Describe Transport Across the Membrane and it's various types ?

 Transport Across the Membrane

 Membrane Transport

1 Passive Transport 

2 Active Transport

Passive transport mechanisms:

 Diffusion

 Facilitated Diffusion

 Osmosis

 Capillary filtration

 Bulk Flow / Solvent drag

 DIFFUSION

 Diffusion is the net movement of molecules (or ions) from a region of their high concentration to a

region of their lower concentration.

Characteristic features of diffusion

 The molecules move down a concentration gradient.

 Requires no energy

 Molecules move about randomly by kinetic energy

 As a result of diffusion, molecules reach equilibrium (no net movement of molecules from either

side)

Factors Affecting Diffusion

 Cell membrane permeability

 Concentration Gradient

 Pressure gradient

 Electrical potential gradient

 Cell membrane permeability depends upon:

 Solubility of the substance in the lipid bilayer

 Molecular size of the particle

 Charge of the particle

 Charge at the pore

 Surface area

 Thickness

 Number of protein channels

 Temperature

Fick’s law of Diffusion:

 J = DA (C1-C2)/T

 J = Rate of Diffusion T = Thickness of membrane

 C1-C2 = concentration gradient D = Diffusion coefficient

 A = Cross sectional area

Types of diffusion:

1. Simple diffusion

2. Facilitated diffusion 

1. Simple Diffusion

 Diffusion without the help of a carrier protein

 Mode of Simple Diffusion

 Through Lipid Bilayer

 Through Channels

Molecules that pass through the lipid bilayer by diffusion

 Gases (oxygen, carbon dioxide)

 Water molecules (rate is slow due to polarity)

 Lipids (steroid hormones)

 Lipid soluble molecules (hydrocarbons, alcohols, some vitamins)

 Small noncharged molecules (NH3)

Molecules that pass through the Channels by diffusion

 Ions (Na+, K+, Cl-)

 Small water soluble molecules

 Water (faster rate)

Gating of Protein channels

 Types of gated channels

 1. Voltage gated channels – open when there is a change in the resting membrane potential

 e.g., Na+ channels along the nerve fiber

 2.Ligand gated channels – open when a chemical binds to the receptor which is attached

 to the channel e.g., channels attached to the acetylcholine receptors in synapse or Neuro-

 muscular junction

Mechanical gated channels – Open when there is a mechanical stretch

 e.g Channels in the smooth muscle fiber

 Drugs that block channels:

 Sodium channels – Tetrodotoxin (TTX) & Saxitoxin

 Potassium channels – TEA (Tetra Ethyl Ammonium)

 Calcium channels – Verapramil (to treat hypertension)

2. Facilitated diffusion

 Movement of solutes from high concentration to low concentration through the membrane

 with the help of a carrier protein

 Features of Facilitated Diffusion

 Occurs along the concentration gradient

 Does not require energy

 Involves carrier protein

 Carrier proteins are highly specific for molecules 

 Have saturation point. Diffusion increases with increase in concentration gradient in simple

diffusion whereas in facilitated diffusion, diffusion depends on availability of carrier proteins.

When the carrier proteins are saturated, facilitated diffusion stops

 Competitive inhibition occurs

 Mechanism of Facilitated Diffusion

 Molecule binds to carrier

 Carrier changes conformation

 Molecule released on other side

 Purely passive process- stops when concentrations are equal 

e.g., Glucose transport in to the cells with the help of a carrier protein GLUT (Glucose Transporter)

 Glucose molecule from interstitial fluid binds to GLUT

 GLUT changes its confirmation

 Glucose molecule is released in to the ICF 

OSMOSIS

The diffusion of water from an area of high concentration of water molecules (high water potential) to an

area of low concentration of water (low water potential) across a partially permeable membrane.

