CH. 4 Biology of the Cell

Biology of the Cell

  • Chapter 4 by Dr. Lawson

Basic Information Regarding Cells

  • Cytology: The study of cells

  • Cells: The smallest biological unit, functional unit of the body

  • Varied sizes and shapes of cells, with structure often determining function

    • Examples:

      • Irregular: Nerve cells

      • Biconcave disc: Red blood cells

      • Cube-shaped: Kidney tubule cells

      • Column-shaped: Intestinal lining cells

      • Spherical: Cartilage cells

      • Cylindrical: Skeletal muscle cells

Size Comparison of Cells

  • Size Range:

    • Ostrich egg: 1 cm

    • Human height: 1 m

    • Muscle/nerve cells: ~0.1 m

    • Most animal cells: ~30 μm

    • Red blood cell: 1 μm

    • Mitochondrion: 100 nm

    • Ribosomes: 1 nm

    • Atoms: 0.1 nm

Common Features and Functions of Cells

  • Plasma Membrane: Acts as a barrier between the inside of the cell and the external environment (interstitial fluid)

  • Nucleus: Largest organelle, containing DNA that controls cell activities

  • Cytoplasm: Encompasses all contents except the nucleus

    • Cytosol: Fluid inside the cell

    • Organelles: Complex organized structures with unique functions (can be membrane-bound or non-membrane-bound)

Non-Membrane-Bound Organelles

  • Ribosomes:

    • Free Ribosomes: Floating in cytosol

    • Fixed Ribosomes: Attached to rough endoplasmic reticulum

  • Centrosome, Proteasome, Cytoskeleton

Membrane-Bound Organelles

  • Nucleus:

    • Nuclear membrane, nucleolus

  • Rough Endoplasmic Reticulum (ER)

  • Smooth Endoplasmic Reticulum

  • Mitochondrion

  • Golgi Apparatus

  • Peroxisome

  • Lysosome

  • Inclusions

  • Vesicles

General Functions of Cells

  • Maintain structure and integrity

  • Obtain nutrients and form chemical building blocks

  • Dispose of waste products

  • Some cells can divide

  • Growth, maintenance, and replacement of dead cells

The Plasma Membrane

  • Structure: Fluid matrix primarily composed of phospholipids and proteins

  • Regulates Entry/Exit: Only small and nonpolar substances can pass without assistance

Lipid Components of Plasma Membrane

  • Phospholipids: Form bilayer; hydrophilic heads face exterior/interior, hydrophobic tails face inward

  • Cholesterol: Provides stability across temperature ranges

  • Glycolipids: Contribute to identity markers and cell adhesion

Protein Components of Plasma Membrane

  • Integral Proteins: Embedded and span the membrane; include glycoproteins

  • Peripheral Proteins: Loosely attached to surfaces, categorized by function

Categories of Membrane Proteins

  1. Transport Proteins: Regulate substance motion across the membrane

  2. Cell Surface Receptors: Bind ligands for intercellular communication

  3. Identify Markers: Recognize cell identity, important for immune responses

  4. Enzymes: Catalyze reactions on either side of the membrane

  5. Anchoring Sites: Secure the cytoskeleton to the plasma membrane

  6. Cell Adhesion Proteins: Facilitate cell-to-cell attachments

Functions of the Plasma Membrane

  1. Physical Barrier: Supports and protects cellular contents

  2. Selective Permeability: Regulates substance entry/exit

  3. Electrochemical Gradients: Maintains electrical charge difference

  4. Communication: Contains receptors for signal recognition

Membrane Transport

Types of Membrane Transport

  • Passive Processes: Do not require energy, rely on gradients

  • Active Processes: Require energy to move substances against gradients

Passive Processes: Diffusion

  • Diffusion: Movement from high to low concentration until equilibrium is reached

  • Influenced by:

