Chapter 7- Membrane Structure and Function
the plasma membrane
boundary of life- separates the cell from its surroundings
selectively permeable membrane
allows some materials to cross it more easily than others
enables the cell to maintain a unique internal environment
fluid mosaic model
biological membranes consist of various proteins that are attached to or embedded in a bilayer of phospholipids
have both a hydrophilic and hydrophobic region
the phospholipid bilayer is fluid- always moving
with a mosaic of proteins floating in it
phospholipid bilayer
phospholipids self-assemble to form a double layer called a bilayer
hydrophobic hydrocarbon tails in the center
hydrophilic heads facing the aqueous solution on both sides of the membrane
internal environment of the cell (cytoplasm)- mostly H2O
external environment of the cell (extracellular matrix)- mostly H2O
fluidity of membranes
membranes held together by weak hydrophobic interactions
allows lipids and some proteins to drift laterally (side to side)
membranes must be fluid to work properly
if a membrane solidifies permeability changes
enzymatic proteins become inactive
unsaturated vs. saturated hydrocarbon tails
phospholipids with unsaturated hydrocarbon tails maintain membrane fluidity at lower temperatures
cholesterol (steroid)
common in plasma membrane of animals
acts as a fluidity buffer
restricts movement of phospholipids
reducing fluidity at warmer temps
prevents close packing of lipids
enhancing fluidity at colder temps
evolution of differences in membrane lipids
variations in membrane lipid composition is an evolutionary adaptation
fish that live in extreme cold have higher percentage of unsaturated hydrocarbon tails
bacteria/archaea in thermal hot springs have a membrane lipid composition that prevent excessive fluidity
ability to change lipid composition in response to changing temperatures is an evolutionary adaptation
some plants (example: winter wheat) adjust percentage of unsaturated tails with seasonal temperature changes
percentage increases in autumn to prevent membrane from solidifying
membrane proteins (mosaic part of fluid mosaic model)
each membrane has its own unique set of proteins
determine specific functions of the membrane
integral proteins (transmembrane proteins)
extend through the membrane
have two hydrophilic ends and a hydrophobic midsection
peripheral proteins
attached to the surface of the membrane
also attach to the cytoskeleton and fibers of the ECM
provides support for the plasma membrane
6 major functions of membrane proteins- transport, enzymatic activity, signal transduction, cell-cell recognition, intracellular joining, attachment
membrane carbohydrates
ability of a cell to distinguish other cells is based on recognition of membrane carbohydrates
covalently bond to lipid or proteins
glycolipid/glycoprotein
vary from species to species, individual to individual, and cell to cell
example- A, B, AB, and O Blood types
red blood cells differ in the carbohydrate part of a glycoprotein on surface
synthesis of membranes
membrane proteins and lipids are synthesized in the ER, modified in the Golgi, and transported by vesicles to the membrane
selective permeability
plasma membrane permits a regular exchange of nutrients, waste products, oxygen, and inorganic ions
ease and rate at which small molecules pass through them differs
hydrophobic, non-polar molecules cna dissolve and cross easily
hydrocarbons, CO2 and O2
transport proteins
ions and polar molecules are stopped by the hydrophobic center
Na+, K+, H2O, and Glucose can not cross easily
channel proteins
provides a hydrophilic passageway through the membrane
aquaporins
carrier proteins
physically bind and change shape to shuttle molecules across
glucose transporter
diffusion
movement of a substance down its concentration gradient
molecules move randomly, but movement of substance is directional
movement is from areas of high concentration to areas of low concentration
the cell does not expend energy when substances diffuse across membranes down their concentration gradient
this is passive transport
osmosis
diffusion of free water across a selectively permeable membrane
water diffuses down its own concentration gradient
affected by concentration of dissolved solutes
clustering of water molecules around solute particles lowers proportion of free water
tonicity
tendency of a surrounding solution to cause a cell to gain or lose water- affected by concentration of dissolved solutes
cellular environments can by hypotonic, isotonic, or hypertonic
isotonic cell environment
solutions that have equal concentrations with no net movement of water across the membrane
iso means same
water goes in and out of the two solutions at the same rate
an animal cell will neither gain nor lose water in an isotonic environment
hypertonic cell environment
solution with a higher concentration of solutes
hyper means more
water will rush out of the cel, into the solution
an animal cell placed in a hypertonic solution will lose water and shrivel
hypotonic cell environment
solution with a lower concentration of solutes
hypo means less
water will rise into the cell, out of the solution
if placed in a hypotonic solution, the cell will gain water, swell, and possibly lyse (burst)
plant cells (cell with cell wall)
the cell wall of plants, fungi, and prokaryotes play a role in water balance in hypotonic environments
