Introduction
The cell is the basic structural and functional unit of all living organisms. It is the smallest part of the body of an organism, capable of independent existence and able to perform all the essential functions of life. Cells vary in number (singlecelled: Amoeba; few-celled: Spirogyra, multi-celled: Man), size (smallest: Bacteria, longest: Nerve cell, largest: Ostrich egg) and shape (columnar: Epithelial cells).
The history of cell science began in 1665, with the observation of a thin section of a bottle cork by the English scientist Robert Hooke.
In 1838, Matthias Schleiden and Theodor Schwann proposed a basic cell theory.
The postulates of the modern cell theory are
- The cell is the smallest unit of structure of all living things.
- The cell is the unit of function of all living things.
- All cells arise from pre-existing cells.
- Also Read Tissues: Plant Tissues and Animal Tissues
Structural Organization Of a cell
Cytoplasm
| NATURE AND OCCURRENCE | Content inside the plasma membrane, excluding the nucleus. |
| MAIN CHARACTERISTICS | Contains a mixture of water and soluble organic and inorganic compounds and various cell organelles. |
| MAIN FUNCTIONS | a. Centre of all metabolic activities. b. Seat of occurrence of glycolysis (production of pyruvic acid). |
Endoplasmic reticulum (ER)
| NATURE AND OCCURRENCE | Irregular network of tubular double membrane. |
| MAIN CHARACTERISTICS | May be smooth (SER) or rough (RER). |
| MAIN FUNCTIONS | a. Supportive framework of the cell. b. RER synthesises proteins, while SER secretes lipids. |
Mitochondria
| NATURE AND OCCURRENCE | Have varied shapes, but usually are sausage-like. |
| MAIN CHARACTERISTICS | a. Double-walled, inner wall thrown into folds, called cristae. b. Contain their own ribosomes and DNA, with several genes. |
| MAIN FUNCTIONS | a. Seat of aerobic respiration. b. Synthesis of respiratory enzymes and energy-rich compounds. |
Golgi Apparatus (In animal cells)
Dictyosomes (In plant cells)
| NATURE AND OCCURRENCE | Stacks of flattened membrane sacs. |
| MAIN CHARACTERISTICS | Consists of a set of membrane bounded, fluid-filled vesicles and vacuoles. |
| MAIN FUNCTIONS | a. Synthesis and secretion of enzymes, hormones etc. b. Involved in the synthesis of plasma membrane, cell wall etc. |
Ribosomes
| NATURE AND OCCURRENCE | Dense, spherical, small granules, either scattered in the cytoplasm or attached to the outside of ER. |
| MAIN CHARACTERISTICS | a. Single-walled, dense, spherical bodies composed mainly of RNA and proteins. b. Not bounded by a membrane. c. Two types: 70S and 80S |
| MAIN FUNCTIONS | Synthesis of proteins. |
Lysosomes
| NATURE AND OCCURRENCE | Simple, tiny, single membrane bound sacs. |
| MAIN CHARACTERISTICS | Contains 40 different types of enzymes. |
| MAIN FUNCTIONS | a. Intracellular digestion. b. Destroy foreign substances. |
Centrosome (In animal cells only)
| NATURE AND OCCURRENCE | Region surrounding the centrioles, located near nucleus. |
| MAIN CHARACTERISTICS | Contains one or two centrioles that are surrounded by radiating microtubules to form an aster shape. |
| MAIN FUNCTIONS | Initiates and regulates cell division. |
Plastids (In plant cells only)
| NATURE AND OCCURRENCE | a. Have their own genome. b. Self-replicating organelles. |
| MAIN CHARACTERISTICS | a. Double membrane, proteinaceous matrix containing DNA. b. Possess disc-like structures called thylakoids, containing chlorophyll. |
| MAIN FUNCTIONS | a. Chromoplasts: Impart colour to flowers and fruits. b. Chloroplasts: Trap solar energy for photosynthesis. c. Leucoplasts: Store starch. |
Nucleus
| NATURE AND OCCURRENCE | Centrally located spherical cellular component. |
| MAIN CHARACTERISTICS | a. Mostly spherical and dense. b. Surrounded by nuclear membrane with pores. |
| MAIN FUNCTIONS | a. Regulates cell functions. b. Regulates cell cycle. |
Nucleolus
| NATURE AND OCCURRENCE | Embedded within the nucleus of the cell. |
| MAIN CHARACTERISTICS | a. One or more in number. b. Round-shaped. |
| MAIN FUNCTIONS | a. Produces ribosomes. b. Participates in proteins synthesis by forming and storing RNA. |
