How small is a cell? If you keep the tip of a ballpoint pen on your skin, that will cover around one thousand cells. In human, cells vary between 10 µm and 100 µm (µm stands for micron, which is 1 millionth of a meter or 1 thousandth of a millimetre). Let us see the structure of cells.
Different cells have different shapes and sizes depending on the function they perform. For example, RBCs are circular in shape, nerve cells are elongated, muscle cells are spindle-shaped, skin cells are flat etc.
A cell is regarded as a fundamental unit of life because the basic structure of all the living organisms is made up of a cell. Hence cell is known as the structural unit of life. Cell performs certain functions which are the characteristic of life and which are important for maintenance of life. Hence cell is known as the functional unit of life.
A cell can be viewed as an enclosed vessel, within which innumerable chemical reactions take place simultaneously. These reactions are under very precise control so that they contribute to the life and formation of new cells.
Structure of Cells
A generalized cell consists of a membrane-bound structure enclosing a nucleus and cytoplasm with many small organelles floating in it. Plant and animal cell have a few differences given in the figure below.
Differences Between Plant Cell and Animal Cell
|Plant Cell||Animal Cell|
|Cell wall is present.||Cell wall is absent.|
|Nucleus is peripheral.||Nucleus is central.|
|Large central vacuole is present.||Vacuoles may be small or absent.|
|Plastids are present.||Plastids are absent.|
|Centriole is absent.||Centriole is present.|
|Golgi bodies present in the form of units known as dictyosomes.||Golgi bodies well developed.|
1. Cell Membrane/ Plasma Membrane:
- Present in both cell structure of the prokaryotic cell and eukaryotic cell
- The cell membrane, or plasma membrane, is a biological membrane that surrounds the cytoplasm of a cell. In animals, the plasma membrane is the outer boundary of the cell, while in plants and prokaryotes it is usually covered by a cell wall.
- It is flexible and elastic
- A cell membrane is called a selectively permeable membrane. Why? This is because it allows the materials in and out of the cell according to the requirement of the cell.
- Structure: It is made up of bilipid layer and protein (Fluid Mosaic Model)
- Functions: Encloses the contents of a cell.
- Provides shape
- Allows transport – Transport is of two types- Diffusion and osmosis
Comparison of Diffusion and Osmosis
|Particles of solid, liquid or gas move from its higher concentration to lower concentration||Solvent particles move from its higher concentration to its lower concentration through a semipermeable membrane.|
|It does not require a semipermeable membrane||Semipermeable membrane is required|
|It occurs in liquid and gaseous medium||It occurs only in liquid medium|
What will happen when a cell is placed in solutions of different concentrations?
- Hypotonic solution: A solution which contains more quantity of solvent and less quantity of solute as compared to the cell is called hypotonic solution. When a cell is placed in hypotonic solution endosmosis takes place. An animal cell will swell and burst. A plant cell will resist swelling and burst due to the presence of hard and rigid cell wall. For example distilled water
- Hypertonic Solution: A solution which contains less quantity of solvent and more quantity of solute as compared to the cell is called hypertonic solution. When a cell is placed in hypertonic solution, exosmosis takes place. An animal cell will shrink. In a plant cell, plasma membrane and cytoplasm will separate from the cell wall. This is known as plasmolysis. When a plasmolyzed cell is placed in a hypotonic solution, endosmosis will occur and the cell will regain its structure. This is called de-plasmolysis. For example, saturated salt solution.
- Isotonic Solution: A solution which contains the same quantity of solvent and solute as compared to the cell is called isotonic solution. When a cell is placed in isotonic solution, no net movement of particles takes place. Example Ringer’s solution is isotonic as compared to human RBC.
2. Cell Wall
Next component of the structure of call is the cell wall. They are present only in plant cell and they are hard and rigid.
- Fully permeable
- The cell wall of two adjacent cells are cemented together by middle lamella made up of Calcium Pectate
- Made up of Cellulose in plant and peptidoglycan in bacteria
- breaks in the cell wall (Pits) have cytoplasmic connections (Plasmodesmata) through which exchange of materials occur
- Function: Protection, gives shape and turgidity
- Colloidal, Viscous, Jelly like fluid inside cells.
- Contains 80-90% water and many organic and inorganic compounds.
- Cytoplasm contains enzymes which are responsible for all the metabolic activity.
- Cytoplasm is responsible for giving a cell its shape.
- Various cell organelles are found floating in the cytoplasm.
4. Nucleus (Director/ Brain of the Cell)
- Covered by a double membranous nuclear membrane in a Eukaryotic Cell.
- contains DNA, RNA, Protein, nucleolus, and Chromatin network.
- Functions: Controls the activity of the cell.
- Starts cell division.
- It has the chromosomes which are made up of DNA which controls the hereditary characters.
Solved Question for You
Q: How does the exchange of materials take place in unicellular organisms?
Answer: In unicellular organisms, the process of diffusion helps in exchange of materials. Respiratory gases like oxygen and carbon dioxide diffuse through general body surface into the surrounding water. Food is taken in through phagocytosis.