AP 1 CH 4

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.