3 abundant ingredients in membranes
lipids, proteins, carbohydrates
have both hydrophobic and hydrophilic regions
Fluid Mosaic Model
a model for structure of a membrane, a fluid structure with various proteins embedded in or attached to the phospholipid bilayer
membranes are fluid and held together primarily by hydrophobic interactions
how are membranes affected by a decrease in temperature?
they remain fluid untin the phospholipids settle into closely packaged arrangements and the membrane solidifies.
how is the temperature at which a membrane solidifies
the types of lipids that the membrane is made of determine the temperature at which it solidifies.
which kind of lipids freeze at lower temperatures?
the membrane remains fluid to a lower temperature if it is rich in phospholipids with insaturated carbon tails because they do not pack as well as saturated carbon tails
what happens to the permeability of the membrane once it has solidified
what determines a membrane's specific function?
the proteins embedded in it
what are the two major populations of membrane proteins?
integral proteins and peripheral proteins
describe integral proteins.
They penetrate the hydrophobic core of the lipid bilayer. Many are transmembrane proteins. The hydrophobic regions of an integral protien consist of one or more stretches of nonpolar amino acids usuallay coiled in (fish) helix. The hydrophillic parts of the molecule are exposed to the aqueous solutions on either side of the membrane.
describe peripheral proteins
they are not embedded in the lipid bilayer at all, they are appendages loosely bound to the surface of a the membrane, often to the exposed parts of the integral proteins
do membranes have disinct insides and outside faces?
yes. Carbohydrates are the only on the outside. The assymetrical distribution of proteins, lipids and carbohydrates is determined as the membrane is being built by the ER.
What is cell-cell recognition?
a cell's ability to distinguish one type of neighboring cell from another
What purposes dooes cell-cell recognition serve?
its crucial for sorting cells, rejection of foreing cells
how does cell-cell recognition work?
cells recognize other cells by keying on surface molecules, often carbohydrates on the plasma membrane
what kind of carbohydrates are typically found on the membrane and used in cell-cell recognition
-some are bonded with lipids: glycolipids
-some bonded with proteins: glycoproteins
they vary from cell to cell, but function as markers to distinguish cells
what are some functions of membrane proteins?
tranport, enzymatic activity, signal transduction, intercellular joing, cell-cell recognition, attachment to the cytoskeleton and extracellular matrix
transport as a function of membrane proteins
a protein that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute; some transport proteins hydrolyze ATP as an energy source to actively pump substances across the membrane
enzymatic activity as a function of membrane proteins
a protein built into the membrane may be an enzyme with its active site exposed to the substances in the adjacent solution. In some cases, several enzymes in a membrane are ordered as a team that carries out sequential steps of a metabolic pathway
signal transduction as a function of membrane proteins
a membrane protein may have a binding site with a specific shapre that fits the shape of a chemical messenger such as a hormone. The external messenger signal may cause a conformational change in the protein that relays the message inside the cell
intracellular joining as a function of membrane proteins
membrane proteins of adjacent cells may be hooked together in various kinds of junctions
cell-cell recognition as a function of membrane proteins
some glycoproteins serve as identification tags that are specifically recognized by other cells
attachment to the cytoskeleton and extracellular matrix as a function of membrane proteins
microfilaments or other elements of the cytoskeleton may be bonded to membrane proteins, a function that helps maintian cell shape and fixes the location of certain membrane proteins. Proteins that adhere to the ECM can coordinate extracellular and intracellular changes
in what way are cell membranes permeable?
The membrane's molecular organization results in its selective permeability
which kind of molecules cross the membrane with ease?
hydrophobic: hydrocarbons, carbon dioxide, oxygen
which kind of molecules cross the membrane with difficulty/cannot cross the membrane
the hydrophobic core if the membrane impedes the transport of ions and polar molecules which are hydrophilic
how do hydrophilic substances cross the membrane
hydrophilic substances avoid the lipid bilayer by passing through transport proteins that span the membrane
what are the two ways that transport proteins move molecules?
some have a hydrophilic channel used as a tunnel while other hold on to their passenger and physically move them across the membrane
what is passive transport?
diffusion across a membrane, called passive because it doesn't have to expend energy
what is diffusion?
-the result of thermal motion (intrinsic kinetic energy of molecules)
-the tendency for molecules of any substance to spread out into the available space
how do substances tend to diffuse?
substances will diffuse from where it is more concentrated to where it is less concentrated (diffuses down its concentration gradient)
is a substance's concentration differences affected by the concentration differences of other substances?
what is osmosis?
the passive transport of water
what is a hypertonic solution?
higher concentration of solute
what is a hypotonic solution?
lower solute concentration
what is a isotonic solution?
equal solute concentration
how does water move?
from hypertonic to hypotonic, if the substance is isotonic, water moves across the membrane at an equal rate in both directions
how does water relate to cell survival?
cell survival depends on the balancing water uptake nd loss
how to cell without walls balance their water uptake/loss?
the cells are stable in isotonic environments, but they cannot tolerate excessive loss or gain of water, cells that live in hyper or hypotonic solutions use osmoregulation which controls the water balance within the cell
how do cells with wall balance their water uptake/loss
needs to be turgid, can survive flaccid but will usually die in plasmolysis
when the cell is very firm because water is pressing on the cell wall, healthy for most plants
limp cells due to lack of water
when a plant shrivels, it's plasma membrane pulls away from the wall, usually fatal
what is facilitated diffusion?
when hydrophilic substances are transported across a membrane by a protein
what are some features that transport proteins share in common with enzymes?
-specialized for the solute it transports
-can be saturated
-has a maximum transport rate
-can be inhibited by imposter molecules
what do transport proteins not do that enzymes do?
they do not catalyze reactions
2 kinds of channels
aquaporins and gated channels
water channel proteins that facilitate massive amounts of diffusion
stimulus causes them to open or close
what is active transport?
the pumping of solutes against their gradient, requires work-energy often supplied by ATP, allows cells to maintain small concentrations of small molecules
what is the sodium potassium pump?
exchanges sodium for potassium across a plasma membrane
which cells have voltage across their membranes?
what is voltage?
electrical potential energy, separation of opposite charges
what is membrane potential?
voltage across a membrane, affects ion movement, -50 to -200 milivolts
what is the charge of the cytoplasm?
what is the electrochemical gradient?
the chemical force and electrical force that affect ion movement
what is an electrochemical pump?
a transport proteins that generate voltage across a membrane
what is a proton pump?
the main electrogenic pump of plants, bacteria, fungi; actively transports hydrogen ions out of the cell which transfers positive charge from the cytoplasm to the extracellular solution
what is cotransport?
when a single ATP powered pump that transports a specific solute can indirectly drive the active transport of several other solutes
what is exocytosis?
when a cell secretes macromolecules by the fusion of veciscles with the plasma membrane
what is endcytosis?
when the cell takes in macromolecules and particulate matter by forming new vesicles from the plasma membrane
the three types of endocystosis
phagocytosis, pinocytosis, receptor-mediated endocytosis
cell engulfs a particle by wrapping it in a sac which is digested by a lysosome
cell gulps droplets of extracellular fluids into tiny vessicles
very specific, extracellular substances bind to receptors on molecule (ligands)