 Cycle

C4 Cycle

• Mesophyll cells contain PEP carboxylase which has no affinity for oxygen• PEP carboxylase adds carbon dioxide to PEP (C3),

producing oxaloacetate – C4 molecule

• Oxaloacetate is reduced to malate by a molecule of NADPH

• Malate from throughout the mesophyll moves into the bundle sheath

• There it breaks down into pyruvate by releasing CO2

which enters Calvin Cycle

• Bundle sheath chloroplasts primarily carry out cyclic electron transport• Without noncyclic electron transport, there is no

breakdown of water or production of oxygen

• RUBISCO - located exclusively in the bundle sheath chloroplasts - ideal conditions for CO2 binding - efficient C3 stroma reactions

• Photorespiration increases with temperature, so selective advantage of C4 metabolism depends on the environment – Discuss crops with C4• Under warm, dry conditions, C4 metabolism has a strong

selective advantage over C3 metabolism

C4 Cycle

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CAM Plants

• Crassulacean Acid metabolism (CAM) is a further metabolic adaptation that improves conservation of water while permitting photosynthesis

• CAM - almost identical to C4 metabolism except:

• Malate (or other acids produced from oxaloacetate) not transported but accumulate in effect storing CO2

• However, this occurs only at night, when stomata are open – stomata closed during hot day

CAM Plants

• Stomata are closed during the hottest periods and open only at night when it is cool to reduces transpiration• In daytime, when stomata close, malate or other acids

break down, releasing CO2 for C3 metabolism

• CAM is not particularly efficient• Total amount of carbon dioxide is so small that it may be

entirely used after just a few hours of sunlight

• CAM is selectively advantageous in a hot, very dry climate where survival rather than rapid growth is most important

• CAM plants have other metabolic and structural adaptations for water conservation

CAM Plants

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