Chapter 5

Types of molecules that make up biological membranes

Lipids and proteins.

Chemical attractions that hold biological membranes together

Association of lipid and protein molecules with each other.

Distinguish between integral and peripheral membrane proteins

Integral proteins penetrate the lipid bilayer while peripheral proteins are held to membrane surfaces by noncovalent bonds.

Many functions of membrane proteins

Transport, recognition, receptors, and cell adhesion.

Distinguish between simple diffusion, facilitated diffusion, and active transport

Simple diffusion (passive) and facilitated diffusion (passive) move molecules down a concentration gradient; active transport moves molecules against a concentration gradient using energy.

Why some types of molecules can diffuse passively through biological membranes and other molecules cannot

Hydrophobic (nonpolar) molecules move freely, while hydrophilic (polar) molecules and ions are impeded by the hydrophobic core.

Series of events in the functioning of carrier proteins

Carrier proteins bind a specific single solute, undergo conformational changes, and move the solute-binding site across the membrane.

Why water moves by osmosis from hypotonic to hypertonic solutions

Water moves from the region with less solutes (higher water concentration) to the region with more solutes (lower water concentration) because solute association reduces free water available to cross the membrane.

Contrast exocytosis and endocytosis with other mechanisms of membrane transport

Exocytosis and endocytosis move large molecules in bulk packaged in vesicles, requiring ATP, while other mechanisms like diffusion and active transport move specific ions or molecules.

Membranes

Aggregates, not polymers.

Plasma membrane

A thin layer of lipids and proteins that separates a cell from its surroundings.

Channels that serve for entry of nutrients and exit of wastes

Ion channels and aquaporins.

Transport proteins

Move particular ions and molecules, including water, in a directed way across the membrane.

Cystic fibrosis transmembrane conductance regulator (CFTR)

A transport protein that pumps chloride ions out of epithelial cells.

Cause of Cystic Fibrosis (Clinical Importance)

All CFTR molecules are mutant, chloride transport is defective, and not enough water leaves epithelial tissues, causing mucus to build up into a thick mass.

Parts of the Phospholipid Molecule

A polar (electrically charged) end containing a phosphate group, and a nonpolar (uncharged) end containing two nonpolar fatty-acid tails.

Phospholipids

Lipids having a polar (hydrophilic) end with a phosphate group, and a nonpolar (hydrophobic) end with two fatty-acid tails.

Amphipathic molecules

Molecules that have both hydrophilic and hydrophobic regions.

Phospholipid bilayer

Arrangement of phospholipids where polar ends face the aqueous environment and nonpolar fatty-acid chains assemble in the nonpolar interior.

The Davson-Danielli Model (Sandwich Model) structure/parts

A phospholipid bilayer sandwiched between two layers of globular proteins; the lipid bilayer is also penetrated by protein-lined pores.

The Trilaminar Organization of Plasma Membrane parts

Darkly staining inner layer, darkly staining outer layer, and lightly staining middle layer.

Fluid Mosaic Model

The currently accepted model proposing the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids.

Integral proteins

Membrane proteins that penetrate the lipid bilayer and pass entirely through it (transmembrane proteins).

Peripheral proteins

Membrane proteins held to membrane surfaces by noncovalent bonds to the polar head groups of the lipid bilayer and/or to an integral membrane protein.

The structure/parts of the Fluid Mosaic Model

A fluid phospholipid bilayer in which proteins (integral and peripheral) are embedded and float freely (a mosaic).

Parts of the Outer Leaflet of the Plasma Membrane

Phosphatidylcholine and Sphingomyelin.

Parts of the Inner Leaflet of the Plasma Membrane

Phosphatidylethanolamine, Phosphatidylinositol, and Phosphatidylserine.

Factors that affect the property of membrane permeability

Length of hydrocarbon tails, saturation state of hydrocarbon tails, and presence of cholesterol molecules.

Consequences of Unsaturated Hydrocarbon Tails

Double bonds cause "kinks" creating spaces; reduces Van der Waals interactions; weakens barrier to solutes; enhances fluidity.

Consequences of Saturated Hydrocarbon Tails

No double bonds, long, straight tails; fewer spaces; more Van der Waals interactions; makes the membrane denser and less permeable (increasing viscosity).

