Lecture 10, The Plasma Membrane and Transport Mechanisms Flashcards
Plasma Membranes
cholesterol = a specific type of fat molecule
glyco- = a sugar molecule (i.e. glycoprotein, glycolipid)
lipid = fat molecule
the plasma membrane is a phospholipid layer
- phospolipids = phosphorylated lipids
- contains proteins, lipids and carbohydrates
◦ either embedded in or associated with the bilayer
◦ ex. ion channels, receptor proteins, structural
support proteins, etc. (associated)
main function of the plasma membrane is as a selective barrier to the passage of molecules in/out of the cell
- having these structures embedded in the membrane allows for function
Transport Across the Plasma Membrane
- plasma membranes separate the internal contents of the cell from the extracellular space ( internal contents ex. intracellular fluid, organelles, proteins and enzymes, hormones…)
- plasma membranes also separate the internal contents of an organelle from the intracellular fluid
(example: oxidative enzymes within the mitochondria - allows for more efficient oxidative metabolism as the enzymes and metabolic intermediates are concentrated in one location) - movement of substances across plasma membranes are tightly controlled and regulated (transport mechanisms can be activated or inhibited based on cellular demands, external signals etc.)
- proteins embedded in the plasma membrane are important in allowing for selective transport of substances across the membrane (also important in detecting chemical signals from other cells, or signal present in the extracellular space)
Types of Plasma Membrane Proteins (2)
proteins associated with the plasma membrane can be categorized into two groups:
1. peripheral proteins
- proteins that only interact with the surface of the plasma membrane (can be either the intracellular surface or extracellular surface)
2. integral proteins
- proteins that are very closely associated with the membrane, and cannot be extracted from the membrane without disturbing the bilayer structure
- many integral proteins may span the plasma membrane, and are called transmembrane proteins
- permanently attached to the plasma membrane
- interact with the phosphilic group but also interact with lipid
membrane proteins are not static (in fact, the entire plasma membrane is dynamic, and proteins can move laterally relative to one another
The Fluid Mosaic Model
the fluid mosaic model is used to describe the fluid and dynamic nature of the plasma membrane
- the phospholipid bilayer allows for fluidity and elasticity to the membrane
mosaic: the layout or framework of something that involves multiple diverse elements
key points:
- the fluid mosaic model explains the structure of the plasma membrane as a mosaic of components (carbohydrates, lipids, proteins, cholesterol), all of which give a fluid character to the membrane
- the components of the plasma membrane all have unique functions, that in sum allow for the compartmentalization of cells and organelles, cell signalling, homeostasis, and metabolic regulations etc.
Membrane Transport Mechanisms
passive transport methods DO NOT require the input of energy
1. diffusion
2. facilitated diffusion
3. osmosis (diffusion of water)
- movement of the substance occurs down its concentration gradient
active transport methods DO require the input of energy
1. primary active transport
2. secondary active transport
- usually, movement of the substance occurs UP its concentration gradient (usually, but not always)
Diffusion
due to random thermal motion (the normal, random movement of molecules within a medium), molecules will undergo a net movement from an area of [high] to [low]
- in the absence of barriers, substances will move DOWN their concentration gradient
◦ net movement of the substance from the region
of [high] to [low]
over time, the concentration of the solute in the solution will be uniformly distributed = equilibrium
Simple Diffusion
the movement of molecules from one location to another solely due to random thermal motion
- do not require an input of energy
- molecules pass across the plasma membrane down their concentration gradient, without the need for a transport protein
- example: movement of oxygen and gasses across vascular epithelial tissue (ex. capillaries)
- the rate of diffusion is proportional to the concentration gradient
Diffusion Equilibrium and factors that affect rate of diffusion
when the net flux across the membrane in zero
- the flux in one direction is equal to that in the opposite direction
factors that affect the rate of diffusion
- temperature
◦ ↑ temp = ↑ rate
- Mass of the substance
◦ ↑ mass = decrease rate
- surface area of the plasma membrane
◦ ↑ SA = ↑ rate
- the solvent or medium (cytosol, intracellular, extracellular fluid)
◦ ↑ density = decrease rate
Facilitated Diffusion - Ion Channels (channel gating definition)
some molecules are unable to directly pass through the plasma membrane, and instead are able to move down their concentration gradient via specialized membrane proteins
- does not require an input of energy, but does require a membrane protein (ion channel)
- the movement of molecules across the plasma membrane depends on the membrane protein
ion channels demonstrate selectivity; only specific ions are able to pass through the channel
- ion channels may be present in either open or closed conformations
channel gating: the process of opening or closing an ion channel in response to a signal, the demands of the cell, or changes in environment [ion]
the rate of diffusion depends of how often and how long the ion channel stays open
- in addition to the factors that influence the rate of simple diffusion
Facilitated Diffusion - Transporter Proteins
another type of diffusion involves the use of transporter proteins
-transporter proteins still move molecules down their concentration gradient, without using energy (ATP)
- the transporter protein binds to the ion or molecule, and undergoes a conformational change that allowed the solute to be passed across to the other side of the membrane (structure dictates function)
Ion Channels vs Transporter Proteins
- ion channels, when open, create an open passageway for molecules to pass through
- transporter proteins directly bind to the molecule, and undergo a conformational change to move the molecule to the other side of the membrane
- both are membrane proteins that move molecules DOWN their concentration gradient, WITHOUT using energy (ATP)
Active Transport
active transport: the movement of a molecule or ion UP its concentration gradient
- moves the substance from a region of [low] to [high]
- requires a transporter protein
- requires an input of energy
an ATP molecule is hydrolyzed, releasing energy that is used to move the molecule up its concentration gradient
Primary and Secondary Active Transport (4 steps)
primary active transport is driven by the hydrolysis of ATP (steps 1 and 2)
- 3 Na+ molecules bind to the transporter protein on the intracellular side
- one ATP molecule is hydrolyzed (one of the phosphate bounds is being broken off), releasing energy that is used to change the conformation of the transporter protein, and releasing Na+ to the extracellular space
◦ ADP is released, but the third phosphate group
remains bound to the transporter protein
secondary active transport is driven by the conformational change induced by primary transport (steps 3 and 4)
3. the new conformation of the transporter protein has a high binding affinity for K+, and 2 K+ ions bind to the protein from the extracellular side
- binding of K+ to the transporter results in dephosphorylation of the transport, resulting in the change back to the original conformation, and release of K+ to the intracellular space
my notes:
primary active transport directly uses ATP whereas secondary active transport indirectly uses ATP
* primary is movement up, use the bounds within ATP to do the process
* primary leaves the transporter in different state - need for secondary one to occur
The Na+/K+ Pump
the transport protein involved in the primary/secondary transport of Na+ and K+ is called the Na+ /K+ pump
- essential in maintaining the proper intra- and extra-cellular concentrations of Na+ and K+
- maintaining proper electrochemical gradient
- important for the membrane potential in neurons
* pumps insure that the resting membrane stays at around -70 so that depolarization and repolarization can occur correctly
Counter-transport & Co-transport
counter-transport: the secondary transport of a substance in the opposite direction as the substance that is moved by primary transport
co-transport: the movement of both substances in the same direction across the membrane
