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Photosynthesis Unit Pg 10
STOMATES LAB
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Photosynthesis occurs wherever there are chloroplasts in cells. You can tell if chloroplasts are
present in cells because the plant part will appear green. Often it is just the leaves of a plant that
contain chlorophyll, but many herbaceous plants also have chloroplasts in their stems. Leaves have
special structures that deliver carbon dioxide (CO2) gas to the leaves and allow for the escape of
oxygen gas (O2). These structures are called stomates. Stomates also help control water loss, or
allow for excess water to be evaporated away from the leaf. Stomates are like little gated openings,
found mostly on the underside of leaves (why the underside and not the top? Think water loss!). The
stomate is the hole. The hole is surrounded by two guard cells.
The cell membrane of guard cells is different on the hole side than it is around the rest of the
cell. On the hole side the membrane is thick and NOT at all flexible. The rest of the cell membrane is
flexible. In this activity you will look for guard cells and stomates, noting that the guard cells have
chloroplasts in them. When light is available, and the plant wants to photosynthesize, it needs the
guard cells to open so the rest of the leaf can get CO2. This is where the chloroplasts in the guard
cells come in. They allow the guard cell to photosynthesize, so sugar is produced. This creates a
hypertonic solution inside the guard cell, and water rushes in. With all that water in the guard cell, it
swells. The thick part of the membrane cannot stretch, but the rest of the membrane can. This causes
the guard cell to become distended, pulling on the thick part and bending it away from the hole (see
sketch below). When both guard cells swell, the hole becomes bigger.
NOTE: This hypothesis of how guard cells open is an old hypothesis. More recent hypothesis
attribute the opening to several different possible factors.
Procedure (Watch the demonstration!)
1. Obtain a small section of a green leaf.
2. Bend the leaf in half so the undersides are touching. You are trying to crack the top side.
3. Carefully pull away one layer of the top side, leaving just the epidermis of the underside. You
need to get a SINGLE layer of cells! A good piece will appear colorless.
4. Cut off this colorless piece while holding the leaf on a slide.
5. Add a drop of water and a coverslip.
6. Observe under low, then medium, then high power of the microscope.
7. Sketch the guard cells around two stomates that are near each other. Sketch the clear
epidermal cells between and around the two stomates.
Sketch:
Questions:
1. About how many epidermal cells are there for every stomate?
2. How could you accurately measure the concentration of stomates in a leaf?
3. In what kinds of plants would you predict there to be a high concentration of stomates? Why?
a. Low concentration of stomates? Why?
4. Explain how stomates help a plant and the (old hypothesis) mechanism that allows them to
open.
5. What must happen to make a stomate close? (THINK!)
EXTRA CREDIT
Look up newer hypotheses that explain how stomates open. Cite your references (better, copy them
and highlight the important information) and write a 1 paragraph summery.