Sterol

Another lipid component of certain membranes; Cholesterol is the main sterol in animal membranes.

Parts of the Sterol (Cholesterol) molecule

Nonpolar carbon rings with a nonpolar side chain at one end and a single polar group (—OH) at the other end.

Consequences of Cholesterol at High Temperatures

Provides structural support for phospholipids preventing the membrane from becoming too fluid.

Consequences of Cholesterol at Cold Temperatures

Inserts between phospholipids, creating space that increases fluidity, hindering solidification.

Cholesterol's effect on membrane permeability

Reduces membrane permeability because bulky cholesterol rings force phospholipid tails closer, increasing their packing density.

Integral membrane protein organization

Hydrophilic parts extend into aqueous cell exterior/cytoplasm; hydrophobic side chains interact with the hydrophobic lipid core.

Lipid-anchored proteins

Located outside the lipid bilayer but covalently linked to a lipid molecule situated within the bilayer.

Types of Membrane Proteins (Functional Classification)

Transport, Recognition, Receptor, and Cell Adhesion proteins.

Transport proteins

Form channels that allow selected polar molecules and ions to pass across a membrane.

Recognition proteins

Identify a cell as part of the same individual or as foreign (often glycoproteins).

Receptor proteins

Recognize and bind molecules from other cells (chemical signals, hormones) triggering a cellular response.

Cell adhesion proteins

Bind cells together by recognizing and binding receptors or chemical groups on other cells.

Glycoproteins

Membrane proteins covalently linked to short chains of sugars.

Glycolipids

Membrane lipids covalently linked to carbohydrate groups.

Glycocalyx

Surface coat formed by carbohydrate groups of cell surface glycolipids and glycoproteins in many animal cells.

Glycosylation

The process of the addition of carbohydrate residues to a protein or a lipid.

Selective permeability

Biological membranes allow only some substances to cross more easily than others.

Molecules that freely diffuse through membranes

Hydrophobic (nonpolar) molecules, nonpolar inorganic gases (O2, N2, CO2), and small lipid soluble molecules.

Molecules that cannot freely diffuse through membranes

Hydrophilic molecules (ions and polar molecules) like amino acids and sugars.

Passive transport

Moves ions and molecules along a concentration gradient (high to low); requires no energy expenditure.

Active transport

Moves ions or molecules against the concentration gradient (low to high); uses energy directly or indirectly from ATP.

Diffusion

Net movement of ions or molecules from a region of higher concentration to a region of lower concentration (Passive transport).

Concentration gradient

The concentration difference that drives diffusion; a form of potential energy.

Osmosis

The diffusion of water across a selectively permeable membrane in response to concentration gradients.

Osmotic pressure

The pressure created by the weight of raised solution that balances the movement of water molecules in response to the concentration gradient.

Tonicity

A property of a solution with respect to a particular membrane.

Hypotonic solution

Solution surrounding a cell that contains nonpenetrating solutes at lower concentrations than in the cell (water enters, cell swells).

Hypertonic solution

Solution surrounding a cell that contains nonpenetrating solutes at higher concentrations than in the cell (water leaves, cell shrinks).

Isotonic solutions

Concentrations of solutes inside and outside the cell are balanced (equal); no net movement.

Effect of Tonicity on Plant Cells in Hypotonic Solution

Strong walls prevent bursting; osmotic pressure (turgor pressure) pushes cells tightly against their walls and supports soft tissues.

Effect of Tonicity on Plant Cells in Hypertonic Solution

Stems and leaves wilt; cells shrink and retract from their walls (plasmolysis).

Aquaporins

Specialized channel proteins through which water molecules pass in single file by diffusion (osmosis).

Facilitated diffusion

Diffusion of polar and charged molecules through transport proteins in the hydrophobic lipid bilayer, down their concentration gradients.

Channel proteins

Integral membrane proteins that form hydrophilic channels through which water and ions can pass.

Gated channels

Ion channels that switch between open, closed, or intermediate states in response to a stimulus.

Types of Ion Channels

Voltage-gated, Ligand-gated, Mechano-sensitive, and Non-gated/Leak channels.