water moving into the cell causes the cel to swell against its cell wall
creates turgor pressure- turgid cells
provides mechanical support for the plant
hypotonic environment is the healthiest environment for a plant
animal cells (cell without cell wall)
cels without rigid walls must either live in an isotonic environment
salt water or isotonic body fluids
or have adaptations for osmoregulation- regulation of water balance
isotonic environment is the healthiest environment for an animal cell
facilitated diffusion
passive transport aided by proteins
polar molecules and ions transported across membrane faster than normal
down concentration gradient
no energy (ATP) required
channel protein- hydrophilic tunnel (aquaporin)
carrier protein- structural match (glucose transporter)
active transport
requires expenditure of energy (ATP) to transport a solute against its concentration gradient
movement of solute from low concentration to high concentration
essential for cell to maintain internal concentrations of small molecules
the transport proteins for active transport are carrier proteins
membrane potential
a voltage across the plasma membrane due to the unequal distribution of ions on either side
electrical potential energy results from the separation of opposite charges
cytoplasm of a cell is negatively charged relative to the ECM fluid
favors diffusion of cations (+) into the cell and anions (-) out of the cell
both the membrane potential and the concentration gradient affect the diffusion of an ion
an ion diffuses down its electrochemical gradient
electrogenic pumps
membrane proteins that generate voltage across a membrane by the active transport of ions
sodium potassium pump
actively transports Na+ ions out and K+ ions into the cell to maintain membrane potential in animal cells
3 Na+ out for every to K+ in (maintains negative cytoplasm)
proton pump is main electrogenic pump for plants, fungi, and protists
bulk transport (active)
like active transport, it requires energy
transports larger biological molecules, packaged in vesicles across the membrane
exocytosis- cell secretes large molecules by the fusion of vesicles with the plasma membrane
exo- out of the cell
endocytosis- a region of the plasma membrane sinks inward and pinches off to form a vesicle containing material that had been outside of the cell
endo- into the cell
3 types of endocytosis
phagocytosis- endocytosis of food particles- cellular eating
pinocytosis- endocytosis of fluids- cellular drinking
receptor- a mediated endocytosis enables a cell to acquire specific substances by ligand binding (molecule to specific receptor)
cholesterol for membrane
the plasma membrane
boundary of life- separates the cell from its surroundings
selectively permeable membrane
allows some materials to cross it more easily than others
enables the cell to maintain a unique internal environment
fluid mosaic model
biological membranes consist of various proteins that are attached to or embedded in a bilayer of phospholipids
have both a hydrophilic and hydrophobic region
the phospholipid bilayer is fluid- always moving
with a mosaic of proteins floating in it
phospholipid bilayer
phospholipids self-assemble to form a double layer called a bilayer
hydrophobic hydrocarbon tails in the center
hydrophilic heads facing the aqueous solution on both sides of the membrane
internal environment of the cell (cytoplasm)- mostly H2O
external environment of the cell (extracellular matrix)- mostly H2O
fluidity of membranes
membranes held together by weak hydrophobic interactions
allows lipids and some proteins to drift laterally (side to side)
membranes must be fluid to work properly
if a membrane solidifies permeability changes
enzymatic proteins become inactive
unsaturated vs. saturated hydrocarbon tails
phospholipids with unsaturated hydrocarbon tails maintain membrane fluidity at lower temperatures
cholesterol (steroid)
common in plasma membrane of animals
acts as a fluidity buffer
restricts movement of phospholipids
reducing fluidity at warmer temps
prevents close packing of lipids
enhancing fluidity at colder temps
evolution of differences in membrane lipids
variations in membrane lipid composition is an evolutionary adaptation
fish that live in extreme cold have higher percentage of unsaturated hydrocarbon tails
bacteria/archaea in thermal hot springs have a membrane lipid composition that prevent excessive fluidity
ability to change lipid composition in response to changing temperatures is an evolutionary adaptation
some plants (example: winter wheat) adjust percentage of unsaturated tails with seasonal temperature changes
percentage increases in autumn to prevent membrane from solidifying
membrane proteins (mosaic part of fluid mosaic model)
each membrane has its own unique set of proteins
determine specific functions of the membrane
integral proteins (transmembrane proteins)
extend through the membrane
have two hydrophilic ends and a hydrophobic midsection
peripheral proteins
attached to the surface of the membrane
also attach to the cytoskeleton and fibers of the ECM
provides support for the plasma membrane
6 major functions of membrane proteins- transport, enzymatic activity, signal transduction, cell-cell recognition, intracellular joining, attachment
membrane carbohydrates
