Photosynthesis

Question 1

What is photosynthesis?

Answer 1

The process whereby light energy from the Sun is transferred to chemical energy from the synthesis of large organic molecules (e.g. sugars) from small inorganic molecules (e.g. carbon dioxide).

Question 2

What are autotrophs?

Answer 2

Organisms that are able to harness light energy or energy from chemical reactions to synthesise large organic molecules from small inorganic molecules.

Question 3

What are the 2 types of autotrophs?

Answer 3

1. Chemoautotrophs: Prokaryotes that are able to synthesise organic molecules using energy from exogenic (reactions that release energy) reactions. They usually live in harsh conditions, examples being nitrifying bacteria living in low oxygen soil.
2. Photoautotrophs: Plants and other organisms that contain chloroplast and are able to photosynthesise, using energy from the sun to make complex organic molecules. They make up most of the producers on earth.

Question 4

What are heterotrophs?

Answer 4

Organisms that are not able to make their own food but instead, rely on breaking down (digesting) pre-made organic compounds from further down the food chain, releasing the potential energy stored in them, or assembling them into new organic molecules.

Question 5

Why does all life on earth depend on photosynthesis?

Answer 5

• Photosynthesis converts abundant light energy from the sun into chemical form which is accessible by all types of organisms.
• Photosynthesis releases oxygen as a bi-product which is essential for photosynthesis.
• Provides energy to create nutrients and complex sugars which can be broken down by plants/consumers further down the food chain in respiration to release energy.

Question 6

How are leaves adapted for photosynthesis?

Answer 6

• Leaf blade: Large surface area to trap maximum light.
• Waxy cuticle & upper epidermis: Fully transparent to allow maximum light to pass through to the chloroplast containing palisade cells that carry out most of the photosynthesis.
• Palisade mesophyll cells: Packed full of chloroplasts that are able to move up and down depending on light level, maximising rate of photosynthesis.
• Spongy mesophyll cells: Air pores and high moisture to allow for efficient gas diffusion and exchange.
• Stomata: Opens and closes depending on light intensity and water level to control rate of gas exchange.
• Xylem: Brings water up from the roots for photosynthesis.
  Phloem: Takes away products of photosynthesis to be stored/used.

Question 7

What is the Endosymbiont theory of chloroplast origin?

Answer 7

A very long time ago, the chloroplast was actually a prokaryotic cell that photosynthesised but was one day engulfed by a eukaryotic cell through endocytosis. However, instead of digesting the chloroplast, a permanent symbiotic relationship was formed between the chloroplast and eukaryotic cell which involved the chloroplast providing the cell with energy and sugars while the cell provided the chloroplast with a stable environment and a steady supply of nutrients.

Question 8

What evidence is there to support the Endosymbiont theory of chloroplast origin?

Answer 8

1. The chloroplast contains a circular ring of DNA, like a prokaryotic cell.
2. Chloroplast contains free-floating 70s ribosomes, like a prokaryotic cell.
3. Inner chloroplast membrane is of similar composition to membrane of prokaryotic cell.
4. Affected by some antibiotics, like prokaryotic cells.

Question 9

What are the features of a chloroplast?

Answer 9

• Chloroplast envelope: Double membrane surrounding chloroplast. Outer membrane partially permeable to small ions whereas inner membrane is less so and has transport proteins embedded.
• Circular ring of DNA and 70s ribosomes, so that the chloroplast is able to produce some of its own proteins.
• Thylakoid: Inner membrane folded into thin lamellae, acts as site of light absorption and ATP synthesis in the light dependent part of photosynthesis.
• Granum: Thylakoids stacked together into densely packed regions.
• Intergranal lamellae: Thylakoid membranes between grana that connect them together.
• Starch grains and lipid droplets: Forms of storage for the products of photosynthesis.
• Stroma: Fluid part of the chloroplast containing all features of the chloroplast. This is the site of the light independent reactions in photosynthesis which produces carbohydrates and all other nutrients.

Question 10

How are chloroplasts adapted for photosynthesis?

Answer 10

• Inner membrane is embedded with transport proteins which control which substances enter and exit form the chloroplast., and the flow of chemicals required in photosynthesis.
• Large inner surface area from the grana and stacked thylakoid membranes allows for maximum number of photosynthetic pigments, ATP synthase and other features of the light dependent reaction to be embedded.
• Photosystems allow for the maximum amount of light to be absorbed and used by the plant.
• Stroma contains an abundance of enzymes required for the light independent reactions, which gives maximum rate of reaction.
• Stroma surrounds the grana so that the required light dependent stage products for the light independent stage can readily diffuse into the stroma.
• Chloroplast can manufacture some essential proteins for photosynthesis by itself, using the genetic information it contains.

Question 11

What are photosystems?

Answer 11

• Funnel-shaped complexes consisting of many coloured pigments that work together to absorb the maximum amount of light energy.
• Consists of the primary pigment at the reaction centre (bottom), then accessory pigments on top which absorb other wavelengths of light and pass to primary pigment.

Question 12

What is chlorophyll?

