Lesson 1 - Transport Mechanisms Flashcards
what is the cell membrane composed of and what molecules can diffuse readily
- Cell membrane = composed of Phospholipid bilayer
- Lipid-soluble molecules and gases diffuse through readily
- Water-soluble molecules cannot cross without help
- Impermeable to organic anions (proteins)
–> selective permeability
what does cell membrane permeability depend on? (3)
Permeability depends on molecular size, lipid solubility, and charge
explain simple diffusion
- what molecules pass through?
- what direction does it move in?
- what is the rate of diffusion
- what energy is needed, if any?
- Small, lipid-soluble molecules and gases (e.g. O2, CO2, ethanol, urea etc…) pass either directly through the phospholipid bilayer or through pores
- Movement of substance is down its [ ] gradient
- The relative rate of diffusion is roughly proportional to the [ ] gradient across the membrane
- Passive: No energy input required from ATP
explain facilitated diffusion
- what is it?
- what molecules pass through?
- what direction does it move in?
- what energy is required, if any?
- Process of diffusion, where molecules diffuse across membrane, with the assistance of carrier protein
- Carrier protein aid the movement of polar molecules (e.g. sugars and amino acids) across cell membrane
- Movement of substance is down its [ ] gradient
- The energy comes from the [ ] gradient of the solute
- Passive: No energy input required from ATP
how do channel proteins work?
- one gate is open
- molecule enters
- undergoes a conformational change
- first gate closes and second opens to release the molecule
what is active transport?
- what is it?
- how does it work
- what energy is needed and how is it acquired?
- Active Transport is a mechanism to move selected molecules across cell
membranes, against their [ ] gradient - Substance binds to protein carrier that changes conformation to move substance across membrane
- Active Requires energy from ATP hydrolysis
what are ATPases and what is a famous example?
- ATPases are a group of enzymes that catalyze the hydrolysis of a phosphate bond in adenosine triphosphate (ATP) to form adenosine diphosphate (ADP)
- the sodium potassium pump is a famous example of how ATP is used to transport Na out and K in
what is secondary active transport?
- what is it?
- how does it work?
- what does sequential binding do?
- when a substance is transported up/against its concentration gradient without using ATP
- it works by harnessing the kinetic energy that one substance generates from moving down its [ ] gradient to move its substance against the [ ] gradient
- Sequential binding of a substance and ions to specific sites in the transporter protein induces a conformational change in the protein
what are channels?
- what is it made of?
- what does it look like in the membrane?
- is it selective? if so how?
- what are pores also called?
- membrane spanning protein (around 4-5 protein subunits) creates a central pore for which specific ions can diffuse
- These ‘Pore loops’ of the protein molecules dangle inside the channel
- Physical properties of the pore loops create a selectivity filter
–> Only specific molecules can diffuse through (by means of size and electric charge) - These ‘pores’ are called Membrane Channels
what are gated channels
- what does the protein shape of gated channels mean for the diffusion of a substance?
- what are the two types of gated channels that determine protein shape?
- Membrane channels (these holes in the membrane) generally are not kept open
- Channels can be closed off by a branch of the protein structure = ‘Gate’
- gate closes = no diffusion, gate open = diffusion (remember that it is still selective)
- The protein components switch between 2 shapes; one creates an open pore, the other blocks the pore
- Factors determining channel protein shape (aka when it opens and when its closed):
– ligand gates channels = binding of chemical agent
– voltage gated channels = voltage across the membrane
ligand gates channels
- what body system are cell membrane receptors part of?
- what does the channel need in order to change shape?
- Cell membrane receptors are part of the body’s chemical signaling system
- The binding of a receptor with its ligand usually triggers events at the membrane, such as activation of an enzyme
voltage gated channels
- explain membrane potential and what it usually is like in a cell?
- how does voltage gated channels work depending on the membrane potential
- what is the voltage sensing mechanisms on these gated channels?
- all cells generate a membrane potential
- usually the voltage inside the cell is negative and outside the cell is positive
- Some membrane channels are sensitive to the potential difference across the membrane (e.g. depolarization) - polarized = gate is open, depolarized = back to natural position - closed
- The voltage sensing mechanism is in the 4th transmembrane domain of the protein, the S4 segment. the S4 segment is what changes its shape
voltage gated channels - S4 segment
- where is the s4 segment and what does it look like naturally and in a polarized cell?
- what does the s4 segment look like in a depolarized cell?
- S4 sticks out to the side of the protein (like a wing)
Naturally:
* The natural position of the S4 ‘wing’ is up towards the outer surface of the cell membrane.
Polarized:
- When the membrane is polarized, the positively charged wing is attracted downwards to the negatively charged inner surface of the membrane
Depolarized:
- when the membrane is depolarized (around -50 mV) (inside is more positive), inner region no longer provides sufficient electrical attraction to hold the S4 wing downwards, so it migrates back-up
* In the up position, S4 removes a structural occlusion from the pore (gate is open) such that ions can now diffuse through it (from inside of the cell to outside)
define endo/exocytosis generally
- Endocytosis (outside to in): inward ‘pinching’ of membrane to create a vesicle; usually receptor- mediated to capture proteins, from outside to inside.
- Exocytosis (inside to out): partial or complete fusion of vesicles with cell membrane for bulk trans-membrane transport of specific molecules, from inside to outside.
how many types of exocytosis are there?
two
exocytosis 1: the more rapid mechanism had been dubbed the ‘Kiss and Run’
exocytosis 2: full exocytosis
