Lab 5: Photosynthesis. Leaves float in water due to presence of air (air spaces) in the leaf. How...

Lab 5: Photosynthesis


Leaves float in water due to presence of air (air spaces) in the leaf. How does one get them to sink?


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).


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?


ADD BAKING SODA…Na+HCO3

- dissociates in aqueous solution to Na+ + HCO3-

HCO3- + H3O+ <-> H2CO3 + H2O

H2CO3 <-> H2O + CO2

Na+HCO3- (sodium bicarbonate)


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…


Review…

Lab 5: Photosynthesishttp://www.youtube.com/watch?v=ZnY9_wMZZWI

http://www.youtube.com/watch?v=ZnY9_wMZZWI


What is the dependent variable in this experiment?

What shall we test (the independent variable)?

The time it takes for each disk to float. This is what you are measuring.

Let’s test the amount of CO2 dissolved in the water!!!!


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.


Why is dilute soap being added in step 3?

The “grease particle” in this case are the leaf discs…


Prepare the disks…


Prepare the disks…

- One set of ten gets degassed in pure water (control)

- One set of ten gets 0.2% bicarbonate (experimental group)


Place both cups under the light source…


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…

The ET50 (Estimated Time it takes for 50% of disks to float…median)


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.


Class DataTrials 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?


Class Data

ET50 (s)

361-370 371-380 381-390 391-400 401-410 411-420 421-4300

0.5

1

1.5

2

2.5

3

3.5

Frequency of trials

Histogram indicating the distribution of ET50 measurements in a baby leaf spinach disk assay

Blue – pure waterGreen – 0.2% bicarbonate

Now what?



(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



(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 5.67And now…

Make sure you recall the meaning of the standard deviation and standard error.


Class Data

Pure Water

Time (s)

370

375

380

385

390

395

400

405

410

415

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. Leaves float in water due to presence of air (air spaces) in the leaf. How...

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