Photosynthesis

Question 1

Chloroplast structure ?

Answer 1

• 2-10 μm diameter
• Surrounded by a double membrane envelope - each envelope membrane is phospholipid bilayer
• outer membrane is permeable to ions/small molecules
• inner membrane has transport proteins - allow certain molecules to enter/leave chloroplast

Question 2

What is stroma?

Answer 2

Fluid filled matrix in chloroplasts (CO2,sugars,enzymes/other molecules dissolved in stroma)

Question 3

What system of membranes are found in stroma?

Answer 3

Consists of a series of flattened fluid-filled sacs - THYLAKOIDS
(contain PHOTOSYSTEMS that contain pigments,enzymes,electron carriers)
- thylakoids stack up to form GRANA —> CREATE LARGE SA
GRANA connected by stroma lamellae

Question 4

What is a photosystem?

Answer 4

System of photosynthetic pigments found in thylakoids of chloroplasts
- different pigment molecules arranged in funnel-like structures in thylakoid membrane
- large no. Pigment molecules in an arrangement —> MAX LIGHT ABSORBED

Question 5

What does the stroma also contain?

Answer 5

70S ribosomes- proteins coded for by the loop of DNA produced at 70S ribosome
Loop of DNA- codes for some chloroplast proteins
Starch grains - sugars formed during photosynthesis stored as starch in starch grains

Question 6

Where does each stage of photosynthesis take place?

Answer 6

Light dependent - thylakoids membranes of the GRANA
Light independent - in stroma

Question 7

Adaptations of chloroplasts to photosynthesis?

Answer 7

STROMA : gel-like fluid contains ENZYMES that catalyse reactions of light independent stage
- rapid transport of products from light dependent stage as surrounds GRANA/thylakoid membranes

GRANA: Granal stacks create LARGE SA-> more photosystems can be present —> max light absorption
- more space for electron carriers/ATP synthase enzymes

DNA: chloroplast DNA (cpDNA) has genes that code for some proteins/enzymes used in photosynthesis

Ribosomes: allows translation of proteins coded by cpDNA

Inner membrane of chloroplast envelope: selective transport proteins control flow of molecules between stroma/cytoplasm of plant cell (cytosol)

Question 8

2 types of pigment?

Answer 8

Chlorophylls (a/b)- absorb blue-violet/red regions of light spectrum
- reflect green
ACCESSORY PIGMENT: Carotenoids - absorb wavelengths in blue-violet region
- reflect yellow/orange

Question 9

Why are accessory pigments useful?

Answer 9

They surround primary pigment + Absorb both similar/different wavelength of light to chlorophyll
- expands the wavelength range that can be absorbed for use in photosynthesis

Question 10

What are the different photosystems?

Answer 10

Photosystem 1 (PSI) - P700
chlorophyll a which has a max absorption of light at 700nm

Photosystem 2 (PSII)- P680
Chlorophyll a has max absorption of light at 680nm

Question 11

Stages of light dependent stage of photosynthesis?

Answer 11

1. Light harvesting at photosystems
2. Photolysis of water - light energy used to breakdown water —>
3. ATP produced in photophosphorylation
4. Reduced NADP (NADPH) produced when H+ ions in the stroma + electrons in ETC combine with carrier molecule, NADP

Question 12

What is photophosphorylation?

Answer 12

Using light energy/electron transport chain to phosphorylate ADP to ATP

Question 13

What is photolysis?

Answer 13

Splitting of water into H+ ions, electrons, oxygen in thylakoid lumen
2H20 —> 4H+ + 4e- + O2

Question 14

Non cyclic photophosphorylation? 1-3

Answer 14

1. Light absorbed by PSII
2. 2 electrons in chlorophyll are excited to higher energy level + emitted from chlorophyll molecule —> PHOTOIONISATION
3. Electron passes down electron transport chain to PSI (chain of electron carriers (proteins with Fe at centre)

Question 15

Non cyclic photophosphorylation 4-6?

Answer 15

4. As excited electrons leave PSII to PSI, they are replaced by electrons from photolysis
   5.Excited electrons lose energy as they pass along ETC - this energy used to transport protons into thylakoid via proton pumps
   Thylakoid has higher conc of protons than stroma- PROTON GRADIENT
5. Protons move down their conc grad into stroma via enzyme ATP synthase - chemiosmosis
   Energy from this movement combines ADP + Pi —> ATP

Question 16

Non cyclic photophosphorylation 7-8?

Answer 16

7. Light energy absorbed by PSI, excites electrons to even HIGHER energy level
8. Electrons transferred to NADP with a proton (H+ ions from stroma) to form reduced NADP

2H+ + 2e- + NADP —> NADPH

Question 17

Cylic photophosphorylation?

Answer 17

INVOLVED PSI ONLY
Light absorbed by PSI
Pair of electrons gain energy/become excited to higher energy level
Electrons pass down electron transport chain and passed back to PSI
As electrons travel along electron carriers, small amount of atp generated (BUT NO PHOTOLYSIS OCCURS SO NO proton,oxygen, NADPH produced)
- energy used to transport H+ from stroma to thylakoid lumen via PROTON PUMP
- build up of proton in thylakoid used to **synthesis ATP from ADP+ Pi **(in chemiosmosis)

Question 18

How does cylic and non cyclic photophosphorylation differ?

