1.2. Structure, permeability and transport functions of the cell membrane. Transepithelial transports Flashcards
Structure of the cell membrane
a/ General composition of cell membrane?
Cells membrane are composed of
- Phospholipids (primary)
- Proteins
- Cholesterols
- Glycolipids
Structure of the cell membrane
b/ What are the types of lipids that make up the cell membrane?
- Phospholipids
- Glycolipids
Structure of the cell membrane
b/ Structure and function of phospholipids?
Structures
- Phospholipids are amphipathic
- hydrophobic fatty acid tails pointing inwards
- hydrophilic glycerol head
(most abundant: choline-containing = sphingomyelin)
Role: form a selectively permeable membrane
Structure of the cell membrane
c/ Structure and function of glycolipids?
Structure: sugar group attached to fatty acids
Function: maintenance of cell stability and also for cellular recognition (e.g, antigens for ABO blood group)
Structure of the cell membrane
d/ 4 general functions of lipids
- Transport of molecules
- Source of 2nd messenger
- E.g, Phosphatidylinositol 4,5-bisphosphate = PIP2) - Signal transduction
- with Gq G-protein coupled
- receptor activation, the PIP2 in the membrane is cleaved by phospholipase C,
- releasing IP3, which leads to increased IC Ca2+ - Provide surface
Structure of the cell membrane
e/ What are the 5 types of phospholipids? What are their localizations and roles?
Structure of the cell membrane
f/ A type of glycolipids? What are their localization and role?
Glycosyl-phosphatidyl-inositol
- Localization: Outer leaflet
- Role: Protein-anchor
Structure of the cell membrane
f/ What are localization and role of cholesterol?
Cholesterol
- Localization: Inner/outer leaflet
- Role: Membrane fluidity, lipid raft
Structure of the cell membrane
g/ What are the 3 types of plasma membrane protein?
- Integral proteins
- Lipid-anchored proteins
- Peripheral proteins
Structure of the cell membrane
h/ What are the location and function of integral membrane proteins?
- Location: Embedded in the membrane by their hydrophobic and hydrophilic parts
- Include Transmembrane proteins
- Function: cross the membrane multiple times, allowing contact with both ECF and ICF
Structure of the cell membrane
i/ What are the location and function of lipid-anchored proteins?
Location: Glycosylphosphatidylinositol (GPI)-bound proteins
Role: Lipid modification (palmytoilation)
Structure of the cell membrane
j/ Location of peripheral proteins
Loosely bound to plasma membrane
- Permeability of the cell membrane?
- The lipid component means that the cell is…
- permeable to lipid- soluble substances
+) CO2, O2, fatty acids, NO and steroid hormones.
- Impermeable to water-soluble substances
+) ions, glucose and amino acids. - The protein component functions as transporters, enzymes, hormone receptors, antigens and ion and water channels.
- Transport function of plasma membrane
a/ Classification of transport processes through the plasma membrane
- Simple diffusion
- Protein-mediated membrane transport
a/ Solute carriers (facilitated diffusion)
b/ ATP-dependent carriers (pumps)
c/ Ion channels
d/ Water channels - Vesicular transport
a. Endocytosis
- Pinocytosis
- Phagocytosis
- Receptor-mediated cytosis
b. Exocytosis
- Transport function of plasma membrane
a/ Characteristics, driving force and mechanism of simple diffusion
- Passive transport (not require energy)
- Linearly related with concentration gradient -> no saturation
- Driving force: chemical gradient (concentration difference)
- Particles diffuse from areas of high concentration to areas of low concentration - Mechanism of diffusion: random movement of solute due to the Brownian motion
- Transport function of plasma membrane
b/ How is Fick’s first law on Diffusion related to simple diffusion?
