7. OBJECTIVESOBJECTIVES
Photosynthesis
A photo-chemical
process
Photosynthesis
A photo-chemical
process
Photosynthesis
experiments
Photosynthesis
experiments
QUIZQUIZ
INTRODUCTIONINTRODUCTION
XXXX
Photosynthesis: A Photochemical
Process – Feedback to Question 2
7
Why photosynthesis is a photochemical process?
Photosynthesis is a photochemical process
because light is required for some of the
reactions to occur.
The word ‘photo’ means ‘light’ so that part of
the word refers to the fact that the light-
dependent phase is initiated by the absorption of
light by chlorophyll.
NOTE: Although the word ‘photon’ is a
measure of light, that is not the word
incorporated into photosynthesis.
(Remember also -- The ‘synthesis’ means to
‘make’/’manufacture’, so that part refers to the
manufacturing reactions of the light-
independent phase, from which organic
molecules are synthesized or made.)
Question 3
9. OBJECTIVESOBJECTIVES
Photosynthesis
A photo-chemical
process
Photosynthesis
A photo-chemical
process
Photosynthesis
experiments
Photosynthesis
experiments
QUIZQUIZ
INTRODUCTIONINTRODUCTION
XXXX
Photosynthesis experiments –
Materials for and Products of
Photosynthesis – Question 2
A common test done, to show that photosynthesis has
occurred, is the starch test. Why do we test for starch, not
glucose? Discuss it in your group, then tell your teacher.
9
Feedback: Glucose, a product of photosynthesis, is soluble and
so can affect the osmotic balance in cells in which it is
produced. So, it is converted to insoluble starch almost
immediately.
The insoluble starch granules can be stored in the leaves
without affecting the osmotic balance of the cells. So, we can
test for this product. See the next slide for more information.CHECKCHECK
19. OBJECTIVESOBJECTIVES
Photosynthesis
A photo-chemical
process
Photosynthesis
A photo-chemical
process
Photosynthesis
experiments
Photosynthesis
experiments
QUIZQUIZ
INTRODUCTIONINTRODUCTION
XXXX
Photosynthesis experiments -
Is light really needed?
3. Write a hypothesis on which to base excluding light
from parts of a green leaf, then testing the entire leaf
for starch.
19
Feedback to Item 3: If light is excluded from a
green leaf, then it will not be able to carry out the
photochemical reactions of photosynthesis, so no
glucose will be made and no starch will be present
when the leaf is tested.
4. Test leaf discs from covered and uncovered areas of
green leaves to determine where starch is present.
Feedback to Item 4: Your results should be that only
the leaf discs from the uncovered , green leaf parts
should give a positive starch test result.FEEDBACKFEEDBACK
FEEDBACKFEEDBACK
23. OBJECTIVESOBJECTIVES
Photosynthesis
A photo-chemical
process
Photosynthesis
A photo-chemical
process
Photosynthesis
experiments
Photosynthesis
experiments
QUIZQUIZ
INTRODUCTIONINTRODUCTION
XXXX
Photosynthesis experiments -
Limiting factors
4. Read and look at the graphs at > http://tinyurl.com/7s7knw7
5. From the graphs , what is meant by ‘limiting factors’ for
photosynthesis?
23
A “limiting factor is any factor beyond which the rate of
photosynthesis can not be increased, until the factor is dealt
with”. These are factors such as light intensity, CO2
concentration and temperature that can limit/control the
increase in the rate of photosynthesis.
Go to section 5) in your SIM.
For example, the first graph (Below) shows
that as light intensity increases, photosynthesis
rate also increases until it reaches a point
where the rate remains constant and is no
longer increased or affected by any further
increase in light intensity.
FEEDBACK TO ITEM 5FEEDBACK TO ITEM 5
25. OBJECTIVESOBJECTIVES
Photosynthesis
A photo-chemical
process
Photosynthesis
A photo-chemical
process
Photosynthesis
experiments
Photosynthesis
experiments
QUIZQUIZ
INTRODUCTIONINTRODUCTION
XXXX
SUMMARY
25
Photosynthesis takes place in two stages:
Light-dependent or photochemical reactions - in which plants
use light energy they absorb by means of chlorophyll to split
the water molecules, which had been absorbed by roots and
transported to the leaves, into hydrogen and oxygen.
