Cell Structure and Function

Definition and Overview of Cells

Cells are the fundamental units of life, serving as microscopic packages that contain all the parts necessary for survival. The human body contains trillions of cells, each with specialized functions, but all share basic components and processes.

• Cellular Homeostasis: Loss of cellular homeostasis leads to disease.

• Generalized Cell Structure: Most cells have a plasma membrane, cytoplasm, and nucleus.

• Cell Diversity: Over 250 different cell types exist in the human body, varying in size, shape, and function.

Major Regions of a Generalized Cell

• Plasma Membrane: Separates intracellular fluid from extracellular fluid.

• Cytoplasm: Contains organelles and cytosol.

• Nucleus: Contains genetic material (DNA).

Plasma Membrane Structure and Composition

Membrane Lipids

The plasma membrane is a dynamic structure composed mainly of lipids and proteins, forming a lipid bilayer.

• Phospholipids: ~75% of membrane lipids; form a bilayer with hydrophobic tails and hydrophilic heads.

• Glycolipids: ~5%; lipids with attached sugar groups, found only on the outer membrane surface.

• Cholesterol: ~20%; stabilizes the membrane by wedging between phospholipid tails.

Membrane Proteins

Proteins embedded in the membrane allow communication and transport.

• Integral Proteins: Firmly inserted into the bilayer; include transmembrane proteins that span both layers.

• Peripheral Proteins: Loosely attached to integral proteins.

• Protein Functions:

  • Transport (channels, carriers)

  • Enzymatic activity

  • Receptors for hormones and neurotransmitters

  • Cell recognition and anchoring

Glycocalyx

The glycocalyx is a "sugar covering" of glycoproteins and glycolipids that forms a fuzzy, sticky area at the cell surface, providing biological markers for cell recognition.

Cell Junctions

Cells are often bound together by specialized junctions:

• Tight Junctions: Prevent leakage between cells.

• Desmosomes: Anchor cells together, providing mechanical strength.

• Gap Junctions: Allow communication between cells via channels.

Membrane Transport Processes

Passive Membrane Transport

Passive transport does not require cellular energy (ATP) and relies on concentration gradients.

Diffusion

• Definition: Movement of molecules from an area of high concentration to low concentration.

• Principles:

  • Based on Brownian movement (random, high-speed motion).

  • Influenced by molecular size, temperature, and concentration.

• Equation: Where J is the flux, D is the diffusion coefficient, and is the concentration gradient.

Types of Passive Transport

• Simple Diffusion: Non-polar and lipid-soluble substances diffuse directly through the lipid bilayer (e.g., O2, CO2).

• Facilitated Diffusion: Polar molecules and ions move through protein channels or carriers.

  • Channel-mediated: Substances pass through aqueous channels.

  • Carrier-mediated: Integral proteins transport specific molecules by changing shape.

• Dialysis: Diffusion of solutes across a selectively permeable membrane.

Osmosis

Osmosis is the diffusion of water across a selectively permeable membrane in response to solute concentration differences.

• Solute Sucks: Water moves toward higher solute concentration.

• Osmotic Pressure: Pressure required to prevent water movement into a solution. Where is osmotic pressure, i is the van 't Hoff factor (number of particles), M is molarity, R is the gas constant, and T is temperature.

• Osmolarity: Measurement of solute concentration, expressed as osmoles.

• Osmotic Imbalances: Can cause cells to swell (hypotonic) or shrink (hypertonic).

Table: Membrane Permeability and Osmosis

Osmolarity Calculations

• Depends on number of particles (ions) dissociated in solution.

• Examples:

  • 1 molar glucose (C6H12O6): 1 osmole (does not dissociate)

  • 1 molar NaCl: 2 osmoles (dissociates into Na+ and Cl-)

  • 1 molar Na2SO4: 3 osmoles (dissociates into 2 Na+ and SO42-)

Additional Info

• Extracellular Fluid (ECF): Fluid outside the cell, including interstitial fluid, blood plasma, and cerebrospinal fluid.

• Intracellular Fluid (ICF): Fluid inside the cell, typically called cytosol.

• Cell Junctions: Specialized structures for cell adhesion and communication, important in tissues subjected to mechanical stress.