Lec-3 Membrane Transport

Lecturer: Dr. Twana A. Mustafa

KEY WORDS• Solvent: (relatively large amount of a substance which is

the dissolving medium; in the body is water).• Solute: (relatively small amount of a substance which is

the dissolved substance and it dissolves in the solvent).• Solution: is a homogenous mixture of a solute in a

solvent. • Concentration: of a solvent is the amount of solute

dissolved in a specific amount of solution. • Concentration gradient: difference in the concentration

of a solute on two sides of a permeable membrane. • Equilibrium: exact balance between 2 opposing forces.• Dynamic: continuous motion or movement.

Membrane Transport Proteins

• Many molecules must move back and forth from inside and outside of the cell

• Most cannot pass through without the assistance of proteins in the membrane bilayer– Private passageways for select substances

• Each cell has membrane has a specific set of proteins depending on the cell

Membrane Permeability• Size

– the smaller the particle, the more permeable– small molecules (O2, CO2, H2O) can– large molecules (protein, DNA) cannot

• Lipid Solubility– YES: non-polar molecules (O2, cholesterol),

– NO: charged atoms/molecules (Na+, Cl-, HCO3-),

large polar molecules (glucose)

Ion Concentrations

• The maintenance of solutes on both sides of the membrane is critical to the cell– Helps to keep the cell from rupturing

• Concentration of ions on either side varies widely– Na+ and Cl- are higher outside the cell– K+ is higher inside the cell– Must balance the the number of positive and negative

charges, both inside and outside cell

Impermeable Membranes

• Ions and hydrophilic molecules cannot easily pass thru the hydrophobic membrane

• Small and hydrophobic molecules can

• Must know the list to the left

2 Major Classes

• Carrier proteins – move the solute across the membrane by binding it on one side and transporting it to the other side– Requires a conformation change

• Channel protein – small hydrophilic pores that allow for solutes to pass through– Use diffusion to move across– Also called ion channels when only ions moving

Proteins

• Channels, if open, will let solutes pass if they have the right size and charge

• Carriers require that the solute fit in the binding site–Why carriers are specific like an enzyme and its substrate

Mechanisms of Transport

• Provided that there is a pathway, molecules move from a higher to lower concentration– Doesn’t require energy– Passive transport or facilitated diffusion

• Movement against a concentration gradient requires energy (low to high)– Active transport– Requires the harnessing of some energy source by the

carrier protein• Special types of carriers

Passive Membrane Transport• Simple Diffusion

– movement of particles along a concentration gradient

• Osmosis – diffusion of water across a semi-permeable membrane

• Facilitated Diffusion – movement of particles along a concentration gradient

through a carrier protein

Passive vs Active Transport

Carrier Proteins

• Required for almost all small organic molecules– Exception – fat-soluble molecules and small uncharged

molecules that can pass by simple diffusion

• Usually only carry one type of molecule

• Carriers can also be in other membranes of the cell such as the mitochondria

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Categories of Transport Across the Plasma Membrane

Cell membrane is selectively permeable to some molecules and ions.

Mechanisms to transport molecules and ions through the cell membrane:

1. Passive transport: Non-carrier mediated transport.

Simple Diffusion. Facilitated Diffusion:

Via CarrierChannel

Voltage, Chemical and Mechanical gating channel

2. Active Transport

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Categories of Transport Across the Plasma Membrane

May also be categorized by their energy requirements:Passive transport:

Net movement down a concentration gradientdoes not need ATP

Active transport:Net movement against a concentration

gradientneeds ATP

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1. Simple DiffusionDiffusion is:1. Passive.2. Requires a concentration gradient.3. Occurs until a dynamic equilibrium is reached.4. Rapid over short distance, slow over long distance.5. Increased at increased temperature.6. Inversely related to molecular size, as molecular size increases the resistance. 7. Can occur in an open system or across a membrane.

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Simple Diffusion Through Plasma Membrane

Cell membrane is permeable to:– Non-polar molecules (O2). – Lipid soluble molecules (steroids). – Small polar covalent bonds (CO2).

– H2O (small size, lack charge).

Cell membrane impermeable to:– Large polar molecules (glucose).– Charged inorganic ions (Na+).

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Rate of DiffusionSpeed at which diffusion occurs.

Dependent upon:The magnitude of concentration gradient.

Driving force of diffusion.Permeability of the membrane.

Neuronal plasma membrane 20 x more permeable to K+ than Na+.Temperature.

Higher temperature, faster diffusion rate.Surface area of the membrane.

Microvilli increase surface area.

Fick’s Law of Diffusion:

Simple Diffusion thru gated channels

• Protein channels are present all the way from the ECF to the ICF, thus substances can move by simple diffusion directly along these channels from one side of the membrane to the other. These channels are distinguished by 2 important features:

1.Selective permeability of the channel2.Presence of gates

Gated channels in Simple Diffusion:

Sodium Channels: •0.3 by 0.5 nm in diameter•Negatively charged on the inside•Because of the negative charges they pull the positively charged sodium ion inside, away from the water molecule. Potassium channel:•0.3 by 0.3 nm in diameter•No negative charge on the inside•Pull the hydrated K ion inside. As no negative charge on the inside of the channel, no attractive forces for the Na ion… also, Na ions hydrated form is far too big….

