Chapter 5: Membrane Transport and Cell Signaling
What is a hypotonic solution?
"Less": solute concentration is less than that inside the cell; cell gains water
What is a hypertonic solution.
"More": solute concentration is greater than inside the cell; cell loses water
What is an isotonic solution?
"Same": Solute concentration is the same as inside the cell; no net water movement across the cell membrane (water diffuses across the cell membrane, but at the same rate in both directions)
Because of this, what two forces drive the diffusion of ions across a membrane?
1) a chemical force (the ion's concentration gradient) 2) an electrical force (the effect of the membrane potential on the ion's movement)
Proteins serve as transportation.
1) a protein that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute 2) a protein can bring a substance from one side of the membrane to the other by changing shape. Some of these proteins hydrolyze ATP as an energy source to actively pump substances across the membrane.
What are two examples of this?
1) fishes that live in extreme cold have membranes with high proportion of unsaturated hydrocarbon tails, enabling their membranes to remain fluid 2) some bacteria and archaea thrive at temperatures greater than 90 degrees Celsius- their membranes include unusual lipids that help prevent excessive fluidity at such high temperatures
Why are two examples of why a cell's ability to distinguish one type of neighboring cell from another (cell-cell recognition) is important?
1) function of an organism- sorting of cells into tissues and organs in an animal embryo 2) basis of rejection of foreign cells by immune system
Proteins attach to the cytoskeleton and ECM.
1) microfilaments or other elements of the cytoskeleton no covalently bound to membrane proteins can maintain cell shape and stabilize location of certain membrane proteins 2) proteins bound to ECM molecules can coordinate extracellular and intracellular changes
What are two examples of this?
1) plants that tolerate extreme cold such as winter wheat increase percentage of unsaturated phospholipids 2) bacteria and archaea change proportion of unsaturated phospholipids in cell membranes depending on temperature
What are two examples of aquaporins (channel proteins Duh!)?
1) red blood cells 2) certain kidney cells have aquaporin molecules that allow them to reclaim water from urine before it is excreted
Animals need a stronger framework than the plasma membrane can provide. How do proteins accommodate this?
1) some proteins are attached to the cytoskeleton on the cytoplasmic side of the membrane 2) some proteins are attached to fibers of the extra cellular matrix on the extracellular side
What are the six major functions of membrane proteins?
1) transport 2) enzymatic activity 3) attachment to the cytoskeleton and extracellular matrix (ECM) 4) cell-cell recognition 5) intercellular joining 6) signal transduction Figure 5.7
Proteins participate in signal transduction (transfer of genetic material from one cell to another by means of a virus)
A membrane protein receptor may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone. The external messenger (signaling molecule) may cause the protein to change shape, allowing it to relay the message to the inside of the cell, usually by binding to a cytoplasmic protein.
Proteins radiate enzymatic activity.
A protein built into the membrane may be an enzyme with its active side exposed to substances in the adjacent solution. In some cases, several enzymes in a membrane are organized as a team that carries out sequential steps of a metabolic pathway.
What is the main electrogenic pump of plants, fungi, and bacteria
A proton pump
What is an example of the statement: a transport protein is specific for the substance it translocates, allowing only a certain substance or small group of related substances to cross the membrane.
A specific carrier protein in the plasma membrane of red blood cells transports glucose across the membrane glucose transporter" is so selective, that it even rejects fructose, which has the same molecular formula as glucose.
What type of transport is may be necessary when electrical forces due to the membrane potential oppose the simple diffusion of an ion down its concentration gradient
Active transport
What type of transport pumps a solute across a membrane against its concentration gradient which requires energy, usually in the form of ATP.
Active transport
What does facilitated diffusion not offer?
Alteration in the direction of transport (such as against concentration gradient)
Active transport enables a cell to maintain internal concentrations of small solutes that differ from concentrations in its environment. What is an example of this?
An animal cell has a much higher concentration of potassium ions and a much lower concentration of sodium ions. The plasma membrane helps maintains these gradients by pumping Na out of the cell and K into the cell.
What is a transport protein that generates voltage across a membrane called?
An electrogenic pump- help store energy that can be tapped for cellular work
What type of cell is stable in an isotonic environment?
