Every living organism is made up of cells. Whether it is a tiny bacterium, a plant, an animal or a human being, the cell is the basic structural and functional unit of life. Some organisms are made of a single cell and are called unicellular, while others are made of many cells and are called multicellular. Robert Hooke first discovered cells in 1665 when he observed thin slices of cork under a microscope and saw small box-like compartments which he named "cells".
The study of the structure, composition and functioning of cells is called cell biology. Understanding the cell is essential to understanding life itself, because all the activities of living organisms - respiration, nutrition, growth, reproduction and excretion - are ultimately performed by cells.
In this chapter we study the discovery of the cell, the cell theory, the various cell organelles and their functions, the differences between plant and animal cells, and the processes that cells perform to stay alive.
In 1665, Robert Hooke observed thin slices of cork under his self-made microscope and described the small empty compartments he saw as "cells". Although what he saw were actually the dead remains of plant cells, his observation led to the word "cell".
Later, in 1674, Antonie van Leeuwenhoek observed living cells such as bacteria and red blood cells for the first time. Robert Brown discovered the nucleus in a cell in 1831. In 1838, Matthias Schleiden and Theodor Schwann proposed the cell theory. Rudolf Virchow later expanded it by adding that new cells arise from pre-existing cells.
The cell theory states: 1. All living organisms are composed of one or more cells. 2. The cell is the basic unit of life. 3. New cells arise from pre-existing cells.
An organism can be unicellular or multicellular. A unicellular organism, such as an amoeba, a paramecium or a bacterium, performs all its life processes within a single cell. A multicellular organism, such as a plant or an animal, has millions of specialised cells that perform different functions.
Most cells are so small that they can be seen only with a microscope. Some cells, however, can be seen with the naked eye, such as the egg of a hen. A nerve cell can be up to a metre long in some animals.
The shape of a cell is related to its function. A nerve cell is long and branched to transmit impulses over long distances. A red blood cell is disc-shaped for maximum surface area. A muscle cell is spindle-shaped to help in contraction and relaxation.
A cell is made up of the following main components:
Plasma membrane (cell membrane): The plasma membrane is the outermost covering of the cell in both plant and animal cells. It is a selectively permeable membrane, which means it allows only certain substances to pass through it. It separates the cell from its external environment and controls the movement of substances into and out of the cell. Because of this property, the plasma membrane is also called a selectively permeable membrane.
Cell wall: The cell wall is present only in plant cells. It is found outside the plasma membrane and is made up of cellulose. It provides rigidity, strength and shape to the plant cell. The cell wall is freely permeable, allowing all substances to pass through it.
Nucleus: The nucleus is the control centre of the cell. It is surrounded by a double membrane called the nuclear membrane, which has tiny pores. Inside the nucleus is the nucleolus and the chromatin material, which contains DNA and becomes chromosomes during cell division. The nucleus controls all the activities of the cell and carries hereditary information from parents to offspring. In some organisms, like bacteria, the nuclear material is not enclosed by a nuclear membrane; such organisms are called prokaryotes, and their nucleus-like region is called the nucleoid.
Cytoplasm: The cytoplasm is a jelly-like substance present between the plasma membrane and the nucleus. It contains many organelles and is the site of many chemical activities of the cell. The cytoplasm plus the nucleus form the protoplasm.
Cell organelles are small, membrane-bound structures present in the cytoplasm which perform specific functions.
Endoplasmic reticulum (ER): The endoplasmic reticulum is a network of membranes throughout the cytoplasm. There are two types - rough ER (with ribosomes attached, involved in protein synthesis) and smooth ER (involved in the manufacture of fats and lipids). The ER serves as a channel for the transport of materials between different parts of the cell.
Golgi apparatus: The Golgi apparatus consists of flattened membrane sacs called cisternae. It packages and transports materials like proteins and lipids made by the cell, and helps in the formation of lysosomes.
