MB 7.3

What happens when a cell is immersed in an environment that is hypertonic?

(Hyper means "more" in this case referring to nonpenetrating solutes). The cell will lose water, shrivel, and probably die. This is one way an increase in the salinity (saltiness) of a lake can kill animals there; if the lake water becomes hypertonic to the animals cells, the cells might shrivel and die.

What happens when a cell is immersed in an environment that is isotonic?

If a cell without a wall, such as an animal cell, is immersed in an environment that is isotonic to the cell (iso means "same") There will be no net movement of water across the plasma membrane. Water diffuses across the membrane, but at the same rate in both directions. In an isotonic environment, the volume of an animal cell is stable.

What happens when a cell is immersed in an environment that is hypotonic? (hypo means "less")

If we place an animal cell in a solution that is hypotonic, water will enter the cell faster than it leaves, and the cell will swell and lyse (burst) like an overfilled water balloon.

Give an example of diffusion.

Imagine a synthetic membrane separating pure water from a solution of a dye in water. Diffusion would result in both solutions having equal concentrations of the dye molecules. Once that point is reached, there will be a dynamic equilibrium, with as many dye molecules crossing the membrane each second in one direction as in the other.

What does it mean for a molecule to be "diffusing down a concentration gradient?"

It means that the molecule is diffusing from where it is more concentrated to where it is less concentrated. This diffusion of molecules down its concentration gradient leads to a dynamic equilibrium. The solute molecules continue to cross the membrane, but at equal rates in both directions.

How are plant cells like animal cells in dealing with water?

Like an animal cell, the plant cell swells as water enters by osmosis. However, the relatively inelastic wall will expand only so much before it exerts a back pressure on the cell. At this point, the cell is turgid (very firm), which is the healthiest state for most plant cells. Plants that are not woody, such as most houseplants, depend for mechanical support on cells kept turgid by a surrounding hypotonic solution. If a plant's cells and their surroundings are isotonic, there is no net tendency for water to enter, and the cells become flaccid (limp).

How do you think a cell performing cellular respiration rids itself of the resulting CO2?

CO2 is a nonpolar molecule that can diffuse through the plasma membrane. As long as it diffuses away so that the concentration remains low \outside the cell, it will continue to exit the cell in this way. (This is the opposite of the case for O2 - which is a polar molecule, therefore will need the help of transport proteins to diffuse across the membrane - this is called facilitated diffusion.

Why do carrier proteins undergo a subtle change in shape?

Carrier proteins, such as the glucose transporter mentioned earlier, seem to undergo a subtle change in shape that somehow translocates the solute-binding site across the membrane. Such a change in shape may be triggered by the binding and release of the transported molecule. Like ion channels, carrier proteins involved in facilitated diffusion result in the net movement of a substance down its concentration gradient. No energy is thus required: This is passive transport.

What is the function of the "channel proteins"?

Channel proteins simply provide corridors that allow specific molecules or ions to cross the membrane. The hydrophilic pathways provided by these proteins can allow water molecules or small ions to diffuse very quickly from one side of the membrane to the other. Aquaporins, the water channel proteins, facilitate the massive amounts of diffusion that occur in plant cells and in animal cells such as red blood cells. Certain kidney cells also have a high number of aquaporins, allowing them to reclaim water from urine before it is excreted. If the kidneys did not perform this function, you would excrete about 180 L of urine per day - and have to drink an equal volume of water!

What is facilitated diffusion?

Facilitated diffusion is the passage of molecules or ions down their electrochemical gradient across a biological membrane with the assistance of specific transmembrane transport, requiring no energy expenditure.

State one of the rules of diffusion.

In the absence of other forces, a substance will diffuse from where it is more concentrated to where it is less concentrated. Put another way, any substance will diffuse down its concentration gradient, the region along which the density of a chemical substance increases or decrease (in this case, decreases). No work must be done to make this happen; diffusion is a spontaneous process, needing no input of energy. Note that each substance diffuses down its own concentration gradient, unaffected by the concentration gradients of other substances.

Is there any advantage for a plant cell that is immersed in a hypertonic environment?

In this case, there is no advantage. A plant cell like an animal cell, will lose water to its surroundings and shrink. As the plant cell shrivels, its plasma membrane pulls away from the cell. This phenomenon, called plasmolysis, causes the plant to wilt and can lead to plant death. The walled cells of bacteria and fungi also plasmolyze in hypertonic environments.

What are ion channels?

