concept 8.2 ap bio: The light reactions convert solar energy to chemical energy of ATP and NADPH

What happens when a photon is absorbed by an electron in a pigment molecule?

The electron is excited and moves to a higher energy level, becoming part of an excited state.

Why do excited electrons quickly return to the ground state?

Excited states are unstable, so the electron releases energy and returns to the ground state, often emitting heat and sometimes light (fluorescence).

What is fluorescence in chlorophyll?

When chlorophyll absorbs light and its electron falls back to the ground state, it emits light, called fluorescence, which is usually red-orange.

What are photosystems in plants?

Photosystems are complexes in the thylakoid membrane, consisting of a reaction-center complex and light-harvesting complexes, where light energy is converted into chemical energy.

What are the main parts of a photosystem?

Reaction-center complex (with a pair of chlorophyll a molecules)

Light-harvesting complexes (with pigments like chlorophyll a, chlorophyll b, and carotenoids).

How do light-harvesting complexes work in photosynthesis?

They absorb light energy and transfer it between pigment molecules until it reaches the reaction-center complex, where energy excites electrons in the special pair of chlorophyll a molecules.

What happens after light excites an electron in the chlorophyll molecules?

The excited electron is transferred to the primary electron acceptor, starting the flow of electrons in the photosystem.

What are the names of the two photosystems in the thylakoid membrane?

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

What are the special chlorophyll molecules in PS II and PS I?

In PS II, the chlorophyll is called P680, and in PS I, it’s called P700.

What happens first in the linear electron flow? STEP 1

A photon excites an electron in a pigment molecule in the light-harvesting complex of PS II, and this energy is passed to the P680 chlorophyll, exciting its electron.

What happens after the P680 electron is excited in PS II? STEP 2

The excited electron is transferred to the primary electron acceptor, creating P680+ (the oxidized form).

How does PS II replace the electron lost by P680? STEP 3

Water is split into 2 electrons, 2 hydrogen ions (H+), and 1 oxygen atom. The electrons are supplied to P680+.

What byproducts are released when water is split in PS II?

Oxygen (O2) and hydrogen ions (H+).

What happens to the excited electron after it is transferred from PS II? STEP 4

The electron moves through the electron transport chain, passing through plastoquinone (Pq), a cytochrome complex, and plastocyanin (Pc).

How is ATP generated during the light reactions? STEP 5

As electrons pass through the electron transport chain, protons (H+) are pumped into the thylakoid space, creating a proton gradient. The energy from this gradient drives ATP synthesis through chemiosmosis.

What happens when light strikes PS I? STEP 6

Light energy excites an electron in the P700 chlorophyll of PS I, and the electron is transferred to the primary electron acceptor of PS I.

What happens to the electrons after they are transferred to the primary electron acceptor in PS I? STEP 7

The electrons are passed through a second electron transport chain via ferredoxin (Fd).

How is NADPH formed? STEP 8

NADP+ reductase transfers electrons from ferredoxin (Fd) to NADP+, reducing it to NADPH.

How is ATP made in both chloroplasts and mitochondria?

Both use chemiosmosis, where protons move down their gradient through ATP synthase, driving the production of ATP.

What is the main difference in how chloroplasts and mitochondria use chemiosmosis?

Chloroplasts use light energy to generate a proton gradient and create ATP, while mitochondria use chemical energy from organic molecules.

How are protons pumped in the mitochondria?

Protons are pumped to the intermembrane space and drive ATP synthesis as they diffuse back into the mitochondrial matrix 

How are protons pumped in the chloroplasts?

Protons are pumped into the thylakoid space and drive ATP synthesis as they diffuse back into the stroma 

Mitochondria transfer chemical energy from…

Food to ATP

Chloroplasts transform….

Light energy to ATP

What happens to the pH in the thylakoid space and stroma during the light reactions?

The pH in the thylakoid space drops (more acidic), while the pH in the stroma increases (more basic), due to the proton gradient.

What do the light reactions produce?

The light reactions convert light energy into chemical energy in the form of ATP and NADPH, and produce O2 as a by-product.

What are photosystems?

Photosystems are large protein-pigment complexes in the thylakoid membrane, responsible for absorbing light energy during photosynthesis. They consist of a reaction-center complex and light-harvesting complexes.

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

The two types are Photosystem II (PS II) and Photosystem I (PS I). PS II comes first in the electron flow chain, followed by PS I.

hat role do the light-harvesting complexes play in photosynthesis?

They capture light energy and transfer it to the reaction-center complex, where it excites electrons to higher energy levels.