16: Photosynthesis

Question 1

Give 1 key environmental factor that could affect enzyme activity and therefore rate of photosynthesis:

Answer 1

Temperature:
- increases KE
- increases movement speed of molecules
- leads to more collisions
- more enzyme-substrate complexes form

Question 2

Where does Photosynthesis take place?
Where does the Light-Dependent Reaction occur?
Where does the Light-Independent Reaction occur?
Which molecule absorbs the light?

Answer 2

1. in the chloroplast of cells in leaves and other green areas of plants
2. thylakoid membrane of chloroplast (which contains tightly packed chlorophyll molecules)
3. stroma of chloroplast
4. pigment called chlorophyll

Question 3

What other organisms can photosynthesise?
What are the benefits of photosynthesis?

Answer 3

1. Algae and cyanobacteria
2. • respires some of the glucose to provide them with energy for metabolic reactions
   • excess organic compounds can be stored or used to make other substances such as cellulose, proteins or lipids

Question 4

Describe the Light-Dependent Reaction:

Answer 4

1. light is absorbed by the chlorophyll molecules
2. a pair of electrons become gain energy (become excited) and become so energetic that they leave the chlorophyll molecule (this is photoionisation)
3. a molecule of water is split into protons, electrons and oxygen in a process called photolysis
4. electrons produced by photolysis are used to replace those lost in the chlorophyll by photoionisation
5. electrons are now passed along a series of electron carrier proteins forming an electron transport chain in the thylakoid membranes
6. via a series of redox reactions, electrons release energy they gained from light at each stage
7. this energy is used to actively pump protons through the thylakoid membrane into the lumen of the thylakoid
8. the protons now form a proton concentration gradient between the lumen of the thylakoid and the stroma of the chloroplast, (the protons can only diffuse back into the stroma through special protein complexes, containing enzyme ATP synthase)
9. as the protons pass through, these enzyme complexes causes the enzyme to attach an inorganic phosphate group to ADP molecules to form ATP (photophosphorylisation)
10. protons along with the electrons released at the end of the electron transfer chain react with NADP, reduced NADP is formed

Question 5

Which 2 molecules are produced in the Light-Dependent Reaction are required in the Light-Independent Reaction?
Why?

Answer 5

1. reduced NADP: provide hydrogen necessary for the reduction reaction of GP to triose phosphate
2. ATP provides energy for this reaction

Question 6

Describe the Light-Independent Reaction:

Answer 6

1. CO2 binds with a five carbon molecule called RuBP to form two molecules of glycerate 3-phosphate, this reaction is catalysed by enzyme RuBisCo
2. ATP and reduced NADP produced in Light-Dependent Reaction are used to reduce the 2 x GP into 2 x triose phosphate
3. 1/6 of the 2 x triose phosphate (3C) is converted into carbohydrates like glucose to be converted into other useful organic substances e.g amino acids
4. 5/6 is used to regenerate RuBP

Question 7

Describe 4 limiting factors of photosynthesis rate:

Answer 7

1. CO2 concentration: provides carbon to synthesis organic molecules
2. Light Intensity: provides energy to excite electrons to produce ATP
3. Temperature: provides kinetic energy
4. Water Availability: provides H+ for reducing NADP

Question 8

What are the Benefits of producing crops in glasshouses?

Answer 8

• achieve higher yields
• allows crops to be grown out of season and so provides a better economic return
• plants could be grown in regions where they would not naturally grow

Question 9

How can Glasshouses increase crop yield?

Answer 9

• growers need to control limiting factors inside commercial glasshouses
• faster rates of photosynthesis, the more carbohydrates the plants can make
• more carbohydrates means more energy and resources available, which leads to increased yield

Question 10

Describe functions of the different structures in the chloroplast for photosynthesis:

Answer 10

· LDR occurs in thylakoids. Chlorophyll molecules in thylakoid membrane absorbs light energy. ETC also embedded in thylakoid membrane

· Grana (thylakoid stacks) increase the efficiency of LDR by capturing most of the light energy that enters the chloroplast

· Stroma: contains enzymes required for LIR

· Outer membrane controls movement of substances into and out of the chloroplast

· Starch grains store excess carbohydrate made in photosynthesis (LIR)

· Circular DNA (with no introns and not associated with histones) and 70S ribosomes, like in prokaryotes.