The study of human anatomy and physiology is a crucial branch of Biology Topics.
What is a Cell: Discovery, Types & Characteristics
All of us have seen the plants like rose, marigold, sunflower, cabbage, mushroom, mango tree, oak tree, coconut tree, banana tree, ferns and many others. We also know the animals like dog, cat, man (human beings), cow, horse, camel, lion, elephant, mosquito, housefly, sparrow, fish, starfish, butterfly, sea anemone, centipede, spider, grasshopper, and many others. In fact, there is a large variety of organisms (plants and animals) around us. They have different shapes and sizes. Their food habits and habitats (living places) are also different. Inspite of great variations in the size, shape, food habits and habitats, etc., all the living organisms have a basic similarity among them : they are made up of tiny units called ‘cells’.
Thus, in biology cells are the structural and functional units of life. In order to understand the structure and functioning of an organisme we should know the structure of cells and functions of the various parts of the cells. In this Chapter, we will study the basic parts of the cells and the role played by these parts in the working of cells. Before we go further and study how cell was discovered, we should know the meaning of the term ‘cork’ which was used in the discovery of cell. Cork is a part of the bark of a tree. Cork is a dead plant material.
In 1665, an English scientist named Robert Hooke used a microscope to investigate the structure of a thin slice of cork. He observed that cork had tiny compartments in it (see Figure). Robert Hooke thought of these compartments as ‘small rooms’ and called them cells’ (because cells are small rooms in which prisoners are locked up or in which monks and nuns sleep).
From this observation Robert Hooke concluded that the cork is made of tiny cells. The dead plant cells of cork observed by Robert Hooke were found to be empty.
The microscope used by Robert Hooke was primitive. When better microscopes (having greater magnifying power) were made, scientists investigated pieces of living plants and found that like cork, they also had a cell structure. The living plant cells were, however, not empty like cork cells, they were found to contain a number of tiny structures (called organelles). In 1838, a German scientist Schleiden suggested that all plants are made of cells. A year later, another German scientist Schwann suggested that all animals are made of cells. These observations led to a general cell theory of organisms. The cell theory states that the basic unit of structure and function of all living organisms is the cell. All cells arise from pre-existing cells by cell division.
The cells have been present in the living organisms (plants and animals) since the origin of life. The cells were, however, not studied or observed for thousands of years because most of them are extremely small and cannot be seen with naked eye. It was only when microscopes having high magnification power were made in the seventeenth century to magnify things greatly that cells could be seen in living organisms. These days, microscopes can enable us to see the objects as small as one-millionth of a metre (10-6 m). Such high magnifying power microscopes have helped the scientists to study the minute details of cells. We will now study the cells in detail.
Cells are the basic units of life. All the living things (plants and animals) are made from cells. Just as a house is made up of bricks, in the same way, a living organism is made up of cells. Thus, cells are the building
blocks of plants and animals. A baby, a banyan tree and a bacterium look very different from one another, but they are all made up of tiny cells. Our nose, skin, muscles and bones are all made up of cells. The simplest organisms like Amoeba consist of only one cell but a complex organism like man (human being) is made up of trillions of cells. We can now define a cell as follows:
A cell is the smallest unit of life which has a definite structure and performs a specific function. Most of the cells are very, very small which cannot be seen with naked eye. They can be seen only with the help of microscope. The diagrams of cells which we see in the books are highly magnified (as seen through the microscope). It is within the cells of our body that all our life activities occur. All living cells come only from other living cells. Since cells are the living units, they require energy. The cells obtain their energy by burning food like glucose in a process called respiration. The cells are of two main types: Animal cells and Plant cells. Though many things are common between animal cells and plant cells but they differ in some ways. We will first give the basic parts of the cells and then describe the general structure of an animal cell and a plant cell.
Parts Of A Cell
Each cell has a number of smaller parts in it. Some of the parts are present in all type of cells (plant cells as well as animal cells). But certain parts are found only in plant cells, they are not present in animal cells.
- The important basic parts of all the cells are : Cell membrane (or Plasma membrane), Cytoplasm, Nucleus and Mitochondria.
- The parts which are present only in plant cells are : Cell wall, Chloroplasts and Large vacuole.
We will now describe all these parts of the cell in detail, one by one. Let us start with the cell membrane.
1. Cell Membrane (or Plasma Membrane)
Every cell is covered by a thin sheet of skin which is called cell membrane (or plasma membrane) (see Figure). The cytoplasm and nucleus are enclosed within the cell membrane. In other words, cell membrane encloses the living part of the cell called ‘protoplasm’. The cell membrane protects the cell and also gives shape to the cell. The cell membrane has tiny pores in it.
The cell membrane controls the movement of substances ‘into the cell’ and ‘out of the cell’. The dissolved substances such as food (glucose) and oxygen can enter into the cell whereas the waste products such as carbon dioxide can go out from the cell through the pores of the cell membrane. The cell membrane separates the cells from one another and also from the surrounding medium. The cell membrane (or plasma membrane) is a living part of the cell.
Cytoplasm is a transparent, jelly-like material which fills the cell between nucleus and cell membrane (see Figure). Cytoplasm is a kind of chemical factory of the cell. Here, new substances are built from materials taken into the cell, and energy is released and stored. In fact, most of the chemical reactions which keep the cell alive take place in the cytoplasm. The cytoplasm of a cell has many tiny structures in it. The various structures present in the cytoplasm of a cell are called ‘organelles’.
The most prominent organelle in the cytoplasm is the nucleus. In addition to nucleus, cytoplasm of all the cells contains other organelles such as mitochondria, golgi bodies and ribosomes, etc. The cytoplasm of plant cells also contains chloroplasts (We will study golgi bodies and ribosomes in higher classes). The cytoplasm and the nucleus taken together make up the protoplasm.
