1.3 Flashcards
What make up membranes?
a bi layer of phospholipid molecules and a patchwork of protein molecules; fluid-mosaic model.
Describe the head region of a phospholipid molecule.
Is charged and therefore hydrophilic.
Describe the tail region of a phospholipid molecule.
is uncharged and non-polar so therefore hydrophobic.
What gives membranes fluidity?
The phospholipids are constantly changing positions.
What two types membrane proteins are there?
integral and peripheral.
How are integral proteins held within phospholipid bi layer?
Regions of hydrophobic R groups allow strong hydrophobic interactions.
integral Proteins…
are held firmly within the membrane.
What are transmembrane proteins?
Integral proteins (some); span the entire width of the membrane; e.g. channels, transporters and many receptors.
Peripheral proteins…
loosely associated with plasma membrane; hydrophilic R groups on surface so bound mainly by ionic and hydrophilic interactions; interact with surfaces of integral proteins.
transport across the membrane:
phospholipid bi layer barrier to ions and most uncharged polar molecules; small molecules such as O₂ and CO₂ pass through by simple diffusion; facilitated diffusion is positive transport of substances through transmembrane proteins.
Channel proteins:
different ones in different cell types due to specialisation; most in animal/plant cells are highly selective; multi sub-unit proteins arranged to form water filled pores extended across membranes; some gated, some free flowing.
How do gated channel proteins work?
change conformation to allow or prevent diffusion; respond to stimulus.
What are ligand gated channels?
they are gated channel proteins controlled by signal molecules; chemical.
What are voltage gated channels?
They are gated channel proteins controlled by ion concentration changes; electrical.
Transporter proteins…
allow specific substance to bind to be transported by undergoing conformational change.
Two types of transporter proteins…
Facilitated (e.g. glucose) and Active (e.g. Na/K-ATPase)
The conformational changes of transporter proteins…
Conformational change requires energy from ATP hydrolysis; alternate between two conformations so binding site for solute is exposed on one bi layer side then the other.
Active transport…
uses pump proteins to transfer substances against concentration gradient; pumps are transporter proteins coupled to an energy source; metabolic energy required; some hydrolyse ATP directly for energy.
What do ATPases do?
The hydrolyse ATP.
What are aquaporins?
allow facilitated diffusion of water across plasma membrane; can allow 3 billion water molecules per second; dependent upon osmotic gradient.
What is the electrochemical gradient?
for solute carrying net charge, concentration gradient and electrochemical potential difference combine to form electrochemical gradient to determine solute transport.
What is membrane potential?
created when difference in electrical charge (ion concentration) either side of membrane.
How is an ion gradient established and maintained?
through ion pumps using energy from ATP hydrolysis
Sodium-Potassium pump (Na/K-ATPase)…
transports sodium and potassium ions actively against steep gradient and accounts for a significant part of basal metabolic rate (25% in humans).
Na/K-ATPase process…
- protein has high affinity for Na+ inside the cell, binding occurs
- phosphorylation through ATP hydrolysis causes conformational change
- affinity for Na+ lowers so released outside cell
- K+ bind outside cell
- de-phosphorylation occurs causing conformational change.
6.K+ released inside cell
establishes concentration and electrical gradients.
ion ratio in Na/K-ATPase…
for every ATP hydrolysed, 3 Na+ transported out and 2 K+ transported in.
What is the function of Na/K-ATPase within the small intestine?
the sodium gradient established drives glucose active transport; glucose symport transports Na+ and glucose at same time in same direction; intestinal epithelial cells; Na+ travels down gradient and glucose against gradient.
What is a symport?
An integral membrane protein involved in simultaneously transporting two substances across membrane in same direction.
Other functions of the sodium-potassium pump…
- maintaining osmotic balance in animal cells
- generation and long term maintenance of ion gradient for resting membrane potential in neurons.
- generation of ion gradient in kidney tubules.