J = net rate of diffusion (in moles per unit time)
D = diffusion coefficient (tells how easy it is for something to diffuse (bigger molecules have smaller D’s))
A = area, across which the diffusion is occurring
∆x= distance, along which the diffusion is occurring
∆c= concentration difference
- Transport function of plasma membrane
c/ What are the Properties of simple diffusion in the case of membrane (5 - Characteristics, driving force, rate depends on., substance that can pass,…)
1/ Diffusion through the lipid bilayer
2/ Driving force: concentration difference between the 2 sides of the membrane (∆c)
3/ Thickness of the membrane (∆x)
4/ Rate depends on the hydrophobicity and the size of the solute (D)
5/ Substances that can participate in simple diffusion
- O2, CO, CO2, NO, urea, hydrophobic hormones
- (H2O, ions, glucose)
- Glucose can use glucose transporters
- No peptides, no proteins, no disaccharide
- Transport function of plasma membrane
- Protein-mediated membrane transport 1
a/ What type of transport proteins can be?
Channels and carriers
- Transport function of plasma membrane
- Protein-mediated membrane transport. 1
b/ Definition, characteristics and role of channels/gates?
Channels: which are gates that can alter between blocking or allowing passive transport
- passive, gated, fast transport
- can be saturated, but only with rare and extremely high ion concentration
- protein must undergo a conformational change (gated)
- Transport function of plasma membrane
- Protein-mediated membrane transport 1
c/ Definition, characteristics and role of carriers=enzymes?
Carriers: are enzymes that allow specific substances to cross
- Passive, cyclic, slow transport
- Can be saturated
- Protein must undergo a conformational change (cyclic)
- Carried charge can be electroneutral or electrogenic (ions)
- Transport function of plasma membrane
- Protein-mediated membrane transport 2
a/ Main classification of protein-mediated membrane transport
1/ Facilitated transport (facilitated diffusion)
- Passive transport not linked to metabolic energy
2/ Active transport, linked to metabolic energy
a/ Direct/primary active transport – pump
b/ Indirect/ secondary active transport - Works together with a pump
- Transport function of plasma membrane
- Protein-mediated membrane transport 2
b/ Other classifications of protein-mediated membrane transport
1/ Based on direction of solutes
- Uniporter
- Contransport: symport and antiport
2/ Based on carried charge
- Electroneutral
- Electrogenic
- Transport function of plasma membrane
- Protein-mediated membrane transport 3
a/ Characteristics and function of facilitated diffusion
1/ Integral plasma membrane protein-mediated
2/ Passive transport, no energy required
3/ Driving force: concentration difference (solutes without charges)
4/ Have saturation kinetics
5/ Chemical specificity
- There can be competition between structurally related molecules
6/ Transport molecules
- Water uses aquaporin channels (Can use simple diffusion but slower)
- Glucose use glucose transporters
- Ions use ion channels
- Transport function of plasma membrane
- Protein-mediated membrane transport 3
b/ Structure of GLUT family
- 12 transmembrane segments
- a single site of N-linked glycosylation
- a relatively large, central, cytoplasmic linker domain
- exhibit topologies with both their N and C termini positioned in the cytoplasm
- Transport function of plasma membrane
- Protein-mediated membrane transport 4
- Active transport
a/ Characteristics of active transport?
- Active transport is linked to metabolic energy in some way, usually going against the concentration gradient
- Using pumps (e.g, ATPase)
- Transport function of plasma membrane
- Protein-mediated membrane transport 4
- Active transport
b/ 4 important examples of active transport?
1/ Na+/K+pump pumps 3 Na+ out of the cell and 2 K+ in
2/ Ca2+-ATPase in plasma membrane, which pumps Ca2+ out of the cytosol and into the ECF
3/ Ca2+-ATPase in sarcoplasmic and endoplasmic reticulum (SERCA), which pumps 2 Ca2+ per ATP from cytosol into the sarcoplasmic reticulum or ER (calcium sequestration)
4/ H+-K+-ATPase in the intercalated cells of renal collecting duct
- Transport function of plasma membrane
- Protein-mediated membrane transport 5
- Active transport – secondary active transport
a/ Characteristics of secondary active transport?
1/ Secondary active transport uses the potential energy stored by the concentration gradient created by primary active transport (e.g, high EC Na+ concentration)
2/ Types of transporter can be used
- Antiporter
- Symporter
- Uniporter
- Transport function of plasma membrane
- Protein-mediated membrane transport 5
- Active transport – secondary active transport
a/ What are some examples of secondary active transport?
