M3a L26-27 Flashcards
Describe the functions of membrane transport and the concept of membrane selective permeability.
different membranes have different permeability depending on various factors. this allows it to allow different things into the cell, non polar gasses, have high permeability, while charge ions and polar ions/molecules have low. cholesterol affects membrane fluidity
Outline the mechanism of action of cardiotonic steroids e.g. digoxin.
they lock out the ATPase on the outside open state affecting Na+/K+ gradients. this gradient is used for glucose and calcium transport. this prevents maintenance of the sodium ion gradient, affecting calcium concentration. If calcium is not exported, because its the on signal for muscle activity, results in increased muscle activity, cardiac muscle contraction
Describe and explain how glucose is absorbed from the small intestine into the circulation (blood), focusing on the relevant membrane transport proteins SGLT, Na+/K+ ATPase and GLUT2.
Glucose is moved from the lumen of the intestine to the cytosol, against its gradient, this requires energy, ATP, from maintaining the Na+ gradient on the other side in the plasma membrane due to Na+/K+ ATPase. Part of the SGLT transporter secondary transport, part of a symport The other part of this symport is sodium, which wants to enter the cell, low to high.
Afterwards, the sodium is taken out of the cytosol into the bloodstream and potassium is transported into the cytosol out of the blood stream in another antiport process, also primary active transport this is from Na+/K+ ATPase transporter ATP is needed
Outside the cell has high sodium, low glucose, high potassium
Describe the mechanism of the Na+/K+ ATPase (Na+/K+ ‘pump’).
the energy required for transport of 3Na+ & 2K+ against their concentration gradients is coupled to ATP hydrolysis. in the middle of the membrane is 2 K+ ions
3Na+ bind on cytosolic side of ATPase (high Na+ affinity state)
Causes phosphorylation of ATPase by ATP
Phosphorylation results in a conformational change (low affinity for Na+) expels 3Na+ outside the cell
Binding of 2K+ (high affinity for K+) promotes dephosphorylation
Loss of phosphate promotes original starting conformation open to cytosol releasing 2K+ (low K+ affinity) inside the cell and resetting the ATPase to bind further 3Na+
Define and describe symport, antiport and uniport transport systems and the differences between them. Describe examples of each of these transport types.
Uniport transport systems
For 1 species, can be facilitated diffusion (with gradient) or primary active transport (against gradient)
Antiport and symport cotransport systems
For 2 species at the same time, obligatory either both (primary active transport) or one (secondary active transport) needs to move against a gradient
Outline how ligand-gated and ion-gated channels function
gated channels give the cell a control mechanism to regulate transport depending on requirements
ligand gated: the ligand binds to its receptor, triggering the channel so it opens.
voltage-gated: it responds to a different in ionic concentration across the membrane
Describe how the processes of active transport (primary and secondary) achieve the transport of molecules/ions across membranes.
Primary active transport uses ATP hydrolysis to transport molecules or ions against their concertation gradient and creates/maintains a concentration gradient of ions. This concentration gradient is a potential energy store.
Secondary active transport uses the chemical potential energy contained within an ion gradient to ‘drive’ the active transport of a different molecule/ion across the membrane against its concentration gradient. This is a co-transport process
Describe and explain the differences between the types of membrane transport processes: non-mediated diffusion, facilitated diffusion and active transport, and their relationship to membrane permeability.
in active transport the cells need to provide energy to move the substances across the membrane gradient. Energy is provided by either ATP or the energy stored in ion gradients across membranes
Facilitated transport is a passive process of transporting molecules/ions through a membrane transporter, also called mediated transport/diffusion. This is a diffusive process which hydrophilic/charged molecules/ions that are otherwise unable to cross the lipid bilayer. No extra energy is required, the concentration gradient determines the direction of transport.
The transport of molecules that don’t need transport systems is called non-mediated transport. Plasma membrane is a physical and chemical barrier. Molecules will diffuse across the membrane if they are lipid soluble (i.e., hydrophobic). No protein transporter involvement. Diffusion relies on membrane permeability and concentration gradient, Molecules move from high to low concentration to reach equilibrium
most common lipid is phospholipid
lipids and proteins can diffuse laterally and rotationally
peripheral membranes proteins are either in the outer or inner membrane surface
phospholipids are amphipathic
Describe how the processes of facilitated diffusion and non-mediated diffusion achieve the transport of molecules/ions across membranes.
Protein pores (protein channels) have a hydrophilic channel or pore running through the middle of the protein structure from one side to the other so the hydrophilic molecules do not have to interact with the hydrophobic fatty acyl tails of the membrane phospholipids in the bilayer.
Protein permeases are transporters that undergo a conformational change to be able to flip molecules to their side and let them go through one side of the membrane to the other by opening their other to let them pass.
Both pores and permeases require a concentration gradient of their cargo molecules to drive transport, aka they are both passive transporters
