Cell: Structure and Functions: Comprehensive NEET Biology Notes

1. Cell: The Unit of Life

1.1 What is a Cell?

The cell is the fundamental structural and functional unit of all living organisms. Unicellular organisms are capable of independent existence and performing essential life functions. The invention of the microscope enabled the detailed study of cells. Anton Von Leeuwenhoek first described a live cell, and Robert Brown discovered the nucleus.

Did You Know?

The cell is the smallest unit of life that can replicate independently, and cells are often called the "building blocks of life."

Mnemonic:

Remember the functions of a cell with "MR.GREW": Metabolism, Reproduction, Growth, Response, Energy processing, and Waste management.

1.2 Cell Theory

Matthias Schleiden and Theodore Schwann formulated the cell theory, which states that all living organisms are composed of cells and products of cells. Rudolf Virchow later added that all cells arise from pre-existing cells, completing the cell theory.

Real-life Application:

Understanding cell theory is fundamental in medical research, especially in areas like cancer research, where the focus is on how cells grow and replicate.

1.3 An Overview of the Cell

Cells can be broadly classified into prokaryotic and eukaryotic cells. Eukaryotic cells have a membrane-bound nucleus and organelles, whereas prokaryotic cells do not. The cytoplasm is the main area for cellular activities in both types of cells.

NEET Tip:

Differentiate between prokaryotic and eukaryotic cells: Prokaryotes lack a nucleus, eukaryotes have a nucleus.

Common Misconception:

Not all cells have the same structure; plant cells, for instance, have cell walls and chloroplasts that animal cells do not.

1.4 Prokaryotic Cells

Prokaryotic cells, such as bacteria and blue-green algae, are generally smaller and lack a true nucleus. They have a simple structure with a plasma membrane, cytoplasm, and ribosomes. Plasmids in prokaryotic cells confer unique traits like antibiotic resistance.

Cell Envelope and its Modifications

The prokaryotic cell envelope consists of the glycocalyx, cell wall, and plasma membrane. Bacteria can be classified into Gram-positive and Gram-negative based on their cell envelope's staining properties.

Did You Know?

The smallest prokaryotes are Mycoplasmas, which can be as small as 0.3 µm.

Ribosomes and Inclusion Bodies

Prokaryotic ribosomes (70S) are sites of protein synthesis. Inclusion bodies store reserve materials and are not membrane-bound.

Mnemonic:

For bacterial shapes: "Bacillus (rod), Coccus (spherical), Spirillum (spiral), Vibrio (comma)."

1.5 Eukaryotic Cells

Eukaryotic cells include protists, plants, animals, and fungi. They have a true nucleus and membrane-bound organelles. Plant cells have additional structures like cell walls, chloroplasts, and large central vacuoles, while animal cells have centrioles.

Real-life Application:

Eukaryotic cells' compartmentalization allows for specialized functions, crucial for multicellular organisms' complex processes.

Cell Membrane

The fluid mosaic model describes the cell membrane's structure, primarily composed of a lipid bilayer with embedded proteins. This model explains the membrane's fluid nature and selective permeability.

Common Misconception:

Not all cells have a cell wall; only plant cells and some prokaryotes do.

Endomembrane System

Includes the endoplasmic reticulum (ER), Golgi apparatus, lysosomes, and vacuoles. These organelles work together to synthesize, package, and transport cellular materials.

Did You Know?

The ER can be rough (with ribosomes) or smooth (without ribosomes), each with distinct functions.

Quick Recap

  • Cell Theory: All living organisms are composed of cells; all cells arise from pre-existing cells.
  • Prokaryotic Cells: Simple structure, no true nucleus, examples include bacteria.
  • Eukaryotic Cells: Complex structure, true nucleus, include protists, plants, animals, fungi.
  • Cell Membrane: Fluid mosaic model, selectively permeable.
  • Endomembrane System: Includes ER, Golgi apparatus, lysosomes, vacuoles.

Practice Questions

  1. Describe the main differences between prokaryotic and eukaryotic cells.
  2. Explain the fluid mosaic model of the cell membrane.
  3. What are plasmids, and what is their significance in prokaryotic cells?
  4. How do ribosomes differ in prokaryotic and eukaryotic cells?
  5. Discuss the role of the Golgi apparatus in eukaryotic cells.

Concept Connection

Link to Chemistry:

The study of cell structures and functions involves understanding biochemical processes, linking biology to biochemistry.

NEET Exam Strategy

  • Time Management: Allocate specific time slots for each topic to ensure comprehensive coverage.
  • Practice Questions: Regularly solve NEET questions on cell structure and functions to get familiar with the exam pattern.
  • Diagrams and Charts: Use diagrams to visualize cell structures for better understanding.

Comprehensive Quick Reference Guide and Glossary

Quick Reference Guide

  • Cell Theory: All living organisms are composed of cells; all cells arise from pre-existing cells.
  • Prokaryotic Cells: No true nucleus, simple structure.
  • Eukaryotic Cells: True nucleus, complex structure.
  • Cell Membrane: Lipid bilayer, fluid mosaic model.
  • Endomembrane System: ER, Golgi apparatus, lysosomes, vacuoles.

Glossary

  • Cell Theory: Fundamental concept stating that all living organisms are composed of cells and all cells come from pre-existing cells.
  • Prokaryotic Cell: A cell lacking a true nucleus and membrane-bound organelles.
  • Eukaryotic Cell: A cell with a true nucleus and membrane-bound organelles.
  • Plasmid: A small, circular DNA molecule found in prokaryotes that is separate from the chromosomal DNA.
  • Ribosome: A molecular machine found within all living cells that performs biological protein synthesis (translation).
  • Endoplasmic Reticulum (ER): A network of membranes involved in protein and lipid synthesis.
  • Golgi Apparatus: A series of stacked membranes that package and transport proteins and lipids.
  • Lysosome: A membrane-bound organelle containing enzymes for digestion.
  • Vacuole: A membrane-bound organelle in plant cells for storage and maintaining turgor pressure.
  • Cell Membrane: The semi-permeable membrane surrounding the cytoplasm of a cell.
  • Chloroplast: An organelle in plant cells responsible for photosynthesis.
  • Mitochondrion: An organelle responsible for energy production through aerobic respiration.