Light independent or ‘dark’ stage – in which plants use the
hydrogen they obtained in the light dependent stage and
combine it with carbon dioxide, which entered the leaves via
stomata, to form simple sugars such as glucose. Oxygen is
given off through stomata.
Glucose can be quickly converted to insoluble starch and
stored in leaf cells, or glucose can be used in respiration to
release energy which the plant cells require for various
processes. Some glucose can be trans-located via phloem tissue
to storage areas.
Photosynthesis experiments can be done (a) to show the
materials needed for photosynthesis (e.g. light, CO2,
chlorophyll) or (b) to confirm the products of photosynthesis
(e.g. starch and O2).
34. OBJECTIVESOBJECTIVES
Photosynthesis
A photo-chemical
process
Photosynthesis
A photo-chemical
process
Photosynthesis
experiments
Photosynthesis
experiments
QUIZQUIZ
INTRODUCTIONINTRODUCTION
XXXX
Feedback to M. C. Question 4
34
The answer is D.
In an experiment to test a green leaf for starch,
the leaf is initially placed in boiling water. Why is
this done? To kill the cells in the leaf.
A. To remove chlorophyll from the leaf, we boil it in alcohol in a
water bath.
B. The starch test does not require high temperatures (e.g. if you
drop iodine on bread or the cut surface of a potato, you’d get the
blue-black colour showing starch is present). The use of heat kill
cells, making their membranes more permeable to the
penetration of the indicator.
C. Starch was already formed from glucose in the cells.
NEXT QUESTIONNEXT QUESTION
36. OBJECTIVESOBJECTIVES
Photosynthesis
A photo-chemical
process
Photosynthesis
A photo-chemical
process
Photosynthesis
experiments
Photosynthesis
experiments
QUIZQUIZ
INTRODUCTIONINTRODUCTION
XXXX
Photosynthesis experiments
Feedback to Questions 1 - 4Feedback to Questions 1 - 4
36
1.
1. The photos at left show the kind of result you would get for
variegated leaves, i.e. only the original green parts of the leaves
would show the blue-black colour when Iodine is added during
the test for starch.
2. This is because only the original green parts contained
chlorophyll. So it is in those cells that photosynthesis occurred.
The glucose made there was converted to insoluble starch and
stored in those leaf cells which showed blue-black when tested
with iodine.
3. A plant is de-starched by keeping it in a dark cupboard or box,
for at least 24 hours and perhaps even up to three days, thus
preventing its green leaves and other green parts from getting any
sun)light, so it will no longer make glucose. Also, any starch that
had been present would most likely be used up in respiration or
changed and trans-located elsewhere for use/storage..
4. De-starching a plant or a leaf prior to photosynthesis tests will
ensure that all starch has been transported away from the leaves,
so that any positive starch test obtained during the test will
indicate that photosynthesis had occurred during the test..
FEEDBACK
NEXT QUESTIONNEXT QUESTION
37. OBJECTIVESOBJECTIVES
Photosynthesis
A photo-chemical
process
Photosynthesis
A photo-chemical
process
Photosynthesis
experiments
Photosynthesis
experiments
QUIZQUIZ
INTRODUCTIONINTRODUCTION
XXXX
Photosynthesis experiments
Review Question 5
37
5. The diagrams show Before and After for a starch test
for photosynthesis. The top diagram (Before) shows that
vaseline was smeared on one half of a green leaf, from
a plant that had been fully de-starched then given the
necessary conditions for photosynthesis and finally
tested for starch. Explain the ‘After’ or results.
Feedback: When vaseline was smeared on the test leaf,
the stomata were covered so no air, containing CO2 could
enter the leaf. CO2 is essential for simple carbohydrates
e.g. glucose, to be made during photosynthesis, so
without it, there was no glucose to be converted to starch
and stored in leaf cells. Therefore in the half of the leaf
from which CO2 entry was prevented, photosynthesis did
not occur, so the starch test there was negative. The
other half acted as a control and showed a positive starch
test.
FEEDBACK TO REVIEW QUESTION 5FEEDBACK TO REVIEW QUESTION 5
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http://www.und.nodak.edu/dept/jcarmich/101lab/lab8/lab8.html
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