bio 107 lab # 4 cell membranes, osmosis & diffusion

BIO 107Lab # 4

Cell membranes, Osmosis & Diffusion

S. Badran

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

The plasma membrane

• Is the boundary that separates the living cell from its nonliving surroundings

• Is the minimal requirement for the formation of a cell

• Was the first cell part to form during cellular evolution


Selective permeability

• The plasma membrane allows some substances to cross it but not others

Figure 7.1


The fluid mosaic model

• States that the cell membrane is a fluid structure with a “mosaic” of various proteins embedded in it

• Fluidity results from the movement of phospholipids & proteins.

There are four primary types of membrane proteins, each of which performs a different function.


The Permeability of the Lipid Bilayer

• Small, hydrophobic molecules can pass through the membrane rapidly

• Example: CO2, O2

• Polar molecules do not cross the membrane rapidly

• Example: H2O, sugars

• Charged ions DO NOT cross the lipid bi-layer on their own


Facilitated Diffusion by Transport Proteins

• Allows passage of hydrophilic substances and ions across the membrane and consist of:

– Channels with ionic amino acids for passage of ions such as Na, K and Ca

– Carriers that are specific for sugar, water and other hydrophilic molecules

• Example: glucose carrier and aquaporins


Passive transport

• Cell does not expend any extra energy for diffusion

• Driving force for diffusion:

– kinetic energy of particles diffusing

– Difference in their concentration (concentration gradient)


Diffusion

• tendency for molecules of any substance to spread out evenly into the available space

• Activity: Diffusion

Figure 7.11 A

Diffusion of one solute. The membrane has pores large enough for molecules of dye to pass through. Random movement of dye molecules will cause some to pass through the pores; this will happen more often on the side with more molecules. The dye diffuses from where it is more concentrated to where it is less concentrated (called diffusing down a concentration gradient). This leads to a dynamic equilibrium: The solute molecules continue to cross the membrane, but at equal rates in both directions.

Molecules of dye Membrane (cross section)

Net diffusion Net diffusion Equilibrium

(a)


Diffusion

• Substances diffuse down their concentration gradient, from high to low concentration

Figure 7.11 B

Diffusion of two solutes. Solutions of two different dyes are separated by a membrane that is permeable to both. Each dye diffuses down its own concen-tration gradient. There will be a net diffusion of the purple dye toward the left, even though the total soluteconcentration was initially greater onthe left side.

(b)

Net diffusion

Net diffusion

Net diffusion

Net diffusion Equilibrium

Equilibrium


Effects of Osmosis on Water Balance

• Osmosis is the diffusion of water across a semi-permeable membrane down its concentration gradient

• Based on osmosis, solutions are classified as:

– Hypotonic (hypo = less)

– Isotonic (iso = same)

– Hypertonic (hyper = more)

• The prefix refers to the SOLUTE concentration NOT the water concentration


Tonicity of a solution

• The solution with higher concentration of solutes is hypertonic.

• The solution with lower concentration of solutes is hypotonic.

– These are comparative terms.

• Tap water is hypertonic compared to distilled water but hypotonic when compared to sea water.

• Solutions with equal solute concentrations are isotonic.


Osmosis • Is affected by the concentration gradient of dissolved substances

• Activity: osmosis and water balance

Lowerconcentrationof solute (sugar)

Higherconcentrationof sugar

Same concentrationof sugar

Selectivelypermeable mem-brane: sugar mole-cules cannot passthrough pores, butwater molecules can

More free watermolecules (higher

concentration)

Water moleculescluster around sugar molecules

Fewer free watermolecules (lowerconcentration)

Water moves from an area of higher free water concentration to an area of lower free water concentration

Osmosis


Pressure due to osmosis


Cells in isotonic solutions

• The concentration of solutes is the same as it is inside the cell

• No net movement of water

• Dynamic equilibrium: water moves but at the same rate in opposite directions


Cells in hypertonic solutions

• The concentration of solutes is greater than it is inside the cell

• The cell will lose water

– Animal cells shrivel

– Plasmolysis occurs in plant cells, when the cytoplasm shrivels and detaches from cell wall


Cells in hypotonic solutions

• The concentration of solutes is less than it is inside the cell

• The cell will gain water

– Animal cells burst (or adapt by using contractile vacuoles)

– Plant cells become turgid (preferred state)


Water Balance of Cells with Walls

• Cell walls help maintain water balance

3.9 Osmosis is the diffus the passive diffusion of water


Paramecium, a protist, is hypertonic when compared to pond waterAdaptations:

• membrane is less permeable to water than usual• specialized organelle (contractile vacuole) pumps excess water out

Animal adaptations


Osmosis & Dialysis in today’s lab

• Dialysis tubing: an artificial semi-permeable membrane

– Molecules can pass if they are small enough to fit through the microscopic pores

– Dialysis: diffusion of solute (not osmosis of water)

• You will observe:

– Osmosis and dialysis using dialysis tubing

– Osmosis leading to plasmolysis of Elodea cells treated with high salt concentration


Osmosis leading to plasmolysis of Elodea cells treated with high salt concentration

Elodea cells in isotonic solution


Question 1

• A solution of 1 M glucose is separated by a selectively permeable membrane from a solution of 0.2 M fructose and 0.7 M sucrose. The membrane is not permeable to the sugar molecules. Which of the following statements is correct?

– Side A is hypotonic relative to side B.

– The net movement of water will be from side B to side A.

– The net movement of water will be from side A to side B.

– Side B is hypertonic relative to side A.

– There will be no net movementof water.


For Question 2

An artificial cell consisting of an aqueous solution enclosed in a selectively permeable membrane has just been immersed in a beaker containing a different solution. The membrane is permeable to water and to the simple sugars glucose and fructose but completely impermeable to the disaccharide sucrose.


Question 3

• Which solute(s) will exhibit a net diffusion into the cell?

– sucrose

– glucose

– fructose


Question 4

• Which solute(s) will exhibit a net diffusion out of the cell?

– sucrose

– glucose

– fructose


Question 5

• Which solution is hypertonic to the other?

– the cell contents

– the environment

bio 107 lab # 4 cell membranes, osmosis & diffusion

Documents

plasma membrane molecules

recognition proteins

receptor proteins

membrane rapidlyexample

transmembrane proteins

badran1the plasma membrane

mosaic of various proteins

pearson education