Cells of Crystal and Color

BACKGROUND

In innumerable ways, plants arc different from all other forms of life, and many of these differences can be seen by closely inspecting the interior of a plant cell. In dm laboratory topic, you will learn about several cellular structures that are unique to plants: cell walls that impart the strength to protect and support; chloroplasts for photosynthesis; chro-moplasts and vacuolar dyes that color fruits and flowers to attract animal helper*; and beautiful crystals that provide protection You will also learn how knowledge of botany has put these cellular assets to practical use—for example, as cell walls become wood tor building, plant pigments arc extracted for natural fabric dyes, and crystals are used as identification tools in archaeology and forensic science.

LEARNING OBJECTIVES

After completing this laboratory topic, students should be able to:

  1. Prepare protoplasts and understand how they can be used to advance improvements in agriculture and horticulture.
  2. Identify the prominent and distinctive components of a plant cell.
  3. Understand how plant crystals have been successfully employed in forensic science and archaeology.
  4. Understand the chemistry of plant pigments and extract them for use as textile dyes.

EXERCISE A: Plant Cells Without Walls

One of the most distinctive features of a plant cell is die wall of cellulose encasing it. Although diis cell wall pro vide* protection and support to the plant, it is often an impediment when manipulating plant cells through genetic engineering. Removing the cell wall barrier results in a protoplast, the entire contents of a plant cell minus the cell wall. To produce protoplasts, isolated plant cells arc treated with enzymes that separate cells and break down the cell walls. The introduction of genetically engi neered DNA is more readily absorbed by protoplasts than by an intact plant cell with a cell wall. Additionally, pro toplasts from desirable species can be tiised to create novel hybrid cells. The genetically engineered protoplasts can then be induced through tissue culture techniques (see Laboratory Topic 2) to grow into hybrid plants. These hybrids might include crops with combined nutritional value or disease resistance. In horticulture, a desired flower color might result from protoplast fusion—for example, a truly yellow African violet.

Materials Needed for Exercise A

Mannitol, 13% Marker pen Microslide (optional) Paraiilm

Petals of geranium Petri dishes Scalpels

Beakers, 50 ml

Compound light microscope

Dissecting microscope

Enzyme powders of cellulose and pectinase

Ethanol, 70%

Forceps

Graduated cylinder or pipct, 10 ml

Procedure for Exercise A

  1. Microbial contamination can be a problem with the following technique, so wipe the lab bench down with 70% ethanol and allow to air dry before beginning.
  2. Wash your hands. Obtain 1-2 petals of geranium. Using a forceps that has been sterilized in 70% ethanol, gently rinse each petal in a pctri dish containing 70% ethanol. Next transfer the petal to a pctri dish containing 13% mannitol, and rinse.
  3. Transfer a sterilized petal to the bottom lid of a sterile pctri dish. Using a sterile scalpel, cut the petal into the thinnest strips possible. Cover with the top lid of the pctri dish.
  4. Measure 10 ml of 13% mannitol into a sterile pctri dish. The high concentration of the mannitol solution draws water out of the cell, making the protoplast shrink away from the cell wall. Add the enzyme powders. The enzyme powders consist of a ccllu-lase to break down the cell walls of cellulose and a pcctinasc to dissolve away the intercellular glue that holds plant cells together. Gently swirl the pctri dish to dissolve the powders.
  5. Using forceps, transfer the petal strips to the pctri dish containing the mannitol/enzyme solution. Seal the pctri dish with parafilm. Gently shake the scaled pctri dish to completely submerge the strips in the solution.
  6. After an hour, you can check the progress of the reaction by viewing the petal cells with a dissecting or a compound light microscope. Do not open the lid of the pctri dish. Be carcflil not to leave the pctri dish on the microscope for too long, as the heat of the lamp will interfere with the protoplast release. As the cells lose water, the protoplast pulls back. Draw some representative cells in the following space:
  7. Set the pctri dish at room temperature, in dim light, overnight. Examine it after 24 hours; approximately 10-20% of the protoplasts will release after that period of time. You may pick up some released protoplasts with a microslidc. Place the microslidc near an isolarcd protoplast. The protoplast will How into the microslidc by capillar) action. View under the compound microscope, first under scanning (4 x objective) or low (10 x objective) power and then under high (40 x objective) power. In the following space, draw and label visible structures in some representative protoplasts. The membrane that surrounds each protoplast is the plasma membrane, which is normally not visible as it adheres to the inner cell wall surface in an intact plant cell.

EXERCISE B: Components of the Plant Cell

In plant cells, structure is partnered to function. In this exercise, you will prepare your own slides from common fruits, vegetables, house and garden plants to see the beauty of form and function within the cells of plants.

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