AW- edge of life Flashcards
Explain why phospholipids are “amphipathic” molecules.
they contain both hydrophobic and hydrophilic regions.
Describe the fluidity of the components of a cell membrane
The lipids of the membrane are held together by relatively weak interactions, thus the lipids are in constant motion and can readily move through the membrane structure laterally. It is unlikely for a lipid to swap from one side to the other of the membrane (because the hydrophilic head would have to penetrate through the hydrophobic interior).
Distinguish between peripheral and integral membrane proteins.
- Peripheral proteins do not penetrate right through the membrane but are located in the lipid on one side of the membrane mostly embedded in the hydrophilic component of the membrane (but not exclusively).
- Integral proteins in contrast have both hydrophilic and hydrophobic regions and penetrate right through the membrane.
List six major functions of membrane proteins.
Transport, Enzymatic activity, Signal transduction, Cell-cell recognition, Intercellular joining, Attachment to the cytoskeleton and extracellular matrix
Explain how hydrophobic molecules cross cell membranes.
Hydrophobic molecules are lipid soluble and can pass through the lipid bilayer rapidly (e.g. CO2, ethanol) by diffusion.
Distinguish between channel proteins and carrier proteins.
Channel proteins have a small, hydrophilic pore that runs through the membrane in the centre of a protein molecule. Looking from the top, the channel protein would look a bit like a donut. The channel protein may be relatively specific for certain sizes and charges on molecules. A carrier protein in contrast does not have a pore but has a site that binds the molecule to be transported at one surface of the membrane. The protein then changes shape and position to expel the substrate on the other surface of the membrane. The carriers are generally more specific for the substrate that they will transport. Both carriers and channels can facilitate diffusion down the concentration gradient (i.e. no other energy than the concentration gradient may be needed).
Define diffusion. Explain why diffusion is a spontaneous process.
Movement of a solute down its own concentration gradient via phospholipid bilayer. This is a spontaneous process as it increases the entropy of the system.
Explain why a concentration gradient of a substance across a membrane represents potential energy.
A concentration gradient is out of equilibrium due to there being a higher concentration of the substance on one side than the other. As a consequence, there is potential energy (energy that can do work) associated with a concentration gradient. This energy may be captured in some circumstances. For instance, an ATP-driven proton pump can accumulate protons on one side of a membrane. The energy associated with this concentration gradient (electrochemical potential) can then be used to drive the transport of molecules across the membrane (e.g. sucrose).
what is tonicity
Tonicity is a measure of the osmotic pressure of two solutions separated by a semipermeable membrane
Define osmosis and predict the direction of water movement based on differences in solute concentrations.
Osmosis is the spontaneous net movement of solvent molecules through a selectively permeable membrane into a region of higher solute concentration, in the direction that tends to equalize the solute concentrations on the two sides.
Explain how transport proteins facilitate diffusion.
Diffusion across a membrane occurs down a concentration gradient. As a consequence, no outside source of energy is required and the process is referred to as being passive. Diffusion of small hydrophobic molecules across membranes occurs readily. Examples could be ethanol or CO2. In contrast, hydrophilic molecules or large molecules cannot cross the hydrophobic lipid bilayer. As a consequence, proteins are required to ‘facilitate’ the movement of such molecules. These proteins can facilitate this passive diffusion either by forming a pore that allows the molecules to pass through the membrane (i.e. a protein channel) or by acting as a carrier across the membrane. In both cases the molecules still move passively down their concentration gradients. The channels allow a higher rate of passage of molecules across the membrane than do the carriers. Both are, however, relatively specific allowing only specific and closely related molecules to diffuse through the membrane.
Distinguish among osmosis, facilitated diffusion, and active transport.
Osmosis is the diffusion of water down its concentration gradient. Facilitated diffusion is the diffusion of substances down their concentration gradients facilitated by a protein that enables the molecules in question to rapidly cross the membrane. These facilitators could be channels or carriers. Active transport is the use of energy (often from ATP) to push a molecule across a membrane against its concentration gradient.
Describe the two forces that combine to produce an electrochemical gradient.
The electrochemical gradient is made up of the difference in concentration of ions across the membrane. This concentration gradient has both a chemical component ( concentration gradient of these atoms ) and an electrical component ( the accumulation of positive charges on one side makes that side have a positive charge relative to the other side).
Explain how an electrogenic pump creates a membrane potential across a membrane.
An electrogenic is a specialised integral protein that spans the membrane and uses ATP as a source of energy to actively transport ions against their concentration gradient. By accumulating ions on one side of a membrane, a membrane potential (or voltage) is created that can be used to do work in the cell
Describe the process of co-transport.
Co-transport, also known as “coupled transport” or “secondary active transport”, refers to the transfer of molecules or ions across biological membranes in a fixed ratio. The gradient of one ion or molecule (e.g. proton) that moves from the side where it is more concentrated to that where it is less concentrated is used as a source of energy for movement of a second molecule
