Cell Structure and Membrane Transport: Study Notes for Anatomy & Physiology

Cells: The Living Units

Why This Matters

Understanding the structure and function of the body's cells is essential for explaining how the plasma membrane regulates permeability and affects treatment outcomes.

Cell Theory

• Cell: The structural and functional unit of life.

• The function of an organism depends on the individual and combined activities of its cells.

• Structure and function are complementary: biochemical functions are dictated by cell shape and subcellular structures.

• Continuity of life arises from preexisting cells.

Cell Diversity

• There are over 200 different types of human cells.

• Cells differ in size, shape, and subcellular components, leading to differences in function.

Generalized Cell Structure

• All cells share common structures and functions.

• Three basic parts of a human cell:

  • Plasma membrane: Flexible outer boundary.

  • Cytoplasm: Intracellular fluid containing organelles.

  • Nucleus: DNA-containing control center.

Extracellular Materials

Substances Outside Cells

Extracellular materials are found outside cells and include:

• Extracellular fluid: Includes interstitial fluid (surrounds cells), blood plasma (fluid of blood), and cerebrospinal fluid (surrounds nervous system organs).

• Cellular secretions: Such as saliva and mucus.

• Extracellular matrix: DNA-containing material outside cells.

Plasma Membrane

Part 1 - Plasma Membrane

• Acts as an active barrier separating intracellular fluid (ICF) from extracellular fluid (ECF).

• Controls what enters and leaves the cell.

• Also known as the "cell membrane".

Structure of Plasma Membrane

• Composed of membrane lipids forming a flexible lipid bilayer.

• Membrane proteins float through the bilayer, creating a dynamic mosaic pattern.

• Glycocalyx: Surface sugars attached to lipids and proteins.

• Cell junctions: Structures that hold cells together.

Membrane Lipids

• Lipid bilayer is made up of:

  • 75% phospholipids: phosphate heads are hydrophilic (water-loving), fatty acid tails are hydrophobic (water-hating).

  • 5% glycolipids: lipids with sugar groups on outer membrane surface.

  • 20% cholesterol: increases membrane stability.

Membrane Proteins

• Allow cell communication with the environment.

• Make up half the mass of the plasma membrane.

• Most have specialized membrane functions.

• Some float freely, others are tethered to intracellular structures.

• Types:

  • Integral proteins: span the membrane.

  • Peripheral proteins: loosely attached to integral proteins.

Integral Proteins

• Firmly inserted into the membrane.

• Most are transmembrane proteins (span the membrane).

• Have both hydrophobic and hydrophilic regions.

• Hydrophobic areas interact with lipid tails; hydrophilic areas interact with water.

Peripheral Proteins

• Loosely attached to integral proteins.

• Include filaments on intracellular surface for plasma membrane support.

• Functions:

  • Enzymes

  • Motor proteins for shape change during cell division and muscle contraction

  • Cell-to-cell connections

Glycocalyx

• Consists of carbohydrates sticking out of the cell surface.

• Functions as specific biological markers for cell-to-cell recognition.

• Allows immune system to recognize "self" vs. "nonself".

• Glycocalyx of some cancer cells can change rapidly, making them unrecognizable to the immune system.

Cell Junctions

Types of Cell Junctions

• Tight junctions: Integral proteins fuse adjacent cells to form an impermeable junction, preventing molecules from moving between cells.

• Desmosomes: Linker proteins anchored to plaques inside cells, providing strength and reducing tearing under tension.

• Gap junctions: Transmembrane proteins (connexons) form tunnels for communication and passage of ions and small molecules between cells.

Membrane Transport

Selective Permeability

• Plasma membranes are selectively permeable: some substances pass easily, others do not.

• Two ways substances cross the membrane:

  • Passive processes: No energy required.

  • Active processes: Energy (ATP) required.

Passive Membrane Transport

• Substances move without energy input.

• Types:

  • Diffusion (simple, facilitated, osmosis)

  • Filtration (across capillary walls)

Diffusion

• Movement of molecules from areas of high concentration to low concentration.

• Driven by concentration gradient.

• No energy required.

Simple Diffusion

• Nonpolar, hydrophobic substances diffuse directly through the lipid bilayer.

• Examples: oxygen, carbon dioxide, fat-soluble vitamins.

Facilitated Diffusion

• Transport of polar molecules (e.g., glucose, amino acids, ions) down their concentration gradient via protein carriers or channels.

• Types:

  • Carrier-mediated: Substances bind to protein carriers.

  • Channel-mediated: Substances move through water-filled channels.

Osmosis

• Movement of water across a selectively permeable membrane.

• Water diffuses through plasma membranes via lipid bilayer or aquaporins (water channels).

• Driven by differences in osmolarity (total concentration of solute particles).

Osmotic and Hydrostatic Pressure

• Osmotic pressure: Pressure of water inside cell pushing on membrane.

• Hydrostatic pressure: Pressure of water outside cell pushing to move into cell by osmosis.

• The more solutes inside a cell, the higher the osmotic pressure.

Tonicity

• Ability of a solution to change the shape or tone of cells by altering their internal water volume.

• Isotonic solution: Same solute concentration as inside cell; no net water movement.

• Hypertonic solution: Higher solute concentration than inside cell; water leaves cell, causing shrinkage (crenation).

• Hypotonic solution: Lower solute concentration than inside cell; water enters cell, causing swelling (lysis).

Clinical Applications

• Intravenous solutions of different tonicities are used to treat various conditions.

• Isotonic solutions are commonly given when blood volume needs to be increased quickly.

• Hypertonic solutions are given to patients with edema to pull water back into the blood.

• Hypotonic solutions can cause cell swelling and lysis, especially in red and white blood cells.

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction	Examples
Simple Diffusion	No	Down concentration gradient	O2, CO2, fat-soluble vitamins
Facilitated Diffusion	No	Down concentration gradient	Glucose, amino acids, ions
Osmosis	No	Down water concentration gradient	Water via aquaporins
Filtration	No	Across capillary walls	Blood plasma filtration in kidneys

Key Equations

• Osmosis: Water moves from areas of low solute (high water) concentration to areas of high solute (low water) concentration.

• Osmotic Pressure: $\Pi = iMRT$ Where $\Pi$ is osmotic pressure, $i$ is the van 't Hoff factor, $M$ is molarity, $R$ is the gas constant, and $T$ is temperature in Kelvin.

Additional info: Academic context and definitions have been expanded for clarity and completeness. Table entries and equations have been inferred and formatted for study purposes.