Photosynthesis

Question 1

Define the term Autotroph

Answer 1

Organism that can use an external energy source and simple inorganic molecules to make complex organic molecules

Question 2

Define the term Heterotroph

Answer 2

Organism that needs to take in complex organic molecules to act as a source of energy

Question 3

What is used to produce complex organic molecules in photosynthesis?

Answer 3

light energy

Question 4

How does respiration depend on the products of photosynthesis?

Answer 4

Chemical potential energy in complex organic molecules formed from photosynthesis is used by animals for respiration. The chemical potential energy of ATP is then used for work and thermal energy
(TP –> respiratory substrates)

Question 5

What are the two stages of photosynthesis and where do they occur?

Answer 5

Light Independent - stroma
Light dependent - thylakoid membranes of the grana
Both occur in chloroplasts

Question 6

How is the structure of a chloroplast adapted for photosynthesis?

Answer 6

Chloroplast envelope keeps reactants for photosynthesis close to their reaction sites.
Thylakoids have a large surface area to allow as much light energy to be absorbed.
Lots of ATP synthase /electron carrier molecules present in thylakoid membranes to produce ATP in the light dependant reaction.
Stroma contains all the enzymes, sugars and organic acids for the light independent reaction e.g. rubisco.

Question 7

Define the term photosynthetic pigment

Answer 7

Coloured substances which absorb the specific wavelengths of light energy needed for photosynthesis

Question 8

What is the importance of photosynthetic pigments?

Answer 8

“Photosynthetic pigments are found in photo systems. There are two types of pigments.
Primary Pigments-Reaction centres where electrons are excited during light dependent reactions
Accessory Pigments-Surround the primary pigments and transfer light energy to them”

Question 9

What is a photo system?

Answer 9

Photosynthetic pigments which are attached to proteins

Question 10

Outline how light energy is converted to chemical energy(ATP+ NADP) in the light dependent stage

Answer 10

“Light energy absorbed by PS II
Electrons excited in the photosynthetic pigment of PSII
These high energy electrons move along the electron transport chain to PS I
As excited electrons leave PS II to move along electron transport chain, they must be replaced
Light energy splits water into protons, electrons and oxygen(Photolysis)
Excited electrons lose energy as they move along electron transport chain
Energy used to transport protons into thylakoid so thylakoid has a higher concentration of protons than the stroma
This forms a proton gradient
Protons move down their concentration gradient into the stroma through ATP synthase
The energy from this movement combines ADP + Pi to form ATP(Chemiosmosis)
Light energy is also absorbed by PS I, which excites the electrons
Electrons are then transferred to NADP along with a proton from the stroma to form reduced NADP”

Question 11

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

Answer 11

Cyclic only uses PS I, non-cyclic uses 1 and 2
Cyclic requires light energy, ADP and Pi to produce ATP
Non-cyclic requires light energy, water, NADP, ADP and Pi, produced ATP, oxygen & reduced NADP

Question 12

What does the ‘cyclic’ mean? (in terms of photophosphorylation)

Answer 12

Cyclic because electrons from chlorophyll are not passed onto NADP, but are passed back to PSI through electron carriers. This means electrons are recycled can flow through PSI.

Question 13

Outline how products of the light dependent stage are used in the light independent stage to produce triose phosphate

Answer 13

“CO2 enters the leaf through the stomata
Combined with ribulose biphosphate using the enzyme ribulose biphosphate carboxylase.
This produces an unstable 6-carbon compound which quickly breaks down to two molecules of glycerate 3-phosphate
ATP from the light dependent reactions provides the energy to turn the glycerate 3-phosphate into a different 3 carbon compound, triose phosphate
This reaction also requires protons which come from the reduced NADP produced in the light dependent reaction
Triose phosphate can then be converted into useful organic compounds
Triose phosphate is also recycled to ribulose biphosphate, which also requires ATP from the “

Question 14

What can Triose phosphate be converted to?

ang GP?

Answer 14

Carbohydrates-hexose sugars made by joining two triose phosphate molecules
Lipids-Made using glycerol from triose phosphate and fatty acids from glycerate 3-phosphate
Amino acids-Made from glycerate 3-phosphate”

Question 15

What is Triose phosphate mostly recycled to?

Answer 15

“Ribulose biphosphate

Five out of every six molecules of triose phosphate are used to regenerate ribulose biphosphate

Question 16

What is the effect on the rate of photosynthesis when the concentration of CO2 changes?

Answer 16

When the concentration of CO2 is decreased , the concentration of ribulose biphosphate increases and the concentrations of glycerate 3-phosphate and triose phosphate both decrease

Question 17

What is the effect on the rate of photosynthesis when the light intensity changes?

Answer 17

At low light intensities, the light-dependent reactions cannot take place so a lack of ATP and of reduced NADP slows the reduction of glycerate 3-phosphate, so the concentration of glycerate 3-phosphate rises and the concentrations of triose phosphate and ribulose biphosphate decrease.
As light intensity increases, the rate of photosynthesis will increase as long as other factors are in adequate supply. As the rate increases, eventually another factor will come into short supply - limiting factor.

Question 18

What is the effect on the rate of photosynthesis when the temperature changes?

Answer 18

All reactions take place at a faster rate but at higher temperatures rubisco functions as an oxidase, therefore oxygen and CO2 start competing for the active sites of rubisco. Concentrations of glycerate 3-phosphate and triose phosphate fall. Stomata also begin to close at higher temperatures to conserve water

Question 19

What is a limiting factor?

Answer 19

This is the least favourable factor out of several factors which influence the rate of the reaction

Question 20

Describe how to investigate experimentally the factors that affect the rate of photosynthesis

Answer 20

“Using a photosynthometer
Bubbles of oxygen collect in the capillary tube of the apparatus
When a suitable volume of gas has been collected in a known time period it can be drawn alongside the scale in the gas syringe
Length of the bubble is measured
The length is directly proportional to the volume
All variables other than the factor being investigated must be kept constant
The plant must be left for some time to adjust to new conditions before any measurements
At least 3 measurements taken and mean value found”
Method can be adapted according to which factor is being measured:
Light intensity - have a lamp at different distances from plant
Temperature - put plant in water bath
CO2 conc. - bubble CO2 gas into water/add sodium hydrogen carbonate

Question 21

What does a photosynthometer measure?

Answer 21

Rate of oxygen production of a water plant (e.g. Elodea Canadensis)

Question 22

why doesn’t rate of photosynthesis increase exponentially as light intensity increases?

Answer 22

• at high light intensity other factors become limiting factors
  e. g:
• temperature becomes limiting as Calvin cycle involves enzymes such as Rubisco and relies on kinetic energy of molecules.
• CO2 conc. becomes limiting as it is required for the Calvin cycle - fixation by Rubisco/reaction with RuBP