Transport mechanisms Flashcards
What is the difference between the capillary wall and the cell membrane/plasma membrane?
The capillary wall separates the interstitial fluid and the plasma, and it surrounds all the vessels in the body. The cell membrane/plasma membrane surrounds all the cells in our body and is not always the same depending on the cell.
Are the following substances highly permeable, less permeable, or impermeable to the cell membrane?
H2O, larger molecules, very large molecules, O2, CO2, charged particles, small uncharged particles.
Highly permeable, less permeable, impermeable, highly permeable, highly permeable, less permeable, highly permeable
Describe the structure of the cell membrane.
It consists of an amphipathic phospholipid bilayer, with polar, hydrophilic heads facing towards the extracellular and intracellular fluid and nonpolar, hydrophobic tails pointing inwards.
What proportion of the plasma membrane’s weight do phospholipids comprise? Name 3 other components of it.
40-50%. It also consists of cholesterol, proteins, and glycocalyx.
What is the role of cholesterol in the lipid bilayer at normal temperatures?
At cooler, more physiological temperatures, it orients itself to separate the lipid tails. By reducing interactions between lipid tails, it resists the packing of the tails and their aggregation. This keeps the membrane more fluid than it would be otherwise. It can also be involved in the formation of vesicles and lipid rafts.
What is the role of cholesterol in the lipid bilayer at high temperatures?
It serves to stabilize the cell membrane and adds firmness.
Is cholesterol hydrophobic, hydrophillic, or amphipathic?
It is slightly amphipathic.
What types of proteins are found in the cell membrane? Explain where they are found.
Integral and peripheral proteins. Integral proteins span across the membrane (those that span all the way across are transmembrane proteins). Peripheral proteins are found mostly along the cytoplasmic side and are less strongly embedded.
Describe the difference between integral and peripheral proteins in terms of a) polarity, b) ease of removal
Integral proteins are amphipathic and are hard to remove without disrupting the membrane. Peripheral proteins are polar and are easier to remove.
What is the glycocalyx?
It is a layer of carbohydrate formed by a chain of monosaccharides that extrend from the extracellular surface of the cell membrane bound to proteins. It is also made of glycoproteins and glycoplipids.
Name 3 functions of the glycocalyx.
It provides protection from infection, enables cells to identify each other, and is a site of interaction between cells.
The model used to describe the structure of the cell membrane is called the _________.
Fluid mosaic model
Name 6 functions of plasma membrane proteins.
Selective transport channel,
Enzyme function to catalyze membrane reactions,
Cell surface receptor for chemical signals
Cell surface identity marker,
Cell adhesion,
Attachment to cytoskeleton.
What are the three main forms of transmembrane transportation? Which ones involve proteins?
Via the phospholipid bilayer, via interaction with a protein channel, or via a protein carrier. The latter 2 involve a protein cluster.
What are the two categories of transport mechanisms across the cell membrane? What is the difference between them?
Passive and active transport. Passive transport is energy independent, while active transport depends on an input of energy.
3 types of passive transport
Diffusion
Carrier-mediated facilitated diffusion
Osmosis
3 forms of active transport.
Carrier-mediated primary active transport
Carrier-mediated secondary active transport
Exo/endosytosis
What is simple diffusion?
It is the movement of molecules from one location to another as a result of random thermal motion.
What is flux?
The amount of particles crossing a surface per unit time.
What is net flux?
It is the movement of particles from high to low concentration.
At equilibrium after diffusion, what is the value of net flux?
Equal to zero because the net movement is equal in all directions.
If you have two compartments of equal size separated by a membrane and then put 20 mml/l of solution into one of them, what will happen?
Over time, the concentration between both will even out such that they both have a concentration of 10 mmol/l.
What does downhill mean?
It means movement from high concentration to low concentration (down the concentration gradient)
In the case of diffusion into a single cell from the surrounding extracellular fluid compartment, how do their respective concentrations change?
The concentration of the cell rises until it is equal to the extracellular concentration. The extracellular volume is so huge that it makes no difference to its concentration.
What law allows us to calculate the rate of diffusion?
Fick’s law
What is the formula for Fick’s law? Define the variables.
J = PA(Co-Ci)
J: net flux: rate of diffusion
P: Permeability coefficient
A: Surface area of the membrane
Co-Ci: Concentration gradient of the diffusing molecule across the membrane
To change the flux, what variable in the Fick’s law equation can we change?
The concentration gradient.
Explain the relationship between diffusion time and distance.
Diffusion time increases in proportion to the square of the distance travelled by the solute molecules.
What would happen if we were to move our capillaries further away from our cells? Why?
Diffusion from the capillaries into the cells wouldn’t be nearly as effective, because diffusion is only effective at very short distances.
