3.3 - Transport across the plasma membrane

Question 1

What is intracellular fluid?

Answer 1

• Fluid contained inside body cells
• 2/3 total fluid volume
• Cytosol

Question 2

What is extracellular fluid? Give 4 examples

Answer 2

• Fluid outside body cells
• Interstitial fluid - between cells
• Plasma - in blood vessels
• lymph - in lymph vessels
• CSF - surrounds the brain and spinal cord

Question 3

Describe the concentration gradient

Answer 3

• Difference in concentration between two areas.
• physiologically: difference in concentration between the ECF and ICF
• Solutes can either move along or against the concentration gradient
• Along: from high to low
• Against: from low to high

Question 4

Describe the difference between a passive and active transport process

Answer 4

Passive: along the concentration gradient
Active: Against the concentration gradient. Requires ATP

Question 5

Describe simple diffusion

Answer 5

• Diffusion of membrane permeable substances
• Directly across the lipid bilayer

Question 6

Give examples of substances that move via simple diffusion

Answer 6

• Gases: O2, CO2, Nitrogen gases
• Lipid soluble compounds: Fatty acids, Steroids, Vitamins A D E K
• small polar molecules: Water, Urea

Question 7

Describe facilitated diffusion

Answer 7

• Movement of substances that are membrane impermeable
• Requires channel or carrier proteins

Question 8

What are the different types of transport proteins

Answer 8

• Ion channels: Movement of Ions, K+, Na+, Ca++, (mostly potassium)
• Carrier proteins: Larger polar molecules, sugars, polar vitamins
• Pump proteins: For active transport

Question 9

Describe gated channel proteins

Answer 9

• Can be opened and closed by changing structure
• Signalling molecules bind to the channel protein to cause the change in shape
• Important in the generation in action potentials

Question 10

Describe how carrier proteins work

Answer 10

1. Substrate binds to a site on one side of the carrier protein
2. Binding of the substrate causes the carrier protein to change shape
3. Substrate moves across the membrane
4. Substrate releases from the protein
5. Protein reverts back to its original shape

Question 11

Describe Osmosis

Answer 11

• Passive movement of water across a membrane
• Against the solute concentration gradient - low to high SOLUTE concentration
• Subject to osmotic pressure

Question 12

Describe osmotic pressure

Answer 12

• Pressure required to stop fluid movement across a membrane
• Osmotic pressure of cytosol and interstitial fluid are the same
• Cell volume remains constant

Question 13

Describe isotonicity, hypotonicity and hypertonicity

Answer 13

Isotonicity: external and internal osmotic pressure are the same. No net movement of water
Hypotonicity: lower extracellular solute concentration. Water enters the cell. Cell swells.
Hypertonicity: higher extracellular solute concentration. Water leaves the cell. Cell shrinks.

Question 14

Describe active transport

Answer 14

• Transport of substances against the concentration gradient
• Requires ATP. ATP is cleaved into ADP + Pi
• ATP used to change the shape of a pump protein
• 40% of cellular ATP used for active transport

Question 15

Give examples of ions transported using active transport

Answer 15

K+, Na+, H+, Ca2+, I-, Cl-

Question 16

What is the sodium-potassium pump?

Answer 16

• Most important active transport pump
• Brings in potassium, expels sodium
• Acts as an enzyme to cleave ATP into ADP+Pi
• Maintains a low cytosol concentration of sodium and a high cytosol concentration of potassium
• Pump acts continuously: Na+ and K+ leak back across the membrane
• Vital in maintaining cell osmolality

Question 17

• Describe the action of the Na/K pump

Answer 17

1. 3 Na+ bind to the Na/K pump.
2. ATP is cleaved and changes the shape of the pump. 3 Na+ expelled
3. 2 K+ bind to the protein. Pi released
4. Protein changes to it’s original shape. 2 K+ imported into the cell

Question 18

Describe the types of vesicular transport

Answer 18

• Intracellular transport between organelles
• Endocytosis: movement of substances into a cell
• Exocytosis: movement of substance out of a cell

Question 19

Describe endocytosis

Answer 19

• Substances are surrounded by a section of membrane
• Membrane section buds off inside the cell to form a vesicle
• Vesicle is inside the cell

Question 20

Describe phagocytosis

Answer 20

• Important part of the immune response
• Phagocytes engulf large structures for digestion
  1. Particle binds to receptors on the cell surface
  2. Pseudopods extend from the membrane and surround the particle
  3. Pseudopods fuse and release a phagosome into the cell
  4. Phagosome fuses with a lysosome
  5. Lysosomal enzymes digest the particle. Undigested material remains in a residual body

Question 21

Describe bulk-phase endocytosis

Answer 21

• Cells take up small droplets of extracellular fluid
  1. Plasma membrane folds inward and forms a vesicle
  2. Vesicle enters the cell and fuses with a lysosome
  3. Solutes are digested by the lysosome
  4. Digested products exit the lysosome to be used by the cell

Question 22

Describe exocytosis

Answer 22

• Vesicles of cellular products fuse to the plasma membrane
• Cellular products are expelled from the cell
• Secretion

Question 23

Describe the two most important examples of exocytosis

Answer 23

1. In secretory cells: liberate digestive enzymes, hormones, mucus etc.
2. In nerve cells: Release of neurotransmitters at the synapse