Class 10

What does photosynthesis do?

Convert solar energy into chemical energy.

Where does photosynthesis occur?

Primarily in leaves (but any tissue that is green) within the chloroplasts.

Where are chloroplast found?

Shoot system tissues, they are the site of photosynthesis.

What do chloroplasts contain?

Chlorophyll, a green pigment.

What makes up the complex internal structure of chloroplasts?

Thylakoids, lumen, grana, stroma, and stroma thylakoids.

What are thylakoids?

Internal membranes in which pigment molecules are embedded.

What are lumens?

Liquid-filled cavities inside thylakoids.

What are grana?

Stacks of thylakoids.

What are stroma?

Fluid surrounding the thylakoids.

What are stroma thylakoids?

Thylakoids that connect grana.

How can excess glucose be stored?

As starch in the chloroplast, during the day.

What happens to starch at night?

It is broken down into sucrose, the sugar is delivered through the phloem to other parts of the plant, supplying energy for metabolic processes.

What powers photosynthesis?

Light energy.

What has the shortest wavelength and highest energy?

Violet.

What has the longest wavelength and the lowest energy?

Red.

What are pigments?

Molecules that absorb light.

When does a pigment appear black?

When a pigment absorbs all wavelength of visible light.

What color do we see?

The color that is reflected.

What is the absorption spectrum?

The light absorption pattern of a pigment.

What makes up a chromophore?

Pigments + proteins.

Why does chlorophyll appear green to us?

It absorbs mostly violet and red light, and reflects green light.

What is evidence that the absorption spectrum of chlorophyll a and the action spectrum for photosynthesis are linked?

Their similarity.

What are the pigments involved in photosynthesis?

Chlorophyll a, chlorophyll b, and carotenoids.

What is chlorophyll a?

Essential for photosynthesis in plants.

What is chlorophyll b?

An accessory (antenna) pigment that is not directly involved in photosynthesis but broadens the range of usable wavelengths (not essential).

What are carotenoids?

Accessory (antenna) pigments, they broaden the range of usable wavelengths and protect chlorophyll from damage form light.

What happens to electrons when pigments absorb light?

They are boosted to a higher energy level (excited state).

What do accessory (antennae) pigments do?

They expand wavelengths absorbed, protect leaves from excess energy, and transfer energy to the reaction center (resonance energy transfer).

What does reaction center chlorophyll a do?

Initiate photosynthesis, loses an electron to an acceptor, it is in a unique position.

What reactions does photosynthesis consist of?

The light reactions and the carbon-fixation reactions.

Where do light reactions occur?

In thylakoid membranes.

What happens in light reactions?

Pigment molecules embedded in the thylakoid membranes absorb light energy, and water in the lumen (interior) of the thylakoid is split into oxygen gas and hydrogen atoms (electrons and protons).

Where do carbon-fixation reactions occur?

In the stroma, sugar is produced.

What do light reactions require?

Light energy.

What occurs chemically in light reactions?

Photolysis (split H₂O), release O₂, reduce NADP⁺ to NADPH, generate ATP from ADP, and photophosphorylation.

What do carbon-fixation reactions use?

ATP and NADPH to convert CO₂ to sugar.

LEO the lion says GER:

Lose electrons oxidized and gain electrons reduced.

What are photosystems?

Pigments embedded in the thylakoid membrane are organized into discrete units.

What are photosystem I and II linked together by?

An electron transport chain.

How are photosystems numbered?

By their order of discovery, not their functional order.

What is photosystem II?

P₆₈₀, the special pair of chlorophyll a molecules in the reaction center.

What does P₇₀₀ mean?

P = pigment and 700 designates the optimal absorption peak in nanometers.

What is photosystem I?

P₇₀₀, the special pair of chlorophyll a molecules in the reaction center.

What does P₆₈₀ mean?

P = pigment and 680 designates the optimal absorption peak in nanometers.

How do photosystems I and II work together?

Simultaneously and continuously.

Where are photosystems I and II spatially separated?

In the thylakoid membrane.

How is oxygen released in photosystem II?

Water is oxidized.

Where is light energy absorbed in photosystem II?

Chlorophyll a of P₆₈₀ in the reaction center.

