Chapter 7: Membrane Structure and Function

Merged flashcards from Chapter 7 of Pearson's Campbell Biology, Twelfth Edition.

Passive transport

The transport of small molecules from areas of high to low concentration that does not require energy

• May require transport proteins in a cell membrane

Active transport

The transport of small molecules from areas of low to high concentration that does require energy and a transport protein

Bulk transport

The transport of large molecules as a form of active transport that utilizes exo- or endocytosis

Membrane

Cellular substance comprised of phospholipids and proteins with some carbohydrates

• Hydrophobilc phospholipid tails remain on the inside of the bilayer

• Hydrophilic heads remain on the outside of the bilayer toward the cytosol and extracellular fluid

Amphiphile

A molecule that contains hydrophobic and hydrophilic regions, such as phospholipids

Fluid mosaic model

A model of the membrane as a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids

• Proteins typically form groups that carry out common functions

• Most lipids and some proteins can move sideways within the membrane

Membrane solidification

Occurs as temperatures cool around a membrane depending on the types of lipids present

• More unsaturated fatty acids have higher fluidity than saturated fatty acids

• Lowers membrane function through higher packing of molecules — but too fluid membranes do not support protein function

• Addressed by some animal behaviors such as hibernation or migration

Cholesterol

Membrane component in animal cells that can affect membrane fluidity

• Warmer temperatures lead to restriained movement

• Cooler temperatures prevent tight packing

Lipid composition

Depends on the environmental conditions of an organism

• Extremely cold environments are more conducive to higher unsaturated fats in the membrane

• Seasonal changes can occur such as in winter wheat’s increased levels to prevent solidification

Membrane proteins

Proteins held in place via the cytoskeleton or materials outside the cell that function in:

• Transport

• Enzymatic activity

• Signal transduction

• Cell-cell recognition

• Intercellular joining

• Attachment to the cytoskeleton and extracellular matrix

HIV

A disease that enters immune cells by binding to a cell-surface protein and co-receptor

• Individuals without the receptor are immune, so drugs are in development to mask the receptor and block entrance in nonimmune individuals

Cell identification

Performed through the binding of molecules on the surface of the membrane; these molecules include glycolipids and glycoproteins for identification markers

Selective permeability

Plasma membrane property that allows substances to cross more easily than others as part of material control

• Dependent on the lipid bilyer and specific transport proteins contained

Hydrophobic molecules

Nonpolar molecules that dissolve in the lipid bilayer and pass through the membrane easily; includes hydrocarbons and gasses like CO₂ and O₂

Hydrophilic molecules

Polar molecules where passage through the membrane is impeded; includes sugars, water, and ions

• Requires transport proteins that enable faster transport

Transport proteins

Proteins that function in the transport of hydrophilic molecules across the cell membrane, includes:

• Channel proteins (tunneling proteins)

• Carrier proteins (polymorphic proteins)

These proteins are molecule-specific, affecting overall selective permeability alongside the membrane

Channel proteins

Proteins that have a hydrophilic channel that certain molecules or ions can use a tunnel

Carrier proteins

Proteins that bind to molecules and change shape to shuttle them across the membrane

Aquaporins

Channel proteins that greatly increase the rate of passage of water molecules

• Over 3 billion pass through per second

Diffusion

The movement of particles of any substance so that they spread out evenly into the available space

• Generally directional until equilibrium is reached

Dynamic equilibrium

State of diffusion where as many molecules cross the membrane in one direction as in the other

Concentration gradient

The region along which the density of a chemical substance increases or decreases

• Substances diffuse down these, unaffected by the concentrations of other substances

Passive transport

Transport of substances that requires no cellular energy expidenture

• Potential energy represented by concentration gradient

• Rate of diffusion depends on membrane permeability

Osmosis

The diffusion of free water across a selectively permeable membrane

• Water molecules diffuse to regions of lower to higher solute concentration until equilibrium is reached

Tonicity

The ability of a surrounding solution to cause a cell to gain or lose water

• Depends on the concentration of solutes that cannot cross the membrane relative to the inside of the cell

• Water will be attracted to higher concentrations of solute, which can lead to water going in or out of each cell

Extreme hypo- or hypertonicity can cause problems for cells without walls

Isotonic

Solution where solute concnetration in a solution is the same as that inside the cell

• No net water movement is done across the membrane

• Volume remains stable in this solution

Hypertonic

Solution where the solute concentration is greater than that inside the cell

• Net water movement goes outside of the cell to the solution

• Cells may shrivel and die as they lose water

Hypotonic

Solution where the solute concentration is less than that inside the cell

• Net water movement goes to the inside of the cell from the solution

• Cells may swell and lyse (burst) as they gain water

Turgid

Term for a normal plant cell in a hypotonic environment that contains more water

• These cells swell until the inelastic wall exert a pressure on the cell, signaling a firm and healthy state

