Biological Membranes Flashcards
are Biomembranes are usually selectively permeable or semi?
- Selectively
-Water moved by osmosis - At least some solutes don’t move through
- solute movement through plasma/cellular membranes is especially complex
- influenced by an interaction between:
- chemistry of the membrane
- chemistry of the solute
Membrane permeability of a solute predicted by:
- solute partition coefficient (ie. lipid/water solubility)
- molecular mass and shape
why are some forms of energy always required to drive solute movement
- to overcome H-bonds with water
-alter protein conformation (shape)
etc.
2 types of Solute exchange mechanisms
- Passive exchange
- potential energy stored in electrochemical gradients drivers solute movement
- solute always moves ALONG the gradient
(includes simple diffusion, ion channels & facilitated diffusion) - Active Exchange
- cellular energy (ATP) is needed to drive the movement of a solute
- solute generally moves AGAINST the gradient (includes active transport & bulk transport)
Passive Exchange - Simple Diffusion:
- restricted to small, non electrolytes solutes
What must solutes have to do simple diffusion/passive exchange?
- solute must have high kinetic energy
- have few (or no) H-bonds with water
- fit through gaps between membrane lipids
- be non-polar or have minimal polarity
What substances can be moved by simple diffusion?
-gases
-fatty acids
-steroid hormones
-a few other SMALL organic molecules
is water polar or non-polar?
polar
is water permeable?
water is freely permeable
How exactly does water get through the cell membrane during osmosis?
-water is small; has high kinetic energy
- some will fit through gaps between fatty acid tails in lipid bilayer of membrane
- plus many cells have special water channels called aquaporins
- intrinsic protein that forms a channel through lipid bilayer
what are ion channels
- are specialized integral (intrinsic) proteins - channels that allow ions through
- traverse the lipid bilayer
- provide a pathway for ions to move
what specific characteristics that ions are usually selective for?
Mass and Charge (+/-; mono/divalent)
are ion channels reversible?
fully reversible, opposites attract
reverse the gradient = reverse the flow
in ion channels where do the proteins go through?
they go through the lipid bilayer - many types of protein channels
non-specifically how many subtypes of channels do ions have?
each ion has several different subtypes of channels
what happens to ions as they pass through the channels
ions are dehydrated as they pass through,
allows selection by ionic radius - selective for + charge
what are the names of channels that are open all the time?
leak channels
opening/closing of other channels are regulated are called what?
gated channels or regulated channels
what are the four types of gated channels
- voltage gated channels
- stretch or tension gated channels (cytoskeleton pulls the channels open, stretching it open)
- phosphorylation gated channels (needs phosphate energy to open)
- ligand gated channels (ie nicotinic receptor for acetylcholine)
an example of how a Na + voltage-gated channel works
- positive charge repels sensor upwards
- sensor movement pulls on channel
- pore opens wider
- Na+ from outside can flow through
- this Na+ magnifies effect on sensor repulsion & cycle repeats
Facilitated Diffusion, what does it require
also requires a membrane protein (called a carrier or transporter)
- transporter has a binding site for the solute
- solute physically interacts with the transporter
- solute binding causes conformational change
Is facilitated diffusion reversible?
It is reversible, needs concentration gradient to drive movement
how does facilitated diffusion differ from channels?
facilitated diffusion only requires solute to react with site to work.
what are the general features of facilitated diffusion
- direction of transport is always along the gradient - reverse the gradient =reverse the direction
- binding the solute to transporter is reversible - small forces, physical interaction
- rate of exchange increases as size of concentration gradient increases
- rate of exchange increases as # of transporters increase
- but number of transporters in a cell are limited - rate of transport can be saturated
physiological examples: facilitated diffusion
-glucose transport
-Band III protein in RBC’s
- CO2 transport
-transport of other substances in the gut, kidney and elsewhere
what is faster, simple diffusion of glucose or facilitated diffusion of glucose in RBC
facilitated diffusion is faster
- plus, glucose transport occurs even when blood concentration of glucose is low
What are three things that the rate of glucose transport into cells depends on?
- Maintaining the glucose concentration gradient
- Different types of transporter proteins
- More membrane transporters = faster rate
why is the concentration of glucose very low in the cytoplasm?
and what helps to maintain the [gradient] for glucose?
glycolysis
- hexokinase helps
What is GLUT 1
Most cells
GLUT 2
LIVER, beta cells, hypothalamus, basolateral membrane of small intestine
- whole body glucose regulation
GLUT 3
Neurons, placenta, testes
- very low Km (~1mM)
GLUT 4
Skeletal and cardiac muscle, adipose cells Activated by insulin
GLUT 5
Mucosal surface in small intestine, sperm fructose specific
What is the difference between active exchange and active transport
Active exchange is the whole picture where as active transport is a specific protein
Active transport
- utilizes membrane proteins
- energy from ATP can move solute AGAINST concentration gradient
What is needed for active transport
cellular energy source (e.g glucose) is needed
What are the two different types of active transport
Primary Active Transport
Secondary Active Transport (which is set up by primary AT)
Primary Active transport
-membrane protein is also a transporter (pump) [pushing something where it doesn’t want to go]
- ATP used to directly phosphorylate the transporter
- Transporter is an ATPase (ATP to ADP & PO4^-2)
What is PAT generally used to transport?
inorganic ions eg. NaK ATPase, K+H+ ATPase, Ca++ ATPase etc.
- overcoming concentration gradient
what is an electrogenic pump
where unequal amount of charges go through membranes ie. 3 Na+ out and 2 K+ in
what is an electroneutral pump?
where equal amount of energy go in and out - ie . 2+ charge in 2+ charge out
all living cells have a small transmembrane potential of what?
-10 mV (inside negative)
the electrogenic (or 3:2) Na+K+ Pump is found in…
what is its primary purpose
found in all cells
- its primary purpose is to maintain the Na+/K+ [gradients]
- without ATP, the gradient disappears
- in neurons and muscle cells, this gradient also exploited for bioelectricity
Secondary Active Transport
- How is the energy from ATP used
- energy from ATP is used more indirectly
- PAT establishes an ion [gradient]
what ion from primary AT is used most often
often but not always Na+ (sodium gradient)
What is the most often used solute moved by secondary active transport
small organic molecules (amino acids, glucose, fructose, vitamin, etc)
Bulk transport- what size of solutes can be moved this way
- bulk transports are used for other organic solutes & colloids
- simple/facilitated diffusion - when larger amounts of substances must be exchanged across the membrane
what are physiological examples of bulk transport
- endocytosis used in cholesterol uptake (IN)
- exocytosis used in neurotransmitter & hormone release (OUT)
Energy is used to produce & relocate the membrane-bound vesicles inside the cell
