Module 2: The Plasma Membrane

Question 1

Cell membrane aka plasma membrane aka plasmalemma

Answer 1

2 layers of lipids with various proteins and carbohydrates embedded into it = a lipid bilayer
All biological membranes are lipid bilayers.

Question 2

Lipids that compose cell membranes

Answer 2

3 major types: phospholipids (most abundant), cholesterol, and glycolipids
They are insoluble in water but soluble in organic solvents
Their key characteristic = amphipathic → have both a hydrophilic and a hydrophobic end

Question 3

Typical structure of a Phospholipid

Answer 3

Has a polar - hydrophilic-head group made of phosphate, glycerol, and choline
And has a non polar - hydrophobic- tail group made of hydrocarbon tails

Question 4

Examples of phospholipids (some)

Answer 4

Phosphatidylethanolamine
Phosphatidylserine
Phosphatidyicholine
Phosphatidylinsitol

Question 5

How amphipathic molecules arrange themselves

Answer 5

Amphipathic molecules “bury” the hydrophobic groups (tails) to protect them from water→ leaves the hydrophilic groups (heads) exposed to the water
2 ways to do this: 1. Form micelles= spheres with hydrophobic tails inwards. 2. Form bilayers = hydrophobic tails sandwiched together between the hydrophilic heads.
The amphipathic nature = spontaneous. formation of the lipid part of the membrane

Question 6

Movement WITHIN the bilayer

Answer 6

Two main types: flip-flop and lateral diffusion
In general, molecules are mostly confined to their own monolayers

Question 7

Flip-flop movement

Answer 7

When molecules from one side of the monolayer migrate to the other side → a molecule moves from the top half to the bottom.
This type of movement is rare

Question 8

Lateral diffusion

Answer 8

When molecules within the same monolayer switch positions with their neighbors → occurs at a very rapid rate
This type of movement happens frequently.

Question 9

How can the fluidity of a cell membrane vary?

Answer 9

Can vary drastically from being extremely rigid and crystalline to being liquid-like.

Question 10

What factors can affect the fluidity of a cell membrane?

Answer 10

1. Outside factors like temperature in the environment.
2. The hydrocarbon tails of the phospholipids
3. Cholesterol in the cell membrane

Question 11

How can temperature affect the fluidity of a cell membrane?

Answer 11

Hotter environments= more fluid membrane
Colder environments= more rigid membrane

Question 12

How does the hydrocarbon tails of phospholipids contribute to membrane fluidity?

Answer 12

1. Can have cis bonds that create a “kink” in the tail, affecting the fluidity
2. The varying lengths of the tails also affect the fluidity
   Conclusion: The longer the hydrocarbon tail/chain and the more cis bonds (kinks) present = the more fluid the membrane

Question 13

What role does cholesterol play in membrane fluidity?

Answer 13

Cholesterol can insert itself between phospholipids → enhances the mechanical stability of the membrane → so it’s less likely to fracture in colder temperatures and keeps it from being too fluid in hotter temps.

Question 14

The plasma membrane is composed of what else in addition to lipids?

Answer 14

Also composed of different proteins → transmembrane proteins, ion pores, channels, and other transport proteins

Question 15

Overall function of the plasma membrane

Answer 15

It serves to separate the intracellular environment from the extracellular environment, and also acts as a solvent for membrane proteins

Question 16

How the plasma membrane separates the intracellular and extracelludar environments.

Answer 16

Due to the lipid bilayer’s hydrophobic interior, the membrane prevents the diffusion of many polar substances into the cell → highly impermeable to these molecules

Question 17

The plasma membrane’s role as a solvent for membrane proteins

Answer 17

Cell membranes can express many different proteins and molecules that can serve as receptors, anchors, or even as identifiers for different cell types

Question 18

Molecules moving across the cell membrane

Answer 18

Larger molecules need the aid of transport proteins to pass across all membrane
Small molecules can diffuse across the membrane → do so at different rates

Question 19

Rate of (small) molecules diffusing across a all membrane

Answer 19

The rate at which the molecules diffuse across the membrane depends on their size and their relative solubility in oil
The smaller the molecule and the more soluble in oil = the more rapid diffusion across the cell membrane

Question 20

Membrane transport proteins

Answer 20

1. Help larger molecules ( ie. Ions, metabolites, sugars, etc) to cross over the cell membrane to the intracellular environment
2. Different kinds of transport proteins: uniports, symports, and antiports

Question 21

Uniport transport protein

Answer 21

Transfer a solute from one side of the membrane to the other.