Ion channels are channel proteins that transport ions across the membrane. Many ion channels function as as gated channels, which open or close in response to a stimulus. For some gated channels, the stimulus is electrical. Other gated channels open or close when a specific substance other than the one to be transported binds to the channel. Both types of gated channels are important in the functioning of the nervous system.

What is the diffusion of a substance across a biological membrane called?

It is called passive transport because the cell does not have to expend energy to make it happen. The concentration gradient itself represents potential energy and drives diffusion. Remember however, that membranes are selectively permeable and therefore have different effects on the rates of diffusion of various molecules. In the case of water, aquaporins allow water to diffuse very rapidly across the membranes of certain cells.

What is passive transport?

It is diffusion of a substance across a membrane with no energy investment.

What is tonicity?

It is the ability of a surrounding solution to cause a cell to gain or lose water. The tonicity of a solution depends in part on its concentration of solutes that cannot cross the membrane (nonpenetrating solutes) relative to that inside the cell. If there is a higher concentration of nonpenetrating solutes in the surrounding solution, water will tend to leave the cell, (due to osmosis - which is the movement of water from an area of higher to lower free water concentration.

What is diffusion?

It is the movement of molecules of any substance so that they spread out evenly into the available space.

What is osmosis?

It is the movement of water from an area of higher to lower free water concentration. It is the diffusion of free water across a selectively permeable membrane. The movement of water across cell membranes and the balance of water between the cell and its environment are crucial to organisms.

What type of solution are most terrestrial (land dwelling) animals bathed in?

Most terrestrial (land-dwelling) animals are bathed in an extracellular fluid that is isotonic to their cells. In hypertonic or hypotonic environments, however, organisms that lack rigid cell walls must have other adaptations for osmoregulation, which is the control of solute concentrations and water balance.

How do missing or defective transport systems affect individuals with inherited disease.

One example of how missing/defective specific transport systems affect inherited disease is in the case of cystinuria, a human disease characterized by the absence of a carrier protein that transport cysteine and some other amino acids across the membranes of kidney cells. Kidney cells normally reabsorb these amino acids from the urine and return them to the blood, but an individual afflicted with cystinuria develops painful stones from amino acids that that accumulate and crystallize in the kidneys. I wonder if it possible to go into the genome and identify the gene that codes for carrier protein, and check if it is constantly function correctly. Is it possible to complete gene therapy on an individual with with this inherited disease using retroviruses, that will permanently install the "normal" gene?

Give an example of diffusion

One important example is the uptake of oxygen by a cell performing cellular respiration. Dissolved oxygen diffuses into the cell across the plasma membrane. As long as cellular respiration consumes the O2 as it enters, diffusion into the cell will continue because the concentration gradient favors movement in that direction.

How does the unicellular protist Paramecium caudatum deal with living in an environment that is hypotonic to itself? (This means that it has the issue of taking into too much water - which possibly means it will be lysed)

P. caudatum has a plasma membrane that is less permeable to water than the membranes of most other cells, but this only slows the uptake of water, which continually enters the cell. ( I believe the P. caudatum must have a decreased amount of aquaporins if any at all) The P.caudatum cell doesn't burst because it is also 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. (Well that is a damn handy mechanism - Amazing how nature has forced life to adapt.)

What type of environment are plant cells generally healthiest?

Plant cells are turgid (firm) and generally healthiest in a hypotonic environment, where the uptake of water is eventually balanced by the wall pushing back on the cell. This is considered normal.

Define isotonic

Referring to a solution that, when surrounding a cell, causes no net movement of water into or out of the cell, because there is a constant equilibrium.

Define hypertonic

Referring to a solution that, when surrounding a cell, will cause the cell to lose water. (This solution has more solutes than the cell.)

What types of cells are surrounded by walls?

The cells of plants, prokaryotes, fungi, and some protists are surrounded by walls. When such a cell is immersed in a hypotonic solution - bathed in rainwater, for example - the wall helps maintain the cell's water balance.

What is the function of the contractile vacuole of Paramecium caudatum?

The vacuole collects fluid from a system of canals in the cytoplasm. When full, the vacuole and canals contract, expelling fluid from the cell (LM).

In the supermarket, produce is often sprayed with water. Explain why this makes vegetables look crisp.

The water is hypotonic to the plant cells, so the plant cells take up the water. Thus, the cells of the vegetable remain turgid rather than plasmolyzing (when the plant cell shrivels, and its plasma membrane pulls away from the wall) and the vegetable (for example, lettuce or spinach) remains crisp and not wilted.

What is thermal energy (heat)?

Thermal energy, which is also known as heat, is a type of energy that molecules have, due to their constant motion.