Photosynthesis Flashcards
Define the term Autotroph
Organism that can use an external energy source and simple inorganic molecules to make complex organic molecules
Define the term Heterotroph
Organism that needs to take in complex organic molecules to act as a source of energy
What is used to produce complex organic molecules in photosynthesis?
light energy
How does respiration depend on the products of photosynthesis?
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)
What are the two stages of photosynthesis and where do they occur?
Light Independent - stroma
Light dependent - thylakoid membranes of the grana
Both occur in chloroplasts
How is the structure of a chloroplast adapted for photosynthesis?
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.
Define the term photosynthetic pigment
Coloured substances which absorb the specific wavelengths of light energy needed for photosynthesis
What is the importance of photosynthetic pigments?
“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”
What is a photo system?
Photosynthetic pigments which are attached to proteins
Outline how light energy is converted to chemical energy(ATP+ NADP) in the light dependent stage
“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”
What is the difference between cyclic and non-cyclic photophosphorylation?
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
What does the ‘cyclic’ mean? (in terms of photophosphorylation)
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.
Outline how products of the light dependent stage are used in the light independent stage to produce triose phosphate
“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 “
What can Triose phosphate be converted to?
ang GP?
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”
What is Triose phosphate mostly recycled to?
“Ribulose biphosphate
Five out of every six molecules of triose phosphate are used to regenerate ribulose biphosphate
What is the effect on the rate of photosynthesis when the concentration of CO2 changes?
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
What is the effect on the rate of photosynthesis when the light intensity changes?
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.
What is the effect on the rate of photosynthesis when the temperature changes?
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
What is a limiting factor?
This is the least favourable factor out of several factors which influence the rate of the reaction
Describe how to investigate experimentally the factors that affect the rate of photosynthesis
“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
What does a photosynthometer measure?
Rate of oxygen production of a water plant (e.g. Elodea Canadensis)
why doesn’t rate of photosynthesis increase exponentially as light intensity increases?
- 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
