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Revision Notes for Class 8 Science Chapter 8 Cell Structure and Functions
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Chapter 8 Cell Structure and Functions in Class 8. These exam notes for
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Chapter 8 Cell Structure and Functions Notes Class 8 Science
INTRODUCTION
A
cell is the structural and functional unit of life. It is the building
block of which all living organisms are made, and the smallest unit of
life capable of all the living functions. It is defined as a mass of
protoplasm bounded by a plasma membrane.
Cell vary in size.
Most cells are very small (microscopic), some may be very large. Some
cells may exist as independent units of life. Some such cells like
Euglena and Amoeba can change their shape, but most cells have a fixed
shape. The number of cells vary from organism to organism. An amoeba is
single-celled, while a human body weighing about 60 kg may have as many
as 60 × 1015 cells. In unicelluar organisms, e.g. Amoeba, Paramoecium or
Chalamydomonas, all the basic functions of a living being are performed
in one cell, while multicellular organisms have well-developed division
of labour. So, their different functions are performed by different
organs. For example, we have a stomach to digest food, a heart to pump
blood and a brain to think.
An English scientist, Robert
Hooke, discovered the cell in 1665 while examining thin sections of cork
under his simple microscope. He observed a mass of hexagonal chambers
like a honeycomb and called them(compartments) cells. Cell is the Latin
word for 'a little room'.
Types of cell
Prokaryotes and Eukaryotes
All
living organisms are made of cells. And based on the kind of cells they
are made up of, they are divided into two groups-prokaryotes and
eukaryotes. Prokaryotes, which include bacteria other organisms are
eukaryotes, and probably evolved from prokaryotes.
There are
some structural differences between prokaryotic and eukaryotic cells.
The most obvious difference is that unlike the eukaryotic cell, the
prokayotic cell lacks a true nucleus. There is no nuclear membrane. The
DNA or nuclear material lies in a circular loop in the cytoplasm. Such
undefined nuclear region is called nucleoid. There are some other
differences too. For instance, in prokaryotic cells the ribosomes are
smaller and scattered in the cytoplasm. Besides, prokaryotic cells lack
membrane-bound orgenelles like plastids, mitochondria and endoplasmic
reticulum.
Structure of a cell
Cell membrane or plasma membrane:
A
cell is essentially a tiny 'bag' of living matter. The covering of this
'bag' is called the cell membrane or plasma membrane. It maintains the
shape and size of the cell and protects its contents. It acts like a
sentry-allowing only some things to enter and leave the cell and
stopping others. For example, it allows oxygen and nutrients to pass
into the cell and lets wastes pass out of it. This is why it is called
selectively permeable.
Cell wall:
Plant
cells have an additional protective wll called the cell wall. It is
thick, rigid and permeable, and is made up of a carbohydrate called
cellulose.
Cytoplasm:
The
matter inside the cell mebrane is called cytoplasm. It consists of a
jellylike fluid with various structures, such as the nucleus, floating
in it. These structures are called organelles.
Salts, proteins, sugar and other substances are dissolved in the fluid.
Nucleus:
Almost
every cell has a nucleus. Red blood cells are among the exceptions. The
nucleus is the largest and the most important organelle of the cell. It
is usually spherical or oval in shape. Inside it there are thread like
structures called chromosomes. Nucleus is the controlling central cell.
Chromosomes have genes arranged in a linear fashion.
Vacuoles:
The
central part of most plant cells is occupied by a large vacuole. You
may have noticed it in some of the plant cells you observed. It is a sac
like structure filled with fluid. Food, wastes pigments and other
substances are dissolved in the fluid. Some plant cells have a number of
large vacuoles.
Vaculoes are not so common in animal cells. When they ocour, they are much smaller in size.
Plastids:
These
organelles are not present in animal cells. Chloroplasts (a type of
plastid) contain the green pigment chlorophyll and are responsible for
photosynthesis. Only green parts of plants have chloroplasts.
