CBSE Class 9 Science The Cell Exam Notes

This Class 9 Science chapter The Cell examination notes cover the history of the cell, unicellular and multicellular organisms, cell size and shape, the ultrastructure of plasma membrane, cell wall, nucleus, cytoplasm and cell organelles including endoplasmic reticulum, Golgi complex, lysosomes, mitochondria, plastids and vacuoles.

HISTORY

In 1665, Robert Hooke observed the cell in a cork slice with the help of a primitive microscope. He saw that cork resembled the structure of a honeycomb, consisting of many little compartments. Robert Hooke called these boxes cells. Cell is a Latin word for 'little room'.

UNICELLULAR AND MULTICELLULAR ORGANISMS

All living organisms are made up of one or more cells. On the basis of cell number, organisms are grouped into two categories:

Unicellular organisms: Organisms in which a single cell makes up the whole body. For example, Amoeba, Paramecium, bacteria and Chlamydomonas.

Multicellular organisms: Organisms in which the body contains more than one cell. For example, plants, animals and fungi.

CELL SIZE

• The size of different cells ranges between broad limits.
• Some plant and animal cells are visible to the naked eye.
• Most cells are visible only with a microscope.
• Prokaryotic cells usually range between 1 and 10 µm.
• Eukaryotic cells usually range between 10 and 100 µm.
• Amoeba proteus may reach a diameter of 0.5 mm.
• The smallest cells are those of Mycoplasma laidlawii (0.1 µ in diameter) or PPLO (pleuropneumonia-like organism).
• The largest cell is the egg of an ostrich.

[Figure: Various cells from the human body.]

CELL SHAPE

• The shape of a cell may be variable or fixed.
• Variable shape occurs in Amoeba, white blood cells (WBC), etc.
• Fixed shape occurs in most plant and animal cells.
• Cells may have diverse shapes, such as polyhedral (with 8, 12 or 14 sides), spherical (e.g., eggs of most animals), spindle shaped (smooth muscle fibres), elongated (e.g., nerve cells), and so on.

ULTRASTRUCTURE OF CELLS

[Figure: An animal cell as shown by an electron microscope. This microscope magnifies the objects 600,000 times.]

[Figure: A plant cell as shown by an electron microscope.]

PLASMA MEMBRANE

Introduction: The cell surface in all cells is enclosed by a living membrane, which was called the cell membrane by C. Nageli and C. Kramer (1855).
Historical account: J. Q. Plowe (1931) coined the term plasmalemma for the cell membrane.

Ultrastructure:
• The plasma membrane forms the outer covering of each cell.
• It is present in both plant and animal cells.
• It is a living, thin, delicate, elastic, selectively permeable membrane.
• It separates the contents of a cell from the surrounding medium.

Fluid Mosaic Model: In 1972, Singer and Nicolson proposed this model. According to it, the cell membrane consists of two layers of phospholipid molecules, and phospholipid and protein molecules are arranged as a mosaic.
• Phospholipid molecules have their polar heads directed outward and their non-polar tails pointing inward.
• Proteins are of two types: peripheral and integral. Peripheral proteins lie on the surface, while integral proteins are embedded in the phospholipid matrix. The protein layer gives elasticity and mechanical support to the lipid matrix.

[Figure: Structural detail of the plasma membrane according to the fluid mosaic model.]

Functions of Plasma Membrane:
• The main function of the plasma membrane is to regulate the movement of molecules into and out of the cell.
• It allows the movement of gaseous substances from high concentration to low concentration, which is known as diffusion.
• Water also obeys the law of diffusion. The movement of water molecules through a selectively permeable membrane is called osmosis.
• The flexibility of the cell membrane also enables the cell to engulf food. This is known as endocytosis, for example, in Amoeba.

CELL WALL

In plants, there is another rigid layer outside the plasma membrane called the cell wall. It is made up of cellulose, which provides structural strength to the plant.

Functions of cell wall:
• It maintains the shape of the cell.
• It protects the cells from mechanical injury and prevents their desiccation.
• It provides mechanical support against gravity. It is due to rigid cell walls that the aerial parts of plants are able to stay erect and expose their leaves to sunlight.
• Cell walls allow the cells to withstand very dilute external media without bursting.