Answer 12

• A group of pigments that are all similar in structure, with a hydrocarbon (phytol) tail and porphyrin tail containing a prosthetic Mg group.
• There are 2 different types of chlorophyll.
• Chlorophyll a is a primary pigment and is found in 2 forms, P680 and P700, with peak light absorption at wavelengths 680nm and 700nm respectively.
• Chlorophyll b has a different absorption spectrum and is best at absorbing light of wavelength 500-640nm.

Question 13

What is an absorption spectrum?

Answer 13

Percentage of light of each wavelength a particular pigment is able to absorb.

Question 14

What is an action spectrum?

Answer 14

Percentage of light of each wavelength which is used by a plant in photosynthesis.

Question 15

What are accessory pigments?

Answer 15

• Other photosynthetic pigments that are not directly involved in the light dependent stage of photosynthesis, but absorb light of wavelengths not absorbed by chlorophyll a and pass the energy to it.
• Examples include chlorophyll b and carotenoids which absorb blue light.

Question 16

Where does the light dependent stage of photosynthesis take place?

Answer 16

• In the thylakoid membranes of the chloroplast.

- PSII is found almost exclusively in the grana whereas PSI is mostly found in the intergranal lamellae.

Question 17

What are the stages in the light dependent stage of photosynthesis?

Answer 17

1. Light absorbed by photosynthetic pigments in PSII raises pairs of electrons in chlorophyll a to a higher energy level which causes them to be emitted from the photosystem. This is a result of the electrons being hit by energetic photons.
2. Photolysis of water (in presence of enzymes) releases O2 molecule, 2H+ ions and 2e- which replace those lost by PSII.
3. Electrons from PSII pass along a sequence of electron carriers, releasing a bit of energy each transfer, which is used to pump H+ ions from the stroma into the thylakoid lumen as part of chemiosmosis.
4. Chemiosmosis decreases pH in thylakoid lumen and sets up H+ gradient across thylakoid membrane.
5. H+ ions flow down the concentration gradient back into stroma through the enzyme ATP synthase, which provides energy for ADP to be joined with Pi to form ATP. This process ultimately utilises light energy absorbed in the photosystems and is thus called photophosphorylation.
6. Light energy absorbed by photosynthetic pigments in PSI raises the energy level of electron pairs in chlorophyll a, which leads to their emission.
7. These electrons are replaced by electrons from PSII (non-cyclic phosphorylation) or PSI (cyclic phosphorylation).
8. Electrons from PSI are eventually transferred to NADP reductase where they are used, along with H+ ions from the stroma, to convert NADP to reduced NADP.

Question 18

What is the difference between cyclic and non-cyclic phosphorylation?

Answer 18

• Non-cyclic phosphorylation involves both PSII and PSI whereas cyclic phosphorylation only involves PSI.
• Photolysis in involved in non-cyclic phosphorylation whereas it is not in cyclic phosphorylation.
• Electron from PSI returns to PSI in cyclic phosphorylation whereas electron in PSII ends up being used to produce reduced NADP in non-cyclic phosphorylation.
• Only small amounts of ATP are produced in cyclic phosphorylation whereas ATP, reduced NADP and O2 is produced for non-cyclic phosphorylation.

Question 19

What is the role of water in photosynthesis?

Answer 19

1. Provides O2 for aerobic respiration.
2. Provides electrons which replaced ones lost from PSII during non-cyclic phosphorylation.
3. Provides H^+ ions which are used to create H^+ gradient in chemiosmosis, which leads to ATP production.
4. Provides H^+ ions for use in making reduced NADP.
5. Keeps turgor pressure in plant cells which prevents wilting and provides maximum surface area for photosynthesis.

Question 20

Where does the light independent stages of photosynthesis take place?

Answer 20

In the stroma, where the enzymes associated with the light independent stage are located.

Question 21

What are the stages in the light independent stage of photosynthesis?

Answer 21

1. Carbon dioxide from the air diffuse into the leaves via the stomata on the underside of the leaves.
2. Carbon dioxide diffuse through the air spaces in the spongy mesophyll to reach the palisade mesophyll.
3. Carbon dioxide diffuses through the cell wall, plasma membrane, cytoplasm, and chloroplast envelope to reach the stroma.
4. In stroma, CO2 combines with Ribulose bisphosphate (RuBP) in the presence of rubisco to become carboxylated.
5. 6-carbon compound formed is unstable and breaks down to 2 molecules of glycerate 3-phosphate (GP).
6. In the presence of ATP and reduced NADP (produced in light dependent reaction), GP is phosphorylated to 2 molecules of triose phosphate (TP).
7. 5/6 molecules of TP is phosphorylated by ATP and recycled to make 3 molecules of RuBP.
8. The cycle repeats

Question 22

How is GP and TP used in plants?

Answer 22

• GP is used to make amino acids and fatty acids.
• 2 molecules of TP can be combined to form hexose sugars like glucose which can be isomerised to form fructose.
• Fructose and glucose can be combined to form the disaccharide sucrose (transport form of sugars in phloem).
• Glucose molecules can be polymerised to form cellulose and starch.
• TP can also be used to make glycerol, which is combined with fatty acids to make triglycerides (lipids).

