Photosynthesis and Leaf Pigments Lab Questions

Photosynthesis

Complete the questions in the lab. 

perform in class! Both our protocol and this video use our favorite underwater plan Elodea to perform photosynthesis.

Our lab protocol used the pH indicator phenol red, this video demo uses bromothyml (BTB). Both are chemicals that change color based on the pH of the solution.

a.At 0:46 in the video (after BTB has been added but before the straw has been put in the beaker), what pH do you estimate the solution is according to this scale?

b.At 1:05 in the video (after bubble are no longer being blown in the beaker), what pH do you estimate the solution is according to this scale? Is it more basic or more acidic?

c.How did a human breathing through a straw into the beaker cause this chemical change?

2.Which of the 4 tubes in the experiment showed the greatest color change after 60 minutes of the experiment? 

a.What pH do you estimate this tube is at the end of the experiment according to this scale? Did the pH of this tube that changed the most become more basic or more acidic?

b.Why did none of the other tubes in the experiment show much of a chemical change? Provide an explanation for each of the 3 tubes that did NOT change.

Like Elodea, seagrass is an aquatic plant that performs photosynthesis and lives entirely underwater. In many coastal areas, seagrass habitat has been destroyed and the plants killed. Will these habitats without seagrass experience ocean acidification more or less strongly than areas with still living seagrass? Explain why.

As discussed in class, chlorophyll is the most important pigment for plants to capture light energy and perform photosynthesis. Specifically chlorophyll a is the pigment responsible for moving an electron up to a higher energy state to power the light reactions. That is not the only pigment that plants have! To make all the different colors we see among different plant leaves and flowers, plants have to create different chemical pigments that reflect various parts of the visible light spectrum.

The in-person lab protocol separated the pigments from spinach leaves, and this video demonstration also separates the pigments of spinach leaves using paper chromatography.

4.After the video demonstration scientist grinds up the spinach leaves, they dissolve the pigments from the leaf bits in ethyl alcohol. 

a.If chlorophyll dissolves in ethyl alcohol (ethanol) but NOT water, is chlorophyll a hydrophobic or hydrophilic pigment?

b.Are there more polar or nonpolar covalent bonds in the molecule?

5. According to page 3 of the lab packet PDF (Additional Photosynthesis Information”), what physical factor separated the different pigments on the paper? What determined which pigment traveled the furthest to the top?

6.According to the video (and the information on page 6 of the lab packet PDF (Additional Photosynthesis Information”), why was it important for the video demonstrator to keep the ethanol away from the boiling water? (Reminder: water boils at 100°C)

7.The first pigments removed by the boiling water bath were anthocyanins that create the red/purple color of the Coleus leaves.

a.Which pigment did the ethanol bath remove?

b.How did that change the leaf color?

8.What reagent (that we've used several times in lab ourselves) tested for starch at the end of the experiment? Did the leaves test positive or negative for starch? Explain why.

9.Recall from an earlier virtual lab, that the onion cells and potato cells viewed under the microscope did NOT contain chloroplasts. How is it possible for onion and potato cells to live without photosynthesis? Where does the energy to survive and sprout come from without sunlight?