Photosynthesis

Question 1

What is photosynthesis?

Answer 1

The production of carbon compounds in cells using light energy and simple inorganic substances

Question 2

What are the two type of reactions in photosynthesis?

Answer 2

Light dependent

Light independent

Question 3

Which reaction is dependent on which?

Answer 3

The light-independent reactions depend on light dependent because light dependent reactions produce ATP and reduced NADP which are needed by light independent reactions

Question 4

Where do light dependent reactions take place?

Answer 4

Thylakoid membrane of chloroplasts.

Question 5

What are photosystems and how many exist?

Answer 5

Photosystems are large groups of pigment molecules

Photosystem 1 and Photosystem 2

Question 6

Which is the main photosynthetic pigment in photosystems?

Answer 6

Chlorophyll

Question 7

What do pigments do?

Answer 7

Absorb certain wavelengths of photons and an electron is raised to a higher energy level- the pigment is photoactivated

Question 8

Describe photosystem II and ATP formation

Answer 8

1. Pigments in photosystem II absorb photons generating excited electrons. Excited electrons pass from pigment to pigment until they reach the chlorophyll molecule at the reaction centre of the photosystem.
2. The chlorophyll passes pairs of excited electrons to a chain of electron carriers in the thylakoid membrane.
3. The electrons give up energy as they pass from one carrier to the next
4. The released energy is used to pump protons across teh thylakoid membrane from the stroma into the the thylakoid space
5. A proton gradient is created which is a store of potential energy.
6. Photolysis takes place in the thylakoid space which contributes to the gradient.
7. ATP synthase located in teh thylakoid membranes allows protons to diffuse back to the stroma and uses the energy that the protons release to produce ATP from ADP and inorganic phosphate.
   The generation of the ATP using energy released by the movement protons across a membrane is called chemiosmosis. Production of ATP in chloroplasts is called photophosphorylation.

At the end of the chain electrons are passed to photosystem I

Question 9

What is photolysis?

Answer 9

Photosystem II must replace excited electrons given away by chlorophyll. So water molecules in the thylakoid space are split. The splitting of water of water molecules using light energy is called photolysis. It gives electrons which replace the lost electrons in photosystem II. Oxygen and hydrogen ions also form. Oxygen is a waste product and is excreted. H+ contribute to the proton gradient.

Question 10

Describe photosystem I and reduction of NADP.

Answer 10

1. Pigments in Photosystem I absorb photons generating excited electrons. Excited electrons reach the chlorophyll at the reaction centre.
2. The chlorophyll passes pairs of electrons to a chain of electron carriers
3. At the end of the chain, electrons are passed to NADP in the stroma
4. NADP is converted to reduced NADP by accepting 2 electrons emitted by Photosystem I plus 2 protons in the stroma.
   Reduced NADP is passed to the light independent reaction

Question 11

What is cyclic photophosphorylation?

Answer 11

When the supply of NADP runs out, the electrons return to the electron transport chain that links the two photosystems instead of being passed to NADP. As the electrons flow back along the electron carriers to photosystem I they cause pumping of protons allowing ATP production

Question 12

Where does the light independent reaction take place?

Answer 12

In the stroma of chloroplasts

Question 13

What is the Calvin cycle?

Answer 13

An anabolic pathway that fixes carbon. The cycle regenerates the five- carbon compound used, ribulose biphosphate (RuBP)

Question 14

Describe in detail the Calvin cycle.

Answer 14

1. CO2 diffuses into the chloroplast
2. In the stroma, CO2 combines with a five- carbon sugar, ribulose biphosphate. Carboxylation reaction.
3. Carboxylation reaction is catalysed by the enzyme Rubisco.
4. The product of RuBP carboxylation is an unstable C6 which immediately splits into 2 molecules of glycerate-3-phosphate. This is the first product of carbon fixation.
5. Glycerate 3-phosphate is converted into the three carbon sugar triose phosphate- reduction reaction
6. For this reaction to happen, hydrogen and energy is needed- hydrogen is supplied by reduced NADP and energy by ATP.
7. Triose phosphate is the first carbohydrate to be produced in the Calvin cycle. It can be converted to diff carbohydrates. Glucose phosphate is produced by linking together 2 triose phosphate molecules. Starch is formed in the stroma linking together many molecules by glucose phosphate by condensation.

