Cell Bio Exam 2

Cell membranes

Act as selective barriers

_______ separates the cell from its surroundings

Plasma membrane

What is the plasma membrane involved in?

A. Cell communication

B. Import and export of molecules

C. Cell growth and mobility

D. All of the above

D. All of the above

List some functions of the internal membranes in eukaryotic cells

Compartmentation

Selective transport

Protein and lipid synthesis

Energy transduction

T/F: All cell membranes are composed of lipids and proteins

True

Lipids are arranged in two closely apposed sheets, forming a ______

lipid bilayer

The most abundant lipids in cell membranes are

A. Sterols

B. Phospholipids

C. Glycolipids

B. Phospholipids

T/F: Phospholipids are diglycerides

True

What is the shape of a diglyceride?

Two fatty acids linked to glycerol; phosphate group on 3rd position

T/F: Phospholipids are amphipathic

True

T/F: The other membrane lipids (sterols and glycolipids) are not amphipathic

False

Pure phospholipids form _____

liposomes (synthetic membranes)

Liposomes can vary in size from ____ to ____ in diameter

A. 50 nm to 1 um

B. 25 nm to 1 um

C. 50 um to 100 um

D. 25 um to 100 um

B. 25 nm; 1 um

Liquid droplets

Unique intracellular organelles surrounded by a phospholipid monolayer

What is the purpose of a liquid droplet?

Store energy in the form of hydrophobic lipids (triacylglycerols and cholesterol esters)

Where does the formation of liquid droplets occur?

Endoplasmic reticulum

Which of the following is NOT a function of plasma membrane?

A. Protecting the integrity of the interior cell

B. Providing support and maintaining the shape of the cell

C. Regulating the transport of substances in and out of the cell

D. Serving as a major site for protein synthesis

E. Involving in cell signaling and communication

D. Serving as a major site for protein synthesis

T/F: Lipid bilayers are rigid

False

Which form of motility rarely occurs in membrane phospholipids

A. Lateral diffusion

B. Flexion

C. Rotation

D. Flipping

D. Flipping

Which of the following factors affect fluidity?

A. Temperature

B. Fatty acid chain length

C. Number of double bonds

D. Cholesterol content

E. All of the above

E. All of the above

____ randomly transfer phospholipids from one monolayer to the other

Scramblases

T/F: The membrane grows symmetrically as a bilayer

True

What produces and inserts new phospholipids when creating the cell membrane

Biosynthetic enzymes

T/F: Most cell membranes are symmetrical

False

Flippases

Selectively removes specific phospholipids from the non-cytosolic monolayer and flips them to the cytosolic side

Floppases

Move specific lipids from the cytosolic side to non-cytosolic side

T/F: Both Flippases and Floppases need energy from ATP hydrolysis

True

Where do Floppases and Flippases occur?

Golgi apparatus

T/F: Membranes don’t retain asymmetry/orientation during transport between cell compartments

False

What is the molecular composition of a phospholipid?

Glycerol backbone; two hydrophobic fatty acid tails; hydrophilic phosphate group head

T/F: Sugar chains of glycolipids are typically exposed to the outside of the cell.

True

Imagine two lipid bilayers. In one, all the hydrocarbon chains are saturated (no double bonds). In the other, all the hydrocarbon chains are unsaturated (with cis-double bonds). Which one of the following predictions about their fluidity and permeability to water-soluble molecules would be most accurate?

Choose one:

A. The bilayer with saturated chains would be more fluid and more permeable.

B. The bilayer with unsaturated chains would be less fluid and more permeable.

C. The bilayer with saturated chains would be less fluid and less permeable.

D. The bilayer with unsaturated chains would be more fluid and less permeable.

C. The bilayer with saturated chains would be less fluid and less permeable.

Which phospholipids are confined to the non-cytosolic monolayer

Glycolipids, phosphatidylcholine, and sphingomyelin

What phospholipids are confined to the cytosolic side of the membrane

Phosphatidylserine, phosphatidylethanolamine, and phosphatidylinositol

T/F: Biological membranes consist of proteins embedded in a fluid phospholipid bilayer

True

In animals, proteins constitute about ____ of the mass of most plasma membranes

A. 25%

B. 80%

C. 72%

D. 50%

D. 50%

What are the major components of a plasma membrane?

Lipids (phospholipids and cholesterol), proteins, and carbohydrates

Example transporter protein

Na+ pump

Example ion channel protein

K+ leak channel

Example anchor protein

integrins

Example receptor protein

Platelet derived growth factor (PDGF) receptor

Example enzyme protein

Adenylyl cyclase

T/F: Peripheral proteins are easier to detach from the cell membrane than integral proteins.

True

How does a protein helix interact with the lipid bilayer?

Through hydrophobic interactions

How many non-polar amino acids are required to completely traverse a cell membrane?

A. 20

B. 32

C. 200

D. 50

A. 20

Hydropathy plots

Graphical representation used to predict the hydrophobic or hydrophilic regions of a protein based on its amino acid sequence

Can help identify possible transmembrane domains

Beta-barrel

Structure composed of beta sheets curved into a barrel to form a water filled channel

Found in outer membranes of bacteria, mitochondria, and chloroplasts

e.g. porin proteins

Membrane proteins can be solubilized in ____

A. Water

B. Oil

C. Detergent

D. Fat

C. Detergent

Strong ionic detergents (e.g. SDS)

A. dissolve membranes and unfold proteins

B. dissolve membranes without unfolding proteins

A. dissolve membranes and unfold proteins

Mild non-ionic detergents (e.g. Triton X-100)

A. dissolve membranes and unfold proteins

B. dissolve membranes without unfolding proteins

B. dissolve membranes without unfolding proteins

T/F: Detergents form micelles and “dissolve” membrane lipids and proteins

True

Which is correct?

