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Workers Unite! Amazing Cells at Work

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About this Unit

This unit is comprised of a two-part student investigation. Although it is not essential that both parts be completed, students will develop more depth of understanding by completing both parts.

Before beginning this activity students should be aware of the following:

  • Groups of similar cells having a common function form a tissue.
  • Groups of tissues having a common function form an organ.
  • Groups of organs having a common function form a system.
  • Tissues, organs, and systems are characteristic to all the higher plants and animals.

By the end of the unit students should be made aware that scientists can often determine the function of tissues, organs and systems by "tagging" molecules that the organism uses, and then tracking the motion of the tagged molecule as it journeys through the organism.

The Beginning

For billions of years the only living things on the Earth were tiny single celled microorganisms, each struggling in the endless cycle of birth, growth, and reproduction for survival.

At some point in the distant past, the accidental association to several slightly different cells lead to an improvement in their collective survival rate compared to their lonely isolated cousins. The dawn of multicelled organisms had begun!

Modern day plants and animals are a marvel of cellular specialization and cooperation.

An Example of Specialized Plant Cells

  • Vascular bundle: a strand of longitudinal conducting tissue within plants, consisting mainly of xylem and phloem.
  • Xylem - tissue within plants which conducts water and mineral salts, absorbed by roots from the soil, throughout the plant. Xylem tissue consists of long continuous tubes formed from columns of cells in which the horizontal cross-walls have disintegrated and the cell contents have died. The vessels thus formed are strengthened by a compound called lignin, and ultimately form the wood of the plant. Associated with xylem vessels, and providing additional strength, are specialized fibrous cells called xylem fibres, some of which are useful to humans, for example, flax. Thus xylem is commercially important as a source of wood and fibres.
  • Phloem - tissue within plants which transports carbohydrate from the leaves throughout the plant. Phloem consists of tubes which are formed from columns of living cells in which the horizontal cross-walls have become perforated. This allows the carbohydrate in aqueous solution to move from one phloem cell into the next and thus through the plant. Because of their structure, phloem tubes are also called "sieve" tubes.

Investigation: "Tracing" a Water Molecule, Part 1

To locate and observe the vascular system in celery stems.

Materials Needed

  1. A stalk of fresh celery
  2. A glass jar or bottle
  3. Red or blue food colouring
  4. A sharp knife
  5. Magnifying glass
  6. Low power optical microscope (optional)

Procedure

Step 1

  • In this activity we will observe the transport of water upwards into the stem and leaves of celery. To begin acquire a stalk of fresh celery.
  • Separate the stalk into individual celery stems.
  • Using a sharp knife, make a clean slice across the bottom of the celery stems. You may leave the leaves of the celery on the top of the stem

Step 2

  • Set a few of the celery stalks into a beaker or jar of clean tap water.
  • Place an ample amount of red food colour dye into the water so that it is tinted a deep red colour.
  • Set the beaker in a warm bright location and observe any changes in the celery stalks and their leaves for a period of several days.
    Before After

    Celery

    Transparency Master

    Celery

    Transparency Master

  • In a very short time (as little as a few hours) red dots begin to appear in the celery leaves. At first the leaves may begin to wilt. Left in the red water for a few days they will become much redder in colour. You will also notice that red lines will begin to form on the stems.

Celery

Transparency Master

Step 3

  • Very carefully, and with a sharp knife, cut a thin slice from the lower part of the coloured celery.
  • Making a bias cut (diagonally) through the stem improves the visibility of the vascular bundles that have become coloured red with dye.
  • Using either a magnifying glass (or a microscope on low power) make a simple sketch of the stem's cross-section showing the location of the vascular bundles in the stem.
  • Try repeating this activity using food dyes of other colours such as blue or yellow.

Notes: To view the stem cross-section with a microscope the slice should be as thin as possible.

Given sufficient time to absorb the dye the leaves of the celery will become almost entirely red.

Adding dyes of different colours to biological specimens is a common technique used by scientists to help them visualize the structures and processes which occur in various organisms.

