Exercise Review Sheet Cell Membrane Transport Mechanisms Name: Lab Time/Date: Choose all answers that apply to items 1 and 2, and place their letters on the response blanks. 1. The motion of molecules: a. reflects the kinetic energy of molecules b. reflects the potential energy of molecules c. is ordered and predictable d. is random and erratic 2. The speed of molecular movement: a. is higher in larger molecules b. is lower in larger molecules c. increases with increasing temperature d. decreases with increasing temperature e. reflects kinetic energy 3. What is the relationship between density of the medium and diffusion rate? The following refer to Activity 2, “Observing Diffusion Through Nonliving Membranes”: Sac 1: 40% glucose suspended in distilled water – Did glucose pass out of the sac? – Test used to determine presence of glucose: – Did the sac weight change? – If so, explain the reason for its weight change: Sac 2: 40% glucose suspended in 40% glucose – Was there net movement of glucose in either direction? – Explanation: – Did the sac weight change? – Explanation: Sac 3: 10% NaCl suspended in distilled water – Was there net movement of NaCl out of the sac? – Test used to determine the presence of NaCl: – Direction of net osmosis:
The Correct Answer and Explanation is:
1. The motion of molecules:
✅ a. reflects the kinetic energy of molecules
✅ d. is random and erratic
2. The speed of molecular movement:
✅ b. is lower in larger molecules
✅ c. increases with increasing temperature
✅ e. reflects kinetic energy
3. What is the relationship between density of the medium and diffusion rate?
As the density of the medium increases, the diffusion rate decreases. A denser medium provides more resistance, making it harder for molecules to move, which slows the diffusion process.
Activity 2: Observing Diffusion Through Nonliving Membranes
Sac 1: 40% glucose in distilled water
- Did glucose pass out of the sac? Yes
- Test used: Benedict’s test
- Did the sac weight change? Yes, it increased
- Explanation: Water moved into the sac via osmosis due to the higher solute concentration inside.
Sac 2: 40% glucose in 40% glucose
- Was there net movement of glucose? No
- Explanation: The concentration gradient was equal; glucose molecules moved equally in both directions, resulting in no net movement.
- Did the sac weight change? No
- Explanation: Osmotic balance was maintained; no net water movement.
Sac 3: 10% NaCl in distilled water
- Was there net movement of NaCl out of the sac? Yes
- Test used: Silver nitrate (AgNO₃) test
- Direction of net osmosis: Into the sac
- Explanation: Water moved into the sac due to the higher salt concentration inside. Salt ions diffused out, and water moved in to balance the solute concentration, showing typical diffusion and osmosis behavior across a selectively permeable membrane.
Explanation:
Cell membrane transport relies on principles of diffusion and osmosis. Molecules are constantly in motion due to their kinetic energy, and this movement is random and erratic. Larger molecules move more slowly, and all molecules move faster at higher temperatures because their kinetic energy increases.
Diffusion occurs from areas of higher concentration to lower concentration, and the rate is influenced by the medium’s density—denser mediums slow molecular motion. In Sac 1, glucose was in higher concentration inside, and water was outside. Glucose diffused out, and water moved in, increasing the sac’s weight. The Benedict’s test confirmed glucose diffusion.
In Sac 2, both inside and outside had equal glucose concentrations, so although molecules still moved, there was no net movement. Thus, the sac’s weight remained constant, illustrating equilibrium.
Sac 3 had a higher salt concentration inside than the surrounding distilled water. NaCl diffused out, and water entered the sac by osmosis to dilute the internal solute concentration. The test for NaCl used silver nitrate, which forms a white precipitate in the presence of chloride ions.
These experiments demonstrate how selectively permeable membranes regulate substance movement, mimicking how cells maintain homeostasis by controlling water and solute flow based on concentration gradients.