NUCLEUS

The nucleus is the most important part of the cell, as it controls all the activities of the cell.

Structure of nucleus:
• The nucleus has a double-layered covering called the nuclear membrane.
• The nuclear membrane has pores that connect the inside of the nucleus to its outside, that is, to the cytoplasm.
• The nucleus contains chromosomes, which are visible as rod-shaped structures only when the cell is about to divide.
• Chromosomes are composed of DNA and protein.
• Functional segments of DNA are called genes.
• In a cell which is not dividing, this DNA is present as part of the chromatin material.

Functions of nucleus:
• The nucleus plays an important role in cellular reproduction.
• DNA contains the information necessary for constructing and organising cells.
• Nucleoid: In some organisms, the nuclear region of the cell may be poorly defined due to the absence of a nuclear membrane. Such an undefined nuclear region is called a nucleoid.

Note:
• Prokaryotic cells are cells which do not have a well defined nuclear region. Pro means primitive and karyon means nucleus.
• Eukaryotic cells are cells which have a well defined nuclear region.
• Apart from the absence of a nuclear membrane, prokaryotic cells lack most cell organelles.

CYTOPLASM

The fluid and semifluid matrix of a cell, between the nucleus and the plasma membrane, containing various organelles, is called the cytoplasm.

CELL ORGANELLES

Small membrane bound structures which perform a lot of chemical activities to support the function and structure of a cell are called cell organelles. Some important cell organelles are given below.

Endoplasmic Reticulum (ER)

The endoplasmic reticulum is noticeable only with an electron microscope.

Structure: The ER is an extensive network of intracellular membrane-bound tubes that occupies most of the cytoplasm in almost all eukaryotic cells. The membranes of this system are lipoproteinic in nature, similar in structure to the plasma membrane. The ER is more prominent in young and dividing cells than in older cells. It is absent in prokaryotic cells.

Types: The ER is of two types:
• Rough endoplasmic reticulum (RER)
• Smooth endoplasmic reticulum (SER)

Rough Endoplasmic Reticulum (RER): These appear rough under a microscope because of the large number of grain-like ribosomes on their cytoplasmic surface. The ribosomes are the sites of protein synthesis. Thus, RER is engaged in the synthesis and transport of proteins. RER is generally more abundant in the deeper part of the cytoplasm near the nucleus, where it is connected with the outer membrane of the nuclear envelope. RER is well developed in cells that synthesise and secrete proteins.

Smooth Endoplasmic Reticulum (SER): It consists mainly of tubules and vesicles. It is free of ribosomes and is more abundant near the peripheral part of the cytoplasm, where it may be attached to the plasma membrane. SER helps in the synthesis of fat or lipid molecules. It is therefore well developed in cells that secrete lipids.

Functions of ER:
• Support: The ER acts as a supporting skeletal framework of the cell and maintains its form.
• Transport of materials: The ER facilitates the transport of materials from one part of the cell to another.
• Exchange of materials: The ER helps in the exchange of materials between the cytoplasm and the nucleus.
• Localisation of organelles: It keeps the cell organelles properly placed and distributed in relation to synthesis.
• Surface for protein synthesis: The RER offers an extensive surface on which ribosomes carry out protein synthesis.
• Surface for synthesis of other substances: The SER provides a surface for the synthesis of lipids, including phospholipids, cholesterol and steroid hormones.
• Packaging: Proteins formed on ribosomes pass into the ER lumen, where they are modified. The modified proteins move into the transitional area, where the ER buds off transport vesicles carrying the proteins to the Golgi apparatus. Here they are further processed and packaged into secretory vesicles for export by exocytosis at the plasma membrane. Examples of secretory products include mucus, digestive enzymes and hormones.
• Detoxification: The SER brings about detoxification in the liver, i.e., it converts harmful materials (drugs, insecticides, pollutants and poisons) into harmless substances for excretion by the cell.
• Formation of organelles: The SER produces the Golgi apparatus, lysosomes and vacuoles.
• Membrane formation: The plasma membrane and other cellular membranes are formed by the ER.

[Figure: Endoplasmic reticulum, showing rough ER with ribosomes and smooth ER.]