Animal cell (no cell wall)
What is the channel protein that allows water molecules to pass through the plasma membrane?
Aquaporins
How does the steroid cholesterol impact the plasma membrane in animal cells?
At high temperatures, cholesterol makes the membrane less fluid by restricting phospholipid movement. However, because cholesterol also hinders the close packing of phospholipids, it lowers the temperature required for the membrane to solidify. Cholesterol helps membranes resist changes in fluidity when the temperature changes. (Figure 5.5b)
Why is the sodium-potassium pump considered an electrogenic pump?
Because it does not translocate Na and K one for one, but pumps three sodium ions out of the cell for every two potassium ions it pumps into the cell. With each "crank" of the pump, there is a net transfer of one positive charge from the cytoplasm to the extracellular fluid, a process that stores energy as voltage
Phospholipids are the most abundant lipids in most membranes. Why is this?
Because of their molecular structure. A phospholipid is an amphipathic molecule and so are most membrane proteins.
What is the change in shape of carrier proteins triggered by?
Binding and release of the transported molecule
What do amphipathic molecules have?
Both a hydrophilic region and a hydrophobic region
How do cells recognize other cells?
By binding to molecules, often containing carbohydrates, on the extracellular surface of the plasma membrane
How is voltage created?
By differences in distribution of positive and negative ions across a membrane
How do certain hydrophilic substances avoid contact with the lipid bilayer in crossing the plasma membrane?
By passing through transport proteins that span the membrane
What is one way ATP can power active transport?
By transferring its terminal phosphate group directly to the transport protein. This can induce the protein to change its shape in a manner that translocates a solute bond to the protein across the membrane
Because the inside of the cell is negative compared with the outside, what does the membrane potential favor the passive transport of?
Cations into the cell and anions out of the cell
How does a hypotonic solution affect an animal cell?
Cell will swell and lyse (burst) like an overfilled water balloon
What are ion channels?
Channel proteins that transport ions
What are the two types of passive transport?
Diffusion and facilitated diffusion
All cells have voltages across their plasma membranes. What is voltage?
Electrical potential energy-a separation of opposite charges
So, in the case of passive transport, the ion diffuses not simply down its concentration gradient but, more exactly, down its...
Electrochemical gradient
What is the combination of forces acting on an ion called?
Electrochemical gradient
How does water diffuse across the memebrane?
From the region of lower solute concentration (higher free water concentration to that of higher solute concentration (lower free water concentration) until the solute concentrations on both sides of the membrane are more nearly equal. (Figure 5.10)
What do ion channels function as?
Gated channels, which open or close in response to a stimulus (electrical stimulus for some gated channels, and another substance other than one transported binds to channel for others)
How are membranes held together? How does this affect the membrane?
Hydrophobic interactions which are much weaker than covalent bonds. This means that most of the lipids and proteins can shift laterally.
Why would a membrane remain fluid to a lower temperature?
If it is rich in phospholipids with unsaturated hydrocarbon tails. Because of kinks in the tails where double bonds are located, unsaturated hydrocarbon tails cannot pack together as closely as saturated hydrocarbon tails and thus looseness makes the membrane more fluid (figure 5.5a)
When is a cell wall of no advantage? What happens to the plant cell?
In a hypertonic solution
What is the difference between diffusion and facilitated diffusion?
In diffusion, hydrophobic molecules and (at a slow rate) very small uncharged polar molecules can diffuse through the lipid bilayer. In facilitated diffusion, many hydrophilic substances diffuse through the membrane with the assistance of transport proteins, with channel proteins or carrier proteins
What are the two major populations of membrane proteins?
Integral proteins and peripheral proteins
What do channel proteins do?
Provide corridors that allow specific molecules or ions to cross the membrane
What is the membrane reaction to temperature?
Remains fluid as temperature decreases until finally the phospholipids settle into a closely packed arrangement and the membrane solidifies.
How do we know that proteins move?
Scientists labeled plasma membranes of a mouse cell and a human cell and fused the cells to find that the mixing of the mouse and human membrane proteins indicates that at least some membrane proteins move sideways within the plane of the plasma membrane.
How are membrane carbohydrates structured?
Short, branched chains of fewer than 15 sugar units
How does a hypertonic solution affect an animal cell?
