Photosynthesis Flashcards
Why is photosynthesis is endothermic?
- Small inorganic molecules are joined together into large organic molecules
- Total energy required to break all bonds in small inorganic molecules greater than the total energy released by making bonds
- Photosynthesis is endothermic, with the energy coming from the Sun’s light energy
C-H bonds:
C-H bond has a fairly low bond energy because it is non-polar (electrons shared equally)
C-O and H-O bonds:
C-O and H-O are have a higher bond enthalpy because they are polar
Covalent bonds in small inorganic and larger organic molecules:
- Small inorganic molecules such as CO2 and H20
- Covalent bonds are strong because they have high bond energies
- Release lots of energy when formed but require a lot of energy to break
- Larger organic molecules such as glucose and amino acids
- Covalent bonds are weaker because they have lower bond energies
- Release less energy when the bonds are made and require less energy to break the bonds
What do cells require energy for?
- Synthesis of molecules, e.g. protein synthesis
- Transport of molecules or ions, e.g. vesicles to the Golgi, transport of pyruvate into mitochondria
- Cellular movement, e.g. contraction of cardiac muscle cells
- Synthesis of sugars by photosynthesis
What do plant cells need to do at the start of photosynthesis?
Need to make ATP from ADP and Pi
Why are plants classed at autotrophs?
They are able to photosynthesise
What are the adaptations, for photosynthesis, of a dicotyledonous leaf?
- Large SA
- Many chloroplasts
- Broad, thin, flat
Why are the apical surfaces of the upper and low mesophyll moist?
Convert CO2 (g) to CO2 (aq)
Why does the lower mesophyll have air spaces?
Allows gas to circulate leaf
What is the structure of a chloroplast?
- Outer membrane
- Inner membrane
- Thylakoid membranes
- Lamellae
- Grana
- Stroma
What are the thylakoid membranes?
Network of membranes, which are flattened sacs which maximise SA
What are grana?
Stack of thylakoids
What are lamellae?
Membranous channels that join the grana together
What is the stroma?
Fluid enclosed in the cytoplasm that is the site of chemical reactions
Where are chlorophyll molecules located in a chloroplast?
Embedded in the thylakoid membrane
How is a large SA of the thylakoid membranes achieved in a chloroplast?
- Joined by lamellae and stacked
- Folding
Why is the upper epidermis thin and have no chloroplasts?
Allow light to pass through so the light energy isn’t used up and priority cells are able to use it instead
What do photosynthetic pigments do?
Transfer light energy into chemical energy
What are the primary and secondary photosynthetic pigments?
PRIMARY: Chlorophyll A- absorbs red and blue light SECONDARY: Chlorophyll B- reflects yellow/green Xanthophyll- reflects yellow Carotene- reflects orange
What is a photosystem?
A functional unit in the thylakoid membranes that absorb light energy and transfer the energy to electrons, to make high energy electrons
What are the two components of a photosystem?
- Reaction centre
2. Antennae complex
What occurs in the reaction centre?
- Contains chlorophyll A molecules bound to chlorophyll A/B binding protein that is embedded in the thylakoid membrane
- Location of generation of high energy electrons (photochemistry) using absorbed light energy
What occurs in the antennae complex?
- Contains secondary photosynthetic pigments bound to proteins (if chlorophyll b, bound to chlorophyll a/b binding protein)
- Pigments absorb light energy of different wavelengths and transfer it quickly and efficiently to the reaction centre
What are the two stages of photosynthesis?
- Light dependent stage
2. Light independent stage
Light dependent stage: PSII
Occurs in thylakoid membrane
- PSI has a reaction centre that absorbs light at 700m and PSII has a reaction centre that absorbs at 680nm
- Both photosystems involved in making ATP from ADP and Pi by non-cyclic phosphorylation and making reduced NADP
- High energy electrons from PSII passed a long an electron transport chain that pumps H+ from the stroma into the thylakoid membrane’s lumen
- This makes lumen acidic (higher conc. H+)
- H+ diffuses down its concentration gradient through ATP Synthase by facilitated diffusion and regenerates ATP from ADP and Pi
Light dependent stage: PSI
- Electrons from PSII that have been passed a long electron transport chain are then passed to PSI
- PSI produces high energy electrons using light energy
- ATP produced by chemiosmosis
- Low energy electrons that leave electron transport chain leave the photosystem entirely (non-cyclic photophosphorylation)
- PSII needs replacements electrons that come from the photolysis of water, keeping PSII in REDOX balance
- Electrons that leave PSI are used, a long with H+ that come from the photolysis of water by PSII to make reduced NADP
Where can cyclic photophosphorylation occur?
- PSI can do cyclic photophosphorylation
- Occurs in the lamellae
- Low energy electrons left over by electron transport chain are recycled back to the reaction centre
- ATP is still made but the electrons are cycled back to PSI
When is cyclic photophosphorylation favoured?