There are two other types of plastids called chromoplasts and leucoplasts.
Chromoplasts
contain pigments which give fruits and flowers their colours.
Leucoplasts store food and are found in the storage organs of plants.
Endoplasmic reticulum (ER)
The
endoplasmic reticulum is a network of tubelike structures running
through the cytoplasm. If ribosomes are attached to it, the reticulm is
rough, otherwise it is smooth.
Function - It gives internal support to the colloidal matrix (cytoplasm).
Rough endoplamic reticulum
(RER) is associated with the synthesis of proteins.
Ribosomes
Ribosomes
are extremely amall, round bodies found either in the state in the
cytoplasm or attached to the surface of the ER. They are composed of
ribonucleoprotein (ribonucleic acid and protein).
Functions - The main function of ribosomes is to act as a platform or work place for the synthesis of proteins.
Mitochondria
Mitochondria
are small, rod-shaped organelles found in large numbers. Each
mitochondrion is bounded by two membranes-outer and inner. The outer
membrane is smooth and the inner membrane is pushed inwards at intervals
forming crests called cristae. The cristae lie in a ground substane
called matrix. Mitochondria process enzymes necessary for the oxidation
of carbohydrates. This process releases energy in the form of ATP. This
is why mitochondria are known as the powerhouses of the cell.
Mitochondria have their own DNA and ribosomes. They can synthesize their
own proteins and thus they are semiautonomous organelles.
Function- Mitochondria provide energy for the vital activities of living cells.
Golgi body
They store, modify, package and condense the proteins synthesized in the ribosomes.
Lysosomes
These
saclike, small spherical, single membrane-bound vesicles contain
enzymes. These enzymes are synthesized in the RER, which are brought to
the Golgi complex.
Lysosomes are formed by the Golgi complex. They occur in animal cells and in the meristematic cells of a few plants.
Function-
They help in breaking down (digesting) large molecules of the cell.
They work in defence againt bacteria and viruses. During stavation,
lysosomes act on their own cellular organelles and digest them. This
results in cell death.
Hence lysosomes are called suicide bags or demonlition squads.
Centrioles
The
centrosome is a distinct region of the cytoplasm close to the nucleus
of animal cells. It usually has two central granules called centrioles.
The centrioles are hollow, cylindrical structure made of microtubules
arranged in a specific manner. They are arranged at right angles to each
other.
Function- At the time of cell division, centrioles
move to the poles and form spindle fibre which help in the movement of
chromatids (daughter chromosomes) in the daughter cells. They help in
the formation of cilia and flagella.
Plant Cell & Animal Cell
Movement of subtances across the cell membrane
Diffusion:
Diffusion
is the process of mixing up or different substances due to the random
motion of their component atoms, molecules and ions. Diffusion takes
place in solids, liquids and gases.
Ex. Burning of incense stick.
Osmosis:
Diffusion
of water across a semipermeable membrane is called Osmosis. The
movement of water in living beings depends on osmosis. The movement of
water molecules across the cell membrane is affected by the amount of
solute dissolved in it. Here also the water molecules are free to pass
across the membrane in both directions. But the net movement of water
molecules takes place from the dilute solution to the concentration one,
i.e., from the region of greater concentration of water towards the
region of lower concentration of water.
Ex. Grains in water.
Plant Tissues
Plant
tissue are basically of two types-meristematic and permanent. This
differentiation is based on the ability of the mature cells of the
tissue to divide and produce new cells. Meristematic tissue cells are
capable of dividing, while permanent tissue cells are not.
Meristematic
Tissue
This tissue consists of actively dividing cells and is present in the
growing regions of plants, e.g., the tips of roots and stems.
The
cells can be round, oval, polygonal or rectangular, but there are a few
things they have in common. They are packed closely without
intercellular spaces, have thin cellulose walls, dense cytoplasm and
prominent nuclei. Vacuoles are almost absent in such cells because they
are completely filled with sap. Depending on the region of the plant
where it is present, meristematic tissue can be of three types- apical,
lateral and intercalary.
