Photosynthesis AP Biology

Summary

Photosynthesis is a vital process in which plants and certain bacteria convert sunlight into energy. It occurs in chloroplasts, involving light-dependent reactions in thylakoids and the Calvin cycle in the stroma. Chlorophyll in photosystems absorbs light energy, exciting electrons that drive ATP production via chemiosmosis and NADPH generation. These molecules power the Calvin cycle, converting CO2 into sugars.

Takeaways

• 🌱 Photosynthesis is a process that captures energy from the sun to produce sugars, occurring in both prokaryotes and eukaryotic autotrophs.
• 🌿 In eukaryotic autotrophs, photosynthesis takes place in the chloroplast, a double membrane organelle with a compartmentalized structure.
• 🌳 The chloroplast is divided into thylakoids and the stroma, with thylakoids stacked into grana, enhancing surface area and efficiency of reactions.
• 💡 The light-dependent reactions of photosynthesis occur in the thylakoid membrane and compartment, involving proteins like photosystem 2 and photosystem 1.
• 💧 Water is split in the light-dependent reactions, producing protons, electrons, and oxygen gas as a byproduct.
• 🌞 Photosystem 2 absorbs light energy to excite electrons, which are then passed through an electron transport chain (ETC).
• 🔋 The proton pump uses energy from electron transport to establish a proton gradient, crucial for ATP production.
• 🔌 ATP synthase creates ATP by joining inorganic phosphate with ADP, powered by chemiosmosis, where protons flow down their concentration gradient.
• 🌀 The Calvin cycle takes place in the stroma, converting CO2 into organic carbohydrates with the help of ATP and NADPH produced in the light-dependent reactions.
• 🔄 NADPH and ATP are used in the Calvin cycle to power reactions, with NADPH providing electrons and ATP releasing energy for metabolic processes.

Q & A

What is photosynthesis and where does it occur?

Photosynthesis is the process by which plants, algae, and certain bacteria convert light energy, usually from the sun, into chemical energy in the form of glucose or other sugars. It occurs in both prokaryotes like cyanobacteria and eukaryotic autotrophs, specifically in the chloroplasts of eukaryotic cells.

What are the main components of a chloroplast?

A chloroplast is a double membrane organelle with thylakoids that are stacked into structures called grana. The fluid region outside of the thylakoids is called the stroma. This compartmentalization helps to increase surface area and decrease competition between reactions.

What are the two main processes of photosynthesis?

The two main processes are the light-dependent reactions, which occur in the thylakoid membrane and compartment, and the Calvin cycle, which takes place in the stroma.

Where do the light-dependent reactions take place and what happens there?

The light-dependent reactions take place in the thylakoid membrane and compartment. Here, water is split, releasing protons, electrons, and oxygen. Electrons are excited by light energy absorbed by chlorophyll in photosystems II and I, and are passed through an electron transport chain.

What is the role of the electron transport chain (ETC) in photosynthesis?

The ETC is involved in the transfer of electrons from photosystem II to photosystem I. As electrons move through the chain, energy is released, which is used by the proton pump to establish a proton gradient across the thylakoid membrane.

How does the proton gradient contribute to ATP production?

The proton gradient, created by the active transport of protons from the stroma to the thylakoid compartment, is necessary for ATP production. An enzyme called ATP synthase uses the energy from the flow of protons down their concentration gradient, a process known as chemiosmosis, to create ATP from ADP and inorganic phosphate.

What is the purpose of re-exciting electrons in photosystem one after they have already been excited in photosystem two?

The re-excitation of electrons in photosystem one is necessary to provide high-energy electrons to NADPH. These high-energy electrons are needed for certain reactions in the Calvin cycle.

What is the role of NADPH in the Calvin cycle?

NADPH carries high-energy electrons to the Calvin cycle, where it acts as an electron carrier molecule. The electrons are used to facilitate reactions that convert CO2 into organic carbohydrates, which become sugars.

How does ATP power the reactions in the Calvin cycle?

ATP powers the reactions in the Calvin cycle by being converted to ADP, releasing energy that can be used to drive metabolic processes and synthesize carbohydrates from CO2.

What is the significance of the term 'photophosphorylation'?

Photophosphorylation refers to the process by which ATP is synthesized using the energy derived from light, specifically during the flow of protons through ATP synthase, which is powered by chemiosmosis.

What happens to ADP and NADP+ after they have been used in the Calvin cycle?

After their role in the Calvin cycle, ADP and NADP+ return to the light-dependent reactions to be re-energized and reused, continuing the cycle of photosynthesis.