Question

how does the structure of the phospholipid bilayer contribute to the cell membrane’s function as a selective barrier?
○ the hydrophobic heads allow all types of molecules to easily cross.
○ the hydrophilic tails prevent water - soluble substances from passing through.
○ the hydrophobic tails create a barrier to water - soluble substances.
○ the hydrophilic heads repel all charged molecules.

which of the following is an example of a passive transport mechanism across the cell membrane?
○ uptake of large food particles by engulfing them with the cell membrane
○ movement of oxygen from an area of high concentration to an area of low concentration
○ movement of glucose against its concentration gradient using a protein pump
○ pumping sodium ions out of the cell where they are already in high concentration

osmosis is the movement of ______ across a selectively permeable membrane from an area of high concentration to an area of low concentration.
○ ions
○ water
○ solutes
○ proteins

facilitated diffusion requires the assistance of ______ to move specific molecules across the cell membrane.
○ nucleic acids
○ carbohydrates
○ transport proteins
○ atp

Explanation:

First Question:

• Analyze each option:
• Option 1: Phospholipid heads are hydrophilic, not hydrophobic, and they don't allow all molecules to cross easily. Eliminate.
• Option 2: Tails are hydrophobic, not hydrophilic. Eliminate.
• Option 3: Hydrophobic tails repel water - soluble substances, creating a barrier. This matches the role of the phospholipid bilayer in being a selective barrier.
• Option 4: Hydrophilic heads attract water - soluble (including many charged) molecules, they don't repel all charged molecules. Eliminate.

• Recall passive transport (moves down concentration gradient, no energy).
• Option 1: Uptake of large particles by engulfing is endocytosis, active (needs energy). Eliminate.
• Option 2: Oxygen moving from high to low concentration is diffusion, a passive transport.
• Option 3: Moving against concentration gradient (using a pump) is active transport (needs energy). Eliminate.
• Option 4: Pumping sodium out against concentration is active transport (needs energy). Eliminate.

• Recall osmosis definition: Osmosis is the movement of water across a selectively permeable membrane from high to low water concentration (or low to high solute concentration). Ions, solutes, and proteins are not what move in osmosis (solutes drive osmosis, but the movement is of water).

Answer:

The hydrophobic tails create a barrier to water - soluble substances.

Second Question: