CELLS

Introduction to Cells

Cells are the smallest microscopic living units and serve as the basic structural and functional units of all living organisms. The human body contains over 200 different cell types, each with specialized structures and functions, but all share some common features.

• Definition: A cell is the fundamental unit of life, capable of performing all necessary biological processes.

• Diversity: Human cells vary in size, shape, and function, reflecting their roles in tissues and organs.

• Examples:

  • Fibroblasts: Cells that produce fibrous connective tissue.

  • Erythrocytes: Red blood cells responsible for oxygen transport.

  • Leukocytes: White blood cells involved in immune defense.

  • Epithelial cells: Cells forming protective layers on body surfaces.

  • Myocytes: Muscle cells, including skeletal and smooth muscle fibers.

  • Lipocytes: Fat storage cells.

  • Neurons: Nerve cells specialized for communication.

  • Spermatozoa: Male reproductive cells.

CELL STRUCTURE

Generalized Cell Anatomy

Cells are typically described in three main sections: plasma membrane, cytoplasm, and nucleus.

• Plasma Membrane: Defines cell boundaries and acts as a semi-permeable barrier.

• Cytoplasm: Contains organelles and cytosol, the site of most cellular activities.

• Nucleus: The control center, housing genetic material and regulating cell functions.

EXTRACELLULAR MATERIALS

Components Outside the Cell

Extracellular materials are substances found outside cells, essential for tissue function and communication.

• Extracellular Fluids:

  • Interstitial fluid: Surrounds tissue cells.

  • Blood plasma: Fluid component of blood.

  • Other fluids: Cerebrospinal fluid, lymph, etc.

• Cellular Secretions: Substances released by cells, such as mucus, saliva, and acids.

• Extracellular Matrix: A network of proteins and polysaccharides that provides structural support and binds cells together, especially abundant in connective tissue.

PLASMA MEMBRANE

Structure and Function

The plasma membrane is a dynamic boundary separating intracellular and extracellular environments. It is described by the fluid mosaic model, consisting of phospholipids, cholesterol, and proteins.

• Phospholipid Bilayer:

  • Composed of hydrophilic (water-attracting) heads facing outward and hydrophobic (water-repelling) tails facing inward.

• Membrane Proteins:

  • Integral proteins: Span the membrane, forming channels or acting as receptors.

  • Peripheral proteins: Attached to the membrane surface, often involved in signaling or structural support.

  • Glycoproteins: Proteins with attached carbohydrates, forming the glycocalyx for cell recognition.

• Cholesterol: Stabilizes membrane fluidity and integrity.

Membrane Junctions

Cells connect and communicate through specialized junctions:

• Tight Junctions: Seal adjacent cells, preventing passage of molecules.

• Desmosomes: Anchor cells together, providing mechanical strength.

• Gap Junctions: Allow direct communication and passage of ions/molecules between cells.

TRANSPORT ACROSS PLASMA MEMBRANE

Selective Permeability and Transport Mechanisms

The plasma membrane regulates entry and exit of substances, maintaining cellular homeostasis. Transport can be passive (no energy required) or active (requires ATP).

• Passive Transport:

  • Simple Diffusion: Movement of small, nonpolar molecules (e.g., O2, CO2) down their concentration gradient.

  • Facilitated Diffusion: Movement of larger or polar molecules (e.g., glucose, Na+) via carrier or channel proteins.

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

• Active Transport:

  • Primary Active Transport: Uses pumps (e.g., sodium-potassium pump) to move ions against their concentration gradients.

  • Vesicular Transport:

    • Endocytosis: Uptake of substances via vesicles (includes phagocytosis, pinocytosis, receptor-mediated endocytosis).

    • Exocytosis: Release of substances from the cell via vesicles.

CHARACTERISTICS OF CYTOPLASM

Cytoplasmic Components

The cytoplasm is the site of most cellular activities and consists of:

• Cytosol: Semi-transparent fluid containing water, proteins, salts, sugars, and ions.

• Inclusions: Chemical substances such as glycogen, lipids, and pigments, varying by cell type.

• Organelles: Specialized structures performing distinct cellular functions.

NUCLEUS

Structure and Function

The nucleus is the control center of the cell, containing genetic material and regulating cellular activities.

• Nuclear Envelope: Double membrane with nuclear pores for transport.

• Nucleolus: Site of ribosome assembly.

• Chromatin: DNA wrapped around histone proteins; condenses into chromosomes during cell division.

OVERVIEW OF THE CELL CYCLE

Phases of the Cell Cycle

The cell cycle describes the series of events from cell formation to division, consisting of interphase and cell division.

• Interphase:

  • G1 phase: Cell growth and protein synthesis.

  • S phase: DNA replication.

  • G2 phase: Preparation for division.

• Cell Division (Mitosis and Cytokinesis):

  • Mitosis: Division of the nucleus in four phases (prophase, metaphase, anaphase, telophase).

  • Cytokinesis: Division of the cytoplasm, resulting in two daughter cells.

Summary Table: Cell Types and Functions

Additional info: These notes expand on brief points with academic context, definitions, and examples to provide a comprehensive study guide for Anatomy & Physiology students.