concept 5.1 cellular membranes are fluid mosaics of lipids and proteins

lipids and proteins are essential to the membranes → carbohydrates too

most abundant lipids are phospholipids

• able to form membranes because of their molecular structure

• amphipathic (hydrophilic and hydrophobic regions)

• a phospholipid bilayer (the actual lipid itself) can be made as a stable boundary between 2 aqueous compartments

  • the molecular arrangement protects the hydrophobic tail from interacting with the liquid and the hydrophilic head exposed to the water

membrane proteins are amphipathic

• attached to phospholipid bilayer with hydrophilic regions sticking out

the orientation of this maximizes contact of hydrophilic regions of the protein with water in cytosol and extracellular fluid, while providing hydrophobic parts with nonaqueous environment

fluid mosaic model: membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids

• describes the cell membrane as a tapestry of several types of molecules (phospholipids, cholesterols, and proteins) that are constantly moving

• membrane could be more packed with proteins

membrane is held together primarily by hydrophobic interactions (weaker than covalent bonds)

most lipids and some proteins can shift sideways → plane to the membrane

• sideways movement is fast

proteins are bigger than lipids so they move slower when they do

• many protein seem to be held immobile (not able to move) because of attachment to cytoskeleton or extracellular matrix ECM

• some move in a highly directed manner → maybe because driven along cytoskeletal fibers by motor proteins

• some drift in membrane

membrane remains fluid as temperature decrease until phospholipid are closley packed and membrane solidifies

• temperature when membrane solidifies depend on type of lipid made of

as temperature decreases, membrane remains fluid to lower temp if rich in phospholipids with unsaturated hydrocarbon tails

• unsaturated hydrocarbon tails have kinks bc of double bonds

  • because of where it is located, unsaturated hydrocarbon tails can not pack together as closely as saturated hydrocarbon tails → this looseness makes membrane more fluid

    • reason why unsaturated fat (ex. olive and canola oil--healthty) are liquidy and saturated fat (ex. butter and cheese--unhealthy) solid

steroid cholesterol (in between phospholipid molecules in plasma membrane of animal cells) has different effects on membrane fluidity at different temperatures

• at high temperature (like 98° human body) cholesterol makes membrane less fluid by restraining phospholipid movement

• because cholesterol interferes with the close packing of phospholipids, it lowers the required temperature for membrane to solidify

  • cholesterol helps membrane resist change in fluidity when temperature changes

• plants have lower levels of cholesterol so related steroid lipids resist fluidity changes in plant cells

membranes must be fluid to function properly → fluid like olive oil

• when solidifies, its permability changes → enzymatic proteins in membrane may become inactive

• membrane too fluid cannot support protein function either

  • extreme environment challenges life → resulting in evolutionary adaptations that include differences in membrane lipid composition

lipid composition variations appear to be evolutionary adaptations that maintain appropriate membrane fluidity under specific environmental conditions

ability to change lipid composition depends on change in temperature → this evolves with organisms where temperature vary

natural selections favor organisms whose mix of membrane lipids ensure an appropriate level of membrane fluidity for environment

a membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer

phospholipids are the main fabric of the membrane, but proteins determine membrane’s function

different types of cell has different sets of membrane proteins

two major populations of membrane proteins:

integral proteins

• penetrate hydrophobic interior of bilayer

• majority: transmembrane proteins (type of integral protein)

  • span (meaning reach down to bottom/inner of cell) membrane

• other integral proteins extend only partway into hydrophobic interior

• hydrophobic regions of intergral protein consist of one or more stretches of nonpolar amino acids (20-30 amino acids) → coiled in α-helices

• the hydrophilic parts are exposed to aqueous solutions on either side of membrane

  • some proteins have one or more hydrophilic channels that allow passage of hydrophilic substaces

peripheral proteins

• not embedded in lipid bilayer at all

• loosely bounded to surface of membrane → often exposed parts of integral protein

some membrane proteins are held in place by attachment to cytoskeleton

certain membrane proteins may attach to materials outside the cell (extracellular side)

• in an animal cell, the membrane protein may be attached to fibers of the ECM

  • gives animal cells a stronger framework than plasma membrane itself

transport:

• left--a protein that spans the membrane may provide a hydrophilic channel across the membrane (very selective of solute)

• right--a type of transport protein that shuttles a substance from one side to the other by changing its shape. some of these proteins hydrolyze ATP as energy source to actively pump substances across membrane

enzymatic activity: a protein built into membrane may be an enzyme with its active site (where reactant binds) exposed to substances in the adjacent solution

• several enzymes in membrane are organized as a team to carry out sequential steps of metabolic pathway

signal transduction: membrane protein (receptor) may have a binding site with specific shape that fits the shape of a chemical messenger [ex. hormone.] the external messenger (signaling molecule) may cause protein to change shape → allowing it to relay the message to the inside of the cell, usually by binding to cytoplasmic protein

cell-cell recognition: some glycoproteins (made out of carbohydrates) serve as identification tags that are specifically recognized by membrane proteins of other cells.

