Basics Of Cell Transport

Question 1

Why do we need transport proteins?

Answer 1

Small polar molecules, such as water, may easily pass through the cell membrane but larger polar molecules, such as glucose, may not.
It is for this reason that we require transport proteins. The membrane is SELECTIVELY PERMEABLE.
For this reason our cells use channel and carrier proteins.

Question 2

Electrochemical gradient

Answer 2

For charged molecules their movement is determined by both the electrical and chemical gradient, e.g. Ions.
For uncharged molecules movement is determined by the chemical gradient, as they move from a high to a low concentration.

Question 3

Passive transport

Answer 3

DOES NOT REQUIRE ENERGY

1. ) Simple diffusion
2. ) Facilitated diffusion

Question 4

Active process

Answer 4

REQUIRES ENERGY (ATP)
Against the concentration gradient!
Active transport

Question 5

Simple diffusion

Answer 5

Down the concentration gradient.
Fick’s law: Proportional to SA, concentration gradient and inversely proportional to the thickness of the membrane.
- Osmosis is also simple diffusion

Question 6

Facilitated diffusion

Answer 6

Passage via a transmembrane protein. 
Transport proteins are often selective --> specificity. 
Down the concentration gradient. 
Does not require energy. 
Affected by: 
- Magnitude of driving force
- Transport rate of carriers
- The number of available carriers
- State of the channels (open or closed)

Question 7

Active transport

Answer 7

REQUIRES ENERGY (ATP) - AGAIST CONCENTRATION GRADIENT.
Requires a transmembrane protein pump.
Pumps are highly selective.
Against the concentration gradient.

Question 8

Primary active transport

Answer 8

• Direct use of energy (derived from ATP hydrolysis)
  E.g. Sodium-potassium pump
  3Na out and 2 K in (both against their concentration gradients).
  Generates a membrane potential
  Membrane potential in animals cells = -65 mV

Question 9

Secondary Active Transport

Answer 9

INDIRECT USE OF ENERGY

• One molecule moves down its electrochemical gradient coupled with the uphill movement of another molecule. The energy of the downhill movement is used to drive the uphill movement.
• 2 types: symport and antiport

Question 10

Active Transport: Symport

Answer 10

Example: Na driven glucose transport into the extra cellular fluid from the gut lumen.
NA moves down its electrochemical gradient whilst glucose moves from low (gut lumen) to high (intestinal epithelium) to low (extracellular).
Na movement is facilitated by a Na-K ATPase. This transports Na out of the intestinal epithelium and K in.

Question 11

Active Transport: Antiport

Answer 11

Example: Na-Ca exchanger
Seen in cardiac muscle.
Na is pumped out by the Na-K ATPase. Ca is then exchanged for Na, Ca moves out of the cell whilst Na moves back in. 1 x Ca out and 3 x Na in.
Na comes back in via its electrochemical gradient.