Transport across cell membrane

Cell membrane major components

1. Proteins (55%)

2. Lipids (42%)

• Phospholipids (25%)

• Cholesterol (13%)

• Other lipids (4 %)

3. Carbohydrates (3 %)

Phospholipids present in cells

membrane spontaneously form sheet

like phospholipid bilayers

The hydrocarbon chains of the

phospholipids in each layer, or leaflet,

form a hydrophobic core.

The lipid bilayer has two important

properties. First,the hydrophobic core is an impermeable barrier that prevents the diffusion of water-soluble (hydrophilic) solutes across the membrane..

Second property of the bilayer is its stability. The bilayer structure is maintained by hydrophobic

A and P I / EssentialsA and P I / Essentials

Membrane proteins can be classified into two categories;Integral membrane proteins, also called transmembrane proteins, cross the phospholipid bilayer and are built of three segments. The cytosolic and exoplasmic domains have hydrophilic exterior surfaces that interact with the aqueous solutions on the cytosolic and exoplasmic faces of the membranePeripheral membrane proteins do not interact with the hydrophobic core of the phospholipid bilayer. Instead they are usually bound to the membrane indirectly by interactions with integral membrane proteins or directly by interactions with lipid head groups. Peripheral proteins are localized to either the cytosolic or the exoplasmic face of the plasma membrane.Glycocalyx (glycoprotein) Sugar attached to proteins and protrudes outside the cell membrane, act as receptors for some hormones as insulin or cellmarkers (antigens) It also repel negative charged molecules away from the cells.

Cell Membrane Functions1. Enclose cell components2. Plasma membrane controls movement of

molecules in and out of the cell and functions in cell-cell signaling and cell adhesion.

3. The phospholipid membranes are permeable to lipid soluble molecules as fatty acids and steroid hormones

4. Membranes are impermeable to water soluble molecules due to structure of phospholipid bilayer

5. Integral proteins form channels, for transporting water-soluble substances as glucose amino acids and electrolytes

6. Internal Peripheral proteins act as enzymes especially membrane ATPase and GTPase

7. Both types of protein may attach to sugars for cell recognition; a glycocalyx and act as receptors for some hormones

8. Site for communication between cells (cell signalling 9. Form specialized junctions that allow cell adhesion or

communication

Factors Affecting Permeability

• Size of molecules - small ones can fit through channels

• Solubility in lipids increases permeability - steroids, alcohol

• Charges - proteins in membranes have charges; opposites attract

• Presence of protein carriers

How Things Enter and Leave the Cell

Soluble Particles

How Things Enter and Leave the Cell

PhagocytosisEndocytosis

Pinocytosis

Exocytosis

Particles

How Things Enter and Leave the Cell

Soluble

a. Voltage gated channelsb. Legend gated channeles

1-Primary active transport

Passive transport Active transport

1. Simple diffusion

2. Facilitated diffusion

2-Secondary active transport

• Substances pass through the membranes according to their concentration gradient from higher concentration to lower concentration (Downhill)

Water-soluble and charged substances pass through protein channel of the membrane

• A-voltage gated channel open when there is an electric impulse. Ex Na channel

• Legend gated channel open when specific ligand bind to receptor on the gate ex. Ach. At the neuromuscular junction

• Passive transport is of two types:-

Passive transport

1-Simple diffusion: depend on electric charge, size and lipid solubility of the moleculeOxygen ,Nitrogen are lipid soluble pass easily through cell membraneCO2 is hydrophilic but because its small size it cross very easily1.Substances can cross membranes by diffusion if they can dissolve in the oily interior of the membrane (hydrophobic)2.Movement of substance from where there is more of it to where there is less of it3.Powered by random movement of molecules in a solution4.Net movement is from regions of high concentration to low.5.Diffusion of different substances do not interfere with each other (no competition).6.Net flux (amount of movement) is proportional to the concentration difference and the permeability of th membrane.7.Diffusion can occur through tight junctions or within bulk solutions.

Alcohol & Urea

Fatty Acids, Glycerol,

The rate of diffusion will be increased when there is :

1.1. ConcentrationConcentration: the difference in [ ] between two areas (the [ ] gradient) causes diffusion. The greater the greater the difference in concentration,difference in concentration, the fasterfaster the diffusion.

2.2. Molecular sizeMolecular size: smaller substances diffuse more quickly.quickly. Large molecules (such as starches and proteins) simply cannot diffuse through.

3.3. Shape of Ion/MoleculeShape of Ion/Molecule: a substance’s shape may preventprevent it from diffusing rapidly, where others may have a shape that aids their diffusion.

4.4. Viscosity of the MediumViscosity of the Medium: the higher the viscosity, the more slowlyslowly molecules can move through it.

Facilitated TransportFacilitated Transport: Some molecules are notnot normally able to pass through the lipid membrane, and need channel or channel or carriercarrier proteinsproteins to help them move across.

This does notnot require energyrequire energy when moving from [H] to [L] (with the concentration gradient).

Molecules that need help to move through the plasma membrane are either charged, charged, polar, or too largepolar, or too large..

1.Diffusion of a uncharged molecules through the membrane via a carrier protein It may be Uniport .ex glucose transport in the basolateral border of the intestinal epithelium

2.Co-transport Symport or co-transport as the transport of Na and glucose in the same direction from intestinal lumen to the cells

3.Counter transport Antiport or counter-transport the carrier protein transport two solutes through the membrane in the opposite direction

4.Proteins act as carriers or pores permit flux of substances that cannot diffuse directly through the membrane.

