18. The table is completed as follows:
| Complex diffusion | Concentration gradient? | Needs ATP? | Movement direction |
| :--- | :--- | :--- | :--- |
| Simple diffusion | ✓ | ✗ | High to low |
| Facilitated diffusion | ✓ | ✗ | High to low |
| Active transport | ✗ | ✓ | Low to high |
19. Explanation:
- Water is moving by osmosis from an area of low solute concentration (outside) to an area of higher solute concentration (inside the cell).
- Diffusion between cytoplasm and endoplasmic reticulum: Molecules move from high to low concentration without energy; no ATP needed.
- Exocytosis: Vesicle fuses with plasma membrane, releasing contents outside; requires ATP for vesicle formation and fusion.
- Endocytosis: Plasma membrane invaginates to form vesicle around external material; requires ATP for membrane reshaping.
20. In the membrane, lipid bilayer allows passive movement of small nonpolar molecules; integral proteins facilitate transport of polar/charged molecules via channels or carriers; peripheral proteins assist in signaling or structural support.
21. Phospholipids have hydrophilic heads facing water and hydrophobic tails facing inward, forming a barrier. Cholesterol modulates fluidity. Integral proteins span the membrane for transport/signaling. Peripheral proteins attach temporarily for enzymatic or structural roles. Glycoproteins aid in cell recognition.
22. Diagram outline:
- Sodium-potassium pump actively transports 3 Na⁺ out and 2 K⁺ in per ATP hydrolyzed.
- This creates a net loss of positive charge inside the cell, establishing a negative resting membrane potential relative to the outside.
23. Completed diagram labels:
- Left side (extracellular): High Na⁺, Low K⁺
- Right side (intracellular): Low Na⁺, High K⁺
- Pump: Uses ATP to move Na⁺ out, K⁺ in
- Result: Membrane potential negative inside
24. Solutions:
- Hypertonic solution: Cell shrinks (crenation) due to water leaving.
- Isotonic solution: No net water movement; cell shape unchanged.
- Hypotonic solution: Cell swells and may lyse due to water entering.
Parent Tip: Review the logic above to help your child master the concept of cell transport worksheet answers.
