Photosynthesis and Pigments Overview

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Questions and Answers

What are the main pigments responsible for trapping sunlight in plants?

Lignins and Starch

Anthocyanins and Flavonoids

Tannins and Saponins

Chlorophylls and Carotenoids (correct)

Which pigment absorbs bluish-green light?

Phycobilins

Chlorophyll a

Chlorophyll b

Carotenoids (correct)

What is the role of the reaction center in the photosynthetic membrane?

It absorbs all wavelengths of light.

It produces chlorophyll pigments.

It acts as an antenna for light harvesting.

It serves as a trap for excitons. (correct)

What is the primary color of chlorophyll pigments?

Green Signup and view all the answers

How quickly does the absorption of a photon by an antenna molecule initiate the photosynthesis process?

About 10 femtoseconds Signup and view all the answers

Which of the following types of light is primarily utilized for photosynthesis?

Visible light Signup and view all the answers

What occurs after the initial transition of an antenna molecule to the excited state?

Excited energy is rapidly transferred to nearby antenna molecules. Signup and view all the answers

What happens to some excitons in the process of photosynthesis?

They are converted back into photons and emitted as fluorescence. Signup and view all the answers

What is the role of the proton gradient across the thylakoid membrane in ATP synthesis?

It creates a proton-motive force for ATP synthesis. Signup and view all the answers

Which enzyme catalyzes the carboxylation of ribulose-1,5-bisphosphate during the Calvin cycle?

RuBisCO Signup and view all the answers

What is produced from the reduction of 1,3-bisphosphoglycerate in the Calvin cycle?

Glyceraldehyde 3-phosphate Signup and view all the answers

What occurs during the regeneration phase of the Calvin cycle?

Reversible conversion of G3P into DHAP. Signup and view all the answers

What is the main product of carbon fixation in the Calvin cycle?

3-phosphoglycerate (3-PGA) Signup and view all the answers

Which statement about cyclic photophosphorylation is correct?

It generates ATP while preventing NADPH production. Signup and view all the answers

What are the four main steps of the Calvin cycle?

Carbon fixation, reduction phase, carbohydrate formation, regeneration phase Signup and view all the answers

Which of the following is a characteristic of 3-phosphoglycerate (3-PGA) in the Calvin cycle?

It is produced during carbon fixation. Signup and view all the answers

What is the primary function of kranz anatomy in C4 plants?

To concentrate CO2 around RuBisCO Signup and view all the answers

Which enzyme is responsible for the conversion of pyruvate to phosphoenolpyruvate (PEP) in the C4 pathway?

Pyruvate orthophosphate dikinase Signup and view all the answers

What occurs to malate in the bundle-sheath cells after it is transported from mesophyll cells?

It is decarboxylated to produce CO2 and pyruvate. Signup and view all the answers

What characteristic distinguishes the chloroplasts in bundle-sheath cells from those in mesophyll cells?

They lack grana and are starch-rich. Signup and view all the answers

In the C4 pathway, why is O2 considered a poor substrate for PEP carboxylase?

It reduces the efficiency of CO2 fixation. Signup and view all the answers

How many ATP molecules are required for every CO2 molecule fixed in the C4 pathway?

30 Signup and view all the answers

Which of the following sequences best represents the steps involved in the C4 pathway?

Conversion of pyruvate to PEP, fixation of CO2 into oxaloacetate, decarboxylation of malate. Signup and view all the answers

What types of plant species commonly exhibit C4 photosynthesis?

Tropical grasses and some sedges Signup and view all the answers

What are the products formed when F6P is acted upon by transketolase?

E4P and Xu5P Signup and view all the answers

Which enzyme cleaves sedoheptulose-1,7-bisphosphate into sedoheptulose-7-phosphate?

Sedoheptulose-1,7-bisphosphatase Signup and view all the answers

How many ATP molecules are required per three CO2 molecules during the Calvin cycle?

9 ATP Signup and view all the answers

Which molecule is produced from RuBP during the process of photorespiration?

3-PGA Signup and view all the answers

What is the role of phosphorus in converting RuP into RuBP?

Provides a phosphate group Signup and view all the answers

What is the main consequence of photorespiration for plants?

Loss of CO2 Signup and view all the answers

What is the main purpose of C4 carbon fixation?

To circumvent photorespiration Signup and view all the answers

Which molecule is formed in the first step of C4 carbon fixation?

Oxaloacetate Signup and view all the answers

What type of metabolism allows plants to fix carbon dioxide at night?

Crassulacean acid metabolism (CAM) Signup and view all the answers

What enzyme catalyzes the conversion of organic acids in CAM plants during the night?

PEP carboxylase Signup and view all the answers

How do CAM plants manage water loss during the day?

By closing their stomata Signup and view all the answers

Which of the following enzymes is NOT mentioned as being involved in the decarboxylation process in different plants?

Alanine decarboxylase Signup and view all the answers

What is the primary storage form of fixed carbon dioxide in CAM plants?

