BIO 100 - Topic 6: Biological Membranes

Cell Membrane Function

Acts as a selectively permeable barrier regulating the movement of substances in and out of the cell

Phospholipid Bilayer

Structure of cell membrane consisting of two layers of phospholipids with hydrophilic heads facing out and hydrophobic tails inward

Amphipathic Molecule

A molecule that has both hydrophilic (water-loving) and hydrophobic (water-fearing) regions, like a phospholipid

Fluid Mosaic Model

Describes the cell membrane as a dynamic, fluid structure with proteins embedded in or attached to a fluid phospholipid bilayer

Membrane Fluidity

Maintained by unsaturated fatty acid tails (prevent packing) and cholesterol (prevents solidification at low temps and too much fluidity at high temps)

Cholesterol in Membrane

Helps stabilize membrane fluidity in animal cells by preventing too much movement or rigidity of phospholipids

Integral Membrane Proteins

Proteins embedded in the membrane, often spanning the bilayer; involved in transport, signaling, and enzymatic activity

Peripheral Membrane Proteins

Proteins attached to the membrane surface; provide structural support or cell signaling roles

Membrane Carbohydrates

Short chains of sugars attached to proteins (glycoproteins) or lipids (glycolipids); function in cell-cell recognition and signaling

Selective Permeability

Property of membranes that allows some substances to cross more easily than others; small nonpolar molecules pass easily

Transport Proteins

Integral proteins that assist specific substances across the membrane; include channels and carrier proteins

Channel Proteins

Provide corridors that allow specific molecules or ions to cross the membrane via diffusion

Carrier Proteins

Bind to molecules and change shape to shuttle them across the membrane; specific to particular substances

Passive Transport

Movement of substances across the membrane without energy input; includes diffusion, osmosis, and facilitated diffusion

Diffusion

Movement of molecules from high to low concentration until equilibrium is reached; driven by kinetic energy

Facilitated Diffusion

Passive transport aided by transport proteins (channel or carrier); used for polar or charged molecules

Osmosis

Diffusion of water across a selectively permeable membrane from low solute concentration to high solute concentration

Isotonic Solution

Equal concentration of solutes inside and outside the cell; no net water movement; cell remains stable

Hypertonic Solution

Higher solute concentration outside the cell; water moves out; cell shrinks (animal: crenates, plant: plasmolyzes)

Hypotonic Solution

Lower solute concentration outside the cell; water moves in; animal cells may lyse (burst), plant cells become turgid

Turgid Plant Cell

Plant cell in a hypotonic solution; cell wall prevents bursting and pressure maintains plant structure

Plasmolysis

Plant cell membrane pulls away from the wall in a hypertonic solution due to water loss

Active Transport

Movement of substances against their concentration gradient using energy (ATP); performed by specific membrane proteins

Sodium-Potassium Pump

Active transport protein that moves 3 Na⁺ out and 2 K⁺ into the cell; helps maintain membrane potential

Membrane Potential

Voltage across a membrane caused by unequal distribution of ions; contributes to nerve signaling and transport

Electrochemical Gradient

Combination of electrical (charge) and chemical (concentration) forces that drive diffusion of ions

Proton Pump

Active transport protein that moves H⁺ ions out of the cell; builds electrochemical gradient for co-transport

Cotransport

Transport protein couples the downhill diffusion of one substance to the uphill transport of another (e.g., H⁺ with sucrose)

Exocytosis

Form of bulk transport where vesicles fuse with the plasma membrane to release contents outside the cell

Endocytosis

Form of bulk transport where cells take in substances by forming vesicles from the plasma membrane

Phagocytosis

Cell engulfs a large particle by wrapping pseudopodia around it and forming a food vacuole ("cell eating")

Pinocytosis

Cell takes in extracellular fluid and dissolved solutes in small vesicles ("cell drinking")

Receptor-Mediated Endocytosis

Highly specific endocytosis where receptors bind to target molecules, triggering vesicle formation

Cell Communication

Essential for coordinating cellular activities; includes local signaling (e.g., paracrine) and long-distance (e.g., hormones)

Ligand

A signaling molecule that binds to a receptor protein, triggering a cellular response

Signal Transduction Pathway

Series of steps by which a signal on a cell’s surface is converted into a specific cellular response

Reception

First stage of cell signaling; ligand binds to a receptor on the cell surface or inside the cell

Transduction

Signal is relayed and amplified through a series of molecular changes, often involving phosphorylation cascades

Response (Cell Signaling)

Final stage of signaling pathway; specific cellular activity is triggered such as gene expression or enzyme activation

Difference Between Plant and Animal Cell Membranes

Both have plasma membranes, but only animal cells use cholesterol to regulate fluidity and only plant cells have a cell wall for structural support