Lysosomes: Lysosomes are sacs filled with digestive enzymes. They digest foreign material that enters the cell and break down worn-out cell organelles. Because they can destroy the cell by digesting its own contents, lysosomes are called the suicide bags of the cell.
Mitochondria: Mitochondria are rod-shaped organelles with a double membrane; the inner membrane is folded into projections called cristae. They are the sites of aerobic respiration, where glucose is broken down to release energy in the form of ATP. Because they generate the energy currency of the cell, mitochondria are called the powerhouse of the cell.
Plastids: Plastids are present only in plant cells. Chloroplasts contain the green pigment chlorophyll and are the sites of photosynthesis. Leucoplasts store food, and chromoplasts give colour to flowers and fruits.
Vacuoles: Vacuoles are storage sacs for solids or liquids. In plant cells, a large central vacuole stores water and maintains the turgidity of the cell. Animal cells have small or no vacuoles.
Plant cells differ from animal cells in several ways: 1. Plant cells have a cell wall made of cellulose; animal cells do not. 2. Plant cells have plastids (like chloroplasts); animal cells do not. 3. Plant cells generally have a large central vacuole; animal cells have small vacuoles or none. 4. Plant cells lack centrioles, while animal cells have centrioles which help in cell division. 5. Plant cells are generally larger and have a definite shape due to the cell wall, while animal cells are generally smaller and have an irregular shape.
Both plant and animal cells have a plasma membrane, nucleus, cytoplasm, mitochondria, ribosomes, endoplasmic reticulum, Golgi apparatus and lysosomes.
Diffusion: The movement of a substance from a region of higher concentration to a region of lower concentration is called diffusion. Gases such as oxygen and carbon dioxide move into and out of cells by diffusion. In plants, diffusion helps in the exchange of gases and the transport of materials over short distances.
Osmosis: Osmosis is the movement of water molecules through a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration. When a cell is placed in a hypertonic solution (higher solute concentration outside), water moves out and the cell shrinks. In a hypotonic solution (lower solute concentration outside), water enters the cell and it may swell. In an isotonic solution, the concentration is equal on both sides and no net movement of water occurs.
Osmosis is important for plants; the absorption of water by roots and the opening and closing of stomata depend on osmosis.
All living cells arise from pre-existing cells through cell division. Cell division is of two main types:
Mitosis: Mitosis produces two daughter cells which are identical to the parent cell, with the same number of chromosomes. It is responsible for growth and for repairing damaged cells.
Meiosis: Meiosis produces four daughter cells, each with half the number of chromosomes of the parent cell. It is involved in the formation of gametes (sex cells) and introduces genetic variation.
Cell division is essential for the growth of an organism, the replacement of worn-out cells and the repair of injuries.
| Organelle | Function |
|---|---|
| Plasma membrane | Selectively permeable; controls entry and exit of substances |
| Cell wall | Provides rigidity and shape (plants only) |
| Nucleus | Control centre; carries hereditary material |
| Mitochondria | Site of respiration; powerhouse of the cell |
| Chloroplast | Site of photosynthesis (plants only) |
| Ribosomes | Protein synthesis |
| Lysosomes | Digestion; suicide bags of the cell |
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Cell wall | Present | Absent |
| Plastids | Present | Absent |
| Vacuole | Large central | Small or absent |
| Shape | Definite | Irregular |
| Centrioles | Absent | Present |
In this chapter we learned that the cell is the fundamental unit of life, discovered by Robert Hooke and studied further by Leeuwenhoek, Schleiden, Schwann and Virchow. We studied the structure of the cell, including the plasma membrane, cell wall, nucleus and cytoplasm, and the functions of the organelles such as the endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria, plastids and vacuoles. We compared plant and animal cells, and understood how cells transport materials through diffusion and osmosis. Finally, we examined the two types of cell division - mitosis and meiosis - which are essential for growth, repair and reproduction. A clear picture of the cell helps us understand how organisms grow, obtain energy and reproduce.