Lab 4: Photosynthesis in Algal Cells

Question 1

Chlorophyll

Answer 1

light absorbing pigment located in the thylakoid membranes of the chloroplasts

Question 2

overall photosynthesis equation

Answer 2

6CO2 + 6H2O —> C6H12O6 + 6O2

Question 3

What is the hill reaction

Answer 3

the light reaction.
photolyzes water and traps energy in the form of ATP and NADPH.
-produces oxygen as a biproduct.

Question 4

Overall reaction for the hill reaction

Answer 4

12H2O + 12NADP+ +18Pi + 18ADP —> 6 O2 + 12 NADPH + 18 ATP

Question 5

Why isn’t the hill reaction dependent on temperature?

Answer 5

the hill reaction uses light energy in order to transfer electrons through photosystems and ultimately to NADPH. It does not need enzymes, and thus, if the heat was to increase, there is no problem of enzymatic denaturation.

Question 6

Why is the hill reaction dependent on light?

Answer 6

the hill reaction uses light energy in order to transfer electrons through photosystems and ultimately to NADPH. Without any light exciting and sparking the photolysis of water, NADPH could not be created.

Question 7

What is the Calvin cycle

Answer 7

the dark reaction.

- NADPH and ATP made in the hill reaction used to make glucose. CO2 is REDUCED/FIXED to form GLUCOSE.

Question 8

Net reaction for the calvin cycle

Answer 8

12CO2 + 12NADPH + 18ATP —-> C6H12O6 + 12 NADP+ + 18 Pi + 18ADP + 6 H2O

Question 9

Is the calvin cycle dependent on temperature? What about light

Answer 9

the calvin cycle is dependent on temperature because it is dependent on temperature-sensitive enzymes, such as rubisco, to fix Co2 into glucose. It is NOT dependent on light because there are no photolysis of any compounds

Question 10

Where do the light reactions of photosynthesis take place? what about the dark reactions?

Answer 10

light reactions: thylakoid membrane

dark reactions: stroma of chloroplasts

Question 11

How did we measure the light reactions of photosynthesis in this lab? Source of error for this sort of measurement?

Answer 11

we used an oxygen electrode to measure the amount of O2 produced by the hill reaction. the higher the concentration of oxygen produced, the higher the rate of O2 diffusion across the electrode membrane, which could be quantified.

source of error: must ensure that the electrode is sealed so that no oxygen escapes or enters the membrane, causing different readings.

Question 12

The actual rate of oxygen evolution will be ____ than the Observed rate of photosynthesis. Why?

Answer 12

The actual rate of oxygen evolution will be HIGHER than the Observed rate of photosynthesis.

• you need to add the value of O2 evolution and Absolute value O2 consumption to get the actual rate of oxygen evolution, because respiration is still occurring while O2 is being consumed. ( the dark reaction can still occur in the light, it just doesn’t rely on light)
• see lab 4 notes for observed vs actual rates of photosynthesis.

Question 13

before measuring oxygen, why do you bubble N2 gas into the system

Answer 13

-in order to displace any remainding O2/present O2 prior to conducting the experiment

Question 14

Terbutrin, a photosynthetic inhibitor, was added to the algal suspension, and O2 evolution was measured in the LIGHT. what do you expect to happen?

Answer 14

even though the light is on and photosynthesis should be able to take place, the photosynthetic inhibitor blocks PHOTOSYSTEM II, inhibiting the Hill reaction. Therefore, even though light is present, no oxygen can be produced. No NADPH would be produced either, since electrons cannot be conveyed down the thylakoid membrane.

Question 15

After homogenizing the algae and isolating the chloroplasts, the pigment concentration was determined by measuring the absorbance at 650 (0.407Abs), 665nm (0.965Abs), and 750 nm (0.004). What is the concentration of chlorophyll in microGRAMS per LITER? what about in micrograms/ml?

Answer 15

1251.16 micrograms/liter, 1.25 microgram/mL. See lab 4 notes for calculations/

Question 16

Why do you need to subtract the absorbance values of chlorophyll at 650 and 665 from 700?

Answer 16

to account for background.

Question 17

at 25 degrees, the solubility of oxygen is 7.37 microg/ml. We used 2 mls of Euglena (2 mls of chlorophyll) in this experiment. If the average chlorophyll content was 1.25 microg/ml, and the total units of the chart of the oxygen electrode was 90 units (to represent the total amount of O2 present), how many micrograms of Oxygen per minute per microgram of chlorophyl is being synthesized at 25 degrees? The amount of oxygen evolved from graph was 6.60 units/min

Answer 17

0.4556 micrograms of oxygen evolved per minute for every 1 microgram of chlorophyll present.

Question 18

which specific enzyme of the calvin cycle is effected by temperature?

Answer 18

the rubisco enzyme. at high temperatures, it cannot distinguish between CO2 and O2, and it will try to fix O2, resulting in the formation of 2-phosphoglycerate toxin. At high temps, the plants will die because of the toxin.

Question 19

why do we calculate the actual value of oxygen evolution?

Answer 19

the actual value is the amount of oxygen evolution plus the amount of oxygen consumption (no negatives). This compensates for the oxygen that is being used for cellular respiration while the light reactions are still taking place and creating oxygen.