Flaccid

Term for a plant cell in an isotonic environment with less water

• No net movement of water occurs, and plant cells will become limp causing wilting

Plasmolyzed

Term for a plant cell in a hypertonic environment with low levels of water

• Water moves out of the plant cell, causing the membrane to pull away from the cell wall causing potential death

Lysed

Term for an animal cell in a hypotonic environment with too much water

Shriveled

Term for an animal cell in a hypertonic environment with too little water

Osmoregulation

The control of solute concentration and water balance

• Paramecium have a contractile vacuole to pump excess water out of the cell due to their hypotonic environment

• Bacteria and archea in hypersaline environments have mechanisms to retain water

Facilitated diffusion

Diffusion where transport proteins speed up the passive movement of molecules across the plasma membrane

• Does not require energy

• Includes transport (channel, carrier) proteins

Channel proteins

Transport proteins that provide a corridor for a specific molecule or ion to cross the membrane

Aquaporin

Channel protein that facilitates the diffusion of water

Ion channel

Channel protein that facilitates the transport of ions

Gated channels

Ion channels that open or close in response to a stimulus

• Potassium ion channels open in response to electrical stimulus in nerve cells

• Others open in response to chemical stimulus, or the binding of a specific substance to the protein

Carrier proteins

Proteins that undergo a substle shape change that moves the solute-binding site across the membrane

• Can be triggered by the binding and release of the transported molecule

• Substances are moved down the concentration gradient with no energy input required

Facilitated diffusion

A passive method of transport that requires no energy due to the molecules only moving down their concentration gradient

Active transport

Transport that requires energy — usually in the form of ATP hydrolysis — to move substances agains their concentration gradient

• Requires carrier proteins specifically

• Allows cells to maintain solute concentrations that differ from their environment

Membrane potential

The voltage across a membrane created by differences in the distribution of positive and negative ions across a membrane

Electrochemical gradient

Two forces that drive the diffusion of ions across the membrane that ions diffuse down:

• Chemical force (the ion’s concentration gradient)

• Electrical force (the effect of the membrane potential on the ion’s movement)

Electrogenic pump

A transport protein that generates voltage across a membrane, storing energy that can be used for cellular work

• Animals have the sodium-potassium pump

• Plants, fungi, and bacteria have the proton pump that transport H⁺ ions out of the cell

Cotransport

Transport that occurs when the active transport of a solute indirectly drives transport of other substances

• Downhill solute movement may couple to uphill transport of another substance against its own concentration gradient

Proton pump

Pump present in plants that generate an H⁺ gradient across the cell membrane

• An H⁺ cotransporter allows sucrose to be coupled to its active transport into the cell, enabling its transport around a plant body

Sodium-potassium pump

Pump present in animals that transport Na⁺ out of the cell to maintain the electrochemical gradient while using ATP in the process

• Sodium in waste is reabsorbed to maintain a constant level in the body

• Drinking concentrated salt and glucose solutions can enable recovery from a rapid, life-threatening loss of sodium too fast for reabsorption

Vesicles

Separated sections of membrane for large molecules like polysaccharides and proteins to cross the lipid bilayer

• Smaller molecules and water are transported through proteins or the layer itself instead

Exocytosis

Process where transport vesicles migrate to the membrane, fuse with it, and release their contents outside the cell

• Secretory cells like the pancreas can export products like insulin via this

Endocytosis

Process where macromolecules are taken into the cell in vescles through a pocket formed and deepened by the membrane that is eventually pinched off; includes

• Phagocytosis (cellular eating)

• Pinocytosis (cellular drinking)

• Receptor-mediated endocytosis

Phagocytosis

Form of endocytosis where a cell engulfs a particle by extending pseudopodia around it and packing it in a membranous sac called a food vacuole

• This food vacuole then fuses with a lysosome for digestion

Pinocytosis

Form of endocytosis where a cell takes up extracellular fluid via small vesicles; this is nonspecific for substance transport and any solutes are taken into the cell

• The inner side of vesicles formed from the plasma membrane are coated with coat proteins

Receptor-mediated endocytosis

Form of endocytosis where vesicle formation is triggered by a solute binding to a receptor

• Receptors bound to specific solutes from the extracellular fluid are clustered in coated pits that form coated vesicles

• Emptied receptors can be recycled into the plasma membrane by the same vesicle

• Used to take in cholesterol within particles called low-density lipoproteins (LDLs) — lack of receptors can lead to hypercholesterolemia, building up in blood vessels