1: Osmosis

Question 1

Are plasma membranes usually freely permeable to water?

Answer 1

yes

Question 2

Why is the passive movement of water across membranes (ie. osmosis) different from diffusion?

Answer 2

because direction of movement of water depends on concentration of solute, rather than the concentration of water molecules

Question 3

What is a solvent?

Answer 3

a liquid* in which particles dissolve

*true for osmosis, but a solvent is not always a liquid

Question 4

What are dissolved particles in liquid called?

Answer 4

solutes

Question 5

What does the direction in which water moves depend on?

Answer 5

depends on the solute, not the concentration of water molecules. It is therefore called osmosis, rather than diffusion.

Question 6

Define ‘osmosis’.

Answer 6

is the passive movement of water molecules from a region of lower solute concentration to a region of higher solute concentration, across a partially permeable membrane

Question 7

Why does water move to regions with a higher solute concentration?

Answer 7

attractions between solute particles and water molecules

Question 8

What does the osmolarity of a solution tell us?

Answer 8

the number of moles of solute particles per unit volume of solution

Question 9

What is the osmolarity of pure water?

Answer 9

0 (moles/unit volume)

Question 10

What does a higher osmolarity tell us?

Answer 10

the solution has a higher concentration of solutes

Question 11

What will happen if two solutions at equal pressure but different osmolarity are separated by a partially permeable membrane?

Answer 11

water will move by osmosis from the solution with the lower osmolarity to the solution of higher osmolarity

Question 12

When do plant cells absorb water from their surroundings? What is the name for the conditions for this to happen?

Answer 12

• gain: if their osmolarity is higher than that of the solution
• hypotonic: “surrounding solution has lower osmolarity than that of plant cell”

Question 13

When do plant cells lose water? What is the name for the conditions for this to happen?

Answer 13

• lose: if their osmolarity is lower than that of the solution
• hypertonic: “surrounding solution has higher osmolarity than that of plant cell”

Question 14

What is the principle of osmolarity? What can this principle be used to estimate?

Answer 14

• assuming pressure equal: water will move by osmosis from the solution with the lower osmolarity to the solution of higher osmolarity
• used to estimate the osmolarity of a type of plant tissue, e.g potato

Question 15

Describe the method for estimating the osmolarity of plant tissue. (look at p11)

Answer 15

1. prepare a series of solutions with a suitable range of solute concentrations, such as 0.0, 0.1, 0.2, 0.3, 0.4, and 0.5 moles/litre
2. Cut the plant tissue into samples of equal size and shape
3. Find the mass of each sample using an electronic balance
4. Bathe tissue samples in each of the range of solutions for long enough to get measurable mass changes, usually between 10 and 60 mins.
5. Remove the tissue samples from the bathing solutions, dry them and find their mass again
6. Calculate % mass change using the formula (another card).
7. Plot results on a graph.
8. Read off the solute concentration which would give no mass change.

Note

• osmolarity of a glucose solution is equal to its molarity because glucose remains as a single molecules when it dissolves
• osmolarity of a sodium chloride solution is double its molarity because one mole of NaCl gives two moles of ions when it dissolves - one mole of Na+ and one mole of Cl- .

Question 16

Quote the formula for calculating the percentage mass change of tissue samples in an osmolarity experiment.

Answer 16

% change = (final mass - initial mass)/ initial mass x100%

Question 17

Practice reading osmolarity results from a graph.

Answer 17

-

Question 18

How accurate can estimates from osmolarity experiments be?

Answer 18

only as accurate as the quantitative measurements

Question 19

The estimates from osmolarity experiments are only as accurate as the quantitative measurements. Which measurements is it important to get right? How should these quantities be measured?

Answer 19

• volume of water used for making solutions should be measured with a volumetric flask
• initial and final mass of tissue samples should be measured with the same electronic balance that is accurate to 0.01g (10mg)

Question 20

What can gaining/losing too much water in human tissue cause organs to do?

Answer 20

• gaining too much: swell up and burst

- losing too much: shrink

Question 21

How can shrinking/swelling of human tissue be prevented in organ donations?

Answer 21

(fyi organ is outside body, body not alive)

• organ must be bathed in a solution with the same osmolarity as human cytoplasm

Question 22

In organ donations, an organ must be bathed in a solution with the same osmolarity as human cytoplasm otherwise it will shrink/swell. What is the solution?

Answer 22

solution of salts called isotonic saline used for some procedures

Question 23

What are donor organs surrounded by when they are being transported?

Answer 23

isotonic slush, with low temperatures helping to keep them in a healthy state