Examples of Na+-cotransporter
1/ in TAL Na+/K+/2Cl—cotransporter
2/ in PT SGLT (Sodium glucose transporter)
3/ Na+/amino acid- cotransporter
- Transport function of plasma membrane
- Protein-mediated membrane transport 6
a/ What is the role of Cystic fibrosis transmembrane conductance regulator (CFTR)?
allows chloride ions to move passively across the cell membrane (water secretes with chloride, maintaining the balance of salt and water on many surfaces of the body)
- Transport function of plasma membrane
- Protein-mediated membrane transport 6
b/ What is the consequence of lack or mutation in the Cystic fibrosis transmembrane conductance regulator (CFTR) gene
1, disrupts the function of the chloride channels
- Preventing them from the regulating the flow of chloride ions and water across cell membranes.
- no water secretion
- production of thick and sticky mucus occurs in the lungs, pancreas or other organs.
(In acini of pancreas, no water secretion to dilute enzyme -> pancreatitis)
- Transport function of plasma membrane
- Protein-mediated membrane transport 7
a/ Characteristics of Water transport through the membrane
- The water permeability of lipid bilayer is low
- Protein-mediated transport (aquaporins – 11 isoforms)
- Passive transport
- Driving force: osmosis
+) Water flows to where osmotic concentration is higher
- Transport function of plasma membrane
- Protein-mediated membrane transport 7
b/ This is a representation of osmotic water movement and the generation of osmotic pressure. What is the reason for this phenomenon?
- Solute concentration in A is higher than in B
- Membrane is impermeable is to solute, but permeable to water
- Transport function of plasma membrane
- Protein-mediated membrane transport 8 - Osmotic measurement
a/ What is Oncotic pressure?
- Oncotic pressure is the osmotic pressure generated by large molecules (especially proteins) in solution.
- Transport function of plasma membrane
- Protein-mediated membrane transport 8 - Osmotic measurement
b/ Give an example of Oncotic pressure based on this figure.
When the hydrostatic pressure (oncotic pressure) applied to the solution in chamber A is equal to the osmotic pressure of that solution, there is not net water flow across the membrane
- Transport function of plasma membrane
- Protein-mediated membrane transport 8 - Osmotic measurement
c/ What is van’t Hoff’s law?
- Transport function of plasma membrane
- Protein-mediated membrane transport 8 - Osmotic measurement
f1/ What is the definition of Reflection coefficient (𝞂)?
The reflection coefficient, sigma, a measure of the relative ability of the molecule to cross the cell membrane
Kelvin
- Transport function of plasma membrane
- Protein-mediated membrane transport 8 - Osmotic measurement
f2/ What is the formula of Reflection coefficient (𝞂)?
π = osmotic pressure
n = number of dissociable particles per molecule
c = total solute concentration
R = gas constant
T = temperature in degrees Kelvin
- Transport function of plasma membrane
- Protein-mediated membrane transport 8 - Osmotic measurement
f3/ What happen to a solute/molecule when their Reflection coefficient (𝞂) in 3 cases:
Case 1: σ = 0
Case 2: σ = 1
Case 3: 0 < 𝞂 <1?
1/ For a molecule that can freely cross the cell membrane, such as urea in the preceding example, σ = 0, and no effective osmotic pressure is exerted (e.g., urea is an ineffective osmole for red blood cells)
2/ σ = 1 for a solute that cannot cross the cell membrane (in the preceding example, sucrose). Such a substance is said to be an effective osmole.
3/ Many molecules are neither completely able nor completely unable to cross cell membranes (i.e., 0 < σ < 1) and generate an osmotic pressure that is only a fraction of what is expected from the molecules’ concentration in solution.
- Transport function of plasma membrane
- Protein-mediated membrane transport 8 - Osmotic measurement
g/ What is tonicity of a solution? 3 types of related solutions?
The tonicity of a solution is related to the effect of the solution on the volume of a cell.
