Photosynthesis Overview and Types

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Questions and Answers

What is the primary process by which green plants convert sunlight into chemical energy?

Fermentation

Photosynthesis (correct)

Respiration

Transpiration

In oxygenic photosynthesis, what happens to water during the electron transfer process?

It acts as a carbon source.

It receives electrons.

It loses electrons. (correct)

It is produced as a byproduct.

Which of the following correctly describes anoxygenic photosynthesis?

It generates carbohydrates without using sunlight.

It produces oxygen from water.

It uses electron donors other than water. (correct)

It is observed only in plants and algae.

What is the main role of chloroplasts in photosynthesis?

To perform the photosynthesis process. Signup and view all the answers

What are thylakoids primarily responsible for in the chloroplast?

Performing light-harvesting. Signup and view all the answers

Which of the following statements about the significance of photosynthesis is false?

It directly provides energy to animals. Signup and view all the answers

What is the primary role of pigments in photosynthesis?

To trap sunlight and bestow color on organisms Signup and view all the answers

What distinguishes chromoplasts from chloroplasts?

They contain carotenoids. Signup and view all the answers

Which pigment is primarily responsible for absorbing blue and red light?

Chlorophyll Signup and view all the answers

What is the wavelength range of visible light that drives photosynthesis?

400 to 700 nm Signup and view all the answers

What is the purpose of plastids in photosynthetic eukaryotic organisms?

To store nutrients or pigments. Signup and view all the answers

Which sub-type of chlorophyll is capable of efficiently absorbing light for photosynthesis?

Chlorophyll a Signup and view all the answers

How quickly does the transition from the electronic ground state to an excited state occur when a photon is absorbed?

10^-15 seconds Signup and view all the answers

What percentage of absorbed quanta are transferred to the reaction center within a few hundred picoseconds?

90% Signup and view all the answers

Which type of pigment absorbs light not well absorbed by chlorophylls and carotenoids?

Phycobilins Signup and view all the answers

What occurs to some excitons during the process of photosynthesis?

They are emitted as fluorescence Signup and view all the answers

What is the role of Photosystem II in the light-dependent reactions of photosynthesis?

To oxidize water and produce oxygen Signup and view all the answers

How does ATP synthase generate ATP during the light-dependent reactions?

By utilizing a proton gradient Signup and view all the answers

What is the final electron acceptor in the photosynthetic electron transport chain?

NADP+ Signup and view all the answers

What is the function of the cytochrome b6f complex in photosynthesis?

To pump protons and transfer electrons Signup and view all the answers

Which component acts as the primary electron donor in Photosystem I?

P-700 chlorophyll dimer Signup and view all the answers

What process is referred to as photophosphorylation?

The conversion of light energy to ATP Signup and view all the answers

Which molecules help create a proton gradient in the thylakoid space during photosynthesis?

Electrons and hydrogen ions Signup and view all the answers

What is the primary role of ferredoxin in the light reactions of photosynthesis?

To facilitate electron transfer to NADP+ Signup and view all the answers

What is the main purpose of cyclic photophosphorylation in plants?

To generate ATP Signup and view all the answers

Which enzyme catalyzes the carboxylation of ribulose-1,5-bisphosphate during carbon fixation?

RuBisCO Signup and view all the answers

What are the products of the reduction phase in the Calvin cycle?

Glyceraldehyde 3-phosphate and NADP+ Signup and view all the answers

During which phase of the Calvin cycle is ADP produced?

Reduction phase Signup and view all the answers

Which molecule is produced from the initial splitting of the six-carbon intermediate during carbon fixation?

3-Phosphoglycerate Signup and view all the answers

What is the role of dihydroxyacetone phosphate (DHAP) during the regeneration phase?

To combine with G3P into fructose 6-phosphate Signup and view all the answers

What are the main inputs for the Calvin cycle?

ATP and NADPH Signup and view all the answers

Which component of ATP synthase is located outside the membrane?

The F1 portion Signup and view all the answers

Which enzyme is responsible for converting sedoheptulose-1,7-bisphosphate into sedoheptulose-7-phosphate?

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

What is the primary product of the reaction between erythrose-4-phosphate and G3P?

Xylulose-5-phosphate Signup and view all the answers

How many ATP molecules are required for the conversion of three CO2 molecules in the process described?

9 ATP Signup and view all the answers

Which product is formed when RuBP reacts with O2 during photorespiration?

3-PGA and 2-phosphoglycolate Signup and view all the answers

What is the role of phosphopentose isomerase in the described process?

Convert xylulose-5-phosphate to ribulose-5-phosphate Signup and view all the answers

What advantage does C4 carbon fixation provide to plants in warm climates?

Reduces photorespiration by increasing CO2 concentration Signup and view all the answers

Which of the following statements about transketolase is true?

It transfers two carbon units from one sugar to another. Signup and view all the answers

What is a consequence of high rates of photorespiration in plants?

Less effective carbon fixation Signup and view all the answers

Study Notes

Photosynthesis

Photosynthesis is the process used by plants, algae, and certain bacteria to convert light energy into chemical energy.
It is the main source of energy for most living organisms.

Photosynthesis Types

Oxygenic photosynthesis: Most common type, occurs in plants, algae, and cyanobacteria. Light energy is used to transfer electrons from water (H2O) to carbon dioxide (CO2), producing carbohydrates and oxygen.
Anoxygenic photosynthesis: Occurs in bacteria like purple bacteria and green sulfur bacteria. Electron donors other than water are used, leading to the production of sulfur when hydrogen sulfide is used.

