Lab 3, Membranes, Diffusion, and Osmosis

What is plasmolysis?

A loss of water to the outside environment

What is a selectively permeable membrane?

A membrane that blocks, slows, or accelerates the passage of specific substances. It regulates the passage of substances.

A net movement of water occurs from the side with the ______ solute to the side with the ______ solute. This diffusion of water through a membrane separating solutions of differing free water concentration is called...

A net movement of water occurs from the side with the *least* solute to the side with the *most* solute. This diffusion of water through a membrane separating solutions of differing free water concentration is called *osmosis*

Define plasmolysis

A phenomenon in walled cells where the cytoplasm shrivels and the plasma membrane pulls away from the cell wall. Occurs when the cell loses water to a hypertonic environment.

Define hypertonic

A solution where the solutes are *more concentrated outside* the cell. It *loses water*. Hyper: outside > inside ; more concentrated solutes outside ; water moves out of cell

According to Fick's Law, substances with _________ molecular weights diffuse slower than those with ________ molecular weights.

According to Fick's Law, substances with *large* molecular weights diffuse slower than those with *small* molecular wegiths.

What does the placement of the white percipitate in the agar water gel solution have to do with molecule size?

According to Fick's law, substances with large molecular weights diffuse slower than molecules with small molecular weights. Because the white precipitate was closer to the barium chloride hole in the agar gel, we can determine that hydrogen sulfate is a smaller and faster molecule than barium chloride.

Why are membranes described as being selectivley permeable?

Because they block, slow, or accelerate the passage of specific substances

Why do polar and ionic molecules dissolve in water?

Because water molecules form hydrogen shells around the charged particles

Why did we use gel in observing diffusion rather than water?

Becuase the cytoplasm of a cell is more gel-like than aqueous

How do central vacuoles work?

Canals collect excess water from the cytoplasm and convey it to the vesicle. Once it reaches a certain size, the CV collapses by expelling water through a pore on the cell's surface. The pore then closes, and the cycle restarts

Define active transport

Certain proteins in the cell membrane bind with the substances to be transported, and use metabolic energy to drive the 'pumping' of an ion or molecule into or out of a cell

What is the reverse process of plasmolysis?

Cytolysis, which can occur if the cell is in a *hypotonic* solution resulting in a lower external osmotic pressure and a net *flow of water into* the cell.

If ____________ is present after performing the Benedict's test, then the solution will turn _______________; if it is not present after performing the test, then the solution will turn ________.

If *glucose* is present after performing the Benedict's test, then the solution will turn *green, yellow, orange, or red*; if it is not present after performing the test, then the solution will turn *blue*.

If I add a drop of water near the edge of my coverslip containing red blood cells, then I am making the environment hypertonic, or hypotonic?

If I add a drop of water near the edge of my coverslip containing red blood cells, then I am making the environment *hypotonic* because hypotonic solutions are ones where the total solute concentration is greater inside the cell than outside the cell.

If there is a greater concentration of solutes, then water will flow __________ the cell, and the solution is ____________

If there is a greater concentration of solutes, then water will flow *out of* the cell, and the solution is *hypertonic*

If there is a lesser concentration of solutes, then water will flow __________ the cell, and the solution is ____________

If there is a lesser concentration of solutes, then water will flow *into* the cell, and the solution is *hypotonic*

Paramecium live in fresh water. It's environment is _________ to its cytoplasm, so the organism continually (gains/looses) water.

Paramecium live in fresh water. Its environment is *hypotonic* to its cytoplasm, so the organism continually *gains* water.

What is a plasmolysis?

Plasmolysis is the process in which *cells lose water* in a *hypertonic* solution.

What is the difference between hypertonic, isotonic, and hypotonic?

*Isotonic* concentration outside = concentration inside *Hypertonic* concentration outsde > concentration inside *Hypotonic* concentration outside < concentration inside

Define cell membrane

- composed of two layers of phospholipids - selective hydrophobic barriers - allow small, uncharged molecules to pass through - excludes all other types of molecules

We perform the Benedict's test to test for the presence of...

...glucose

What are the four factors the rate of diffusion is dependent on?

1. *Cross-sectional area* molecules can move through 2. *Concentration differences* between regions 3. *Temperature* (influences speed of molecular motion) 4. *Molecular mass* of involved substance

How can cells control what substances enter and leave their citoplasm?

1. By regulating what proteins are in their membranes 2. By metabolically regulating the function of those proteins

Water, whether inside or outside the cell, exists in two states:

1. Free to diffuse 2. Less mobile (because bound in hydrogen shells)

Red blood cells in an isotonic solution were ______________; in a hypotonic solution were ________________; and in a hypertonic solution were _______________.

Red blood cells in an isotonic solution were *normal*; in a hypotonic solution were *turgid*; and in a hypertonic solution were *plasmalyzed*.

Red onion cells in an isotonic solution were ______________; in a hypotonic solution were ________________; and in a hypertonic solution were _______________.

Red onion cells in an isotonic solution were *flaccid*; in a hypotonic solution were *turgid*; and in a hypertonic solution were *plasmolyzed*.

What substances can easily pass through cell walls?