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2 Facilitated Diffusion

Definition: the diffusion of lipid insoluble or water

soluble substance across the membrane down their concentration gradients by aid of

membrane proteins (carrier or channel)

Substances: K+, Na+, Ca2+, glucose, amino acid, urea etc.

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2. Facilitated Diffusion

1- Facilitated diffusion via carrier2- Facilitated diffusion through channel

2.1 Voltage-gated ion channel2.2 Chemically-gated ion channel2.3 Mechanically-gated ion channel2.4 Water channel

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2.1 Facilitated Diffusion via carrier

Concept: Diffusion carried out by carrier protein

Substance: glucose, amino acid

Passive Transport by Glucose Carrier

• Glucose carrier consists of a protein chain that crosses the membrane about 12 times and has at least 2 conformations – switch back and forth

• One conformation exposes the binding site to the outside of the cell and the other to the inside of the cell

How it Works

• Glucose is high outside the cell so the conformation is open to take in glucose and move it to the cytosol where the concentration is low

• When glucose levels are low in the blood, glucagon (hormone) triggers the breakdown of glycogen (e.g., from the liver), glucose levels are high in the cell and then the conformation moves the glucose out of the cell to the blood stream

• Glucose moves according to the concentration gradient across the membrane

• Can move only D-glucose, not mirror image L-glucose

Calcium Pumps

• Moves Ca2+ back into the sarcoplasmic reticulum (modified ER) in skeletal muscle

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Facilitated Diffusion via CarrierCharacteristics of carrier mediated

diffusion:1. Down concentration Gradient2. Chemical Specificity:

Carrier interact with specific molecule only.

3. Competitive inhibition:Molecules with similar chemical

structures compete for carrier site.4. Saturation:

Vmax (transport maximum):Carrier sites have become

saturated.

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2.2 Facilitated diffusion through channels

Definition

Some transport proteins

have watery spaces all the way through the molecule

allow free movement of certain ions or molecules. They are called channel proteins.

Diffusion carried out by protein channel is termed channel mediated diffusion.

3 Types of Channels• Voltage-gated channels – controlled by membrane potential• Ligand-gated channels – controlled by binding of a ligand to a

membrane protein (either on the outside or the inside)• Stress activated channel – controlled by mechanical force on the

cell

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Voltage-gated Channel The molecular conformation of the gate responds to the

electrical potential across the cell membrane

this type of channel is called voltage gate (or dependent) channel.

Electrochemical Gradient• This gradient determines the direction of the solute during

passive transport

Ligand-Gated ChannelExample: Na+-K+ gated channel

Closed when a neurotransmitter is not bound to the extracellular receptor

Open when a neurotransmitter is attached to the receptor - Na+ enters the cell and K+ exits the cell

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Ion channel runs through receptor. Receptor has 5 polypeptide

subunits that enclose ion channel.

2 subunits contain ACh binding sites.

Channel opens when both sites bind to ACh. Permits diffusion of Na+ into

and K+ out of postsynaptic cell.

Inward flow of Na+ dominates . Produces EPSPs.

Ligand-Operated ACh Channels

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2.3 Mechanically-gated channel

Some protein channel gates may be opened by the mechanical deformation of the cell membrane. mechanically-gated channel.

It plays a very important role in the genesis of excitation of the hair cells

Auditory Hair Cells

• Stress activated• Sound waves cause the stereocilia to tilt and this causes the

channels to open and transport signal to the brain • Hair cells to auditory nerve to brain

OSMOSIS

OSMOSISDefinition: The diffusion of water down its concentration gradient (that is, an area of higher water concentration to an area of lower water concentration) thru a semi-permeable membrane is called Osmosis. Concept: Because solutions are always referred to in terms of concentration of solute, water moves by osmosis to the area of higher solute concentration. Despite the impression that the solutes are “pulling,” or attracting, water, osmosis is nothing more than diffusion of water down its own concentration gradient across the membrane.

Osmotic pressure: is the pressure that is required to stop osmosis. It is the pressure necessary to prevent osmosis into a given solution when the solution is separated from the pure solvent by a semipermeable membrane. The greater the solute conc. of a solution, the greater its osmotic pressure. (HYDROSTATIC PRESSURE = OSMOTIC PRESSURE)

An osmole is one mole of dissolved particles in a solution. E.g. glucose when dissolved in solution does not dissociate, so 1 mole of glucose is also 1 osmole of glucose. On the other hand, NaCl dissociates into 2 ions (Na and Cl) so is taken as 2 moles. Osmolarity is the number of osmoles of solute per liter of solution. Simply put, osmolarity is a measure of total solute conc. given in terms of number of particles of the solute in 1 liter of solution. The osmolarity of body fluids is usually expressed in milliosmoles per liter (mOsm/L). (The normal osmolarity of body fluid is 300 mOsm.) It is usually employed in clinical settings.

Osmolality is the number of milliosmoles of solute per kg of solvent. It is usually calculated in laboratories using an osmometer.

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Isotonic Solution

NO NET MOVEMENT OF

H2O (equal amounts entering

& leaving)

Hypotonic Solution

CYTOLYSIS

Hypertonic Solution

PLASMOLYSIS

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Cytolysis & PlasmolysisCytolysis & Plasmolysis

Cytolysis Plasmolysis

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What Happens to Blood Cells?

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