TRANSPORT ACROSS CELL MEMBRANE

بسم الله الرحمن الرحيم

Prepared byDr.Mohammed Sharique Ahmed Quadri

Assistant prof. PhysiologyAl Maarefa College

Objectives

• Define permeability and list factors influencing permeability.

• Basic principles of various mechanisms of transport across cell membrane (Diffusion, facilitated diffusion, secondary active transport, and primary active transport.)

• define diffusion and describe the factors that influence the rate of diffusion

• osmosis, describe the conditions required for it to occur, and explain how osmosis relates to osmolality and osmotic pressure

• Significance of these mechanisms in various parts of body & and organs

Proteins

Polar heads love water & dissolve.

Non-polar tails hide from water.

Carbohydrate cell markers

Fluid Mosaic Model of the cell

membrane

• 4. Cell membranes have pores (holes) in it• Selectively permeable: Allows some molecules

in and keeps other molecules out• The structure helps it be selective!

About Cell Membranes (continued)

Pores

• Plasma membrane is selectively permeable that means it allows some particles to pass while other can not pass.

• Things which can pass through Plasma membrane easily are:

1. Lipid soluble particles e.g. O2, CO2, fatty acid.

2. Size of the particle—less than 0.8nm.

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MEMBRANE TRASPORT

Types of Cellular Transport

• Passive Transport cell doesn’t use energy

1. Diffusion2. Facilitated Diffusion3. Osmosis

• Active Transportcell does use energy

1. Protein Pumps2. Endocytosis3. Exocytosis

high

low

This is gonna be

hard work!!

high

low

Weeee!!!

Passive Transport• Energy is not required

• Molecules move randomly• Molecules spread out from an area of high

concentration to an area of low concentration.

(HighLow)

3 Types of Passive Transport

1. Diffusion

2. Facilitative Diffusion – diffusion with the help of transport proteins

3. Osmosis – diffusion of water

Simple Diffusion(diffusus means “to spread out”)

• Molecules/ions are in constant state of random motion due to their thermal energy.

• Physical process that occurs when • Concentration difference across the membrane

• Membrane is permeable to the diffusing substance.

Passive Transport: 1. Diffusion

1. Diffusion: random movement of particles from an area of high concentration to an area of low concentration.

(High to Low)• Diffusion continues until all

molecules are evenly spaced (equilibrium is reached)-Note: molecules will still move around but stay spread out.

http://bio.winona.edu/berg/Free.htm

Net Rate of Diffusion factors affecting net rate of diffusion are• Concentration gradient( C)

• Permeability of membrane (P)

• Surface area of the membrane (A)

• Molecular wt of the substance(MW)

• Thickness of membrane( X)

Fick’s law of diffusion

XMW.C.P.A Q diff of ratenet

Diffusion of electrolyte

• If diffusing sub is an ion or electrolyte there r two consequences

1. Potential difference will alter the net rate of diffusion

2. This diffusion itself generate potential diff acc membrane called a diffusion potential

• When there is Electrical gradient (charge) and Concentration gradient (chemical) for ion it is referred ELECTROCHEMICAL GRADIENT.

• REMEMBER THREE WORDS 1. Chemical gradient – concentration of ion 2. Electrical gradient – charge of ion 3. Electrochemical gradient – Both Electrical

charge and concentration gradient

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Electrochemical Gradient

2. Facilitated diffusion: diffusion of specific particles through transport proteins found in the membrane a. Transport Proteins are

specific – they “select” only certain molecules to cross the membrane

b.Transports larger or charged molecules

Facilitated diffusion (Channel Protein)

Diffusion (Lipid

Bilayer)

Passive Transport: 2. Facilitated Diffusion

Carrier Protein

A B

Carrier-Mediated Transport

High Concentration

Low Concentration

Cell Membrane

Glucosemolecules

Proteinchannel

Passive Transport: 2. Facilitated Diffusion

Go to Section:

Transport Protein

Through a

Cellular Transport From a- High

Low

Diffusion through protein channels &gating of these channels

• Ion channels are integral proteins, tubular pathway all the way from extra cellular to intracellular fluid, substance can move by simple diffusion directly along these channels

• These channels are distinguish by two imp characteristics

-selectively permeable -many channels open n closed by gates

• 3.Osmosis: diffusion of water through a selectively permeable membrane

• Water moves from high to low concentrations

•Water moves freely through pores.•Solute (green) to large to move across.

Passive Transport: 3. Osmosis

The force that would have to be exerted to prevent osmosis.

Osmotic Pressure

Effects of Osmosis on Life

• Osmosis- diffusion of water through a selectively permeable membrane

• Water is so small and there is so much of it the cell can’t control it’s movement through the cell membrane.

Osmolarity

• Osmoles: concentration of osmotically active particles is expressed as osmoles

• Osmolarity: number of osmoles per liter of solution

• Osmolality: number of osmoles per kg of solvent osmolarity

Hypotonic Solution

Hypotonic: The solution has a lower concentration of solutes and a higher concentration of water than inside the cell. (Low solute; High water)

Result: Water moves from the solution to inside the cell): Cell Swells and bursts open (cytolysis)!

Hypertonic Solution

Hypertonic: The solution has a higher concentration of solutes and a lower concentration of water than inside the cell. (High solute; Low water)

Result: Water moves from inside the cell into the solution: Cell shrinks (Plasmolysis)!

shrinks

Isotonic SolutionIsotonic: The concentration of solutes in the solution is equal to the concentration of solutes inside the cell.

Result: Water moves equally in both directions and the cell remains same size! (Dynamic Equilibrium)

What type of solution are these cells in?

A CB

Hypertonic Isotonic Hypotonic

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References

• Human physiology by Lauralee Sherwood, fifth edition

• Text book physiology by Guyton &Hall,11th edition

• Text book of physiology by Linda .s contanzo,third edition

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