Cellular Form and Function: Structure and Transport Mechanisms

Chapter 3: Cellular Form and Function

3.1 Concepts of Cellular Structure

This section introduces the foundational concepts of cell biology, including the development of cell theory, cell types, and the basic components of a cell.

• Development of Cell Theory:

  • Robert Hooke made significant improvements to the compound microscope, was the first to see and name "cells," and published Micrographia in 1665.

  • Cell theory states:

    1. Cells are the building blocks of all plants and animals.

    2. All cells come from the division of pre-existing cells.

    3. Cells are the smallest units that perform all vital physiological functions.

    4. Each cell maintains homeostasis at the cellular level.

• Main Classes of Cells:

  • Somatic cells: All body cells except those produced by meiosis (e.g., neurons, osteocytes, muscle cells, epithelial cells).

  • Sex cells (germ cells): Reproductive cells (male sperm cells, female oocytes/eggs).

• Basic Components of a Cell:

  • Plasma (cell) membrane: Defines cell boundaries, separates inside from outside, composed of proteins and lipids.

  • Cytoplasm: Region between plasma membrane and nucleus; contains organelles, cytoskeleton, inclusions, and cytosol (intracellular fluid, ICF).

  • Extracellular fluid (ECF): Fluid outside cells, including tissue (interstitial) fluid, blood plasma, lymph, and cerebrospinal fluid (CSF).

3.2 The Cell Surface

This section explores the structure and function of the plasma membrane, including its molecular components and specialized surface structures.

3.2a The Plasma Membrane

• Functions:

  1. Physical barrier separating inside and outside of the cell.

  2. Regulates entry/exit of ions, nutrients, and wastes.

  3. Responds to extracellular signals via membrane receptors.

  4. Provides structural support and facilitates cell attachment to other cells or the extracellular matrix (ECM).

Membrane Lipids

• Phospholipids: 75% of membrane lipids; amphipathic molecules arranged in a bilayer with hydrophilic heads facing water and hydrophobic tails avoiding water. They drift laterally, maintaining membrane fluidity.

• Cholesterol: 20% of membrane lipids; stabilizes membrane by stiffening or loosening it as needed.

• Glycolipids: 5% of membrane lipids; phospholipids with short carbohydrate chains on the extracellular face, contributing to the glycocalyx (carbohydrate coating on the cell surface).

Membrane Proteins

• Constitute 2% of membrane molecules but 50% of its weight.

• Transmembrane proteins: Span the entire membrane; most are glycoproteins with hydrophilic regions contacting cytoplasm and ECF, and hydrophobic regions passing through the lipid bilayer.

• Peripheral proteins: Adhere to one face of the membrane, often tethered to transmembrane proteins and the cytoskeleton.

Functions of Membrane Proteins

• Receptors: Bind chemical signals and trigger internal changes, often via second messenger systems.

• Enzymes: Catalyze reactions, including digestion and production of second messengers.

• Channel proteins: Allow hydrophilic solutes and water to pass; include leak channels (always open) and gated channels (open in response to stimuli such as ligands, voltage, or mechanical stress).

• Carriers: Bind and transport solutes across the membrane; pumps use ATP to move substances against their concentration gradient.

• Cell-identity markers: Glycoproteins that act as identification tags.

• Cell-adhesion molecules (CAMs): Mechanically link cells to each other and to the ECM.

3.3 Membrane Transport

This section covers the mechanisms by which substances move across the plasma membrane, including passive and active processes.

3.3b Simple Diffusion

• Definition: Net movement of particles from high to low concentration without energy input, due to spontaneous molecular motion.

• Factors affecting diffusion rate:

  • Temperature: Higher temperature increases particle motion and diffusion rate.

  • Molecular weight: Smaller molecules diffuse faster.

  • Concentration gradient: Steeper gradients increase diffusion rate.

  • Membrane surface area: Greater area increases rate.

  • Membrane permeability: More permeable membranes allow faster diffusion.

3.3c Osmosis

• Definition: Net flow of water through a selectively permeable membrane from an area of higher water (lower solute) concentration to lower water (higher solute) concentration.

• Facilitated by aquaporins (water channel proteins).

• Osmotic pressure: Hydrostatic pressure required to stop osmosis; increases with the amount of nonpermeating solute.

• Reverse osmosis: Mechanical pressure overrides osmotic pressure, used in water purification.

3.3d Osmolarity and Tonicity

• Osmolarity: Total concentration of non-permeating solutes per liter of solution. Normal body fluids are about 300 mOsm/L.

• Tonicity: The ability of a solution to affect cell volume and pressure, depending on nonpermeating solute concentration.

  • Hypotonic solution: Lower solute concentration than ICF; causes cells to absorb water, swell, and possibly lyse (burst).

  • Hypertonic solution: Higher solute concentration than ICF; causes cells to lose water and shrivel (crenate).

  • Isotonic solution: Same solute concentration as ICF; no net change in cell volume (e.g., 0.9% NaCl).

3.3e Carrier-Mediated Transport

• Definition: Carrier proteins in the membrane transport solutes into or out of the cell.

• Specificity: Each carrier transports specific solutes.

• Saturation: Transport rate increases with solute concentration up to a maximum (transport maximum, $T_m$).

• Mechanisms:

  1. Facilitated diffusion: Carrier moves solute down its concentration gradient without ATP.

  2. Primary active transport: Carrier moves solute up its concentration gradient using ATP (e.g., calcium pump, sodium-potassium pump).

3.3f Vesicular Transport

• Definition: Movement of large particles, fluid droplets, or many molecules at once via vesicles.

• Endocytosis: Brings material into the cell; exocytosis releases material from the cell. Both require ATP.

• Forms of endocytosis:

  1. Phagocytosis: "Cell eating"; cell engulfs large particles using pseudopods to form a phagosome, which fuses with a lysosome for digestion.

  2. Pinocytosis: "Cell drinking"; cell takes in droplets of ECF containing useful molecules.

  3. Receptor-mediated endocytosis: Specific particles bind to receptors, forming a clathrin-coated vesicle for targeted uptake.

Summary Table: Types of Membrane Transport

Transport Type	Energy Required?	Direction	Example
Simple Diffusion	No	Down concentration gradient	Oxygen, CO2
Osmosis	No	Water down its gradient	Water movement in/out of cells
Facilitated Diffusion	No	Down concentration gradient	Glucose transport
Primary Active Transport	Yes (ATP)	Against concentration gradient	Sodium-potassium pump
Vesicular Transport	Yes (ATP)	Bulk movement	Phagocytosis, exocytosis

Additional info: The notes above are structured to provide a comprehensive overview of cellular structure and membrane transport, suitable for introductory Anatomy & Physiology students. Key terms are defined, and examples are provided for clarity.