DEMO

Cell structure and function

By The End Of The Subtopic Learners Should Be Able To;
  1. Identify cells as basic units of living organisms.
  2. Identify cell parts.
  3. Compare plant and animal cells.

Cells

  • Living organisms are made up of microscopic units called cells.
  • A cell is defined as the smallest structural and functional unit of an organism.
  • Cells have the following  characteristics:
    1. Respire to obtain energy
    2. Irritability (sensitivity) – response to stimuli
    3. Nutrition to absorb nutrients
    4. Growth and development
    5. Excretion to remove metabolic wastes
    6. Reproduce to make more of same kind
                    [RINGER]

  • Most cells cannot be seen by the naked eye but they can be viewed under a microscope.
  • Fig.1.1.1 below shows the structure of a light microscope, commonly used in school science laboratory.
1. Fig 1.1.1.jpg (132 KB)

Table 1.1.1: Parts and functions of a microscope 
PART FUNCTION
Objective lenses Magnification lenses - usually x4, x10, x40 and x100 magnification
Light source Projects light up through the diaphragm
Diaphragm Regulates the amount of light on the specimen
Eye piece Magnifies the image
Stage Supports the slide with the specimen to be viewed
Stage clips Holds the slide in position
Base Supports the microscope, specimen, and the lenses 


Experiment: Examination of onion epidermis

Materials

  • quarter onion (freshly cut)
  • iodine solution
  • distilled water
  • slide
  • slide cover
  • microscope

Procedure

1. Prepare the onion epidermis as illustrated in stages below.cell.JPG (35 KB)
To obtain a clear view of the specimen:
      • Use a small and thin specimen to allow light penetration
      • Ensure that the specimen does not dry.
      • Avoid folding and trapping air between the cover slip and the slide - lower the cover slip slowly.
      • Do not press the cover slip to avoid damaging the specimen tissue.
      • Stain the specimen with iodine solution; it improves the contrast between the different structures in the observed image.
2. Observe under low and high magnification of microscope.
3. Select the magnification which provides as much detail as possible for the cells.
4. Calculate the magnification used.

      • Magnification (M)= Eyepiece magnification x Objective lens magnification, for example:
      M  = Eyepiece (x10) x Objective lens (x40)
           = x400
(NB: magnification should always be expressed with “x” before the digits)

5. Given that the onion specimen is viewed under a total magnification of x1000 and one of the cells measures 15mm in length. Calculate the actual size of the cell using the formula below. magnification = Image size Actual size


Expected observations

Diagram below shows the onion cell when viewed under a microscope.
3. Fig 1.1.3.jpg (353 KB)

Structure and function of plant and animal cells

  • The living matter inside a cell is called protoplasm.
  • Within the protoplasm are structures known as organelles.
  • Organelles are structures in both plant and animal cells which perform specific functions.
  • They enable the cells to carry out the basic processes of life.
  • Organelles common to both plant and animal cells are cytoplasm, nucleus, vacuole, and cell membrane.
  • In addition to these, plant cells have cell walls, chloroplasts and starch granules.
4.jpg (159 KB)

Structure and function of cell organelles

1. Cytoplasm

  • This is a gel-like fluid where all organelles are suspended.
  • Cytoplasm is mostly composed of water and salts.
  • It is surrounded by a selectively permeable membrane to maintain optimum conditions for chemical reactions.
  • The chemical reactions include protein synthesis and respiration.

2. Nucleus

  • It contains the genetic material (DNA) which determines nature of the cell.
  • The nucleus controls all the activities of the cell.

3. Cell membrane

  • The cell membrane is a semi-permeable covering which surrounds the cytoplasm separating cell's contents from its surroundings.
  • It controls the entrance and exit of dissolved substances.

4. Cellulose cell wall

  • Cellulose is tough carbohydrate which provides structural support to the cell.
  • The cell wall is freely permeable to water and dissolved substances.
  • It also protects the cell from damage by osmotic absorption of water.

5. Vacuole

  • In animal cells, the vacuoles are relatively small and temporary.
  • Some are involved in digestion (phagocytes) while others are for removing excess water.
  • In plant cells, the vacuole is large and permanent, and it contains cell sap (solution of salts and sugars).
  • Cell sap helps to draw water into the cell through osmosis.

6. Chloroplasts

  • It is the site where photosynthesis takes place.
  • Chloroplasts contain chlorophyll - a green pigment which absorbs light for photosynthesis.
  • They also contain enzymes which are needed for photosynthesis.

    7. Starch granules

    • They are located in the cytoplasm or in the chloroplasts.
    • They serve as storage structures for the insoluble carbohydrates.

    Cell structure Plant Animal
    Nucleus   Present   Present  
    Cytoplasm Present Present
    cell membrane Present Present
    Vacuole Present (large, permanent) Present (small, temporary)
    Cell wall Present Absent
    Chloroplasts Present Absent
    Starch granules Present Absent