concept 8.1 ap bio: Photosynthesis converts light energy to the chemical energy of food

What is the main purpose of photosynthesis?

Photosynthesis converts light energy into chemical energy stored in organic compounds, such as sugars.

In which types of organisms did photosynthesis likely originate?

Photosynthesis likely originated in a group of bacteria with infolded regions of the plasma membrane.

What is the endosymbiont theory?

It proposes that chloroplasts evolved from photosynthetic prokaryotes that lived inside eukaryotic cells.

Where does photosynthesis primarily occur in plants?

In the chloroplasts of mesophyll cells, which are found in the interior of leaves.

What are stomata, and what is their function?

Stomata are microscopic pores that allow carbon dioxide to enter the leaf and oxygen to exit.

Describe the three main structural components of chloroplasts.

• Outer and inner membranes surround the chloroplast.

• Stroma is a dense fluid inside the chloroplast.

• Thylakoid membranes contain chlorophyll and form stacks called grana.

What is the role of chlorophyll in photosynthesis?

Chlorophyll absorbs light energy, which drives the synthesis of organic molecules.

What are grana and thylakoids?

Grana are stacks of thylakoid sacs, and thylakoids are the membranes where the light reactions occur.

Write the overall chemical equation for photosynthesis.

6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂

What is the simplified equation for photosynthesis?

CO₂ + H₂O + light energy → [CH₂O] + O₂ (where [CH₂O] represents a carbohydrate).

What happens to water molecules during photosynthesis?

Water is split into hydrogen and oxygen, with oxygen released as a by-product.

Where do the light reactions occur?

In the thylakoid membranes of the chloroplast.

What are the main inputs and outputs of the light reactions?

• Inputs: Light, water, ADP, and NADP⁺.

• Outputs: Oxygen, ATP, and NADPH.

What is photophosphorylation?

The process of generating ATP using light energy in the light reactions.

You have ADP, you add a phosphate to it and make ATP.

What is the role of NADPH in photosynthesis?

NADPH acts as a reducing agent, carrying high-energy electrons to the Calvin cycle.

Where does the Calvin cycle occur?

In the stroma of the chloroplast.

What are the main inputs and outputs of the Calvin cycle?

• Inputs: CO₂, ATP, and NADPH.

• Outputs: Sugar (G3P), ADP, and NADP⁺.

What is carbon fixation?

The process of incorporating CO₂ into organic molecules in the Calvin cycle.

Why is the Calvin cycle also called "light-independent reactions"?

Because it does not directly require light, though it relies on products of the light reactions (ATP and NADPH).

How is photosynthesis different from cellular respiration?

Photosynthesis stores energy by reducing CO₂ into sugars, while respiration releases energy by oxidizing sugars into CO₂.

What role do redox reactions play in photosynthesis?

• Water is oxidized (loses electrons) in the light reactions.

• CO₂ is reduced (gains electrons) in the Calvin cycle to form sugar.

What happens to electrons during the light reactions?

Electrons from water are energized by light and transferred to NADP⁺, forming NADPH.

Why is photosynthesis an endergonic process?

Because it requires energy input (from light) to drive the production of sugars.

How are ATP and NADPH used in the Calvin cycle?

ATP provides energy, and NADPH provides electrons for the reduction of CO₂ into sugars.

How many chloroplasts are typically found in a mesophyll cell?

About 30–40 chloroplasts.

Why are leaves green?

Because chlorophyll absorbs red and blue light but reflects green light.

What is the main carbohydrate produced by photosynthesis?

Glucose, though the Calvin cycle directly produces a 3-carbon sugar (G3P) that can be used to form glucose.

What is the role of the outer and inner chloroplast membranes?

The membranes act as a barrier, controlling the exchange of materials between the chloroplast and the cytoplasm.

What is the lumen in a chloroplast?

The lumen is the internal space of the thylakoid, important for the light reactions of photosynthesis.

What pigment is most abundant in chloroplasts?

Chlorophyll a, the primary pigment involved in photosynthesis.

What accessory pigments assist chlorophyll a?

Chlorophyll b and carotenoids, which absorb additional light wavelengths.

How does light energy become chemical energy in photosynthesis?

Light excites electrons in chlorophyll, raising them to higher energy levels; these electrons are then transferred to electron carriers.

What is the role of the electron transport chain in the light reactions?

The chain uses high-energy electrons to pump protons across the thylakoid membrane, creating a proton gradient for ATP synthesis.

What is chemiosmosis in photosynthesis?

It’s the process of using a proton gradient across the thylakoid membrane to drive the synthesis of ATP.

What happens to the oxygen produced in photosynthesis?

It diffuses out of the plant through the stomata and is released into the atmosphere.

Where do plants obtain water for photosynthesis?

From the soil, through the roots and xylem.

How is carbon dioxide used in the Calvin cycle?

It’s fixed by the enzyme RuBisCO and incorporated into a 3-carbon compound.

What is the function of photosystem II (PSII)?

PSII captures light energy to split water molecules, releasing oxygen and supplying electrons to the electron transport chain.

What is the function of photosystem I (PSI)?

PSI uses light energy to energize electrons and transfer them to NADP⁺, forming NADPH.

How does ATP synthase work in photosynthesis?

Protons flow through ATP synthase from the thylakoid lumen to the stroma, driving the phosphorylation of ADP to ATP.

What are the three main stages of the Calvin cycle?

• Carbon fixation: CO₂ is fixed into a 3-carbon compound.

• Reduction: ATP and NADPH are used to convert the 3-carbon compound into G3P.

• Regeneration: Some G3P molecules are used to regenerate RuBP, enabling the cycle to continue.

What enzyme catalyzes the first step of the Calvin cycle?

RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase).

How many turns of the Calvin cycle are required to produce one glucose molecule?

Six turns, because each turn fixes one molecule of CO₂ and produces a small amount of G3P.

Why is photosynthesis considered a fundamental process for life on Earth?

It provides the base of the food chain and produces oxygen, which is essential for most living organisms.

What evolutionary advantage do accessory pigments provide?

They allow plants to use a broader spectrum of light for photosynthesis.

How do C4 and CAM plants differ from C3 plants?

They have adaptations to minimize water loss and photorespiration in hot, dry environments.

How does photosynthesis impact the global carbon cycle?

It removes CO₂ from the atmosphere and incorporates it into organic molecules, balancing carbon levels.

What is the importance of photosynthetic organisms in aquatic ecosystems?

Algae and phytoplankton perform most of the photosynthesis, supplying oxygen and serving as a primary food source.

What happens to the hydrogen ions generated when water is split?

They contribute to the proton gradient used for ATP synthesis.

Why is ATP unstable and not stored in plants?

ATP is used immediately for energy transfer and is converted into stable sugars for storage.

What is the role of plastoquinone in photosynthesis?

It’s an electron carrier that transports electrons from PSII to the cytochrome complex.

How is the proton gradient used differently in photosynthesis compared to respiration?

In photosynthesis, the gradient is across the thylakoid membrane, while in respiration, it’s across the inner mitochondrial membrane.

Why is photosynthesis considered the reverse of cellular respiration?

Photosynthesis builds glucose from CO₂ and water, while respiration breaks down glucose into CO₂ and water.

What happens to sugars produced by photosynthesis?

They are used for energy, growth, and building cell structures like cellulose.

How does light intensity affect photosynthesis?

Higher light intensity increases the rate of photosynthesis up to a point, after which it plateaus.

Why do leaves change color in the fall?

Chlorophyll breaks down, revealing carotenoids and anthocyanins.