Cell Membrane, Diffusion, Osmosis, Active Transport, and Bulk Transport

Basic Parts of a Cell

The cell is the fundamental unit of life, composed of several key structures that perform essential functions.

• Plasma membrane: The outer boundary of the cell, controlling entry and exit of substances.

• Cytoplasm: The region between the plasma membrane and the nucleus, containing cytosol (fluid portion) and organelles.

• Organelles: Specialized structures within the cell, such as the nucleus (which has its own membrane, the nuclear envelope, and contains DNA).

Eukaryotic Cell Membrane (Plasma Membrane)

Structure and Function

The plasma membrane is a dynamic structure that separates the cell from its environment and regulates the movement of substances.

• Physical separation: Maintains the internal environment of the cell.

• Selective permeability: Controls exchange of materials between the cell and its surroundings.

• Communication: Facilitates signaling and interaction with other cells.

• Sensitivity: Responds to environmental changes.

Fluid Mosaic Model

• The membrane is composed of phospholipids, cholesterol, and proteins in a constantly shifting arrangement.

• Described as an "ever moving sea of fluid lipids with proteins that float like icebergs or are anchored like boats at a dock."

• Fluidity allows for cell shape changes and movement of materials within the cell.

Phospholipid Bilayer

• Consists of a double layer of phospholipids (about 75% of membrane lipids).

• One fatty acid is unsaturated, maintaining membrane fluidity.

• Polar heads face outward (toward extracellular fluid) and inward (toward cytosol), forming hydrogen bonds with water.

• Hydrophobic tails face inward, held together by hydrophobic interactions.

• Permeability:

  • Permeable to small, non-polar, uncharged molecules (e.g., CO2, O2, sterols, alcohol).

  • Impermeable to large, polar, or charged molecules (e.g., glucose, amino acids, proteins, ions).

Other Membrane Components

• Cholesterol (20% of lipids): Adds strength and stability, reducing fluidity at higher temperatures.

• Glycolipids (5% of lipids): Contribute to the glycocalyx, involved in cell recognition.

• Proteins (55% of membrane mass): Diverse functions including transport, communication, and structural support.

Membrane Proteins: Types and Functions

• Receptor proteins: Detect environmental changes and initiate cellular responses.

• Enzymes: Catalyze chemical reactions at the membrane surface.

• Recognition proteins: Identify the cell as "self" or "non-self" (important for immune function).

• Linkers and anchors: Attach the membrane to other cells or the extracellular matrix.

• Transport proteins:

  • Channels: Allow specific substances to pass through the membrane.

  • Carriers (transporters): Change shape to move substances across the membrane.

Movement of Substances Across the Membrane

Key Definitions

• Fluid: A substance that can flow and change shape in response to pressure without breaking apart.

• Concentration: The number of molecules in a given volume.

• Gradient: A difference in concentration, pressure, or charge between two regions.

Diffusion

• Net movement of substances from areas of higher to lower concentration (down their concentration gradient).

• Continues until dynamic equilibrium is reached.

• Factors affecting diffusion rate:

  • Steepness of concentration gradient

  • Temperature

  • Mass of diffusing substance

  • Surface area available

  • Diffusion distance

Passive vs. Active Transport

• Passive transport: Substances move down their concentration gradients without energy input (driven by Brownian motion/kinetic energy).

• Active transport: Substances move against their concentration gradients, requiring energy (usually ATP).

Types of Passive Transport

• Simple diffusion: Movement of small, non-polar molecules directly through the lipid bilayer.

• Facilitated diffusion: Movement of substances through channels or carrier proteins; no energy required, but transport proteins are needed.

• Osmosis: Diffusion of water across a semipermeable membrane.

Osmosis and Tonicity

• Isotonic solution: No net movement of water; cell volume remains stable.

• Hypotonic solution: Water enters the cell, causing swelling and possible lysis (bursting).

• Hypertonic solution: Water leaves the cell, causing shrinkage (crenation).

Osmotic Terms

• Iso-osmotic: Equal solute and water concentrations inside and outside the cell.

• Hyper-osmotic: Higher solute, lower water concentration (relative to another solution).

• Hypo-osmotic: Lower solute, higher water concentration (relative to another solution).

Active Transport

• Moves substances against their concentration gradients; requires cellular energy.

• Essential for nutrient absorption, nerve function, and muscle contraction.

Types of Active Transport

• Primary active transport: Direct use of ATP to change the shape of a transport protein and move substances.

• Secondary active transport: Uses the energy stored in a concentration gradient (established by primary active transport) to move other substances; ATP is used indirectly.

Bulk Transport

• Used to move large particles or volumes of fluid across the membrane.

• Pinocytosis: Cell takes in droplets of fluid.

• Phagocytosis: Cell engulfs large particles (e.g., bacteria); forms a phagosome that fuses with a lysosome for digestion.

• Receptor-mediated endocytosis: Specific molecules bind to receptors, triggering vesicle formation for selective uptake.

Key Equations

• Fick's Law of Diffusion:

• Where J is the rate of diffusion, D is the diffusion coefficient, and \frac{dC}{dx} is the concentration gradient.

Summary Table: Membrane Transport Mechanisms

Example Applications

• Osmosis in red blood cells: Placing cells in hypotonic solutions causes swelling and lysis; in hypertonic solutions, cells shrink (crenate).

• Na+/K+ pump: Maintains electrochemical gradients essential for nerve impulse transmission.

• Phagocytosis by white blood cells: Engulfment and digestion of pathogens as part of the immune response.

Additional info: The notes have been expanded to include definitions, examples, and summary tables for clarity and completeness, following a mini-textbook style suitable for college biology students.