Chapter 7: Membrane Structure and Function

Active Transport

Pumping solutes against their concentration gradient using energy from ATP hydrolysis

Amphipathic Molecule

Molecule having both a hydrophilic and a hydrophobic region

Amphipathic Proteins Location

Embedded in the lipid bilayer, with hydrophilic regions exposed and hydrophobic regions within the membrane

Aquaporin

Transport proteins that facilitate the passage of water molecules through the membrane

Are proteins randomly distributed?

Proteins are usually found in specialized patches (lipid rafts) for common functions, although this is debated

Bacteriorhodopsin

Pigmented protein found in abundance in the plasma membrane of the salt-loving archaeon Halobacterium halobium; pumps protons out of the cell in response to light

Bulk Transport

Movement of large molecules via vesicles, bypassing diffusion and transport proteins, usually happens through exo- and endo- cytosis

Can one protein carry out multiple functions?

Yes, one protein may itself carry out multiple functions.

Carrier Protein

Transport proteins that bind passengers and change shape to shuttle them across, move solutes against their concentration gradient

Cell Membranes in Varying Temperatures

Ability to change lipid composition of membranes in response to changing temperatures

Cell-Cell Recognition

Ability of a cell to distinguish one type of neighboring cell from another, crucial for sorting cells into tissues, organs, and organ systems

Cellular Membrane

Fluid mosaic of lipids and proteins; structure results in selectively permeabilityChannel Protein

Channel Protein

Protein providing hydrophilic corridors for molecules or ions to cross membrane

Chemical Force

The force driving ion diffusion based on its concentration gradientCh

Cholesterol

Steroid lipid that regulates membrane fluidity at different temperatures

Cholesterol at Different Temperatures

At moderate temps, it reduces fluidity; at low temps, it hinders solidification.

Cholesterol in Membranes at Low Temperatures

It hinders solidification by disrupting regular packing of phospholipids

Cholesterol in Membranes at Moderate Temperatures

Reduces membrane fluidity by restraining phospholipid movement

Coated Pit

Vesicle-forming regions of plasma membrane lined on cytoplasmic side by fuzzy coat protein

Concentration Gradient

Region along which the density of a substance increases or decreases

Contractile Vacuole

Organelle that pumps water out of a cell to prevent bursting

Cotransport

Coupling of one solute's downhill diffusion to uphill transport of another solute

Diffusion

Movement of particles to spread out in available space via thermal energy

Does facilitated diffusion alter the direction of transport?

No, it does not alter the direction of transport; substances still move down their concentration gradient

Electrical force

The effect of membrane potential on the movement of an ion

Electrochemical gradient

The combined force of chemical and electrical gradients acting on an ion

Electrogenic pump

Transport protein that generates a voltage across a membrane

Endocytosis

Cell takes in molecules and particulate matter by forming new vesicles from plasma membrane, Pocket forms in plasma membrane, deepens, and pinches in to form a vesicle

Examples of Amphipathic Molecules

Phospholipids, most membrane proteins, and membrane lipids.

Exocytosis

Secretion of molecules by fusing internal vesicles with the plasma membrane

Extremophiles Cell Membranes

Cell membranes of extremophiles contain unusual lipids to prevent excessive fluidity at high temperatures.

Facilitated Diffusion

Passive transport of polar molecules or ions via transport proteins

Flaccid

Limp state of plant cells in isotonic solutions

Fluid Mosaic

Membrane described as proteins bobbing in a fluid bilayer of phospholipids

Gated Channel

Channel protein that opens or closes in response to electrical or chemical stimuli

Glycolipid

Carbohydrate covalently bonded to a lipid

Glycoprotein

Carbohydrate covalently bonded to a protein

How are proteins attached?

On cytoplasmic side, some are held by attachment to cytoskeleton On extracellular side, some attach to materials outside cell, but to ECM in animal cells

How do carrier proteins function in facilitated diffusion

They undergo subtle shape changes to translocate the solute-binding site down its concentration gradient.

How do cells recognize other cells?

Cells recognize others by binding to molecules, often carbohydrates, on their extracellular membrane surface.

How do hydrophilic substances avoid contact with the lipid bilayer when crossing the membrane?

They pass through transport proteins that span the membrane.

How do large molecules generally enter and leave the cell?

They enter and leave in bulk, packaged in vesicles through endocytosis or exocytosis.

How do LDLs enter cells?

They bind to LDL receptors on plasma membranes and enter via endocytosis.

How do membrane carbohydrates vary?

They vary by species, individual, and cell type, function as markers that distinguish one cell from another

How do polar molecules like glucose and water cross the lipid bilayer?

They cross slowly through the bilayer or rapidly via transport proteins, do not cross rapidly without assistance

How do transport vesicles travel to the plasma membrane during exocytosis?

They travel along microtubules of the cytoskeleton.

How do vesicle and plasma membranes fuse during exocytosis?

Specific proteins in both membranes rearrange lipids, causing fusion and release of contents.

How does cell-cell recognition function in the immune system?

It recognizes foreign cells, triggering an immune response to reject them.

How does free water concentration relate to solute concentration?

Higher solute concentration has a lower free water concentration because water clusters around solutes.

How does membrane potential affect the traffic of charged substances?

It favors passive transport of cations into the cell and anions out of the cell.

How does Paramecium caudatum survive in a hypotonic pond environment?

It has a contractile vacuole to pump excess water out of the cell.

How fast do adjacent phospholipids switch?