Apical (from
apex) meristem, as the name suggests, is present at the growing tips of
stems and roots. Apical meristem is primary meristem.
Lateral meristematic tissue occurs along the sides of the cental (longitudinal) axis of the plant. It gives rise to v ascular tissues.
Intercalary meristem occurs at the base of leaves or internodes. These cells grow fast and soon change into permanent tissues.
Permanent Tissue
The
div ision and differentiation of the cells of meristematic tissues
given rise to permanent tissues. Cell division is the formation of two
or more daughter cells from one mother cell. Simple permanant tissue
consist of similar permanent cells that perform the same function or a
similar set of functions. Parenchyma, collencyma and sclerenchyma are
three types of simple permanent tissues. Complex permanent tissue are a
group of different types of cells that perform a common function. Xylem
and phloem are two types of complex permanent tissue.
Parenchyma
This
tissue is composed of large, thin-walled cells which are generally oval
or spherical. The cells are not packed closely, i.e., there are
intercellular space. These living cells with a nucleus and a v acuole
are found in the soft parts of the plant. They store food, fill up
spaces between other tissue and provide temporary support to the plant. W
hen they contain chloroplasts, as in leaves, they help manufacture
food.
Collenchyma
This
tissue is composed of cells that are elogated and thickened with
cellulose at the corners. There is no intercellular space. Collenchyma
provides mechnical support to plant organs and is found in leaf stalks
and below the epidermis of stems. It helps leav es and stems bend
without breaking. It prov ides support, protection and flexibility ot
plant organs. It is generally absent in roots.
Sclerenchyma
This
tissue is composes of long, narrow cells whose walls are ev enly
thickened with lignin. Lignin is a chemical that acts like cement,
sticking fibres and hardening them. Sclerenchyma cells are dead. They
are packed together closely, and prov ide strength and flexibility to
plant parts. They are present in stems, v eins of leav es, the hard
covering of seeds and nuts, and the husk of coconut. Fibre-yielding
plants like jute and flax contain this tissue in abundance.
Xylem
Xylem,
or wood, as it is often called, is a complex tissue. The cells are
thick-walled, tubular and often dead. This tissue has four types of
cells– tracheids, v essels, xylem parenchyma and xylem fibres. Of these
only tracheids and vessels transport sap.
Phloem
Phloem
too is a complex tissue made up of four types of cells, or
elements–siev e tubes, companion cells, phloem fibres and phloem
parenchyma. It is not necessary for the phloem to contain all four types
of cells. Phloem to contain all four types of cells. Phloem cells are
living cells (except phloem fibres) which help transport food from leav
es to the storage organs and growing regions of the plant.
Animal Tissues
While
doing the activities in this chapter, you have come across two types of
animal tissue, the cheek cells are a type of epithelial tissue, while
blood is a kind of connective tissue. There are two other types of
animal tissue–muscular and nerv ous.
Epithelial Tissue:
This
tissue covers the surface of the body and lines the internal organs.
Its main function is protection. The cells that form the different types
of epithelial tissue differ in shape. Some are thin and flat, some
cubelike, while others are columnar.
Connective Tissue:
Blood,
bones cartilages, tendons (which connect muscles with bones) and
ligaments (which tie bones together) are diferent types of connectiv e
tissue. Adipose tissue, or what is generally known as fat, is also a
kind of connective tissue. Though different in structure, connective
tissues have one thing in common-the cells are suspended or embedded in a
matrix. In blood, the matrix is liquid and is called the plasma.
Muscular Tissue:
Muscular
tissue is also of different types. However, the different types of
muscular tissue (or muscles) have the same basic functoin. They contract
and relax to make different parts of the body mov e. The muscles in our
arms, legs thighs, back and so on help us move. The muscles in the
heart help it pump blood. The muscles in the alimentary canal help the
passage of food. The muscles in the blood v essels help them dilate and
get constricted.
