Transport across plasma membrane

Question 1

Why can only a few molecules cross the plasma membrane?

Answer 1

Charge/polarity substances would have to be able to interact with the hydrophilic and hydrophobic parts of the bilayer
Size: Only very small molecules could squeeze between the head groups and lipid tails
Plasma membrane is semi-permeable–allows controlled passage of solutes & ions–Essential for cell function (nutrients, activity)

Question 2

Define passive diffusion?

Answer 2

No protein required, e.g. glycerol
Unaided net movement of solute molecules through the lipid bilayer from [high] to [low]
Very small non-polar molecules move across plasma membrane such as carbon dioxide and oxygen.

Question 3

Define facilitated diffusion?

Answer 3

Protein required, substance flows along concentration gradient e.g. glucose transporters

Specific proteins carry solutes through the hydrophobic lipid bilayer i.e. facilitating the diffusion
Solutes diffuse across the membrane
Uncharged molecules: concentration gradient-Ions: electrochemical gradient
Saturable-Reaches a maximum rate (speed) maximum capacity of the carrier proteins is reached

Question 4

Define primary active transport?

Answer 4

Protein required, substance flows against concentration gradient, energy required provided directly by ATP hydrolysis. e.g. Na+K+ATPase

Question 5

Define secondary active transport?

Answer 5

Protein required, substance flows against concentration gradient, energy required provided indirectly by ATP hydrolysis. Gradient of ion used to couple transport (usually sodium)
Can be symport or antiport mechanism

Question 6

What is the driving force for transport?

Answer 6

The concentration gradient (difference in concentration) is the driving force for transport.
Until equilibrium reached

Question 7

Give a clinical example of passive diffusion?

Answer 7

[O2] is high in lungs and [O2] is low in RBCs- O2 is taken up by RBCs in circulatory system- Is released in body tissues

Question 8

What is a clinical example of facilitated diffusion that occurs within the body?

Answer 8

Example: glucose movement across the plasma membrane Glucose concentration is higher in blood than in red blood cell
Glucose cannot cross the plasma membrane by passive diffusion
Transport protein is required-GLUT1

Question 9

What re the two types of transport proteins?

Answer 9

Channel and carrier proteins.

Question 10

Define carrier proteins?

Answer 10

Facilitate traffic in either direction (inward or outward)

Bind one or more solute molecules on one side of the PM, then undergo a conformational change

Question 11

What are channel proteins?

Answer 11

Categorised based on the number of solutes transported and the direction they move

1) Uniport: Single solute
2) Symport: 2 solutes / simultaneously
3) Antiport: 2 solutes / opposite directions

Form hydrophilic transmembrane channels
Allow specific solutes to pass PM / no change in shape
Channel proteins
1) Ion channels
2) Porins
3) Aquaporins
Large pores. Porins in mitochondria. Aquaporins for water transport.

Question 12

Explain the process in which glucose is facilitated into the cell into a RBC?

Answer 12

1. Glucose binds to a GLUT1 transporter which has a binding site on the outside of the cell.
2. This binding causes a conformation change in the protein transporter causing a t2 conformation change where the binding allows to one open to the inside of the cell.
3. This allows glucose to be released to the inferior side of the cell which initiated a second conformation change in shape of GLUT1.
4. Loss of bounded glucose allows the GLUT1 protein to restore itself back to its original conformation.

Question 13

Define ion channels use in the plasma membrane?

Answer 13

Small pores lined with hydrophilic amino acid side chains
Allow rapid passage of specific ions - Na+, K+, Ca2+, and Cl-
Channels are gated-Pore opens/closes in response to stimulus
Voltage gated:change in membrane potential
Ligand gated:binding of specific molecules
Mechano-sensitive:mechanicalforces

Question 14

What are the three major functions of active transport?

Answer 14

1. Uptake of nutrients when conc higher inside cell
   2) Secretory products & waste materials removed
   3) Maintainance/restoration of intracellular concentrations of ions and resting membrane potential ATP

Question 15

Explain the process of primary active transport?

Answer 15

E.g. Na+K+ ATPase present on plasma membrane•Involves direct hydrolysis of ATP
The pump is an enzyme (ATPase). Energy liberated by hydrolysis of ATP results in K+pumped inward & Na+ outward, both against their concentration gradients
Maintains electrochemical ion gradients: increase [ K+] decrease [ Na+]= inside decrease [K+] increase [Na+]=outside

Question 16

Explain the process of secondary active transport?

Answer 16

E.g. Gut epithelial glucose transporter SGLT1
Uses electrochemical ion gradients (Na+) to ‘push’ transport of solute against concentration gradient
Indirectly uses the energy expended by the sodium pump (ATPase), which maintains the Na+concentration gradient

Question 17

How are large materials transported across PM ?

Answer 17

Exocytosis and Endocytosis

Question 18

What is exocytosis?

Answer 18

Process by which the contents of secretory granules (intracellular molecules) are released to the exterior of the cell
Vesicles fuse with plasma membrane to release contents

Question 19

Give examples of substances the are released via exocytosis?

Answer 19

Peptides and protein hormones
Enzymes
Neurotransmitters
water soluble hormones

Question 20

What is endocytosis?

Answer 20

Process by which external materials are internalised by cells (uptake of extracellular materials)
A small segment of plasma membrane progressively folds inward
It pinches off to form an endocytic vesicle containing ingested substances or particles

Question 21

What is endocytosis important in?

Answer 21

Ingestion of nutrients by some organisms
Defence against microorganisms by white blood cells
Signalling to nucleus and recycling/disposal of membrane components

Question 22

What are the different types of endocytosis?

Answer 22

-Clathrin-mediated endocytosis
Vesicles 100 nm diameter
-Caveolae 
Vesicles 50 nm diameter
Enriched in cholesterol
-Pinocytosis
Vesicles 500 nm
 5 mm diameter
Only in some cell types – allows cells to ‘drink’, take up extracellular fluid.
-Phagocytosis–Take up particlulate matter up to 0.75 mm diameter

Question 23

What are phagocytes?

Answer 23

Large and solid particles are ingested

aggregates of macromolecules, parts of other cells, and even whole microorganism or other cells)