Photosynthesis

Question 1

Describe the method of studying the density of stomata

Answer 1

1. Apply clear nail polish to the lower epidermis of a leaf between the veins and allow it to dry
2. Using forceps, peel the nail polish from the leaf to produce a replica of the lower epidermis
3. Place the replica on a microscope slide and count the number of stomata using the microscope

Question 2

Other than the presence of stomata, give 3 adaptations of the leaf for photosynthesis

Answer 2

1. Large surafce area: absorbs as much light as possible
2. Thin: light penetrates all levels
3. Air spaces in the spongey mesophyll: allows CO2 to diffuse into photosynthesising cells

Question 3

Biochemical reactions in cells often involve cyclic metabolic pathways which occur in compartments (such as chloroplasts) inside cells. Why is this important?

Answer 3

• Reactions can occur in small volume/ isolation of enzymes/reactants
• Membranes isolate reactions within organelles, keeping products seperate from the cytoplasm
• Mitochondria can be positioned close to areas needing ATP, and organelles ca move within the cytoplasm

Question 4

State the location and function of the light dependent stage of photosynthesis

Answer 4

Thylakoid membrane and thylakoid space of chloroplast
Involves the conversion of light energy into chemical energy (ATP and NADPH)

Question 5

State the location and function of the light independent stage of photosynthesis

Answer 5

Stroma of chloroplast
Uses the prducts of the light dependent reaction (ATP and NADPH) to produce organic molecules such as glucose

Question 6

Where are photosynthetic pigments located?

Answer 6

Within the thylakoid membranes of chloroplasts

Question 7

During photosynthesis, different wavelengths of light strike the leaf and are absorbed by varous pigments. What is the role of the pigments?

Answer 7

Capture light energy from the sun and convert it into chemical energy

Question 8

What are the 2 main types of photosynthetic pigments in flowering plants?

Answer 8

Primary pigments and accessory pigments

Question 9

What is the primary pigment and which wavelengths does it absorb?

Answer 9

Chlorophyll a absorbs blue and red wavelengths of light

Question 10

What are the secondary pigments and which wavelengths do they absorb?

Answer 10

• Chlorophyll b absorbs blue and red wavelengths of light
• Carotenoids absorb violet/blue light (xanthophyll & carotene)

Question 11

Why are accessory pigments important?

Answer 11

They absorb wavelengths of light not absorbed by the primary pigments
This ensures a wider range of wavelengths are absorbed, increasing the efficiency of photosynthesis

Question 12

What macronutrient is needed to produce chlorophyll a?

Answer 12

Magnesium ions

Question 13

Describe how chromatography can be used to separate photosynthetic pigments

Answer 13

1. Tear up leaves and grind with acetone to form a dark green pigment solution
2. Use capillary tube to spot the pigment on to chromatography paper
3. Place the chromatography paper into a solvent
4. Remove the chromatography paper once the solvent has travelled to the top of the strip. Observe the different distances the pigments have travelled
5. Calculate the Rf value for each pigment
6. Identify each pigment by comparing Rf values to known values

Question 14

Why do we add acetone when grinding leaves during chromatography?

Answer 14

Acetone dissolves the phospholipid membrane to extract the pigment

Question 15

How do you calculate the Rf value?

Answer 15

distance travelled by the pigment/distance travelled by solvent

Question 16

Where are photosystems located?

Answer 16

On the thylakoid membranes of chloroplasts

Question 17

What is the role of photosystems?

Answer 17

• Absorb photons of light energy
• Act as transducers, transferring this light energy to high energy electrons
• This energy is then used to fuel proton pumps and synthesise ATP

Question 18

Describe the photosystem structure

Answer 18

Accesssory pigments are grouped into clusters with associated proteins, forming an antenna complex
These pigments absorb photons of light energy and funnel this energy down the antenna complex to the reaction centre
Chlorophyll a molecules are found in the reaction centre, below the antenna complex
When photons of light energy reach chlorophyll a, electrons are excited to become high energy electrons

Question 19

At what wavelength does photosystem I best absorb light?

Answer 19

700nm

Question 20

At what wavelength does photosystem II best absorb light?

Answer 20

680nm

Question 21

What 6 things does the light dependent reaction involve?

Answer 21

• Requires light energy
• Takes place in the thylakoid membranes and thyakoid space
• Involves the synthesis of ATP from ADP and Pi (via photophosphorylation)
• Involves the splitting of water using light (photolysis)
• Involves the reduction of NADP to form redNADP
• Releases oxygen gas as a by product

Question 22

What are the 2 light dependent reactions in higher plants?

