The Fundamental Unit of Life
The cell is the basic structural and functional unit of life.
While examining a thin slice of cork, Robert Hooke observed a honeycomb-like structure of many little compartments. This observation was made in 1665 using a self-designed microscope. Hooke called these boxes cells, derived from the Latin word for 'a little room'. This marked the first time someone observed that living things appear to consist of separate units.
Other important discoveries in cell biology include:
Living organisms are made up of cells. Some organisms, like Amoeba, Chlamydomonas, Paramecium, and bacteria, are unicellular organisms (uni = single), meaning they consist of only one cell. On the other hand, multicellular organisms (multi = many), such as some fungi, plants, and animals, are made up of many cells that group together and perform different functions.
Every multicellular organism originates from a single cell, which divides to produce more cells of its kind. Therefore, all cells come from pre-existing cells.
To observe the structure of cells in an onion peel.
You will observe a structure that looks like Fig. 5.2, showing cells of the onion peel. The cells will look similar to each other, forming a big structure like an onion bulb. Onion bulbs of different sizes have similar small structures visible under a microscope. The cells of the onion peel will all look the same, regardless of the size of the onion they came from.
The basic building units of the onion bulb are cells.
The shape and size of cells are related to their specific functions. For example, Amoeba cells have changing shapes, while nerve cells have a typical, fixed shape. Each living cell performs basic functions characteristic of all living forms. This is achieved through a division of labor within the cell, where specific components called cell organelles perform specialized functions.
The cell is called the structural and functional unit of life because all living organisms are made up of cells (structural), and all life processes occur at the cellular level (functional).
If we study a cell under a microscope, we would come across three features in almost every cell; plasma membrane, nucleus and cytoplasm. All activities inside the cell and interactions of the cell with its environment are possible due to these features.
The plasma membrane or cell membrane is the outermost covering of the cell, separating its contents from the external environment. It allows or permits the entry and exit of some materials while preventing the movement of others. Therefore, it is called a selectively permeable membrane.
Substances move into and out of the cell through processes like diffusion and osmosis.
Diffusion is the spontaneous movement of a substance from a region of high concentration to a region of low concentration. For example, carbon dioxide (), a cellular waste, accumulates in high concentrations inside the cell. Because there is a lower concentration of outside the cell, it moves out by diffusion. Similarly, oxygen () enters the cell when its concentration inside the cell decreases.
Osmosis is the movement of water molecules through a selectively permeable membrane. It is affected by the amount of substance dissolved in water (solute concentration). Osmosis is the net diffusion of water across a selectively permeable membrane toward a higher solute concentration.
One of three things could happen:
To observe the effect of osmosis on a de-shelled egg in different solutions.
(a) Remove the shell of an egg by dissolving it in dilute hydrochloric acid. The shell is mostly calcium carbonate. A thin outer skin now encloses the egg. Put the egg in pure water and observe after 5 minutes.
(b) Place a similar de-shelled egg in a concentrated salt solution and observe for 5 minutes.
(a) The egg swells because water passes into it by osmosis.
(b) The egg shrinks.
(a) In pure water (hypotonic solution), water moves into the egg due to osmosis, causing it to swell.
(b) In a concentrated salt solution (hypertonic solution), water moves out of the egg due to osmosis, causing it to shrink.
To observe the effect of osmosis on dried raisins or apricots in different solutions.
(a) Each gains water and swells when placed in water.
(b) However, when placed in the concentrated solution it loses water, and consequently shrinks.
(a) In plain water (hypotonic solution), water moves into the raisins or apricots due to osmosis, causing them to swell.
(b) In a concentrated solution (hypertonic solution), water moves out of the raisins or apricots due to osmosis, causing them to shrink.
Unicellular freshwater organisms and most plant cells tend to gain water through osmosis. Absorption of water by plant roots is also an example of osmosis.
The plasma membrane is flexible and made up of organic molecules called lipids and proteins. The flexibility of the cell membrane also enables the cell to engulf in food and other material from its external environment. Such processes are known as endocytosis. Amoeba acquires its food through such processes.
moves out of the cell and moves into the cell by the process of diffusion, driven by concentration gradients. Water moves in and out of the cell by the process of osmosis, which is the diffusion of water through a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration.
The plasma membrane is called a selectively permeable membrane because it allows the passage of certain substances into and out of the cell while preventing the movement of other substances. This selective permeability is crucial for maintaining the cell's internal environment and carrying out its functions.
Plant cells, in addition to the plasma membrane, have another rigid outer covering called the cell wall. The cell wall lies outside the plasma membrane and is mainly composed of cellulose. Cellulose is a complex substance that provides structural strength to plants.
