Module 2

Question 1

What are the four main functions of the plasma membrane?

Answer 1

1. Regulates the passage of substances into and out of cells and organelles.
   2.Detects chemical messengers at the cell surface.
   3.Links adjacent cells through membrane junctions.
   4.Anchors cells to the extracellular matrix.

Question 2

Describe the structure of the phospholipid bilayer.

Answer 2

• Composed of amphipathic phospholipids.
• Hydrophobic fatty acid tails face inward, forming a nonpolar core.
• Hydrophilic polar heads face outward, interacting with the extracellular fluid and cytosol.

Question 3

What is the role of cholesterol in the cell membrane?

Answer 3

• Limits the ordered packing of fatty acids to maintain membrane fluidity.
• Provides structural integrity and flexibility.

Question 4

What are the two types of membrane proteins?

Answer 4

1. Integral Proteins: Amphipathic and embedded in the lipid bilayer. Includes transmembrane proteins for ion channels and signalling.
2. Peripheral Proteins: Non-amphipathic and located on the membrane surface, bound to integral proteins or polar lipid regions.

Question 5

What is the function of transmembrane proteins?

Answer 5

They facilitate ion/water transport, transmit chemical signals, and anchor extracellular/intracellular protein filaments.

Question 6

What are the three types of membrane junctions?

Answer 6

Desmosomes, Tight junctions, and Gap junctions.

Question 7

What does desmosomes do as a membrane junction.

Answer 7

Provide mechanical strength by anchoring adjacent cells, especially in stretch-prone tissues like skin

Question 8

What do tight junctions do?

Answer 8

Seal adjacent cells to limit molecule movement between cells and support trans-epithelial transport.

Question 9

What do gap junctions do?

Answer 9

Allow direct communication between cells via connexon protein channels.

Question 10

How do desmosomes anchor cells?

Answer 10

• Dense plaques on the cytoplasmic surface anchor cadherins (proteins) that extend into the extracellular space and bind with cadherins from adjacent cells.
• Provide mechanical strength in stretch-prone tissues.

Question 11

What is the function of tight junctions?

Answer 11

1. Form barriers to limit the movement of ions and molecules between cells.
2. Ensure substances pass through cells via transcellular pathways.
3. Maintain specialized membrane surface functions by blocking protein movement between apical and basolateral surfaces.

Question 12

Describe the structure of gap junctions.

Answer 12

1. Formed by connexin proteins that assemble into connexons, creating a channel for direct molecule passage between cells.
2. Allow communication of ions and small molecules, critical in cardiac muscle and electric synapses.

Question 13

What are the main types of transport across the membrane?

Answer 13

Simple Diffusion, Facilitated Transport, and Active Transport

Question 14

What is simple diffusion?

Answer 14

Movement of molecules from high to low concentration without energy use.

Question 15

What is Facilitated Transport?

Answer 15

Utilizes specific protein channels or carriers for selective molecule transport.

Question 16

What is Active Transport ?

Answer 16

Moves molecules against the gradient using ATP.

Question 17

Why is diffusion across a membrane slower than in water?

Answer 17

The hydrophobic interior of the lipid bilayer limits the movement of polar and charged molecules.

Question 18

What factors influence ion diffusion across a membrane?

Answer 18

Ion diffusion depends on:

1.	Concentration Gradient: Difference in ion concentration across the membrane.
2.	Electrical Gradient: Charge difference influencing ion movement.

Question 19

What are the two main pathways for epithelial transport?

Answer 19

1. Transcellular Pathway: Molecules pass through cells via membrane transporters.
2. Paracellular Pathway: Molecules pass between cells through junctional gaps.

Question 20

What is simple diffusion?

Answer 20

The net movement of molecules from a region of higher concentration to a region of lower concentration until equilibrium is reached.

Question 21

How does distance affect diffusion time?

Answer 21

Diffusion time increases with the square of the distance, making diffusion inefficient over long distances.