Golgi Complex

Introduction: Golgi bodies are absent in prokaryotic cells. The Golgi complex is found in all eukaryotic cells except RBCs.
Historical account: Camillo Golgi (1898), a zoologist, observed Golgi bodies in the form of a network in the nerve cells of the barn owl.
Ultrastructure: It is also called the Golgi complex, Golgi apparatus or dictyosome (in plant cells). It is made up of cisternae. Golgi bodies are interconnected with tubules.

[Figure: Golgi apparatus in section.]

Functions of Golgi apparatus:
• The main function of the Golgi apparatus is secretory.
• It produces vacuoles or secretory vesicles that contain cellular secretions like enzymes, proteins and cellulose.
• The Golgi apparatus is also involved in the synthesis of the cell wall, plasma membrane and lysosomes.

Lysosomes

Introduction: Lysosomes are generally found in the cytoplasm of animal cells. They exhibit polymorphism.
Historical account: The term lysosome was introduced by De Duve in 1955.
Ultrastructure:
• Lysosomes are also called demolition squads, scavengers, cellular housekeepers and suicidal bags.
• They are simple, tiny, spherical sac-like structures evenly distributed in the cytoplasm.
• A lysosome is a small vesicle surrounded by a single membrane and contains powerful enzymes.

Functions of lysosomes:
• Lysosomes serve as the intracellular digestive system, and hence are called digestive bags.
• They remove worn out and poorly working cell organelles by digesting them, to make way for new replacements.

Mitochondria

Introduction: A single mitochondrion is present in the unicellular green alga Micrasterias. The number of mitochondria varies from 50 to 50,000 per cell. Mitochondria of a cell are collectively known as the chondriome.
Historical account: C. Benda (1897) gave the name mitochondria (mitos, thread, and chondrion, granules). The term 'bioplast' for mitochondria was used by Altman.
Ultrastructure:
• Mitochondria are rod-shaped organelles bounded by a double membrane envelope.
• The outer membrane is smooth. The inner membrane surrounds a central cavity filled with a jelly-like substance called the matrix.
• Inner membrane folds are called cristae. These folds are tubular and called microvilli.
• Mitochondria contain electron transport systems aggregated into compact structures. F1 particles, or oxysomes, are tennis racket-like bodies on the inner membrane, involved in oxidation and phosphorylation.
• Each F1 particle is made up of a base, a stalk and a head.
• The Krebs cycle occurs in mitochondria.

[Figure: Mitochondria, showing the outer membrane, inner membrane, cristae, matrix, DNA, ribosomes and ATP synthase particles.]

Functions of mitochondria:
• Mitochondria are called power plants, power houses or cellular furnaces.
• The synthesis of ATP (adenosine triphosphate) in mitochondria is called oxidative phosphorylation.
• Mitochondria as the site of cellular respiration were first observed by Hogeboom.

Plastids

Introduction: Plastids are organelles enclosed by a double membrane and found in all plant cells.
Historical account: E. Heckel (1865) gave the term plastid. Plastids are the largest cell organelles.
Ultrastructure:
• Plastids occur in most plant cells and are absent in animal cells.
• Plastids are self-replicating organelles, like mitochondria, i.e., they have the power to divide.
• Schimper divided plastids into three types:
    (a) Chromoplasts: coloured plastids (except green)
    (b) Chloroplasts: green coloured plastids
    (c) Leucoplasts: colourless plastids
• Plastids also have a double membrane but no cristae.

Functions of plastids:
• Chloroplasts trap solar energy and use it to manufacture food for the plant.
• Chromoplasts impart various colours to flowers to attract insects for pollination.

Vacuoles

Vacuoles are storage sacs for solid or liquid contents. Vacuoles are small in animal cells, while plant cells have very large vacuoles. The central vacuole of some plant cells may occupy 50 to 90 percent of the cell volume.

Functions of vacuoles:
• They are storage sacs of the cell. The stored material may be solid or liquid food, toxic metabolic by-products or end products of cells.
• In some unicellular organisms, specialised vacuoles maintain the water balance of the body (osmoregulation).
• In plants, they provide turgidity and rigidity to the cells.

Ø ULTRA STRUCTURE OF CELLS

• Cell walls permit the cells to with stand very dilute external media without bursting.