Plant Cell Structure

BY105.  Biology of Plants.  Lab Study I. 

 

The cell is the basic structural unit of all organisms.  As in all cells, structure is partnered to function in plant cells.  In this exercise, you will prepare your own slides from common fruits, vegetables, house, and garden plans to see the beauty of form and function within the cells of plants.  Please follow the instructions in your syllabus on how to write up this study in your lab notebook.

 

A.  Cork Cells

The existence of cells was first noted in 1665 when Robert Hooke described the cellular nature of cork tissue.  For this he is given credit for discovering cork cells.  This exercise is designed for you to duplicate his observations.

 

1.  Prepare a dry mount of a small, VERY THIN section of cork tissue.  (The section does not have to be much larger than the period at the end of this sentence.)  Do not use water or add a cover slip.

 

2.  Observe this tissue with both 10X and 40X.

 

3.  Notice the fairly uniform size and general appearance of these cells.  These are dead cells with no internal contents, just a thick, waxy cell wall.

 

4.  Diagram a few connected cork cells and label the CELL WALL.  As for all your drawings in your lab notebook, carefully label what you have drawn, including the name of the organism, any specific parts asked for, and the magnification.

 

 

B.  Elodea leaf

1.  Obtain a small leaf from the tip of an Elodea stem.  Place it in a drop of water on a microscope slide and carefully add a coverslip, lowering one edge first, then slowly lowering the rest of the slide (to avoid trapping air bubbles under the coverslip).  Following the guidelines below, draw several cells and label the cell parts that are underlined.

 

2.  Scan under 4X or 10X power for a section that is only one or two cell layers thick.  Focus on a small group of cells.  Bring to 40X.  Note that the plant cells have a regular shape.  They look like rectangles.  The outline of the cell is clear because the outermost boundary of a plant cell is the CELL WALL.  The cell wall is rigid and gives support to the cell.  In these cells, the PRIMARY CELL WALL is composed mainly of the polysaccharide cellulose.  In some other types of plant cells, additional layers are laid down inside the primary wall.  This is the SECONDARY CELL WALL, which is much thicker and contains LIGNIN in addition to cellulose.  The strength and character of wood is due to lignified secondary cell walls.

 

The cohesiveness of plant tissue is due to the MIDDLE LAMELLA, intercellular glue composed of pectins (and other substances) that holds the cell walls of neighboring cells together.  The middle lamella is most pronounces when the corners of plant cells adjoin.

3.  View the interior of one of these cells.  Note the spherical bright green discs that appear to hug the interior side of the cell wall.  These are the CHLOROPLASTS, and the green pigment contained within them is CHLOROPHYLL.  Chloroplasts are the sites of PHOTOSYNTHESIS, in which the energy of the sun drives the joining of water and carbon dioxide to make glucose.  Although not visible, a PLASMA MEMBRANE encases the PROTOPLAST (everything inside the cell wall).  The plasma membrane is a lipid bilayer with embedded proteins.  It is selectively permeable and regulates what passes into and out of the cell.

 

 

 

4.  Scan several cells to spot a NUCLEUS.  The nucleus is a perfect sphere and clearly outlined by a nuclear envelope, a double membrane layer.  It is larger than a chloroplast, ant it may be adjacent to the cell wall or somewhere in the center of the cell.  The interior is granular because of CHROMATIN, the form DNA takes in a non-dividing cell DNA is the genetic blueprint of life.

 

     You may also see one or two smaller circular regions within the nucleus.  These are NUCLEOLI (nucleolus sigular).  The nuceolus is rich in RNA and is involved in assembly of the ribosomal subunits.

 

 

 

5.  Although the interior of the cell looks empty, that is really not the case.  The center is occupied by the large central VACUOLE.  The TONOPLAST is the vacuolar membrane and will not be visible.  The vacuole serves as a reservoir for materials, including water, that the cell can draw upon when needed.  It may also be a place to sequester waste products.  In fact, some products accumulate in such great quantities that they precipitate as crystals.  Some plant pigments (anthocyanins) are also stored in the vacuole, giving (usually purple) color to fruits, flowers, and sometimes leaves and stems. 

 

 

 

6.  Note the difference between the CYTOSOL and the CYTOPLASM.  The cytoplasm is everything inside the plasma membrane except for the nucleus.  The cytosol is the gel-like matrix inside the plasma membrane that does not include any membrane-bound organelles.

 

 

 

7.  You may be able to observe CYTOPLASMIC STREAMING in your leaf.  If you are able to see cytoplasmic streaming, are all cellular components moving in the same direction and rate within a single cell?  Do all cells have the same direction and rate of cytoplasmic streaming or does it vary from cell to cell?

 

 

C.  Red Cabbage

1. To see anthocyanins stored in the vacuole, examine a leaf from red cabbage.  Peel off the epidermis of a red cabbage leaf by folding a small section of leaf in half and grabbing the thin purple layer with forceps.  Place a small piece in a drop of water on a slide.  Cover.  Examine at 10X, then 40X power.

 

2.  Note that the interior of each cell is purple.  This purple color is due to the presence of a pigment called anthocyanin, which is contained within the VACUOLE of the cell.  Note also the CELL WALLS that define the boundary of each cell.

 

3.  Locate another cell structure prominent in the cells.  It is spherical and centrally located.  Note that it has one or two small, dark-stained circles within it.  What is it?  Label it in your drawing.