Salts, water, sugars, and most other dissolved substance

Define isotonic solution

Solution where the total concentratin of solutes is *equal* on both sides of the selective membrane. It neither gains nor loses water by osmosis

Define hypotonic solution

Solution where the total solute concentration is *lower outside* the cell than inside. It *gains water* by osmosis Hypo: outside < inside ; less concentrated solutes outside ; water moves into the cell

What evidence do you have from experiment 3 to indicate that molecular weight influences the rate of diffusion?

The agar gel experiment: according to Fick's law, substances with large molecular weights diffuse slower than molecules with small molecular weights. Because the white precipitate was closer to the barium chloride hole in the agar gel, we can determine that hydrogen sulfate is a smaller and faster molecule than barium chloride.

Define osmosis

The diffusion of water through a membrane separating solutions of differing free water concentration

State Fick's Law

The rate of diffusion *increases* as concentration increases; cross-sectional area becomes larger; and temperature increases The rate of diffusion *decreases* as molecular mass of involved substances increases

Define contractile vacuole

The subcellular strucutre that removes water from the cell of a paramecium

What are transport proteins?

They are proteins embedded in, and spanning, the cell membrane, which act as selective channels through which large organic molecules and ions may diffuse. They can be directional

True or False Many cells in multicellular animals lack mechanisms to cope with osmotic challenges.

True

When would turgor pressure increase? When would it decrease?

Turgor pressure is the pressure of water pushing the plasma membrane against the cell wall - it's what makes a plant cell turgid. Therefore, turgor pressure would *increase in hypotonic solutions*, because water is being added to the cell; it would *decrease in hypertonic solutions*, because water is leaving the cell

What was the purpose of using gels and agar in our 3rd experiement?

When agar is added to water, it becomes a gel, which is similar to the cytoplasm environment of a cell. We bore two holes in the agar solution and placed a solution of hydrogen sulfate in one, and barium chloride in the other. The ions of the compounds diffused through the gel (agar + water) and chemically reacted to form a white precipitate, barium sulfate.

When deprived of water, plant cells undergo ________, allowing the cell walls to bend because they are no longer supported by...

When deprived of water, plant cells undergo *plasmolysis*, allowing the cell walls to bend because they are no longer supported by the *hydrostatic pressure of a full vacuole* in the cytoplasm. When water is available again, the vacuoles fill, and the drooping branches straighten

When the cells are in a hypotonic environment, the cytoplasm is ___________ against the cell wall

When the cells are in a hypotonic environment, the cytoplasm is *closely pressed* against the cell wall

When does plasmolysis occur?

When the cytoplasm pulls away from the cell wall due to water loss from the central vacuole in hypertonic solutions

What does Fick's Law describe?

it describes the relationship among the four factors affecting the rate of diffusion.

What regulates what passes into or out of the cytoplasm of cells?

plasma membrane

Define kinetic energy

the energy an object possesses due to its motion

Define turgor pressure

the pressure created by water pushing the plasma membrane against the cell wall of a plant cell (ie. when it's turgid)

Diffusion is molecular movement for a region of high concentration to a region of low concentration due to...

thermal motion

Hypotonic: ________________ > ________________ therefore water moves _______________

Hypotonic: *inside* > *outside* therefore water moves *in*

Diffusion and facilitated transport are ___________ processes driven by _____________________ and _________________

Diffusion and facilitated transport are *passive processes* driven by *thermal motion* and *concentration gradients*

Hypertonic: ________________ > ________________ therefore water moves _______________

Hypertonic: *outside* > *inside* therefore water moves *out*

Why are cells hypertonic to pure water?

In general, pure water will be hypotonic in comparison with any solution, because it contains no solute whatsoever.

We count the number of CV contractions to be 7 per minute. Then we put the paramecium in a slide containing 3% sodium chloride and notice a rate change. What does this suggest?

It suggests that the CV has an osmoregulatory function

What are aquaporins?

Membrane proteins that function as specific channels for the passage of water

Define diffusion

Molecular movement for a region of high concentration to a region of low concentration due to thermal motion

Do the cell walls of plants, algae, bacteria, or fungi act as barriers to diffusion?

No

What is facilitated diffusion?

Often, transport proteins are embedded in, and span, the cell membrane. The transport proteins act as selective channels through which large organic molecules and ions may diffuse. Therefore, because diffusion (or passage) is assisted by a protein (transport protein), the process is called facilitated diffusion.

What is osmotic pressure?

aka Turgor Pressure With increasing pressure, the kinetic energy of the free water molecules in the cytoplasm increases so that every molecule of water osmotically entering a walled cell is matched by one leaving as a result of increased molecular motion. The pressure at which this occurs is called the *osmotic pressure*

The solution with the ____________ solute concentration has the ___________ amount of free water. a) greatest; greatest b) greatest; least c) least; least

b) greatest; least The solution with the *greatest* solute concentration has the *least* amount of free water.

Plasmolysis occurs when the cell _______ water to a ____________ environment. a) gains; hypertonic b) gains; hypotonic c) loses; hypertonic d) loses; hypotonic

c) loses; hypertonic Plasmolysis occurs when the cell *loses* water to a *hypertonic* environment.

What was the dialysis tubing we used in the osmosis lab made of?

cellulose