Answer 22

• Non-cyclic photophosphorylation
• Cyclic photophosphorylation

Question 23

Describe what happens at photosystem II during non-cyclic photophosphorylation

Answer 23

• Photons of light are absorbed by PSII and passed to chloropyll a in the reaction centre
• This excites 2 electrons on chlorophyll a to a higher energy level, causing them to be emitted, passed to electron acceptors and then transferred along a chain of carriers (including a proton pump) to photosystem I
• As the electrons move along the chain, they release energy, which is used to pump hydrogen ions (protons) from the stroma across the thylakoid membrane into the thylakoid space
• This creates an elecrochemical gradient due to the high H+ concentration in the thylakoid space and low concentration in the stroma
• H+ flow down this proton gradient, through a proton channel connected to ATP synthase providing energy for the formation of ATP from ADP and Pi

Question 24

Describe when happens at photosystem I during non-cyclic photophosphorylation

Answer 24

• Photons of light energy are absorbed by PSI, causing 2 electrons in chlorophyll a to be raised to a higher energy level, emitted, and passed to another electron acceptor
• The elecron acceptor transfers the electrons to protons outside the thylakoid membrane into the stroma, which then reduce NADP to NADPH
• The use of H+ to reduce NADP lowers their concentration in the stroma, helping to maintain the electrochemical gradient between the thylakoid space and the stroma
• The emitted electrons do not return to their original source so this process is known as non-cyclic photophosphorylation

Question 25

Describe **photolysis** (happens during non-cyclic photophosphorylation)

Answer 25

- Photolysis is the splitting of water using light - It occurs in the thylakoid space - Water splits into oxygen, protons and electrons - The electrons replace those lost by photosystem II, making the chlorophyll molecule stable - The protons help to maintain the electrochemical gradient between the thylakoid space and the stroma

Question 26

Where/when does cyclic photophosphorylation occur?

Answer 26

Occurs in bacteria and primitive plants. Higher plants also use this pathway to provide extra ATP , especially when CO2 is in short supply, or if levels of reduced NADP are high

Question 27

Describe cyclic photophosphorylation

Answer 27

- Only photosystem I is involved. Light energy is absorbed at PSI and channelled to chlorophyll a - Electrons in the chlorophyll a become excited, energised and released. They are accepted by an electron acceptor and eventually passed along the same tranfer chain as that used by the electrons from PSII - Protons are pumped into the thylakoid space and ATP is produced from ADP + Pi - The same chhemiosmotic theory as that used to describe the Z scheme is believed to be involved in the productionn of ATP - Electrons excited and released form PSI return to PSI

Question 28

How is the light independent stage linked to the light dependent stage?

Answer 28

The light independent stage (Calvin cycle) uses **ATP** and **reduced NADP** from the light dependent stage to fix carbon dioxide into organic molecules

Question 29

Describe the light independent stage (Calvin Cycle)

Answer 29

- CO2 from the atmosphere is fixed with RuBP (5C) - The enzyme RuBisCo catalyses this reaction - Results in the formation of an unstable 6C compound which splits into two molecules of Glycerate-3-phosphate (3C) - Glycerate-3-phosphate is then reduced using reduced NADP produced during the light dependent stage - ATP from the light dependent stage is also required. ATP is hydrolysed and the phosphate transferrred to Glycerate-3-phosphate, resulting in the formation of 2 molecules of triose phosphate (3C) - One carbon atom of the 6 available is removed from the Calvin cycle and can be used to produce organic molecules such as glucose - Five carbon atoms remain in the Calvin cycle and are used to regenerate RuBP so the cycle can begin again

Question 30

Name 2 symptoms of a deficiency of nitrates

Answer 30

- Reduced growth of organs - Chlorosis - yellowing leaves due to inadequate chlorophyll production so plant no longer able to absorb light energy

Question 31

Name a symptom of a deficiency of magnesium

Answer 31

Pronounced chlorosis between veins of older leaves as existing magnesium is moved and transported to new leaves

Question 32

Name a symptom of deficiency of phosphates

Answer 32

Stunting of plant growth

Question 33

What are the 3 limiting factors of photosynthesis?

Answer 33

- Temperature - CO2 concentration - Light intensity

Question 34

How does an increase in temperature affect the rate of photosynthesis?

Answer 34

An increase in temperature generally enhances enzyme activity, thereby increasing the rate of photosythesis

Question 35

The light dependent stage is much less dependent on enzyme activity than the light independent stage. Explain the implications of this on the overall rate of photosynthesis

Answer 35

An increase in temperature speeds up the LIS more than it speeds up the LDS. The supply of products from the LDS will not be able to keep up with the demand from the LIS. The overall rate of photosynthesis will become limited by the supply of reduced NADP and ATP

Question 36

Describe a physical change that could occur in the leaf that might result in the rate pf photosynthesis decreasing at very high carbon dioxide concentrations

Answer 36

- Carbon dioxide diffuses into leaves and dissolves in water - The enzyme carbonic anhydrase catalyses this reaction - Carbonic acid is formed which dissociates to form H+ ions and HCO3- ions - The accumulation of H+ ions causes cells to become more acidic - The acidity denatures enzymes in mesophyll cells in the leaves

Question 37

Explain the effect that an increase in light intensity will have on the yield of photosynthates

Answer 37

As the light dependent reaction speeds up, ATP and reduced NADP are produced at a faster rate. They enter the Calvin cycle and organic molecules are produced at a faster rate

Question 38

Describe a physical change that could occur in a leaf that might result in the plateau at high light intensity. Explain the advantage of this to the plant

Answer 38

At high light intensity, temperatures are also high so stomata close to reduce transpiration/wilting