Presentation on theme: "Lab 5: Photosynthesis. Leaves float in water due to presence of air (air spaces) in the leaf. How does one get them to sink?"— Presentation transcript:
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 CO 2 …what can we add to the water to put lots of dissolved CO 2 ?
Lab 5: Photosynthesis ADD BAKING SODA… Na + HCO 3 - dissociates in aqueous solution to Na + + HCO 3 - HCO H 3 O+ H 2 CO 3 + H 2 O H 2 CO 3 H 2 O + CO 2 Na + HCO 3 - (sodium bicarbonate)
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 O 2 and producing CO 2 gas as so you are actually also observing cell resp though the amount of CO 2 = O 2 used…
Lab 5: Photosynthesis Review…
Lab 5: Photosynthesis
Lab 5: Photosynthesis 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. Lets test the amount of CO 2 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 ET 50 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 ET 50 (Estimated Time it takes for 50% of disks to float…median)
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 ET 50 for both.
Lab 5: Photosynthesis Class Data TrialsET 50 – Pure Water (s)_ ET 50 – 0.2% bicarbonate (s) 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 ET 50 (s) Frequency of trials Histogram indicating the distribution of ET 50 measurements in a baby leaf spinach disk assay Blue – pure water Green – 0.2% bicarbonate Now what?
Lab 5: Photosynthesis Class Data TrialsET 50 – Pure Water (s)_ ET 50 – 0.2% bicarbonate (s) MEAN SD (σ) SE
Lab 5: Photosynthesis Class Data TrialsET 50 – Pure Water (s)_ ET 50 – 0.2% bicarbonate (s) MEAN SD (σ) SE And now… Make sure you recall the meaning of the standard deviation and standard error.
Lab 5: Photosynthesis Class Data Pure Water Time (s) 0.2% Bicarbonate Figure X. Mean ET 50 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).