Membrane Transport I Passive transport Cell Biology for Clinical Pharmacy Students MD102 Module II: Cell Functions (Lecture # 6 ) Membrane Transport I Passive transport Dr. Ahmed Sherif Attia ahmed.attia.e.learning@gmail.com https://sites.google.com/site/ahmedsattia/

Objectives By the end of this lecture you should be familiar with: Types of membrane transporters Passive transport mechanisms: -Simple diffusion -Facilitated diffusion Osmosis.

The cell membrane is a barrier Nutrients must get in. Products and wastes must get out. Permeability determines what moves in and out of a cell.

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.

Bases of Selective Permeability Selective permeability restricts materials based on: Size Molecular shape Lipid solubility Electrical charge

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

Transport Through Cell Membranes Transport through a cell membrane can be: Passive (no energy required). Active (requiring energy).

Types of Transporters Uniport: A membrane protein that mediates transport of a single solute across the bilipid membrane. An example is the GLUT1 glucose carrier.

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

Symport The two substrates are transported in the same direction.

Antiport: The two substrates travel in opposite directions.

Passive Transport Passive Transport involves moving things through membranes without the expenditure of cell energy. Passive transport in cells involves the process of diffusion.

Diffusion in Solution Molecules in solution move randomly. 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 Molecules mix randomly. Solute spreads through solvent. Solutes move down a concentration gradient: Molecules mix randomly. Solute spreads through solvent. Eliminates concentration gradient.

Factors Affecting Diffusion Rates Distance the particle has to move Molecule size Smaller is faster Temperature More heat, faster motion Gradient size The difference between high and low concentration Electrical forces: Opposites attract, like charges repel

Diffusion and the Cell Membrane Diffusion can be simple, channel, or carrier mediated.

Simple diffusion 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.

Facilitated Diffusion Channel & 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.

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, 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.

Osmosis 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.

Osmotic Solutions 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.

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 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.

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.

Summary Types of membrane transporters Passive transport mechanisms: -Simple diffusion -facilitated diffusion Osmosis.