Name the 5 factors that affect diffusion across the cell membrane.
- Mass of the molecule
- Concentration gradient across the cell membrane
- Lipid solubility
- Electrical charge
- Availability of selective ion channels or membrane carriers.
Explain the difference between how nonpolar molecules and ions diffuse.
Non-polar molecules diffuse by dissolve in the lipid component of the membrane, while ions diffuse through channels.
Describe the structure of ion channels.
They usually consist of clusters of proteins with transmembrane domains that also extend into the extra- and intracellular fluid. They cluster together to form a channel in the middle.
Ion channels show selectivity based on _____ and ____.
Their diameter, the distribution of charges in the channel
What is the electrochemical gradient?
It is the simultaneous existence of an electrical gradient caused by the membrane potential (extra negative charge in the cell) and by the concentration gradient of the molecule. These forces form an equilibrium that determines the direction of diffusion.
What are the three mecanisms of ion channels?
Ligand-gated, voltage-gated, mechnically-gated.
What is a ligand-gated channel?
It is a channel with a receptor to which a molecule binds and changes the shape of the channel.
What is a voltage-gated ion channel?
The voltage across the cell membrane determines whether the channel will open or close.
What is a mechanically-gated channel?
It is regulated by the stress on the membrane caused by stretching or pain.
Ion channels can exist in ____ or ____ state as they undergo ______ changes.
Open, closed, conformational
What are the 4 most popular ions that flow through voltage-gated ion channels?
Na+, K+, Ca+, Cl-
What are the main types of mediated transport systems?
Facilitated diffusion, primary active transport, secondary active transport
What is mediated transport?
It is the movement of ions and other molecules by integral membrane proteins called transporters or carriers.
Name the 3 major characteristics of mediated transport systems.
Specificity, saturation, competition
Explain how mediated transport systems are specific.
Usually a carrier only transport one particular type of molecule.
Explain saturation in mediated transport systems.
The rate of transport reaches a maximum as solute concentration increases when all transporters’ binding sites are occupied (e.g. all the seats on the bus are full). This is unlike diffusion, which doesn’t have a maximum flux into the cell.
Explain competition in mediated transport systems.
Structurally similar substances can compete for the same binding site on a membrane carrier.
What 4 factors affect the flux magnitude of mediated transport system?
- Solute concentration
- Affinity of transporter for the solute
- Numbers of transporters
- Rate of transporter conformational change
What is facilitated diffusion?
It involves the presence of a carrier molecule that allows a solute to penetrate more readily than it would by simple diffusion (along its concentration gradient)
What are the 4 steps involved in facilitated diffusion?
- Solute binds to transporter
- Transporter changes configuration
- Solute is delivered to the other side of the membrane
- Transporter resumes original configuration.
How do hormones affect facilitated diffusion?
They can increase the number and/or affinity of transporters in some membrane.
What is active transport?
It requires a supply of chemical energy (usually ATP) and uses it to transport a solute against its concentration gradient. It also requires a transporter/carrier.
What does primary active transport require?
It is active transport that involves the hydrolysis of ATP by a transporter. The phosphorylation of the transporter changes its conformation and solute binding affinity.
What is the most famous active transportation mechanism? Explain what would happen if it didn’t exist.
The Na+/K+-ATPase pump. Due to the membrane potential, the natural tendency is for Na+ to move into the cell and for K+ to move out of the cell. Without the pump, both ions would just accumulate in these respective places. This would also make secondary active transport impossible.
Describe the 5 steps in the Na+/K+-ATPase.
- Start with ATP binding to transporter. This makes it favourable for Na+ molecules to bind to the transporter’s 3 designated sites.
- When Na+ binds, ATP is hydrolyzed with the release of ADP and the bonding of a high-energy phosphate.
- This causes a conformational change in the protein, exposing the 3 Na+ to the extracellular solution.
- Once Na+ is released, K+ binds to high affinity sites that become exposed on the extracellular side.
- K+ is released and the molecule returns to the original conformation. ATP eventually binds again, restarting the cycle.
Aside from the Na+/K+-ATPase, name 3 other significant primary active transport mechanisms and their function.
- Ca2+-ATPase: maintains low intracellular Ca2+
- H+-ATPase: maintains low lysosomal pH
- H+/K+-ATPase: acidification of the stomach
What is secondary active transport?
The transport of Na+ down its concentration gradient is coupled to the transport of another solute molecule uphill against its concentration gradient. It uses the energy stored in the electrochemical gradient generated by the Na+/K+-ATPase, so it is reliant on primary active transport.
Describe the 3 steps in secondary active transport.
- Na+ binds to a transporter outside the cell, allowing the solute to bind to the same carrier.
- Through a change in configuration, the transporter “delivers” both molecules into the cell. This is driven by the energy gained by transporting Na+ along its concentration gradient.
