Photosynthesis: Processes, Pigments, and Adaptations in Plants

What is photosynthesis?

The process that converts solar energy into chemical energy within chloroplasts.

What are autotrophs?

Organisms that sustain themselves without eating anything derived from other organisms; they produce organic molecules from CO2 and inorganic molecules.

What are photoautotrophs?

Almost all plants that use sunlight to make organic molecules.

What role do heterotrophs play in the biosphere?

They obtain organic material from other organisms and are the consumers of the biosphere.

What are decomposers?

Organisms that eat dead organic material or feces.

How are fossil fuels related to photosynthesis?

Fossil fuels were formed from the remains of organisms that died hundreds of millions of years ago, representing ancient stores of the sun's energy.

What are chloroplasts?

Organelles in plants and other photosynthetic organisms where photosynthesis occurs.

Where does most photosynthesis occur in plants?

In the leaves, specifically in the mesophyll cells.

What is the function of stomata?

Microscopic pores through which CO2 enters and O2 exits the leaf.

What is the structure of a chloroplast?

It has an envelope of two membranes surrounding a dense fluid called the stroma, with thylakoids that may be stacked in columns called grana.

What is the overall equation for photosynthesis?

6 CO2 + 12 H2O + Light energy → C6H12O6 + 6 O2 + 6 H2O.

What does photosynthesis reverse compared to cellular respiration?

The overall chemical change during photosynthesis is the reverse of cellular respiration.

What is the role of water in photosynthesis?

Chloroplasts split H2O into hydrogen and oxygen, incorporating hydrogen into sugar molecules and releasing O2 as a by-product.

What are the two stages of photosynthesis?

The light reactions and the Calvin cycle.

What occurs during the light reactions?

They split H2O, release O2, reduce NADP+ to NADPH, and generate ATP from ADP.

What is the Calvin cycle responsible for?

Making sugar from CO2 using ATP and NADPH generated during the light reactions.

What is light energy?

Electromagnetic energy that travels in rhythmic waves.

What is the electromagnetic spectrum?

The entire range of electromagnetic energy, including visible light which drives photosynthesis.

What are pigments?

Substances that absorb visible light; different pigments absorb different wavelengths.

What types of pigments are found in chloroplasts?

Chlorophyll a, chlorophyll b, and carotenoids.

What is the absorption spectrum of chlorophyll a?

It indicates that violet-blue and red light are most effective for photosynthesis, while green is the least effective.

Who first demonstrated the action spectrum of photosynthesis?

Theodor W. in 1883.

What method did Engelmann use to measure O2 production in filamentous algae?

He exposed different segments of the algae to different wavelengths of light and measured the growth of aerobic bacteria.

How does the action spectrum for photosynthesis compare to the absorption spectrum of chlorophyll?

The action spectrum is broader than the absorption spectrum of chlorophyll.

What role do accessory pigments like chlorophyll b play in photosynthesis?

They broaden the spectrum of light used for photosynthesis.

What are the newly discovered forms of chlorophyll that absorb higher wavelengths of light?

Chlorophyll d and chlorophyll f.

Which cyanobacterium uses chlorophyll f instead of chlorophyll a?

Chroococcidiopsis thermalis.

What are carotenoids and their role in photosynthesis?

Carotenoids are accessory pigments that absorb violet and blue-green light, broadening the spectrum for photosynthesis and providing photoprotection.

What happens to an electron in a pigment molecule when it absorbs light?

It goes from a ground state to an excited state, which is unstable.

What is fluorescence in the context of chlorophyll excitation?

It is the release of excess energy as heat or light when excited electrons fall back to the ground state.

What is a photosystem composed of?

A reaction-center complex associated with light-harvesting complexes.

What is the function of the primary electron acceptor in a photosystem?

It accepts excited electrons and is reduced as a result.

What are the two types of photosystems found in the thylakoid membrane?

Photosystem II (PS II) and Photosystem I (PS I).

What is the wavelength at which Photosystem II (PS II) absorbs light best?

680 nm.

What is the wavelength at which Photosystem I (PS I) absorbs light best?

700 nm.

What is the primary pathway for electron flow during the light reactions?

Linear electron flow.

What are the eight steps involved in linear electron flow?

1. Photon hits PS II. 2. Energy excites P680. 3. Electron transferred to primary electron acceptor. 4. H2O is split. 5. Electrons reduce P680+. 6. Electrons passed down an electron transport chain. 7. Proton gradient created. 8. ATP and NADPH produced.

