Chapter 7-Lecture Notes Flashcards
_______________; is the boundary that separates the living cell from its surroundings; exhibits _____________________, allowing some substances to cross it more easily than others
The plasma membrane
selective permeability
________________ are the most abundant lipid in the plasma membrane
Phospholipids
Phospholipids are _________________ containing hydrophobic and hydrophilic regions
amphipathic molecules,
A phospholipid bilayer can exists as a stable boundary between
two aqueous compartments
_______________________ states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it; Proteins are not randomly distributed in the membrane
The fluid mosaic model
_________________: in the plasma membrane can move within the bilayer
Phospholipids
Most of the lipids, and some proteins, drift laterally
Rarely, a lipid ___________________________
may flip-flop transversely across
the membrane
As temperatures cool, membranes switch from a fluid state to a solid state
The temperature at which a membrane solidifies depends on the types of lipids
Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids
Membranes must be fluid to work properly; they are usually about as fluid as salad oil
The steroid cholesterol has different effects on membrane fluidity at different temperatures
At warm temperatures (such as 37°C), cholesterol restrains movement of phospholipids
At cool temperatures, it maintains fluidity by preventing tight packing
Evolution of Differences in Membrane Lipid Composition
Variations in lipid composition of cell membranes of many species appear to be adaptations to specific environmental conditions
Ability to change the lipid compositions in response to temperature changes has evolved in organisms that live where temperatures vary
Membrane Proteins and Their Functions
A membrane is a collage of different proteins, often grouped together, embedded in the fluid matrix of the lipid bilayer
Proteins determine most of the membrane’s specific functions
_____________: are bound to the surface of the membrane
_____________ penetrate the hydrophobic core
Peripheral proteins
Integral proteins
Integral proteins that span the membrane are called
transmembrane proteins
The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar
amino acids, often coiled into ______________
alpha helices
Six major functions of membrane proteins
Transport Enzymatic activity Signal transduction Cell-cell recognition Intercellular joining Attachment to the cytoskeleton and extracellular matrix (ECM)
HIV must bind to the immune cell surface protein CD4 and a “co-receptor” CCR5 in order to infect
a cell
HIV cannot enter the cells of resistant individuals that lack CCR5
Cells recognize each other by binding to molecules, often containing carbohydrates, ___________________________________
on the extracellular surface of the plasma membrane
Membrane carbohydrates may be covalently bonded to lipids (forming ________________) or more commonly to proteins (__________________)
glycolipids
forming glycoproteins
_______________ on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual
Carbohydrates
The asymmetrical distribution of proteins, lipids, and associated carbohydrates in the plasma membrane is
determined when the membrane is built by the ER and Golgi apparatus
A cell must exchange materials with its surroundings, a ________________ control by _______
process controlled by the
plasma membrane
Plasma membranes are ________________, regulating the cell’s molecular traffic
selectively permeable
Hydrophobic (nonpolar) molecules, such as hydrocarbons, can
dissolve in the lipid bilayer and pass through the membrane rapidly
Hydrophilic molecules including ions and polar molecules
do not cross the membrane easily
allow passage of hydrophilic substances across the membrane
transport proteins
Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel
Channel proteins called
——————————facilitate the passage of water
aquaporins
Other transport proteins, called carrier proteins, bind to molecules and change shape to shuttle them across the membrane
A transport protein is specific for the substance
_________________________ is the tendency for molecules to spread out evenly into the available space
Although each molecule moves randomly, diffusion of a population of molecules may be directional
At dynamic equilibrium, as many molecules cross the membrane in one direction as in the other
Diffusion
Substances diffuse down their ______________________, the region along which the density of a chemical substance increases or decreases
concentration gradient
No work must be done to move substances ________ the concentration gradient
down
The diffusion of a substance across a biological membrane is _______________ because no energy is expended by the cell to make it happen
passive transport
________________ is the diffusion of water across a selectively permeable membrane
Osmosis
______________________ solution: Solute concentration is greater than that inside the cell; cell loses water
Hypertonic
In a hypertonic environment, plant cells lose water
____________ solution: Solute concentration is less than that inside the cell; cell gains water
Hypotonic
In a hypertonic environment, plant cells take water
Hypertonic or hypotonic environments create
osmotic problems for organisms
—————————the control of solute concentrations and water balance, is a necessary adaptation for life in such environments
Osmoregulation,
The protist Paramecium, which is hypertonic to its pond water environment, has a
contractile vacuole that acts as a pump
A plant cell in a hypotonic solution swells until the wall opposes uptake; the cell is now _________________
turgid
If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell;
the cell becomes
flaccid
The membrane pulls away from the cell wall causing the plant to wilt, a usually lethal effect called
plasmolysis
In_________________ transport proteins speed the passive movement of molecules across the plasma membrane
facilitated diffusion,
the solute concentration is equal both in and outside the cell. No water leaves or exits
Isotonic
______________ facilitate the diffusion of ions
Ion channels
Some ion channels, called gated channels, _____________________
open or close in response to a stimulus
_____________________ moves substances against their concentration gradient; requires energy, usually in the form of ATP; is performed by specific proteins embedded in the membranes
Active transport
Some ion channels, called _________________, open or close in response to a stimulus
gated channels
Carrier proteins undergo a subtle change in shape that
translocates the solute-binding site across the membrane
—————— is still passive because the solute moves down its concentration gradient, and the transport requires no energy
Facilitated diffusion
Some transport proteins, however,
can move solutes against their concentration gradients
moves substances against their concentration gradients
Active transport
____________________: requires energy, usually in the form of ATP; is performed by specific proteins embedded in the membrane
Active transport
Active transport allows cells to maintain concentration gradients that differ from their surroundings
The _________________ is one type of active transport system
sodium-potassium pump
______________________ is the voltage difference across a membrane
Membrane potential
__________ is created by differences in the distribution of positive and negative ions across a membrane
Voltage
___________________ collectively called the electrochemical gradient, drive the diffusion of ions across a membrane
Two combined forces,
A chemical force (the ion’s concentration gradient)
An electrical force (the effect of the membrane potential on the ion’s movement)
__________________ is a transport protein that generates voltage across a membrane
The sodium-potassium pump is the major electrogenic pump of animal cells
An electrogenic pump
The main electrogenic pump of plants, fungi, and bacteria is a ____________ This Electrogenic pumps help store energy that can be used for cellular work
proton pump
__________________ occurs when active transport of a solute indirectly drives transport of other substances
Cotransport
Plants commonly use the
gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell
______________: across the
plasma membrane occurs by exocytosis and endocytosis
Small molecules and water enter or leave the cell through the lipid bilayer or via transport proteins
Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles
Bulk transport
Bulk transport requires energy
______________: transport vesicles migrate to the membrane, fuse with it, and release their contents outside the cell
Many secretory cells use exocytosis to export
their products
exocytosis,
____________: the cell takes in macromolecules by forming vesicles from the plasma membrane
Endocytosis is a reversal of exocytosis, involving different proteins
endocytosis,
There are three types of endocytosis
Phagocytosis (“cellular eating”)
Pinocytosis (“cellular drinking”)
Receptor-mediated endocytosis
___________________: a cell engulfs a particle in a vacuole
The vacuole fuses with a lysosome to digest the particle
phagocytosis
_______________: molecules dissolved in droplets are taken up when extracellular fluid is “gulped” into tiny vesicles
pinocytosis,
_________________: , binding of ligands to receptors triggers vesicle formation
receptor-mediated endocytosis
________________________ is any molecule that binds specifically to a receptor site of another molecule
A ligand