Class

Transparency Master

Observations and Conclusion

  1. The images show the basic effects of dying the water red.
  2. As water is transported up the celery stem, the red dye highlights the location of the vascular bundles.
  3. These specialized cells form part of a system responsible for getting water from the roots to the leaves via the plant's stem.
    Before After

    Celery

    Transparency Master

    Celery

    Transparency Master

Discussion

  1. Examine the diagram that you have made of your celery stem. How does it compare to the image shown here?
  2. Where, in relation to the ridges that run up and down outside of the celery stem, are the vascular bundles located?
  3. Are the vascular bundles located nearest to the inside or outside of the stem? Can you suggest a reason for their location?
  4. The vascular system of the plant transports vital water and the dissolved materials it contains to cells throughout the plant. What do the living cells of the vascular system get in return from the rest of the plant?

Celery

Transparency Master

Extended Activities

  1. Using a scientific dictionary, or a suitable science textbook, research definitions for the following terms and then write an explanation explaining these terms in your own words.

    i. Diffusion
    ii. Osmosis

  2. Make a list of the basic systems found in humans (such as the circulatory system, the skeletal system, digestive system and so on). Use your textbook to help.
  3. For each of the systems that you identify, create a list of the organs which make up that system.
  4. Using the school library, the Internet, and other classroom resources, make a list of some of the nutrients that are essential for the maintenance and good health of each of these body systems. (Hint: these might include vitamins, minerals, carbohydrates etc.)
  5. Read Biodiagnostics to find out how scientists at the National Research Council of Canada are using biodiagnostics to gather important information about disease processes.

Note to Teacher:

In research, scientists sometimes 'tag' molecules. In this activity, we are not tagging a water molecule; rather, we are adding a coloured molecule that is transported through the plant along with the water. These are more commonly referred as "tracers". To tag a water molecule we would actually have to modify the water molecule itself, perhaps by replacing the oxygen molecule with a radioactive isotope of oxygen.

Nevertheless, the basic concept is the same, that is, to create an effect whereby we can observe the biological process(es) that we are interested in studying.

Investigation: "Tracing" a Water Molecule, Part 2
To locate and observe the vascular system in the petals of a flower.

Materials Needed

  1. A small flower such as a carnation or lily with its stem intact
  2. A glass jar or bottle
  3. Red or blue food colouring
  4. A sharp knife
  5. Magnifying glass
  6. Low power optical microscope (optional)

Procedure

Step 1

  • Obtain a bunch of freshly cut flowers. White flowers work best. Carnations, lilies and daffodils (especially the white ones) work well.
  • Select a fresh flower (or a flower that is not quite fully opened) from the bunch.
  • Carefully remove any leaves from the stem of the flower.

Flower

Step 2

  • Place several flower stems in a beaker or jar of clean tap water.
  • Add sufficient red food colouring to the water to tint it a deep red.
  • Set the flowers in a warm bright location where they be easily observed at regular intervals.
Before After

Flowers

Flowers

Step 3

  • Check the petals of the flowers several times each day for a period of several days.
  • Record your observations and note any changes in the appearance of the flowers from one observation to the next.

Classe

  • Note the difference in the petal changes of the different coloured flowers.
  • Note the difference in the petal changes of the different kinds of flowers

Flower

  • Select a specific petal on one of the flowers.
  • Make a sketch of this petal each day for several days to document any changes that you might observe.
  • Use coloured pencils on your diagram to facilitate the documenting of your observations.

Flowers

Observations and Conclusion

  1. If you have been observant you will have noticed that the flowers begin to form red "blobs" near the end of the petals after only a few hours.
  2. After a few days, red lines delineating the vascular bundles in the petals begin to form.
    Before After

    Flower

    Flower

  3. The vascular system in plants seems to be extremely efficient with respect to its petals. After only a few hours water has made its way to the tips of the petals as seen in the image here.
    Flower
  4. After a few days the vascular system is well defined by red lines throughout the entire petal.

Flower