Transports Accross The Membrane

Question 1

Which molecules can move freely through the membrane ?

Answer 1

Small and uncharged molecules

Hydrophobic substances

Question 2

Which molecules cannot move freely through the membrane ?

Answer 2

Ions and large hydrophilic substances

Question 3

What is passive transport ?

Answer 3

Transportation through the membrane which doesn’t require energy :

• diffusion
• osmosis
• facilitated diffusion

Question 4

What is active transport ?

Answer 4

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

Question 5

Concentration gradient definition

Answer 5

A difference in concentrations. It’s present when there is an unequal distribution of the solute across the plasma membrane.

Question 6

Equilibrium definition

Answer 6

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.

Question 7

Diffusion definition

Answer 7

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.

Question 8

Osmosis definition

Answer 8

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.

Question 9

Osmotic pressure definition

Answer 9

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.

Question 10

Tonicity definition

Answer 10

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.

Question 11

Hypertonic solution definition

Answer 11

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

Question 12

Hypotonic solution definition

Answer 12

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.

Question 13

Isotonic solution definition

Answer 13

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.

Question 14

Facilitated diffusion definition

Answer 14

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.

Question 15

Define and name channel proteins

Answer 15

A trans membrane protein that forms a pore for a specific molecule.

• constitutively open
• ligand gated
• voltage gated
• mechanically gated

Question 16

Constitutively open channel protein definition

Answer 16

Always open to a specific molecule. Ex : K+ channel in neurons called “resting potassium channel”. They set the negative membrane potential.

Question 17

Ligand gated channel protein definition

Answer 17

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.

Question 18

Voltage gated channel protein definition

Answer 18

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.

Question 19

Mechanically gated channel proteins definition

Answer 19

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.

Question 20

Describe how the carrier protein works

Answer 20

• 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.

Question 21

Protein pumps definition

Answer 21

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.

Question 22

Describe the functioning of the Sodium - Potassium pump

Answer 22

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

Question 23

Co-transport definition

Answer 23

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.

Question 24

Describe the Na+/glucose co-transporter

Answer 24

• 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.

Question 25

Describe the functioning of the sodium calcium exchanger

Answer 25

Anti porter
The sodium calcium exchanger (NCX) transports calcium very rapidly and can restore calcium concentration after the nerve impulse or a muscular contraction.
You can find NCX in the heart and muscles where it contributes to the maintenance of Ca2+ concentration in the endoplasmic reticulum.

Question 26

Endocytocis definition and types

Answer 26

Is a form of active transport in which a molecule causes the cell membrane to bulge inward forming a vesicle.

• phagocytosis
• pinocytosis
• receptor mediated endocytosis

Question 27

Phagocytosis definition

Answer 27

An entire cell or a big particule is engulfed (=eating). In humans, it’s a process used to by cells of the immune system to remove pathogens (bacterias) or dead cells. The phagocytosis is our first line of defense. The phagosome usually fuses with a lysosome so the content can be digested and degraded.

Question 28

Define Pinocytosis

Answer 28

The cell takes in extra cellular fluid and solute.
Non specific mean to internalize fluids and dissolved nutrients.
The content of the vesicles is slowly transferred to the cytosol and they become progressively smaller.
This process is important for the immune system as dendritic cells use it to capture antigens.

Question 29

Describe receptor mediated endocytosis

Answer 29

The cell takes in specific molecules such as hormones, proteins, metabolites or even viruses. Ex : cholesterol which is a composent of the plasma membrane and a precursor for steroid hormones enters the cell this way.

Question 30

Definition of Exocytosis

Answer 30

Vesicles budding from the golgi appartus and containing large molecules fuse with the cell membrane in order to release their content out of the cell.
1- vesicle trafficking : use of motor protein and cytoskeletal track to get closer to the membrane
2- vesicle tethering and docking
3 - vesicle fusion

Question 31

Constitutive Exocytosis definition

Answer 31

Release components of the extra cellular matrix

Delivery of membrane proteins to the surface

Question 32

Regulated Exocytosis definition

Answer 32

Neurotransmitter secretion

Hormone release