Unit 2 Cell Membranes and Homeostasis Flashcards
selective permeability
plasma membranes allow some substances to cross more easily than others
phospholipids
the most abundant lipids in most membranes
-amphipathic; have both a hydrophilic and hydrophobic region
fluid mosaic model
membrane is a fluid structure with a “mosaic” of various proteins embedded or attached to a double (bilayer) of phospholipids
The Fluidity Membranes
- membrane is held together mainly by weak hydrophobic interactions
- most lipids and some proteins can shift laterally (this occurs rapidly)
- some proteins seem to be moved and directed by cytoskeleton while others are held in place
- membranes must be within a certain range of fluidity to function properly
The Fluidity of Membranes and Temperature
membranes solidify at low temperatures
- membranes with bent, unsaturated tails will remain fluid at lower temperatures because they cannot be backed closely together
- cholesterol in animal cells resists changes in plasma membrane fluidity caused by change in temperature
integral proteins
penetrate the hydrophobic interior of the lipid bilayer
- hydrophobic part contains stretches of nonpolar amino acids
- hydrophilic parts are exposed to aqueous solutions
peripheral proteins
appendages loosely bound to the surface of a membrane
Functions of Proteins
- Transport
- Enzymatic Activity
- Signal transduction
- Cell-cell recognition
- Intercellular joining
- attachment of cytoskeleton and ECM
Proteins and transport
may provide hydrophilic channel across membrane or shuttle a substance across by changing shape
proteins and enzymatic activity
may be enzyme with active sight exposed to substances in adjacent solution
proteins and signal transduction
may have binding sight that fits the shape of a chemical messenger, protein changes shape and relays message
proteins and cell-cell recognition
some proteins serve as ID tags that are recognized by membrane proteins of other cells
proteins and intercellular joining
membrane proteins of adjacent cells hook together
proteins and attachment of cytoskeleton and ECM
elements of both bound to membrane proteins
The Role of Membrane Carbohydrates in cell-cell recognition
- cells recognize other cells by minding to molecules, often containing carbohydrates, on the extracellular surface of the plasma membrane
- diversity of molecules allow them to serve as markers
glycolipids
carbohydrates bonded to lipids
glycoproteins
carbohydrates bonded to proteins
The permeability of the lipid bilayer
- nonpolar, hydrophobic molecules can dissolve in the lipid bilayer and easily cross the membrane
- polar molecules pass through very slowly
transport proteins
allow hydrophobic substances to pass through
-types: channel proteins and carrier proteins
channel proteins
function by having a hydrophilic channel that certain molecules or ions use as a tunnel through the membrane
aquaporins
allows water to pass through plasma membrane rapidly
carrier proteins
hold on to passengers and shuttle them across the membrane by changing shape
-specific for the substance it transports
passive transport
diffusion of a substance across a membrane with no energy investment
diffusion
the movement of molecules of any substance so that they spread out evenly into the available space
-a substance will naturally diffuse from where it is more concentrated to where it is less concentrated
dynamic equilibrium
reached when molecules cross a membrane at equal rates in both directions
concentration gradient
the region along which the density of a chemical substance increases or decreases
osmosis
diffusion of free water molecules across a selectively permeable membrane
-water diffuses from region of lower solute concentration (more water molecules) to the region of higher solute concentration until both concentrations are equal
tonicity
the ability of a surrounding solution to cause a cell to gain or lose water
-depends in part on concentrations of solutes that cannot cross the membrane
isotonic
an environment in which water will diffuse across a cell membranes at the same rate in both directions. Volume of animal cell remains stable
hypertonic
more solute outside of cell; cell will lose water and shrivel
hypotonic
loss solute outside, cell will swell and burst
osmoregulation
the control of solute concentrations and water balance
-some organisms have less water permeable membranes or have vacuoles that pump out water
turgid
health, firm state of a plant cell that occurs when it is in a hypotonic solution
flaccid
limp plant cells in isotonic solutions
plasmolysis
in a hypertonic environment, a plant cell lose water, shrivels, and its plasma membrane pulls away from the wall
facilitated diffusion