Significance of Photosynthesis

Converts solar energy into organic matter, sustaining life.
Consumes atmospheric carbon dioxide and releases oxygen, contributing to atmospheric balance.
Produces carbohydrates, the basic building blocks for sugars, proteins, fats, and other key biomolecules.
Provides energy for various metabolic activities and physical processes in organisms.

Photosynthetic Requirements

Eukaryotic photosynthetic organisms contain organelles called plastids, responsible for pigment storage and nutrient storage.
Chloroplasts: Contain chlorophyll, the main pigment for trapping light energy.
Chromoplasts: Contain carotenoids, pigments that absorb blue-green light.
Leucoplast: Non-pigmented, store starch and fats.

Chloroplasts

Double-membrane bound organelles where photosynthesis takes place.
The grana are stacks of disc-shaped membranes called thylakoids.
Thylakoids contain the light-harvesting complex for electron transfer.
The empty spaces between grana columns are called the stroma.
The photosynthetic membrane, within the thylakoid, contains pigments and proteins required for light reactions.

Pigments

Molecules responsible for trapping sunlight and giving color to plants, algae, and bacteria.
Different pigments absorb different wavelengths of light.
Chlorophylls: Green pigments that absorb blue and red light.
- Chlorophyll a, chlorophyll b, and chlorophyll c types exist.
Carotenoids: Red, orange, or yellow pigments that absorb bluish-green light.
Phycobilins: Red or blue pigments found in cyanobacteria and red algae that absorb wavelengths not efficiently absorbed by other pigments.

Absorption of Light Energy

Plant photosynthesis relies mostly on visible light (400-700 nm) absorbed by pigments like chlorophyll a, b, and carotenoids.
Pigment molecules are bound to light-harvesting protein complexes in the photosynthetic membrane, collecting light energy.
These complexes surround reaction centers, acting as an antenna system to capture photons.

Light Reactions

Initiated by the absorption of a photon by an antenna molecule, exciting electrons.
The excited electrons are transferred through an electron transport chain (ETC), leading to a series of redox reactions.
The electron flow goes from Photosystem II (PSII) to cytochrome b6f complex to Photosystem I (PSI).
In PSI, electrons gain energy from another photon.
The final electron acceptor is NADP+, which is reduced to NADPH.
Cytochrome b6f and ATP synthase work together to create ATP through photophosphorylation in two ways:
- Non-cyclic photophosphorylation: Generates ATP and NADPH.
- Cyclic photophosphorylation: Generates only ATP.

Photosystem II (PSII)

The first protein complex in oxygenic photosynthesis.
Captures photons to energize electrons, which are then transferred to reduce plastoquinone to plastoquinol.
Oxidizes water, releasing hydrogen ions (protons) and molecular oxygen.
These protons contribute to the proton gradient used by ATP synthase.

Cytochrome b6f Complex

Found in the thylakoid membrane of chloroplasts, cyanobacteria, and green algae.
Transfers electrons from PSII to PSI, pumping protons into the thylakoid space.
This proton gradient is later used to create ATP.

Photosystem I (PSI)

The second photosystem in oxygenic photosynthesis.
Electron transfer components include a primary electron donor P-700 and five electron acceptors.
Mediates electron transfer from plastocyanin to ferredoxin.
Ferredoxin reduces NADP+ to NADPH through Ferredoxin-NADP+ reductase (FNR).

ATP Synthase

An enzyme that produces ATP, the energy storage molecule.
Has two regions:
- FO portion: Embedded within the membrane.
- F1 portion: Outside the membrane.
The proton gradient across the thylakoid membrane drives ATP synthesis.

Cyclic Photophosphorylation

Cytochrome b6f uses energy from PSII and PSI to create more ATP, preventing NADPH production.
Helps maintain the correct balance of ATP and NADPH for the dark reactions.

Calvin Cycle (Dark Reactions)

The process where plants and algae convert carbon dioxide into sugar.
Occurs in the stroma and has four main steps:
- Carbon Fixation: RuBisCO enzyme carboxylates ribulose-1,5-bisphosphate (RuBP), forming 3-phosphoglycerate (3-PGA).
- Reduction Phase: 3-PGA is phosphorylated and reduced to glyceraldehyde 3-phosphate (G3P) using ATP and NADPH.
- Carbohydrate Formation: G3P molecules are used to make glucose and other carbohydrates.
- Regeneration Phase: RuBP is regenerated to continue the cycle.

Photorespiration

RuBisCO can react with oxygen instead of carbon dioxide, leading to photorespiration.
This process is more prevalent at high temperatures and reduces carbon fixation efficiency.
Photorespiration results in the release of carbon dioxide, negatively impacting the plant.

C4 Carbon Fixation

A more efficient process that minimizes photorespiration in plants adapted to warm climates.
Named for the four-carbon molecule oxaloacetate, the first product of carbon fixation in this pathway.

C4 Pathway Key Features

Occurs in plants like maize, sugarcane, and sorghum.
Involves two cell types: mesophyll cells and bundle sheath cells.
Initial carbon fixation occurs in mesophyll cells, forming oxaloacetate, which is later converted to malate.
Malate is transported to bundle sheath cells, where it is decarboxylated, releasing carbon dioxide for use in the Calvin cycle.
This mechanism concentrates carbon dioxide in bundle sheath cells, minimizing photorespiration.

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Description

This quiz explores the fascinating process of photosynthesis, including its types and significance. Learn about the differences between oxygenic and anoxygenic photosynthesis and understand how these processes contribute to sustaining life on Earth. Test your knowledge on the conversion of solar energy into chemical energy.