- The transporter then reverts to its original configuration and Na+ is extruded from the cell by Na+/K+-ATPase.
What are the two subtypes of secondary active transport? Explain the difference between them. What type of solute is typically transported by each means?
Cotransport (symport): solute X is transported in the same direction as Na+, into the cell. This is typically done for amino acids.
Countertransport (antiport): solute X is transported in the opposite direction to Na+, out of the cell. This is typically done for Ca2+.
Give 2 significant examples of antiports.
The Na+/H+ exchanger and the Na+/Ca2_ exchanger
Name the typical ions that use:
a) Diffusion through the lipid bilayer
b) Diffusion through protein channel
c) Facilitated diffusion
d) Primary active transport
e) Secondary active transport
a) Nonpolar molecules (O2, CO2, fatty acids)
b) Ions: Na+, K+, Ca2+
c) Polar molecules (glucose)
d) Ions: Na+, K+, Ca2+, H+
e) Polar molecules: amino acids, glucose, some ions
Review diagram showing all the transportation mechanisms we’ve learned so far
slide 50
What do endocytosis and exocytosis have in common?
They are both active transport mechanisms that involve the participation of the cell membrane itself.
What is endocytosis?
It is when the cell membrane invaginates and pinches off to bring the contents into the cell.
What is exocytosis?
It is when an intracellular vesicle fuses with the cell membrane and its contents are released into the ECF.
What are the 2 types of exocytosis? Explain the difference between them.
Constituitive exocytosis: the normal turnover of the plasma membrane to deliver proteins to the membrane and get rid of substances from the cell
Regulated exocytosis: triggered by extracellular signals and the increase of cytosolic Ca2+. Secretes hormones, digestive enzymes, and neutrotransmitters.
What are the 3 types of endocytosis?
Pinocytosis (cell drinking), phagocytosis (cell eating), and receptor-mediated endocytosis.
What is pinocytosis?
An endocytotic vesicle engulfs the extracellular fluid including whatever solutes are present and bring them into the cytoplasm.
What is phagocytosis?
It is the process by which phagosomes engulf large particles just outside the cell such as dust, bacteria, and cell debris. Extensions of the cell membrane called pseupobodia fold around the particle and then form large vesicles called phagosomes that pinch off the membrane. They travel through the cell and fuse with lysosomes to degrade the contents.
What is receptor-mediated endocytosis?
It is when molecules called ligands in the extracellular fluid bind with high affinity to protein receptors on the plasma membrane. This can trigger several different response types.
Name the main 2 response types triggered by receptor-mediated endocytosis.
Clathrin-dependent receptor-mediated endocytosis, potocytosis
Explain how clathrin-dependent receptor-mediated endocytosis works.
The ligand binds to the receptor in the cell membrane, causing the receptor to undergo conformational change and the recruitment of clathrin, which is linked to the legand-recept by adaptor proteins. The clathrin proteins form a clathin-coated pit, which invaginates and forms a clathrin-coated vesicle. The vesicle pinches off and sheds the clathrin coat and travels to fuse with organelles such as endosomes and lysosomes. The receptors and clathrin protein are recycled back to the cell membrane.
What is transcytosis?
In the case of clathrin-dependent receptor-mediated endocytosis, the clathrin-coated vesicle travels across the cell to fuse with the membrane on the other side instead of fusing with an organelle.
In the case of potocytosis, it is when the caveolae cross to the opposite side of the cell to deliver its contents into the plasma membrane on the other side.
Explain potocytosis.
Molecules are sequestered and transported by vesicles called caveolae. The contents are delivered directly into the cell cytoplasm as well as to the endoplasmic reticulum or other organelles (or to the plasma membrane across the cell - see transcytosis).
Give an example of clathrin-dependent receptor-mediated endocytosis.
The LDL receptor.
Give an example of an application of potocytosis.
It is implicated in the uptake of low molecular weight molecules like vitamins and folates.
Water is able to diffuse freely across most cell membranes. Explain why this shouldn’t be the case and why it is possible.
This shouldn’t be the case because water is polar, meaning that it is hydrophilic and insoluble in the hydrophobic cell membrane. it can freely pass through because of the presence of aquaporins in the membrane, which form water permeable channels.
Is the quantity of aquaporins everywhere in the body the same? Explain why or why not.
They are found in all cells of the body but the quantity can vary based on how much water gets transported. Kidneys have a lot of aquaporins, for example, compared to other organs.
What is osmosis?
Osmosis is the net diffusion of water across a semipermeable membrane (not permeable to all solute).
Explain why a semipermeable membrane is essential for osmosis to work.
