Physiology 2130 Lecture Notes - Lecture 3: Digoxin, Electrochemical Gradient, Integral Membrane Protein
29 May 2018
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Module 3 – Outcomes
Make a diagram of a cell membrane, showing the two parts of the phospholipid bilayer, the positions of membrane
proteins, cholesterol, and carbohydrate moieties.
• The cell membrane separates the intracellular enviro from the extracellular
enviro
• Cell membrane is made up of proteins that form channels and pores (passage),
carbohydrates (cell recognition) and cholesterol (stability)
• The most abundant component of the cell membrane are the phospholipid
molecules
• Figure
o 1 – hydrophilic head of a phospholipid molecule
▪ Phosphate head is situated towards the external and internal environments
▪ Hydrophilic
o 2 – hydrophobic tail of the phospholipid molecule
▪ Lipid tail is situated away from the external environment and into each other
▪ Hydrophobic
o 3 – cholesterol
▪ Found inserted into the non-polar lipid layer of the membrane
▪ Helps makes the membrane impermeable to some molecules and keeps the membrane flexible over
a wider range of temperatures
o 4 – enzymes
▪ Associated protein that act as catalysts for certain reactions immediately inside or outside the
membrane
o 5 – carbohydrate molecule
▪ Can be found on inside or outside surface
▪ Form a protective layer called the glycocalyx which plays a key role in the immune response of the
cell and recognition of other cells
o 6 – membrane spanning protein
▪ Proteins that are embedded in the bilayer such that they span the entire width
▪ Act as gates or channels that control the movement of specific substances in/out of the cell
o 7 – structural associated proteins
▪ Generally attached to the inside surface of the cell membrane
▪ Structural proteins can support and strengthen the membrane and can anchor cell organelles
Discuss the permeability of the lipid bilayer.
• Selectively permeable
• Larger molecules cannot penetrate – proteins, nucleotides, etc.
• Smaller molecules and ions can penetrate
• Polar → hydrophilic, hard to penetrate
• Nonpolar → hydrophobic, can penetrate
List five functions of the membrane proteins.
• Receptors for the attachment of chemical hormones and
neurotransmitters
• Enzymes that help with chemical reactions or breakdown molecules
• Ion channels or pores that allow water-soluble substances into the cell
• Membrane-transport carriers that transport molecules across the
membrane
o Includes gated channels
• Cell-identity markers
o Like antigens or glycoproteins
o Antigens – forgiven particles that can stimulate the immune system
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List five major ways substances cross membranes.
• Endocytosis/exocytosis (pinocytosis for small molecules)
• Diffusion through the lipid bilayer (in the case of fat-soluble molecules)
• Diffusion through protein channels (in the case of water and water-soluble molecules)
• Facilitated diffusion
• Active transport
Describe the mechanism of diffusion.
• Diffusion: the movement of molecules from an area of high concentration to low concentration due to the molecules
random thermal motion
• Electric charges and diffusion
o Electrically charged molecules tend to move toward areas of opposite charges, down their electrical gradient
▪ +ve moves to -ve, and vice versa
o Charged ions can move down both their chemical and electrical gradient
▪ If the chemical and charged gradients are in opposite directions, the movement will depend on the
balance of the two gradients
▪ And will stop moving when the molecules reach electrochemical equilibrium: when the electrical
force is equal and opposite to the chemical force
• Diffusion of Lipid-Soluble Substances
o Substances that are lipid soluble can pass right through the cell membrane
▪ Ex. oxygen, carbon dioxide, fatty acids and steroid hormones
o Substances that are water soluble have a tougher time
▪ Cannot diffuse directly, but may still be able to cross through channels or pores
List four factors that affect the rate of movement of substances through protein channels.
• Size: the size of the protein channel
o Is ~0.8 nm
o This places limits on the size of molecules that can pass
o E. suga oleules ae too ig ad a’t diffuse
• Charge: the charge on the molecule
o the charge of the molecule will react with the +ve charge of the protein channel
o +ve ion will not be able to move through +ve channel
• Electrochemical gradient:
o The greater the gradient the greater the rate of movement
• Number of channels in the membrane:
o More channels that exists the more ions will diffuse across
Describe facilitated diffusion. How does it differ from simple diffusion?
• Substances that cannot diffuse through the lipid bilayer or proteins can still cross the membrane through facilitated
diffusion
o These molecules attach to specific protein carriers on the membrane and cause a change in the proteins
shape
o This change in shape results in either an opening of the protein channel or the rotation of the protein
• Facilitated vs. simple:
o Siila to siple diffusio i that it does’t euie eeg ad is poweed the oetatio gadiet of
the molecule
o Differs from simple diffusion because the rate of transport is limited by the number of available proteins
▪ Once the carriers are all occupied it becomes saturated and cannot operate any faster
▪ The speed at which the carrier can change shape is also limited
• Facilitated diffusion shows chemical specificity – a given carrier protein will interact only with a specifically shaped
molecule
• Facilitated diffusion can be competitively inhibited by molecules that are very similar in shape
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