TRANSPORT ACROSS PLASMA MEMBRANE

Membrane Function

Membranes organize the chemical activities of cells.

The outer plasma membrane – forms a boundary between a living cell and its surroundings – Exhibits selective permeability

Controls traffic of molecules in and out

Internal membranes provide structural order for metabolism

Form the cell’s organelles

Compartmentalize chemical reactions

Role of flippase and floppases

The barrier to rapid spontaneous flip-flop contributes to the maintenance of lipid asymmetry

To the maintenance and dissipation of transbilayer lipid asymmetry are proteins that catalyze the movement of lipids across the membrane.

These activities are classified according

1.substrate specificity

2.requirements for energy and direction of transport.

1.Flippase——————— A P-type ATPase, transports glycerophospholipid from the outer monolayer to the cytoplasmic surface of the plasma membrane.

2. A second ATP-dependent activity, catalyzed by floppases, ABC transport protein, transports lipids in the opposite direction.

3. The plasma membrane also contains phospholipid translocator termed scramblase, which ensures that both monolayers remain equally populated with phospholipids.

4. It moves phospholipid non-specifically in either direction, along the concentration gradient. It does not require ATP but is activated by calcium.

Motion of lipid molecules Lipid bilayer is not rigid and static structure.

In a lipid bilayer, lipid molecules can rotate freely around their long axis (rotational motion) and diffuse laterally (lateral motion) within each leaflet.

The ability of lipids to diffuse laterally in a bilayer indicates that it can act as a fluid. The degree of bilayer fluidity depends on the lipid composition

The transition of a lipid molecule from one leaflet to the other is called transverse diffusion (flip-flop).

Motion to occur, the lipid head group, which is polar and may be charged, must move into the hydrophobic interior of the bilayer.

The half time for phospholipids depends on the nature of the lipid and the membrane.

Flip-flop ——–depends on the nature of the lipid and the membrane rates are also dependent on phospholipidacyl chain length and degree of unsaturation.

Fluidity of a lipid bilayer

The fluidity of the lipid bilayer, the optimal value of which -depends on lipid composition and temperature.

In general, an increase fluidity. Lowering of temperature causes a freely-flowing, low-v rigid, gel-like organization.

This change is described as phase Transition temperature. happens is called the transition temperature.

for normal cell growth and function, temperature. In general, an increase in the temperature tends to elevate membrane

Fluidity of lipid bilayer

1.  depends on chain length and the degree of unsaturation of its component fatty residues.

2. Lipids with short or unsaturated fatty acyl chains undergo the phase transition at lower temperatures the lipids with long or saturated chains.

3. Short chains have less surface area with which to form vander Waals interaction with each other.

4. A shorter chain length reduces the tendency of the hydrocarbon tails to interact with one another.

Transport across plasma membrane

The internal composition of the cell is maintained because the plasma membrane is a selectively permeable structure.

The plasma membrane forms a barrier that blocks the free exchange of molecules between the cytoplasm” and the external environment of the cell.

Most biological molecules are unable to diffuse through the protein-free phospholipid bilayer for synthetic lipid bilayer.

Small non-polar molecules, such as o, and CO,, readily dissolve in synthetic lipid bilayers and, therefore, diffuse rapidly across them. Small uncharged polar molecules, such as water or urea, also diffuse across a synthetic lipid bilayer. By contrast, synthetic lipid bilayers are highly impermeable for charged molecules

Transport of Substances Across the Plasma Membrane (PM)

1.Passive Transport – (Simple)

Diffusion (5.3)

Facilitated diffusion (5.6)

Osmosis (5.4, 5.5)

2. Active Transport (5.8)

2. Bulk Flow (5.9) – Endocytosis – Exocytosis

Passive Transport

In passive transport substances cross the membrane by diffusion – Diffusion – net movement of substances from an area of high concentration to low concentration

No energy required

Mechanism of transport

Passive transport occurs

1.concentration gradient

2.without the use of metabolic energy substance carries a net charge

3. its movement is influenced by both its concentration gradient potential, the electric potential across the membrane.

4.Thus, a simply down its concentration gradient but, more exactly, down its electrochemical gradient.

The combination of these two forces, called electrochemical  gradient determines the direction of transport of a charged molecule across a membrane.

Types of passive

1.Simple diffusion: A molecule simply dissolves in the phospholipid bilayer. No membrane proteins are involved and the direction of transport is determined a relative concentrations of the molecule inside and outside of the cell. The relative diffusion rate of an across a pure phospholipid bilayer is proportional to its concentration gradient across the layer hydrophobicity and size. The hydrophobicity of a substance is measured by its partition coefficient equilibrium constant for partition of the molecule between oil and water.

            The higher the substance coefficient—— the more lipid soluble it is.

            Movement of solutes by diffusion is always from a higher to a low concentration,

The rate is described by Fick’s Law of Diffusion.

2 Facilitated diffusion:

1.facilitated diffusion involves the movement of solutes along the concentrations gradient.

2. the passage is mediated by transport protein (carriers and channels and selective in nature.