• this type of binding is short-lived

intercellular joining: membrane proteins of adjacent cells may hook together in various kinds of junctions (ex. gap junctions or tight junctions). this type of binding is more long-lasting than cell-cell recognition

attachment to the cytoskeleton and extracellular matrix (ECM): microfilaments or other elements of cytoskeleton may be noncovalently bound to membrane proteins

• function that helps maintain cell shape and stabilizes the location of certain membrane proteins

proteins that can bind to ECM molecules can coordinate extracellular and intracellular changes

cell-cell recognition (ability to distinguish one type of neighboring cell from another) is important to functioning of an organism

• sort cells into tissues and organs in animal embryo

• the basis for the rejection of foreign cells by immune system

cells recognize other cells by binding to molecules, often containing carbohydrates--on extracellular surface of plasma membrane

concept 5.2 membrane structure results in selective permeability

small molecules and ions can move across plasma membrane in both directions

sugar, amino acids, and other nutrients enter the cell and metabolic waste leaves the cell. cell takes in O2 for cellular respiration and releases CO2. cell also regulates Na+, K+, Ca2+, and Cl- by shuttling them one or other across plasma membrane

cell membranes are selective in permeability

• substances can not across the barrier randomly. it takes up some molecules and ions and excludes others

nonpolar molecues (ex. CO2 and O2) are hydrophobic and so are lipids

• can dissolve in lipid bilayer of membrane cross easily w/o help

hydrophobic interior of membrane prevents direct passage through membrane of ions and polar molecules (which are hydrophilic)

polar molecules (ex. glucose and sucrose) pass by slowly through lipid bilayer

• water also cross very slowly too

  • water is a polar molecule (hydrophilic substance)

lipid bilayer is only one aspect of the selective permeability

• proteins built in the membrane play key roles in regulating transport

hydrophilic substances can pass plasma membrane and avoiding contact with lipid bilayer by passing through transport proteins

channel proteins: function by having hydrophilic channel that certain molecules or atomic ions to use as a tunnel through membrane

aquaporins: a type of channel protein that specifically facilitates osmosis (diffusion of free water molecules across membrane)

carrier proteins: hold onto their passengers and change their shape in a way that moves them across membrane

transport proteins are specific to which substances can pass through the membrane

• ex. a glucose transport protein can move glucose at a faster pace than glucose itself out of the membrane

selective permeability depends on the lipid bilayer and transport protein built into membrane

concept 5.3 passive transport is diffusion of substance across membrane with energy investment

molecules have thermal energy (from constant motion)

diffusion: movement of particles of any substance so that they spread out into available space

molecules move in random motion but diffusion is directional movement

a substance will diffuse from a higher concentration to a lower concentration

• it diffuses down its concentration gradient

diffusion is a spontaneous proccess → need no input energy

each substances diffuses down its own concentration gradient, unaffected by the concentration gradient of other substances

most of the traffic across cell membrane occur by diffusion

• when a substance is more concentrated on one side of the membrane, a tendency forms to diffuse across (down its concentration gradient)

  • example: dissolved oxygen diffuses into the cell across the plasma membrane. as long as cellular respiration consumes the O2 as it enters, diffusion will continue because of concentration gradient favors the movement in that direction

passive transport: diffusion of a substance across a biological membrane with no energy

concentration gradient is potential energy itself and drives diffusion

• water can diffuse quickly through aquaporins compare to without it

• movement of water has important consequences for cell

solution with higher concentration has lower free water molecules

water diffuses across membrane from region of high free water concentration (lower solute concentration) to region of low free water concentration (high solute concentration)

• until solute concentrations on both sides are more nearly equal

osmosis: diffusion of free water across selective permeable membrane

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 that inside the cell

  • isotonic: no net movement of water across plasma membrane

  • hypertonic: shrivels up because not enough water entering cell

    • reason for increase in saltiness

  • hypotonic: bursts from too much water entering

a cell without rigid cells can tolerate neither excessive uptake of water nor excessive loss of water

