AP Biology Unit 3 Topic(s) 5

What is photosynthesis?

The conversion of light energy into chemical energy.

What type of autotrophs are plants?

Photoautotrophs.

What is the difference between autotrophs and heterotrophs?

Autotrophs produce their own food from simple substances; heterotrophs cannot make their own food and live off other organisms.

Where did photosynthesis first evolve?

In prokaryotic organisms.

Which early prokaryotes were capable of photosynthesis?

Cyanobacteria.

How did cyanobacteria change Earth’s atmosphere?

They oxygenated it.

What provided the foundation for eukaryotic photosynthesis?

Prokaryotic photosynthetic pathways.

Where does photosynthesis primarily occur in plants?

In the leaves.

What organelle is the site of photosynthesis?

The chloroplast.

In what leaf cells are chloroplasts found?

Mesophyll cells.

What are stomata and their function?

Pores in leaves that allow CO₂ in and O₂ out.

What surrounds chloroplasts?

A double membrane.

What is the stroma?

The aqueous fluid inside the chloroplast.

What are thylakoids and grana?

Thylakoids are membrane sacs; grana are stacks of thylakoids.

What pigment is found in thylakoid membranes?

Chlorophyll.

What is the simplified formula for photosynthesis?

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

In photosynthesis, what molecule is split?

H₂O is split into hydrogen and oxygen.

What type of reaction is photosynthesis?

A redox reaction.

In photosynthesis, where are electrons transferred?

Electrons and H⁺ from H₂O are transferred to CO₂, reducing it to sugar.

In terms of redox, what happens in photosynthesis?

CO₂ is reduced to sugar; H₂O is oxidized to O₂.

What are the two stages of photosynthesis?

The light reactions and the Calvin cycle.

What part of photosynthesis is the “photo” part?

The light reactions.

What part of photosynthesis is the “synthesis” part?

The Calvin cycle.

What is light?

Electromagnetic energy.

What are photons?

Particles of light energy.

How does light travel?

In waves.

What is wavelength?

The distance between the crests of two waves.

What is the electromagnetic spectrum?

The entire range of electromagnetic radiation.

What is the range of visible light?

About 380 nm to 750 nm.

What is the relationship between wavelength and energy?

Shorter wavelengths have higher energy; longer wavelengths have lower energy.

When light interacts with matter, what can happen?

It can be reflected, transmitted, or absorbed.

What determines the color we see in pigments?

The wavelengths of light that are reflected.

Why are leaves green?

Chlorophyll absorbs violet

What is chlorophyll a?

The primary pigment involved in light reactions; it appears blue

What is chlorophyll b?

An accessory pigment that is yellow

What are carotenoids?

Accessory pigments that expand the light absorption spectrum.

What colors are carotenoids?

Yellow and orange.

What is the photoprotective role of carotenoids?

They absorb and dissipate excess light energy that could damage chlorophyll or react with oxygen.

Where do the light reactions occur?

In the thylakoid membrane, within photosystems.

What do the light reactions convert?

Solar energy into chemical energy.

In what forms is chemical energy stored in the light reactions?

NADPH and ATP.

How do the light reactions begin?

Photons excite electrons in chlorophyll.

What happens when chlorophyll absorbs a photon?

An electron is boosted from a ground state to an excited state.

What happens to an electron in the excited state?

It is unstable and falls back to the ground state.

What happens when the electron falls back to the ground state?

Energy is released as heat or fluorescence.

What are photosystems?

Complexes of proteins, pigments, and electron acceptors in thylakoid membranes.

What is the reaction center in a photosystem?

A protein complex with chlorophyll a and a primary electron acceptor.

What are light

harvesting complexes?

What are the two types of photosystems?

Photosystem II (P680) and Photosystem I (P700).

What wavelength does Photosystem II absorb best?

680 nm.

What wavelength does Photosystem I absorb best?

700 nm.

Which photosystem was discovered first?

Photosystem I.

What is the first step in Photosystem II?

Light excites electrons in pigments, and energy is transferred until it excites the P680 pair of chlorophyll a molecules.

What happens to electrons in P680?

They are transferred to the primary electron acceptor, making P680⁺.

How is P680⁺ reduced back to P680?

Water is split, releasing 2 electrons to reduce P680⁺, 2 H⁺ into the thylakoid space, and ½ O₂ as a by

What is linear electron flow?

The process where electrons excited in PSII move through the ETC to PSI.

What happens to electrons as they move from PSII to PSI?

They fall down an ETC through PQ, cytochrome complex, and PC.

What does the “fall” of electrons between PSII and PSI provide?

Energy to form ATP.

What gradient forms during electron flow between PSII and PSI?

An H⁺ gradient across the thylakoid membrane.

How is ATP generated in the light reactions?

ATP synthase couples H⁺ diffusion to ATP formation.

What excites electrons in Photosystem I?

Light absorbed by P700 chlorophyll molecules.

What happens when P700 chlorophyll is excited?

Electrons are transferred to the primary electron acceptor, creating P700⁺.

Where do electrons from PSI go?

They pass down a second electron transport chain via Fd.

What enzyme catalyzes the transfer of electrons to NADP⁺?

NADP⁺ reductase.

What is the product of electron transfer at the end of PSI?

NADPH.

What are the inputs of the light reactions?

H₂O, ADP, and NADP⁺.

What are the outputs of the light reactions?

O₂, ATP, and NADPH.

What are the main functions of the light reactions?

Split H₂O, release O₂, reduce NADP⁺ to NADPH, and generate ATP.

What is the Calvin cycle?

A cycle that uses ATP and NADPH to reduce CO₂ into sugar (G3P).

How many turns of the Calvin cycle are required for one net G3P?

Three turns.

What are the three phases of the Calvin cycle?

Carbon fixation, reduction, and regeneration of RuBP.

What happens during carbon fixation?

CO₂ attaches to RuBP, catalyzed by rubisco, forming 3

What happens during the reduction phase of the Calvin cycle?

3

How many G3P molecules are formed per 3 CO₂?

Six total, but only one is a net gain.

What happens to the other 5 G3P molecules in the Calvin cycle?

They are used to regenerate RuBP.

What happens during the regeneration phase of the Calvin cycle?

5 G3P regenerate 3 RuBP, using 3 ATP.

What are the inputs of the Calvin cycle?

3 CO₂, 9 ATP, 6 NADPH.

What are the outputs of the Calvin cycle?

1 G3P, 9 ADP, 6 NADP⁺.

What enzyme catalyzes CO₂ fixation in the Calvin cycle?

Rubisco.

What is photorespiration?

A process where rubisco binds O₂ instead of CO₂, consuming ATP and releasing CO₂ without producing sugar.

Why is photorespiration harmful to plants?

It uses ATP and produces no sugar, reducing efficiency.

Why do plants close their stomata on hot days?

To conserve water.

What effect does closing stomata have on CO₂ and O₂ levels?

CO₂ decreases and O₂ increases.

What adaptations do C4 plants have?

Spatial separation of steps: CO₂ is fixed into a 4

What are examples of C4 plants?

Maize (corn), grasses, sugarcane.

What adaptations do CAM plants have?

Temporal separation of steps: stomata open at night to take in CO₂, which is stored as organic acids, then used during the day in the Calvin cycle.

What are examples of CAM plants?

Pineapples, cacti, succulents, jade.