An Overview of Cell
* Cell is the fundamental unit of life in all living organisms.
* Modern cell theory states that:
• Living organisms are composed of cells and products of cells.
• Cells arise from pre-existing cells.
* The plant cell is surrounded by a cell wall on the outside and a plasma membrane on the inside.
* Plant and animal cells contain a spherical structure called the nucleus, which contains thread-like structures called chromosomes.
* If the nucleus is membrane-bound, then the cell is called an eukaryotic cell.
* Cells without a membrane-bound nucleus are called prokaryotic cells.
* Eukaryotic cells contain membrane-bound organelles such as:
• Endoplasmic reticulum
• Golgi complex
• Lysosomes
• Mitochondria
• Microbodies
• Vacuoles
• Ribosomes
* The cytoplasma of prokaryotic cells is embedded only with ribosomes and plasmids.
* Cells can be disc-like, polygonal, columnar, cuboid or thread-like.
Prokaryotic Cells
* Prokaryotes are organisms without a well-defined nucleus.
* Prokaryotes possess a cell envelope, consisting of the outermost glycocalyx, followed by the cell wall and then the plasma membrane.
* The plasma membrane extends into the cell, forming the mesosome, a membranous structure.
* Motile prokaryotes have thin filamentous extensions called flagella, which consist of a filament, hook and basal body.
* Motile as well as non-motile bacteria also possess pili and fimbriae, surface structure that attach the prokaryotes to rocks in streams and also to host tissues.
* The cytoplasm hosts the genomic DNA-a single chromosome made up of circular DNA.
Eukaryotic Cells
* Eukaryotic cells have an organised nucleus surrounded by a nuclear envelope.
* The cytoplasm of eukaryotic cells contains a cytoskeleton, an elaborate network of filamentous proteinaceous structures.
* Cytoplasm also contains numerous single-membrane-bound minute vesicles called microbodies, which are filled with enzymes that aid in the synthesis of amino acids.
* Cytoplasm also contains several organelles which lead to its extensive compartmentalisation.
* Animal cells contain an organelle called a centrosome made up of two cylindrical structures called centrioles.
* Plant cells differ from animal cells as they possess a cell wall, a central vacuole and plastids.
* The cell wall of a plant is made of cellulose, hemicelluloses, pectins and proteins.
* The cell wall consists of the following three layers:
• Middle lamella
• Primary wall
• Secondary wall
The Cell Membrane
* The cell membrane is composed of lipds, proteins and carbohydrates.
* Lipids are arranged in a bilayer, with the charged polar head of both layers pointing outward and the non-polar, hydrophobic tail made of saturated hydrocarbons pointing inward.
* The Fluid Mosaic Model put forth by Singer and Nicolson states that lipids are found in the form of a fluid bilayer.
* Substances get transported across the cell membrane either through passive transport or through active transport.
* Cilia and flagella are hair-like structures projecting from the cell’s surface.
The Endomembrane System
* The endoplasmic reticulum, Golgi apparatus, Lysosomes and Vacuoles are referred to as an endoplasmic system.
* The Endoplasmic reticulum is a network or reticulum of tiny tubular structures.
* The Golgi apparatus consist of a series of flat, disc-shaped sacs or cisternae stacked one above the other.
* The Golgi apparatus acts as a packaging and forwarding centre and is an important site for the formation of glycoproteins and glycolipids.
* Lysosomes are single-membrane vesicular structures formed during the packaging process in the Golgi apparatus.
* Vacuoles are enveloped in a membrane called the tonoplast and contain water, sap, excretory products and other materials not useful for the cell.
Ribosomes, Mitochondria and Plastids
* Ribosomes are granular and non-membrane bound structures composed of RNA and proteins.
* Mitochondria, called the “Powerhouse of the Cell” are sites of aerobic respiration and produce cellular energy in the form of ATP.
* The membranes of mitochondria divide its lumen into the outer compartment and the inner compartment called the matrix.
* Plastids, the largest double-membrane-bound cytoplasmic organelles, contain different pigments that impart specific colours to plants.
* On the basis of the pigments they possess, plastids can be classified as chloroplasts, chromoplasts and leucoplasts.
* Based on the type of nutrient stored, leucoplast can be classified as amyloplasts, elaioplasts and aleuroplasts.
Nucleus
* The nucleus is surrounded by a nuclear envelope or nuclear membrane.
* The nuclear envelope consists of the outer membrane and inner membrane.
* At a number of places, the two membranes fuse and give rise to minute pores called nuclear pores.
* Present inside the nuclear envelope is the nucleoplasm, the nuclear matrix that contains the nucleolus, a spherical structure that lacks a membrane.
* The nucleoplasm also contains chromatin, a network of highly extended and elaborate nucleoprotein fibres.
* When a cell begins to divide, the chromatin condenses and coils to form rod-like structures called chromosomes.
* A chromosome consists of a primary constriction or centromere that has disc-shaped structures called kinetochores attached to its side.
* Based on the position of the centromere, chromosomes can be classified as metacentric, sub-metacentric, acrocentric and telocentric.