L2: Functional organization of the cell membrane and transport through the cell membrane Flashcards
What is the cell membrane?
Very thin elastic semi-permeable membrane (allowing some substances to pass through it and prevent others) that surrounds the cell.
What is the thickness of the cell membrane?
: 7-9nm (70 - 90 Ao = Angstrom = 10-10 of meter)
What is the function of the cell membrane?
«STM GRC»
- Separates the cytoplasm from ECF.
- Maintains the cell’s internal environment.
- Transports of macromolecules into and out of the cell.
- Controls distribution of ions e.g. Na, K extracellular ICF, and ECF.
- Contains receptors for hormones and transmitter substances.
- Generates transmembrane membrane potentials.
What is the structure of the cell membrane?
Lipids: 42%
Proteins: 55%
Carbohydrates: 3%
Lipids in the cell membrane
-They form the basic structure of the membrane.
Include:
- Phospholipids.
- Cholesterol.
- Glycolipids.
What are the types of proteins in the cell membrane?
- Integral or intrinsic proteins
2. Peripheral or extrinsic proteins
What is the site of integral or intrinsic proteins?
• Bind to the hydrophobic center of the lipid bilayer.
What are the types and functions of integral or intrinsic proteins?
i) Transmembrane proteins → span the entire bilayer which acts as:
➢ Channels → for the diffusion of small ions
➢ Carriers transport substances e.g. glucose
➢ Pumps actively transport ions
➢ Receptors initiate
intracellular reactions when activated.
ii) Present only on one side of the membrane: act as enzymes e.g. adenyl cyclase that forms cyclic AMP from ATP
What is the site of peripheral or extrinsic proteins?
Bind to hydrophilic polar heads of lipids or to integral proteins.
What are the types of peripheral or extrinsic proteins?
i) Peripheral proteins: bind to the intracellular surface of the membrane & contribute to the cytoskeleton.
ii) Peripheral proteins: bind to the extracellular surface of membrane & contribute to glycocalyx or cell coat.
What are the protein ion channels?
They are protein channels that allow the passage of ions e.g. Na+ ion through the cell membrane.
What are the types of protein ion channels?
1- leak ion channel
2- chemical-gated ion channels
3- voltage-gated ion channels
What is the definition of leak ion channels?
Channels that are always open
What are examples of leak ion channels?
K channels
What is the importance of the leak ion channel?
Resting membrane potential
What is the definition of chemical-gated ion channels?
Channels open when a chemical substance bind to its receptor
What are examples of chemical-gated ion channels?
K & Na channel at NMJ.
What is the importance of chemical-gated ion channels?
Graded membrane potential e.g. motor endplate potential
What is the definition of voltage-gated ion channels?
Channels open by changes in cell membrane potential
What are examples of voltage-gated ion channels?
Na & K channels
What is the importance of voltage-gated ion channels?
Action potential
What is the definition of diffusion?
Movement of substances across the cell membrane down its electrochemical gradient due to the continuous random motion of its particles.
What are the types of transporting across the cell membrane?
- Diffusion
- Active transport
- vesicular transport
What are the types of diffusion?
Simple, facilitated, and osmosis.
What is the definition of simple diffusion?
Movement of substances across cell membrane down its electrochemical gradient by simple movement without the necessity of binding with carrier proteins.
What are the characters of the simple diffusion?
- It occurs down an electrochemical gradient.
- Passive i.e. no external energy is required.
- Not rate-limiting i.e. linear with concentration gradients.
- The diffusion process is not saturable.
What are the mechanisms of simple diffusion?
- Interstices of the lipid bilayer e.g. diffusion of O2, nitrogen, CO2, and alcohol.
- Watery proteins channels e.g. diffusion of ions
What is the definition of facilitated diffusion?
As simple diffusion but it needs the presence of carrier proteins.
What are the examples of facilitated diffusion?
Transport of glucose into the cells.
What is the definition of osmosis?
The passive flow of water across a semi-permeable membrane down a concentration gradient of water
What are examples of osmosis?
from high concentration of water to low concentration of water or low concentration of solute to high concentration of solute.
What are the Factors affecting Net Rate of Diffusion
(Fick’s Law)?
a. Concentration gradient for the solute (Cin - Cout in mmol/L)
b. Diffusion Coefficient (D) or permeability coefficient of the membrane
c. Membrane surface area (A in cm2).
- The rate of diffusion is directly proportional to these factors.
d. Membrane thickness (X in cm) or distance, the rate of diffusion is inversely proportional to the thickness of the membrane.
What is the definition of active transport?
Movement of substances across cell membranes against an electrochemical gradient.
What are the characters of active transport?
- Occurs against the electrochemical gradient
- Active i.e. energy is required.
- Requires presence of a transport carrier protein, so its rate is limited,
saturable, and shows competition and stereospecificity
What are the types of active transport?
Primary and secondary.
Primary active transport
Use energy directly from ATP hydrolysis
What are examples of primary active transport?
Na-K Pump, Ca ATPase Pump , H-k Pump
Sodium-potassium pump (Na+-K+ ATPase) :
➢ Transports 3 Na+ from ICF to ECF & 2 K+ from ECF to ICF.
➢ This maintains low intracellular Na and high intracellular K.
➢ It utilizes about 40% - 50% of energy
Secondary active transport
Use energy generated by sodium gradient created by Na-K pump e.g. Na-Ca exchanger and Na-glucose cotransport
What is the definition of vesicular transport?
The mechanism by which the large-sized substances can cross the cell membranes.
What are the types of vesicular transport?
- Endocytosis and exocytosis
What is the definition of endocytosis?
The extracellular material is trapped within vesicles that are formed by the invagination of the cell membrane and pushed inside the cell.
What are the types of endocytosis?
Phagocytosis: Endocytosis of bacteria & dead tissue.
Pinocytosis: Endocytosis of substances in solution e.g. proteins.
Receptor-mediated endocytosis: The material to be transported first binds to a receptor e.g: Iron and cholesterol
What is the definition of exocytosis?
the intracellular material is trapped within vesicles, then the vesicles fuse with the cell membrane and release their contents to the ECF.
What are the examples of exocytosis?
release of hormones, digestive enzymes, and synaptic transmitters