3.Facilitated diffusion may be carrier proteins or channel proteins mediated.

4.The rate of  transport of the molecule across the membrane is far greater in facilitated diffusion as compared to simple diffusion.

5.Facilitated diffusion allows polar and charged molecules, such as carbohydrates, amino acid,  nucleosides and ions to cross the plasma membrane.

Classes of protein that mediated facillated diffusion

1.T he Carrier proteins mediated Carrier proteins (called transporters or permeases) 

●

Non-covalently bind specific molecules to be transported one side of the membrane.

Undergo conformational changes  that allow the molecule to pass through membrane and be released on the other side.

Example is the movement of glucose mediated  (GLUT  glucose transporter )

various isoforms of the glucose transporter (GLUT) family.

Composed of ~500 amino acids and possess 12 transmembrane segments

2. Channel proteins mediated

Channel proteins form open pores through the membrane, allowing the free diffusion of any molecules of the appropriate size and charge

Rate of diffusion mediated by channel protein is higher than those mediated by carriers.

Channels typically show less stereospecificity than carriers and are usually non-saturable.

Ion channel——Channel proteins concerned specifically with inorganic ions transport organic ions transport are called ion channels.

Ion channels are highly selective.

Most of the ion channels are not permanently open.

 1. voltage gated channels————- open in response to change in electric potential.

2.  ligand gated channels—————open in response to the binding of ligands.

The properties of ion channels can be studied by means of the patch clamp technique.

Erwin Neher and Bert Sakmann, were awarded the Nobel Prize in 1991.

Aquaporins

Specialized water channel in  plasma membrane

Family of transport proteins that allow

   -water

    -Glycerol

Aquaporins (AQP) assemble as homotetramer  consists of six membrane-spanning a-helical domains with cytoplasmically oriented an  amino acid  and Carboxyl terminal.

Each monomer functions as an independent pore.

Two hydrophobic loops contain conserved Asparagine -proline-alanine (NPA) motif for selectivity of the channel.

Ionophores

Small hydrophobic lipid-soluble molecules that dissolve in lipid bilayers and increase their permeability  to specific inorganic ions.

Ionophores shield the charge of the ion to be transported, enabling it to penetrate the hydrophobic interior of the lipid bilayer.

                There are two classes of ionophores

1.Mobile ion carriers

2.Channel formers.

Yalinomycin (isolated from Streptomyces fulvissimus) a mobile ion carrier.

It transports K+ down its electrochemical gradient.

Momensin acts as a carrier for Na+  ions.

Gramicidin A (produced by the bacterium Bacillus brevis)

      – 15-residue peptide with alternating D- and L-amino acids.

       – In membranes it forms a channel.

        -The channel permits the passage of water and univalent cations, but not anions.

Active transport

Occurs against the concentration gradient and is mediated by carrier proteins

Metabolic energy used to move ions or molecules against a concentration gradient.

Active transport results in the accumulate solute on one side of membrane.

Selective Passive

Active Occurs along the concentration gradient

Occurs against the concentration gradient

Transport protein involved

Saturable

Entropy decreases

Active transport is of two types

1.Primary active transport —-Directly coupled with metabolic energy

                                                          ATP hydrolysis

        Absorption of light  by carrier protein (In Holobacteria)

1.Secondary active transport- Indirect

Transport ATPase

Transport ATPase are the ATP powered pumps, which transport ions and various small molecules against  concentration gradients.

Four major types of ATPases  P- V-, F-ATPases and ABC transporter

P-ATPases have a simple polypeptide composition and are phosphorylated during catalysis.

Example– the Na+ K+-ATPase of the plasma membrane of animal cells

The H+-ATPase of the plasma membranes of fungi and plants

The Ca2+-ATPase of the sarcoplasmic reticulum.

The plasma membrane Na+ K+ ATPase and HT-ATPase function to generate and maintain the plasma membrane electrical potential and maintain the plasma membrane electrical potential difference (inside negative).

‘V-ATPases couple ATP hydrolysis to transport of protons against a concentration to transport of protons against a concentration gradient.

These transport ATPase are found on the vacuolar membranes (from which the ‘V’ is derived in plant cells

Endosomal and lysosmal membranes in animal cells

Plasma membrane or osteoclasts

V of osteoclasts, V-class pumps generally function to maintain the cytosolic to the exoplasmic face of the membrane against a proton electrochemical gradient

vF-ATPases (also known as F,-F, ATPase) have a complex polypeptide component known V and F-ATPases transport only protons, in a process that does not involve complex polypeptide composition very similar to V-ATPases.

Does not involve phosphorylation during catalysis.

ABC transporter

ATP dependence

ATP Binding Cassette) transporters constitute a large family of ATP dependent (ATP Binding Cassette) transporters wide range of molecules, including ions, sugars, and amino acids.

Most ABC transporters are plasma membrane, but sometimes are also found in membrane of ER, mitochondrid and lysosome. AR utilize energy derived from ATP hydrolysis to transport molecules.