Nervous Tissue:
Nerve
cells make up nerv ous tissue. A nerve cell has a long tail and short
branches coming out of it.The tail too has branches. These help to carry
message from one cell to the other. The brain and spinal cord are make
up of nervous tissue.
DISCOVERY OF CELL
1. Robert Hooke (1665) :– An English man and first curator of Royal society of London.
Observed a thin transverse section of bark of a tree under self designed microscope.
He noticed honey - comb like compartments.
He coined the term cell.
He wrote a book - Micrographia.
He actually observed dead cells.
2. Antony Van Leeuwenhoek (1674) was first to observe living cells like bacteria [from tartar of teeth]
erythrocytes [fish], sperms and protozoans [eg. Vorticella]
3. N. Grew (1682) :– Proposed cell concept which states that cell is unit of structure of organisms.
4. Rudolf Virchow (1858) :– Proposed that new cells formed from the pre-existing cells.
MICROSCOPE
A
microscope is an instrument to view small objects by magnifying them.
It enables us to see the different types of living cells and the
structures they contain.
TYPES OF MICROSCOPES
There are mainly three types of microscopes. They are :
♦ Light microscope : The light microscope uses light to produce images.
♦ Transmission
Electron Microscope (TEM) : The electron microscope was designed by
Knoll & Ruska (1932). A TEM makes use of a beam of highly energetic
electrons to examine objects. The image produced is of a very fine
scale.
♦ Scanning Electron Microscope (SEM) : Like the TEM, the SEM also uses electrons to produce images.
In
the case of a SEM, electrons are reflected off the surface of the
specimen, because of which SEM images usually manage to capture the
physical features of a cell in great detail.
STRUCTURE OF A CELL
♦ Cell membrane or plasma membrane:
A
cell is essentially a tiny 'bag' of living matter. The covering of this
'bag' is called the cell membrane or plasma membrane. It maintains the
shape and size of the cell and protects its contents. It acts like a
sentry-allowing only some things to enter and leave the cell and
stopping others. For example, it allows oxygen and nutrients to pass
into the cell and lets wastes pass out of it. This is why it is called
selectively permeable.
♦ Cell wall:
Plant
cells have an additional protective wll called the cell wall. It is
thick, rigid and permeable, and is made up of a carbohydrate called
cellulose.
♦ Cytoplasm :
The
matter inside the cell mebrane is called cytoplasm. It consists of a
jellylike fluid with various structures, such as the nucleus, floating
in it. These structures are called organelles.
Salts, proteins, sugar and other substances are dissolved in the fluid.
♦ Nucleus:
Almost
every cell has a nucleus. Red blood cells are among the exceptions. The
nucleus is the largest and the most important organelle of the cell. It
is usually spherical or oval in shape.
Inside it there are
thread like structures called chromosomes. Nucleus is the controlling
central cell. Chromosomes have genes arranged in a linear fashion.
It is the most important part of the living cell.
It is usually spherical or oval in shape.
It controls all the vital functions of the cell.
It has four components:
(i) Nulcear Membrane (ii) Nulceoplasm
(iii) Nulceolus (iv) Chromosomes
(i) Nuclear membrane:
Surrounds the nucleus and separates it from the cytoplasm. It is
permeable and controls the passage of materials between cytoplasm and
nucleoplasm
(ii) Nucleoplasm: The
part of protoplasm which is enclosed by nuclear membrane is called
nucleoplasm. It contains chromatin threads and nucleolus.
(iii) Nulceolus: It is a spherical body in the nucleus. It is composed of RNA and is responsible for protein synthesis.
(iv) Chromosomes: Nucleus contains thread like structures called chromosomes.
The
hereditary units of chromosomes are the genes. They are responsible for
the transmission of characters from parents to the offsping.