Shrivel and die
What happens to the plant cell in a hypertonic environment and what is this called?
Shrivels, plasma membrane pulls away from wall, causes plant to wilt and can lead to death
What is the major electrogenic pump for animal cells?
Sodium potassium-pump
It is logical why the solution with the higher concentration of solute has the lower concentration of water and water would if fuse into it from the other side for that reason, but why?
Solutes do not affect the water concentration significantly. Tight clustering of water molecules around the hydrophilic solute molecules makes some of the water unavailable to cross the membrane. So, it is the difference in free water concentration that is important.
Proteins participate in cell-cell recognition.
Some glycoproteins serve as identification tags that are specifically recognized by membrane proteins of other cells. This type of cell to cell binding is short lived compared to intercellular joining.
The hydrophilic parts of the integral protein are exposed to the aqueous solutions on either side of the membrane. What does this allow for?
Some integral proteins have one or more hydrophilic channels that allow passage of hydrophilic substances (even water, itself)
What is tonicity?
The ability of a surrounding solution to cause a cell to gain or lose water
What is an adaption that organism that live where temperatures vary have?
The ability to change the lipid composition of their cell membranes in response to the changing temps
What us an example of an emergent property of the plasma membrane?
The ability to regulate transport across cellular boundaries
What does the fluid mosaic model represent?
The cell membrane and its lipids and proteins
Why does the cell not have to expend energy in passive transport?
The concentration gradient itself represents potential energy and drives diffusion
What does tonicity depend on?
The concentration of solutes that cannot cross the cell membrane (non-penetrating solutes) relative to that inside the cell
What is and example of both electrical and chemical contributions to the electrochemical gradient acting in the same direction across the membrane?
The concentration of the cation Na inside a resting nerve cell is much lower than outside of it. When the cell is stimulated, gated channels open that facilitate Na diffusion. Sodium ions then "fall" down their electrochemical gradient, driven by the concentration gradient of Na and by the attraction of these cations to the negative side (inside) of the membrane.
What is osmoregulation?
The control of solute concentrations and water balance, is a necessary adaption for life in hypertonic or hypotonic environments
How is the cytoplasmic side of the membrane charged in relation to the extracellular side?
The cytoplasmic side is negative relative to the extracellular side because of an unequal distribution of anions and cations on the two sides
What is osmosis?
The diffusion of free water across a selectively permeable memebrane, whether artificial or cellular
What is an example of the fact that membrane carbohydrates function as markers that distinguish one cell from another?
The four human blood types (A,B,AB,O) reflect variation in the carbohydrate part of glycoproteins on the surface of red blood cells
Membrane carbohydrates can be covalently bonded to lipids (forming glycolipids) or, more commonly, to proteins (forming glycoproteins). So what does glyco refer to?
The presence of carbohydrate
How does the sodium- potassium pump work?
The pump oscillates between two shapes in a cycle that moves 3 Na out of the cell for every 2 K pumped into the cell. The two shapes have different affinities for Na and K. ATP powers the shape change by transferring a phosphate group to the transport protein (phosphorylating the protein). (Figure 5.14)
A simple rule of diffusion is that any substance will diffuse down its concentration gradient. What is a concentration gradient?
The region along which the density of a substance increases or decreases. No work must be done to make this happen; diffusion is a spontaneous process, needing no input of energy. Each substance diffuses down its own concentration gradient, unaffected by the concentration gradients of other substances.
What is one specific type of an active transport system?
The sodium-potassium pump
One result of this motion is diffusion. What is diffusion?
The tendency for molecules to spread out evenly into the available space
Describe an example of osmoregulation?
The unicellular protist Paramecium caudatum lives in pond water, which is hypotonic to the cell, but the Paramecium doesn't burst because it is equipped with a contractile vacuole, an organelle that functions as a bilge pump to force water out of the cell as fast as it enters by osmosis.
Describe an example of diffusion
The uptake of oxygen by a cell performing cellular respiration: dissolved oxygen diffuses into the cell across the cell membrane. As long as cellular respiration consumes the oxygen as it enters, diffusion into the cell will continue because the concentration gradient favors movement in that direction.
What is membrane potential?