• can be solved if the cell lives in isotonic surroundings

  • seawater isotonic to many marine animals

organisms that lack rigid cells walls must adapt for osmoregulation (control of solute concentrations and water balance)

the cell of plants, prokaryotes, fungi and other protists

• when it is submerged in rainwater, the cell wall helps maintain the cell wall’s balance

  • plant cell will swell as water enter by osmosis

    • a cell becomes turgid (very firm) which is a healthy state of cells

if a plant’s cell and surroundings are isotonic, theres no net tendency for water to enter and cells become flaccid (limp); plant will wilt

as a plant cell shrivels, its plasma membrane will pull away from the cell wall

• plasmolysis cause plants to wilt and can lead to plant death

facilitated diffusion: many polar and ion molecules impeded by lipid bilayer diffuse passively with help of transport protein that span membrane

concept 5.4 active transport uses energy to move solutes against their gradients

facilitated diffusion considered passive transport because solute is moving down its concentration gradient (process that requires no energy)

• speeds transport of a solute by providing efficient passage through membrane but does not alter direction of transport

some other transport proteins can move against their own concentration gradient from side of less concentration to more concentration

Active transport: needs energy to pump solute against their concentration gradient

transport protein that move against their own concentration gradient are all carrier proteins rather than channel proteins

ATP hydrolysis supplies energy for most active transport

• one way ATP can power active transport is when the terminal phosphate group is transferred directly to the transport protein

  • induce the protein to change its shape in a manner that translocate a solute bound to protein acros membrane

    • ex of this is sodium potassium pump → exchange Na+ for K+ across plasma membrane

all cells have voltages across their plasma membrane

• voltage is electrical potential energy

voltage across a membrane is called membrane potential

• behaves like a battery → an energy source that affects the traffic of all charged substances across membrane

  • membrane potential favors passive transport of cations into cell and anions out of cell

2 forces drive diffusion of ions across membrane: chemical force and electrical force

• combination called electrochemical gradient

ions diffuses down its electrochemical gradient

• usually this in passive transport but also active transport may be neccessary because electrochemical force oppose simple diffusion of an ion down its concentration gradient

electrogenic pump: a transport protein that generates voltage across membrane

main pump of electrogenic pump of plants, fungi, & bacteria is proton pump

cotransport: a transport protein can couple the “downhill” diffusion of the solute to the “uphill” transport of a second substance aginst its own concentration (or electrochemical) gradient

concept 5.5 bulk transport across the plasma membrane occurs by exocytosis and endocytosis

water and small solutes enter and leave the cell by diffusing through the plasma membrane or by being moved across it by transport protein

larger molecules cross membrane in bulk → packaged in vesicles

• require energy

cell scretes certain molecues byu fusion of vesicles with plasma membrane → exocytosis

transport vesicle budded from golgi apparatus moves along microtubules of cytoskeleton to plasma membrane

vesicle membrane and plasma membrane fuse when it comes in contact

• the lipid molecules from the lipid bilayer rearrange themselves to fuse membranes

  • as a result, vesicle membrane becomes part of plasma membrane

many cells use exocytosis to export products

• ex. pancreas make insulin to secreite it into extracellular fluid by exocytosis.

exocytosis deliver some neccessary proteins and caryboyhydrates from golgi vesciles to outside of the cell to create cell wall

endocytosis: cell takes in molecules and particular matter by forming new vesicles from plasma membrane

• endocytosis looks like the reverse of exocytosis to form a pocket

  • as pocket deepens, it pinches in, forming a vesicle containing material that had been outside cell

3 types of endocytosis: phagocytosis (cellular eating), pinocytosis (cellular drinking), receptor-mediated endocystosis

humans use recepter-mediated endocytosis to take in cholesterol for membrane synthesis and the synthesis of other steroids

endocytosis and exocytosis provide mechanisms for rejuvenating or remodeling plasma membrane

• occur continually in most eukaryotic cells

phagocytosis: cell engulfs a particle by extending pseudopodia around it and packaging it within a membranous sac called a food vacuole

• will be digested after food vacule fuses with a lysosome containing hydrolytic enzymes

pinocytosis: a cell continually “gulps” droplets of extracellular fluid into tiny vesicles (formed by infoldings of plasma membrane)

• cell obtains molecules dissolved in droplets

receptor-mediated endocytosis: specialized type of pinocytosis that enables the cell to acquire bulk quantities of specific substances, even tho those substances may not be very concentrated in extracellular fluid

proteins with receptor sites are embedded in plasma membrane are exposed to extracellular fluid

• receptor proteins cluster in coated pits and form a vescile containing the bound molecules