Cell Theory and Cellular Structure

Cell Theory

The cell theory is a fundamental concept in biology that describes the properties of cells, the basic unit of life.

• Cells are the basic structural and functional units of life.

• The activity of an organism depends on the collective activities of its cells.

• Biochemical activities of cells are dictated by their organelles.

• Continuity of life has a cellular basis.

Characteristics of Cells

• Cells vary in their size, shape, and function.

• All cells are composed primarily of carbon, hydrogen, nitrogen, and oxygen.

• All cells have a plasma membrane, cytoplasm, and nucleus (in eukaryotes).

Plasma Membrane

Structure of the Plasma Membrane

• Fluid Mosaic Model: The plasma membrane is a phospholipid bilayer with embedded proteins and cholesterol.

• Phospholipids: Form the basic structure, with hydrophilic heads facing outward and hydrophobic tails inward.

• Proteins: Integral (span the membrane) and peripheral (attached to the surface).

• Cholesterol: Stabilizes membrane fluidity.

Specializations of the Plasma Membrane

• Microvilli: Fingerlike extensions that increase surface area for absorption.

• Membrane Junctions:

  • Tight Junctions: Prevent leakage between cells.

  • Desmosomes: Anchor cells together, providing mechanical strength.

  • Gap Junctions: Allow communication between adjacent cells.

Functions of the Plasma Membrane

• Acts as a selectively permeable barrier.

• Facilitates communication and transport between the cell and its environment.

Membrane Transport

Passive Processes

• Simple Diffusion: Movement of molecules from high to low concentration.

• Facilitated Diffusion: Movement via membrane proteins.

• Osmosis: Diffusion of water across a selectively permeable membrane.

Active Processes

• Active Transport: Movement against a concentration gradient, requires ATP.

• Vesicular Transport:

  • Endocytosis: Uptake of materials into the cell.

  • Exocytosis: Release of materials from the cell.

  • Phagocytosis: "Cell eating" of large particles.

  • Pinocytosis: "Cell drinking" of fluids.

Cytoplasm and Organelles

Cytoplasm

The cytoplasm is the fluid-filled region inside the cell, containing organelles and the cytosol.

Major Organelles

• Nucleus: Contains DNA; control center of the cell.

• Nucleolus: Site of ribosome synthesis.

• Ribosomes: Sites of protein synthesis.

• Rough Endoplasmic Reticulum (ER): Studded with ribosomes; synthesizes proteins.

• Smooth ER: Synthesizes lipids and detoxifies chemicals.

• Golgi Apparatus: Modifies, packages, and ships proteins and lipids.

• Lysosomes: Contain digestive enzymes for breakdown of waste.

• Peroxisomes: Detoxify harmful substances.

• Mitochondria: Powerhouse of the cell; site of ATP production.

• Cytoskeleton: Network of protein filaments for structural support.

• Centrosome & Centrioles: Organize microtubules during cell division.

The Cell Cycle

Phases of the Cell Cycle

• Interphase: Period of cell growth and DNA replication.

  • G1 phase: Cell growth.

  • S phase: DNA replication.

  • G2 phase: Preparation for mitosis.

• Mitotic Phase (M phase): Includes mitosis and cytokinesis.

Mitosis

• Prophase: Chromosomes condense, nuclear envelope breaks down.

• Metaphase: Chromosomes align at the cell equator.

• Anaphase: Sister chromatids separate to opposite poles.

• Telophase: Nuclear envelopes reform, chromosomes decondense.

• Cytokinesis: Division of the cytoplasm.

Control of the Cell Cycle

• Regulated by checkpoints and proteins such as cyclins and cyclin-dependent kinases (CDKs).

• Loss of control can lead to uncontrolled cell division (cancer).

Protein Synthesis

Central Dogma

• Transcription: DNA is transcribed to RNA in the nucleus.

• Translation: RNA is translated to protein at the ribosome.

Key Steps

1. DNA → mRNA (transcription)

2. mRNA → Protein (translation)

Tissues

Definition and Types

Tissues are groups of similar cells performing a common function. The four primary tissue types are:

• Epithelial tissue (covering)

• Connective tissue (support)

• Muscle tissue (movement)

• Nervous tissue (control)

Epithelial Tissue

• Characteristics: Closely packed cells, little ECM, avascular, high regeneration.

• Functions: Protection, absorption, filtration, secretion.

• Classification:

  • By layers: Simple (one layer), Stratified (multiple layers)

  • By shape: Squamous (flat), Cuboidal (cube-shaped), Columnar (tall)

• Special Types: Pseudostratified, Transitional, Glandular (endocrine/exocrine)

Connective Tissue (CT)

• Functions: Binding, support, protection, insulation, transportation.

• Common Characteristics: Derived from mesenchyme, varying vascularity, abundant ECM.

• Types:

  • Loose CT (areolar, adipose, reticular)

  • Dense CT (regular, irregular, elastic)

  • Cartilage (hyaline, elastic, fibrocartilage)

  • Bone

  • Blood

Membranes

• Cutaneous membrane: Skin; keratinized stratified squamous epithelium.

• Mucous membrane: Lines body cavities open to the exterior.

• Serous membrane: Lines closed body cavities; produces serous fluid.

Muscle Tissue

• Types:

  • Skeletal muscle: Voluntary, striated, attached to bones.

  • Cardiac muscle: Involuntary, striated, found in heart.

  • Smooth muscle: Involuntary, non-striated, found in walls of hollow organs.

Nervous Tissue

• Function: Conducts electrical impulses, controls body functions.

• Cell Types: Neurons (transmit signals), Neuroglia (support and protect neurons).

Tissue Repair

• Steps:

  1. Inflammation: Prepares area for repair.

  2. Organization: Restores blood supply.

  3. Regeneration and fibrosis: Effect permanent repair.

• The regenerative capacity varies among tissue types.

Summary Table: Major Tissue Types

Additional info:

• Equations for osmotic pressure and membrane potential are relevant but not explicitly listed in the notes. For example, the Nernst equation for membrane potential:

• Further details on cell signaling, apoptosis, and differentiation can be found in advanced texts.