how transport across the membrane is regulated Flashcards
The cell membrane is selectively permeable. what molecules is it permeable to?
The lipid bilayer is permeable to non-polar molecules such as O2, CO2 and hormones
what is the cell membrane impermeable to?
It is impermeable to ions and large molecules such as Na+ and glucose. They need trans-membrane channel and carrier proteins
is it permeable to water?
It is slightly permeable to small uncharged polar molecules such as water
Membrane permeability: Gradients
why is selective permeability important in terms of gradient
Selective permeability allows cells to build concentration gradients
pH can be regulated
Electrical gradients are built (the inside of a cell is more –ve) creating membrane potentials
give an example of concentration gradient.
E.g. O2and Na+ are more concentrated in the extracellular fluid: the opposite is true for K+ & CO2
transport can occur in 3 different ways . what are they ?
passive
active
vesicular
what processes does passive transport carry out?
diffusion,
osmosis
facilitated diffusion
what processes does active transport carry out?
primary active transport
secondary active transport
what processes does vesicular transport carry out?
endocytosis
phagocytosis
pinocytosis
trancytosis
what is passive transport
No cellular energy is used as substances move down their own concentration gradient.
• It is of the following types
– 1. Diffusion through lipid bilayer
– 2. Channel mediated facilitated diffusion
– 3. Carrier mediated facilitated diffusion
How Is diffusion affected by: Temperature Concentration gradient Mass Surface area Diffusion distance
temp: increase will increase rate of diffusion
increase in conc will increase diffusion
mass increase will decrease rate of diffusion
surfave area depends
diffusion distance will decrease diffusion rate
what are the two types of facilitated diffusion?
Channel mediated
- Can be gated
- Can be timed or signal regulated
Carrier mediated
Subject to transport maximum and saturation
how is faciltated diffusion regulated? give an example
The selective permeability can be regulated to maintain homeostasis
E.g. the hormone insulin, via its receptor, up-regulates
give an example of a disease which is due to malfunction in facilitated diffusion .
Diabetic patients can lack the ability to up regulate GluT
what is osmosis?
what transmembrane protein is used to transport water?
• It is the diffusion of water through a semi-permeable membrane.
– diffusion through the lipid bilayer occurs through specific transmembrane protein channels called aquaporins
what does the following words mean?
Hypertonic
Isotonic
Hypotonic
hypertonic causes water to move out of a cell causing cell to shrivel up.
isotonic is when there is a balance in water potential inside and outside the cell causing the cell not to be affected by osmosis.
hypotonic causes water to move into the cell due to unbalanced water potential.
give an example of Osmosis & tonicty
RBC’s for transfusion are destroyed by hypertonic and hypotonic solutions
Most intravenous solutions are isotonic (0.9% saline or 5% dextrose)
give a medical use for hypertonic solutions
Infusion of hypertonic solution to relieve cerebral edema
what is active transport ?
- It involves expenditure of energy from hydrolysis of ATP.
- It is used to transport essential ions against their concentration gradient. (Helps maintain tonicity, volume and charge)
what are the two types of active transport?
Primary active transport (uses energy from the hydrolysis of ATP)
– Secondary active transport (uses energy stored by an ionic concentration gradient)
what is primary active transport?
Movement against a concentration gradient (mainly ions)
ATP changes the shape of the carrier protein, pumping the substance across
Cells typically use 40% of their generated ATP this way
Must work non-stop due to passive leaking
give an example of primary active transport.
sodium-potassium pump
removes 3 Na+ by binding from cytosol. ATP is hydrolysed by ATPase.
2K+ is gained via carrier protein.
what is secondary active transport (co-transport )
Secondary active transportis a form ofactive transportacross a biological membrane in which a transporter protein couples the movement of an ion (typically Na+or H+) down its electrochemical gradient (this gradient has been made by primary active transport) to the uphill movement of another molecule or ion against a concentration/electrochemical gradient.
what is symport and antiport ?
Symport is where the molecules travel in the same direction (used for Ca2+ regulation and H+ regulation)
Antiport is where they go in opposite directions
explain how glucose is transported into a kidney or small intestine cell.
- low conc of glucose in the lumen compared to the epithelial cell.
- high conc of Na+ in the lumen
- Na+ and glucose go together due to diffusion gradient and are co-transported into epithelial
- this occurs as Na+ is at low conc in epithelial due to the primary active transport of Na+ by a sodium-potassium pump into the blood stream.