Osmosis occurs through water channels called “Aquaporins”

Osmotic pressure

The minimum pressure which when applied on the side of higher solute concentration prevents osmosis Colloidal osmotic pressure

 The osmotic pressure exerted by colloidal substances in the body is colloidal osmotic pressure

 The colloidal osmotic pressure of blood due to plasma proteins is called ONCOTIC PRESSURE

 Oncotic pressure or colloidal osmotic pressure of blood is normally 25 – 30 mmHg

 80% of the oncotic pressure is determined by albumin

 colloidal osmotic pressure of blood is one of the factors influencing the capillary filtration

 colloidal osmotic pressure of blood opposes filtration and thereby prevents edema

 A decrease in colloidal osmotic pressure due to hypoproteinemia increases capillary filtration

and causes development of edema. E.g., Malnutrition, Liver cirrhosis and nephrotic syndrome

Osmolarity

 Number of osmotically active substances per Liter of a solution is called osmolarity

 Normal osmolarity of body fluid is 290 milliosmoles / liter

 Osmolarity of body fluid is mainly determined by sodium, Chloride and bicarbonate

 Osmolarity is also influenced to some extent by glucose, urea, plasma proteins etc.,

Tonicity

 The effective osmotic pressure of a solution relative to the plasma is called tonicity

 Isotonic solutions:

 Solutions having same osmolarity as that of plasma ie., 290 milliosmoles/lt

 Examples of isotonic solution:

 0.9% sodium chloride solution (isotonic saline)

 5% glucose solution

 20% urea solution

 10% mannitol solution

 Significance of isotonic solution:

 Isotonic solutions (0.9% NaCl / 5% glucose / 20% urea) can be infused in case of dehydration to

 restore the body fluid volume without upsetting the osmotic equilibrium of the cells

 Hypertonic solutions:

 Fluids whose osmolarity is greater to that of plasma ie., > 290 milliosmoles/lt

 Hypotonic solutions

 Fluids whose osmolarity is lesser to that of plasma ie., < 290 milliosmoles/lt

 CAPILLARY FILTRATION

 It is a process by which a fluid is forced through a membrane due to variation in the hydrostatic

pressure

 Major route of transport between blood and interstitial space.

 Filtration is determined by difference in various pressures of blood & interstitial space.

 The normal rate of net filtration in the entire body is only about 2ml/min.

 This fluid is drained by the lymphatics from the interstitial space.

Starling’s hypothesis:

 At any capillary bed the rate of filtration is determined by forces acting across the capillary

membrane.

 Any imbalance in these forces may lead to edema (accumulation of fluid in tissue spaces) or

dehydration (fluid loss from the body)

Starling’s Forces: The forces acting on the capillary membrane and influencing the rate of capillary

 filtration are called Starling’s forces.

 The forces are:

 Capillary hydrostatic pressure – favours filtration

 Capillary osmotic pressure – oppose filtration

 Interstitial fluid hydrostatic pressure – oppose filtration

 Interstitial fluid osmotic pressure – favours filtration 

At arterial end

 Hydrostatic pressure of blood = 30 mmHg

 Oncotic pressure of blood = 28 mmHg

 Interstitial hydrostatic pressure = - 3 mmHg

 Osmotic pressure of interstitial fluid = 8 mmHg

 OUTWARD FORCES - INWARD FORCE = 30 + 8 – (-3) +28

 = 30 + 8 + 3 – 28

 = 13 mmHg (net filtration pressure)

At venous end

 Hydrostatic pressure of blood = 10 mmHg

 Oncotic pressure of blood = 28 mmHg

 Interstitial hydrostatic pressure = - 3 mmHg

 Osmotic pressure of interstitial fluid = 8 mmHg

 OUTWARD FORCES = 10 + 8 + 3 = 21 mmHg

 INWARD FORCE = 28 mmHg

 28 – 21 = 7 mmHg (net absorption pressure)

 SOLVENT DRAG

Transfer of solutes by being carried along with the water flow driven by osmotic gradients across cell

membranes.

 Example: Transfer of fluid along with its constituents across the intestinal wall & Capillary