بسم الله الرحمن الرحيم
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بسم الله الرحمن الرحيم. TRANSPORT ACROSS CELL MEMBRANE. Prepared by Dr.Mohammed Sharique Ahmed Quadri Assistant prof. Physiology Al Maarefa College. Objectives . Define permeability and list factors influencing permeability .

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Transport across cell membrane

بسم الله الرحمن الرحيم


  • Prepared by

  • Dr.MohammedSharique Ahmed Quadri

  • Assistant prof. Physiology

  • Al Maarefa College



  • 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

Transport across cell membrane

Fluid Mosaic Model of the cell membrane

Polar heads love water & dissolve.

Non-polar tails hide from water.

Carbohydrate cell markers


About cell membranes continued

About Cell Membranes (continued)

  • 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!


Membrane trasport


  • 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.

Types of cellular transport




This is gonna be hard work!!



Types of Cellular Transport

  • Passive Transport

    cell doesn’t use energy

    • Diffusion

    • Facilitated Diffusion

    • Osmosis

  • Active Transport

    cell does use energy

    • Protein Pumps

    • Endocytosis

    • Exocytosis

Passive transport

Passive Transport

  • Energy is not required

  • Molecules move randomly

  • Molecules spread out from an area of high concentration to an area of low concentration.


3 types of passive transport

3 Types of Passive Transport

  • Diffusion

  • Facilitative Diffusion– diffusion with the help of transport proteins

  • Osmosis – diffusion of water

Simple diffusion

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 permeableto the diffusing substance.

Passive transport 1 diffusion

Passive Transport:1. Diffusion

  • Diffusion: randommovement 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.

Net rate of diffusion

NetRate 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

Fick’s law of diffusion

Diffusion of electrolyte

Diffusion of electrolyte

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

  • Potential difference will alter the net rate of diffusion

  • This diffusion itself generate potential diff acc membrane called a diffusion potential

Electrochemical gradient

Electrochemical Gradient

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


    1. Chemical gradient – concentration of ion

    2. Electrical gradient – charge of ion

    3. Electrochemical gradient – Both Electrical charge and concentration gradient

Transport across cell membrane

Passive Transport: 2. Facilitated Diffusion



2. Facilitated diffusion: diffusion of specific particles through transport proteins found in the membrane

  • Transport Proteins are specific – they “select” only certain molecules to cross the membrane

  • Transports larger or charged molecules

Diffusion (Lipid Bilayer)

Facilitated diffusion(Channel Protein)

Carrier-Mediated Transport

Carrier Protein

Transport across cell membrane

Passive Transport: 2. Facilitated Diffusion



Cellular Transport From a-


High Concentration

Cell Membrane



Low Concentration


Transport Protein

Through a 

Go to Section:

Diffusion through protein channels gating of these channels

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

Transport across cell membrane

Passive Transport: 3. Osmosis

  • 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.

Osmotic pressure

Osmotic Pressure

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

Effects of osmosis on life

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.

O smolarity


  • 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


Hypotonic solution

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 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)!

Isotonic solution

Isotonic Solution

Isotonic: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)

Transport across cell membrane

What type of solution are these cells in?









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