Photosynthesis Flashcards by Lucy Shu

What is the formula for photosynthesis?

6 H2O + 6 CO2 + light (hv) –> C6H12O6 + 6 O2 : redox rxn. H2O –> O2 (donate) is oxidation and CO2 –> C6H12O6 (accept) is reduction. Or water + carbon dioxide + light –> glucose and oxygen.

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What is photosynthesis?

The transfer of light energy into chemical energy. It is an endergonic, redox reaction and an energy source from the sun that provides electromagnetic energy stored in chemical bonds in carbohydrates.

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How is light captured in plants?

Via pigments.

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What are the three photosynthesizing pigments?

Chlorophyll A = blue, chlorophyll B = green, carotenoids = yellow/orange

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What are pigments and how do they work?

They are light-absorbing compounds that absorb certain wavelengths to visible light and reflect/transmit the rest.

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What is an absorbance spectra?

Each pigment within a chloroplast has its own absorption spectrum. It is the absorbance response of a pigment exposed to a series of wavelengths of light.

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What is an action spectrum?

The physiological response of an organism to specific wavelengths.

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What is the chlorophyll molecule made of?

A hydrocarbon tail and porphyrin ring

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What is the function of the hydrocarbon tail inside the chlorophyll molecule?

It interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts.

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What is the function of the porphyrin ring inside the chlorophyll molecule?

It is the light-absorbing head of the molecule; conjugated double bonds allow electrons to flow in the ring and are held in place by the positive charge of the magnesium (Mg) in the centre.

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What are thylakoids?

Site of light-dependent reactions of photosynthesis.

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What is the stroma?

Where light-independent reactions of photosynthesis occur; colourless fluid in the inner membrane space.

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What are grana?

Stacks of thylakoids in the inner membrane space, shape allows for maximal surface area for thylakoids to maximize photosynthesis.

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What are the parts of the chloroplast?

Stroma, grana, thylakoids, outer membrane, inner membrane, intermembrane space.

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What are the three steps of photosynthesis?

1) Photochemistry (light reaction): H2O -> O2 in thylakoids. 2) Production of NADH and ATP (light reaction): moves to power calvin cycle in stroma from thylakoids. 3) Incorporation of CO2 into sugar (dark reaction/Calvin cycle): takes place in stroma, releases NADP+ and ADP + Pi to start photosynthesis again in thylakoids.

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How are the three steps of photosynthesis powered?

1) Photochemistry; Pigments: transfer radiant energy from pigments to chemical bonds. 2 & 3) Production of NADH and ATP, Calvin cycle; Enzymes (proteins) transfer chemical energy.

What is the primary function of light reactions?

The conversion of light energy to H+ (pH) gradient used for ATP synthesis and NADPH for dark reactions.

What are the parts of the light reaction?

Photosystem II –> Cytochrome complex + electron transport chain –> Photosystem I –> second electron transport chain + NADP+ reductase = NADPH to Calvin Cycle.

What happens to the H+ produced by photosystem II (h2o redox)?

H+ produced by Photosystem II and other H+ that diffuses through stroma to thylakoid space go to ATP synthase that turns ADP to ATP for Calvin Cycle.

What are the photosystems made of?

Protein complexes and pigments (chlorophyll a, b, cartenoids) organized in light-harvesting complexes and reaction centers.

Where are photosystems II and I located?

Within the thylakoid membrane

What are the light-harvesting complexes and what is their function?

Light-harvesting complexes connect the pigments in the photosystems together. Chlorophyll a, b, and carotenoids transfer energy (not e-) embedded in the protein complex. They are connected to reaction centers.

What are reactions centers and their function?

Reaction centres are connected to light-harvesting complexes, and they transfer electrons to the primary electron acceptor in the middle.

How many electron transport chains are there in photosynthesis and what is their function?

There are two electron transport chains, one from PSII to PSI and one from PSI to NADP+ reductase. They are used to generate energy rich compounds.

What are the steps of photosynthesis in photosystem II?

1) Light excites pigments in PSII, making excited e- which falls to the ground state and causes other nearby pigments to be excited. This makes the e- in pigment P680 excited. 2) e- are transformed from chlorophyll a pair P680 w/in reaction center to primary e- acceptor. P680 = P680+ + e-. 3) The e- gap in P680 must be filled and pulls e- from split h2o, releasing H+ for ATP synthesis. (h2o redox). 4) 1st e- transport chain through cytochrome complex to PSI

What are the steps of photosynthesis in photosystem I?

1) PSI is excited by light and P700 (similar to P680 but in PSI) looses e-. 2) The e- gap hole is filled with e- from PSII (electron transport chain) 3) 2nd e- transport chain to NADP+ reductase to made NADPH for dark reactions.

When do dark reactions occur?

Dark reactions occur when light reactions cannot.

What reactions stop without the presence of light?

Photochemistry, e- transport chain, splitting of h2o and o2 production, h+ gradient generation

Can CO2 absorption and sugar production occur even w/o light?

Yes - as long as substrates are available. Welcome darkness my old friend.

What is the function of the Calvin Cycle/dark reactions?

The incorporation of CO2 into carbohydrates. Enzymes (proteins) transfer chemical energy using ATP and NADPH from light reactions.

What are the three main steps of the Calvin Cycle?

1) Carbon fixation. 2) Reduction. 3) Regeneration of CO2 acceptor.

Where does the Calvin Cycle take place?

W/in the stroma

Describe Phase 1 of the Calvin Cycle: Carbon Fixation

Enzyme RuBisCO catalyzes reaction between CO2 and RuBP, forming two molecules of 3-PGA. (Atoms form new bonds, so CO2 is 'fixed' from inorganic into organic).

During Phase I of the Calvin Cycle, how many molecules of 3-PGA are formed?

Since the Calvin Cycle starts with the addition of 3 CO2 molecules and 3 RuBP, 6 molecules of 3-PGA are formed. (2 * 3 = 6).

Describe Phase 2 of the Calvin Cycle: Reduction

Reduction is the gain of an e- by an atom or molecule. 6 molecules of ATP and NADPH are used to convert the 6 molecules of 3-PGA to 6 molecules of G3P. ATP turns to ADP and NADPH is turned to NADP+ ; return to light reactions to be reused.

Describe Phase 3 of the Calvin Cycle: Regeneration of CO2 acceptor

1 G3P molecule leaves and the other 5 are used to regenerate 3x RuBP to continue to fixate CO2 which requires 3x ATP to 3x ADP.

How is glucose made from the Calvin Cycle?

G3P is produced after three turns of the cycle, and since it is a single three-carbon molecule and glucose is a six-carbon molecule, the cycle needs to be completed 6 times to form one glucose molecule. (1 G3P = 3 turns, 2 G3P = 1 glucose = 6 turns).

What is the importance of G3P?

It is the starter for all biological organic matter (proteins, lipids, carbohydrates, DNA/RNA, hormones, etc.)