Photosynthesis Lab Modified from pasco probeware protocols. Purpose




Дата канвертавання19.04.2016
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Photosynthesis Lab
Modified from PASCO probeware protocols.
Purpose: To explore the production of oxygen by photosynthesis from an aquatic plant (Elodea), phytoplankton (Thalassiosira weissflogii) and Cyanobacterium (Anabaena) and to observe the consumption of oxygen by respiration.
Terms:

Carbohydrates – a source of food

Concentration – the amount of a substance

Dissolved – a substance staying in liquid

Saturation – containing the maximum amount of a substance that can be dissolved

Equilibrate – to stabilize


Background: The process of photosynthesis is the chemical pathway by which all plants make food from carbon dioxide - CO2, water - H2O, and sunlight. The process to transform CO2, H2O and light is a complex series of transformations that take place inside of a cell. During the transformation, hydrogen from water is added to molecules of carbon dioxide to make carbohydrates. The generalized equation for photosynthesis is given below:
6CO­2 + 6H2O light C6H12O6 + 6O2


Carbon

dioxide

Water

Carbohydrate

Oxygen

The process of respiration is the metabolic pathway in which most living organisms generate energy from carbohydrates – C6H12O6 and oxygen – O2. The generalized equation for respiration is the reverse of the photosynthesis equation.


During photosynthesis by aquatic plants, phytoplankton and Cyanobacteria, oxygen is released into the water where it dissolves, forming dissolved oxygen. Water can only hold a limited amount of oxygen, when the concentration reaches this limit (saturation) oxygen is released (out-gases) into the air.
There are a series of biochemical events that comprise photosynthesis that are divided up into the light-dependent (require light) and light-independent reactions. The light-dependent reactions are involved in capturing light energy into energized molecules that are then used in the light-independent reactions to fuel the synthesis of sugars (carbohydrate) from carbon dioxide and water. The light-independent reactions do not require light and they can occur and do occur under light conditions.
Pre-Lab Questions

How do you think the oxygen concentration will change when exposed to light?


Oxygen concentrations should go up
How will the oxygen concentration change when kept in the dark?

Oxygen concentrations should go down



Materials

1 – Elodea couple of shoots

1 – PASCO Xplorer GLX with oxygen probe

1 – Tank

1 – Light

1 – Black garbage bag

1 – Magnetic stir bar and stir plate

Tap water, equilibrated overnight


Procedure

  1. Turn on the probeware.

  2. Turn on the magnetic stirrer and place the stir bar in the flask.

  3. Insert the dissolved oxygen probe into the Elodea culture until the silver band is under the water.

  4. Allow the probe to equilibrate and record the value (background).

  5. Turn the lamp on and record the dissolved oxygen concentration every minute for 15 minutes.

  6. After 15 minutes turn off the lamp and place the garbage bag over the entire flask.

  7. Record the dissolved oxygen concentration every minute for 15 minutes.

  8. After 15 minutes turn off the probeware and clean off the probe.


Data

Time

O2 concentration

Time

O2 concentration

Time

O2 Concentration

Time

O2 concentration

Background

4.0

11

4.3

22

4.0







1

4.0

12

4.4

23

3.9







2

4.0

13

4.4

24

3.9







3

4.1

14

4.4

25

3.8







4

4.1

15

4.4

26

3.8







5

4.1

16

4.3

27

3.7







6

4.2

17

4.2

28

3.7







7

4.2

18

4.2

29

3.6







8

4.2

19

4.1

30

3.6







9

4.3

20

4.1













10

4.3

21

4.0
















  1. What are the units for O2 concentration? mg/L – milligrams per liter

  2. Make a graph of the data you recorded below. Label the units for each axis.


4.5


4.0

3.5


0

15

30

Time (min)


Questions



  1. What happened to the oxygen concentration when you turned on the light? Why do you see this change?

The oxygen concentration increased because the plant started to photosynthesize.



  1. What happened to the oxygen concentration when you turned off the light? Why do you see this change?

The oxygen concentration decreased because the plant stopped photosynthesizing and respiration consumed the oxygen.



  1. During photosynthesis carbon is removed from the water – the CO2 is dissolved and is stored in the plant .




  1. How has photosynthesis from aquatic plants changed the atmosphere over time?

They have produced oxygen gas and in the process consumed/decreased the amount of carbon dioxide in the atmosphere.


  1. If there was no carbon dioxide in the atmosphere or in water could photosynthesis occur? Why or why not?

No, photosynthesis could not occur because one of the necessary starting components to photosynthesis is carbon dioxide.


  1. Does photosynthesis and respiration in the photosynthetic organism occur at the same time?

Yes, they occur at the same time. During light conditions photosynthesis produces more oxygen than is consumed by respiration and that is why we see a net increase in oxygen concentration. During dark conditions more oxygen is being used by respiration than is being generated by photosynthesis and that is why we see a net decrease in oxygen concentration.


  1. What differences do you see in the Elodea oxygen concentrations in 30 min compared to the 24 hr cycle of the marine organisms (Thalassiosira weissflogii, and Anabaena )?

The changes in oxygen concentration for Elodea are much smaller than the 24 hr cycle of the marine organisms. All the organisms have a similar pattern of increasing oxygen concentration during the light period and decreasing in the dark.


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