Most of the triose phosphate molecules produced are used to regenerate RuBP.

Question 15

Which techniques were used to discover the Calvin cycle?

Answer 15

1. Radioactive labelling
2. Double way paper chromatography.
3. Autoradiography

Question 16

What is autoradiography?

Answer 16

Production images on X-ray films to locate radioisotopes. When radioactive isotopes decay they give off radiation. This makes a small spot in an adjacent X-ray film.

Question 17

Describe Calvin’s experiment

Answer 17

A suspension of the unicellular alga chlorella was placed in a glass lollipop vessel.
Chlorella was illuminated and supplied with CO2 and HCO3
At the start teh 12C in both carbon sources are replaced by radioactive 14C
Samples of Chlorella were obtained at very short time intervals and immediately killed with hot methanol
Carbon compounds were extracted and separated by double- way paper chromatography and then were found by autoradiography and quantified.

Question 18

Draw a chloroplast and describe its adaptations.

Answer 18

1. Chloroplast envelope consisting of inner and outer membrane- separates the contents of the chloroplast with the rest of the cell
2. Granum- Stack of thylakoids for absorption of photons of light
3. Thylakoid membranes- extensive system of internal membrane which provides a large SA for light- absorbing photosystems - generate proton gradient and chemiosmosis
4. Thylakoids space- Protons are being pumped to create the proton gradient needed for ATP production by photophosphorylation. The volume is very small to build a steep gradient
5. Stroma- Fluid that contains the enzymes for the Calvin cycle
6. 70S ribosomes- protein synthesis
7. Starch and lipid droplets- starch grains store the carbohydrates made by photosynthesis.

Question 19

What is the visible light spectrum?

Answer 19

Between 400 and 700 nm. Blue has the shorter wavelength than red. Blue= 450-500 and Red= 650-700

Question 20

Draw an absorption spectrum

Answer 20

Question 21

Draw an action spectrum

Answer 21

Question 22

State the properties of absorption spectra.

Answer 22

1. A graph showing % of light absorbed at each wavelength by a pigment
2. X axis shows wavelength and y axis % absorption
3. The absorption spectrum of chlorophyll has a higher peak at blue light and a lower peak at red light.

Question 23

State the properties of action spectra

Answer 23

1. An action spectra is a graph showing the rate of photosynthesis at each wavelength of light
2. X axis shows wavelength and the y axis the rate of photosynthesis
3. The maximum photosynthesis rate is in blue light at 430nm with another lower peak at 660nm. The peak in the blue-violet range is broader.
4. The trough in the green range does not reach zero showing there is some use of green light as well.

Question 24

Outline the effects of photosynthesis on the Earth

Answer 24

Primordial Earth had very low oxygen levels–> Photosynthesis led to an increase in the % of oxygen in the atmosphere–> first organisms to photosynthesise were bacteria causing the oxygen content to rise to 2%–> photosynthesis caused a decrease in CO2 and a rise in O2 which meant a decrease in methane (co2 and methane are greenhouse gases so reduction of temp) –> Increase in oxygen caused the oxidation of dissolved iron in water to form iron oxide which formed banded iron formations

Question 25

What are the three factors affecting the rate of photosynthesis?

Answer 25

Temperature, light intensity and co2 concentration

Question 26

What are limiting factors?

Answer 26

A factor that is not at its optimal level which limits the rate of photosynthesis.

Question 27

Describe the effect of light intensity as a limiting facor.

Answer 27

As light intensity increases the rate of photosynthesis increases. Light is needed for photolysis and production of ATP and high energy electrons. It reaches a plateau because another factor becomes limiting and increasing light intensity has no effect

Question 28

Describe the effect of CO2 concentration as a limiting factor

Answer 28

As CO2 conc increases rate of photosynthesis increases. Below 0.01% Rubisco which fixes CO2 is not effective so no net photosynthesis. Between 0.01% and 0.04% CO2 conc is often the limiting factor bc the rate of collisions between CO2 molecules and the active site of Rubisco is still lower. At high CO2 conc, fixation is at a maximum and the rate reaches a plateau.

Question 29

Describe the effect of temperature on the limiting factor.

Answer 29

As temperature increases, rate of photosynthesis increases. This happens to an optimal temperature. At temp over 30C the efficiency of Rubisco decreases .