 

 

I.  Plastids

Plant color can also be imparted by another organelle, the CHROMOPLAST.  Chromoplasts contain the carotenoid pigments of yellow, orange, and red.  Chromoplasts are related to chloroplasts; in fact, both originate from a common precursor called a proplastid that gives rise to all of the plastids.  Plastids include not only chloroplasts and chromoplasts, but leucloplasts.  Leucoplasts are colorless and include AMYLOPLASTS, which accumulate starch grains, and ELAIOPLASTS, which accumulate lipids. 

 

D.  Avocado fruit

     To see elaioplasts, obtain a tiny piece of avocado fruit and smear  to a thin layer on a slide.  Add a drop of water and a drop of Sudan III solution.  Sudan III will stain the lipid-containing elaioplasts.  Focus under 4X or 10X and then bring up on 40X. What color do the elaioplasts stain in the presence of Sudan III?  Draw a few cells and label the ELAIOPLASTS.

 

E.  Bell peppers

     Chromoplasts  come in a variety of shapes and sizes.  You will know them by their color and abundance in certain fruits, flowers, and in some cases, roots.  To view a chromoplast, take a small piece of bell pepper skin.  Place it on a slide, add a drop of water, and cover with a cover slip.  Draw a few cells and label the CHROMOPLASTS.

 

Look closely at the cell wall.  Can you spot the cytoplasmic bridges through the walls?  These are the PLASMODESMATA that interconnect plant cells.

 

F.   Potato (Solanum tuberosum) Tuber Cells

Amyloplasts are plastids that store starch and therefore stain darkly with iodine.  Use a razor blade to make a thin section of a potato tuber.  Make the section as thin as you can.  Place the section in a drop of water on a microscope slide, add a drop of iodine stain, and add a coverslip.  Iodine is a stain specific for starch.  Draw a cell or two, clearly labeling the AMYLOPLASTS.  What can you conclude about the location of starch in storage cells of potato?  Why is a potato a good source of carbohydrates?

 

 

II.  Crystals

Plants produce metabolic waste products just as animals do, however, they lack excretory systems as a way of eliminating these chemical from their cells for from the entire plant.  One way that plants have evolved to escape the harmful effects of the buildup of wastes is to convert them to insoluble substances.  These insoluble materials are then stored in the cells without doing any damage.  One such harmful waste product is oxalic acid, which is converted to crystals of calcium oxalate.  Both raphides and druses are crystals composed of calcium oxalate.  Why they differ in shape is not known.  We’ll only look at raphides today.

 

G.  Raphides.  Take a small sample of crushed pineapple and spread it thinly on a slide with a drop of distilled water.  Cover with cover slip.  Look at low power first.  Look for bunches of needlelike crystals.  Focus under 40X for a closer look.  These are called RAPHIDES.  Composed of calcium oxalate, they are common in many plants.  You may also find these crystals in the leaves of dumb cane (Dieffenbachia)—this is the plant in the window of the classroom.  This common houseplant got its nickname because, if the leaves are eaten, the sharp raphide crystals stab the tongue and throat, causing inflammation to such an extent that the person is struck dumb or mute. 

 

 

 

Additional Questions

 

1.  Why should the coarse adjustment know NOT be used when the high-power objective is in place?

 

2.  Compare the size, shape, and color of chromoplasts with chloroplasts.

 

3.  At one time it was thought that vegans, those whose diet is based solely on plants, would not get sufficient quantities of calcium.  After completing these lab exercises, what do you think of that assumption?  Why?

 

4.  Some plants, for example the jack-in-the-pulpit, have high concentrations of calcium oxalate crystals in their cells.  Taking a bite of the raw plant tissue produces a sever burning sensation in the lining of the mouth.  Drinking water does not alleviate this pain.  Can you suggest a reason why drinking water does not help?

 

 

 

This exercise was modified by M. Olney for BY105, block 4, 2002 from the following sources:

Laboratory Manual for Applied Botany.  Estelle Levetin, Karen McMahon, Robert Reinsvold.  McGraw-Hill.  2002. 

Exercises for the Botany Laboratory.  Joel Kazmierski.  Morton Publishing Company.  1999.  Biology Laboratory Manual.  Darrell Vodopich, Randy Moore.  WCB McGraw-Hill.  1999.  Laboratory Manual for Plant Biology.  Deborah Canington.  Wadsworth Publishing Company. 

1998.

Even more to write up in your lab notebook (and to help you study for the exam):

 

Define/describe the functions of the following cell wall components:

 

Cell wall:         middle lamella

                        primary cell wall

                        secondary cell wall

                        plasmodesmata

 

Protoplast:

            Nucleus:          nuclear envelope

                                    nucleolus

                                    chromatin

                                    nuclear pore

 

            Cytoplasm:

                                    plasma membrane

                                    cytosol

double membrane-bound organelles               

                                                            mitochondria                                                  

                                                            plastids                                                           

                                                                        proplastid

                                                                        chloroplast                              

                                                                        chromoplast                                        

                                                                        leucoplast                                           

                                                                        amyloplast                              

                                                                        elaioplast

single-membrane bound organelles

peroxisome

vacuole/tonoplast

endomembrane system

endoplasmic reticulum (smooth and rough)

dictyosome (Golgi body)

vesicles

non-membrane bound components

                                                            cytoskeleton

                                                                        microtubules

                                                                        actin filaments

                                                                        (intermediate filaments in plants?)

                                                            ribosomes

                                                                        what are these composed of?

                                                            oil bodies

                                                                        especially found in seeds