Lecture 22: Transport Of Solutes Across Cell Membrane Flashcards
What is non mediated transport?
Transport across them membrane that does not directly use a protein
What is mediated transport?
Transport that moves material with the help of the protein
What is passive transport?
Transport that moves substances down their concentration gradients with only their kinetic energy
What is active transport?
Transport which uses energy to drive substances against their concentration or electrochemical gradient
What is vesicular transport?
Transport which moves materials across membranes in small vesicles either by exocytosis or endocytosis
What is a non polar molecule?
A molecule without electrical pores (does not have charge)
What is the process of non mediated transport?
Diffusion through the lipid bilayer
Why is non mediated diffusion important?
Because it’s used for the absorption of nutrients and the secretion of wastes
What type of molecules use non mediated transport and give examples
Many non-polar, hydrophobic molecules
E.g: oxygen, carbon dioxide, nitrogen, fatty acids, steroids, small alcohols, ammonia, fat-soluble vitamins (A, E, D and K)
Three properties of channels?
- Ion selectivity
- Gating
- Electrical current
What do channels do?
They form cores that span across the membrane but they do not bind to solutes
What are the channels made from?
Proteins that fold to form the core
What do the channels form?
A water-filled pore that shields the ions from the hydrophobic pore of the lipid bilayer
What does the channel allow through?
Ions and small molecules
What is the water filled pore of the channel lined with?
Hydrophilic amino acids
What is the outside of the folded protein lined with?
Hydrophobic amino acids
Is the channel the only way ions can get to the cell?
Yes, it’s the only way ions can pass through the hydrophobic core
What law is the channel governed by?
The laws of diffusion
Is ion channel transport rapid? How many ions pass through per second? Why is this?
Yes, about 1 million ions per second.
Fast because ions do not bind
How is the channel ion selective?
By having specific amino acids lining the core, in which the ion is determined by charge (eg a positively charged amino acid attracts negatively charged ions)
What does the channel selectivity allow for?
It allows the channel to harness the energy stored in the different ion gradients
What do ion gates do?
They control the opening and closing of the pore
What controls the opening/closing of the pore, and give examples?
Different stimuli, such as: voltage, ligand binding, cell volume (stretch), pH, phosphorylation
How do you measure the electrical current of the channel gate?
The patch clamp technique
What is the measurable current of the channel gate?
10^-12 amps
What do current fluctuations represent I’m regards to channel gates?
The opening/closing of a single ion channel (like pulses- no current when shut)
What do the current fluctuations represent?
The conformational changes in channel structure that are associated with channel gating
What are three types of passive transport?
- Diffusion through a lipid bilayer
- Diffusion through a channel
- Facilitated diffusion
What is carrier transport?
When the substrate to be transported directly interacts with the transporter protein
Is the carrier transport slower than channel transport?
Yes, because the transporter undergoes a conformational change
Is carrier transport active or passive?
It can be both (whereas channel transport is only passive)
What is the key difference between channels and carriers?
Binding.
What are two essential sequences for carrier transport?
- Molecule must bind to transport protein
2. Change in conformation , which opens a gate and allows molecule to move across the cell membrane
Can the protein carry any sized molecule in carrier transport?
No, it must fit in the binding pocket
What are carrier-mediated transport properties similar to?
Enzyme properties
What four properties do carrier mediated transport exhibit?
- Specifity
- Inhibition
- Competition
- Saturation
What is carrier-mediated transport specificity?
Is specific to a particular structure (eg glucose)
What is inhibition in carrier mediated transport?
If a molecule binds to a transport site, it stops transport
What is competition in carrier-mediated transport ?
If two types of substrates can bind to a transport site, they will compete (similar to enzymes)
What is saturation (transport maximum) of carrier- mediated transport?
If there’s a big concentration of molecules, transport eventually saturates as there’s only so many binding sites
What is an example of the enzymatic kinetics that carriers display?
The transport of glucose that occurs until all binding sites are saturated
What is facilitated diffusion?
Passive carrier-mediated transport
What is the glucose transport protein called?
GLUT
What causes and where does glucose get converted during facilitated diffusion?
The enzyme kinase, and it occurs inside the cell (once released from protein)
What does glucose get converted to in facilitated diffusion?
Glucose-6-phosphate
Why does glucose get converted in facilitated diffusion?
So equilibrium is never reached, it maintains the concentration gradient for glucose entry (otherwise no more glucose would enter the cell which would be bad)
What are the steps of glucose facilitated diffusion?
- Glucose binds to GLUT
- The transport protein (GLUT) changes shape. Glucose moves across cell membrane (but only down concentration gradient)
- Inside the cell, kinase enzyme reduces glucose concentration by converting glucose into glucose-6-phosphate
- Conversion of glucose maintains concentration gradient for glucose entry
What is active transport?
An energy requiring process that moves molecules and ions against their concentration or electrochemical gradients (utilises metabolic energy)
What is metabolic energy?
The energy from the process of generating ATP
What are the two types of active transport?
Primary and secondary
How do the two types of active transports differ?
They way the use energy: primary transport uses ATP directly: secondary uses ATP indirectly
What is Primary active transport?
When the energy is directly derived from the hydrolysis of ATP
How much energy does a typical cell use on primary active transport?
