Photosynthesis Flashcards
What is photosynthesis?
The process where energy from the light is used to make glucose and other complex organic molecules from water (H2O) and carbon dioxide which are simple inorganic molecules
What is the photosynthesis equation?
What are autotrophs and heterotrophs?
1) autotrophs - able to carry out photosynthesis and use simple inorganic molecules to synthesise complex organic molecules such as glucose. Energ is obtained from light. Eg plants and algae
2) heterotrophs - organisms which obtain organic molecules by digesting and absorbing complex organic molecules. The organic molecules are used for growth and respiration. Eg animals and fungi
What are the stages photosynthesis can be divided into?
1) light dependent stage (LDR) - occurs in thylakoid
2) light independent stage (LIR) - occurs in stroma
What do chloroplasts contain?
1) thylakoids = site of LDR, contains photosynthetic pigments
2) granum = stacks of thylakoids, stacked to maximise likelihood of light striking photosynthetic pigments
3)stroma = sight of LIR, contains enzymes eg RUBISCO
4) starch grains = store products of photosynthesis as starch
5) 70s ribosomes = site of protein synthesis from the chloroplast
6) circular DNA = carries genes coding for proteins/enzymes found in the chloroplast
7) inner and outer membranes = enclose stroma and separate contents from cytoplasm, control passage of substances in + out
8) lipid globules = for chloroplast membrane formation and some chlorphyll formation
What are photosynthetic pigments?
Chemicals that absorb light energy (photons), found in the thylakoids membranes of chloroplasts. Each pigment absorbs a particular range of wavelength of light energy
What are the several types of chlorophyll molecule?
- chlorophyll a
- chlorophyll b
- carotene
- xanthophyll
Each pigment absorbs a different range of wavelengths of light energy
Why do plants have several photosynthetic pigments?
So that they can absorb more wavelengths of light for the LDR to increase the rate of photosynthesis.The action spectrum is the rate of photosynthesis at each wavelength.
What are photosystems?
The photosynthetic pigment molecules are arranged in clusters in the thylakoids membrane called photosystems. There are to photosystems:
a) photosystem II (PSII) - absorbs light best at 680nm
b) photosystem I (PSI) - absorbs light best at 700nm
What two types of photosynthetic pigments do photosystems consist of
1) accessory pigments = surround the primary pigments and absorb light energy and pass the energy from one to another and then onto the primary pigment
2) primary pigments = reaction centres where electrons are excited to higher energy levels during the light dependant reaction
What are redox reactions ad what is the acronym to remember?
Redox reactions involve oxidation and reduction
We use the acronym OILRIG in reference to electrons
OIL = oxidation is loss (lost hydrogen, gained oxygen)
RIG = reduction is gain (gained hydrogen, lost oxygen)
Define co - enzymes
These are molecules that aid the function of an enzyme. They usually work by transferring a chemical from one molecule to another. Eg NADP transfers hydrogens from one molecule to another
What are 4 main stages that occur in the light dependant reaction?
1) excitation and photoionisation
2) photolysis of water
3) non-cyclic photophosphorylation
4) chemiosmosis
Describe the process of excitation and photoionisation in LDR
Light energy is absorbed by chlorophyll and other photonsynthetic pigments present in the photosystems which causes excitation of electrons in the chlorophyll and results in the electrons having more energy and being released from the molecule.
The chlorophyll molecule is oxidised and becoming positively charged. An electron acceptor gains the electron and becomes reduced
Describe the photolysis of water
As a result of electrons being lost from chlorophyll due to photoionisation, these electrons must be replaced. Replacement electrons are supplied from the photolysis of water. Water molecules are split using light energy into oxygen, hydrogen and electrons
Describe the process of non- cyclic phosphorylation
1) accessory pigments in PSII absorbs light energy and direct it at the primary pigments causing excited electrons to escape PSII.
2) these electrons are accepted by electron accepter and are passed through the electron transport chain to PSI. As they do so the electrons lose energy ( this energy is used to add a phosphate to ADP to produce ATP)
3) the excited electrons from PSI reduce the co-enzyme NADP to produce NADPH (reduced NADP)
Describe the process of non- cyclic phosphorylation
1) accessory pigments in PSII absorbs light energy and direct it at the primary pigments causing excited electrons to escape PSII.
2) these electrons are accepted by electron accepter and are passed through the electron transport chain to PSI. As they do so the electrons lose energy ( this energy is used to add a phosphate to ADP to produce ATP)
3) the excited electrons from PSI reduce the co-enzyme NADP to produce NADPH (reduced NADP)
What are the 3 stages of the Calvin cycle (LIR) ?
1) carbon dioxide fixation
2) reduction of glycerate-3-phosphate to form triose phosphate
3) regeneration of RuBP
How does carbon dioxide concentration affect the Calvin cycle
When CO2 is reduced, less is available to react with RuBP, therefore less glycerate phosphate and triose phosphate are formed and the levels of RuBP increase
How does light intensity affect Calvin cycle intermediates
When light is removed the light dependant reactions cannot take place and consequently there is no ATP or NADPH available for the reduction of glycerate phosphate to triose phosphate; therefore the concentration of GP rises but the concentration of Triose phosphate and RuBP falls.
How does temperature affect the Calvin cycle intermediates?
- As the temperature increases, the rate of the reaction increases but the relative levels of the intermediates are unchanged.
- At higher temperatures, oxygen competes with carbon dioxide for the Rubisco enzyme and so less carbon dioxide is fixed.
- This results in photorespiration.
- The concentration of RuBP rises and the concentration of glycerate phosphate and triose phosphate fall.
- At very high temperatures, rubisco enzyme may denature.
Describe the graph
- between the points A and B the rate of photosynthesis is limited by the light intensity
- as light intensity increases the rate of photosynthesis increases
- point B is the saturation point, any further increase in light intensity will not increase the rate of photosynthesis further
- some other factor such as low CO2 concentrations or temperature is now the limiting factor
(Remember there is only ever one limiting factor at a time)
Describe the graph
- line A = at high light intensities, low CO2 is the limiting factor
- line B = CO2 concentration is increased further, the maximum rate of photosynthesis increases and at high light intensities the lower temps (20C) now becomes the limiting factor
- line C = the maximum rate of photosynthesis is achieved when there is high light intensities, highCO2 concentration and high temperatures (35C)
What is the compensation point?
The light intensity where the rate of photosynthesis exactly equals the rate of respiration and there is no net gas exchange in or out of the plant
