1. Write a brief note on cell theory. (02) marks
ANSWER
In biology, cell theory is a scientific theory which describes the properties of cells. These cells are
the basic unit of structure in all organisms and also the basic unit of reproduction. Cell theory was
eventually formulated in 1838. This is usually credited to Matthias Schleiden and Theodor
Schwann. The three tenets to the cell theory are as described below:
1-All living organisms are composed of one or more cells.
2-The cell is the most basic unit of life.
3-All cells arise from pre-existing, living cells
2. Discuss the types of microscopes, and their attributes. (02) marks
ANSWER
A microscope is an instrument used to see objects that are too small for the naked eye. The most
common is the optical microscope, which uses light to image the sample. Other major types of
microscopes are the electron microscope (both the transmission electron microscope and the
scanning electron microscope) and X-ray microscope.
An electron microscope is a microscope that uses a beam of accelerated electrons as a source of
illumination. Because the wavelength of an electron can be up to 100,000 times shorter than that
of visible light photons, the electron microscope has a higher resolving power than a light
microscope and can reveal the structure of smaller objects.
3. What is a technique to isolate cell contents? (02) marks
ANSWER
Tissue is typically homogenized in an isotonic buffer solution, as well as a pH buffer by use of a
variety of mechanisms such as grinding, mincing, chopping, pressure changes, osmotic shock,
freeze-thawing, and ultra-sound homogenization. This is done to stop osmotic damage.
Differential centrifugation is a common procedure used to separate certain organelles from whole
cells for further analysis of specific parts of cells. The homogenate is then subjected to repeated
centrifugations, each time removing the pellet and increasing the centrifugal force. Separation is
based on size and density, with larger and denser particles pellet at lower centrifugal forces. As an
example, unbroken whole cells will pellet at low speeds and short intervals such as 1,000g for 5
minutes. Smaller cell fragments and organelles remain in the supernatant and require more force
and greater times to pellet.
4. Why is the structure of plasma membrane important? (02) marks
ANSWER
The cell membrane is selectively permeable to ions and organic molecules and controls the
movement of substances in and out of cells. The basic function of the cell membrane is to protect
the cell from its surroundings. It consists of the phosphor-lipid bi-layer with embedded proteins.
Lipid bi-layers form through the process of self-assembly. The cell membrane consists primarily of
a thin layer of amphipathic phospholipids which spontaneously arrange so that the hydrophobic
"tail" regions are isolated from the surrounding polar fluid, causing the more hydrophilic "head"
regions to associate with the intracellular (cytosolic) and extracellular faces of the resulting bilayer.
5. How does fluid and mosaic model explain the structure of plasma-membrane? (02) marks
ANSWER
According to the fluid mosaic model of S. J. Singer and G. L. Nicolson (1972) biological membranes
can be considered as a two-dimensional liquid in which lipid and protein molecules diffuse more
or less easily. Although the lipid bi-layers that form the basis of the membranes do indeed form
two-dimensional liquids by themselves, the plasma membrane also contains a large quantity of
proteins, which provide more structure. While plasma membranes are commonly described as
phosphor-lipid bi-layers, they are actually composed primarily of protein can be either peripheral
or integral to the membrane. Peripheral membrane proteins are anchored to the surface of the
membrane, while integral membrane proteins contain trans-membrane regions that pass
completely through the bi-layer.
6. Write a short note on nucleus. (02) marks
ANSWER
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells.
Eukaryotes usually have a single nucleus, but a few cell types have no nuclei, and a few others
have many. Cell nuclei contain most of the cell's genetic material, organized as multiple long linear
DNA molecules in complex with a large variety of proteins, such as histones, to form
chromosomes. The genes within these chromosomes are the cell's nuclear genome. The main
structures making up the nucleus are the nuclear envelope, a double membrane that encloses the
entire organelle and isolates its contents from the cellular cytoplasm. Because the nuclear
membrane is impermeable to large molecules, nuclear pores are required that regulate nuclear
transport of molecules across the envelope.
7. Discuss the different modes of transportation across plasma-membrane. (02) marks
ANSWER
The cell employs a number of transport mechanisms that involve biological membranes:
Passive osmosis and diffusion: Some substances (small molecules, ions) such as carbon dioxide
(CO2) and oxygen (O2), can move across the plasma membrane by diffusion, which is a passive
transport process. Such molecules diffuse passively through protein channels such as aquaporins
(in the case of water (H2O)).
Endocytosis is the process in which cells absorb molecules by engulfing them. The plasma
membrane creates a small deformation inward, called an invagination, in which the substance to
be transported is captured. Endocytosis is a pathway for internalizing solid particles ("cell eating"
or phagocytosis), small molecules and ions ("cell drinking" or pinocytosis), and macromolecules.
Endocytosis requires energy and is thus a form of active transport. In Exocytosis, the membrane of
a vesicle can be fused with the plasma membrane, extruding its contents to the surrounding
medium.
8. Write a short note on cell-wall. (02) marks
ANSWER
The walls of plant cells must have sufficient tensile strength to withstand internal osmotic
pressures of several times atmospheric pressure that result from the difference in solute
concentration between the cell interior and external water. Plant cell walls vary from 0.1 to
several μm in thickness. Up to three layers may be found in plant cell walls:
1-The middle lamella, a layer rich in pectins. This outermost layer forms the interface between
adjacent plant cells and glues them together.
2-The primary cell wall, generally a thin, flexible and extensible layer formed while the cell is
growing.
3-The secondary cell wall, a thick layer formed inside the primary cell wall after the cell is fully
grown. It is not found in all cell types. Some cells, such as the conducting cells in xylem, possess a
secondary wall containing lignin, which strengthens and waterproofs the wall.
9. Discuss briefly the importance of mitochondria. (02) marks
ANSWER
The mitochondrion (plural mitochondria) is a double membrane-bound organelle found in most
eukaryotic cells. Mitochondria range from 0.5 to 1.0 μm in diameter. A considerable variation can
be seen in the structure and size of this organelle. Unless specifically stained, they are not visible.
These structures are described as "the powerhouse of the cell" because they generate most of the
cell's supply of adenosine triphosphate (ATP), used as a source of chemical energy. The organelle
is composed of compartments that carry out specialized functions. These compartments or
regions include the outer membrane, the inter-membrane space, the inner membrane, and the
cristae and matrix.
10. Write a short note on plastids. (02) marks
ANSWER
Plastids are the site of manufacture and storage of important chemical compounds used by the
cell. They often contain pigments used in photosynthesis. In plants, plastids may differentiate into
several forms, depending upon which function they play in the cell. Undifferentiated plastids
(proplastids) may develop into any of the following variants:
Chloroplasts (green plastids): for photosynthesis
Chromoplasts (coloured plastids): for pigment synthesis and storage
Leucoplasts (colour-less plastids): for starch storage
11. What is the importance of ribosomes? (02) marks
ANSWER
The ribosome is a large and complex molecular machine, found within all living cells, that serves
as the site of biological protein synthesis. A ribosome is made from complexes of RNAs and
proteins and is therefore a ribo-nucleo-protein. Each ribosome is divided into two subunits: a
smaller subunit which binds to the mRNA pattern, and a larger subunit which binds to the tRNA
and the amino acids. When a ribosome finishes reading an mRNA molecule, these two subunits
split apart. When a ribosome begins to synthesize proteins that are needed in some organelles,
the ribosome making this protein can become "membrane-bound". In eukaryotic cells this
happens in a region of the endoplasmic reticulum (ER) called the "rough ER".
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