What unique ability does photosystem II have?

It can extract electrons from water (water photolysis).

Where are energized electrons transferred to in photosystem II?

The cytochrome b6/f complex.

What produces the proton gradient needed for generation of ATP in photosystem II?

Protons released into the thylakoid lumen and the protons released from water.

What links photosystem I and photosystem II?

The cytochrome b6/f complex.

Where are electrons from photosystem II donated?

To plastocyanin via cytochrome b6/f.

What is plastocyanin?

A mobile electron carrier, more protons pumped into the lumen.

Why is photolysis required?

For ATP production across the thylakoid membrane.

How do photosystem II and cytochrome b6/f contribute to the electrochemical proton gradient?

They pump protons into the lumen.

What does ATP synthase provide?

A channel for protons to move downhill out into the stroma.

What is photophosphorylation?

The resulting potential energy yields ATP from ADP and P_i.

How do electrons pass through photosystem I and photosystem II?

Via noncyclic electron flow.

What happens after each excitation step?

High-energy electrons flow “downhill” via electron transport chains.

What is the formation of ATP by photosystem II/photosystem I called?

Noncyclic photophosphorylation.

What happens to NADP+ in photosystem I?

It is reduced to NADPH.

What happens to light energy in photosystem I?

It is passed to the P₇₀₀ molecules at the reaction center.

Where do energized electrons flow in photosystem I?

“Downhill” to ferredoxin.

In photosystem I where are electrons from ferredoxin transferred to?

NADP+.

What does the reduction of NADP+ yield?

NADPH and oxidizes P₇₀₀.

What happens to the lost electrons in photosystem I?

They are replaced with those moved down from photosystem II.

What is noncyclic electron flow?

The unidirectional flow of electrons from water to NADP+ (with noncyclic photophosphorylation to yield to ATP).

Where are electrons transferred in photosystem I?

To the cytochrome b6/f complex (instead of NADP+).

What is the cyclic electron flow in photosystem I?

Light energy is passed to the P₇₀₀ molecules at the reaction center, energized electron flows “downhill” to ferrodoxin, electrons are transferred to cytochrome b6/f complex (instead of NADP+), electrons go back to the PSI reaction center, and causing more protons to enter the lumen for more ATP production.

Why have two systems?

To satisfy the calvin cycle requirement.

How much ATP and NADPH does the calvin cycle need?

3 ATP and 2 NADPH.

What does noncyclic electron flow produce?

6 ATP and 6 NADPH.

What does cyclic electron flow produce?

Additional ATP.

Where is the calvin cycle present?

In all photosynthetic plants, it occurs in 3 stages.

What is the first reaction of the calvin cycle catalyzed by?

RUBP carboxylase oxygenase = rubisco.

What is rubisco?

The most abundant protein on earth, it is one of largest, most complex enzymes.

What is the balanced equation of photosynthesis?

6CO2 + 6H2O + energy -------→ C6H12O6 + 6O2.

What are the three stages of the calvin cycle?

Fixation, reduction, and regeneration.

What happens in fixation?

Carbon is “fixed” (covalently bonded to) RuBP, rapidly hydrolyzed into 3-PGA, 3-PGA is the first detectable product of the Calvin cycle which is also why it is called C3 pathway for the 3 carbons in PGA.

What happens in reduction?

PGA is reduced to PGAL in 2 steps (it takes 3 molecules of CO2 + 3 molecules RuBP = 6 molecules of PGAL).

What happens in regeneration?

5 molecules of PGAL are used to make 3 molecules of RuBP.

What happens at each full turn of the calvin cycle?

1 molecule of CO₂ enters and is reduced to form 1 molecule of RuBP.

What is required to produce 1 molecule of PGAL?

Three turns of the cycle.

Why does the calvin cycle need cyclic photophosphorylation?

It requires more ATP than NADPH.

What is PGAL?

The primary molecule transported form the chloroplast into the cytosol.

What does PGAL serve as?

The starting point for the synthesis of sugars, starches and other cellular components.

What happens to PGAL in the chloroplasts?

It is converted to starch and stored temporarily.

What does starch produce in darkness?

Sucrose.

Where is most PGAL exported to?

The cytosol, where it is converted into sucrose.