Cell Theory
Introduction
The Cell Theory is a fundamental principle in biology that explains the properties and structure of all living organisms. This theory was initially proposed by Schleiden and Schwann in 1838-1839 and later refined by Rudolf Virchow.
Key Points of the Cell Theory:
1. All living organisms are composed of one or more cells: Cells are the basic units of life in both unicellular and multicellular organisms.
2. The cell is the basic unit of life: All the functions essential to life occur within cells.
3. All cells arise from pre-existing cells: This concept was introduced by Rudolf Virchow in 1855, emphasizing that cells do not spontaneously generate but come from division of pre-existing cells.
Exceptions to Cell Theory:
NEET Focus: Know the key scientists associated with the development of cell theory, its principles, and the exceptions.
Characteristics of Prokaryotic Cells
Introduction
Prokaryotic cells are primitive cells that lack membrane-bound organelles. Bacteria and archaea are the primary examples of prokaryotic organisms.
Characteristics of Prokaryotic Cells:
1. Lack of Nucleus: Prokaryotes have a nucleoid region where their circular DNA is located, but they lack a true membrane-bound nucleus.
2. Plasmids: Small, circular DNA molecules that are separate from chromosomal DNA and often carry genes for antibiotic resistance.
3. Cell Wall: Most prokaryotes have a rigid cell wall made of peptidoglycan (in bacteria), providing structural support and protection.
4. Ribosomes: Prokaryotes have 70S ribosomes, which are smaller than eukaryotic ribosomes (80S).
5. Lack of Membrane-Bound Organelles: Organelles like mitochondria, Golgi apparatus, and endoplasmic reticulum are absent.
6. Binary Fission: Prokaryotes reproduce asexually through a process called binary fission.
NEET Focus: Characteristics of prokaryotic cells, especially structural components like the cell wall, ribosomes, and nucleoid, are commonly asked questions.
Characteristics of Eukaryotic Cells
Introduction
Eukaryotic cells are complex cells that contain membrane-bound organelles, including a true nucleus. All multicellular organisms and some unicellular organisms (like protists) are eukaryotic.
Characteristics of Eukaryotic Cells:
1. Nucleus: Eukaryotic cells have a well-defined, membrane-bound nucleus that contains linear chromosomes made of DNA and histone proteins.
2. Membrane-Bound Organelles: Organelles such as the mitochondria, Golgi apparatus, endoplasmic reticulum (ER), and lysosomes are present.
3. Cell Wall: In plant cells and fungi, a cell wall is present (composed of cellulose in plants and chitin in fungi). Animal cells lack a cell wall.
4. Cytoskeleton: A network of microfilaments, microtubules, and intermediate filaments that maintain cell shape and help in intracellular transport.
5. 80S Ribosomes: Larger ribosomes (80S) in the cytoplasm, with 70S ribosomes present in mitochondria and chloroplasts.
6. Mitosis and Meiosis: Eukaryotic cells divide by mitosis (for growth and repair) and meiosis (for sexual reproduction).
NEET Focus: Comparisons between prokaryotic and eukaryotic cells are commonly tested. Focus on structural differences, organelles, and reproduction.
Structure and Function of Plasma Membrane
Introduction
The plasma membrane is a selectively permeable barrier that surrounds the cell, controlling the movement of substances in and out of the cell. It is composed primarily of lipids and proteins.
Structure of Plasma Membrane:
1. Fluid Mosaic Model: Proposed by Singer and Nicolson in 1972, the plasma membrane is described as a dynamic structure with lipids and proteins that move laterally within the bilayer.
2. Phospholipid Bilayer: The plasma membrane consists of two layers of phospholipids, with hydrophilic heads facing outwards and hydrophobic tails facing inwards.
3. Proteins:
o Integral Proteins: Span the entire membrane and are involved in transport and signaling.
o Peripheral Proteins: Loosely attached to the outer or inner surface of the membrane, involved in cell signaling and maintaining cell shape.
4. Carbohydrates: Attached to proteins (glycoproteins) or lipids (glycolipids), functioning in cell recognition and communication.
Functions of Plasma Membrane:
1. Selective Permeability: Regulates the entry and exit of ions, nutrients, and waste products.
2. Cell Signaling: Receptors in the membrane receive and transmit signals from other cells or the environment.
3. Cell-Cell Recognition: Glycoproteins and glycolipids act as identification markers for cell recognition.
4. Transport: Facilitates passive (diffusion, osmosis) and active (requiring energy) transport processes.
NEET Focus: The structure (fluid mosaic model) and function of the plasma membrane are critical topics. Questions on transport mechanisms (diffusion, facilitated diffusion, active transport) are frequently asked.
Structure and Function of Cell Wall
Introduction
The cell wall is an additional, rigid layer found in plant cells, fungi, and some bacteria. It provides structural support and protection.
Structure of Cell Wall:
1. Composition in Plants: Made of cellulose, hemicellulose, and pectin.
2. Middle Lamella: The region between adjacent plant cells, rich in pectin, helping to cement cells together.
3. Primary Cell Wall: Thin, flexible, and forms first in young cells. Composed of cellulose and other polysaccharides.
4. Secondary Cell Wall: Thicker and stronger, formed inside the primary wall in mature cells. Contains cellulose, lignin, and other strengthening materials.