5.4: Passive Transport Across Membranes

because of ions' repulsion to the interior of the plasma membrane, they cannot move between the cytoplasm of a cell and the extracellular fluid without the assistance of...

membrane transport proteins

diffusion

molecules and ions dissolved in water are in constant random motion. This random motion causes a net movement of these substances from regions of high concentration to regions of lower concentration

if a concentration difference exists for a nonpolar molecule it will..

move across the membrane until the concentration is equal on both sides. At this point, movement in both directions still occurs, but there is no net movement in either direction

the concentration of all solutes in a solution determines the...

osmotic concentration of the solution

ion channels

possess a hydrated interior that spans the membrane. Ions can diffuse through the channel in either direction, depending on their relative concentration across the membrane

hydrostatic pressure

pressure that is exerted by a fluid at equilibrium at a given point within the fluid, due to the force of gravity

facilitated diffusion provides the cell with a mechanism to...

prevent the buildup of unwanted molecules within the cell or to take up needed molecules that may need to be present outside the cell in high concentrations

rate of transport: simple diffusion vs when a carrier protein is put in the picture

simple diffusion: as concentration increases, transport by simple diffusion shows a linear increase in the rate of transport -but when a carrier protein is involved, a concentration increase means that more of the carriers are bound to the transported molecule. At high enough concentrations all carriers will be occupied, and the rate of transport will be constant. This means that the carrier exhibits *saturation*

when 2 regions are separated by a membrane, what happens depends on whether the...

solutes can pass freely through that membrane

many important molecules required by the cells cannot easily cross the plasma membrane, but these molecules can enter by diffusion through...

specific channel proteins or carrier proteins embedded in the plasma membrane if there is a higher concentration of the molecule outside the cell than inside

some substances also move in response to a gradient, but do so through...

specific channels formed by proteins in the membrane

a cell in any environment can be thought of as a plasma membrane separating 2 solutions...

the cytoplasm and the extracellular fluid. The direction and extent of any diffusion of water across the plasma membrane is determined by comparing the osmotic strength of these solutions

the amount of water that enters the cell depends on

the difference in solute concentration between the cell and the extracellular fluid. this is measured as osmotic pressure

osmosis

the diffusion of water across a selectively permeable membrane (a membrane that permits the free passage of water but prevents the passage of a solute) in the absence of differences in pressure or volume, the net movement of water is from the side containing a lower concentration of solute to the side containing a higher concentration

the major barrier to crossing a biological membrane is..

the hydrophobic interior that repels polar molecules but not nonpolar molecules

turgor pressure // hydrostatic pressure

the internal pressure inside a plant cell, resulting from the osmotic intake of water, that presses its cell membrane tightly against the cell wall, making the cell rigid

solvent

the medium in which one or more solutes is dissolved

passive transport

the movement of substances across a cell's membrane without the expenditure of energy

if the membrane is strong enough, the cell reaches an equilibrium, at which...

the osmotic pressure, which tends to drive water into the cell, is exactly counterbalanced by the hydrostatic pressure, which tends to drive water back out of the cell

osmotic pressure (osmotic potential)

the potential pressure developed by a solution separated from pure water by a differentially permeable membrane. The higher the solute concentration, the greater the osmotic potential of the solution. This is the force needed to stop osmotic flow.

osmotic concentration

the property of a solution that takes into account all dissolved solutes; if two solutions with different osmotic concentrations are separated by a water-permeable membrane, water will move from the solution with lower osmotic concentration to the solution with higher osmotic concentration.

when thinking about diffusion, we need to consider...

the relative concentrations both inside and outside the cell, as well as how readily a molecule can cross the membrane

the cell membrane is considered selectively permeable because,,

these channels and carriers are usually selective for one type of molecule

RBCs keep their internal concentration of glucose low through a chemical trick...

they immediately add a phosphate group to any entering glucose molecule, converting it to a highly charged glucose phosphate that can no longer bind to the glucose transporter, and therefore cannot pass back across the membrane -the glucose transporter that assists the entry of glucose into the cell doesn't appear to from a channel in the membrane. Instead, this transmembrane protein appears to bind to a glucose molecule and then to flip its shape, dragging the glucose through the bilayer and releasing it on the inside of the plasma membrane. After it releases the glucose, the transporter reverts to its original shape and is then available to bind the next glucose molecule that comes along outside the cell.

the side of the membrane with higher solute concentrations has... what are the consequences in terms of the movement of water??

tied up more water molecules in hydration shells and thus has fewer free water molecules as a consequence of this difference, free water molecules move down their concentration gradient, toward the higher solute concentration. This net diffusion of water across a membrane toward a higher solute concentration is called osmosis

changes in membrane potential form the basis for...

transmission of signals in the nervous system and some other tissues

most solutes, including ions and sugars, are not lipid-soluble and, therefore are

unable to cross the lipid bilayer. The concentration gradient of these solutes can lead to the movement of water

this is what happens in a hypotonic solution (the cell's cytoplasm is hypertonic relative to the extracellular fluid)

water diffuses into the cell from the extracellular fluid, causing the cell to swell. -the pressure of the cytoplasm pushing out against the cell membrane, or *hydrostatic pressure*, increases

what happens to water in a hypertonic solution

water diffuses out of a cell (the cytoplasm of the cell is hypotonic, compared with the extracellular fluid). the loss of water causes the cell to shrink until the osmotic concentrations of the cytoplasm and the extracellular fluid become equal

isotonic/isosmotic

when 2 solutions have the same osmotic concentration

is transport through a carrier still a form of diffusion?