When a living plant cell loses water through osmosis, there is shrinkage or contraction of the contents of the cell away from the cell wall. This phenomenon is known as plasmolysis.
To observe plasmolysis in the cells of a Rhoeo leaf.
Plasmolysis is observed in the living cells when a strong solution of sugar or salt is added. No plasmolysis is observed in the dead cells.
Only living cells are able to undergo plasmolysis, demonstrating the importance of a functional cell membrane in osmosis.
Cell walls permit the cells of plants, fungi, and bacteria to withstand very dilute (hypotonic) external media without bursting. In such media, the cells tend to take up water by osmosis. The cell swells, building up pressure against the cell wall, which exerts an equal pressure against the swollen cell. Because of their walls, such cells can withstand much greater changes in the surrounding medium than animal cells.
The nucleus is a darkly colored, spherical or oval, dot-like structure near the center of each cell. It has a double-layered covering called the nuclear membrane, which has pores that allow the transfer of material from inside 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 contain information for inheritance of characters from parents to next generation in the form of DNA (Deoxyribo Nucleic Acid) molecules. Chromosomes are composed of DNA and protein. DNA molecules contain the information necessary for constructing and organizing cells. Functional segments of DNA are called genes. In a cell which is not dividing, this DNA is present as part of chromatin material, visible as an entangled mass of thread-like structures. Whenever the cell is about to divide, the chromatin material gets organized into chromosomes.
The nucleus plays a central role in cellular reproduction, the process by which a single cell divides and forms two new cells. It also plays a crucial part, along with the environment, in determining the way the cell will develop and what form it will exhibit at maturity, by directing the chemical activities of the cell.
In some organisms like bacteria, the nuclear region of the cell may be poorly defined due to the absence of a nuclear membrane. Such an undefined nuclear region containing only nucleic acids is called a nucleoid. Such organisms, whose cells lack a nuclear membrane, are called prokaryotes (Pro = primitive or primary; karyote = karyon = nucleus). Organisms with cells having a nuclear membrane are called eukaryotes.
Prokaryotic cells also lack most of the other cytoplasmic organelles present in eukaryotic cells. Many of the functions of such organelles are also performed by poorly organized parts of the cytoplasm. The chlorophyll in photosynthetic prokaryotic bacteria is associated with membranous vesicles (bag-like structures) but not with plastids as in eukaryotic cells.
The cytoplasm is the fluid content inside the plasma membrane. It contains many specialized cell organelles, each of which performs a specific function for the cell.
Cell organelles are enclosed by membranes. In prokaryotes, beside the absence of a defined nuclear region, the membrane-bound cell organelles are also absent. On the other hand, the eukaryotic cells have a nuclear membrane as well as membrane-enclosed organelles.
The significance of membranes can be illustrated with the example of viruses. Viruses lack any membranes and hence do not show characteristics of life until they enter a living body and use its cell machinery to multiply.
| Prokaryotic Cell | Eukaryotic Cell |
|---|---|
| 1. Size: generally small () | 1. Size: generally large () |
| 2. Nuclear region: undefined and known as nucleoid | 2. Nuclear region: well defined and surrounded by a nuclear membrane |
| 3. Chromosome: single | 3. More than one chromosome |
| 4. Membrane-bound cell organelles absent | 4. Membrane-bound cell organelles present (e.g., mitochondria, ER, Golgi apparatus) |
Every cell has a membrane around it to keep its own contents separate from the external environment. Large and complex cells, including cells from multicellular organisms, need a lot of chemical activities to support their complicated structure and function. To keep these activities of different kinds separate from each other, these cells use membrane-bound little structures (or 'organelles') within themselves. This is one of the features of the eukaryotic cells that distinguish them from prokaryotic cells. Some of these organelles are visible only with an electron microscope.
Some important examples of cell organelles are: endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria and plastids.
The endoplasmic reticulum (ER) is a large network of membrane-bound tubes and sheets. It looks like long tubules or round or oblong bags (vesicles). The ER membrane is similar in structure to the plasma membrane. There are two types of ER: rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER).
RER looks rough under a microscope because it has particles called ribosomes attached to its surface. The ribosomes, which are present in all active cells, are the sites of protein manufacture. The manufactured proteins are then sent to various places in the cell depending on need, using the ER.
SER helps in the manufacture of fat molecules, or lipids, important for cell function. Some of these proteins and lipids help in building the cell membrane. This process is known as membrane biogenesis. Some other proteins and lipids function as enzymes and hormones.
One function of the ER is to serve as channels for the transport of materials (especially proteins) between various regions of the cytoplasm or between the cytoplasm and the nucleus. The ER also functions as a cytoplasmic framework providing a surface for some of the biochemical activities of the cell. In the liver cells of vertebrates, SER plays a crucial role in detoxifying many poisons and drugs.
