Session 2: Photosynthesis

Question 1

What is chemiosmosis?
and what is phosphorylation?

Answer 1

C –> movement of protons to create a gradient
P –> addition of a phosphate

Question 2

What is chlorophyll and where is it found?

Answer 2

It is the main photosynthetic pigment but it does have smaller assosiated pigments (e.g. carotenoids). It is found in the chloroplasts.

Question 3

Describe visible light.

Answer 3

Overall, visible white light is made up of colours with their own individual wave lengths, and what we see depends on what colours are absorbed or reflected. Reflected colours are the ones we see.

Question 4

What pigments are used for light absorption?

Answer 4

chlorophyll a, chlorophyll b, and carotenoids

Question 5

Describe the electromagnetic spectrum.

Answer 5

First of all, light is a form of energy known as electromagnetic radiation. Visible light is between 380 and 750 nanometres and visible light is what drives photosynthesis. Blue light is closer to 380 nm, and has higher energy. Red light is closer to 750 nm, and has higher wavelength.

Question 6

Describe photosynthetic pigments overall and their role in photosynthesis.

Answer 6

Biological pigments absorb visible light, they are chlorophylls or carotenoids. All pigments have different absorption spectrums which provide information about their role in photosynthesis since light can only perform work if it is absorbed.

Question 7

Describe the photosynthetic pigments in detail.

Answer 7

Chlorophylls: absorb red and blue-violet light, they are the main photosynthetic pigment in plants, and give the green colour because green wavelengths are not absorbed.
Carotenoids: absorb mainly blue-violet light and appear orange, yellow, or red because those are not absorbed. Carotenoids are considered to be associate pigments.

Question 8

What is it and what factors affect the photosynthetic rate?

Answer 8

The photosynthetic rate is the rate at which plants make food. The main factors are the amount of light available, level of carbon dioxide, and temperature (mainly in relation to enzyme reactions)

Question 9

Overall what is photosynthesis and where does it occur?

Answer 9

The conversion of carbon dioxide and water into glucose and oxygen using sunlight. It occurs in membranous organelles called chloroplasts found in plants, algae, and some bacteria.

Question 10

Describe chloroplasts.

Answer 10

Chloroplasts are responsible for photosynthesis in plants and so have a complex structure of stroma, thylakoids, grana, lamella, and a double membrane. Stroma contain enzymes and a suitable pH for the Calvin cycle, it is a supportive tissue similar to cytoplasm. Thylakoids are flattened disks with small lumen for chemiosmosis (it maximises the electrochemical gradient upon proton accumulation) and they are also the site of light dependent reactions. Grana or granum are thylakoids arranged in stacks to increase the SA:V ratio. And lamella connect and separate thylakoid stacks.

Question 11

What are light dependent reactions? Where do they occur and what are the steps? How are the products of these reactions used?

Answer 11

Light dependent reactions convert light energy into chemical energy (ATP and NADPH). They occur in the intermembrane spaces of thylakoids. There are three main steps photoactivation (excitation of light absorbing pigments), photophosphorylation (ATP formed via the electron transport chain), and photolysis of water (formation of NADPH). Products are used by light independent reactions.

Question 12

What are photosystems?

Answer 12

Pigments are clustered in groups called photosystems because photosynthetic plants do not rely on a single pigment to absorb light, but rather use the combined action of many.

Question 13

Describe the actions of pigment molecules within photosystems. Reference photosystems I and II.

Answer 13

When a pigment molecule is excited by light it will release electrons. Accessory pigments then transfer energised electrons to a central reaction centre containing chlorophyll a. Energised electrons are released from photosystems. Photosystem II comes first, it released electrons into an electron transport chain (makes ATP). Photosystem I releases electrons to reduce a hydrogen carrier (makes NADPH)

Question 14

Describe the process of Photosystem II.

Answer 14

Electrons enter the electron transport chain (ETC), then lose energy as they are shuttled between electron carriers. This energy is used to pump protons into the thylakoid lumen from the stroma, which then pass through ATP synthase to produce ATP.
The protons pumped into the thylakoid make a gradient that produces ATP via chemiosmosis (the process of moving protons to the other side of a biological membrane).

Question 15

Describe the process of Photosystem I.

Answer 15

De-energised electrons from Photosystem II replaced the electrons lost in photosystem I (fuel it). Electrons from photosystem I may be used to reduce NADP and create NADPH which is required for light independent reactions.
Electrons lost from Photosystem II are replaced using the photolysis of water and formation of oxygen.

Question 16

What is the difference between cyclic and non-cyclic photophosphorylation?

Answer 16

Non-cyclic: electrons are not recycled/it is not a cycle (e.g. PSI and PSII).
Cyclic: electrons are recycled and water is not needed (e.g. PSI can independently make ATP if its electrons are recycled and NADPH is not produced).
Cyclic photophosphorylation is useful for generating stocks of ATP during daylight, however it cannot be used to synthesise organic molecules.

Question 17

What are light independent reactions? Where do they occur and what are the steps?

Answer 17

Light independent reactions are where ATP and NADPH are converted to carbon compounds. Light independent reactions are also referred to as the Calvin Cycle and they occur in the stroma of the chloroplast. LI reactions involve carbon fixation (carboxylation of RuBP), reduction of glycerate-3-phosphate (GP), and regeneration of RuBP. The Calvin Cycle must occur twice to synthesise one full molecule of a hexose sugar, e.g. glucose.

Question 18

Describe the first step of the Calvin Cycle: Carbon Fixation.

Answer 18

1. The enzyme Rubisco catalyses the attachment of CO2 to RuBP (1 carbon + 5 carbon molecules).
2. The 6 carbon compound formed is unstable and breaks down into two 3 carbon molecules called GP molecules.

Question 19

Describe the second step of the Calvin Cycle: Reduction of GP.

Answer 19

The two GP molecules are converted into alternative 3 carbon compounds called triose phosphate (TP) - this restructuring is mediated by ATP and NADPH.

Question 20

Describe the third step of the Calvin Cycle: Regeneration of RuBP.

Answer 20

1. One of the TP molecules is used to produce carbon compounds (carbohydrates).
2. The remaining TP molecules are used to regenerate RuBP stocks (which requires ATP).

Question 21

Summarise Photosynthesis.

Answer 21

Solar energy/light, ADP, NADP, and water fuel the light dependent phase which produces oxygen, ATP and NADPH2. Light independent phase produces the main product of glucose and also water, ADP and NADP, it is also fuelled by CO2, ATP, and NADPH2.

Question 22

What are the absorption spectrum and the action spectrum?

Answer 22

The absorption spectrum deals with wavelengths of light absorbed by each pigment, where the action spectrum tells us the overall rate of photosynthesis at each wavelength of light.