C

The plasma membrane is reinforced by the underlying _____

cell cortex

What is the cell cortex?

A meshwork of fibrous proteins attached to the cytosolic side of the plasma membrane

How can the lateral mobility of membrane proteins be restricted?

Tethering proteins or diffusion barriers

FRAP

Fluorescence recovery after photobleaching

What is FRAP used for?

To measure the rate of lateral diffusion

____ cells are coated with carbohydrates

Eukaryotic

_____ and ____ are proteins covalently attached to sugars on the extracellular side

Glycoproteins; proteoglycans

What is the carbohydrate layer of the cell membrane called?

Glycocalyx

What is the function of cell-surface carbohydrates?

Protection from mechanical damage, immune response and cell signaling, cell-cell recognition and adhesion

T/F: Protein-free, artificial lipid bilayers are permeable to most water-soluble molecules

False

What do specialized membrane transport proteins do?

Facilitate passage of certain small water-soluble molecules across the cell membrane

T/F: Small molecules always cross the lipid bilayer of a liposome more easily than larger molecules.

True

Rank the permeability of the solutes across an artificial lipid bilayer.

1. Ions

2. Small Uncharged polar molecules

3. Small Nonpolar molecules

4. Large uncharged polar molecules

A. 1, 2, 3, 4

B. 2, 4, 3, 1

C. 2, 3, 4, 1

D. 3, 2, 4, 1

D. 3, 2, 4, 1

Ion concentrations generate _____

Membrane potential

What are the two types of membrane transport proteins?

Transporters and Channels

Channel

Proteins pores that extend through the membrane

Water and ions typically transported

Rely on pore size/electric charge for specificity

Transporter

Span the membrane

Directly bind to the solute

Binding induces a conformational change, moving solute to other side of membrane

Releasing solute reverts transporter to original conformation

T/F: Transport mediated by transporters involves conformational changes in the transport protein to move molecules across the membrane.

True

A liposome is a synthetic membrane that does not contain any protein components. Some uncharged, polar molecules are found to diffuse across these membranes, to varying degrees. Which of the following has the lowest rate of diffusion across the artificial membrane?

A. water

B. glucose

C. glycerol

D. ethanol

B. Glucose

Simple diffusion

Small nonpolar molecules move down concentration gradient without a membrane transporter

Passive transport

Solutes spontaneously move down their concentration gradients with the help of a membrane transporter/channel

Active transport

Solutes move against the concentration gradient with the help of a transporter and energy

Electrochemical gradient

Sum of the force from the concentration gradient of the solute and the force from membrane potential

What does electrochemical gradient determine?

Direction a charged solute will flow by passive transport

T/F: In passive transport, ions always move down their concentration gradient.

True

Aquaporin

Specialized membrane protein for transporting water

Work as a tetramer

Osmosis

The process by which water moves passively down its concentration gradient

Osmolarity

Total concentration of solutes in a solution

T/F: Generally the solute outside the cell is greater than the solute inside the cell

False

Isotonic

Solute concentration is the same inside and outside the cell

Hypotonic

Solute concentration outside the cell is lower than inside the cell (Hypo is low)

Hypertonic

Solute concentration outside the cell is higher than inside the cell (Hyper is high)

What happens if an animal cell is placed in a hypertonic solution?

It will shrivel

How do protozoans resist osmotic swelling?

Special water collecting vacuoles that periodically remove excess water

How do plant cells resist osmotic swelling?

Cell wall prevents cell from expanding excessively and vacuoles

How do animal cells resist osmotic swelling?

Na+/K+ pump

How do transporters and channels select which solutes they help move across the membrane?

A. Both channels and transporters discriminate between solutes mainly on the basis of size and electric charge.

B. Transporters discriminate between solutes mainly on the basis of size and electric charge; channels bind their solutes with great specificity in the same way an enzyme binds its substrate.

C. Channels allow the passage of solutes that are electrically charged; transporters facilitate the passage of molecules that are uncharged.

D. Channels will allow the passage of any solute as long as it has an electrical charge; transporters bind their solutes with great specificity in the same way an enzyme binds its substrate.

E. Channels discriminate between solutes mainly on the basis of size and electric charge; transporters bind their solutes with great specificity in the same way an enzyme binds its substrate.

E. Channels discriminate between solutes mainly on the basis of size and electric charge; transporters bind their solutes with great specificity in the same way an enzyme binds its substrate.

Passive Transporters

Move solute along electrochemical gradient

Can adopt several conformations; switch reversibly and randomly btwn them

Highly selective

Active transporters (Pumps)

Move solute against electrochemical gradient

Always coupled to an energy source

Na+ pump

Accounts for 30%+ of ATP consumption

Carries 3 Na+ out, 2 K+ in

NOKIA

(3) Na+ out; (2) K+ in, ATP used

What is the function of the Na+ pump

To create a concentration gradeitn

Ca²⁺ pump

Located in plasma and ER membrane

Important for muscle contraction, fertilization, and nerve cell communication

Maintain extremely low Ca²⁺ in cytosol

requires ATP

What is the transport direction of Ca²⁺ pumps?

Against concentration gradient (low con → high con)

T/F: Gradient driven pumps can be a symport or antiport

True

Symport

transports both solutes in same direction

Antiport

Transports both solutes in opposite directions (one goes up, other goes down)

T/F: Uniports are pumps

False