Nucleus along with tis role in inheritance regulates and controls differeny metabolic activities of the cell.
On the basis of well organised nucleus, cells can be of two types
(i) Prokaryotic cell (ii) Eukaryotic cell
(i) Prokaryotic cells: These are cells having primitive nucleus without nuclear membrane.
Organism with primitive nucleus are known as prokaryotes.
Ex. Bacteria and blue green algae.
(ii) Eukaryotic cells: These are cells having a well organised nucleus with nuclear membrane.
Organsims with true nucleus are known as eukaryotes.
Ex. Man, elephant, onion.
¶ Vacuoles:
The
central part of most plant cells is occupied by a large vacuole. You
may have noticed it in some of the plant cells you observed. It is a sac
like structure filled with fluid. Food, wastes pigments and other
substances are dissolved in the fluid. Some plant cells have a number of
large vacuoles.
Vaculoes are not so common in animal cells. When they ocour, they are much smaller in size.
¶ Plastids:
These
organelles are not present in animal cells. Chloroplasts (a type of
plastid) contain the green pigment chlorophyll and are responsible for
photosynthesis. Only green parts of plants have chloroplasts.
There
are two other types of plastids called chromoplasts and leucoplasts.
Chromoplasts contain pigments which give fruits and flowers their
colours. Leucoplasts store food and are found in the storage organs of
plants.
¶ Endoplasmic reticulum (ER)
The endoplasmic reticulum is a network of tube-like structures running through the cytoplasm.
If ribosomes are attached to it, the reticulm is rough, otherwise it is smooth.
Function - It gives internal support to the colloidal matrix (cytoplasm).
Rough endoplamic reticulum (RER) is associated with the synthesis of proteins.
¶ Ribosomes
Ribosomes
are extremely amall, round bodies found either in the state in the
cytoplasm or attached to the surface of the ER. They are composed of
ribonucleoprotein (ribonucleic acid and protein).
Functions - The main function of ribosomes is to act as a platform or work place for the synthesis of proteins.
¶ Mitochondria
Mitochondria
are small, rod shaped organelles found in large numbers. Each
mitochondrion is bounded by two membranes-outer and inner. The outer
membrane is smooth and the inner membrane is pushed inwards at intervals
forming crests called cristae. The cristae lie in a ground substane
called matrix. Mitochondria process enzymes necessary for the oxidation
of carbohydrates.
This process releases energy in
the form of ATP. This is why mitochondria are known as the powerhouses
of the cell. Mitochondria have their own DNA and ribosomes. They can
synthesize their own proteins and thus they are semiautonomous
organelles. Function- Mitochondria provide energy for the vital
activities of living cells.
¶ Golgi body
They store, modify, package and condense the proteins synthesized in the ribosomes.
¶ Lysosomes
These
saclike, small spherical, single membrane-bound vesicles contain
enzymes. These enzymes are synthesized in the RER, which are brought to
the Golgi complex. Lysosomes are formed by the Golgi complex. They occur
in animal cells and in the meristematic cells of a few plants.
Function-
They help in breaking down (digesting) large molecules of the cell.
They work in defence againt bacteria and viruses. During stavation,
lysosomes act on their own cellular organelles and digest them. This
results in cell death. Hence lysosomes are called suicide bags or
demonlition squads.
¶ Centrioles
The
centrosome is a distinct region of the cytoplasm close to the nucleus
of animal cells. It usually has two central granules called centrioles.
The centrioles are hollow, cylindrical structure made of microtubules
arranged in a specific manner. They are arranged at right angles to each
other.
Function- At the time of cell division, centrioles
move to the poles and form spindle fibre which help in the movement of
chromatids (daughter chromosomes) in the daughter cells. They help in
the formation of cilia and flagella.
• Movement of subtances across the cell membrane
• Diffusion:
Diffusion
is the process of mixing up or different substances due to the random
motion of their component atoms, molecules and ions. Diffusion takes
place in solids, liquids and gases.
