Cell Structure and Types

Overview

This lecture covers the chapter "Cell: The Unit of Life," focusing on cell structure, classification, organelles, and differences between prokaryotic and eukaryotic cells, including key points relevant for NEET 2025 exam preparation.

Introduction to Cells

• A cell is the fundamental structural and functional unit of life.
• All living organisms are made up of cells; non-living things lack cells.
• Cells can exist as unicellular (one cell) or multicellular (many cells) organisms.

Cell Theory & Discovery

• Anton van Leeuwenhoek first observed living cells; Robert Brown discovered the nucleus.
• Schleiden (plants) and Schwann (animals) formulated the cell theory: all organisms are composed of cells and their products.
• Rudolf Virchow added: new cells arise from pre-existing cells.

Types of Cells: Prokaryotic vs. Eukaryotic

• Prokaryotes: no membrane-bound nucleus or organelles; DNA is free in cytoplasm (e.g., bacteria, cyanobacteria).
• Eukaryotes: have membrane-bound nucleus and organelles (e.g., plants, animals, fungi, protists).
• Ribosomes are present in both types but differ structurally.

Cell Size, Shape, and Function

• Prokaryotic cells are generally smaller than eukaryotic cells.
• Cell shape and size relate to function (e.g., biconcave RBCs for gas transport, amoeboid WBCs for defense).
• Bacteria have various shapes: cocci (spherical), bacilli (rod), spirilla (spiral), vibrio (comma-shaped).

Prokaryotic Cell Structure

• Cell envelope has three layers: glycocalyx (outer, capsule/slime), cell wall (peptidoglycan), plasma membrane.
• Gram-positive bacteria: thick peptidoglycan layer; gram-negative: thin, with outer membrane.
• Mesosomes (plasma membrane infoldings) for ATP production; chromatophores hold photosynthetic pigments in cyanobacteria.
• Motile bacteria have flagella (locomotion); pili (conjugation) and fimbriae (attachment).
• Ribosomes (70S) synthesize proteins; inclusion bodies store food; gas vacuoles provide buoyancy.

Eukaryotic Cell Structure

• Eukaryotes include plants, animals, fungi, and protists.
• Plant cells: have cell wall, chloroplasts, and large central vacuole; animal cells: centrioles, no cell wall or chloroplasts.
• Cell wall in plants provides shape, protection, and consists of cellulose (plants), chitin (fungi).
• Key wall layers: middle lamella (calcium pectate), primary wall (cellulose), secondary wall (lignin).
• Plasmodesmata connect adjacent plant cells for communication.

Plasma Membrane Structure & Function

• Plasma membrane is a phospholipid bilayer with proteins (integral, peripheral) and carbohydrates.
• Follows the fluid mosaic model: quasi-fluid nature allows protein movement.
• Functions: selective transport (simple diffusion, osmosis, facilitated diffusion—passive; active transport requires ATP).

Endomembrane System

• Organelles with coordinated functions: endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vacuoles.
• ER: rough (protein synthesis, ribosomes attached), smooth (lipid synthesis).
• Golgi: modifies, packages proteins/lipids, forms glycoproteins/glycolipids.
• Lysosomes: contain hydrolytic enzymes, degrade waste ("suicide bags").
• Vacuoles: storage, large in plants (tonoplast = membrane).

Other Eukaryotic Organelles

• Mitochondria: double membrane, site of ATP synthesis, semi-autonomous (own DNA, 70S ribosomes).
• Plastids (plants): chloroplasts (photosynthesis), chromoplasts (color), leucoplasts (storage).
• Ribosomes: 80S in cytoplasm/ER, 70S in mitochondria/chloroplasts.

Cytoskeleton & Locomotion

• Cytoskeleton: protein-based microtubules, microfilaments, intermediate filaments for shape, support, movement.
• Cilia and flagella: 9+2 arrangement of microtubules, covered by plasma membrane, for movement.
• Centrioles (in centrosome): only in animal cells, organize spindle fibers during cell division (9+0 structure).

Nucleus & Chromosomes

• Nucleus: double membrane, contains nucleoplasm, chromatin (DNA+protein), nucleolus (rRNA synthesis).
• Nuclear pores allow exchange of RNA/proteins.
• Chromatin condenses into chromosomes during cell division; centromere position classifies chromosome type (metacentric, submetacentric, acrocentric, telocentric).
• Some cells are multinucleate or enucleated (e.g., RBCs).

Microbodies

• Membrane-bound vesicles (e.g., peroxisomes) in plant/animal cells contain enzymes for specific metabolic reactions.

Key Terms & Definitions

• Cell — the smallest structural and functional unit of life.
• Prokaryote — organism lacking a membrane-bound nucleus (e.g., bacteria).
• Eukaryote — organism with a nucleus and membrane-bound organelles.
• Organelle — specialized cell structure with a specific function.
• Ribosome — non-membranous organelle for protein synthesis.
• Plasmodesmata — cytoplasmic connections between plant cells.
• Mesosome — membranous infolding in prokaryotes for respiration.
• Glycocalyx — outermost polysaccharide layer in bacteria.
• Lysosome — organelle containing digestive enzymes.
• Fluid Mosaic Model — model describing the plasma membrane's structure.
• Centrosome/Centriole — animal cell structures for spindle formation during division.
• Chromosome — condensed DNA structure visible during cell division.

Action Items / Next Steps

• Review diagrams of prokaryotic and eukaryotic cell structure.
• Memorize major organelle functions and differences between cell types.
• Study the structure and function of plasma membrane, ER, Golgi, mitochondria, and chloroplasts.
• Prepare for upcoming chapters by revising molecular basis of inheritance (chromatin/chromosome structure).