Cell Membrane Part 2

Small uncharged molecules

Molecules like O₂, CO₂, and ethanol that can freely diffuse across the lipid bilayer.

Oily/lipid-soluble molecules

Molecules that dissolve in the membrane’s hydrophobic core and diffuse without proteins.

Ions

Atoms or molecules with a positive or negative charge that cannot cross the lipid bilayer without help.

Polar molecules (>100 Da)

Electrically imbalanced molecules too large to cross membranes unaided.

Monosaccharides

Simple sugar molecules that cannot diffuse freely across membranes.

Amino acids

Protein building blocks that require facilitated diffusion or transport proteins to enter cells.

Nucleosides

Molecules that are precursors to nucleotides; they cannot freely cross the lipid bilayer.

Polysaccharides

Large carbohydrate polymers that cannot pass through membranes unaided.

Proteins

Large, complex molecules that require specific transport or endocytosis to enter cells.

Nucleic acids

very large, negatively charged polymers like DNA/RNA that cannot cross membranes unaided.

Water

A small polar molecule that can pass through membranes slowly or via aquaporins.

Urea

A small polar molecule that can diffuse through membranes due to its small size.

Phospholipid

An amphipathic lipid with a hydrophilic head and hydrophobic tails; major membrane component.

Hydrophilic

Water-loving; attracted to polar environments like extracellular or intracellular fluids.

Hydrophobic

Water-fearing; tends to avoid water and interact with nonpolar environments.

Amphipathic

A molecule with both hydrophilic and hydrophobic regions (e.g.

Bilayer formation

Self-assembly of phospholipids in water with tails inward and heads facing water.

Thermodynamically stable

The lowest energy arrangement; why bilayers form spontaneously.

Flip-flop (phospholipids)

Rare movement of phospholipids from one leaflet of the membrane to the other.

Leaflets (membrane)

The two layers of phospholipids that make up the bilayer.

Membrane protein

A protein embedded in or associated with the cell membrane.

Integral membrane protein

A protein embedded within the membrane that cannot be removed without disruption.

Transmembrane protein

A type of integral protein that spans the entire bilayer.

Hydrophobic region (protein)

Part of a transmembrane protein that interacts with the lipid bilayer interior.

Hydrophilic region (protein)

Part of a membrane protein exposed to aqueous environments inside or outside the cell.

Cell adhesion

Membrane protein function that connects neighboring cells.

Desmosome

A type of cell junction that mechanically binds adjacent cells using cadherins and intermediate filaments.

Tight junction

A seal between adjacent cells that prevents passage of water and solutes between them.

Gap junction

A channel formed between adjacent cells allowing small molecule and ion exchange.

Transport protein

A membrane protein that helps move substances across the membrane.

Binding site protein

A protein with a region where external substances like hormones attach.

Membrane enzyme

A protein that catalyzes reactions at the membrane surface.

Recognition protein

A protein used for cell identification or immune recognition.

Membrane channel

A pore-like membrane protein allowing diffusion of solutes or water.

Transporter (carrier)

A protein that binds and moves molecules across the membrane

Enzyme

Catalyzes a chemical reaction in which covalent bonds are made or broken.

Structural protein

Attaches to other molecules to anchor internal structures or form junctions.

Glycoprotein

A membrane protein with carbohydrate chains attached.

Glycolipid

A membrane lipid with a carbohydrate group attached.

Proteoglycan

A molecule with long carbohydrate chains and a small protein core.

Carbohydrate role in membranes

Helps in cell recognition, adhesion, protection, and maintaining membrane asymmetry.

Membrane leaflet asymmetry

The outer and inner layers of the membrane have different components, including carbohydrates.

Hydrophilic carbohydrate groups

Water-attracting sugar chains that face the extracellular fluid.

Simple diffusion

Passive movement of particles from high to low concentration until evenly distributed.

Passive transport

Movement across membranes without using energy (ATP).

Concentration gradient

Difference in the amount of a substance across a membrane; ions move from high to low concentration.

Kinetic energy (diffusion)

The energy causing molecules to move, collide, and spread out.

Lipid-soluble molecule

A molecule that can dissolve in lipids and pass freely through the membrane.

Membrane-soluble substances

Molecules like oxygen, CO₂, and alcohol that can diffuse through the lipid bilayer.

Barbiturates

Lipid-soluble drugs that can diffuse through membranes; used as anesthetics.

Non-membrane-soluble

Molecules that cannot pass through the bilayer and need help crossing the membrane.

Channel-mediated transport

Movement of lipid-insoluble molecules across membranes via protein channels.

Permeable

The ability of a membrane to allow a specific molecule to pass through.

Voltage-gated channel

A membrane channel that opens or closes in response to changes in electrical potential.

Electrical potential (membrane)

Difference in charge across the membrane that can trigger gated channels to open.

Conformational change

A structural change in a protein (like a channel) that affects its function (open/close).

Lipid-insoluble substance

A molecule that cannot pass through the lipid bilayer without help (e.g., ions, glucose).

Ion

An atom or molecule with a positive or negative charge.

Ion channel

A membrane protein that forms a pore specific for certain ions to pass through the membrane.

Electrochemical gradient

The combined effect of the chemical gradient (concentration) and electrical gradient (charge) that determines ion movement.

Chemical gradient

Difference in solute concentration across a membrane.

Electrical gradient

Difference in electrical charge across a membrane; ions move toward areas with opposite charge.

Synergistic effect (gradient)

When both gradients move the ion in the same direction, increasing movement.

Antagonistic effect (gradient)

When gradients oppose each other, potentially reducing or reversing ion movement.

Calcium channel blocker

A drug that regulates calcium entry into cells, used in treating cardiac arrhythmias.

Facilitated diffusion

Passive transport of molecules across a membrane with the help of a carrier protein.

Carrier protein

A transmembrane protein that binds to specific molecules and changes shape to transport them.

Glucose facilitated diffusion

ransport of glucose into cells using a carrier protein, without ATP.

Insulin

A hormone that increases glucose uptake by adding more carrier proteins to the cell membrane.