Ex. Burning of incense stick.
¶ Osmosis:
Diffusion
of water across a semipermeable membrane is called Osmosis. The
movement of water in living beings depends on osmosis. The movement of
water molecules across the cell membrane is affected by the amount of
solute dissolved in it. Here also the water molecules are free to pass
across the membrane in both directions. But the net movement of water
molecules takes place from the dilute solution to the concentration one,
i.e., from the region of greater concentration of water towards the
region of lower concentration of water.
Ex. Grains in water.
¶ Plant Tissues
Plant
tissue are basically of two types-meristematic and permanent. This
differentiation is based on the ability of the mature cells of the
tissue to divide and produce new cells. Meristematic tissue cells are
capable of dividing, while permanent tissue cells are not.
¶ Meristematic Tissue
This
tissue consists of actively dividing cells and is present in the
growing regions of plants, e.g., the tips of roots and stems. The cells
can be round, oval, polygonal or rectangular, but there are a few things
they have in common. They are packed closely without intercellular
spaces, have thin cellulose walls, dense cytoplasm and prominent nuclei.
Vacuoles are almost absent in such cells because they are completely
filled with sap. Depending on the region of the plant where it is
present, meristematic tissue can be of three types- apical, lateral and
intercalary.
Apical (from apex) meristem, as the name suggests, is
present at the growing tips of stems and roots. Apical meristem is
primary meristem.
Lateral meristematic tissue occurs along the
sides of the cental (longitudinal) axis of the plant. It gives rise to
vascular tissues.
Intercalary meristem occurs at the base of leaves or internodes. These cells grow fast and soon change into permanent tissues.
¶ Permanent Tissue
The
division and differentiation of the cells of meristematic tissues given
rise to permanent tissues. Cell division is the formation of two or
more daughter cells from one mother cell. Simple permanant tissue
consist of similar permanent cells that perform the same function or a
similar set of functions. Parenchyma, collencyma and sclerenchyma are
three types of simple permanent tissues. Complex permanent tissue are a
group of different types of cells that perform a common function. Xylem
and phloem are two types of complex permanent tissue.
¶ Parenchyma
This
tissue is composed of large, thin-walled cells which are generally oval
or spherical. The cells are not packed closely, i.e., there are
intercellular space. These living cells with a nucleus and a vacuole are
found in the soft parts of the plant. They store food, fill up spaces
between other tissue and provide temporary support to the plant. When
they contain chloroplasts, as in leaves, they help manufacture food.
¶ Collenchyma
This
tissue is composed of cells that are elogated and thickened with
cellulose at the corners. There is no intercellular space. Collenchyma
provides mechnical support to plant organs and is found in leaf stalks
and below the epidermis of stems. It helps leaves and stems bend without
breaking. It provides support, protection and flexibility ot plant
organs. It is generally absent in roots.
¶ Sclerenchyma
This
tissue is composes of long, narrow cells whose walls are evenly
thickened with lignin. Lignin is a chemical that acts like cement,
sticking fibres and hardening them. Sclerenchyma cells are dead. They
are packed together closely, and provide strength and flexibility to
plant parts. They are present in stems, veins of leaves, the hard
covering of seeds and nuts, and the husk of coconut. Fibre-yielding
plants like jute and flax contain this tissue in abundance.
• Xylem
Xylem,
or wood, as it is often called, is a complex tissue. The cells are
thick-walled, tubular and often dead. This tissue has four types of
cells– tracheids, vessels, xylem parenchyma and xylem fibres. Of these
only tracheids and vessels transport sap.
• Phloem
Phloem
too is a complex tissue made up of four types of cells, or
elements–sieve tubes, companion cells, phloem fibres and phloem
parenchyma. It is not necessary for the phloem to
contain all four
types of cells. Phloem to contain all four types of cells. Phloem cells
are living cells (except phloem fibres) which help transport food from
leaves to the storage organs and growing regions of the plant.
