Membrane Transport and the Membrane Potential In Lecture Today: Cell membrane - Mechanisms of transport across the cell membrane: –Diffusion, and rate of diffusion –Osmosis Molarity and Molality Osmolality Tonicity Carrier-mediated transport Membrane potential CHAPTER 6

Cell Membrane Separates intracellular fluid from extracellular fluid. Composed primarily of phospholipids and proteins. Proteins may serve as carriers, channels and receptors. SELECTIVLY PERMEABLE

Mechanisms of Transport Across Cell Membrane According to the means of transport there are two categories: 1- Non-carrier-mediated transport - Simple diffusion 2- Carrier-mediated transport - Facilitated diffusion - Active transport Passive Active

Diffusion Random motion of molecules due to their thermal energy is called diffusion. Molecules in a solution tend to reach a uniform state. For example a drop of ink in a water container spreads uniformly.

Diffusion

Diffusion Through the Cell Membrane Two major groups of molecules can pass the cell membrane by simple diffusion: 1- Molecules that can dissolve in the lipid bilayer membrane, non-polar molecules such as: O 2, Hormones (Steroids) 2- Small polar molecules which are uncharged such as: CO 2, alcohol, and urea phospholipid bilayer of

Diffusion Through Protein Channels Small ions can use ion channels in the membrane:

Rate of Diffusion Rate of diffusion = number of diffusing molecules passing through the membrane per unit time. Rate of diffusion depends on: 1- Concentration difference across the membrane. 2- Permeability of the membrane to the diffusing molecule. 3- Surface area of the membrane. 4- Molecular weight of the diffusing molecule. 5- Distance. 6- Temperature. Rate of diffusion  Concentration gradient x Surface area x Temperature MW x distance

Osmosis The net diffusion of water across the membrane is called osmosis. Osmosis can occur only if the membrane is semipermeable. Semipermeable means that the membrane must be more permeable to water than the solute dissolved in water.

Osmotic Pressure What is osmotic pressure? The force needed to prevent osmotic movement of water from one area to another across a semipermeable membrane.

Molarity and Molality Equivalent of one molecular weight (g) of a substance dissolved in water to make a total one liter solution is called a Molar solution (1 M). When equivalent of one molecular weight (g) of a substance is added to one liter (Kg) of water, this solution is called Molal solution (1 m). Molal solution is a better indication of solute to solvent ratio, therefore it is a better indicator of osmosis. However, in the body since the differences between Molal and Molar concentration of solutes is very small, Molarity is often used.

Molarity and Molality

Osmolality Total molality of substances in a solution is called osmolality (Osm). e.g A solution containing 1 m glucose and 1 m fructose has osmolality of 2 osmol/L (2 Osm). Electrolytes such as NaCl are ionized when in solution, therefore one molecule of NaCl in solution yields two ions. So 1 m of NaCl has osmolality of 2 Osm.

Tonicity Solutions that have the same total concentration of osmotically active solutes and the same osmotic pressure as plasma* are said to be isotonic. Solutions that have a lower total concentration of osmotically active solutes and a lower osmotic pressure than plasma are said to be hypotonic. Solutions that have a higher total concentration of osmotically active solutes and a higher osmotic pressure than plasma are said to be hypertonic. * In the body plasma has osmolarity of 0.28 Osm (280 mOsm).

Tonicity

Regulation of Blood Osmolarity Blood osmolarity is maintained within a narrow range and when this osmolarity changes several regulatory mechanisms come into action. Negative feedback

Carrier-Mediated transport Unlike the simple diffusion, carrier-mediated transport shows: 1- Specificity 2- Competition 3- Saturation Simple diffusion

Carrier-Mediated transport There are two major types of carrier-mediated transport: a) Facilitated diffusion: like simple diffusion facilitated diffusion is powered by thermal energy of the diffusing molecules. But the transport of molecules across the membrane is helped by a carrier protein. For example glucose is transported to the cells of the body by faciliteted difussion. the net transport is along the concentration gradient. b) Active transport: Movement of molecules against their concentration gradient which requires energy (ATP). For example movement of calcium from inside to outside of the cell. Passive Active

Facilitated Diffusion Conformational change

Active Transport a) Primary active transport: ATP is directly needed for the carrier protein in the following sequences: 1- Binding of molecule to the carrier protein 2- ATP is hydrolysed to provide energy for transport. 3- Carrier changes its shape and moves the molecule across the membrane. Conformational change

a)Primary active transport: e.g Transport of Ca ++ from inside to outside of the cell.

a)Primary active transport: e.g Na/K pump.

b) Secondary active transport (Co-transport): The energy required is obtained from downhill transport of Na + into cell: ECFICF Na K K Glucose Active Transport Na

b) Secondary active transport (Co-transport): e.g Transport of glucose in kidney. Secondary Active Transport Primary Active Transport Facilitated Diffusion

b) Secondary active transport (Co-transport): e.g Co-transport of Na + and glucose.

The difference in ionic distribution between inside and outside of the cell result in electrical potential difference across the cell membrane which is called membrane potential. Membrane potential is produced by: Membrane Potential 1- The action of Na/K pump at the cell membrane is essential for the production of membrane potential. 2- Proteins, ATP and other organic molecules in the cell are negatively charged, and can not cross the cell membrane therefore this makes inside of the cell negative.

Membrane Potential