Question 23

What is the other name for the light independent stage of photosynthesis?

Answer 23

The Calvin cycle.

Question 24

What is CO2 used for in the Calvin cycle?

Answer 24

CO2 provides carbon and oxygen which is used by the cycle to generate amino acids, fatty acids, hexose sugars and glycerol.

Question 25

What is the role of ATP in the calvin cycle?

Answer 25

• Provides energy and phosphate for the phosphorylation of GP to TP.
• Provides phosphate for the phosphorylation of TP to RuBP.

Question 26

What is the role of reduced NADP in the calvin cycle?

Answer 26

Acts as a reducing agent in the conversion of GP to TP.

Question 27

What are the environmental limiting factors of photosynthesis?

Answer 27

1. Carbon dioxide concentration.
2. Availability of water.
3. Temperature.
4. Light intensity.

Question 28

What are the non-environmental limiting factors of photosynthesis?

Answer 28

1. Surface area of leaves.
2. Stomata density.
3. Disease.
4. Number of chloroplast.
5. Range of photosynthetic pigments present.

Question 29

What is a limiting factor?

Answer 29

The single factor that is present at the lowest or least favourable value and thus determines the maximum rate of reaction.

Question 30

How does light intensity affect rate of photosynthesis?

Answer 30

• Rate of photosynthesis increases in proportion as light intensity increases up to a maximum when another factor becomes limiting factor, then it plateaus.
• Light intensity determines the rate of electron emission from PSII and PSI which determines the rate of photophosphorylation and thus rate of ATP and reduced NADP production, which limits rate of calvin cycle as these products are required.
• Increasing light intensity causes stomata to open wider, which allows greater volume of CO2 to diffuse into the leaves to be used for the calvin cycle.
• Increasing light intensity increases rate of photolysis and H+ production, which limits rate of chemiosmosis, phosphorylation, ATP and reduced NADP production.

Question 31

How does carbon dioxide concentration affect rate of photosynthesis?

Answer 31

• Increasing concentration of CO2 increases rate of photosynthesis up to when another factor becomes the limiting factor.
• Increasing concentration of CO2 increases availability of carbon for the calvin cycle, therefore increasing rate of calvin cycle and photosynthesis.

Question 32

How does temperature affect rate of photosynthesis?

Answer 32

• Increasing temperature increases rate of photosynthesis up to a maximum at optimum temperature, then plummets beyond optimum temperature (reminiscent of enzyme reaction-temperature curve).
• Increasing temperature up to optimum increases rate of enzyme-catalysed reactions in plants and thus photosynthesis. Beyond optimum, enzymes get denatured and thus rate of photosynthesis decreases.
• Increasing temperature also increases rate of transpiration and may cause the stomata to close. This decreases availability of CO2 for photosynthesis and thus rate of photosynthesis.
• Efficiency of rubisco decreases as temperature increases beyond optimum as O2 outcompetes CO2 for photorespiration.

Question 33

What happens to level of RuBP, TP and GP in dim/dark environments?

Answer 33

• Low light decreases rate/stops light dependent reactions as no energy is available to excite electrons in PSII and I. Rate of photophosphorylation decreases and rate of ATP and reduced NADP also decreases.
• RuBP is converted to GP, but GP cannot be phosphorylated to TP and no ATP or reduced NADP is present to carry out process, so GP levels increase.
• TP in converted to hexose sugars, glycerol… but no more is being produced as GP cannot be phosphorylated, so TP levels decrease.
• RuBP is converted to GP by rubisco, but no more TP is being phosphorylated to TP as:
  1. No more GP is being converted to TP due to lack of ATP and reduced NADP.
  2. No ATP is available to phosphorylate TP back into RuBP.
  So RuBP levels decrease.

Question 34

What happens to levels of RuBP, GP and TP in low CO2 concentration environments?

Answer 34

• Less RuBP is being carboxylated to to GP due to lack of CO2; more RuBP is being made de to conversion of TP to RuBP, so RuBP accumulates, and levels increases.
• Less GP is being formed since less RuBP is being carboxylated, so GP levels decrease.
• Less TP is being formed since less GP is being formed to be converted to TP, plus TP is still being converted back to RuBP, so TP levels decrease.

Question 35

What happens if CO2 concentrations are too high?

Answer 35

• Stomata open for extended periods of time and rate of transpiration is very high.
• If rate of water uptake cannot keep up with loss by transpiration, plant wilts and responds with stress.
• Stomata closes to minimise water loss by transpiration and, but gas exchange is cut off.
• No CO2 is available for photosynthesis so photosynthesis effectively stops.

Question 36

What are the limitations with measuring rate of O2 production in aquatic plants to determine rate of photosynthesis?

Answer 36

• Some O2 may be used for respiration in plants.
• Some of the gas collected may contain N2 dissolved in the water from the atmosphere, especially as temperature increases.
• Some O2 may dissolve in water and not be collected.
• Some O2 may not be collected by the apparatus.