Chromatin fibres
| NATURE AND OCCURRENCE | Embedded within the nucleus of the cell. |
| MAIN CHARACTERISTICS | Network of thread-like structures, made up of DNA. |
| MAIN FUNCTIONS | Chromosomes carry hereditary information or genes. |
Vacuoles
| NATURE AND OCCURRENCE | Fluid-filled or solid-filled and membrane-bounded spaces. |
| MAIN CHARACTERISTICS | a. Non-living structures. b. Larger in plant cells and fewer and smaller in animal cells. |
| MAIN FUNCTIONS | a. Storage of water and other substances, food, pigments and waste products. b. Gives turgidity to the cells. c. Helps to maintain an osmotic pressure in a cell. |
Granules
| NATURE AND OCCURRENCE | Non-living structures. |
| MAIN CHARACTERISTICS | Small particles, crystals or droplets. |
| MAIN FUNCTIONS | Starch (in plant cells), glycogen (in animal cells) and fat-containing granules serve as food for the cell. |
Differences between plant and animal cells
Structural differences:
| FEATURE | PLANT CELL | ANIMAL CELL |
|---|---|---|
| Cell wall | Presence of a definite cell wall, made up of cellulose | Absence of a cell wall |
| Cell membrane | Present internal to the cell wall | Forms the boundary of the cell |
| Centrosome | Absence of centrosome | Presence of centrosome |
| Centriole | Absence of centriole | Presence of centriole |
| Vacuole | Presence of one or more prominent vacuoles | Presence of small and temporary vacuole |
| Plastid | Presence of plastids | Absence of plastids |
Functional differences:
| FEATURE | PLANT CELL | ANIMAL CELL |
|---|---|---|
| Size | Usually larger with distinct outlines | Usually smaller with less distinct boundaries |
| Cytoplasm | Not so dense | Denser and more granular |
| Arrangement of cytoplasm | Only a thin lining of cytoplasm which is mostly pushed to the periphery | Cytoplasm fills up almost the entire cell |
Differences between prokaryotic and eukaryotic cells
| FEATURE | PROKARYOTIC CELL | EUKARYOTIC CELL |
|---|---|---|
| Nucleus | Absence of a well-defined nucleus | Presence of a well-defined nucleus with a nuclear membrane |
| Nucleolus | Absent | Present |
| Genetic material | Presence of a single length of only DNA | Presence of several lengths of DNA, wound around certain protein |
| Ribosomes | Presence of smaller ribosomes | Presence of larger ribosomes |
| Cell organelles | Absence of other cell organelles | Presence of several other cell organelles such as mitochondria, ER, chloroplasts etc |
| Cell division | Cell division takes place by fission or budding, but not by mitosis | Cell division takes place by mitosis or meiosis |
| Examples | Bacteria, blue green algae | Euglena, Amoeba, plants, animals |
- All organisms grow due to the growth in size and increase in the number of cells. This growth is because of the production of more body substance and cell substance.
- Repair of an injury or regeneration of a lost part is because of cell division.
- Movement of the body is because of contractility of the cells or cellular parts.
- Feeding and nutrition have many steps, and each step is a result of cellular activities.
- Circulation of blood as well as the movement of other fluids in the body takes place through various forces that are set up due to the contraction of muscle cells of the heart and other parts of the body.
- Respiratory gases are transported from the lungs to different parts of the body by blood cells such as RBCs.
- Protection of the body from different germs and diseases occurs through cells called white blood cells (WBCs). These cells either devour the germs or give out antibodies or antitoxins, which destroy the germs and neutralize their effect.
- The sensation of touch, pain, heat, cold etc. occurs due to sensory cells. The brain orders the muscles to contract or glands to secrete through their cells.
- Thermoregulation or maintenance of our body heat also takes place through cellular activity.
- All living organisms reproduce through cellular activity.
- In plants, transportation takes place due to cellular activity
Follow us