yes, and it therefore requires a concentration difference across the membrane because this process is facilitated by a carrier protein, it is often called facilitated diffusion

a voltage difference is..

an electrical potential difference across the membrane called a membrane potential -potential is the quantity determining the energy of a mass in a gravitational field

water flow in living cells is facilitated by...

aquaporins, which are specialized channels for water

the cytoplasm of a cell contains ions and molecules, such as sugars and amino acids, dissolved in water....this is called a... what is water termed? the substances dissolved in it?

aqueous solution water is termed the solvent and the substances dissolved in the water are the solutes

concentration gradient

-a difference in concentration of a substance from one location to another, often across a membrane -some ions and molecules can pass through the membrane fairly easily and do so because of this

what's an ion? what are the positive and negative ones called?

-atoms with an unequal number of protons and electrons have electric charge and are called ions -ions with a positive charge are called cations -ions with a negative charge are called anions

Turgor (maintaining osmotic balance)

-most plant cells are hypertonic to their immediate environment, containing a high concentration of solutes in their central vacuoles. The resulting internal hydrostatic pressure, known as turgor pressure, presses the plasma membrane firmly against the interior of the cell wall, making the cell rigid. -most green plants depend on turgor pressure to maintain their shape, and thus they wilt when they lack sufficient water

Isosmotic Regulation (maintaining osmotic balance)

-some organisms that live in the ocean adjust to their internal concentration of solutes to match that of the surrounding seawater. Because they are isosmotic with respect to their environment, no net flow of water occurs into or out of these cells -many terrestrial animals solve this problem in a similar way, by circulating a fluid through their bodies that bathes cells in an isotonic solution -for example, the blood in a human body contains a high concentration of the protein albumin, which elevates the solute concentration of the blood to match that of the cells' cytoplasm.

Extrusion (maintaining osmotic balance)

-some single-celled eukaryotes use organelles called *contractile vacuoles* to remove water -each vacuole collects water from various parts of the cytoplasm and transports it to the central part of the vacuole, near the cell surface -the vacuole possesses a small pore that opens to the outside of the cell. By contracting rhythmically, the vacuole pumps out (excludes) through this pore the water that is continuously drawn into the cell by osmotic forces

facilitated diffusion has three essential characteristics

1. *it is specific.* A given carrier transports only certain molecules or ions 2. *it is passive.* The direction of net movement is determined by the relative concentrations of the transported substance inside and outside the cell. The direction is always from high concentration to low concentration 3. *it saturates.* If all relevant protein carriers are in use, increases in the concentration gradient do not increase the rate of movement

three conditions to determine the direction of the net movement of ions

1. their relative concentrations on either side of the membrane 2. the voltage difference across the membrane 3. (for gated channels) the state of the gate (open or closed)

more than 11 different kinds of aquaporins have been found in mammals consisting of 2 main types...

1. those that are specific for only water 2. those that allow other small hydrophilic molecules, such as glycerol or urea, to cross the membrane as well

aquaporins

a membrane channel that allows water to cross the membrane more easily than by diffusion through the membrane

carrier proteins

a membrane protein that binds to a specific molecule that cannot cross the membrane and allows passage through the membrane (kind of like an enzyme binds to a substrate). Molecules bind to a carrier protein on the extracellular side of the cell and pass through the plasma membrane via a conformational change in the carrier protein

hypertonic/hyperosmotic

a solution with a higher concentration of solutes than the cell. A cell in a hypertonic solution tends to lose water by osmosis

hypotonic/hypoosmotic

a solution with lower concentration of solutes than the cell. A cell in a hypotonic solution tend to take in water by osmosis

channel proteins

a transmembrane protein with a hydrophilic interior that provides an aqueous channel allowing polar molecules to pass when the channel is open. Usually allows passage of specific ions such as K+, Na+, or Ca2+ across the membrane

carrier proteins can help transport..

both ions and other solutes, such as some sugars and amino acids, across the membrane

diffusion of water and da solutes

both water and solutes tend to diffuse from regions of high concentration to ones of low concentration (they diffuse down their concentration gradients)

gated channels

can be opened or closed in response to a stimulus (stimulus can be either chemical or electrical)

facilitated diffusion

carrier-assisted diffusion of molecules across a cellular membrane through specific channels from a region of higher concentration to one of lower concentration; the process is driven by the concentration gradient and doesn't require cellular energy from ATP

how does carrier diffusion and simple diffusion differ?

carriers have to bind to the molecule they transport, so the relationship between concentration and rate of transport differs from that due to simple diffusion

selectively permeable

condition in which a membrane is permeable to some substances but not to others

studies on artificial membranes show that water, despite its polarity, can...

cross the membrane, but this flow is limited

when a membrane separates 2 solutions having different concentrations of solutes,

different concentrations of free water molecules exist on the two sides of the membrane

water molecules interact with dissolved solutes by...

forming hydration shells around the charged solute molecules

due to their charge, how do ions interact with polar/nonpolar molecules?

ions interact well with polar molecules such as water, but are repelled by nonpolar molecules such as the interior of a plasma membrane