If the membrane were permeable to all solute, the solute would simple redistribute itself according to its concentration gradient without the water having to move. When the solute can’t diffuse through, it forces the water to move to even out its own concentration gradient.
What is osmotic pressure?
It is the pressure required to prevent the movement of water across a semi-permeable membrane. It is equal to the difference in hydrostatic pressure between the two solutions on either side of the semipermeable membrane.
What is the major determinant of osmotic pressure of a solution?
The number of particles in the solution per unit volume (unrelated to size, configuration, or charge).
What is osmolarity?
It is the total solute concentration of a solution, including both penetrating and non-penetrating solutes.
Calculate the osmolarity of physiological saline.
Physiological saline: 0.9% NaCl = 0.9 g NaCl/100 ml = 9 g NaCl/L
9 g NaCl = 9g/58.5 g = 0.15 mol NaCl
Osmolarity:
0.15 M NaCl = 0.15 mol Na+ + 0.15 mol Cl- = 0.30 Osm = 300 mOsm
Solutions which have the same osmolarity as normal extracellular or intracellular solution have an osmolarity of ____ and are called ____.
300 mOsm, isoosmotic.
Solutions which have an osmolarity lower than 300 mOsm are called ______.
Hypoosmotic.
Name 2 examples of nonpenetrating solutes in the cell and explain why they are designated as such.
Na+ and K+. They are designated as a nonpenetrating solute because even if they diffuse in/out of the cell through an ion channel, this gets reverse by the Na+/K+-ATPase, essentially making it as though they were not penetrating the membrane.
If you place red blood cells in extracellular solution with a concentration of 300 mOsm of nonpenetrating solute particles, what will happen? Why?
There will be no net shift of water because red blood cells also have an osmolarity of 300 mOsm. The solute they have been placed in is an isotonic solution.
If you place red blood cells in extracellular solution with a concentration of nonpenetrating solute particles less than 300 mOsm, what will happen? Why?
Water will enter the cell and cell will swell, because it is at a lower concentration inside the cell. The solute is hypotonic.
Solutions which have an osmolarity greater than 300 mOsm are called ______.
Hyperosmotic.
If red blood cells are placed in a solution with a concentration of nonpenetrating solute particles greater than 300 mOsm, what will happen? Why?
Water will flow out of the cell and the cell will shrink, because water is at a higher concentration in the cell than outside of it. The solution is hypertonic.
Is an isoosmotic solution necessarily isotonic? Explain why or why not.
No, because isoosmotic just means that the total solute concentration is the same on both sides, including penetrating and nonpenetrating solutes, and is equal to 300 mOsm. But isotonic would require that the concentration of nonpenetrating solute is equal to 300 mOsm, which is not necessarily the case.
Where does the main exchange between ICF and ECF take place?
At the cell membrane
Describe the makeup of the capillary wall.
The capillaries are made of up a single layer endothelial cells and basement membrane. They have clefts sometimes between one another. Clefts like these can be totally absent in certain parts of the body, like the brain, where we don’t want anything diffusing, or more present like in the liver, where you want to exchange molecules
What are the four different transport mechanisms through the capillaries?
Diffusion of water-soluble molecules through water-filled channels
Passing through lipid bilayer directly
Endo/exocytosis for exchangeable proteins
Bulk flow
What is the most important transport mechanism across the capillary wall?
Diffusion
What is bulk flow?
It distributes the extracellular fluid volume between the plasma and the interstitial fluid. Its magnitude is proportional to the hydrostatic pressure difference between the plasma and the ISF. The capillary wall allows plasma to move into the ISF and filters out large proteins.
How do pinocytosis and phagocytosis differ in terms of specificity and purpose?
Pinocytosis is non-specific and is constituitive, while phagocytosis is specific and must be triggered.
How does excessive H2O intake affect ECF Osm and ECF volume?
It will decrease ECF Osm and increase ECF volume.
How does excessive H2O intake affect ICF Osm and ICF volume?
ICF Osm will decrease and ICF volume will increase.
How does an IV infusion of 0.9% NaCl affect ECF Osm and ECF volume?
It will increase the ECF volume but the ECF Osm will be the same.
How does an IV infusion of 0.9% NaCl affect ICF Osm and ICF volume?
It will have no effect.
How does hemorrhage affect ECF Osm and ECF volume?
It will decrease ECF volume and have no effect on ECF Osm.
How does hemorrhage affect ICF Osm and ICF volume?
It will have no effect.
How does drinking sea water affect ECF Osm and ECF volume?
It will increase ECF Osm and ECF volume.
How does drinking sea water affect ICF Osm and ICF volume?
It will decrease ICF volume and increase ICF Osm
How does severe sweating affect ECF Osm and ECF volume?
It will decrease ECF volume and increase ECF Osm (we’re not considering loss of electrolytes)