Question 22

How do tight junctions affect molecule passage in epithelial tissues?

Answer 22

Tight junctions force substances to move transcellularly (through cells) rather than between them, allowing for selective and controlled transport.

Question 23

Why are membranes critical for maintaining homeostasis?

Answer 23

Membranes regulate the internal environment by controlling molecule movement, facilitating communication, and providing structural integrity.

Question 24

What does it mean for phospholipids to be amphipathic?

Answer 24

Amphipathic molecules have both hydrophobic (nonpolar fatty acid tails) and hydrophilic (polar phosphate head) regions.

Question 25

How does the amphipathic nature of phospholipids contribute to membrane structure?

Answer 25

It allows phospholipids to self-assemble into a bilayer, with hydrophobic tails forming the core and hydrophilic heads facing the aqueous surroundings.

Question 26

What is the fluid mosaic model of the plasma membrane?

Answer 26

The model describes the membrane as a fluid structure with proteins embedded within or attached to a bilayer of phospholipids, allowing dynamic movement of lipids and proteins.

Question 27

Why is membrane fluidity important?

Answer 27

Fluidity allows for membrane flexibility, repair, and the proper functioning of membrane proteins involved in transport and signaling.

Question 28

How do saturated and unsaturated fatty acids affect membrane fluidity?

Answer 28

• Saturated fatty acids pack tightly, reducing fluidity.
• Unsaturated fatty acids have kinks due to double bonds, preventing tight packing and increasing fluidity.

Question 29

What are integrins, and what is their function?

Answer 29

Integrins are transmembrane proteins that connect the extracellular matrix to the intracellular cytoskeleton, aiding in cell signaling and structural stability.

Question 30

How does cholesterol interact with membrane phospholipids?

Answer 30

Cholesterol fits between fatty acid tails, reducing tight packing at low temperatures and preventing excessive fluidity at high temperatures, maintaining an intermediate fluid state.

Question 31

What is the main function of gap junctions in cardiac muscle?

Answer 31

They allow direct ion flow between cells, enabling synchronized contractions in cardiac tissue.

Question 32

What specific role do tight junctions play in the gut epithelium?

Answer 32

Tight junctions prevent the passage of harmful substances between gut epithelial cells and regulate nutrient absorption

Question 33

Why is diffusion not effective over long distances?

Answer 33

Diffusion time increases with the square of the distance, making it inefficient for large organisms or distances.

Question 34

Which molecules diffuse easily through the lipid bilayer?

Answer 34

Small nonpolar molecules like oxygen, carbon dioxide, and lipid-soluble molecules.

Question 35

What are ion channels, and how do they work?

Answer 35

Ion channels are transmembrane proteins that form pores for specific ions to move down their concentration gradients, enabling passive transport.

Question 36

What is the driving force for ion diffusion across membranes?

Answer 36

The combination of concentration gradients and electrical gradients (electrochemical gradient).

Question 37

How does the paracellular pathway differ from the transcellular pathway?

Answer 37

• Paracellular: Movement occurs between cells, limited by tight junctions.
  Transcellular: Movement occurs through cells via active or passive transport mechanisms.

Question 38

What differentiates passive transport from active transport?

Answer 38

• Passive transport moves molecules down their concentration gradient without energy.
• Active transport requires ATP to move molecules against their concentration gradient.

Question 39

What limits diffusion rates through the cell membrane?

Answer 39

The hydrophobic interior of the lipid bilayer slows the diffusion of polar and charged molecules.

Question 40

Why do membranes act as barriers for diffusion?

Answer 40

The lipid bilayer selectively restricts molecule movement based on size, charge, and polarity.

Question 41

Selectivity for the type of ion ( Na+ or K+) that can diffuse through a voltage gated channel is determined by ———

Answer 41

the P- loops

Question 42

In the animal cell, the overall concentration effects on Na+ diffusion causes Sodium ions to _______ the cell.

Answer 42

influx