Question 22

Co-transporters

Answer 22

Transport proteins that allow for two solutes to be transferred together.
Symports and antiports are both types of co-transporters.

Question 23

Symport transport proteins

Answer 23

A type of co-transporter → transport 2 solutes in the same direction

Question 24

Antiport transport proteins

Answer 24

A type of co-transporter → transport 2 solutes in opposite directions

Question 25

Passive transport

Answer 25

Depends on the concentration gradient or electrochemical gradient of the moleculein question
This type of transport does not require energy

Question 26

Active transport

Answer 26

Used when the molecule in question needs to be transported against its electrochemical/ concentration gradient → like pushing a ball uphill
This type of transport requires energy

Question 27

Types of passive transport

Answer 27

Simple diffusion and facilitated diffusion

Question 28

Simple diffusion

Answer 28

When the molecules flow passively across the membrane without the assistanceof membrane proteins

Question 29

Facilitated diffusion

Answer 29

Occurs via the action of channel proteins or via the action of carrier proteins

Question 30

Facilitated diffusion via channel proteins

Answer 30

Allow passive transport of the molecules that are appropriate in size

Question 31

Facilitated diffusion via carrier proteins

Answer 31

Allow passive transport by binding to specific molecule and helping it cross the membrane

Question 32

The 2 types of active transport

Answer 32

1. Primary active transport = the direct use of ATP
2. Secondary active transport= the use of another solute that is moving down its concentrationgradient → favorable process = produces energy which is used in secondary active transport

Question 33

Membrane potential

Answer 33

Every membrane has an electrochemical potential that is maintained by specific ions and their concentrations

Question 34

Maintenance of a cell’s membrane potential

Answer 34

Uses 2 channels→ the Na+/K+ pump and the K+ leak channel
These channels give the cell a net negative charge inside and a net positive charge outside
The resting membrane potential = -75mV (in general)

Question 35

How the Na+/K+ pump works

Answer 35

Pumps 3 Na+ ions out of the cell and 2 K+ ions into the cell via active transport

Question 36

How the K+ leak channel works

Answer 36

Allows K+ ions to move down its concentration gradient out of the cell

Question 37

The Na+/K+ pump is an ATPase

Answer 37

This pump directly hydrolyzes ATP to ADP → it’s a form of primary active transport
This ATPase also helps control cell volume via controlling the number of solutes inside of the cells

Question 38

Changes in the solute concentrations in the cell

Answer 38

Responds via osmotic forces changing and causing the cell to shrink or lyse depending on whether the concentration of ions is hypertonic v isotonic v hypotonic in the extracellular environment

Question 39

Hypertonic environment

Answer 39

Cell will shrink

Question 40

Hypotonic environment

Answer 40

Cell will swell and lyse

Question 41

Isotonic environments

Answer 41

Cell does not change size or shape = a normal cell

Question 42

Transport of macromolecules

Answer 42

transport requires the formation of membrane- bound vesicles
2 types of membrane - bound vesicular transport = exocystosis and endocytosis

Question 43

Exocytosis

Answer 43

When a cell needs to secrete a substance, whether it’s a waste product or a protein product like insulin
The vesicles formed fuse with the plasma membrane intracelluarly to release the contents that they’re holding

Question 44

Secretory vesicles

Answer 44

Found in cells that secrete a specific product → these vesicles store the product within the cell
The vesicles are only released once there’s an appropriate rise in the intracellular Ca2+concentrations

Question 45

Endocytosis

Answer 45

When cells take in macromolecules and particles → 2 types: pinocytosis and phagocytosis
The vesicles used are usually coated with proteins, most common= clathrin → formed from coated pits that are used for the uptake of specific macromolecules via receptor-mediated endocytosis
The vesicles created via endocytosis usually fuse with lysosomes → used for intracellular digestion

Question 46

Pinocytosis aka cell drinking

Answer 46

Uses small vesicles for the uptake of other microorganisms or cell debris so they can be digested by the cell

Question 47

Phagocytosis

Answer 47

Uses large vesicles for the uptake of microorganisms or cell debris so they can bedigested by the cell