Organic acids Signup and view all the answers

During which process do CAM plants release carbon dioxide for fixation in the Calvin cycle?

Deacidification Signup and view all the answers

Which family of plants is characterized by Crassulacean acid metabolism?

Crassulaceae Signup and view all the answers

What happens to the stomata of CAM plants during the night?

They open to allow gas exchange Signup and view all the answers

Study Notes

Photosynthetic Membrane

The photosynthetic membrane forms a closed space with an outer water space (stromal phase) and an inner water space (lumen).
The photosynthetic membrane contains pigments and proteins required for light reactions.

Pigments

Pigments absorb light and give plants, algae, and bacteria their color.
Different pigments absorb different wavelengths of light.
Chlorophylls are green pigments that absorb blue and red light.
Carotenoids are red, orange, or yellow pigments that absorb bluish-green light.
Phycobilins are red or blue pigments that absorb wavelengths not well absorbed by chlorophylls and carotenoids. They are found in cyanobacteria and red algae.

Light Absorption

Photosynthesis is driven by visible light (wavelengths 400-700 nm) absorbed by pigment molecules (mainly chlorophyll a, b, and carotenoids).
200-300 pigment molecules bind to light-harvesting protein complexes located in the photosynthetic membrane.
These light-harvesting complexes surround the reaction centers, which serve as an antenna.

Photosynthesis Initiation

The absorption of a photon by an antenna molecule initiates photosynthesis within a femtosecond (10⁻¹⁵ s), causing a transition from the ground state to an excited state.
The excited state decays by vibrational relaxation to the first excited singlet state within 10⁻¹³s.
The excited state energy has a high probability of being transferred by resonance energy transfer to a nearby antenna molecule with similar energy states.
Over 90% of absorbed quanta are transferred from the antenna system to the reaction center within a few hundred picoseconds.

Chlorophyll Fluorescence

Some excitons are converted back into photons and emitted as fluorescence.

ATP Synthase

The F1 portion of ATP synthase is outside the membrane.

Proton Gradient

The proton gradient across the thylakoid membrane creates a proton-motive force, used by ATP synthase to form ATP.

Cyclic Photophosphorylation

In cyclic photophosphorylation, cytochrome b6f uses the energy of electrons from PSII and PSI to create more ATP and stop the production of NADPH.
Cyclic phosphorylation is important for creating ATP and maintaining the right proportion of NADPH for light-independent reactions.

Calvin Cycle

The Calvin cycle converts carbon dioxide from the air into sugar in plants and algae.
It is also known as the carbon fixation reaction or dark reaction.
The Calvin cycle occurs in the stroma and has four main steps:
- Carbon fixation
- Reduction phase
- Carbohydrate formation
- Regeneration phase
The energy for this sugar-generating process is provided by ATP and NADPH produced in light reactions.

Carbon Fixation

The enzyme RuBisCO catalyzes the carboxylation of ribulose-1,5-bisphosphate (RuBP), a 5-carbon compound, by carbon dioxide in a two-step reaction.
The product of the first step is an enediol-enzyme complex that can capture CO₂ or O₂. This makes the enediol-enzyme complex the real carboxylase/oxygenase.
The CO₂ captured by enediol in the second step produces a six-carbon intermediate that immediately splits in half, forming two molecules of 3-phosphoglycerate (3-PGA), a 3-carbon compound.

Carbon Reduction

The enzyme phosphoglycerate kinase catalyzes the phosphorylation of 3-PGA by ATP, producing 1,3-bisphosphoglycerate (1,3BPGA) and ADP.
The enzyme glyceraldehyde 3-phosphate dehydrogenase catalyzes the reduction of 1,3BPGA by NADPH, producing glyceraldehyde 3-phosphate (G3P) and oxidizing NADPH to NADP⁺.

Regeneration Phase

Triose phosphate isomerase converts all of the G3P reversibly into dihydroxyacetone phosphate (DHAP), also a 3-carbon molecule.
Aldolase and fructose-1,6-bisphosphatase convert a G3P and a DHAP into fructose 6-phosphate (6C), releasing an inorganic phosphate ion.
Transketolase removes two carbons from F6P, giving erythrose-4-phosphate. The two carbons on transketolase are added to a G3P, giving the ketose xylulose-5-phosphate (Xu5P).
Aldolase enzyme converts E4P and a DHAP into sedoheptulose-1,7-bisphosphate (7C).
Sedoheptulose-1,7-bisphosphatase cleaves sedoheptulose-1,7-bisphosphate into sedoheptulose-7-phosphate and releases an inorganic phosphate ion.
Transketolase removes two carbons from S7P, giving ribose-5-phosphate (R5P) and transfers the two remaining carbons to one of the G3P, giving another Xu5P.
Phosphopentose isomerase converts R5P into ribulose-5-phosphate (Ru5P) and phosphopentose epimerase converts Xu5P into RuP.
Phosphoribulokinase phosphorylates RuP into RuBP, completing the Calvin cycle. This requires one ATP molecule.

