Classroom adventure - plant cells and pollen

Learning how to use the compound microscope can be awkward at first.  Trying to get your specimen in the field of view, focusing, changing objective lenses, losing your specimen and having to find it all over again. But students eventually get it and I love seeing the excitement on their faces when they finally figure it out and see cells for the first time.  

Elodea, an aquatic plant species, is useful to look at plant cells under the microscope.  The leaves are thin so you can see cells.  There are two features that are easily seen that distinguish plant cells from animal cells - the rigid cell walls and chlorophyll - the pigment that allows plants to absorb and use energy from sunlight and gives plants their green color.

This set up is also a good way to demonstrate the process of osmosis.  Cells are surrounded by a semi- permeable membrane, that is, the membrane is very picky about what it lets in and out of the cell.  Water inside and outside a plant cell have different concentrations of solutes in it.  Solutes are the "stuff" diluted into the water, such as sugar or salt.  Water will move either into or out of the cell to equalize the concentrations on both sides of the membrane.  The movement of water for this purpose is called osmosis.

In this first slide, we used distilled water, water that has had most of its solutes removed.  In this case, there are more solutes inside the cell than outside, so water enters the cell to equalize the concentrations.  You can see the vacuole (the structure that holds the water inside the cell) is expanded.  The vacuoles are so full, they squish the chlorophyll all the way to the edges of the cell.

Elodea cells in distilled water

In this second slide, we added salt water, so in this case, there are more solutes outside the cells.  Water then rushes out of the cell to try to equalize the concentrations.  In many cases, so much water has left the cell, the cell membrane, which is located just inside the cell wall, has collapsed.  Osmosis is the reason plants suffer damage when salt is used to de-ice roads and then washes into the soil.  Too much salt in the soil makes water leave the plant roots instead of entering and the plants wilt and often die. 

Elodea cells in salt water

Plants with hairs on their leaves or stems are also useful for looking at individual cells.  Hairs on plants are called trichomes.  In the slide below you see the trichomes from a sunflower stem.  Each hair shows the individual cells that make it up.  And if you look closely, you can see the individual cells on the stem tissue as well.

Trichomes on sunflower stem

For some of us, pollen is just a nuisance that gives us allergies.  But for plants, it is vital for reproduction. Pollen grains, made in the anthers, carry the male sex cells of plants.  Their only concern is to get to the female organ of the plant (the carpel, or pistil) and fertilize the egg.  Some plants use animals to transfer the pollen from the male to female organs - think bees, butterflies, birds, but also bats and other animals. Animal pollinated plants do not give us allergies since their pollen never gets airborne.  Some plants use wind to transfer their pollen.  They produce an abundance of pollen which they release into the air and let the wind carry it off to other plants.  These wind pollinated plants are the ones that give us our allergies.  

Pollen grains are different for each species, like a plant fingerprint.  A palynologist is a scientist who studies pollen (and other microscopic particles) and he or she can tell you what plant produced the pollen you are looking at.  Some pollen grains are plain, some have complex structures.  We looked at two samples in class.  Both of these are animal pollinated plants, so neither are responsible for your allergy.

Gladiola pollen

Sunflower pollen