Study Notes - Cells as Units of Life

ZOOLOGY - UNIT I, LESSON 3: CELLS AS UNITS OF LIFE

Overview

  • The cell is the smallest unit that can carry out all activities associated with life.

    • Most prokaryotes and many protists and fungi consist of a single cell.

    • Most plants and animals have millions of cells.

    • In multicellular organisms, cells are modified in various ways to fulfill specialized functions.

Historical Contributions to Cell Theory

  • Robert Hooke (1665):

    • English scientist; inventor of the compound microscope.

    • Viewed slices of cork and leaves and termed their box-like cavities “cells”.

  • Antonie van Leeuwenhoek:

    • Dutch microscopist; detailed descriptions of various cells laid the foundation for cell theory.

  • Matthias Schleiden (1838):

    • Announced that all plant tissue was composed of cells.

  • Theodor Schwann (1839):

    • Described animal cells as similar to plant cells.

  • Johannes Purkinje (1840):

    • Coined the term protoplasm to describe cell contents; discovered “Purkinje cells”.

  • Rudolf Virchow (1858):

    • Recognized that all cells come from preexisting cells.

Cell Theory

  • Cell Theory Definition:

    • All living organisms are made up of cells.

    • Life cannot exist without cells.

    • A cell represents the basic structural and functional unit of life.

Biological Organization

  • Cells are small and well organized, which aids in maintaining homeostasis.

  • Plasma Membrane:

    • A selective barrier that controls the entry and exit of substances.

  • Most cells have organelles, with each performing specific metabolic jobs.

  • Each cell contains DNA, which holds genetic instructions found in the nucleus.

Cellular and Organismal Levels of Organization

  • Cellular Level:

    • Atoms → Molecule → Macromolecule → Organelle → Cell

  • Organismal Level:

    • Cell → Tissue → Organ → Organ system → Organism

Cell Size and Shape

  • Cell components measured in nanometers (nm), which equals 1/1,000,000,000 of a meter.

  • The surface area-to-volume ratio is critical in limiting cell size. Variations in cell shape help increase this ratio (e.g., microvilli).

Examples of Surface Area-to-Volume Ratio Calculations

  • 2 mm Cube:

    • Surface Area (mm²): 2 imes 2 imes 6 imes 1 = 24 .

    • Volume (mm³): 2 imes 2 imes 2 imes 1 = 8 .

    • Ratio: 24:8 or 3:1 .

  • 1 mm Cube:

    • Surface Area (mm²): 1 imes 1 imes 6 imes 8 = 48 .

    • Volume (mm³): 1 imes 1 imes 1 imes 8 = 8 .

    • Ratio: 48:8 or 6:1 .

Methods of Studying Cells

  • Various techniques used to study cells:

    • Light Microscopes (LM) - basic observations through lenses.

    • Electron Microscopes (EM) - provide the highest resolution.

    • Transmission Electron Microscope (TEM): Passes electron beams through specimens.

    • Scanning Electron Microscope (SEM): Scans surfaces to create 3D images.

    • Biochemical Techniques such as cell fractionation separate the components of cells.

Cell Structure Overview

Prokaryotic Cells

  • Lack a nucleus and membranous organelles.

    • Includes Bacteria and Cyanobacteria.

    • Genetic material (DNA) located in the nucleoid.

    • Features:

    • Size: Typically ext{~1/10} the diameter of eukaryotic cells.

    • Have ribosomes, a cell wall outside plasma membrane, and often flagella for movement.

Eukaryotic Cells

  • Characterized by the presence of a nucleus and membranous organelles.

    • Includes Animals, Plants, Fungi, and Protists.

    • Features:

    • Multiple organelles including mitochondria and the Golgi complex.

    • Specialized structures and a supporting framework (cytoskeleton).

Cell Organelle Functions

Nucleus

  • Enclosed by a nuclear envelope; stores and processes genetic information, with chromatin containing DNA and DNA-binding proteins.

  • Contains nucleolus, which synthesizes rRNA.

Ribosomes

  • Synthesize proteins; may float freely in the cytoplasm or be attached to endoplasmic reticulum (ER).

  • Composed of rRNA and proteins synthesized by nucleolus.

Endoplasmic Reticulum (ER)

  • Rough ER: Studded with ribosomes; synthesizes proteins intended for secretion or for organelles.

  • Smooth ER: Involved in lipid synthesis and detoxification.

Golgi Complex

  • Modifies, sorts, and packages proteins from the ER.

  • Has an entry (cis) and exit (trans) face.

Lysosomes

  • Membrane-bound vesicles containing digestive enzymes; dissolve cellular debris.

Vacuoles

  • Large sacs in plant cells for storage, maintaining turgor pressure.

Mitochondria and Chloroplasts

Mitochondria

  • Known as the powerhouse of the cell; site of cellular respiration, ATP generation.

  • Structure includes a double membrane with cristae increasing surface area.

Chloroplasts

  • Site of photosynthesis; contains chlorophyll and thylakoids.

  • Also has a double membrane; converts light energy into chemical energy.

Cytoskeleton

  • Network of protein fibers providing mechanical strength and shape to cells.

  • Composed of

    • Microtubules: Provide structural support and assist with cell division.

    • Microfilaments: Involved in movement and shape changes.

    • Intermediate filaments: Provide stability and support.

Membrane Function

  • Fluid-Mosaic Model: Describes the plasma membrane as a dynamic and flexible structure composed of a phospholipid bilayer with embedded proteins.

  • Selective Permeability: Regulates the flow of molecules; permeable to gases, less so to larger molecules (hydrophilic/lipophilic factors).

Types of Transport

  • Diffusion and Osmosis: Movement from areas of higher to lower concentration across membranes.

  • Carrier-Mediated Transport: Involves specific transporter proteins for facilitated diffusion and active transport.

  • Endocytosis and Exocytosis: Mechanisms for intake and release of macromolecules and materials.

Cell Division

  • Mitosis: Process where a single cell divides to produce genetically identical daughter cells. Involves phases: Prophase, Metaphase, Anaphase, and Telophase.

  • Meiosis: Reduces chromosome numbers to produce gametes for sexual reproduction.

Cell Cycle Regulation

  • The cell cycle is regulated by cyclins and cyclin-dependent kinases (CDKs) that manage checkpoints to ensure cells divide correctly.