Chapter 6 Photosynthesis Flashcards
What is autotrophic nutrition?
Organisms using chemical energy to synthesise large organic molecules from simple inorganic molecules, such as water and CO2
What are organisms that photosynthesise called?
photoautotrophs (producers)
What is the general equation for photosynthesis?
6CO2 + H2O (+energy from photons) —> C6H12O6 + 6O2
What is the compensation point?
When photosynthesis and respiration occur at the same rate so there is no net loss or gain of carbohydrate
Where does the light-dependent stage take place?
In the grana in the thylakoid membrane
Where does the light-independent stage take place?
Stroma
What is a photosynthetic pigment?
Pigment that absorbs specific wavelengths of light and traps the energy associated with light (examples include chlorophyll a, chlorophyll b and carotene), They are used in the light dependent stage and are located in the thylakoid membrane in photosystems.
Where are photosystems located?
Thylakoid membrane
What is the role of oil droplets in a chloroplast?
They act as a reserve of raw materials for the synthesis of new chloroplast membranes
What connects separate granum?
Intergranal lamellae
What holds the photosystems in place?
Proteins embedded in the thylakoid membrane
What does the stroma contain?
Enzymes needed to catalyse reactions of the light-dependent stage, starch grains, oil droplets, small ribosomes and DNA
What colour do chlorophyll a and b appear?
Chlorophyll a: blue-green
Chlorophyll b: yellow-green
What are the stages of the light-dependent stage of photosynthesis?
- light harvesting at photosystems
- photolysis of water
- photophosphorylation - production of ATP in the presence of light
- formation of reduced NADP
What is the equation for photolysis?
2H2O —> 4H+ + 4e- + O2
What are the steps of non-cyclic phosphorylation?
- When a photon of light strikes photosystem ll its energy is transferred to the primary pigment reaction centre
- The light excites a pair of electrons in the photosystem
- The energised electrons escape from the chlorophyll and are captured by an electron carrier/ acceptor
- These electrons are replaced by electrons generated by photolysis
- They are carried along the electron transport chain via a series of oxidation and reduction reactions involving Fe ions
- These reactions form ATP which is used to pump H+ ion into the thylakoid space
- The electrons reach another chlorophyll molecule in PSI, replacing those lost by excitation of PSI electrons.
- A protein-iron-sulphur complex called Ferredoxin accepts the electrons from PSI and passes them to NADP in the stroma
- As H+ ions accumulate in the thylakoid space, a concentration gradient builds up, causing H+ ions to move across the membrane through ATP synthase, forming ATP
- The ions are accepted by NADP which becomes reduced (catalysed by enzyme NADP reductase)
- Reduced NADP and ATP move to the stroma to take part in the light-dependent stage
What are the steps of cyclic phosphorylation?
- When a photon of light hits a chlorophyll molecule in photosystem I an electron is excited and leaves the molecule
- It is taken up by an electron acceptor and passed along the electron transport chain
- When an electron returns to the chlorophyll in PS I, it can be excited again
- Only a small amount of ATP is generated (no H+ ions, oxygen or reduced NADP)
What are the steps of the Calvin Cycle?
- CO2 combines with a CO2 acceptor, a 5 carbon compound called Ribulose Biphosphate (RuBP), catalysed by the enzyme RuBisCO
- This produces an unstable 6 carbon compound, which immediately breaks down into 2 molecules of the 3 carbon compound Glycerate 3-phosphate (GP)
- GP is then reduced to form triose phosphate - ATP and hydrogen from reduced NADP, both from the light-dependent stage, are used for this reaction
- 10 in every 12 triose phosphate produced goes back to replace the ribulose biphosphate needed in the 1st stage of the cycle. 2 molecules of TP in every 12 is synthesised into glucose and used to synthesise amino acids, fatty acids and glycerol, or are converted to sucrose, starch and cellulose
What are some uses of triose phosphate (TP)?
- Glucose can be converted into starch and cellulose
- TP is used to synthesise amino acids, fatty acids and glycerol
- The rest of TP is recycled to regenerate supply of RuBP
What is the name of the cycle in the light-independent stage of photosynthesis?
Calvin Cycle (CO2 is converted into organic molecules)
How many turns of the Calvin Cycle are required to form one molecule of glucose?
6 turns
What are some examples of limiting factors of photosynthesis?
- Light intensity
- Carbon dioxide concentration
- Temperature
- Water availability
- Availability of chlorophyll
What are the effects of low light intensity on the Calvin Cycle?
- GP cannot be reduced to TP as products of the light-dependent stage (ATP and reduced NADP) will be in short supply
- TP levels fall and GP accumulates
- If TP levels fall, RuBP cannot be regenerated
What are the effects of low carbon dioxide concentration on the Calvin Cycle?
- RuBP accumulates (as it cannot combine with CO2)
- GP levels will fall
- TP levels will fall
What are the effects of low temperatures on the Calvin Cycle?
Enzymes such as RuBisCO work more slowly so reactions are slower and levels of RuBP, GP and TP will fall
What are the effects of very high temperatures on the Calvin Cycle?
Enzymes such as RuBisCO may denature so reactions are slower/ stop and levels of RuBP, GP and TP will fall
What is the effect of water stress on the rate of photosynthesis?
- Roots are unable to take up enough water to replace that lost via transpiration
- Cells lose water and become plasmolysed
- Plant roots produce abscisic acid that, when translocated to leaves, causes stomata to close, reducing gaseous exchange
- Tissues become flaccid and leaves wilt
- The rate of photosynthesis greatly reduces
What is the apparatus used to measure the rate of photosynthesis?
Photosynthometer
What are the limitations of finding the rate of photosynthesis by measuring volume of oxygen produced per minute?
- Some of the oxygen produced by the plant will be used for its respiration
- There may be some dissolved nitrogen in the gas collected
What is the difference between primary and accessory pigments?
Primary pigments act as reaction centres where electrons get excited. Accessory pigments surround the reaction centre and transfer energy to the primary pigments.
