Chapter 11 - Photosynthesis

Question 1

Adoptions of leaves for photosynthesis

Answer 1

• large surface area - To absorb sunlight.
• Being thin - To keep the diffusion distance short.
• A transparent cuticle and epidermis - To allow light to enter the leaves.
• Chloroplast-packed mesophyll cells - To carry out photosynthesis.
• Stomata - For gas exchange
• Xylem and phloem - To transport reactants and products of photosynthesis between the leaves and the rest of the plant.

Question 2

Three main stages of photosynthesis

Answer 2

1) Capturing of light energy - This is done by pigments such as chlorophyll.
2) The light-dependent reaction - This is when light energy is converted into chemical energy.
3) The light-independent reaction - This is when sugars and other organic molecules are produced.

Question 3

Role of chloroplast

Answer 3

• absorb light energy to drive photosynthesis.
• To convert light energy into chemical energy.

Question 4

Structure of chloroplasts

Answer 4

• Thylakoids - contain pigments like chlorophyll in their membranes to absorb light for the light-dependent reaction.
• Grana - These are stacks of thylakoids.
• Lamellae - These are membranous extensions that connect thylakoids.
• Stroma - This is a fluid surrounding the thylakoids where the light-independent reaction occurs.
• Starch grains - These store sugars.
• Ribosomes - These are for protein synthesis within the chloroplast.
• Chloroplast DNA - This has genes that code for proteins involved in photosynthesis.

Question 5

Where did the light-dependent reaction occur

Answer 5

In the Thylakoid membranes inside the chloroplast

Question 6

Thylakoids adaptions for photosynthesis

Answer 6

• Large surface area - So they can contain more chlorophyll and electron carriers.
• ATP synthase channels - This allows the synthesis of ATP.
• Selectively permeable - This allows a proton gradient to be set up.

Question 7

Reactants and products of light dependent reaction

Answer 7

Reactants : water and light energy
Products : ATP and reduced NADP

Question 8

What is photophosphorylation

Answer 8

The process in which ATP is formed using light energy e.g light-dependent reaction is also known as photophosphorylation

Question 9

Prices of light dependent reaction (WARNING ITS A LOT)

Answer 9

1st stage - photoionisation and ETC (electron transport chain )
1) light energy is absorbed by chlorophyll in the stoma
2) Photoionisation occurs, as due to the absorption of light energy 2 electrons inside the chlorophyll become excited, causing the chlorophyll to become oxidized
3) electrons are transferred to an electron carrier protein, which causes the protein to be reduced
4) electrons are passed along the ETC releasing energy as they go

2nd Stage Photolysis of water
1) light is used to split water into electrons, protons and oxygen ( 2H2O —> 4H+ + 4e- + O2)
2) The electrons replace those lost from the chlorophyll during Photoionisation
3) oxygen gas is released as a waste products

3rd Stage chemiosmosis generating ATP and reduced NADP
1) the energy lost by the electrons along the ETC is used to pump protons across the thylakoid membrane into the thylakoid space
2) produces a proton gradient where there is a higher conc of protons inside the thylakoid space compared to the stroma
3) protons diffuse through the ATP synthase into the stroma
4) this movement catalyzes ATP synthase to produce ATP from ADP and water due to the change in structure of the enzyme when protons pass through it
5) NADP takes up the protons and electrons in the stroma to form reduced NADP
6) reduced NADP is then used for the light independent reaction

Question 10

What is the Chemiosmotic theory

Answer 10

The precise process in which ATP is produced

Question 11

Reactants and products of light-independent reaction

Answer 11

Reactants: carbon dioxide, ATP and reduced NADP from light-dependent reaction
Products: glucose and organic molecules

Question 12

Processes of light independent reaction or Calvin cycle (A LOT)

Answer 12

Stage 1 Carbon fixation
1) carbon dioxide reacts with a 5-carbon compound (5C), RubBP to form an unstable 6C compound
2) This 6C compound splits into 2 3C glycerate - 3 - phosphate (GP) molecules
3) This is catalyzes by the enzyme rubisco

Stage 2 reduction of GP
1) GP is reduced triose phosphate
2) this uses energy from the hydrolysis of ATP
3) this also requires protons and electrons from reduced NADP, which causes if to become oxidized to reform back into NADP
4) The NADP them returns back to the light dependent reaction

Stage 3 regeneration of RuBP
1) most triose phosphate is used to regenerate RuBP using ATP
2) the rest of the triose phosphate is used to make other organic molecules (e.g glucose, amino acids and lipids)

Question 13

How many times does the Calvin cycle have to undergo tomprodice one molecule of glucose and why

Answer 13

Six full turns of the Calvin cycle is needed to make one molecule of glucose
As for each turn of the Calvin cycle, five carbons are used to regenerate RuBP (5C) and only one carbon, from CO2 (1C), is available to make new organic compounds

Question 14

What is TP used for

Answer 14

To make:
- RuBP
- simple sugars (e.g glucose)
- Larger carbohydrates (e.g. starch, sucrose, and cellulose)
- Amino acids
- Lipids
- Nucleotides

Question 15

Requirements for photosynthesis

Answer 15

• Photosynthetic pigments (e.g. chlorophyll) - These absorb light energy.
• Carbon dioxide - This provides carbon to make glucose in Calvin cycle
• Water - This provides electrons and is a source of hydrogen ions (protons).
• Light - This provides energy to split water, to produce ATP, and to reduce NADP.
• A suitable temperature - This provides kinetic energy for reactions.

Question 16

Limiting factos for photosynthesis

Answer 16

• Light intensity
• carbon dioxide concentration
• temperature

Question 17

How does light intensity affect photosynthesis

Answer 17

• Low light intensity - This limits the light-dependent stage, so not much ATP and reduced NADP are produced. This slows the light-independent stage, and the rate of photosynthesis is low.
• Intermediate light intensity - This produces more ATP and reduced NADP in the light-dependent stage, which regenerates RuBP in the light-independent stage more quickly. More GP gets converted to TP, and the rate of photosynthesis increases.
• Very high light intensity - There is more light than needed, so another factor like temperature becomes limiting and the rate plateaus.

Question 18

How does carbon dioxide dioxide concentration afffect rate of photosynthesis

Answer 18

• Low CO2 concentration - This limits the light-independent reaction as less CO2 is fixed, reducing the production of GP and TP. The rate of photosynthesis is low.
• Intermediate CO2 concentration - This allows faster production of GP and TP, increasing the rate of photosynthesis.
• High CO2 concentration - Another factor like light becomes limiting.

Question 19

How does temperature affect photosynthesis

Answer 19

Low temperature - This provides little kinetic energy, slowing the enzyme-catalysed stages of photosynthesis, like the fixation of carbon dioxide controlled by rubisco. The rate of photosynthesis is low.
Intermediate temperature - This increases the kinetic energy supplied to these reactions, increasing the rate of photosynthesis up to an optimum point.
Very high temperature - This denatures enzymes, so the rate of photosynthesis drops sharply.

Question 20

What is the saturation point

Answer 20

The saturation point is when the factor being measured is no longer limiting the reaction and something else is the limiting facto

Question 21

What is the light compensation point

Answer 21

The light compensation point is when the volume of oxygen produced and carbon dioxide absorbed in photosynthesis is balanced by the oxygen absorbed and the carbon dioxide produced by respiration.

Question 22

Where does the light independent reaction take place

Answer 22

The stroma