The Golgi apparatus, first described by Camillo Golgi, consists of a system of membrane-bound vesicles (flattened sacs) arranged approximately parallel to each other in stacks called cisterns. These membranes often have connections with the membranes of ER and therefore constitute another portion of a complex cellular membrane system.
The material synthesized near the ER is packaged and dispatched to various targets inside and outside the cell through the Golgi apparatus. Its functions include the storage, modification, and packaging of products in vesicles. In some cases, complex sugars may be made from simple sugars in the Golgi apparatus. The Golgi apparatus is also involved in the formation of lysosomes.
Structurally, lysosomes are membrane-bound sacs filled with digestive enzymes. These enzymes are made by RER. Lysosomes are a kind of waste disposal system of the cell. These help to keep the cell clean by digesting any foreign material as well as worn-out cell organelles. Foreign materials entering the cell, such as bacteria or food, as well as old organelles end up in the lysosomes, which break complex substances into simpler substances. Lysosomes are able to do this because they contain powerful digestive enzymes capable of breaking down all organic material. During the disturbance in cellular metabolism, for example, when the cell gets damaged, lysosomes may burst and the enzymes digest their own cell. Therefore, lysosomes are also known as the 'suicide bags' of a cell.
Mitochondria are known as the powerhouses of the cell. Mitochondria have two membrane coverings. The outer membrane is porous while the inner membrane is deeply folded. These folds increase surface area for ATP-generating chemical reactions. The energy required for various chemical activities needed for life is released by mitochondria in the form of ATP (Adenosine triphosphate) molecules. ATP is known as the energy currency of the cell. The body uses energy stored in ATP for making new chemical compounds and for mechanical work.
Mitochondria are strange organelles in the sense that they have their own DNA and ribosomes. Therefore, mitochondria are able to make some of their own proteins.
Plastids are present only in plant cells. There are two types of plastids: chromoplasts (colored plastids) and leucoplasts (white or colorless plastids). Chromoplasts containing the pigment chlorophyll are known as chloroplasts. Chloroplasts are important for photosynthesis in plants. Chloroplasts also contain various yellow or orange pigments in addition to chlorophyll. Leucoplasts are primarily organelles in which materials such as starch, oils, and protein granules are stored.
The internal organization of the Chloroplast consists of numerous membrane layers embedded in a material called the stroma. These are similar to mitochondria in external structure. Like the mitochondria, plastids also have their own DNA and ribosomes.
Vacuoles are storage sacs for solid or liquid contents. Vacuoles are small-sized in animal cells while plant cells have very large vacuoles. The central vacuole of some plant cells may occupy 50-90% of the cell volume.
In plant cells, vacuoles are full of cell sap and provide turgidity and rigidity to the cell. Many substances of importance in the life of the plant cell are stored in vacuoles. These include amino acids, sugars, various organic acids, and some proteins. In single-celled organisms like Amoeba, the food vacuole contains the food items that the Amoeba has consumed. In some unicellular organisms, specialized vacuoles also play important roles in expelling excess water and some wastes from the cell.
The two organelles that contain their own genetic material (DNA) are mitochondria and plastids.
If the organization of a cell is destroyed, the cell will no longer be able to perform its functions properly. This can lead to cell death and, if enough cells are affected, can harm the entire organism.
Lysosomes are known as suicide bags because they contain powerful digestive enzymes that can break down all organic material. If a cell is damaged or needs to be destroyed, the lysosomes can burst and release these enzymes, which digest the cell itself.
Proteins are synthesized inside the cell on ribosomes. Ribosomes are found freely floating in the cytoplasm and attached to the rough endoplasmic reticulum (RER).
Each cell thus acquires its structure and ability to function because of the organization of its membrane and organelles in specific ways. The cell thus has a basic structural organization. This helps the cells to perform functions like respiration, obtaining nutrition, and clearing of waste material, or forming new proteins.
Thus, the cell is the fundamental structural unit of living organisms. It is also the basic functional unit of life.
New cells are formed in organisms in order to grow, to replace old, dead, and injured cells, and to form gametes required for reproduction. The process by which new cells are made is called cell division. There are two main types of cell division: mitosis and meiosis.
The process of cell division by which most of the cells divide for growth is called mitosis. In this process, each cell called mother cell divides to form two identical daughter cells. The daughter cells have the same number of chromosomes as the mother cell. It helps in growth and repair of tissues in organisms.
Specific cells of reproductive organs or tissues in animals and plants divide to form gametes, which after fertilization give rise to offspring.
They divide by a different process called meiosis, which involves two consecutive divisions. When a cell divides by meiosis, it produces four new cells instead of just two. The new cells only have half the number of chromosomes than that of the mother cells.
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