Introduction
The study of cells is fundamental to biology, as cells are the basic units of life. This topic is crucial for NEET aspirants because understanding the structure and function of cells lays the foundation for more advanced concepts in biology. This study note will break down complex ideas into manageable sections, ensuring a comprehensive understanding of the topic.
Cell Theory
Historical Background
- Robert Hooke (1665): First person to observe cells in a cork slice using a primitive microscope.
- Anton van Leeuwenhoek: Improved the microscope and observed living cells, including bacteria and protozoa.
- Matthias Schleiden and Theodor Schwann (1838-1839): Proposed that all plants and animals are composed of cells.
- Rudolf Virchow (1855): Stated that all cells arise from pre-existing cells (Omnis cellula e cellula).
Tenets of Cell Theory
- All living organisms are composed of one or more cells.
- The cell is the basic unit of structure and function in living organisms.
- All cells arise from pre-existing cells.
Cell theory is one of the foundational principles of biology, emphasizing that the cell is the fundamental unit of life.
Types of Cells
Prokaryotic Cells
- Characteristics:
- Lack a membrane-bound nucleus.
- DNA is located in a nucleoid region.
- Absence of membrane-bound organelles.
- Generally smaller in size (1-10 µm).
- Examples:
- Bacteria (e.g., Escherichia coli)
- Archaea
Escherichia coli is a well-studied prokaryotic organism that lives in the intestines of humans and other animals.
Eukaryotic Cells
- Characteristics:
- Possess a membrane-bound nucleus.
- Contain membrane-bound organelles (e.g., mitochondria, chloroplasts).
- Larger in size (10-100 µm).
- Examples:
- Animal cells
- Plant cells
- Fungi
- Protists
Eukaryotic cells have a higher level of structural organization compared to prokaryotic cells.
Cell Structure
Plasma Membrane
- Composition:
- Phospholipid bilayer with embedded proteins.
- Carbohydrates attached to proteins and lipids.
- Functions:
- Selective permeability to regulate the entry and exit of substances.
- Communication through receptor proteins.
Remember the fluid mosaic model to understand the dynamic nature of the plasma membrane.
Cytoplasm
- Components:
- Cytosol: The fluid part of the cytoplasm.
- Organelles: Specialized structures within the cell.
Nucleus
- Structure:
- Nuclear envelope: Double membrane with nuclear pores.
- Nucleolus: Site of ribosome synthesis.
- Chromatin: DNA and protein complex.
- Function:
- Controls cellular activities by regulating gene expression.
Confusing the nucleolus with the nucleus. The nucleolus is a substructure within the nucleus responsible for ribosome production.
Mitochondria
- Structure:
- Double membrane with inner membrane folded into cristae.
- Matrix containing enzymes and mitochondrial DNA.
- Function:
- ATP production through cellular respiration.
Endoplasmic Reticulum (ER)
- Types:
- Rough ER: Studded with ribosomes; involved in protein synthesis.
- Smooth ER: Lacks ribosomes; involved in lipid synthesis and detoxification.
Golgi Apparatus
- Structure:
- Stacks of flattened membranous sacs.
- Function:
- Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.
Lysosomes
- Structure:
- Membrane-bound vesicles containing hydrolytic enzymes.
- Function:
- Digestive system of the cell, breaking down waste materials and cellular debris.
Chloroplasts (in Plant Cells)
- Structure:
- Double membrane with internal thylakoid membranes forming grana.
- Stroma containing enzymes and chloroplast DNA.
- Function:
- Photosynthesis: Conversion of light energy into chemical energy.
Cytoskeleton
- Components:
- Microfilaments: Actin filaments involved in cell movement.
- Intermediate filaments: Provide structural support.
- Microtubules: Tubulin filaments involved in cell division and intracellular transport.
The cytoskeleton is essential for maintaining cell shape, enabling movement, and facilitating cell division.
Cell Division
Mitosis
- Phases:
- Prophase
- Metaphase
- Anaphase
- Telophase
- Cytokinesis
- Function:
- Produces two genetically identical daughter cells.
Meiosis
- Phases:
- Meiosis I: Reductional division.
- Meiosis II: Equational division.
- Function:
- Produces four genetically diverse haploid cells (gametes).
Confusing mitosis with meiosis. Remember that mitosis results in identical cells, while meiosis results in genetically diverse gametes.
Conclusion
Understanding the cell as the unit of life is crucial for mastering biology. This topic encompasses the basic structure and function of cells, the differences between prokaryotic and eukaryotic cells, and the processes of cell division. Mastery of these concepts is essential for success in the NEET exam and for a deeper understanding of biological sciences.
