2.9 Photosynthesis

Question 1

What is photosynthesis?

Answer 1

the ability of plants to capture the energy of light and to turn it into the chemical energy of food molecules
two steps: light-dependent reactions (thylakoid) and light-independent reactions (stroma)

Question 2

Equations of photosynthesis

Answer 2

carbon dioxide + water + sunlight —–> glucose + oxygen

6CO2 + 6H2O —–> C6H12O6 + 6O2

Question 3

Parts of a chloroplast

Answer 3

Outer membrane
Inner membrane
Stroma: clear fluid that organelles are embedded in, has appropriate enzymes and suitable pH for Calvin Cycle
Thylakoid: small disks, has electron transport chain and ATP synthase needed for photophosphorylation
Granum: stacks of thylakoids, increases SA:Vol ratio and the small internal volume quickly accumulates ions
Lamella: membranous extensions that join the granum, connects and separates thylakoid stacks

Question 4

What is the absorption spectrum?

Answer 4

• shows the % of wavelength absorbed by the common chlorophyll pigments (chlorophyll a and b)
• shows the greatest absorption for violet, blue and red light
• lowest for yellow and green
  graph: touches the x axis at green (550) area and peaks at red (450) and violet (650)

Question 5

What is the action spectrum?

Answer 5

• shows the % use of different wavelengths of light by chlorophyll (all pigments)
• shows most efficiently used light is red, blue and violet
• least efficient is yellow and green
  graph: peaks at red, blue and violet (400-500 & 600 - 700), lower at yellow/green (500-600) but doesn’t touch axis as xanthophylls and carotenes still absorb light and hence can be used for photosynthesis

Question 6

Experiment to show rate of PS: CO2

Answer 6

• CO2 uptake can be measured by placing a plant in an enclosed space with water
• CO2 interacts with the water molecules, producing bicarbonate and hydrogen ions, which increases the acidity of the resulting solution
• the change in pH can therefore provide a measure of the CO2 uptake by a plant
  (increased CO2 uptake=more alkaline pH)

Question 7

Experiment to show rate of PS: O2

Answer 7

• O2 gas can be measured submerging a plant in an enclosed space with water attached to a sealed gas syringe
• any O2 gas produced will bubble out of solution and can be measured by a change in water level

Question 8

Experiment to show rate of PS: glucose production

Answer 8

two different ways:

• can be measured indirectly by a change in a plant’s biomass
• this required plant to be completely dehydrates prior to weighing to ensure the change in biomass reflects a change in organic matter, not water content
• determine the change in starch levels (as glucose is stored as starch in plants)
• iodine staining then data quantitated using a colorimeter

Question 9

Factors that affect photosynthesis

Answer 9

Enzymes:

• PS is run by enzymes
• any factor that affects enzymes will affect the rate of PS

Temperature:

• at cold temps PS is reduced as the enzymes that catalyse the reaction work slowly
• it speeds up as the temp rises until is becomes optimum
• when it gets too hot it slows down until the enzymes become denatured and PS stops

Amount of CO2:

• at low/med conc it is the combining of CO2 to make glycerate 3 phosphate that controls the rate of PS
• CO2 exists at low levels (0.03%) so changes in CO2 levels can have a bif effect
• at high conc other factors will limit rate so increased CO2 will increas PS but only up to a point

Light Intensity:
- at low light intensity the light dependent reactions are slow and there is a shortage of NADPH+HT and ATP