ICSE Class 7 Biology Chapter 01 Cell The Structural and Functional Unit of Life

Unit 1: Basic Biology

Cell - The Structural and Functional Unit of Life

Syllabus

1. The cell - the basic unit of life - its structure. Cell organelles - cell membrane, plastids, mitochondria, vacuole, centrosomes, nucleus - their functions (in brief). Differences between plant and animal cells. Cell division - needed for growth and repair in both plants and animals. (Note: Mitosis and meiosis need not be mentioned). * Looking at onion peel and cheek cells under a microscope and drawing the same (D/E). (D = demonstration; E = experiment).

Teacher's Note

Understanding cells helps us appreciate why we grow, heal from cuts, and why plants need sunlight to survive - all processes happening at the cellular level in our bodies.

Introduction to Cells

You have learnt from your previous class that all living organisms, whether plants, animals or human beings, are made up of minute cells. Of these, some are single-celled i.e., unicellular, e.g. amoeba, while others are many-celled i.e., multicellular, e.g. rose, frog, etc.

Every organism starts life as a single cell. Even the largest banyan tree, or an elephant or we ourselves start life as a single cell. This single cell undergoes repeated divisions to become a complete living organism. Thus, we say that the cell is the basic unit of life, both structurally as well as functionally. For this reason, cells are also known as the building blocks of life.

Discovery of Cells

First Discovery: Antony Von Leewenhoek (1632-1723) developed a simple microscope (Fig. 1.1), using only a single biconvex lens. He was the first to see the cells (blood cells in the capillaries of the foot-web of frog) and recognized them as living units of all living beings. However, he did not call them cells.

First to coin the term "cell": In 1660, Robert Hooke developed a microscope to observe different objects using two lenses, hence his microscope was called a compound microscope (Fig. 1.2).

He examined a very thin slice of a dead cork and observed a cluster of box-like cubicles piled up together (Fig. 1.3). This reminded him of "cells" of monks in a monastery, and he was the first to call them cells.

Robert Hooke had only seen the walls of dead cells that had once surrounded the living portion of cells. This living substance is called "protoplasm", and it has two portions - the cytoplasm and the nucleus.

Teacher's Note

The word "cell" comes from the Latin word "cella" meaning compartment - just like how we might organize books in compartments on a shelf, nature organizes life into these tiny cellular compartments.

The branch of biology which deals with the study of cells is called cytology.

Do You Know?

The word cell is derived from the latin word cella (compartment)

Cells - Their Numbers, Shapes and Sizes

Cells - How Numerous?

On the basis of the number of cells, the organisms are categorised as unicellular or multicellular.

(i) Unicellular (single-celled) organisms: Many tiny plants and animals are formed of just one single cell. Such organisms are called unicellular (uni: single). Examples - Bacteria, Yeast, Amoeba, Paramecium, Chlamydomonas (Fig. 1.4).

(ii) Multicellular (many-celled) organisms: All plants and animals, which we see around us, are made up of millions and billions of cells. They are called multicellular (multi: many). Rose, peepal, neem, hydra, earthworm, fish, frog, lion, deer, humans, etc., are all multicellular organisms.

Cell Shape

(Fig. 1.5)

There is a great variety in the shape of cells - oval, spherical, rectangular, irregular, elongated, etc. The cell shape is also determined by the function of the cell.

(i) Irregular: The single-celled body of fresh water Amoeba, is irregular in shape.

(ii) Oval: In Chlamydomonas (an aquatic organism), the single-celled body has an oval shape. The red blood cells are thin and oval shape so that they can slide through easily even in the finest blood capillaries.

(iii) Oblong: In Paramecium (usually found in stagnant water), the single-celled body is oblong (slipper-shaped).

(iv) Elongated: The muscle cells are elongated and contractile, thus enabling them to provide movements to the bones.

(v) Very long (thread-like): The nerve cells are long. Their long shape enables them to send messages over long distances in the body. Imagine the length of a nerve fibre running from your toe in the foot, upto the brain inside the head.

(vi) Cubical or rectangular: The cells of the leaf are cubical or rectangular. In fact, plants also have a variety of cell shapes from cubical to polygonal as well as elongated and tubular.

Cell Size

There is a huge variety in the size of the cells. The great majority of cells are very small, and they can be seen only with the help of a microscope. However, there is a great range in their sizes:

(i) The smallest cells are found among bacteria (02 - 05 micrometre). (A micrometre is equivalent to one-thousandth of a millimetre).

