BIO 1140 - Cell Transport Across Plasma Membrane (Pt. 1)

Question 1

What do Cell Membranes contain?

Answer 1

Cell membranes contain specialized membrane transport proteins
- Membrane transport proteins facilitate the passage of selected small water-soluble molecules
- Liposomes are impermeable to most water-soluble molecules

Question 2

What determines diffusion rate?

Answer 2

The molecule’s size and solubility determine the diffusion rate
- Simple diffusion

Question 3

What are the most important Inorganic ions for Cells?

Answer 3

Na+, K+, Ca2+, Cl–, and H+ (protons) are among the most important inorganic ions for cells.
- Na+ is most abundant outside the cell.
- K+ is most abundant inside the cell.

Question 4

What are the Electrical Charges inside and out?

Answer 4

Electrical charges inside and outside are generally balanced (Na+ by Cl- and K+ by organic molecules).

Question 5

What is a Voltage Difference?

Answer 5

Small excesses of positive or negative charge on the two sides of the plasma membrane create a voltage difference

Question 6

What is a Membrane Potential?

Answer 6

This voltage difference across the membrane is called the membrane potential
- The activity of membrane transport proteins create and maintain the membrane potential

Question 7

What is a Resting Membrane Potential?

Answer 7

For a cell at rest, the exchange of anions and cations across the membrane is steady (resting membrane potential)

Question 8

What are the Two Classes of Membrane Transporter Proteins?

Answer 8

Membrane transport is mediated by two classes of membrane proteins

1. Transporters: transfer small organic molecules or inorganic ions
2. Channels: form tiny hydrophilic pores and allow substances to pass by diffusion

• Most channels are ion channels

Question 9

What is Passive Transport?

Answer 9

Molecules flow from a region of high concentration to a region of low concentration (downhill movement)
- Downhill movement is passive and requires no energy(passive transport)
- Passive transporters change conformation to mediate transport across the membrane

Question 10

What is Active Transport?

Answer 10

The uphill movement requires energy (active transport)
- against a concentration / electrical gradient requires an input of energy
- Only transporters can carry out active transport

Question 11

What are Active Transporters / Pumps?

Answer 11

Transmembrane proteins responsible for active transport are called active transporters or pumps

Question 12

What is Movement Direction of an Uncharged Molecule determined by?

Answer 12

For an uncharged molecule movement direction is determined only by its concentration gradient

Question 13

What does Concentration Gradient and Membrane Potential of a Charged Molecule exert?

Answer 13

For a charged molecule, both concentration gradient and the membrane potential exert forces

Question 14

What is an Electrochemical Gradient?

Answer 14

The net driving force moving a charged solute is called electrochemical gradient
- The electrochemical gradient is the sum of a force from the membrane potential and a force from the concentration gradient of the solute

Question 15

What is Osmosis?

Answer 15

Osmosis is the diffusion of water across membranes
- Water molecules diffuse rapidly through aquaporin channels in the plasma membrane of some cells

Question 16

What are Aquaporins responsible for?

Answer 16

Aquaporin channels responsible for water reabsorption in our kidneys
- A nephron tissue

Question 17

What is an important example of a Passive Transporter?

Answer 17

Glucose Transporter

Question 18

What are the Three types of Proteins for Active Transport?

Answer 18

1. Uniporter: transports one substance in one direction
2. Symporter: transports two different substances in the same direction
3. Antiporter: transports two different substances in opposite directions

Question 19

What are the Three Types of Energy Sources used in Active Transport?

Answer 19

• ATP (used by primary active transporters)
• Electrochemical gradient (used by secondary active transporters)
• Light

Question 20

What do ATP-driven Ca2+ Transporters maintain?

Answer 20

ATP-driven Ca2+ transporters (pumps) keep the cytosolic Ca2+ concentration low
- Ca2+ binds to a variety of proteins in the cell and alters their activities
- An influx of Ca2+ into the cell serves as an intracellular signal
- It triggers cell processes, such as muscle contraction, fertilization and nerve cell communication

Question 21

What do ATP-driven Na+ Transporters use?

Answer 21

ATP-driven Na+ transporters (pumps) uses energy supplied by ATP to expel Na+ and bring in K+
- The ATP-driven Na+ pump plays a pivotal role in the energy economy of animal cells
- This pump accounts for 30% or more of their total ATP consumption
- This pump is also called Na+-K+ ATPase or the Na+-K+ pump

Question 22

What do ATP-driven Na+ Transporters generate?

Answer 22

ATP-driven Na+ transporter generates a steep concentration gradient of Na+ across the plasma membrane
- With the Na+ pump, Na+ concentration remains about 10–30 times lower and K+ concentration about 10–30 times higher inside the cell
- High Na+ concentration outside the cell represents a huge store of energy

Question 23

What are Coupled Transporters?

Answer 23

Coupled transporters (pumps) are secondary active transporters
- Coupled transporters mediate coupled transport using electrochemical gradients.
- Symport & Antiport

Question 24

What does Glucose–Na+ transporters use to drive Active Import of Glucose?

Answer 24

Glucose–Na+ transporters use the electrochemical Na+ gradient to drive the active import of glucose

Question 25

What are Both Active and Passive Transporters involved in?

Answer 25

Active and passive transporters are involved in glucose transfer across the epithelial lining of the gut

Question 26

How do Larger Molecules Enter and Leave the Cell?

Answer 26

Large molecules enter and leave the cell by endocytosis and exocytosis
- Proteins, polysaccharides, and nucleic acids are too large and too charged or polar to pass through biological membranes
- import extracellular materials by endocytosis
- secrete intracellular materials by exocytosis.

Question 27

What are the Three Types of Endocytosis?

Answer 27

1. Receptor-Mediated: Specific uptake of large molecules
   - Receptor-mediated endocytosis is a highly specific process
2. Pinocytosis: Nonspecific uptake of extracellular fluid (fluids and dissolved substances from blood)
3. Phagocytosis: Nonspecific uptake of large undissolved particles (invading bacteria by cells of the immune system

Question 28

What is Exocytosis?

Answer 28

Release of large molecules:
- Vesicle fusion with cell membrane (digestive enzymes in the pancreas)

Release of small molecules:
- Vesicle fusion with cell membrane (Neurotransmitters at synapse)

Question 29

What is Receptor Mediated Endocytosis responsible for?

Answer 29

Receptor-mediated endocytosis is responsible for LDL uptake

Question 30

What is Cholesterol and LDL?

Answer 30

Cholesterol is a hydrophobic molecule that is transported in the blood as part of huge lipoprotein complexes such as the low-density lipoprotein (LDL).

Question 31

What is Phagocytosis Responsible for?

Answer 31

Smaller cells and large particles are taken up by phagocytosis

Question 32

What is Insulin Secreted from?

Answer 32

Insulin is secreted from pancreatic β cells by exocytosis

Question 33

What happens to Secretory Vesicles during Exocytosis?

Answer 33

During exocytosis, secretory vesicles fuse with the plasma membrane and releases (secretes) its content into the extracellular space.