- Anaerobic conditions
- Very high light levels
Light independent stage:
Occurs in the stroma
- CO2 diffuses into the stroma of the chloroplasts
- CO2 combined with 5 carbon molecule called Ribulose Bisphosphate (RuBP) to make an unstable 6 carbon intermediate
- Enzymes that does this reaction is called Ribulose Bisphosphate Carboxylase (Rubisco)
- Unstable 6 carbon intermediate breaks down immediately into two molecules of Glycerate 3-phosphate (GP)
- Each molecule of GP converted to a molecules of Triose Phosphate (TP) - hydrogen added
- Conversion requires ATP and reduced NADP made by light dependent stage
- Most of the TP is used to regenerate RuBP
- Some of TP is used to make glucose, lipids and amino acids
What is the light independent stage also known as?
Calvin cycle
How many times does the Calvin cycle need to turn to produce one molecule of glucose?
Calvin cycle has to turn fully 6 times
How many of each molecule does 6 turns of the Calvin cycle produce?
- Six CO2 molecules enter so 12 TP molecules are produced
- Two molecules of TP used to make one molecule of glucose
- Remaining 10 TP molecules used to regenerate six molecules of RuBP
What is the fault with Rubisco?
- Rubisco is competitively inhibited by O2
- This produces phosphoglycerate instead of the 6 carbon intermediate
- Phosphoglycerate is a two carbon molecule that is toxic to plants
- Plants get rid of it by converting it into non toxic waste chemicals, which requires lots of ATP and reduced NADP
- Greatly reduces efficiency of Calvin cycle
- Magnified when plant closes stomata
- Less CO2 but more photolysis of water produces more O2, so more competitive binding and more toxic phosphoglycerate produced
- Therefore, more ATP and NADPH wasted
What are the limiting factors for photosynthesis?
- Light intensity
- Light quality
- CO2 concentration
- Temperature
- ‘Stomatal status’ -open or closed
How does light intensity affect rate of photosynthesis?
LOW LIGHT INTENSITY
1. Rate of reaction of the light dependant stage will reduce
2. Concentration of ATP and NADPH will reduce
3,. Concentration of GP will increase as there is not enough ATP or NADPH to convert it to TP
4. Concentration of TP will reduce
5. Concentration of RuBP will reduce because of the lower concentration of TP that it is made (cannot be regenerated)
Effect of light quality on rate of photosynthesis?
-If light shone on the plant is the wrong wavelength, the rate of the light dependent stage will be reduced or may even stop as plant cannot use it
Effect of stomatal status on rate of photosynthesis:
If stomata are closed then rate of photosynthesis will decrease as CO2 concentration is decreased as it cannot diffuse through the stomata
What is the effect of CO2 concentration on the rate of respiration?
LOW CO2 CONCENTRATION
- Rate of Calvin cycle will decrease
- Concentration of GP will be reduced as there is less CO2 to be fixed
- Concentration of TP will be reduced as there is less GP that it is made from
- Concentration RuBP will be increased because it is still being made from TP but not being used up in CO2 fixation
What is the effect of temperature on rate of respiration?
LOW TEMPERATURE
1. Rate of Calvin cycle will be reduced
2. Kinetic energy of all enzymes and substrates of Calvin cycle will be lower
3. Reduced concentration of GP, TP and RuBP
TOO HIGH TEMPERATURE
1. Enzymes of Calvin cycle will be irreversibly denatured
2. Reduced concentrations of GP, TP and RuBP
3. Rate of Calvin cycle decreased
What is C4 photosynthesis?
- CO2 molecules are fixed to make a 4 carbon molecules and not by RUBISCO
- CO2 fixed by an enzyme called Pep Carboxylase and produces oxaloacetate
- Pep Carboxylase has no affinity for oxygen
- Oxaloacetate is transported into bundle sheath cells around the vascular bundles
- Oxaloacetate then decarboxylated to produce CO2
- Rubisco then converts CO2 and RuBP into 6 carbon intermediate
- This means RUBISCO is shielded from the oxygen produced by photolysis in the chloroplast
What is CAM photosynthesis?
- CAM plants close their stomata all day to reduce water loss by transpiration and only open them during the cooler night
- CO2 absorbed at night is converted to a 4 carbon molecule called Malic Acid
- Malic acid builds up during the night as a store of CO2
- During the day, when the stomata close, the malic acid is transported into the chloroplasts
- Malic acid converted back into CO2
- RUBISCO uses CO2 and RuBP to make 6 carbon intermediate and the Calvin cycle functions as normal
- CAM plants are adapted to high light levels and arid conditions, where the stomata only open at night
- e.g. Cacti
How can the rate of photosynthesis be measured?
- Volume of O2 produced per unit time
- Aquatic plant submerged in water in conical flask attached to gas syringe - Measure movement of air bubble in capillary tube and calculate rate of respiration by multiplying distance moved x internal diameter
- Cut out discs from leaf and count how many discs float in certain period of time as O2 production makes leaf buoyant