30% of its energy (ATP)
What does primary active transport create?
Stored ion gradients to be harnessed by secondary active transport (proteins take ATP to utilise it for primary active transport, that makes the ion gradients which can represent stored energy)
What is secondary active transport?
Energy stored in an ionic concentration gradient is used to drive the active transport of a molecule against its gradient
What is a key example of primary active transport?
The NA/K ATPase aka the sodium/potassium pump
What does the NA/K pump maintain?
A low concentration of NA and high concentration of K in the cytosol (inner cell)
How many Na are pumped out/K pumped in?
3 Na are pumped out for every 2 K ions pumped in
What does the exchange of ions create? Why?
It generates a net current and is electrochemical due to the extra Na ion
What is the net difference created by the Na/K pump?
The charge of 1
Steps of the Na/K ATPase pump?
- 3 Na ions bind to Na/K ATPase (the protein)
- This causes ATP to split: inside the cell, ATP releases 1 phosphate which joins to the ATPase- the phosphate has a large negative charge which causes a change of ATPase conformation. This exposes the Na binding site to outside of cell
- On outside, 2 K ions bind to altered binding site, which causes phosphate to fall off
- This changes the conformation, so K is released into the inside of the cell
What does the repeated NA/K pump do?
Maintains the Na/K gradient
What are other examples of primary active transport?
The Ca/K ATPase pump
The H/K ATPase pump
Discuss the Ca/K ATPase pump?
Muscle SR
Takes Ca out, so there’s a low level in the cell so that an influx of Ca can be a signal for many cellular processes
Discuss H/K ATPase pump?
In stomach
Is a very strong pump, can cause an acidic environment of pH2 by continuous pumping of hydrogen out
6 reasons why the Na/K concentration is important?
- Maintains resting membrane potential
- Electrical excitability
- Contraction of muscle
- Maintenance of steady state cell volume
- Uptake of nutrients via secondary active transporters
- Maintenance of intracellular pH by secondary active transporters
What is the pump leak hypothesis?
Because Na and K are continuously leaking back into/out the cell down their respective gradients, the pump works continuously
What are the three important electrochemical gradients?
Sodium
Calcium
Hydrogen
What is secondary active transport?
It uses energy stored in an ion gradient created by primary active transporters to move other substances against their concentration gradients
-thus, the transporter indirectly uses energy obtained by the hydrolysis of ATP
What powers many secondary active transporters?
The Na gradient, initially established by Na pump (as strong electrochemical)
Two types of secondary active transport?
- Na antiporters or exchangers
2. Na cotransporters or symporters
Describe Na antiporters or exchangers
Na rushes in (passive movement down electrochemical gradient) but is secondary because Ca2+ and hydrogen are going actively against their electrochemical gradient to get out of the cell
What is the net effect of Na antiporters or exchangers?
Active removal of Ca or H
Describe Na cotransporters or symporters
Glucose or amino acids rush into the cell with Na ions, driving uptake of glucose or amino acid in cell above or against its concentration gradient
What is Na cotransporters or symporters utilised for?
To accumulate key nutrients inside cells
Two types of vesicular transport?
Endocytosis and exocytosis
Three types of endocytosis?
- Phagocytosis
- Pinocytosis
- Receptor mediated transport
Discuss phagocytosis
“Cell-eating”
Molecules cross membrane but don’t interact with it
Foreign particle binds to a membrane receptor protein, pseudopods extend to form a vesicles (phagosome) and engulfs the particle
Phagosome fuses with lysosome to degrade particle
What cells carry out phagocytosis?
Leukocytes eg macrophages and neutrophils
Discuss pinocytosis
“Cell drinking”
No receptor protein involved so a vesicle forms around the particle and it is ingested directly without specialised structures such as pseudopods. Non selective drinking of extra cellular fluids. Still fuses with lysosomes to digest.
What is receptor mediated transport?
Mechanism for uptake of specific substances as ligands bind specifically- used to take up trace molecules- process of recycling
Steps of receptor mediated transport?
- Desired substance binds to receptor protein in clathrin-coated pit reign of cell membrane
- Binding causes membrane to fold inwards forming a vesicle
3-4. Vesicle becomes uncoated and combine with endosome - Receptor proteins separate from ligand and return to surface.
Particles degraded in lysosomes
Describe exocytosis
Substances (neurotransmitters, enzymes) are exported from the cells by fusion with vesicles with the cell membrane
Where is the receptor protein located in receptor mediated endocytosis?
In the clathrin coated pot region of the cell membrane
What does the binding between the substance, the ligand and receptor protein cause in receptor mediated endocytosis?
The membrane to fold inwards and form a vesicle
What happens to w vesicle once it’s formed in receptor mediated endocytosis?
It becomes uncoated and combines with an endosome
What happens once the vesicle has bound to an endosome in receptor mediated endocytosis?
The receptor proteins separate from ligands and return to surface
What happens once receptor proteins are separated from ligand in receptor mediated endocytosis?
Ligands are digested by Lysozymal enzymes or transported across the cell- epithelial cell crossing accomplished
What sort of substances using ligand receptor-mediated endocytosis?
Trace elements such as iron (you can have the process of recycling)