(ii) The longest cells are the nerve cells (upto 3 metre).

(iii) The largest (bulkiest) cells are bird's eggs (actually the yellow sphere inside is the cell which gets surrounded by the white albumen and the egg shell outside). Ostrich eggs are the largest eggs and thus the largest cells (about 170-200 mm in diameter).

Teacher's Note

When you look at a grain of salt under a microscope, you realize that a single cell is thousands of times smaller - yet it contains an entire living system with all the machinery needed for life.

Cell Theory

The cell-theory explains that:

1. Every living organism is made up of one or many cells.

2. The cell is the structural unit of all living organisms.

3. The cell is the functional unit of all living organisms. Activities of an organism are due to the activities of its cells.

4. All cells arise from the pre-existing cells.

Three scientists contributed in framing the cell theory:

First, a German botanist, Matthias Jacob Schleiden (1838), said that every plant is made up of numerous cells, each of which performs various functions.

Second, a German zoologist, Theodor Schwann (1839), confirmed the presence of nucleus surrounded by the cytoplasm and the outer membrane as well. He declared that all plants and animals are made up of cells which serve as the units of structures and functions.

Third, Rudolph Virchow (1858) added that all cells arise from the pre-existing cells.

Cell - The Structural Unit of Life

Every living part of the body of all living organisms (plants and animals) is made up of cells.

Cells may vary in shape and size but basically, they are just the same in their fundamental structure.

If you examine even a tiny part of a plant or animal's body under a microscope, irrespective of whether it is the root, stem or leaf of a plant, or the skin, muscle or bone of an animal, you will find that they are all made up of cells.

Teacher's Note

Every part of your body, from your hair to your fingernails, is made of cells working together - it's like a city where millions of tiny workers coordinate to keep everything functioning.

Cell - The Functional Unit of Life

Every function inside the body of all living organisms (plants or animals) is the result of cell activity.

In Plants

Absorption of water and minerals: It is the cells of the roots through which plants absorb water and dissolved minerals from the soil. If the root cells die for any reason, the plants will dry up.

Preparation of food: It is the chlorophyll (green pigment) containing cells of the leaves which utilise sunlight, carbon dioxide and water to prepare food (glucose) for the plant. This process of preparing food by the plants is known as photosynthesis.

In Animals

Movement: It is the ability of the muscle cells to contract and relax. This enables an animal to move its body as well as fold and unfold its body parts.

Digestion: It is the cells of various glands which secrete enzymes that digest the food. For example, the saliva in your mouth is the secretion of the cells of the salivary glands, that converts starch, present in the food, into sugar (maltose).

Release of energy: Every activity inside the body of living organisms, needs energy. We move our muscles, our nerves convey messages, our body synthesises a large variety of proteins and other organic compounds. All these need energy. Besides, it also keeps our body warm. The energy required for all these activities is produced by oxidising food (glucose) inside the cells.

Teacher's Note

When you eat food and your body converts it to energy for movement and thought, that's cells working at the functional level - digesting, converting, and powering every action you take.

Structure of a Cell

(Fig. 1.6)

An animal cell or a plant cell as seen under a compound microscope show three essential basic parts:

1. The outermost cell membrane (also called plasma membrane),

2. The cytoplasm, and

3. The nucleus.

1. Cell Membrane or Plasma Membrane

Each cell is surrounded by a cell membrane also called plasma membrane.

It is very thin, delicate and flexible.

It is a living structure.

It has fine pores in it, through which only certain substances can pass in and out, while others cannot. Therefore, the cell membrane is called selectively permeable. This allows the entry of certain molecules only, while holding back the others.

It is freely permeable, allowing substances in solution to enter and leave the cell without any hindrance.

Cell wall

Only the plant cells have a cell wall outside in addition to the cell membrane.

It is made up of cellulose.

It gives shape and rigidity to the plant cell.

It is a non-living structure.

It protects the cell from the entry of disease-causing agents, as well as the underlying protoplasm against mechanical injuries.

Note: Animal cells have no cell walls.

2. Cytoplasm

The cytoplasm is a semi-liquid, colourless and translucent fluid. It forms the entire living portion of the cell inside the cell membrane, except the nucleus

3. Nucleus

(Fig. 1.7)

Nucleus is a small spherical mass located in the centre of the cytoplasm. It is the most important part of the cell.

It regulates and coordinates various life processes of the cell.

It plays an important part during cell division.

It contains genes, which determine the heredity of the organism.

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