Lecture 15: Cellular Membrane Processes

Question 1

How thick is the cell membrane?

Answer 1

8 nm

Question 2

What is the model for the cell membrane called?

Answer 2

Fluid mosaic model

Question 3

What is the composition of the cell membrane?

Answer 3

50% lipid and 50% protein held together by hydrogen bonds

Question 4

What are the 3 types of lipid molecules found in cellular membrane?

Answer 4

Glycolipid
Phospholipid
Cholesterol

Question 5

What is a phospholipid?

Answer 5

A phosphate head and two fatty acid tails 0 amphipathic

Makes up 75% of lipids

Question 6

What does amphipathic mean?

Answer 6

Contains both a polar and non-polar region

Question 7

What does membrane fluidity mean?

Answer 7

The lipids and proteins are able to move around within the plane of the membrane leaflet - on a side of phospholipids however cannot move between phospholipid layers

Question 8

What are the determinants of fluidity?

Answer 8

Lipid tail length
Cholesterol
Double bonds in fatty acid tails

Question 9

How does lipid tail length affect fluidity?

Answer 9

Longer lipid tail lengths decrease fluidity

Question 10

How does cholesterol level affect fluidity?

Answer 10

More cholesterol molecules within the cell membrane make it less fluid

Question 11

How does number of double bonds affect fluidity?

Answer 11

More double bonds in the fatty acids of the phospholipids increase fluidity

Question 12

What are integral proteins?

Answer 12

Proteins in the cell membrane which extend completely across the membrane

Question 13

What are peripheral proteins?

Answer 13

Attached to either inner or outer surface of cell membrane and are easily removed

Example is proteins that bind to the cytoskeleton of the cell determining the shape of the membrane

Question 14

What are features of integral membrane proteins?

Answer 14

Amphipathic - have hydrophobic and hydrophilic regions

The hydrophobic regions usually coil up into helices making up the inner part of the protein

Question 15

What is the fluid mosaic model of membrane structure?

Answer 15

The fluid mosaic model describes the cell membrane as a “sea of lipids in which proteins float like icebergs,” with the membrane being approximately 50% lipid and 50% protein, held together by hydrogen bonds.

Question 16

How does the molecular structure of the membrane result in selective permeability?

Answer 16

The lipid bilayer acts as a barrier to the entry or exit of polar substances, while proteins function as “gatekeepers” to regulate traffic across the membrane.

Question 17

What are the main components of the lipid bilayer?

Answer 17

The lipid bilayer consists of two back-to-back layers of three types of lipid molecules, including phospholipids (75% of lipids), cholesterol, and glycolipids.

Question 18

What is the role of phospholipids in the cell membrane?

Answer 18

Phospholipids are amphipathic molecules that form the bilayer, with polar heads facing outward and nonpolar tails facing inward, creating a hydrophobic core that contributes to selective permeability.

Question 19

What factors determine membrane fluidity?

Answer 19

Lipid tail length
Number of double bonds in the tails
Amount of cholesterol present

Longer tails and more cholesterol decrease fluidity, while more double bonds increase it.

Question 20

What are the two types of membrane proteins?

Answer 20

Integral proteins extend into or completely across the cell membrane (transmembrane proteins), while peripheral proteins are attached to either the inner or outer surface and can be easily removed.

Question 21

What are integral membrane proteins and their characteristics?

Answer 21

Integral membrane proteins are amphipathic, having hydrophobic regions that span the hydrophobic core of the lipid bilayer, usually consisting of nonpolar amino acids coiled into helices, with hydrophilic ends interacting with the aqueous solution.

Question 22

What are the functions of membrane proteins?

Answer 22

Membrane proteins can function as channels, transporters, receptor proteins, cell identity markers, linkers, enzymes, and ion channels, facilitating various processes across the membrane.

Question 23

What is selective permeability in the context of cell membranes?

Answer 23

Selective permeability refers to the membrane’s ability to allow some substances to cross while excluding others, based on molecular organization.