About 10^7 times per second meaning they can travel 2 micrometers in 1 sec

How is a particle engulfed by phagocytosis digested?

The particle is packaged in a food vacuole, which then fuses with a lysosome for digestion.

How is energy transferred from ATP to a transport protein in active transport?

A phosphate group is transferred from ATP to the transport protein, causing it to change shape.

How many water molecules can pass through an aquaporin per second?

Up to 3 billion water molecules per second.

How mobile are proteins?

Most move, some held immobile by attachment to cytoskeleton or ECM while some move in highly directed manner along cytoskeletal fibers by motor proteins

How much faster does the glucose carrier protein transport glucose than simple diffusion?

50,000 times faster and it is selective enough to reject fructose

How must membranes be built to work properly?

Membranes must be fluid to work properly, affecting permeability and ability of membrane proteins to move to where function is needed

Hydrophilic Channels in Integral Proteins

Hydrophilic channels in integral proteins allow passage of hydrophilic substances across the membrane.

Hypertonic

Solution with higher solute concentration outside the cell; causes cell to shrivel

Hypotonic

Solution with lower solute concentration outside the cell; causes cell to swell

Integral Protein

Protein that penetrates the hydrophobic interior of the lipid bilayer, some only extend partway

Ion Channel

Channel protein specifically designed to transport ions

Is pinocytosis specific or nonspecific for the substances it transports?

Pinocytosis is nonspecific for the substances it transports, taking in dissolved solutes and extracellular fluid.

Isotonic

Two solutions having the same osmotic pressure across a semipermeable membrane; no net water movement

Low-Density Lipoproteins (LDLs)

Particles carrying cholesterol in blood; bind to LDL receptors for endocytosis, Complexes of lipids and protein that transport cholesterol in the blood.

Main Function of Membrane

Determined by its proteins

Membrane potential

Voltage across a cell membrane, typically ranging from -50 to -200 mV

Osmoregulation

Control of solute concentrations and water balance

Osmosis

Diffusion of water molecules across a semi-permeable membrane

Passive Transport

Movement of substances down their concentration gradient without using cellular energy

Peripheral Protein

Protein not embedded in the lipid bilayer; loosely bound to the surface, often to exposed parts of integral proteins

Phagocytosis

Engulfing a particle by extending pseudopodia to form a food vacuole

Phospholipid Bilayer

stable boundary between two aqueous compartments as molecular arrangement shelters hydrophobic tails from water while exposing hydrophilic heads to water

Pinocytosis

Nonspecific "gulping" of extracellular fluid and dissolved solutes into tiny vesicles

Plasmolysis

Process where a plant cell in a hypertonic solution loses water, causing the membrane to pull away from the cell wall.

Proton pump

Main electrogenic pump for plants, fungi, and bacteria; translocates hydrogen ions

Receptor-mediated endocytosis

Selective uptake of specific substances via receptor proteins in coated pits, Enables cell to acquire bulk quantities of specific substances using extracellular receptor proteins.

Saturated Hydrocarbon Tails in Membrane

Saturated tails pack together, increasing membrane viscosity and reducing fluidity.

Selective Permeability

Property allowing some substances to cross the membrane more easily than others, Ability to regulate transport across cellular boundaries in essential to the cell's existence

Sodium-potassium pump

Electrogenic pump in animal cells that exchanges sodium for potassium ions, main electrogenic pump for animal cells

Structure of Aquaporin

Four identical polypeptide subunits form a channel for water molecules to pass through individually.

Structure of Membrane Carbohydrates

Carbohydrates are usually short, branched chains of fewer than 15 sugar units covalently bonded to lipids (glycolipids) or proteins (glycoproteins).

Till what point to membranes remain fluid?

Membranes remain fluid until the temperature decreases enough for phospholipids to pack closely and solidify.

Tonicity

Ability of a surrounding solution to cause a cell to gain or lose water

Trans-Membrane Protein

Integral protein that spans the entire membrane

Transport Protein

Protein that spans the membrane to allow hydrophilic substances to cross

Turgid

Rigid state of plant cells in hypotonic solutions due to wall pressure

Unsaturated Hydrocarbon (Kinked) Tails in Membranes

Prevent packing enhancing membrane fluidity

Voltage

Electrical potential energy created by the separation of opposite charges

What are membranes held together by?

Held together by many weak hydrophobic interactions, not covalent bonds.

What are some common ions that move across the cell membrane?

Na+, K+, Ca2+, and Cl- are common ions that move across the cell membrane.

What causes diffusion?

Molecules have thermal energy and are in constant motion.

What coreceptor must HIV bind to in addition to CD4 to infect immune cells?

HIV must bind to CCR5 as a coreceptor in addition to CD4 to infect immune cells.

What determines the asymmetrical arrangement of membrane components?

The way the membrane is built determines the asymmetrical arrangement of proteins, lipids, and carbohydrates.

What determines the temperature at what the membrane solidifies?

The types of lipids it's made of; unsaturated tails lower solidification temperature because of kinks where double bonds located

What do hydrophobic regions in integral proteins consist of?

Hydrophobic regions consist of nonpolar amino acids, typically 20-30 amino acids in length, usually coiled into alpha helices

What does a concentration gradient represent?

A region along which the density of a substance increases or decreases, representing potential energy.

What does the sodium-potassium pump enable animal cells to maintain?

Internal concentrations of small solutes differing from their environment.

What does tonicity depend on?

Tonicity depends on the concentration of nonpenetrating solutes that cannot cross the membrane relative to inside the cell.