- glucose travels into the blood stream as there is a lower concentration of glucose in the blood stream compared to the epithelial.
what is endocytosis and exocytosis?
Used for endocytosis (bringing substances into the cell) and exocytosis (expelling from the cell)
Used for inter-organelle transport
Both use ATP
Endocytosis is further classified into:
Receptor mediated endocytosis
Phagocytosis and
Pinocytosis
what is the use of Receptor mediated endocytosis?
Typically used for transporting LDL or some vitamins and hormones
what is clathrin?
Clathrin is a protein for enriching membrane domains
describe the steps that occur in Receptor mediated endocytosis
Binding lateral diffusion invagination Vesicle formation Unseating(uncoating) Fusion with endosome either transcytosis Recycling of receptor Degradadtion in lysosomes
example of Receptor mediated endocytosis
CD4 Receptor mediated endocytosis in a T helper cell
what is pahgocytosis
Engulf large particles such as worn out cells and bacteria
Is carried out by macrophages and neutrophils
describe the process of phagocytes
- Chemotaxis and adherence of microbe to phagocyte
- Ingestion of microbe to phagocyte
- Formation of phagosome
- Fusion of the phagosome with a lysosome to form a phagolysosome
- Digestion of ingested microbe by enzymes
- Formation of residual body containing indigestible materials
- Discharge of waste materials
what is Bulk phase endocytosis (pinocytosis = to drink)
Small droplets of extracellular fluid are taken in
No receptors are needed
All solutes in the extracellular fluid are brought in (non-selective)
where does pinocytosis occur the most?
Occurs in most cells but especially absorptive cells in the intestines and kidney
what is Exocytosis
It is used to transport material out of the Cell.
which cells exhibit exocytosis?
It is exhibited by all cells but very important in
– Secretory cells (digestive enzymes and hormones)
– Nerve cells (neurotransmitters)
what is trancytosis ?
Combination of endo and exocytosis
Substances are passed through a cell
where is trancytosis expressed?
Is common in endothelial cells which line the blood vessels
How are gap junctions formed?
where are they found? give two examples.
how do they work? i.e: talk about the type of molecules they allow to pass through. and anything else you know about how they function.
Gap junctions are formed between adjacent cells which are connected by hollow cylinders known as connexons. They are found in electrically excitable tissues such as heart and smooth muscles.
They directly connect the cytoplasm of two cells together enabling small molecules/ ions and electrical impulses to pass through. These channels act like tunnels although not with a big opening but large enough for movement of molecules from cytoplasm of one cell to the next.
What is the role of tight junctions?
How are they formed?
Where can they be found?
Tight junctions prevent movement of substances.They prevent anything passing through extracellular space between them
Tight junctions are formed by the fusion of integral proteins of adjacent cells.
They can be found in cells lining the digestive tract.
How are tight junctions formed?
Where is it common?
Tight junctions are formed when two membrane bond into one, making a strong barrier between two cells. they form solid walls.
It is common where one area needs to protected from the contents of other areas, sealing epithelial cells together.
Is there any intra-cellular space where tight junctions are found and explain your answer?
There is no intra-cellular space found where there is a tight junction, because there re long rows of tight junction proteins that form a complex mesh-work.
where are adherens junctions found?
Found at sites where cell-to-cell interactions occur of epithelial cells, which are subject to stretching and pulling, yet require tight contact between them so they can function as one unit.
what is the function of adherens junction?
Give an example.
To maintain cellular position. For example they resist separation in contractile movements such as peristalsis.
Describe the structure of adherens junction.
PLAQUE which attaches to membrane proteins and the microfilaments of the cytoskeleton.
Often form adhesion belts
They connect actin filaments in neighbouring cells.
how does a hemidesmosome differ from a desmosome?
hemisdesmosome athough has a similar structure to desmosome yet it JOINS a cell to its BASILAR membrane (basement membrane) rather than to another cell.
describe the structure of desmosome/ hemisdesmosome.
they are anchoring junctions held together by linker protein filaments called cadherins or integrins extending from button like thickenings called plaques, this distributes tension and prevents tearing e.g: skin and heat muscle.
what is the role of desmosomes?
they physically connect cells like the gap junction but no opening is created. proteins that bond the membrane of one cell to its neighbour creates desmosomes: cell-to-cell adhesion.
