Introduction To Photosynthesis

Q: What do some organisms do to make their own food?

A: They make their own food by using energy from the sun.

Q: What are organisms make their own food by using energy from the sun called?

A: Photoautotrophs.

Q: What do photoautotrophs do?

A: Convert light energy into chemical energy, which can then assemble organic materials.

Q: What are the two stages of photosynthesis?

A: Light-dependent stage and Light-independent stage.

Q: What happens in the light-dependent stage?

A: Photons of light are absorbed.

Q: What happens in the light-independent stage?

Does not use light directly

Uses products of light-dependent reactions

Q: Where do both stages occur?

A: In the chloroplasts.

Q: How many membranes does a chloroplast have?

A: Three; Outer, Inner, Thlyakoid Membrane 

Q: What is the function of the outer membrane?

A: Covers the entire surface of the organelle.

Q: What does the inner membrane contain?

A: The stroma, where chemical reactions (e.g. Calvin cycle) take place.

Q: What happens in the thylakoid membrane?

A: Light-dependent reactions occur; produces chlorophyll and carotenoids (pigments).

Q: What happens when a photon of light strikes chlorophyll?

A: An electron becomes “excited” and jumps to a higher energy level.

Q: From where to where does the electron move?

A: From ground state (lower level) to a higher energy level.

Q: What must be true for energy to be absorbed?

A: The energy difference between the ground and excited state must be equivalent.

Q: What happens if it is not equivalent?

the energy difference between the energy

level of a ground state to the energy level of the

excited state must be equivalent

If it is not, no energy will be absorbed

Q: What are the three possible outcomes once energy is absorbed?

Q: What are the three possible outcomes once energy is absorbed?
1⃣ The excited electron returns to its ground state → releases thermal/fluorescent energy.
2⃣ Energy is transferred to a neighboring pigment → new pigment excited, original returns to ground state.
3⃣ Excited electron is accepted by a primary electron acceptor → continues photosynthesis process.

Q: What are the two types of chlorophylls?

A: Chlorophyll a and Chlorophyll b.

Q: What does chlorophyll a do?

• Becomes oxidized during photosynthesis

• Donates an electron to the primary electron acceptor

• Produces green pigment

Q: What does chlorophyll b (carotenoids) do?

• Known as an accessory pigment

• Transfer excited energy to chlorophyll a

• Produces orange, yellow and red pigments

Q: What is the antenna complex?

A: A cluster of light-absorbing pigments embedded in the thylakoid membrane.

Q: What does the antenna complex do?

A: Captures and transfers energy to chlorophyll a and other molecules in the reaction center. This is the area where the primary electron acceptor is found

Q: What is found in the reaction center?

A: The primary electron acceptor.

Absorption Spectrum Q: Do pigments absorb all wavelengths of light?

A: No, only certain ones.

Q: What happens to colours not absorbed?

Colours that are not absorbed are either

transmitted through the object or reflected

Q: What gives an object its colour?

A: Reflected light.

Q: What is the absorption spectrum?

A: A plot showing the amount of light energy of various wavelengths that a substance absorbs.

Absorption Spectrum

A pigment does not absorb all the wavelengths

of light

• Colours that are not absorbed are either

transmitted through the object or reflected

• Reflected light is what gives off a distinctive colour

Absorption Spectrum: a plot of the amount of

light energy of various wavelengths that a

substance absorbs

Q: What is a spectrophotometer?

A: A device used to measure the absorption spectrum.

Q: How do pigments behave with light?

A: Each pigment strongly absorbs some colours and reflects/transmits others.

Q: Which colours does chlorophyll a absorb and reflect?

A: Absorbs blue and red; reflects green.

Q: How is pigmentation often reached?

A: From the combination of both chlorophyll a and b.

Q: Where are pigments found?

A: Within the photosystems of the plant.

Q: How many pigments surround each antenna complex?

A: Around 250–400 pigments.

Q: What happens when photons strike the reaction centre?

A: Chlorophylls are excited and convert light energy to chemical energy.

Q: What happens to the electrons?

A: Transferred to the primary electron acceptor, then move to the ETC (Electron Transport Chain).

What happens in a photosystem?

•  Photons of light strike the reaction centre

• The chlorophylls are excited

• Converts light energy to chemical energy

• Electrons are transferred to the primary electron acceptor

• Moves to ETC

Q: How many types of photosystems are there?

2

Q: Describe Photosystem I.

A: Contains P700 nm chlorophyll a; absorbs light at 700 nm wavelength.

Q: Describe Photosystem II.

A: Contains P680 nm chlorophyll a; absorbs light at 680 nm wavelength.

Q: Which photosystem occurs first?

A: Photosystem II occurs before Photosystem I.