UNIT 3: CELL STRUCTURE
The three basic, structural parts of a compound microscope are:
–– head/body houses the optical parts in the upper part of the microscope,
–– base of the microscope supports the microscope and houses the illuminator;
–– arm connects to the base and supports the microscope head
The different parts of light microscope are described below:
–– Base: supports and stabilizes the microscope on the table or any other working place
–– Light source: It is made by lamp or mirror which provides light for viewing the slide.
–– Stage: is a platform used to hold the specimen in position during observation.
–– Stage clips: are pliers used to fix and hold tightly the slide on stage.
–– Arm: supports the body tube of microscope
–– Body tube: maintains the proper distance between the objective and ocular lenses
–– Arm: used for holding when carrying the microscope and it holds the body tube which bears the lenses.
–– Coarse focus adjustment moves stage up and down a large amount for coarse focus
–– Fine focus adjustment moves stage up and down a tiny amount for fine focus
–– Objective lenses: focuses and magnifies light coming through the slide
Under electron microscope, it is possible to identify a range of organelles in
plant and animal cells. Ultrastructure is the detailed of cell as revealed bythe electron microscope.
–– Gives the membranes of some eukaryotic cells the mechanical stability.
–– It fits between fatty acid tails and helps make the barrier more complete,
so substances like water molecules and ions cannot pass easily and directly through the membrane.
Channel proteins
–– Allow the movement of some substances across the membrane.
–– Large molecules like glucose enter and leave the cell using these protein channels.
Carrier proteins
–– Actively move some substances across the cell membrane. For
example, magnesium and other mineral ions are actively pumped into
the root’s hair cells from the surrounding soil.
–– Nitrate ions are actively transported into xylem vessels of plants. Receptor sites
–– Allow hormones to bind with the cell so that a cell response can be carried out.
–– Glycoproteins and glycolipids may be involved in cells signalling and they allow the immune system to recognize foreign objects to the cells.
–– Some hormone receptors are glycoprotein, and some are glycolipid.
3.2.2. Cytoplasmic constituents and their functions
plant and animal cells contain a variety of cell organelles including nucleus,
mitochondria, Golgi apparatus, endoplasmic reticulum, ribosomes, centrioles,vacuoles, chloroplasts, lysosomes and cytoskeleton.
The ER consists of a series of flattened membrane-bound sacs called
cisternae. The rough ER is surrounded with ribosomes. The rough ER
transports proteins made on attached ribosomes. The smooth ER is made
of tubular cavities lacks ribosomes, and it involves in synthesis of lipids that
the cell needs.
NB: Glandular cells are seen to have several RER for synthesis of hormones
and enzymes. Examples include liver cells, plasma cells, and pancreaticcells.
These are spherical sacs surrounded by a single membrane. They contain
powerful digestive enzymes. Their role is to break down materials such
as white blood cells, and destroy invalid microorganisms. In acrosome,
lysosomes help the sperm to penetrate the egg by breaking down thematerial surrounding the egg.
A vacuole is a saclike structure that is used to store materials such as water,
salts, proteins, and carbohydrates. In many plant cells there is a single, and
large central vacuole filled with liquid. The pressure of central vacuole in
this cells makes it possible for plants to support heavy structures such as
leaves and flowers. Some animals and some unicellular organisms contain
contractile vacuoles which contract rhythmically to pump excess water outof the cell.
The cell nucleus contains nearly all the cell’s DNA with the coded
instructions for making proteins and other important molecules. The nucleus
is surrounded by a double nuclear envelope, which allow materials to move
into and out of the nucleus through nuclear pores. The granules found in the
nucleus are called chromatin which consist of DNA bound to protein. When
a cell divides, the chromatin condenses into chromosomes containing the
genetic information. The nucleus contains a dense spherical structure callednucleolus in which assembly of ribosomes occurs.
Chloroplasts are the site of photosynthesis in plant cells. These are found
in plant cells and in cells of some protoctists. They also have two membranes
separated by a fluid-filled space. The inner membrane is continuous, with
thylakoids. A stalk of thylakoids is called a granum (plural: grana). A
chloroplast contains many sets of disc like sacs called thylakoids, which
are arranged in stacks known as grana. Each granum looks like a stack of
coins where each coin being a thylakoid. The thylakoid contains chlorophyll
molecules which capture the light energy that is needed for in the process of
light-dependent reactions of photosynthesis. A typical chloroplast contains
approximatively 60 grana, each consisting of about 50 thylakoids. The space
outside the thylakoid membranes are made by watery matrix called stroma.
The stroma contains enzymes responsible for light-independent reactions ofphotosynthesis.
Mitochondrion have two membranes separated by a fluid-filled intermembrane
space. The inner membrane is highly folded to form cristae that plays a big
role in aerobic respiration. The central part of the mitochondrion is called
matrix. The mitochondria are the site where Adenosine triphosphate(ATP= cellular energy) is produced during aerobic respiration.
3.3.1. Similarities between animal cell and plant cell
–– Both animal and plant cells have a cell membrane, a cytoplasm and a nucleus.
–– Both animal and plant cells have a true nucleus bounded by an envelope.
–– Both animal and plant cells have mitochondria, Golgi apparatus, Reticulum endoplasmic, lysosome, big ribosomes (80S), peroxisome, microtubules.
–– The protoplasm is enveloped by a bounding cell membrane called plasmalemma.
–– The protoplasm is composed of a dense round structure called nucleus which is surrounded by a less dense jelly-like cytoplasm.
–– Vacuoles contain secretions, food- particles, or decomposing organic substances.
–– Chemically, both plant and animal cells are made up of water (80-90%), proteins (7-13%), lipids (1-2%), carbohydrates (1-1.5%) and inorganic salts.
–– The cytoplasmic organelles are suspended in a semi-fluid jelly matrix called cytosol.3.3.2. Difference between animal and plant cells
End unit assessment
A. Multiple choice questions
1. Which organelle converts the chemical energy in food into a form
that cells can use?
a) Chromosome
b) Chloroplast
c) Nucleus
d) Mitochondrion
2. The cell membranes are constructed mainly of:
a) Carbohydrate gates
b) Protein pumps
c) Lipid bilayer
d) Free-moving proteins
3. In many cells, the structure that controls the cell’s activities is the:
a) Nucleus
b) Nucleolus
c) Cell membrane
d) Organelle
4. Despite differences in size and shape, all cells have cytoplasm and a
5.a) Cell wall
b) Cell membrane
c) Mitochondria
d) Nucleus
If a cell of an organism contains a nucleus, the organism is a (an)
a) Plant
b) Eukaryote
c) Animald) Prokaryote
6.Match each part of the cell (left column) to corresponding statement(right column):
7. How does a cell membrane differ from a cell wall?
8. Name the structures that animal and plant cells have in common,
those found in only plant cells, and those found only in animal cells.
9. List:
a) Three organelles each lacking a boundary membrane
b) Three organelles each bounded by a single membrane
c) Three organelles each bounded by two membranes (an envelope
10. The diagram below shows the structure of a liver cell as seen usingan electron microscope.
a) Name the parts labelled A, B, C and D.
b) The magnification of the diagram above is 12 000. Calculate the
actual length of the mitochondrion labelled M, giving your answer in μm. Show your working.
c) Explain the advantage to have a division of labor between different cells in the body.