Answer 18

Cyclic only involved PSI , non clyclic involved both PS1/PS11
Cyclic does not produced reduced NADP (non cylic does)

Question 19

Which products of the light dependent stage are needed for light independent reaction and what does this stage produce?

Answer 19

Energy from ATP and hydrogen from REDUCED NADP used for light independent reaction (Calvin cycle)
Needed to produce carbohydrates and other organic molecules :
Starch - storage
Sucrose - translocation around plant
Cellulose- make cell walls

Question 20

Why cant light independent stage continue indefinitely in darkness?

Answer 20

Requires inputs of ATP and reduced NADP

Question 21

Stages of Calvin cycle?

Answer 21

1. CO2 combines with RuBP (5C) - catalysed by enzyme RuBisCo
   By accepting COO- group RuBP becomes 6C unstable intermediate , which breaks down
2. Product of this is 2x GP (3C) - CO2 has now been fixed
3. hydrogens from NADPH and ATP (from light dependent stage) used to reduce GP —> TP
4. 1/6 of TP molecules used to produce useful organic products needed by the plant (e.g glucose)
5. 5/6 of TP molecules used to regenerate RuBP —> NEED ATP FOR THIS
   TURNS 6 TIMES TO MAKE ONE HEXOSE SUGAR(needs to produce 2xTP molecules)
   - 18 ATP and 12 NADPH needed from light dependent reaction

Question 22

Uses of TP?

Answer 22

• TP can condense to become hexose phosphates (6C) —> used to produce starch,sucrose,cellulose
• TP can be converted to glycerol/GP can be converted to fatty acids (MOLECULES JOIN TO FORM LIPIDS FOR CELL MEMBRANES)
• TP used in production of amino acids for protein synthesis
• TP recycles to regenerate RuBP

Question 23

Limiting factors of photosynthesis? What are limiting factors?

Answer 23

Factors that Prevents rate from increasing when at a lower level
CO2 Supply
Light
Temperature

Question 24

How does rate of photosynthesis depend on light intensity (given CO2/temp are constant)?

Answer 24

Rate of photosynthesis increases as light intensity increase
- greater light intensity, more energy supplied to the plant , so faster light dependent stage
Produces more ATP/reduced NADP for Calvin cycle , so light independent can also happen at greater rate

Question 25

What happens if light intensity continues to increase ?

Answer 25

Rate of photosynthesis will plateau - no longer a limiting factor of photosynthesis

Question 26

How does CO2 conc affect photosynthesis (if light/temp are constant)?

Answer 26

**The rate of photosynthesis increases as carbon dioxide concentration increases** - needed for **light independent** stage of photosynthesis (carbon fixation) - so the more CO2 , the **faster this step** is in the Calvin cycle —> **faster rate of photosynthesis** - Trend continues unless another factor is in limit/prevent rate from increasing

Question 27

How does temperature affect photosynthesis (if light/CO2 are constant)?

Answer 27

**As temperature increases the rate of photosynthesis increases, as the reaction is controlled by enzymes** - trend only continues to certain temp as ***enzymes will denature**/rate of reaction will decrease TEMP affects **CALVIN CYCLE as enzyme controlled** (RuBisCO) Increasing temp can also cause: - **stomata to close** to reduce water loss —> CO2 cannot enter leaves —> photosynthesis slows down - **Membrane permeability** - dissipation of proton gradients in thylakoid (for light dependent) —> SLOWER

Question 28

Why does temp not have much effect on light dependent reactions?

Answer 28

As thye are driven by energy from light instead of KE of reacting molecules

Question 29

What is the effect of light intensity on GP,TP and RuBP levels?

Answer 29

LIGHT : decrease in light intensity causes **DECREASE IN TP/RuBP conc + slight INCREASE IN GP** -Less light so **light dependent stage STOPS**/ doesn’t form **ATP and NADPH for LI STAGE** -So **GP builds up** as **not converted to TP** -Lack of TP means **RuBP wont form** -**Fixation of CO2 will stop**/conc of GP will **plateau**

Question 30

What is the effect of CO2 on GP,TP and RuBP levels?

Answer 30

Low CO2 CONC - **decreases conc of GP + TP, increase RuBP conc** RuBP accepts CO2 so when lack of CO2, it **remains unfixed/builds up*** Lack of carbon fixation **prevents GP/TP from forming**

Question 31

How are limiting factors balanced in agriculture ?

Answer 31

**Glasshouses** - sensors monitor **light intensity , humidity, CO2 conc** So they grow through **night /all year round** **Computer** manages all factors/adjustable to maximise photosynthesis rate **Pesticides/natural pests** used to MAX YIELD Must find balance between cost/environment /yields

Question 32

Differences between photosynthesis and respiration?

Answer 32

- ATP produced by **photophosphorylation** in photosynthesis but produced by **substrate level and oxidative phosphorylation** in respiration - Occurs in **chloroplasts** , aerobic in **mitochondria**, anaerobic in **cytosol/cytoplasm** - **NADP vs NAD** used - **ETC** involved in photosynthesis/aerobic , **not in anaerobic**

Question 33

How many TP molecules are made in relation to CO2 molecules in Calvin cycle?

Answer 33

2 molecules of TP made for every CO2 molecule used