Transports Accross The Membrane Flashcards
Which molecules can move freely through the membrane ?
Small and uncharged molecules
Hydrophobic substances
Which molecules cannot move freely through the membrane ?
Ions and large hydrophilic substances
What is passive transport ?
Transportation through the membrane which doesn’t require energy :
- diffusion
- osmosis
- facilitated diffusion
What is active transport ?
Transportation through the membrane which requires energy under the form of ATP :
-protein pomps
-co- transport
-Endocytosis and Exocytosis
The molecules move against the concentration gradient
Concentration gradient definition
A difference in concentrations. It’s present when there is an unequal distribution of the solute across the plasma membrane.
Equilibrium definition
When the concentrations on the two sides of the plasma membrane are equal. There is an equal movement of molecules in both direction which means no net movement.
Diffusion definition
Movement of the solute down its concentration gradient.
Molecules that can move freely across the membrane.
It’s the random movement of particules of a solute from an area of higher concentration to an area of lower concentration.
Diffusion stops at equilibrium.
Osmosis definition
Movement of the solvent.
Passage of water through the membrane.
When the solute cannot pass through the membrane, water moves to where there is a greater amount of solute until the amounts of solute are (almost) equal on the two sides.
Osmotic pressure definition
The external pressure that must be exerted to prevent net movement of solvent across the membrane. It’s a force that can sustain a column of water.
Tonicity definition
The ability of an extra cellular solution to make water move into or out of a cell by osmosis.
A solution’s tonicity depends on the difference of osmotic pressures inside and outside of a cell.
Hypertonic solution definition
Has a higher osmotic pressure or contain a higher concentration of solute compared to the cell’s cytosol.
When a cell is placed in an hypertonic solution, the water diffuses out of the cell, causing the cell to shrivel
Hypotonic solution definition
Has a smaller osmotic pressure or contains a lower concentration of solute compared to the cell’s cytosol.
When a cell is placed in an hypotonic solution, water diffuses into the cell causing the cell to swell and explode.
Isotonic solution definition
Has the same osmotic pressure or contains the same concentration of solute as the cell’s cytosol.
When a cell is placed in an isotonic solution, water diffuses into and out of the cell at the same rate. The fluid that surrounds the body cells is isotonic.
Facilitated diffusion definition
Diffusion of specific particles through transport proteins found in the membrane. These proteins are specific as the let only certain molecules cross the membrane. Ex : large hydrophilic molecules like amino acids or glucose or ions like Na+ or Cl-
Facilitated diffusion stops at equilibrium.
Define and name channel proteins
A trans membrane protein that forms a pore for a specific molecule.
- constitutively open
- ligand gated
- voltage gated
- mechanically gated
Constitutively open channel protein definition
Always open to a specific molecule. Ex : K+ channel in neurons called “resting potassium channel”. They set the negative membrane potential.
Ligand gated channel protein definition
Opens in response to a chemical signal. The binding of the ligand changes the shape of the protein. Ex : Na+ channel in muscle cells open in response to acetylcholine. They depolarize the cell and lead to muscular contraction.
Voltage gated channel protein definition
Opens in response to a change in the membrane’s electrical potential.
Ex : Na+ channels in neurons play the major in the transmission of the nerve impulse.
Mechanically gated channel proteins definition
They open in response to a mechanical stress, pressure or distorsion.
Ex: cation selective channel that acts as sensors in many system as the sense of touch or hearing.
Describe how the carrier protein works
- bind to a specific molecule on one side of the membrane
- the protein undergoes a conformational change
- release of the molecule on the other side of the membrane
The molecules move down their concentration gradient
Ex: the glucose transporter GLUT1 is a carrier protein that facilitates the transport of glucose inside the cell. It maintains the basal glucose level in all cells.
Protein pumps definition
Also named primary active transport use ATP to move ions from a region of low concentration a region of high concentration.
They are membrane proteins that actively move specific molecule to where they are needed. They create a concentration gradient. Energy through hydrolysis of ATP must be provided.
Describe the functioning of the Sodium - Potassium pump
1) cytoplasmic Na+ binds to the pump as the pump has high affinity for sodium and will take up to 3 of them.
2) Na+ binding will stimulate the phosphorylation of ATP
3) phosphorylation causes the protein change its affinity for Na+ and to change its conformation expelling Na+ outside of the cell.
4) in its phosphorylated form the protein has high affinity for potassium, therefore extra cellular K+ binds to the protein triggering release of the phosphate group
5) upon dephosphorylation, the protein looses its affinity for potassium and restores itself to its original conformation
6) K+ ions are released in the cell so Na+ sites are receptive again and the cycle can repeat
The sodium-potassium pump transpo
Co-transport definition
Also called secondary active transport.
A carrier protein that moves two substances at the same time :
Symporter = same direction
Anti porter = different directions
The movement of one solute down its concentration gradient will provide energy to transport a molecule against its concentration gradient. This movement doesn’t require ATP itself but energy was spent to creat the concentration gradient in the first place.
Describe the Na+/glucose co-transporter
- symporter
- once sodium and glucose bind to the Sodium Glucose Link Transporter (SGLT) on the extra cellular side of the membrane, the protein changes shape and release them into the cytoplasm.
You can find this protein in the kidneys where where it contributes to renal glucose reabsorption.