The voltage across a membrane which ranges from -50 to -200 milli volts (mV) the minus side indicates that he inside of the cell is negative relative to the outside
How does the cell wall help maintain the cell's water balance in a hypotonic solution?
The wall will expand only so much before it exerts a back pressure on the cell
What is the energy that molecules have that is associated with their constant motion?
Thermal energy
Where are peripheral proteins located?
They are loosely bound to the surface of the membrane, often exposed to parts of integral proteins
How are aquaporin proteins structured?
They consist of four identical subunits. The polypeptide making up each subunit forms a channels that allows single-file passage of up to 3 billion (3x10^9) water molecules per second to cross the membrane
A membrane is a collage of different proteins, often grouped together, embedded in the fluid matrix of the lipid bilayer. Why are the specific proteins in the bilayer so important?
They determine most of the membrane's functions even though phospholipids form the main fabric
How does the specific transport protein, channel proteins, function?
They have a hydrophilic channel that certain molecules or atomic ions use as a tunnel through the membrane.
How does the specific transport protein, carrier proteins, function?
They hold onto their passengers and change shape in a way that shuttles them across the membrane
Where are integral proteins located?
They penetrate the hydrophobic interior of the lipid bilayer
Why does each molecule move randomly, but diffusion of a population of molecules may be directional?
To maintain a dynamic equilibrium, with cross the membrane each second in one direction as the other.
The majority of integral proteins are transmembrane proteins. What do transmembrane proteins do and how are they different from the rest of the integral proteins?
Transmembrane proteins span the membrane while other integral proteins extend only partway into the hydrophobic interior
What si this pressure called?
Turgor pressure
What does the temperature at which a membrane solidifies depend on?
Types of lipids
How does evolution play in the composition of the cell membrane?
Variations in lipid composition of cell membranes of many species appear to be adaptions to specific environmental conditions
What does turgid mean?
Very firm- healthy state for plant cells
Why do extreme environments pose a challenge for for life concerning the plasma membrane?
When a membrane solidifies (cold), it's permeability changes, and enzymatic proteins in the membrane may become inactive. Membranes that are too fluid (warm) cannot support proteins function either.
What is facilitated diffusion?
When transport proteins speed the passive movement of molecules across the plasma membrane
How is the asymmetric arrangement of proteins, lipids, and their associated carbohydrates in the plasma membrane determined?
It is determined as the membrane is being built by the endoplasmic reticulum and Golgi apparatus 1) membrane proteins and lipids are synthesized in the ER. Carbohydrates are added to the transmembrane proteins, making them glycoproteins. The carbohydrate portions may then be modified 2) inside the Golgi apparatus, the glycoproteins undergo further carbohydrate modification, and some lipids acquire carbohydrates, becoming glycolipids 3) The glycoproteins, glycolipids, and secretory proteins are transported in vesicles to the plasma membrane 4) as vesicles fuse with plasma membrane, the outside face of the vesicle becomes continuous with the cytoplasmic face of the plasma membrane. This releases the secretory proteins from the cell, a process call exocytosis and positions the carbohydrates of membrane glycoproteins and glycolipids on the extracellular side of the plasma membrane
An isotonic solution will cause the cells to become flaccid. What does this mean?
Limp
Proteins participate in intercellular joining.
Membrane proteins of adjacent cells may hook together in various kinds of junctions such as gap junctions or tight junctions
What are examples of nonpolar molecules and what can they do in relation to the plasma membrane?
Molecules such as hydrocarbons, carbon dioxide, and oxygen are hydrophilic and can therefore dissolve in the lipid bilayer of the membrane and cross it easily, without the aid of membrane proteins
What do the hydrophobic regions of the integral protein consist of?
One or more stretches of non-polar amino acids usually coiled in alpha helices
What is the diffusion of a substance across a biological membrane called?
Passive transport
What is the difference between the rate in which phospholipids move in comparison to proteins?
Phospholipids move rapidly. Proteins move slowly.
The cells of what are surrounded by walls?
Plants, prokaryotes, fungi, and some protists
The hydrophobic interior of the membrane impedes the direct passage of ions and polar molecules, which are hydrophilic, through the membrane. How does this affect polar molecules?
Polar molecules such as glucose and other sugars pass only slowly through the lipid bilayer. Even water, and extremely small polar molecule, crosses slowly.
