CBSE Class 8 Science Cell Structure and Functions Assignment Set B

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'.

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.
Cork section shown in
Robert Hooke's Microscope
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.

MCQs

Question. Blood is a type of
(A) Epithelial tissue
(B) Connective tissue
(C) Nervous tissue
(D) Muscular tissue
Answer. B

Question. The covering of an animal cell is:
(A) Cell wall
(B) Cell membrane
(C) Nuclear membrane
(D) Cytoplasm
Answer. B

Question. Organisms lacking a nucleus and membranebound organe celles are called:
(A) Diploid
(B) Haploid
(C) Prokaryotes
(D) Eukaryotes
Answer. C

Question. Relatively large vacuoles are present in:
(A) all prokaryotes
(B) all eukaryotes
(C) Plant cells
(D) Animal cells
Answer. C

Question. The growth of cells is regulated by
(A) mitochondrion
(B) vacuole
(C) nucleus
(D) Golgi complex
Answer. C

Question. The centriole is associated with:
(A) DNA synthesis
(B) cell division
(C) Circulation
(D) respiration
Answer. B

Question. Proteins are formed in the:
(A) Golgi complex
(B) mitochondria
(C) plastids
(D) ribsosomes
Answer. D

Question. The name suicide bag has been given to:
(A) Centroiles
(B) Ribosome
(C) Mitochondrion
(D) Lysosome
Answer. D

Question. Genes are located on the:
(A) Nuclear membrane
(B) Chromosomes
(C) Lysosomes
(D) Cell membrane
Answer. B

Question. Gaseous exchange in plants takes place through:
(A) epidermal cells
(B) stomata
(C) stem
(D) vascular tissue
Answer. B 

Question. When a cell is placed in strong salt solution, it shrinks because:
(A) salt solution enters the cell
(B) cytoplasm of the cell begins to decompose
(C) water comes out of the cell to develop equilibrium
(D) all of these
Answer. C

Question. Tissue is defined as:
(A) group of similar cells having a common function
(B) different types of cells performing the same functions
(C) different types of cells performing different functions
(D) organised group of cells performing many functions
Answer. A

Fill in the blanks

1. Cell wall is found only in .............. cells.
2. Ribosomes are located on the surface of .............. .
3. All the components of a cell including the cell membrae is called .............. .
4. A true nucleus is absent in bacteria and .............. .
5. Two types of nucleic acids are .............. and............. .
Answer. 
1. plant
2. endoplasmic reticulum
3. protoplasm
4. BGA (Blue green algae)
5. RNA & DNA

Click on link below to download CBSE Class 8 Science Cell Structure and Functions Assignment Set B