Membrane Transport IPassive transport

Cell Biology for Clinical Pharmacy StudentsMD102

Module II: Cell Functions (Lecture # 6 )

Dr. Ahmed Sherif Attiaahmed.attia.e.learning@gmail.comhttps://sites.google.com/site/ahmedsattia/

By the end of this lecture you should be familiar with:

Types of membrane transporters Passive transport mechanisms:

-Simple diffusion-Facilitated diffusion

Osmosis.

Objectives

Nutrients must get in.

Products and wastes must get out.

Permeability determines what moves in and out of a cell.

The cell membrane is a barrier

A membrane is: Impermeable if it lets nothing in or out. Freely permeable if it lets anything pass. Selectively permeable if it restricts movement.

Cell membranes are selectively permeable: Allow some materials to move freely. Restrict other materials.

Selective permeability restricts materials based on:

SizeMolecular shapeLipid solubilityElectrical charge

Bases of Selective Permeability

Keep in mind: The same molecule can be transported via different transport types in different cells.

Transport through a cell membrane can be:

Passive (no energy required). Active (requiring energy).

Transport Through Cell Membranes

Uniport:

A membrane protein that mediates transport of a single solute across the bilipid membrane.

An example is the GLUT1 glucose carrier.

Types of Transporters

Bind two dissimilar solutes (substrates) and transport them together across a membrane. Transport of the two solutes is obligatorily coupled.

Cotransport (coupled transport)

Symport The two substrates are transported in the same

direction.

Antiport: The two substrates travel in opposite directions.

Passive Transport involves moving things through membranes without the expenditure of cell energy.

Passive transport in cells involves the process of diffusion.

Passive Transport

All molecules are constantly in motion:

Molecules in solution move randomly.

Random motion causes mixing.

Concentration is the amount of solute in a solvent.

Concentration gradient: More solute in one part of a solvent than

another.

Diffusion in Solution

Solutes move down a concentration gradient:

Molecules mix randomly. Solute spreads through solvent. Eliminates concentration gradient.

Diffusion

Distance the particle has to moveMolecule size Smaller is fasterTemperature More heat, faster motionGradient size The difference between high and low

concentrationElectrical forces: Opposites attract, like charges repel

Factors Affecting Diffusion Rates

Diffusion can be simple, channel, or carrier mediated.

Diffusion and the Cell Membrane

It happens when a small, non-polar molecule passes through a lipid bilayer. 

In it, a hydrophobic molecule can move into the hydrophobic region of the membrane without getting rejected.

It does not involve a protein.

It is not subject to saturation.

Simple diffusion

Facilitated DiffusionChannel & carrier mediated diffusion are kinds of facilitated diffusion. No energy is involved, so it is still a passive process.

However they are:Specific: to size, charge, & interaction with the channel or the carrier.Subject to saturation: only in carrier mediated diffusion.

Materials that move through membranes by facilitated diffusion include:Glucose, many small ions, and amino acids.

Glucose Carrier

• Glucose carrier crosses the membrane and has at least 2 conformations.

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

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, 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.

How it Works

Osmosis is the movement (diffusion) of water across a differentially permeable membrane in response to solute (dissolved substances) gradients that are maintained by the membrane.

The "force" to move water through membranes is called osmotic pressure. It is comparable to physical pressure.

Osmosis

Hypertonic The solution has a higher solute concentration (less water) than the cell so water will leave cells by osmosis, and cells may shrink.

Hypotonic The solution has a lower solute concentration (more water) than the cell so water will enter cells by osmosis, causing the cells to swell.

Isotonic Equal proportions of solutes to water on both sides of the membrane. So there is no net movement of water.

Osmotic Solutions

In an isoosmotic environment, the volume of an animal cell will remain stable with no net movement of water across the plasma membrane.

In a hyperosmotic environment, an animal cell will lose water by osmosis and crenate (shrivel).

In a hypoosmotic environment, an animal cell will gain water by osmosis, swell and perhaps lyse (cell destruction).

In a hypoosmotic environment, water moves by osmosis into the plant cell, causing it to swell until internal pressure against the cell wall equals the osmotic pressure of the cytoplasm. A dynamic equilibrium is established (water enters and leaves the cell at the same rate and the cell becomes turgid).

Turgid = Firmness or tension such as found in walled cells that are in a hypoosmotic environment where water enters the cell by osmosis. Ideal state for most plant cells. Turgid cells provide mechanical support for

plants. Requires cells to be hyperosmotic to their

environment.

Turgid

In a hyperosmotic environment, walled cells will lose water by osmosis and will plasmolyze, which is usually lethal.

Plasmolysis = Phenomenon where a walled cell shrivels and the plasma membrane pulls away from the cell wall as the cell loses water to a hypertonic environment.

• In an isoosmotic environment, there is no net movement of water into or out of the cell. Plant cells become flaccid or limp. Loss of structural support from turgor pressure causes plants to wilt.

Types of membrane transporters

Passive transport mechanisms:-Simple diffusion-facilitated diffusion

Osmosis.

Summary