Question 24

What types of molecules can permeate the lipid bilayer?

Answer 24

The lipid bilayer is permeable to nonpolar, uncharged molecules (e.g., O2, N2), lipid-soluble molecules (e.g., steroids), and small uncharged polar molecules (e.g., water, urea),

Impermeable to large uncharged polar molecules (e.g., glucose) and ions (e.g., Na+, K+).

Question 25

How do membrane proteins assist in selective permeability?

Answer 25

Membrane proteins mediate the transport of substances that cannot permeate the hydrophobic core of the lipid bilayer, allowing ions and large uncharged molecules to cross with assistance.

Question 26

What is the process of diffusion?

Answer 26

Diffusion is the net movement of molecules from an area of high concentration to an area of low concentration.

Question 27

What are the principles of diffusion?

Answer 27

Diffusion is the random mixing of particles in a solution due to kinetic energy, with molecules moving from high to low concentration. Factors affecting diffusion include concentration difference, temperature, size of the substance, surface area, and diffusion distance.

Question 28

What physical consequences does diffusion have on cell size?

Answer 28

The rate of diffusion limits cell size to about 20 μm. To enhance diffusion, cells can increase membrane area, while thicker membranes slow diffusion.

Question 29

What are the types of gradients across the cell membrane?

Answer 29

Concentration gradients influence non-charged molecules, while electrical gradients affect ions, governed by the electrochemical gradient.

Question 30

Do cells maintain concentration and electrical gradients?

Answer 30

Cells maintain differences in charged ions across the membrane, establishing an electrical gradient or membrane potential, which represents stored energy.

Question 31

What is osmosis?

Answer 31

Osmosis is the diffusion of water across membranes due to solute obstruction, moving from areas of high water concentration to low water concentration through a selectively permeable membrane.

Question 32

What are the three types of solutions in relation to osmosis?

Answer 32

Hypotonic solution: net inflow of water, lower solute concentration Isotonic solution: no net flow of water, equal solute concentration Hypertonic solution: net outflow of water, higher solute concentration.

Question 33

What factors influence membrane permeability to water?

Answer 33

Membrane permeability to water (PW) is influenced by diffusion through the lipid bilayer (Pd) and through water channels (Pf), with Pf being larger and more significant for water transport.

Question 34

What is the equation for membrane permeability to water (PW)?

Answer 34

Membrane permeability to water (PW) is calculated as PW = Pd + Pf, where Pd is permeability through the lipid bilayer and Pf is permeability through water channels.

Question 35

What are the properties of Pd in relation to water permeability?

Answer 35

Pd (permeability through the lipid bilayer) is small, mercury insensitive, and temperature dependent

Question 36

What are the properties of Pf in relation to water permeability?

Answer 36

Pf (permeability through water channels) is large, mercury sensitive, and temperature independent.

Question 37

How do aquaporins affect water permeability in cells?

Answer 37

Aquaporins, which have 9 isoforms, mediate Pf, allowing cells to have different water permeability (PW) based on the expression of these isoforms.

Question 38

What is osmotic pressure?

Answer 38

Osmotic pressure is the pressure applied to a solution to prevent the inward flow of water across a semi-permeable membrane, influenced by differences in osmolarity.

Question 39

What role does cholesterol play in cell membranes?

Answer 39

Cholesterol, with its stable ring structure, inserts itself between phospholipids, maintaining membrane fluidity and acting as a buffer.

Question 40

What are the differences between channel proteins and carrier proteins?

Answer 40

Channel proteins facilitate passive transport down the concentration gradient without energy, while carrier proteins can transport substances against the concentration gradient, sometimes using ATP.

Question 41

What are glycoproteins and their function?

Answer 41

Glycoproteins are proteins with sugar chains attached, playing a role in cell recognition.

Question 42

What are peripheral and lipid-bound proteins?

Answer 42

Peripheral proteins are located on the outside of the membrane, while lipid-bound proteins are embedded within the membrane.