Cell Membranes And Transport Flashcards
How do cells communicate with the extracellular fluid?
Semi-permeable barrier of the plasma membrane
Describe the structure of cell membranes.
Fluid lipid structure 2 layers (bilayer): hydrophilic external layer + hydrophobic internal layer
What organelles in the cell have double membranes?
Nucleus
Mitochondria
What organelles in the cell have single membranes?
ER
Golgi apparatus
Others
What can the endoplasmic reticulum membrane be continuous with?
Outer nuclear membrane
How do organelles communicate with the intracellular fluid i.e. cytoplasm?
Membranes
What is the most important feature of cell membranes?
Semi-permeable - allow some substances to pass across easily but not others
What type of molecules can go across the cell membrane? How do they move across?
Small, uncharged and/or fat-soluble things e.g. O2, water + steroids via simple diffusion down a concentration gradient
What is diffusion?
The movement of molecules from an area of high concentration to low concentration i.e. down a concentration gradient
What is osmosis?
The movement of water across a semi-permeable membrane from an area of low-solute concentration to high-solute concentration
What are the mechanisms of movement of molecules across the cell membrane?
Simple diffusion
Facilitated diffusion
Active transport
Exo/endocytosis
Explain facilitated diffusion.
Requires a transporter (carrier molecule or channel) to facilitate transport but no extra energy is needed as this is passive - molecules move down a concentration gradient
What molecules move via facilitated diffusion?
Larger molecules e.g. glucose + charged molecules
Explain active transport.
Uses ATP to move molecules against their concentration gradients
What is secondary active transport?
Where a molecule e.g. Na+ is actively pumped out of a cell then allowed back in via transporter that also takes in another substance e.g. sugars, AAs
What type of transport is endocytosis and exocytosis?
Bulk i.e. large molecules or other substances can be transported in bulk
How can the knowledge of this lecture be applied to physiological processes?
Absorption of food molecules in the gut as glucose is absorbed via co-transport with Na+
Secretion of acid into stomach - active process of pushing more acid into an already acidic environment
Excretion of substances from kidney
What are transporters?
Integral membrane proteins which are channels or carriers that are selective + regulated
Can be passive or active
What is the difference between channels and carriers?
Channels are faster + less specific than carriers
How are voltage-gated transporters regulated?
Respond to changes in membrane potential
How are ligand-gated transporters regulated?
Binding of ligands e.g. EC (neurotransmitters) or IC (2nd messengers)
How are mechanically-gated transporters regulated?
Respond to sound, touch + stretch etc.
What is the difference between passive and active transporters?
Passive transporters do not use metabolic energy + transport is by facilitated diffusion from [high] to [low] whereas active transporters used metabolic energy derived from hydrolysis of ATP to move molecules against a concentration gradient
What are the different names for co-transport systems?
Secondary
Coupled transport system
What is the difference between symport and antiport?
Symport is where compounds go in the same direction whereas antiport is where the compounds are transported in the opposite direction to eachother - 1 compound will go down concentration gradient whilst the other goes against
What are the different types of co-transport?
Symport
Antiport
Why do molecules move into and out of cells?
Take in nourishment
Get rid of waste
Communicate with ‘outside world’
What is an ion?
Charged particle
What is an anion?
-tively charged particle
What is a cation?
+vely charged particle
Do ions need aid moving in and out of cells?
Yes because they are charged
What is an electrochemical potential?
Combines chemical and electrical potential:
- Chemical potential is concentration difference
- Electrical potential is the charge difference
Where is there an electrochemical potential?
Across plasma membrane
What is membrane potential?
Charge difference across the membrane
What is the resting membrane potential?
Inside membrane is more negative than outside so typical value in a resting cell is ~70mV
What ions contribute to the membrane potential?
Ions close (<1nm) from membrane
What are the main intracellular and extracellular ions?
IC: K+
EC: Na+ but also Cl- & Ca2+
How is a resting membrane potential established?
Cell membrane more permeable to K+ than other ions as it contains K+ leak channels that are constantly open
More K+ inside the cell so it moves out but outside of cell is +vely charged which makes K+ want to move in - system reaches equilibrium where there is no net K+ movement
Why is the cell membrane potential more negative on the inside?
Inside of cell contains large negatively charged molecules like DNA + RNA
What ion movement occurs in hyperpolarisation?
K+ moves out of cells whilst Cl- moves in = making the cell more -ve
What ion movement occurs in depolarisation?
Na+ & Ca2+ move into cell = making cell more +ve
What are action potentials?
Cycles of depolarisation, hyperpolarisation + return to resting state due to opening + closing of voltage gated Na+ & K+ channels
What is the function of action potentials?
Powerful way of signalling to a cell e.g. getting a muscle to contract
What ion movement occurs during an action potential?
Na+ influx into cell
K+ efflux out of cell