1/ Isotonic solutions that do not change the volume of a cell are said to be isotonic.
2/ Hypotonic solutions are solutions that cause a cell to swell
3/ Hypertonic solutions are solutions that cause a cell to shrink
- Transport function of plasma membrane
- Protein-mediated membrane transport 8 - Osmotic measurement
h/ What is the Relationship between the concentration of plasma proteins in solution and the osmotic pressure (oncotic pressure) that they generate?
The magnitude of the osmotic pressure generated by a solution of protein does not conform to van’t Hoff’s law.
- it appears to be related to the size and shape of the protein molecule.
- For example, the correlation to van’t Hoff’s law is more precise with small, globular proteins than with larger protein molecules.
- Transport function of plasma membrane
- Vesicular transport 2 - Endocytosis
a/ What is endocytosis?
Endocytosis is the process whereby a piece of the plasma membrane pinches off and is internalized into the cell interior
- It involves a number of accessory proteins, including adaptin, clathrin and the GTPase dynamin
- Transport function of plasma membrane
- Vesicular transport 2 - Endocytosis
b/ What are the 3 mechanisms of endocytosis?
1/ Pinocytosis
2/ Phagocytosis
3/ Receptor-mediated endocytosis
- Transport function of plasma membrane
- Vesicular transport 2 - Endocytosis
c/ Characteristics of pinocytosis
1/ consists of the nonspecific uptake of small molecules and water into the cell
2/ a prominent feature of the endothelial cells that line capillaries and is responsible for a portion of the fluid exchange that occurs across these vessels.
- Transport function of plasma membrane
- Vesicular transport 2 - Endocytosis
d/ Characteristics of phagocytosis?
1/ allows for the cellular internalization of large particles (e.g., bacteria, cell debris).
2/ an important characteristic of cells in the immune system (e.g., neutrophils and macrophages).
- Transport function of plasma membrane
- Vesicular transport 2 - Endocytosis
e/ Characteristics of Receptor-mediated endocytosis?
1/ Allows the uptake of specific molecules into the cell
2/ Molecules will bind to receptors on the surface of the cell
- Transport function of plasma membrane
- Vesicular transport 3 - Exocytosis
b/ Classification of exocytosis?
Constitutive and Regulated
- Transport function of plasma membrane
- Vesicular transport 3 - Exocytosis
c/ Characteristics of Constitutive exocytosis?
Location: Plasma cells
Example
- Constitutive exocytosis occurs in plasma cells that are secreting immunoglobulin or in fibroblasts secreting collagen.
- Histamine release from mast cell
- Transport function of plasma membrane
- Transepithelial transport
a/ Classification
2 types
1/ Transcellular transport
2/ Paracellular transport
- Transport function of plasma membrane
- Transepithelial transport
b/ Characteristics of transcellular transport
1/ The solute or water is transported across both the apical and basolateral membrane, which may be either a passive or an active process.
2/ Uses
- Pumps, carriers
- Ion channels
- Water channels
3/ Examples
- uptake of Na+ into the cell across the apical membrane via Na+ channel is passive, the exit of Na+ from the cell across the basolateral membrane is primary active transport via Na+-K+ ATPase channel.
- Transport function of plasma membrane
- Transepithelial transport
c/ Characteristics of paracellular transport
1/ The solute or water is transported across the tight junction, which is passive
2/ Not regulated
3/ The driving forces are transepithelial concentration gradient for the solute and the transepithelial voltage.
- Leaky example: epithelium of the proximal tubule of renal nephron, duodenum and jejunum.
- Tight example: epithelium of collecting duct of renal nephron, urinary bladder and the terminal portion of colon
3/ Not regulated
- Transport function of plasma membrane
- Transepithelial transport
d/ What are the 5 important examples of transepithelial transport?
1/ Skin of the frog - NaCl reabsorption
2/ Small intestine / proximal tubule - glucose resorption
3/ Salivary gland - chloride secretion
4/ Stomach - proton secretion
5/ Regulation of the transports is primary active transport via Na+-K+ ATPase channel.