5.Movement is still passive i.e… from high concentration to low

6.Occurs across cell membranes only 7.Related substances can compete for the

same carrier or pore .

2-Facilitated Diffusion

If molecules are POLAR, CHARGED, or TOO LARGE they need a protein CARRIER help them across the

membrane

EXAMPLES: sugars, amino acids, ions, sugars, amino acids, ions, nucleotidesnucleotides ….

Each protein channel or protein carrier will allow only ONE TYPE OF ONE TYPE OF MOLECULEMOLECULE to pass through it.

http://nutrition.tufts.edu/img/information/matters/muscle.jpg

Facilitated Diffusion

1. Movement of a substance from a lower concentration to a higher concentration using a carrier and energy

2. Allows cells to transport materials against a concentration gradient. (uphill)

3. Moving solutes against a concentration gradient requires free energy coupling to a favorable process, such as hydrolysis of ATP, or co-transport of another solute "down" its concentration gradient

Active transport

1. Primary active transport: "uphill" solute transport directly coupled to an exergonic chemical reaction, e.g., ATP hydrolysis

2. Secondary active transport: "uphill" solute transport is coupled to "downhill" transport of a different solute whose gradient was established (is maintained) by primary active transport.

Co-transport

Co-transport Processes that utilize a favorable gradient for one compound to drive the uptake of a second compound. Sodium-glucose co-transport across apical surface of intestinal epithelial cells is one example, accumulating Glucose in cell against its concentration gradient

Glucose import from intestine made possible by Na+-K+ ATPase which generates/maintains both high Na+ concentration outside cell and charge gradient (electrical potential) that both favor Na+ import through Na+-glucose symporter. Permits epithelial cells to concentrate glucose from intestine to 30,000x

this intestinal concentration Resulting in high concentration of glucose within cell passes "down" its concentration gradient through basal surface of cell into blood via GluT2 transporter (facilitated diffusion, uniport system).

Hydrophobic molecules (oil soluble)

O2, N2

Nonpolar Benzene

Small uncharged Polar  molecules

 H2O, Urea,

glycerol, CO2

Molecules that pass through the phospholipid bilayer easily...

Large uncharged Glucose

Polar molecules  Sucrose

Ions (charged)H+ , Na+ , HCO3 , K+Ca2+ , Cl- , Mg2+

Molecules that do not pass through the phospholipid bilayer easily...

Molecules that diffuse through cell membranes easily

oxygen – Non-polar (hydrophobic or lipophilic) so diffuses very quickly.

Carbon dioxide – Polar but very small so diffuses quickly.

Water – Polar but also very small so diffuses quickly.

35

How Things Enter and Leave the Cell

PhagocytosisEndocytosis

Pinocytosis

Exocytosis

Particles

Phagocytosis - cell eating It is the intake of a solid material by a cell. Just like pinocytosis, it involves the invagination of cell membrane in the region where the macromolecule is present. The invagination traps the solid and gets pinched off to form a phagocytic vesicle (or phogosome). This is the process by which an Amoeba ingests its solid food. This is the process by which WBC engulf pathogenic bacteria.

Pinocytosis - is when the external fluid is engulfedIt is called cell drinking; occurs in the kidney, bladder

and intestine

Receptor-Mediated Endocytosis The ligand binding to specific cell surface receptor leads to a selective recruitment of ligand-receptor complexes into clathrin-coated pits. The coated pit represents a small area of the plasma membrane, which invaginates and pinch-off into vesicle in the cytosol.

Exocytosis - vesicle fuses with the

plasma membrane and then ruptures; used in hormone

and neurotransmitter

release

OsmosisOsmosis is the diffusiondiffusion of waterof water across a selectively permeable membrane driven by a difference in the concentration of solutesdifference in the concentration of solutes on the two sides of the membrane.

A selectively permeable membrane is one that allows unrestricted passage of waterwater, but not solute molecules or ions.

So it is the WATER THAT MOVESWATER THAT MOVES to create equilibrium!!!

• Osmosis requires NONO ENERGY.

• Osmosis is the net movement of WATERWATER molecules from the area of [high][high] of water to the area of [low][low] of water until it is equally equally distributed.distributed.

• Because membranes often restrict or preventrestrict or prevent the movement of some molecules, particularly large ones, the water (solventsolvent) must be the one to move.move.

•To cross the membrane, water must move through a protein protein ion channelion channel.

•In certain cellular conditions, these protein channels can be opened or closed (ie: in the kidneyskidneys, large large intestinesintestines) depending on how much water is needed by the body.

• Osmosis – Is the passive transport of water across a membrane from lower osmotic pressure to higher osmotic pressure

• Types of solution• Isotonic solution • If the concentration of solute (salt) is equal on both

sides

• Hypotonic solution - in this case there are less solute (salt) molecules outside the cell, since salt sucks, water will move into the cell.The cell will gain water and grow larger and cell may be in danger of bursting

• Hypertonic solution - there are more solute (salt) molecules outside the cell, which causes the water to be sucked in that direction.

•Oncotic pressureThe osmotic pressure produced due to

the presence of the colloidal substance (plasma proteins) in the blood.

It is also called colloidal osmotic pressure and it equals about 25 mm Hg.

•Filtration - movement of solvent and solute through a membrane via mechanical (hydrostatic) pressure; occurs at the glumrular capillary in the kidney