Nucleus is a large, spherical organelle present in all the cells (see Figure 6 on page 138). In animal cells, nucleus lies in the centre of the cell whereas in plant cells the nucleus may be on the periphery (near the edge) of the cell. Nucleus is the largest organelle in a cell. Nucleus can be stained (with a dye) and seen easily with the help of a microscope. Nucleus is separated from the cytoplasm by a membrane called nuclear membrane.
Nucleus controls all the activities of cell.The transmission of characteristics (or qualities) from the parents to the offspring is called inheritance. Nucleus plays a role in inheritance. This can be explained as follows: Nucleus contains thread-like structures called chromosomes (Chromosomes can be seen only when the cell divides). Chromosomes contain genes (which are the units of inheritance).
The function of chromosomes is to transfer the characteristics from the parents to the offsprings through the genes (which is called inheritance). The nucleus also contains a tiny round structure called nucleolus (which can be seen under the high magnifying power of a microscope). We will study the functions of nucleolus in higher classes. The nucleus containing chromosomes and nucleolus is bound by a membrane called ‘nuclear membrane’. The nuclear membrane has tiny pores for the exchange of materials with cytoplasm. Red blood cells, however, do not have a nucleus.
Before we end the discussion on nucleus, it will be good to say a few words about genes’ which are held on the chromosomes present in the nucleus of a cell. Gene is a unit of inheritance in living organisms (which is transferred from a parent to offspring during reproduction and determines some characteristic feature of the offspring). Thus, genes control the transfer of hereditary characteristics from parents to offsprings.
This means that our parents pass on some of their characteristic features to us through their genes. It is the genes inherited (or received) from our parents which are responsible for our various body features. In other words, it is the genes on the chromosomes which decide all sort of things about us, say, the colour of our eyes, the look of our hair, the shape of our nose, our complexion, our appearance, etc.
For example, if our father has brown eyes, we may also have brown eyes (due to the gene of brown eye colour coming from him). And if our mother has curly hair, then we may also inherit curly hair (through the gene coming from her). However, usually the different combinations of genes from both the parents (father and mother) result in different characteristics in the offspring. Let us discuss the protoplasm now.
All the living matter in a cell is called protoplasm. Protoplasm is a liquid substance which is present inside the cell membrane. Protoplasm includes cytoplasm, nucleus and other organelles. Most of the protoplasm is made up of compounds of only four elements : carbon, hydrogen, nitrogen and oxygen. It also contains some other elements. Some of the compounds present in protoplasm are water, carbohydrates, proteins, fats, nucleic acids and minerals salts. A unique combination of elements and compounds provides living nature to protoplasm.
Mitochondria are the tiny rod-shaped or spherical organelles which are found in all the cells (see Figure). Mitochondria provide energy for all the activities of the cell. This energy is produced by the process of respiration in which food (such as glucose) is broken down by oxygen. Thus, mitochondria use glucose and oxygen to produce energy. Mitochondria are found in large numbers in the cytoplasm in all the cells.
5. Cell Wall
The plant cells have a thick cell wall around them (outside the cell membrane) (see Figure). The cell wall is made of a tough material called cellulose. Cell wall gives shape and support to the plant cell. Cell walls also hold the plant cells together and give plants most of their strength. Plants need protection against high wind speed, variations in temperature and atmospheric moisture, etc.
Since plants are fixed at a place, so they cannot move to protect themselves from the various changes in their surroundings. The tough cell wall present in plant cells provides protection to plants. Please note that cell wall is present only in plant cells. Cell wall is not there in animal cells. The cell wall is a non-living part of the plant cells.
Chloroplasts are the green coloured organelles present in the cytoplasm of plant cells (see Figure). The process of food making by plants known as photosynthesis takes place in chloroplasts. The green colour of chloroplasts is due to the presence of a green pigment called ‘chlorophyll’ in them. Chlorophyll can absorb sunlight energy. In the chloroplasts, carbon dioxide and water combine in the presence of sunlight energy to produce food such as glucose. And this process of food making is called photosynthesis. Thus, chloroplasts help in the making of food by green plants. Please note that chloroplasts are found only in those plant cells which carry out photosynthesis (like the cells in green leaves). So, chloroplasts will not be found in the root cells of a plant.
The organelles containing pigments (coloured matter) present in the cytoplasm of plant cells are called plastids. Plastids can be of many different colours. The plastids containing green pigment (chlorophyll) are known as chloroplasts. The green coloured plastids (called chloroplasts) present in the cells of leaves provide
green colour to the leaves (see Figure). The plastids can also have pigments of other colours. For example, the red colour of tomatoes is due to the presence of ‘plastids having red pigment’ in the cytoplasm of its cells (see Figure). The different colours of flowers are due to the presence of plastids containing pigments of different colours.
7. Large Vacuole
Vacuole is a space in the cytoplasm of a cell which is enclosed by a membrane and usually contains substances dissolved in water. A vacuole appears as an empty space under the microscope. All the plant cells have a large vacuole (see Figure). The vacuole is filled with a liquid called ‘cell sap’, which contains dissolved sugars and salts. The pressure of liquid (cell sap) pushes on the outer parts of the plant cell keeping the plant cell firm (or turgid). The function of vacuole in a cell is to store various substances including waste products of the cell. Most of the animal cells do not have vacuoles. Some animal cells have vacuoles but- they are much smaller than those found in plant cells. For example, Amoeba is an animal cell which has very small vacuoles. In Amoeba, vacuoles contain food particles, so they are known as food vacuoles.