Learning Outcomes - Week 1 Flashcards
Appreciate the significance of the plasma membrane for physiological systems. Examples?
The transport of materials across the plasma membrane of cells is a fundamental requirement for the normal function of virtually every physiological system. For example, the ability of the digestive tract to absorb nutrients, the capacity of the kidneys to maintain water balance and the electrical events that enable the nervous system to function, all require very precise regulation of the movement of materials across the plasma membrane of cells.
Understand what is meant by the terms solvent, solute and aqueous solution as they relate to physiological systems.
- the substance that dissolves is called the solute
- the liquid in which it dissolves is called the solvent
- the liquid formed is a solution
- a saturated solution is a solution into which no more solute can be dissolved
- An aqueous solution is one in which the solvent is liquid water
Know the basic structural components of the plasma membrane and how these regulate the movement of materials between the extracellular and intracellular fluid compartments.
Phospholipid bilayer consisting of two opposing layers of phospholipids - hydrophilic head and hydrophobic tails.
Hydrophobic nature of the tails is what prevents water soluble substances moving across the membrane
Membrane proteins acting as pumps, receptors, or channels allow substances to move across the layer in a highly regulated fashion
Be able to explain the fundamental difference between passive and active processes of membrane transport.
passive processes which don’t require the use of metabolic energy and active processes, which do. In subsequent modules in this lesson we will consider the features of these two types of membrane transport in more detail.
Understand what is meant by the term diffusion and how collisions between molecules and thermal energy are important to this physiological process.
All passive processes involve the movement of substances by diffusion which refers simply to the tendency for substances to distribute themselves evenly in the environment.
Diffusion occurs because of the continual random motion of the molecules due to the thermal energy of the system. This motion causes the molecules to collide with each other and ultimately ensures that they become evenly distributed throughout the environment.
Be able to explain what is meant by the term concentration gradient.
A concentration gradient occurs when the concentration of particles is higher in one area than another. In passive transport, particles will diffuse down a concentration gradient, from areas of higher concentration to areas of lower concentration, until they are evenly spaced.
Understand how and why molecular size, distance, concentration gradient and cell size affect the rate of diffusion.
Smaller molecules bounce faster during collisions
Molecular distance determines how long diffusion takes to occur (if the distance between two cells is large then it’ll take longer for the molecules to get there)
In passive transport molecules diffuse down a concentration gradient
When a cell’s surface area increases, the amount of substances diffusing into the cell increases. As volume and surface area increase, the volume increases faster, so much so that the surface area available to allow substances in halves each time the cell volume doubles.
Understand the two major mechanisms by which solutes can traverse the plasma membrane by simple diffusion and be able to identify solutes that travel by each mechanism.
Simple diffusion:
- happens without ATP, straight through lipid bilayer itself or through protein channels - happens due to concentration gradients
- Simple diffusion through bilayer:
Studies of artificial membranes (either synthetic lipid bilayers or cell membranes with all the proteins removed) have revealed that substances which have a high lipid solubility (such as O2, CO2, N2, alcohols and fatty acids) are able to dissolve in the phospholipids of the lipid bilayer and diffuse directly through the plasma membrane. In general, the higher the lipid solubility of a substance, the faster it will diffuse through the plasma membrane.
- Simple Diffusion through Protein Channels:
Small water-soluble substances are repelled by the phospholipids and are unable to cross the lipid bilayer directly. Instead these substances take advantage of the presence of protein channels which traverse the membrane and permit movement by simple diffusion. However because the channels have a relatively small diameter, simple diffusion by this route is largely restricted to small ions such as H+, Na+, K+ and Ca2+. An interesting feature of these channels is that they often only permit the movement of one ion species (i.e. are ion selective) and may be gated (i.e. may be open or closed).
Facilitated diffusion:
- Facilitated diffusion is the mechanism by which most essential metabolic substrates (glucose, some amino acids and soluble vitamins), which are either lipid insoluble or too large to pass through protein channels, traverse the plasma membrane.
A carrier protein undergoes a conformational change, still WITHOUT ATP (form of passive transport), in order to move substance in either direction through membrane depending on concentration gradient
Be able to explain the difference between simple and facilitated diffusion and be able to identify substances that move across the membrane by each mechanism.
Simple = substances with high lipid solubility (O2, CO2, N2, alcohols, fatty acids) straight through lipid bilayer
Facilitated = proteins on surface act as channels, pumps, receptors - undergo conformational change when exposed to low lipid soluble solutions and allows substances to pass through - still depending on concentration gradient (H+, Na+, K+ and Ca2+) - An interesting feature of these channels is that they often only permit the movement of one ion species (i.e. are ion selective) and may be gated (i.e. may be open or closed).
Describe an experiment that would allow you to determine whether a substance moved across the plasma membrane of a cell by simple or facilitated diffusion.
If you change the concentration of a solute outside a cell and then measure the rate at which it diffuses across the membrane into a cell then you get two quite different results depending upon whether the solute is moving by simple if facilitated diffusion. If the solute is moving by simple diffusion then there is a fairly linear relationship between solute concentration and rate of diffusion. But if the solute is moving by facilitated diffusion then the rate of diffusion will plateau because once all the carrier proteins are occupied there can be no further increase in the rate of diffusion. This is known as saturation kinetics and is a simple way to distinguish between simple and facilitated diffusion experimentally.
