Cells and Cell Structure

nChapter 4: Cells and Cell Structure

Cell Theory

  • Definition: An overarching principle of biology that describes the nature and function of cells.

  • Key Points:

    • Cells are the basic units of life, representing the smallest functional unit of all living organisms.

    • All living organisms are composed of cells.

    • New cells arise from preexisting cells, emphasizing the continuity of life.

Common Components of Cells

  • Four universal components of all cells:

    1. Enclosing Plasma Membrane:

    • Acts as a barrier, separating the interior of the cell from its external environment.

    1. Cytoplasm:

    • Composed of cytosol, a jelly-like substance in which cell components are suspended.

    1. DNA:

    • The genetic material that encodes the information necessary for cellular functions and heredity.

    1. Ribosomes:

    • Cellular structures that synthesize proteins, essential for various cellular functions.

Types of Cells: Prokaryotes vs. Eukaryotes

  • The Two Major Types of Cell Types:

    1. Prokaryotes:

    • Characteristics: Lack a defined nucleus.

    1. Eukaryotes:

    • Characteristics: Have a defined nucleus containing the cell's genetic material.

Factors Limiting Cell Size

  • Surface Area-to-Volume Ratio:

    • As cells increase in size, the volume of the cell increases at a faster rate than the surface area.

    • This limited ratio poses challenges for nutrient uptake and waste elimination.

    • Example of Surface Area and Volume Calculations:

    • For a cube with a side length of 1 mm:

      • Surface Area:

      • Total surface area = height x width x number of sides x number of boxes

      • Result: 6 mm²

      • Volume:

      • Total volume = height x width x length x number of boxes

      • Result: 1 mm³

      • Surface-to-Volume Ratio:

      • (Surface area) / (volume) = 6 mm² / 1 mm³ = 6

    • Larger cells do not increase surface area sufficiently to meet metabolic demands.

Size Differences: Prokaryotic vs. Eukaryotic Cells

  • Prokaryotic Cells:

    • Generally smaller than eukaryotic cells.

    • Reasons for Small Size:

    • Favorable surface area-to-volume ratio for efficient material transport.

    • Absence of complex internal structures found in eukaryotic cells that facilitate transport processes, leading to increased efficiency.

General Structure of Prokaryotic Cells

  • Components:

    • Chromosomal DNA:

    • Localized within a nucleoid region.

    • Ribosomes:

    • Present in the cytoplasm, responsible for protein synthesis.

    • Cell Membrane:

    • Enclosed by a cell wall, which may contain various structures not universally present in all bacteria.

Structure of Eukaryotic Cells

  • Key Organelles and Components:

    • Nucleus:

    • Typically one per cell; usually the largest organelle.

    • Larger than most prokaryotic cells; surrounded by a double membrane (nuclear envelope).

    • Plasma Membrane Structure:

    • Composed of a phospholipid bilayer with embedded proteins, including:

      • Glycoproteins:

      • Proteins attached to carbohydrates.

      • Glycolipids:

      • Lipids attached to carbohydrates.

      • Peripheral Membrane Proteins:

      • Located on the membrane's surface.

      • Integral Membrane Proteins:

      • Embedded within the membrane.

      • Cholesterol:

      • Embedded to maintain membrane fluidity and stability.

      • Protein Channels:

      • Allow selective transport across the membrane.

    • Cell Components Summary:

    • Cytoplasm:

      • Contains cytosol (aqueous, jelly-like solution), ribosomes, and cytoskeleton.

      • Role: Provides cell structure, facilitates cell division and transport.

Endomembrane System in Eukaryotic Cells

  • Components of the Endomembrane System:

    1. Rough Endoplasmic Reticulum (RER):

    • Has ribosomes attached, involved in protein synthesis and modification.

    1. Smooth Endoplasmic Reticulum (SER):

    • Lacks ribosomes, involved in lipid synthesis and calcium ion storage.

    1. Lysosomes (in animals):

    • Contains enzymes for digestion of large molecules and organelles.

    1. Vacuoles (in plants):

    • Stores water, food, ions, and facilitates digestion.

    1. Golgi Apparatus:

    • Processes, modifies, and sorts proteins and lipids for distribution.

  • Functionality:

    • Interconnected membranes and sacs that play roles in synthesis, modification, and transport of cellular materials.

Semiautonomous Organelles and Endosymbiotic Theory

  • Definition of Semiautonomous Organelles:

    • Organelles believed to have evolved from engulfed cells.

  • Main Examples:

    1. Mitochondria:

    • Has two membranes; involved in cellular respiration which converts stored energy in molecular bonds to ATP.

    1. Chloroplast:

    • Contains two membranes; responsible for photosynthesis, converting light energy into chemical energy (ATP and carbohydrates).

    1. Peroxisome:

    • Has a single membrane; involved in lipid metabolism and detoxification of hydrogen peroxide (H₂O₂).

  • Functionality Implication:

    • The evolution of these organelles relates to the endosymbiotic theory, suggesting a symbiotic origin contributing to the complexity of eukaryotic cells.