membrane transport and the membrane potential in lecture today: cell membrane - mechanisms of...
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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
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Cell Membrane• Separates intracellular fluid from extracellular fluid.
• Composed primarily of phospholipids and proteins.
• Proteins may serve as carriers, channels and receptors.SELECTIVLYPERMEABLE
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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
Passive
Active
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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.
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Diffusion
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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:
O2, Hormones (Steroids)
2- Small polar molecules which are uncharged such as:
CO2, alcohol, and urea
phospholipid bilayer of
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Diffusion Through Protein Channels
• Small ions can use ion channels in the membrane:
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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
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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.
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Osmotic Pressure
• What is osmotic pressure?
The force needed to prevent osmotic movement of water from one area to another
across a semipermeable membrane.
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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.
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Molarity and Molality
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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.
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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).
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Tonicity
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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
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Carrier-Mediated transport
• Unlike the simple diffusion, carrier-mediated transport shows:
1- Specificity
2- Competition
3- Saturation
Simple diffusion
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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
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Facilitated Diffusion
Conformationalchange
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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.
Conformationalchange
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a) Primary active transport:e.g Transport of Ca++ from inside to outside of the cell.
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a) Primary active transport:e.g Na/K pump.
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b) Secondary active transport (Co-transport):
The energy required is obtained from downhill transport of Na+ into cell:
ECF ICF
NaNaK K
NaNa
GlucoseGlucose
Active Transport
Na
Na
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b) Secondary active transport (Co-transport):e.g Transport of glucose in kidney.
Secondary Active Transport
Primary Active Transport
Facilitated Diffusion
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b) Secondary active transport (Co-transport):e.g Co-transport of Na+ and glucose.
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The difference in ionic distribution between inside and outside of the cellresult in electrical potential difference across the cell membrane which iscalled 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.
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Membrane Potential