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Biochem Ch. 12
Membrane Structure and Function
| Question | Answer |
|---|---|
| Characteristics of membranes: Membranes are _____-______ structures, two molecules thick, that form closed boundaries. | sheet-like |
| Characteristics of membranes: Membranes are composed of ______ and _______, either of which can be decorated with ______________. | lipids and proteins, carbohydrates |
| Characteristics of membranes: Membrane lipids are small _____________ molecules that form closed bimolecular sheets that prevent the movement of polar or charged molecules. | amphipathic |
| Characteristics of membranes: _______ serve to mitigate the impermeability of membranes and allow movement of molecules and information across the cell membrane. | Proteins |
| Characteristics of membranes: Membranes are held together by ___________ interactions | noncovalent |
| Characteristics of membranes: Membranes are asymmetric. Explain | the outer surface is always different from the inner surface |
| Characteristics of membranes: Membranes are fluid structures. Explain | lipid molecules diffuse rapidly in the plane of the membrane, as do proteins, unless they are anchored by specific interactions |
| Characteristics of membranes: Most cell membranes are electrically ___________ such that the inside is negative | polarized |
| Phospholipids and glycolipids form ______ _________ in aqueous solutions | lipid bilayer |
| What is the formation of membranes powered by? | they hydrophobic effect |
| Membrane formation is a consequence of the amphipathic nature of the molecules. Explain the amphipathic structure | polar head groups favor contact with water while hydrocarbon tails are hydrophobic |
| What are some other chemical forces, other than hydrophobic interactions, stabilize lipid bilayers? | van der Waals attractive forces between hydrocarbon tails favor close packing. electrostatic interactions lead to hydrogen bonding between polar head groups and water molecules |
| Lipid bilayers are highly ___________ to ions and most polar molecules | impermeable |
| Because lipid bilayers are quite fluid, they can act as a ___________ for membrane proteins | solvent |
| Membrane processes depend on the _________ of the membrane | fluidity |
| What is membrane fluidity controlled by? | fatty acid composition and cholesterol content |
| The temperature at which a membrane transitions from being highly ordered to very fluid is called the ___________ temperature (half of the lipids are solid, half are fluid) | melting (T_m) |
| As the temperature increases, lipids move from the ______ phase to ______ phase | solid to liquid |
| What two things is the melting temperature dependent on? | length of the fatty acid chains in the membrane lipid and the degree of cis unsaturation |
| Long saturated fatty acids interact more strongly than do short ones. why? | inceased number of van der Waals interactions |
| What does a cis double bond to in the hydrocarbon chain that a trans double bond does not? | produces a kink in the acyl chain with decreases the number of interactions between chains and Tm is lowered (disrupts tight packing of fatty acid chains) |
| What is the key modulator of membrane fluidity for animals? | cholesterol |
| Cholesterol contains a bulky steroid nucleus with a hydroxyl group at one end and a flexible hydrocarbon tail at the other end. How does cholesterol effect membrane fluidity? | it inserts into the biplayers, disrupting the regular interactions between fatty acid chains and helps maintain membrane fluidity. |
| While membrane lipids establish a permeability barrier, membrane _______ allow transport of molecules and information across the membrane | proteins |
| Membranes vary in protein content from as little as __ % to as much as ___ %. | 18-75% |
| Proteins ‘__________’ membranes, i.e., catalytic as in membrane bound enzymes, transport function as in ‘pores or channels’, receptors as in signaling cascades, etc. | functionalize |
| Proteins associate with the lipid bilayer in a variety of ways: membrane proteins can be classified as being either _________ or __________ on the basis of their interaction with the hydrophobic interior of the membrane | peripheral or integral |
| Integral membrane proteins | embedded in the hydrocarbon core of the membrane |
| Peripheral membrane proteins | bound to the polar head groups of membrane lipids or to the exposed surfaces of integral membrane proteins. |
| Some proteins are associated with membranes by attachment to a ____________ chain (such as a fatty acid) that is inserted into the membrane. | hydrophobic |
| Membrane-__________ α helices are a common structural feature of integral membrane proteins | spanning |
| Other means of embedding integral membrane proteins is by using β strands to form a ___________ in the membrane or by embedding part of the protein into the membrane | Channel (or pore) |
| lipids and many membrane proteins are constantly in motion, a process called _________ diffusion | lateral |
| lateral diffusion | lateral movement of membrane constituents |
| _________ diffusion, or flip-flopping is very rare without the assistance of enzymes | transverse |
| Because lateral diffusion is rapid while transverse diffusion is very rare, membrane __________ can be preserved for long periods | asymmetry |
| A major role of membrane proteins is to function as __________. | Transporters |
| What two factors determine whether a small molecule will cross a membrane? | the concentration gradient of the molecule across the membrane, and the molecule's stability in the hydrophobic environment of the membrane |
| Explain the concentration gradient conditions that must be met for a small molecule to spontaneously cross a membrane | The concentration of the molecule is higher on one side of the membrane than the other. |
| Explain the stability conditions of the molecule that must be met for a small molecule to spontaneously cross a membrane | The molecule must be lipophilic (or soluble in nonpolar solutions.) |
| Molecules meeting these two criteria can simply _______ across the cell membrane | diffuse |
| ______ molecules can diffuse across a membrane down their concentration gradient only with the assistance of proteins (channels) | Polar |
| Polar molecules can diffuse across a membrane down their concentration gradient only with the assistance of proteins. Such movement is called what? | facilitated diffusion or passive transport. |
| Movement of molecules against a concentration gradient requires a source of energy (ATP) and is called what? | active transport. |
| What is an important pump in many cells? | the Na+/K+ ATPase (sodium potassium pump) |
| The Na+-K+ ATPase or Na+-K+ pump uses the energy of ATP hydrolysis to simultaneously pump three Na+ ions out of the cell, and two K+ ions into the cell _______ their concentration gradients. | against |
| Many active-transport processes are not directly driven by the hydrolysis of ATP. Instead, the ____________ uphill flow of one type of ion is coupled to the downhill flow of a different type of ion | thermodynamically (uphill and downhill is talking about conc. gradient) |
| Membrane proteins that move ions or molecules uphill by this means are termed __________ ____________ | secondary transporters (or cotransporters) |
| Secondary transporters can transport two substrates in opposite directions (called __________) or two substrates in the same directions (called __________) | antiporters or symporters |
| Other membrane proteins, the passive-transport systems called ___ _________, are capable of transporting even faster than active-transport systems | ion channels |
| Ion channels are passive transport systems that allow specific and rapid transport of ions down their concentration gradients, from ____ concentration to ____ concentration | high to low |
| ion channels can be activated. ________-gated channels are opened in response to changes in membrane potential. _______- gated channels open in response to the binding of small molecules | voltage-gated and ligand-gated |
| ______________ produced by the puffer fish, is a lethal inhibitor of the Na+ channel. | Tetrodotoxin |
| The structure of the potassium ion channel reveals the basis of ion ___________ | specificity |
| The potassium channel selectively and rapidly transports K+ across the cell membrane. Larger ions are not transported. why? | because they are too big to enter the channel. |
| A K+ ion can fit in the pore without losing its shell of bound water molecules. About two-thirds of the way through the membrane, the pore becomes more constricted. At that point, What must K+ ions do to enter? | the K+ ions must give up their water molecules and interact directly with groups from the protein |
| The free-energy cost of dehydrating these ions are considerable. The channel pays the cost of dehydrating K+ by providing compensating interactions with the carbonyl atoms lining the selectivity filter. Why are smaller ions excluded from entering? | the energy required to dehydrate them can not be recovered |
Created by:
cmccartney2