Chapter 11 - Photosynthesis

Question 1

What is the equation for photosynthesis?

Answer 1

Carbon dioxide + water = glucose + oxygen

Question 2

What are autotrophs?

Answer 2

Plants that use light energy from the sun and is transferred into chemical energy within organic molecules

Question 3

When molecules are described as organic, what does this mean? Give the 4 main examples

Answer 3

They are carbon containing

Nucleic acids
Protein
Carbohydrates
Lipids

Question 4

List the main structures of the internal leaf cell (top to bottom)

Answer 4

Waxy cuticle, upper epidermis, palisade mesophyll, spongy mesophyll with air spaces, vascular bundle (xylem and phloem), lower epidermis, stomata and guard cells

Question 5

List 3 macroscopic adaptations of the leaf

Answer 5

1. Large SA:V ratio as it is FLAT to absorb as much sunlight as possible
2. Shorter diffusion pathways for CO2 and O2 as it is thin
3. Leaf arrangement on plants avoids shadowing so all leaves receive sunlight

Question 6

List some microscopic adaptations of the leaf cell

Answer 6

• Waxy cuticle and upper epidermis is transparent to let light in + reduce water loss
• turgid cells to absorb more light as plant is upright
• lots of chloroplasts in palisade mesophyll to absorb sunlight
• stomata open/ close in response to light to carry out photosynthesis
• air spaces so O2 and CO2 can diffuse quickly
• xylem brings water in and phloem carries produced sugars away

Question 7

Where is chlorophyll found in chloroplast?

Answer 7

Embedded in the thylakoid membrane

Question 8

What are the 3 main stages of photosynthesis?

Answer 8

1. Capturing of light energy by pigments in thylakoid membrane
2. Light- dependant reaction involves light energy into chemical energy (in thylakoids membrane)
3. Light- independent reaction where sugars and organic molecules are produced (in stroma)

Question 9

Step 1

Why are there different pigments in thylakoid membranes?

Answer 9

Different pigments are specialised to absorb different wavelengths of light from the sun.

This maximises photosynthesis by being able to absorb more wavelengths for more growth

Question 10

Exam Tip

What should be mentioned when describing pigment graphs?

Answer 10

• State wavelength values not colours
• Mention the ‘peaks’ value
• Mention values they also DON’T absorb

Question 11

What is the action spectrum?

Answer 11

A graph that shows the rate of photosynthesis at different wavelengths of light.

• usually compared to absorption spectrum

Question 12

Why is having a stack of thylakoids better than a long single one?

Answer 12

Larger surface area of thylakoid membrane when in stacks.

This means more pigments are embedded in the thylakoid membrane for light absorption.

Therefore there is a faster rate of photosynthesis which allows more growth.

Question 13

Chromatography Core Practical

Why use propane solvent?

Answer 13

It is organic which means that the hydrocarbon and hydrophobic tail of the pigments will dissolve better in the solvent

Question 14

Chromatography Core Practical

Why do you mark the origin line with pencil and not pen?

Answer 14

The ink pigments in pen would mix with the leaf pigments.

Pencils have no pigments so they do not mix

Question 15

Chromatography Core Practical

Why is the running solvent added to below the origin line?

Answer 15

Running solvent will move up the TLC paper via capillary action.

When it gets to the origin line, it dissolves the pigment and therefore the pigment moves up solvent.

Question 16

Chromatography Core Practical

When is the chromatography stopped?

Answer 16

When the pigments get to the solvent frontline, take the TLC paper out so RF values can be calculated

Question 17

Describe the process of the light dependant reaction [7]

Answer 17

1. Light hits the photo system and chlorophyll a absorbs the light
2. Light excites 2 electrons in chlorophyll a which are then lost from chlorophyll
3. Electrons move along the electron transfer chain releasing energy
4. This energy is used to power proton pumps in order to set up an electrochemical gradient in the thylakoid lumen
5. Energy from movement of protons is used to phosphorylate ADP into ATP
6. NADP+ is reduced by electrons and protons to form NADPH
7. Photolysis of water splitting to produce 2 protons, 2 electrons and oxygen whereby 2 electrons are used to replace those lost from chlorophyll

Question 18

What is photoionisation?

Answer 18

Step 2 - when chlorophyll is left positively charged as the 2 electrons are lost

Question 19

What is chemiosmosis?

Answer 19

Protons move down electrochemical gradient through ATP synthase which results in production of ATP from ADP and Pi

Question 20

What is photophosphorylation?

Answer 20

Phosphorylation (adding of phosphate group) to ADP to from ATP USING LIGHT

Question 21

What is the terminal electron acceptor?

Answer 21

In LDR it is the NADP+
- final acceptor of electrons

Question 22

Describe the steps within the light independent reaction

Answer 22

(In stroma)

1. CO2 combines with RuBP
2. This produces 2 GP using rubisco enzyme
3. GP is reduced to triose phosphate
4. Using NADPH
5. Also using energy from ATP
6. Triose phosphate is converted to glucose/ RuBP

Question 23

CORE PRACTICAL 8

What is the purpose of the hill reaction?

Answer 23

Shows that both chloroplasts and light are required for photosynthesis

Chloroplast - site
Light - to energise electrons

Question 24

CORE PRACTICAL 8

What is DCPIP?

Answer 24

An artificial electron acceptor in place of NADP+

Question 25

How does DCPIP change when there is chloroplasts suspension and light?

Answer 25

Blue - Colourless/ green

The light excites electrons in the chloroplast which begins the light dependant reaction. The electrons are then emitted from chloroplast into the DCPIP solution which then becomes reduced. This makes it become colourless and reduced DCPIP is colourless.

It will stay green because of the chloroplasts

Question 26

Describe how light is a limiting factor in terms of the graph.

Answer 26

As light intensity increases so does the rate of photosynthesis. It then plateaus because another factor becomes limiting e.g. temperature

Question 27

How does the concentrations of RuBP, TP and GP change in the dark?

Answer 27

• GP accumulates and TP and RuBP levels decrease
• this is because the reduction of GP to triose phosphate requires ATP and reduced NADP from the light dependant reaction
• the formation of triose phosphate is prevented and regeneration of RuBP cannot take place

Question 28

How is temperature a limiting factor in photosynthesis?

Answer 28

• As temperature increases, so does the rate of photosynthesis because molecules have more kinetic energy so more collisions occur
• then reaches optimal temp
• beyond this point rate decreases because enzymes bonds break so active site changes shape = denaturation

Question 29

Which process does temperature affect?

Answer 29

Light independent reaction - as enzymes are involved so they denature

No effect on LDR

Question 30

How is CO2 a limiting factor in photosynthesis?

Answer 30

• as CO2 levels increase so does rate of photosynthesis
• then plateaus as light or temperature becomes limiting factor

Question 31

Explain how chemicals which inhibit the decolorisation of DCPIP could slow growth of weeds [2]

Answer 31

Less ATP produced ;
Less reduced NADP produced ;
So less GP reduced to triose phosphate ;