Osmosis

Question 1

Osmosis - basics

Answer 1

Process of cells exchanging water with their environments

Question 2

Osmosis - definition

Answer 2

The net, random movement of free water molecules from a high water potential to a low water potential across a semi-permeable membrane until dynamic equilibrium is reached it

Question 3

Semi-permeable membrane

Answer 3

Permeable to water and to certain solutes

Question 4

Water potential (ψ)

Answer 4

• measured in kPa
• pressure exerted by free water molecules in a system under standard conditions
• takes into account different solute effects
• water potential of pure water = 0kPa (convention)
• high water potential (low negative number) = high number of FWPs
• more negative with addition of solutes

Question 5

Why does water potential fall with addition of solutes?

Answer 5

Water molecules cluster (form hydrogen bonds) around the solute molecules

Question 6

Solute potential

Answer 6

• aka solute pressure (ψs)
• contribution of solutes to the water potential of a system
• always lowers ψ; always = 0
• more negative as more solutes are added

Question 7

Pressure potential

Answer 7

• greater pressure -> less negative water potential
• ψp - always positive
• in plants: results from cell wall exerting pressure on cytoplasm

Question 8

equation linking ψ, ψs and ψp

Answer 8

ψ = ψs + ψp

Question 9

Osmosis in unicellular, aquatic organisms/ cells in bloodstream

Answer 9

• bathed in fluid which has the same water potential as cytosol
• sea water is isotonic
• no change in volume in cell; no net osmosis

Question 10

Animal cell osmosis in fresh water

Answer 10

• less solutes - high ψ
• hypotonic to the cytosol -> osmosis into cell
• cell volume increases
• if a cell cannot eliminate excess water (e.g. RBC), it lyses
• if natural environment, it will have a mechanism for eliminating excess water to keep cell volume constant -> e.g. contractile vacuole in amoebae

Question 11

Animal cell osmosis in very salty water

Answer 11

• hypertonic to cytosol -> osmosis out
• cell volume decreases
• can cause creation -> cannot complète chemical reactions due to lack of water (solvent and metabolite)

Question 12

Plant cell osmosis in hypotonic solutions

Answer 12

• into cell - fills vacuole to capacity
• cell surface membrane pushes against cell wall -> turgid
• no danger of lysis because cell wall exerts pressure on cell membrane, restricting water inflow

Question 13

Plant cell osmosis in hypertonic solutions

Answer 13

• water moves out of cell
• vacuole shrinks
• cell membrane pulls away from cell wall (plasmolysed; flaccid cell)

Question 14

Flaccid cell

Answer 14

• no net pressure against or away from cell membrane and cells all
• low turgour pressure: plant loses stability

Question 15

When turgid, ψp is

Answer 15

Positive

Question 16

When flaccid, ψp is

Answer 16

Zero

Question 17

Isotonic

Answer 17

• same strength
• equal water potential as cytosol
• no net movement of water

Question 18

Hypotonic

Answer 18

• under strength
• more watery
• less negative water potential than cytosol
• water moves into cell

Question 19

Hypertonic

Answer 19

• over strength
• less watery
• more negative water potential than cytosol
• water moves out of cell