many polar molecules and ions diffuse passively with the help of transport proteins that span the membrane
- channel proteins allow quick diffusion of some hydrophilic substances
- carrier proteins also carry substances down concentration gradient
ion channels
channel proteins that transport ions
gated channels
open or close in response to a stimulus (electric stimulus or binding of substances)
active transport
uses energy to move solutes against their gradients
- uses carrier proteins
- enables a cell to maintain internal concentrations of small solutes that differ from concentrations in the environment
- ATP can transfer its terminal phosphate group directly to the protein, inducing the protein to change shape
sodium-potassium pump
exchanges sodium ions for potassium ions across the plasma membrane (using active transport) of animal cells
membrane potential
the voltage across a membrane
- typically -50 to -200V
- the inside of the cell is negative relative to the outside
- influences traffic, favors passive transport of cations (+) into the cell and anions out of the cell
- some proteins protons, such as NA-K pump, contribute to membrane potential
electrochemical gradient
the combination of the chemical gradient and electrical force acting on an ion
-ion diffuses down its electrochemical gradient
electrogenic pump
a transport protein that generates voltage across a membrane
-help store energy that can be tapped for cellular work
proton pump
an electrogenic pump that actively transports protons (H+) out of a plant cell
contransport
a mechanism that occurs when an ATP powered pump that transports a specific solute can indirectly drive the active transport of several other solutes
-couples the diffusion of a previously pumped substance with the transport of a second substance against its gradient.
exocytosis
transport vesicle from the Golgi moves to the plasma membrane. Proteins rearrange lipids so that the vesicle membrane and plasma membranes fuse. Contents of vesicle are spilled outside the cell
-used by secretory cells to export products
endocytosis
the cell takes in biological molecules and matter by forming new vesicles from the plasma membrane
phagocytosis
cell wraps psuedopodia around a particle and packaging it in a food vacuole
pinocytosis
cell gulps droplets of extracellular fluid into tiny vesicles. Nonspecific in the substance it transports
receptor-mediated endocytosis
specific substances bind to receptor proteins on the membrane. Proteins cluster in coated pits (which are lined by coat proteins) and form a vesicles. Vesicles is emptied and receptors are recycled to plasma membrane by the same vesicle
ligands
any molecule that binds specifically to a receptor site on another molecule
apoplast
in plants, consists of everything external to the plasma membrane of living cells
-includes cell walls, extracellular space, and dead cells
symplast
consists of the entire mass of cytosol of all living cells in a plant as well as the plasmodes (the cytoplasmic channels that interconnect cells)
three routes of transport within a plant tissue or organ
- Apoplastic route
- Symplastic route
- transmembrane route
apoplastic route
water and solutes move along the continuum of cell walls and extracellular space
symplastic route
water and solutes move among continuum of cytosol,
-substance must pass a plasma membrane once then can move from cell to cell with the plasmodesmata
transmembrane route
movement out of one cell, across cell wall, and into neighboring cell
H+ rather than NA+ plays the primary role in basic transport processes in plant cells
- h+ established membrane potential
- H+ most often contransported in plants, helps with absorption of neutral solutes (like sucrose) and movement of ions
water potential
predicts the direction in which water will flow. Includes the effects of solute concentration and physical pressure
- free water moves from higher to lower water potential
- potential refers to waters capacity to perform work
- equal 0 when pure water in an open containers is at standard conditions
solute potential
directly proportional to molarity
-expressed as a negative number as the greater the solutes, the fewer free water molecules and the less water potential
water potential equation
solute potential+ physical pressure= water potential
pressure potential
physical pressure on a solution. Can be + or - relative to atmospheric pressure
protoplast
the living part of the cell, which includes the plasma membrane
Long Distance transport
diffusion is too slow to function in long-distance transport within a plant
-bulk flow plays large role
bulk flow
the movement of a liquid in response to pressure gradient
- moves from higher to lower pressure
- independent of solute concentration