Appreciate the importance of passive process in terms energy-expenditure by the human body.
- Substances which are lipid-soluble are able to readily diffuse across the plasma membrane by dissolving in the lipid component of the plasma membrane.
- Small water-soluble ions are repelled by the lipid bilayer but are able to diffuse through protein channels which traverse the membrane.
Substances which are lipid-insoluble or are too large to fit through the protein channels may still diffuse across the membrane providing there is a specific carrier protein to facilitate this process. - One very important consequence of the movement of a large number of physiologically important substances by passive processes is that no cellular energy needs to be expended. This is a very significant economy which allows this cellular energy to be used for other purposes some of which we will consider in the next section.
Be able to describe the key functional elements of active transport (solute pumping) and know some of the substances that are transported across the membrane by this mechanism.
A membrane protein undergoes a confomational change when exposed to a substance with a particular ‘shape’ on the low concentration side of the membrane (such as Na+, K+, Ca2+, some sugars and most amino acids).
Requires ATP to ‘pump’ and can involve more than one substance at a time.
Examples of the pumps: hydrogen ion pump, the sodium-potassium exchange pump and the calcium pump
Appreciate that solutes can move across the plasma membrane by more than one physiological process.
Small water-soluble molecules, for example, can also be transported via active transport (can be transported a number of ways depending on conditions and requirements of the cell)
Understand the fundamental difference between exocytosis and endocytosis and the types of materials that are transported by each mechanism.
Exocytosis is moving substances out of the cell and endocytosis is moving substances into the cell.
(i) Exocytosis
In this instance hormones, neurotransmitters, enzymes and other macromolecules that are synthesised within the cell and stored in intracellular membrane-bound vesicles are secreted into the extracellular space - vesicle containing substance migrates to cell membrane, attaches to membrane and opens up, secretes substance, then vesicle breaks down
(ii) Endocytosis
Endocytosis is essentially the reverse of exocytosis and involves the transfer of large particles, macromolecules and extracellular fluid INTO the cell. The process involves a fold of the plasma membrane forming around the substance to be ingested which then goes on to form a membrane-bound vesicle. There are three variations on this basic theme which differ according to the nature of the substance being ingested
Appreciate that there are three types of endocytosis and be able to describe each as well as name substances that are transported by each.
a) phagocytosis
b) Pinocytosis: Unlike phagocytosis, most cells in the body perform pinocytosis which simply involves a small volume of the extracellular fluid being incorporated into a membrane-bound vesicle formed by a small fold in the plasma membrane
c) Receptor-mediated Endocytosis: Both phagocytosis and pinocytosis are relatively non-specific in terms of the substances which are ingested. Receptor-mediated endocytosis on the other hand is a much more selective form of bulk transport because the membrane expresses extracellular receptors to which a particular macromolecule binds. This then triggers endocytosis of the portion of the membrane containing the receptor/macromolecule complex
Substances which are carried into cells in this manner include cholesterol, iron and the hormone insulin.
One interesting consequence of these mechanisms of bulk transport is that portions of the cell membrane are constantly being removed by endocytosis and subsequently replaced by exocytosis. The mechanisms by which cells monitor this have not yet been determined but it is clearly an important consideration if the cell is to maintain its functional integrity.
Be able to explain why water moves across the plasma membrane of cells.
Semipermeable membranes that allow only water molecules to move between them allow water to move from an area of low solute concentration to an area of high solute concentration in order to balance the concentration of the solution across the system.
Because the membrane is water permeable, water moves from the left compartment to the right by osmosis.
Note that because of the net flow of water from left to right, the volume of the right compartment increases and the left compartment decreases.
Understand the concepts of osmotic pressure, osmolarity and tonicity.
Osmotic pressure:
The the minimum pressure that must be applied to a solution to halt the flow of solvent molecules through a semipermeable membrane
Osmolarity:
the number of particles of solute per liter of solution, whereas the term osmolality refers to the number of particles of solute per kilogram of solvent.
For example, NaCl dissociates into Na+ and Cl- when in solution and therefore contributes two particles. Glucose on the other hand doesn’t dissociate in solution and therefore only gives rise to one particle. For this reason NaCl contributes to a greater extent to the osmotic pressure of a solution than the same weight of glucose.
Osmolarity therefore provides us with a measure of the osmotic effectiveness of a solution.
Osmolarity is measured in Osmoles/Litre where 1 Osmole (Osm) is simply the molarity of the solute multiplied by the number of particles into which the solute dissociates when in solution. Here are a couple of simple examples it help clarify this:
Example One: If we have a 1M solution of glucose then the osmolarity of this solution = 1 Osm/L because glucose doesn’t dissociate in solution (i.e. for substances which don’t dissociate osmolarity and molarity are the same).
Example Two: If we have a 0.5 M solution of NaCl then the osmolarity of this solution = 1 Osm/L because NaCl dissociates into Na+ and Cl- when is solution (i.e. the osmolarity is 2 x molarity).
Example Three: If we have a 0.1 M solution of MgCl2 then the osmolarity of this solution = 0.3 Osm/L because MgCl2 dissociates into a Mg+and two Cl- ions when is solution (i.e. the osmolarity is 3 x molarity).
Tonicity:
The ability of an extracellular solution to enable water to migrate into or out of a cell via osmosis is referred to as tonicity
Be able to calculate the osmolarity of a number of simple solutions.