What is the role of NADP+ reductase in the light reactions?

It catalyzes the transfer of electrons from ferredoxin (Fd) to NADP+, reducing it to NADPH.

What is cyclic electron flow?

It is a process where photoexcited electrons cycle back from ferredoxin to the cytochrome complex instead of being transferred to NADP+.

What is the main product of cyclic electron flow?

ATP, but no NADPH or oxygen.

How do chloroplasts and mitochondria generate ATP?

Both generate ATP by chemiosmosis, using electron transport chains to pump protons across membranes.

What is the Calvin cycle's primary function?

To use the chemical energy of ATP and NADPH to reduce CO2 to sugar.

What are the three phases of the Calvin cycle?

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

What enzyme catalyzes the binding of CO2 to ribulose bisphosphate (RuBP) in the Calvin cycle?

RuBP carboxylase-oxygenase, or rubisco.

What is the net gain of carbohydrate for every three CO2 molecules fixed in the Calvin cycle?

One G3P sugar.

What is photorespiration?

A process that occurs when plants close stomata to conserve water, leading to reduced CO2 access and increased O2, which can waste energy.

What trade-off do plants face in arid climates regarding photosynthesis?

The balance between photosynthesis and water conservation.

What type of plants are most common and what compound do they initially fix CO2 into?

Most plants are C3 plants, which initially fix CO2 into a three-carbon compound (3-phosphoglycerate).

What occurs during photorespiration?

In photorespiration, rubisco binds with O2 instead of CO2, producing a two-carbon compound.

Why is photorespiration considered costly for plants?

Photorespiration is costly because it consumes O2 and organic fuel without producing any ATP or sugar.

What is the evolutionary significance of photorespiration?

Photorespiration is thought to be an evolutionary relic from a time when the atmosphere had less O2 and more CO2.

How much carbon fixed by the Calvin cycle can be drained by photorespiration in some plants?

In many plants, photorespiration can drain away as much as 50% of the carbon fixed by the Calvin cycle.

What adaptations have some plants evolved to minimize photorespiration?

Some plant species have evolved alternate modes of carbon fixation to minimize photorespiration and optimize the Calvin cycle.

What is the primary function of C4 plants in relation to photorespiration?

C4 plants minimize the cost of photorespiration by incorporating CO2 into a four-carbon compound as the first product of the Calvin cycle.

How many times has C4 photosynthesis evolved and in how many families?

C4 photosynthesis has evolved several times and is used by several thousand species in at least 19 different families.

Name two important agricultural examples of C4 plants.

Sugarcane and corn.

How do C4 plants manage water loss during hot, dry weather?

C4 plants partially close their stomata to conserve water, which reduces CO2 intake.

Where does photosynthesis begin and where is it completed in C4 plants?

Photosynthesis begins in mesophyll cells and is completed in bundle-sheath cells.

What enzyme catalyzes the production of four-carbon precursors in C4 plants?

The enzyme PEP carboxylase catalyzes the production of four-carbon precursors in mesophyll cells.

What advantage does PEP carboxylase have over rubisco?

PEP carboxylase has a higher affinity for CO2 than rubisco, allowing it to fix CO2 even at low concentrations.

What happens to the four-carbon compounds in bundle-sheath cells?

In bundle-sheath cells, CO2 is released from the four-carbon compound and used in the Calvin cycle.

What is the fate of pyruvate in C4 plants after the Calvin cycle?

Pyruvate is transported back to mesophyll cells where one ATP is used to convert it back to PEP.

What is the impact of increased CO2 levels since the Industrial Revolution on plant species?

Increasing CO2 and temperature may affect C3 and C4 plants differently, potentially changing their relative abundance.

What is the significance of genetically modifying rice to perform C4 photosynthesis?

Genetically modified rice, a C3 plant, could achieve a 30-50% increase in yield for given water and resources.

What is the function of CAM in certain plants?

CAM (crassulacean acid metabolism) allows some plants to conserve water by fixing carbon at night.

How do CAM plants utilize CO2 during the day?

During the day, CAM plants close their stomata and use CO2 released from organic acids in the Calvin cycle.

How do the C4 and CAM pathways differ in terms of carbon fixation?

The C4 pathway separates the initial steps of carbon fixation from the Calvin cycle structurally, while the CAM pathway separates them in time.

What is the overall importance of photosynthesis for life on Earth?

Photosynthesis is essential as it stores energy from sunlight as chemical energy in organic compounds, providing energy and carbon skeletons for cells.