Regeneration Summary

Of the six G3P produced, five are used to make three RuBP (5C) molecules (totaling 15 carbons). Only one G3P is available for subsequent conversion to hexose.
This requires nine ATP molecules and six NADPH molecules per three CO₂ molecules.

Photorespiration

RuBisCO can react competitively with O₂ instead of CO₂ in photorespiration.
The rate of photorespiration is higher at high temperatures.
Photorespiration turns RuBP into 3-PGA and 2-phosphoglycolate, a 2-carbon molecule that can be converted via glycolate and glyoxalate to glycine.
Two glycines are converted into serine and CO₂ via the glycine cleavage system and tetrahydrofolate.
Serine can be converted back to 3-phosphoglycerate.
Photorespiration negatively impacts plants, leading to the loss of CO₂ instead of fixation.

C4 Carbon Fixation

C4 carbon fixation gets its name from the 4-carbon molecule oxaloacetate present in the first product of carbon fixation.
C4 carbon fixation evolved to circumvent photorespiration and occurs in plants native to warm or tropical climates, such as corn.
It involves shuttling CO₂ via malate or aspartate from mesophyll cells to bundle-sheath cells.
In bundle-sheath cells, RuBisCO is isolated from atmospheric oxygen and saturated with CO₂ released by the decarboxylation of malate.

C4 Leaf Anatomy

C4 plants possess kranz anatomy, where vascular bundles are surrounded by two rings of cells: the inner ring, called bundle sheath cells, contains starch-rich chloroplasts lacking grana; the outer ring, called mesophyll cells, contains chloroplasts with grana.
The chloroplasts in C4 plants are called dimorphic.
Kranz anatomy provides a site where CO₂ can be concentrated around RuBisCO, avoiding photorespiration.
The kranz boundary layer has low conductance to CO₂, enhanced by the presence of suberin, which helps maintain a higher CO₂ concentration in the bundle sheath.

C4 Pathway

C4 carbon fixation was elucidated by Marshall Davidson Hatch and C.R. Slack in 1966. It is sometimes called the Hatch-Slack pathway.
The first step is the conversion of pyruvate to phosphoenolpyruvate (PEP) by the enzyme pyruvate orthophosphate dikinase.
The next step is the fixation of CO₂ into oxaloacetate by the enzyme PEP carboxylase.
Both steps occur in the mesophyll cells.
O₂ is a poor substrate for PEP carboxylase, so at low CO₂ concentrations, most CO₂ will be converted into bicarbonate and fixed by this pathway.
The product is usually converted to malate, which is transported to the bundle-sheath cells.
In bundle-sheath cells, malate is decarboxylated by malic enzyme to produce CO₂ and pyruvate.
The CO₂ enters the Calvin cycle and the pyruvate is transported back to the mesophyll cell.
In mesophyll chloroplasts, pyruvate orthophosphate dikinase (PPDK) converts pyruvate back to PEP, completing the C4 cycle.

C4 Pathway Energy Usage

The C4 pathway uses more energy than the C3 pathway because each CO₂ molecule is fixed twice: first by a 4-carbon organic acid and then by RuBisCO.
The C4 pathway requires 30 molecules of ATP.

C4 Pathway Variations

The 4-carbon acid transported from mesophyll cells can be malate, as described above, or aspartate.
The 3-carbon acid transported back from bundle-sheath cells can be pyruvate, as described above, or alanine.
The enzyme that catalyses decarboxylation in bundle-sheath cells can vary. In maize and sugarcane, the enzyme is NADP-malic enzyme; in millet, it is NAD-malic enzyme; and, in Panicum maximum, it is PEP carboxykinase.

Crassulacean Acid Metabolism (CAM)

This pathway, named after the Crassulaceae family, occurs commonly in other families, such as Cactaceae, Euphorbiaceae, Orchidaceae, and Bromeliaceae.
CAM plants fix CO₂ at night using PEP carboxylase as the primary carboxylating enzyme.
Malate, made by the enzyme malate dehydrogenase, accumulates in the large vacuoles of their cells.
Deacidification occurs during the day when CO₂ is released from malate and fixed in the Calvin-Benson cycle using RuBisCO.

CAM Plant Water Usage

CAM plants have closed stomata during the day to prevent water loss.
Their stomata are open at night when the air is cooler and more humid, allowing the leaves to assimilate CO₂.
CAM plants require less water than C3 and C4 plants to fix the same amount of CO₂ in photosynthesis.

CAM CO₂ Fixation

The stomata of CAM plants are open at night.
CO₂ is fixed in the cytoplasm of mesophyll cells by a PEP reaction similar to the C4 pathway.
Unlike the C4 mechanism, the resulting organic acids are stored in vacuoles for later use, meaning they are not immediately passed on to the Calvin cycle.

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Description

This quiz tests your knowledge on the photosynthetic membrane and the role of various pigments in photosynthesis. Learn about how pigments like chlorophylls and carotenoids absorb light and contribute to the process of photosynthesis. Assess your understanding of light absorption and its significance in plant biology.