Lab 5

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Lab 5: Photosynthesis
Lab 5: Photosynthesis
Lab 5: Photosynthesis
Leaves float in water due to presence of air (air spaces) in the leaf. How
does one get them to sink?
Lab 5: Photosynthesis
Put leaf discs under vacuum by placing them in an aqueous solution
inside a syringe, cover the hole of the syringe with your thumb, and then
pull back on the syringe…negative pressure…sucks air out of leaf (fig
6a). Disks will sink (fig. 6b).
Lab 5: Photosynthesis
We want leaf discs to still be able to do photosynthesis in the
solution…but you just removed the air and replaced it with water,
which has little dissolved CO2…what can we add to the water to
put lots of dissolved CO2?
Lab 5: Photosynthesis
Na+HCO3- (sodium bicarbonate)
ADD BAKING SODA…
Na+HCO3- dissociates in aqueous solution to Na+ + HCO3HCO3- + H3O+ <-> H2CO3 + H2O
H2CO3 <-> H2O + CO2
Lab 5: Photosynthesis
Now hit them with light…what happens?
They will float after some amount of time because O2 is being produced by
photosynthesis.
However, this is only an INDIRECT method of measuring CO2 rate…why?
Cell Respiration is occurring, using some of the O2 and producing CO2 gas as so you are
actually also observing cell resp though the amount of CO2 = O2 used…
Lab 5: Photosynthesis
Review…
Lab 5: Photosynthesis
http://www.youtube.com/watch?v=ZnY9_wMZZWI
Lab 5: Photosynthesis
What is the dependent variable in this experiment?
The time it takes for each disk to float. This is what you are
measuring.
What shall we test (the independent variable)?
Let’s test the amount of CO2 dissolved in the water!!!!
Lab 5: Photosynthesis
Why is dilute soap being added in step 3?
The cuticle of the leaf is hydrophobic and prevents the leaf discs from ever sinking.
The soap, as you know, is amphiphilic (its essentially a fatty acid). It will coat the
leaf discs (tails interacts with cuticle and carboxyl head interacts with water). This
will allow the discs to sink after the gas is sucked out.
Lab 5: Photosynthesis
Why is dilute soap being added in step 3?
The “grease particle” in this case are the leaf discs…
Lab 5: Photosynthesis
Prepare the disks…
Lab 5: Photosynthesis
Prepare the disks…
- One set of ten gets degassed in pure water (control)
- One set of ten gets 0.2% bicarbonate (experimental group)
Lab 5: Photosynthesis
Place both cups under the light source…
Lab 5: Photosynthesis
The ET50 (Estimated Time it takes for 50% of disks to float…median)
The ET50 is the estimated time it takes for
50% of the disks to float, which has been
shown to be a reliable measure to compare
between experiments if you repeat this…
Lab 5: Photosynthesis
Class Data
Every group should have a chart like this, but with two lines…one for pure water
and one for 0.2% bicarbonate solution from which they will determine the ET50 for
both.
Lab 5: Photosynthesis
Class Data
Trials
ET50 – Pure Water
(s)_
ET50 – 0.2%
bicarbonate (s)
1
415
388
2
420
364
3
408
371
4
395
395
5
424
410
6
398
381
7
416
379
8
410
405
Here is our data…now what do we do to analyze it and see if there if a difference
between these two groups?
Lab 5: Photosynthesis
Class Data Histogram indicating the distribution of ET
measurements
in a baby leaf spinach disk assay
50
3.5
Frequency of trials
3
Blue – pure water
Green – 0.2% bicarbonate
2.5
2
1.5
1
0.5
0
361-370
371-380
381-390
391-400
ET50 (s)
Now what?
401-410
411-420
421-430
Lab 5: Photosynthesis
Class Data
Trials
ET50 – Pure Water
(s)_
ET50 – 0.2%
bicarbonate (s)
1
415
388
2
420
364
3
408
371
4
395
395
5
424
410
6
398
381
7
416
379
8
410
405
MEAN
SD (σ)
SE
Lab 5: Photosynthesis
Class Data
Make sure you recall
the meaning of the
standard deviation
and standard error.
And now…
Trials
ET50 – Pure Water
(s)_
ET50 – 0.2%
bicarbonate (s)
1
415
388
2
420
364
3
408
371
4
395
395
5
424
410
6
398
381
7
416
379
8
410
405
MEAN
410
387
SD (σ)
10.2
16.0
SE
3.60
5.67
Lab 5: Photosynthesis
420
Class Data
410
Time (s)
400
390
380
370
360
Pure Water
0.2% Bicarbonate
Figure X. Mean ET50 values for leaf disks evacuated and submerged in pure
water and 0.2% bicarbonate solution. Error bars indicate standard error for
eight trials (n=8).
Lab 5: Photosynthesis
Lab 5: Photosynthesis
Lab 5: Photosynthesis

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