Cell Structure, Membrane Components, and Fluid Transport Mechanisms

Cell Structure & Components

Cell Membrane & Related Terms

The cell membrane is a dynamic structure that separates the cell from its environment and regulates the movement of substances in and out of the cell. It is composed of a phospholipid bilayer with embedded proteins and carbohydrates, each contributing to membrane function and cellular communication.

• Phospholipid: Molecule with a hydrophilic (water-loving) head and hydrophobic (water-fearing) tail. Forms the bilayer of membranes.

• Hydrophobic: Repels water (phospholipid tails).

• Hydrophilic: Attracts water (phospholipid heads).

• Amphipathic: Molecule with both hydrophilic and hydrophobic regions (like phospholipids).

• Plasma membrane: Outer boundary of cell, made of phospholipid bilayer with proteins and carbohydrates. Controls what enters/exits.

• Membrane proteins: Embedded or attached proteins; can act as channels, receptors, enzymes, anchors, or transporters.

• Membrane carbohydrates: Attached to proteins/lipids = form glycocalyx, used for recognition, signaling, and protection.

Internal Cell Components

Cells contain specialized structures called organelles, each with distinct functions necessary for cellular survival and activity.

• Cytoplasm: All material between the plasma membrane and nucleus (cytosol + organelles).

• Cytosol: Fluid portion of cytoplasm (water, ions, molecules).

• Cytoskeleton: Network of protein fibers; provides shape, movement, and internal transport.

• Endoplasmic reticulum (ER):

  • Rough ER: Studded with ribosomes; site of protein synthesis and transport to rough ER.

  • Smooth ER: No ribosomes; lipid synthesis, detoxification, calcium storage.

• Golgi apparatus: Modifies, packages, and ships proteins and lipids for secretion or membrane insertion.

• Mitochondria: Double-membrane organelle; site of ATP production via cellular respiration.

Nucleus & Genetic Material

The nucleus is the control center of the cell, housing genetic material and directing cellular activities through gene expression.

• Nucleus: Control center; houses DNA and directs cell activity.

• Nucleoplasm: Gel-like fluid inside the nucleus.

• Deoxyribonucleic acid (DNA): Double-stranded genetic material storing instructions for proteins.

• Ribonucleic acid (RNA): Single-stranded; carries DNA instructions to ribosomes for protein synthesis.

Feedback Mechanisms

Homeostatic Regulation

Feedback mechanisms are essential for maintaining homeostasis, allowing the body to respond to internal and external changes.

• Negative feedback: The body reverses a change to maintain homeostasis.

  • Example: Body temperature regulation (sweating when hot, shivering when cold).

• Positive feedback: The body amplifies a change until completion.

  • Example: Blood clotting, labor contractions.

Fluids & Transport at Microscopic Level

Fluid Compartments

Body fluids are distributed in distinct compartments, each playing a role in cellular function and transport.

• Extracellular fluid (ECF): Fluid outside cells (includes plasma + interstitial fluid).

• Intracellular fluid (ICF): Fluid inside cells.

Transport Mechanisms

Transport across cell membranes occurs via several mechanisms, driven by concentration gradients and membrane properties.

• Solute: Homogeneous mixture of solute in solvent.

• Concentration gradient: Difference in solute concentration across a membrane.

• Diffusion: Passive movement of molecules from high to low concentration.

• Permeability: Ability of a membrane to allow substances to cross.

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

• Osmotic pressure: Pressure exerted by water moving into a solution due to solute concentration.

Tonicity

Tonicity describes the effect of extracellular fluid on cell volume, influencing water movement and cell shape.

• Isotonic: Equal solute concentration; no net water movement.

• Hypotonic: Lower solute outside → water enters cell → swelling/lysis.

• Hypertonic: Higher solute outside → water leaves cell → shrinking (crenation).

Key Equations

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

Example Table: Tonicity Effects on Cells

Solution Type	Solute Concentration	Water Movement	Cell Effect
Isotonic	Equal inside & outside	No net movement	Cell remains normal
Hypotonic	Lower outside	Into cell	Cell swells/lyses
Hypertonic	Higher outside	Out of cell	Cell shrinks (crenates)

Additional info: The notes have been expanded to include definitions, examples, and a table for clarity and completeness.