Cell Theory and Eukaryotic Cell Structure

3.1 Cell Theory

  • Definition of Cell Theory: A fundamental principle in biology developed in the mid-19th century, based on contributions from Matthias Schleiden, Theodor Schwann, Rudolf Virchow, and others.

  • Core Principles:
      - All organisms are made of cells: Microscopic organization similarities led to this observation.
        - Types of Organisms:
            - Unicellular: Comprises a single cell (e.g., bacteria).
            - Multicellular: Comprises multiple cells (e.g., humans).
        - Cell Specialization: Different cells perform specialized functions:
            - Skin cells: Provide protection.
            - Skeletal muscle cells: Facilitate movement.
            - Liver cells: Aid metabolism.
            - Nerve cells: Process information.
      - Cell as Fundamental Unit of Life:
        - Definition: The simplest entity that can be defined as living.
        - Characteristics of Life: Ability to:
            - Reproduce
            - Respond to the environment
            - Harness energy
            - Evolve
        - Distinction from Non-living Entities: Anything smaller (like a membrane or molecule) does not exhibit all the characteristics of life.
      - Cells come from Preexisting Cells: Cells arise from the division of a parent cell, leading to daughter cells.

  • Origin of the First Cell: The topic of Case 1 Life’s Origin raises the question of how the first cell originated.

  • Structure and Function Relationship: The structure of cells is intricately linked to their functions at various biological scales.
      - Example: Different cell types exhibit distinct structures that reflect their functions.
        - Red Blood Cell (Fig. 3.2a): Biconcave shape aids in oxygen transport.
          - High surface area-to-volume ratio facilitates gas exchange.
        - Muscle Cell (Fig. 3.2b): Long and slender for contraction.
        - Neuron (Fig. 3.2c): Extensive branching for communication.
        - Intestinal Cell (Fig. 3.2d): Specialized for nutrient absorption.

3.2 Classification of Cells

  • Prokaryotic vs. Eukaryotic Cells:
      - Prokaryotic Cells (e.g., bacteria, archaea):
        - Lack a nucleus; genetic material is in the nucleoid region.
        - Typically 1–2 micrometers in diameter, small size increases surface area-to-volume ratio.
        - Surrounded by a cell wall; some possess flagella for motility.
      - Eukaryotic Cells (e.g., animals, plants, fungi, protists):
        - Contain a nucleus where DNA is organized as multiple linear chromosomes.
        - Larger size compared to prokaryotic cells.
        - Feature extensive internal membranes defining compartments (organelles).
        - Organelles play specialized roles within the cell, akin to departments in a factory.

3.3 Nucleus and Gene Regulation

  • Nucleus:
      - Stores the majority of the cell’s DNA and is the site of RNA synthesis.
      - Structure: Has a double membrane known as the nuclear envelope.
        - Composed of two lipid bilayers with proteins, continuous at nuclear pores that regulate molecule movement between the nucleus and cytoplasm.
      - Transcription and Translation:
        - In eukaryotes, transcription occurs in the nucleus, while translation occurs in the cytoplasm, allowing for separate regulation of each process.
        - In prokaryotes, transcription and translation occur simultaneously, as soon as mRNA is produced.

3.4 The Endomembrane System

  • Definition: A system of membranes that compartmentalizes the cell, including the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, and vesicles.

  • Structure of Eukaryotic Cells:
      - Total surface area of internal membranes is approximately ten times greater than the cell membrane.
      - The internal environment surrounding organelles is referred to as the cytosol.

  • Functions:
      - Organelle membranes are interconnected, allowing communication and transportation of materials.
      - Vesicles: Transport substances between organelles or to/from the exterior of the cell via processes like exocytosis and endocytosis.

3.5 Organelles of the Endomembrane System

  • Nucleus: Houses the genome and regulates RNA synthesis.

  • Endoplasmic Reticulum (ER):
      - Continuous with the outer nuclear envelope.
      - Types:
        - Rough ER: Ribosome-studded, synthesizes proteins for export or membrane incorporation.
            - Well-developed in cells that secrete proteins, e.g., pancreatic cells.
        - Smooth ER: Lacks ribosomes, involved in lipid biosynthesis and detoxification of harmful substances.

  • Golgi Apparatus: Modifies, sorts, and packages proteins and lipids received from the ER.
      - Composed of flattened membrane sacs (cisternae), with distinct regions facilitating different modifications like glycosylation.

  • Lysosomes: Derived from the Golgi, they degrade macromolecules and play a role in cellular digestion and the recycling of organic compounds.
      - Contain hydrolytic enzymes that function optimally at a pH of about 5.

  • Functionality of Lysosomes: Essential for managing cellular waste, recycling components, and controlling apoptosis due to environmental stimuli.