    • Steepness of gradient

    • Temperature

Diffusion in Cells

  • Occurs through the plasma membrane

    • Simple Diffusion: Nonpolar substances

    • Facilitated Diffusion: Requires transport proteins for polar substances

Passive Processes: Osmosis

  • Passive movement of water through a selectively permeable membrane

  • Water moves from low to high solute concentration

Osmotic Pressure and Tonicity

  • Osmotic Pressure: Pressure due to water movement

  • Tonicity: Describes relative solute concentration which affects cell volume

    • Isotonic, hypotonic, hypertonic solutions defined

Hypotonic Solution

  • Lower concentration of solutes outside the cell; can cause swelling or lysis

Hypertonic Solution

  • Higher concentration of solutes outside the cell; can cause cell shriveling

Active Processes: Active Transport

  • Requires energy, classified into primary and secondary active transport

Primary Active Transport

  • Energy from ATP breakdown; ion pumps maintain gradients (e.g. Na+/K+ pump)

Secondary Active Transport

  • Energy harvested from moving a second substance down its gradient

Active Processes: Vesicular Transport

  • Movement of large substances via vesicles

  • Exocytosis: Movement out of the cell

  • Endocytosis: Movement into the cell; includes phagocytosis and pinocytosis

Cell Communication

  • Achieved through direct contact or ligand-receptor signaling

Forms of Ligand-Receptor Signaling

  1. Channel-Linked Receptors: Allow ion passage

  2. Enzymatic Receptors: Serve as enzymes to regulate activity

  3. G Protein-Coupled Receptors: Activate second messengers for cell response

Membrane-Bound vs. Non-Membrane-Bound Organelles

  • Membrane-Bound: Specialized with membranes; part of the endomembrane system

  • Non-Membrane-Bound: Lack membranes; often protein complexes

The Endoplasmic Reticulum (ER)

  • A network of membranes, classified into:

    • Rough ER: Modifies/proteins stores proteins

    • Smooth ER: Synthesizes and detoxifies substances

Golgi Apparatus

  • Functions in modifying, packaging, and sorting proteins received from rough ER

Lysosomes and Peroxisomes

  • Lysosomes: Break down unwanted substances

  • Peroxisomes: Detoxify by removing hydrogen atoms; important in metabolism

Mitochondria

  • Produces ATP through cellular respiration; operates on a double membrane

Cytoskeleton

  • Provides structural support, aids in cell movement, and composed of microfilaments, intermediate filaments, microtubules

Types of Cytoskeletal Components

  1. Microfilaments: Smallest; maintain shape and structure

  2. Intermediate Filaments: Provide rigidity

  3. Microtubules: Largest; involved in organizing organelles and mitosis

External/Surface Cell Structures

  • Cilia and Flagella: Extensions for movement, supported by microtubules

  • Microvilli: Extensions to increase surface area for absorption

Membrane Junctions

  • Important for cell adherence and tissue formation

    • Tight Junctions: Prevent leakage

    • Desmosomes: Hold cells together under stress

    • Gap Junctions: Allow communication between cells

The Nucleus

  • Largest organelle, houses DNA, with nuclear envelope and nucleoplasm

Inside the Nucleus

  • Contains genetic material organized into chromatin or chromosomes during division

Cellular Division

  • Somatic Cells: Divide by mitosis, excluding sex cells

  • Sex Cells: Divide by meiosis

The Cell Cycle and Mitosis

  • Interphase: Preparation phase, divided into three subphases (G1, S, G2)

  • Mitotic Phase: Division phase including prophase, metaphase, anaphase, and telophase

Interphase Details

  • G1: Growth stage, preparation for replication

  • S Phase: DNA replication occurs here

  • G2: Preparation for mitosis, completion of organelle production

DNA Replication Steps

  1. DNA unwinding

  2. Complementary strand assembly

  3. Restoration of DNA double helix

The Mitotic Phase

  • Mitosis consists of four key phases: prophase, metaphase, anaphase, telophase, leading to cytokinesis

Apoptosis

  • Programmed cell death; essential for removal of damaged or potentially harmful cells

  • Activated by ligands binding to receptors, leading to self-destructive pathways

Importance of Cell Death

  • Prevents cancer, limits tissue growth, and encourages proper development

To Be Continued

  • Future discussions will include genetics, DNA transcription, translation and synthesis of proteins.