Plasma membranes
1. What is the primary function of the plasma membrane?
A) To store genetic information
B) To regulate the passage of substances in and out of the cell
C) To synthesize proteins
D) To provide structural support to the nucleus
The plasma membrane is selectively permeable, controlling what enters and exits the cell.
1. According to the fluid mosaic model, why are nonpolar molecules (e.g., O₂, CO₂, N₂) able to diffuse directly through the plasma membrane?
A) They are repelled by hydrophobic phospholipid tails.
B) They are small enough to pass through integral proteins.
C) They dissolve in the hydrophilic core of the membrane.
D) They are uncharged and soluble in the hydrophobic lipid bilayer.
D – Nonpolar molecules diffuse freely because they dissolve in the hydrophobic tails of phospholipids.
Which process describes the movement of molecules from an area of high concentration to low concentration without energy input?
A) Active transport
B) Facilitated diffusion
C) Endocytosis
D) Sodium-potassium pump
B – Facilitated diffusion is a passive process (no ATP) where molecules move down their gradient via channel/carrier proteins.
Which type of transport across the plasma membrane requires energy (ATP)?
A) Osmosis
B) Facilitated diffusion
C) Active transport
D) Simple diffusion
C – Active transport moves substances against their concentration gradient, requiring ATP.
2. Which statement best explains why polar molecules (e.g., Na⁺, glucose) struggle to cross the membrane without assistance?
A) Their charges are repelled by peripheral proteins.
B) The hydrophobic interior of the membrane blocks their passage.
C) They are too large to fit between phospholipids.
D) Cholesterol selectively excludes polar substances.
B – Polar/charged molecules are hydrophilic and cannot pass the membrane’s hydrophobic core without transport proteins.
How does water primarily cross cell membranes?
A) Simple diffusion through the lipid bilayer
B) Active transport via aquaporins
C) Facilitated diffusion through aquaporins
D) Exocytosis
C) Facilitated diffusion through aquaporins
. What is the primary difference between integral and peripheral proteins in the plasma membrane?
A) Integral proteins are only found on the extracellular surface, while peripheral proteins are inside the cell.
B) Integral proteins span the entire membrane, while peripheral proteins are outside, attached to one side.
C) Peripheral proteins are always glycoproteins, while integral proteins are not.
D) Integral proteins require ATP to function, while peripheral proteins do not.
B) Integral proteins span the entire membrane, while peripheral proteins are outside, attached to one side.
3. How do channel proteins differ from carrier proteins in facilitating transport?
A) Channel proteins hydrolyze ATP; carrier proteins do not.
B) Channel proteins form pores for passive diffusion; carriers undergo conformational changes.
C) Carrier proteins only transport nonpolar molecules; channels transport ions.
D) Channels require a concentration gradient; carriers work against gradients.
B – Channels create hydrophilic pores for passive diffusion (e.g., aquaporins), while carriers bind molecules and change shape (e.g., glucose transporters).
What distinguishes active transport from facilitated diffusion?
A) Active transport moves nonpolar molecules.
B) Active transport requires energy to move solutes against their gradient.
C) Facilitated diffusion only occurs in prokaryotes.
D) Facilitated diffusion uses ATP.
B – Active transport requires ATP to pump molecules uphill; facilitated diffusion is passive.
How are integral proteins held in the plasma membrane?
A) By covalent bonds to peripheral proteins
B) By hydrophobic interactions with phospholipid tails
C) By hydrogen bonds with the aqueous environment
D) By ionic bonds with cholesterol
B) By hydrophobic interactions with phospholipid tails
Because Integral proteins have hydrophobic regions that embed them in the lipid bilayer, avoiding contact with water.
5. Why can water (a polar molecule) cross the membrane more easily than other polar substances?
A) It ionizes to form H⁺ and OH⁻, which diffuse freely.
B) Its small size and partial charge allow slow diffusion but very low in concentration or use of aquaporins.
C) Cholesterol creates pores specifically for water.
D) Peripheral proteins transport water actively.
B – Water’s small size enables slow diffusion, and aquaporins (channel proteins) greatly enhance its permeability.
A red blood cell placed in a hypertonic solution will likely:
A) Swell and burst due to osmosis.
B) Shrink as water leaves the cell.
C) Actively transport ions to balance the gradient.
D) Remain unchanged due to rigid cell walls.
B – In hypertonic solutions, water moves out passively (osmosis), causing shrinkage.
The fluid mosaic model describes the plasma membrane as:
A) A rigid, static bilayer of phospholipids with embedded proteins.
B) A dynamic structure where proteins and phospholipids can move laterally within the membrane.
C) A single layer of phospholipids coated with peripheral proteins on one side.
D) A porous sheet of cellulose with fixed channels for transport.
B – The fluid mosaic model emphasizes:
Fluidity: Phospholipids and proteins can move laterally (like a "fluid").
Mosaic: Proteins are scattered unevenly, creating a patchwork ("mosaic").
What is the main purpose of the cell wall?
_____________________________? (provide your answer)
It provides support and structure to the cell when water pressure is high (Plant)
A Paramecium living in a freshwater pond (hypotonic environment) constantly gains water by osmosis. How does it prevent lysing (bursting)?
A) By actively pumping water out using contractile vacuoles.
B) By increasing membrane cholesterol to reduce permeability.
C) By absorbing ions to balance internal and external solute concentrations.
D) By secreting a rigid cell wall to resist swelling.
A – Paramecium uses contractile vacuoles to actively expel excess water, maintaining osmotic balance.