• Animal Tissues
While
doing the activities in this chapter, you have come across two types of
animal tissue, the cheek cells are a type of epithelial tissue, while
blood is a kind of connective tissue. There are two other types of
animal tissue–muscular and nervous.
• Epithelial Tissue:
This
tissue covers the surface of the body and lines the internal organs.
Its main function is protection. The cells that form the different types
of epithelial tissue differ in shape. Some are thin and flat, some
cubelike, while others are columnar.
• Connective Tissue:
Blood,
bones cartilages, tendons (which connect muscles with bones) and
ligaments (which tie bones together) are diferent types of connective
tissue. Adipose tissue, or what is generally known as fat, is also a
kind of connective tissue. Though different in structure, connective
tissues have one thing in common-the cells are suspended or embedded in a
matrix. In blood, the matrix is liquid and is called the plasma.
• Muscular Tissue:
Muscular
tissue is also of different types. However, the different types of
muscular tissue (or muscles) have the same basic functoin. They contract
and relax to make different parts of the body move. The muscles in our
arms, legs thighs, back and so on help us move. The muscles in the heart
help it pump blood. The muscles in the alimentary canal help the
passage of food. The muscles in the blood vessels help them dilate and
get constricted.
• Nervous Tissue:
Nerve
cells make up nervous tissue. A nerve cell has a long tail and short
branches coming out of it.The tail too has branches. These help to carry
message from one cell to the other. The brain and spinal cord are make
up of nervous tissue.
Question. Make a sketch of the human nerve cell. What funtion do nerve cells perform?
Answer. Functions of human nerve cell:
(i) Nerve cells receive message from different parts of body.
(ii)
They further transfer these messages to brain and accordingly brain
send commands for functioning of different organs of body.
Question. Write short notes on the following:
(i) Cytoplasm (ii) Nucleus of a cell
Answer.
(i) Cytoplasm: Cytoplasm is a jelly like substance which is present
between the cell membrane and the nucleus. Various other organelles of
cells are present in the cytoplasm. Cytoplasm is made up of chemical
substances like carbohydrates, proteins and water. These chemical
substances are present in cells of all types and sizes. Cytoplasm
contains many important tiny substances called Organelles.
(ii)
Nucleus of a cell: Nulceus is the master of the cell. It commands all
the functioning of the cell. It is generally located in the center of
the cell and is spherical in shape. A membrane called nuclear membrane
separates it from cytoplasm. It contains the genetic material DNA and
RNA in it. This porous membrane allows the transfer of material in the
nucleus and cytoplasm. Nucleus contains a dense body called Nucleolus
which actually contains Chromosomes, the genetic material.
Question. Which part of the cell contains organelles?
Answer. Cytoplasm.
Question. State a difference between eukaryotes and prokaryotes.
Answer. Prokaryotes do not have a well designed nuclear membrane while, eukaryotes have a well designed nuclear membrane.
Question. Where are the chromosomes found in cell? State their functions?
Answer.
Chromosomes are found in the nucleus of a cell. Their function is to
carry characteristic features of parent cells to the daughter cell
means, from parent to offspring.
Question. Cells are the basic structural units of living organism. Explain.
Answer.
In Biology, the basic unit of which all living thins are composed is
knows as cell. The cell is the smallest structural unit of living matter
that is capable of functioning independently. A single cell can be a
complete organism in itself, as in bacteria and protozoans. A
unicellular organism also captures and digests food, respires, excretes,
grows, and reproduces. Similar functions in multi-cellular organisms
are carried out by groups of specialized cell which are organized into
tissues and organs such as, the higher plants and animals. Hence, ‘cell’
is known as the basic structural and functional unit of life.
Question. Explain why chloroplast are found only in plant cells.
Answer. Chloroplasts are found only in plant cells because they are required for photosynthesis.