ability of a cell to distinguish other cells is based on recognition of membrane carbohydrates
covalently bond to lipid or proteins
glycolipid/glycoprotein
vary from species to species, individual to individual, and cell to cell
example- A, B, AB, and O Blood types
red blood cells differ in the carbohydrate part of a glycoprotein on surface
synthesis of membranes
membrane proteins and lipids are synthesized in the ER, modified in the Golgi, and transported by vesicles to the membrane
selective permeability
plasma membrane permits a regular exchange of nutrients, waste products, oxygen, and inorganic ions
ease and rate at which small molecules pass through them differs
hydrophobic, non-polar molecules cna dissolve and cross easily
hydrocarbons, CO2 and O2
transport proteins
ions and polar molecules are stopped by the hydrophobic center
Na+, K+, H2O, and Glucose can not cross easily
channel proteins
provides a hydrophilic passageway through the membrane
aquaporins
carrier proteins
physically bind and change shape to shuttle molecules across
glucose transporter
diffusion
movement of a substance down its concentration gradient
molecules move randomly, but movement of substance is directional
movement is from areas of high concentration to areas of low concentration
the cell does not expend energy when substances diffuse across membranes down their concentration gradient
this is passive transport
osmosis
diffusion of free water across a selectively permeable membrane
water diffuses down its own concentration gradient
affected by concentration of dissolved solutes
clustering of water molecules around solute particles lowers proportion of free water
tonicity
tendency of a surrounding solution to cause a cell to gain or lose water- affected by concentration of dissolved solutes
cellular environments can by hypotonic, isotonic, or hypertonic
isotonic cell environment
solutions that have equal concentrations with no net movement of water across the membrane
iso means same
water goes in and out of the two solutions at the same rate
an animal cell will neither gain nor lose water in an isotonic environment
hypertonic cell environment
solution with a higher concentration of solutes
hyper means more
water will rush out of the cel, into the solution
an animal cell placed in a hypertonic solution will lose water and shrivel
hypotonic cell environment
solution with a lower concentration of solutes
hypo means less
water will rise into the cell, out of the solution
if placed in a hypotonic solution, the cell will gain water, swell, and possibly lyse (burst)
plant cells (cell with cell wall)
the cell wall of plants, fungi, and prokaryotes play a role in water balance in hypotonic environments
water moving into the cell causes the cel to swell against its cell wall
creates turgor pressure- turgid cells
provides mechanical support for the plant
hypotonic environment is the healthiest environment for a plant
animal cells (cell without cell wall)
cels without rigid walls must either live in an isotonic environment
salt water or isotonic body fluids
or have adaptations for osmoregulation- regulation of water balance
isotonic environment is the healthiest environment for an animal cell
facilitated diffusion
passive transport aided by proteins
polar molecules and ions transported across membrane faster than normal
down concentration gradient
no energy (ATP) required
channel protein- hydrophilic tunnel (aquaporin)
carrier protein- structural match (glucose transporter)
active transport
requires expenditure of energy (ATP) to transport a solute against its concentration gradient
movement of solute from low concentration to high concentration
essential for cell to maintain internal concentrations of small molecules
the transport proteins for active transport are carrier proteins
membrane potential
a voltage across the plasma membrane due to the unequal distribution of ions on either side
electrical potential energy results from the separation of opposite charges
cytoplasm of a cell is negatively charged relative to the ECM fluid
favors diffusion of cations (+) into the cell and anions (-) out of the cell
both the membrane potential and the concentration gradient affect the diffusion of an ion
an ion diffuses down its electrochemical gradient
electrogenic pumps
membrane proteins that generate voltage across a membrane by the active transport of ions
sodium potassium pump
actively transports Na+ ions out and K+ ions into the cell to maintain membrane potential in animal cells
3 Na+ out for every to K+ in (maintains negative cytoplasm)
proton pump is main electrogenic pump for plants, fungi, and protists
bulk transport (active)
like active transport, it requires energy
transports larger biological molecules, packaged in vesicles across the membrane
exocytosis- cell secretes large molecules by the fusion of vesicles with the plasma membrane
exo- out of the cell
endocytosis- a region of the plasma membrane sinks inward and pinches off to form a vesicle containing material that had been outside of the cell
endo- into the cell
3 types of endocytosis
phagocytosis- endocytosis of food particles- cellular eating
pinocytosis- endocytosis of fluids- cellular drinking
receptor- a mediated endocytosis enables a cell to acquire specific substances by ligand binding (molecule to specific receptor)
cholesterol for membrane