Ligand-gated channels

Channels that open in response to binding of a specific ligand (neurotransmitter, drug, hormone, growth factor).

Mechano-sensitive ion channels

Channels that respond to changes in mechanical forces on the cell membrane, transducing external forces into intracellular signals.

Voltage-gated ion channels

Channels that respond to perturbations/changes in cell membrane potential and are highly selective for a specific ion.

Leak channels (Non-gated channels)

Ion channels that are always open and simply allow ions to pass through the channel without impedance.

Carrier proteins

Membrane proteins that bind a specific single solute and transport it across the lipid bilayer, undergoing conformational changes.

Active transport functions (Three main functions)

Uptake of essential nutrients, removal of waste materials, and maintenance of intracellular concentrations of H+, Na+, K+, and Ca2+.

Types of Membrane Proteins that carry out Active Transport

Uniporter, Symporter, and Antiporter.

Uniporter

Moves a single substance in one direction.

Symporter

Moves two substances in the same direction.

Antiporter

Moves two substances in opposite directions, one into the cell (or organelle) and the other out of the cell (or organelle).

Primary active transport

The protein hydrolyzes ATP directly to power the transport.

Secondary active transport

Transport indirectly driven by ATP hydrolysis; uses a favorable ion concentration gradient established by primary active transport as energy.

Primary Active Transport Pumps move these positively charged ions

H+, Ca2+, Na+, and K+.

The Sodium/Potassium Pump (Na$^+$/K$^+$-ATPase) process

Moves 3 Na$^+$ ions out of the cell and 2 K$^+$ ions into the cell in the same pumping cycle, powered by ATP hydrolysis.

Importance of Na$^+$/K$^+$ pump (Electrochemical Gradient)

Very important in generating neuronal impulses or action potentials; maintains the membrane potential.

Electrochemical gradient

Differences in concentration of ions and electrical charge on two sides of the membrane; a form of potential energy.

Membrane potential

Electrical charge difference (voltage) across the plasma membrane, contributed to by active transport of ions.

Proton pump

A membrane protein that moves protons (H$^+$) across a cell membrane, creating a proton gradient using ATP hydrolysis.

Functions of Proton (H$^+$ ion) Pumps

Maintaining pH balance, acidification of lysosomes and endosomes, acid secretion in gastric parietal cells, and maintaining membrane bioenergetics.

Purpose of Calcium Ion Pump

Moves Ca$^{2+}$ from the cytoplasm to the cell exterior, and from the cytosol into the vesicles of the ER to maintain low cytosolic Ca$^{2+}$ concentration.

Secondary Active Transport: Symport

Solute moves through the membrane channel in the same direction as the driving ion.

Secondary Active Transport: Antiport

Solute and driving ion move through the membrane channel in opposite directions.

Cotransport

A molecule moves against its concentration gradient coupled with an ion moving down its concentration gradient (uses ATP indirectly).

Bulk transport mechanism

Transport of large molecules (like proteins and polysaccharides) packaged in vesicles.

Exocytosis

Cell secretes molecules by the fusion of transport vesicles (budded from the Golgi) with the plasma membrane.

The Exocytosis process/steps

Transport vesicle (from Golgi) moves along microtubule, vesicle membrane and plasma membrane fuse, contents spill out, vesicle membrane becomes part of the plasma membrane.

Motor protein responsible for vesicle movement along microtubules

Kinesin.

Endocytosis

Substances are trapped in pit-like depressions that bulge inward from the plasma membrane and pinch off as an endocytic vesicle.

Endocytosis pathways

Bulk endocytosis and Receptor-mediated endocytosis.

Bulk endocytosis (Pinocytosis)

Takes in a drop of aqueous extracellular fluid; nonspecific absorption.

Receptor-mediated endocytosis (RME)

Target molecules bind to specific receptor proteins (integral membrane proteins) on the outer cell surface before being internalized.

Role of Clathrin in Receptor-Mediated Endocytosis

Network of proteins that coat and reinforce the cytoplasmic side of the coated pit.

Phagocytosis

Taking in large particles or whole cells; often a protective mechanism carried out by cells like macrophages and monocytes.

The plasma membrane regulates the movement of molecules in and out of the cell

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