Passive Transport Across the Cell Membrane

Solutes: Dissolved particles within a solution.
Concentration: The amount of solute particles in a given volume of solvent (water).
High concentration: A solution with a larger amount of solute compared to another.
Low concentration: A solution with a smaller amount of solute compared to another.
Concentration gradient: A difference in concentration across a distance/space.

The purpose of the lab is to investigate the movement of molecules (starch, iodine, and water) through a membrane (represented by a plastic bag).
Starch: A polysaccharide.
Iodine: A small diatomic molecule that serves as a starch indicator, turning purplish-black upon contact with starch to facilitate easy detection of molecule movement.

Fill a beaker with approximately 250 mL of water.
Measure 25 mL of water and pour it into the bag.
Add one spoonful of cornstarch to the water in the bag and mix until dissolved.
Close the bag.
Add 5 mL of iodine solution to the beaker and stir.
Place the bag into the beaker ensuring the cornstarch solution is as submerged as possible.

What will occur after 30 minutes?
Drawing Task: Divide paper in half with a pencil line and sketch the setup of the demonstration on the left side.

Particles tend to move from areas of high concentration to areas of low concentration.
Movement of particles down their concentration gradient is termed diffusion.
Diffusion is a passive process; it does not require energy since particles move naturally.
Question: How do non-living particles move on their own?

Molecules will continue to diffuse until the concentration gradient is eliminated (the concentration of particles is equal on both sides). This state is known as equilibrium.
At equilibrium, molecules continue to move equally in both directions, resulting in a net movement of zero.

Diffusion can also occur across the cell membrane, leading solute particles from high concentration on one side to low concentration on the other side until equilibrium is reached.

For solutes to cross the cell membrane via diffusion, they must be small enough to fit through the lipid bilayer spaces, termed simple diffusion.
Solutes must be nonpolar to navigate through the hydrophobic tail region of the lipid bilayer. Polar solutes would face repulsion.
Examples of small, nonpolar molecules: Oxygen gas (O₂) and Carbon dioxide (CO₂).

Larger solutes or polar/charged solutes (such as glucose and ions) cannot traverse the lipid bilayer via simple diffusion.
An alternative method called facilitated diffusion is needed for these solutes to cross the cell membrane.

Channel Proteins: Tubular structures that extend through the membrane, allowing large, polar, or charged solutes to pass through. Specific to the molecules they transport.
Carrier Proteins: Require solute binding specifically into them, similar to how substrates bind to enzymes. Upon binding, they change shape to release the solute on the other side of the membrane.

Concentration: Higher concentrations lead to more rapid diffusion due to increased collisions among molecules.
- In simple diffusion, the diffusion rate increases with concentration.
- In facilitated diffusion, diffusion rate will plateau as membrane proteins become saturated.
Temperature: Increased temperature heightens energy levels and results in faster diffusion due to more frequent collisions.
Molecular Size: Larger diffusing molecules move slower due to increased friction, leading to a decreased rate of diffusion.

Osmosis: The process of water diffusing across a membrane, vital for maintaining balance when solutes cannot pass through.
Water, although polar, can pass through the bilayer due to its small size. Specialized proteins called aquaporins assist in this movement in certain cells/tissues.

Water moves from areas of higher water concentration and lower solute concentration to areas of lower water concentration and higher solute concentration until equilibrium is achieved.
At equilibrium, there is an equal concentration of water and solute on both sides of the membrane (solutes remain stationary).
Random movement of water continues in both directions, resulting in no net movement.

A hypotonic solution has a higher water concentration and lower solute concentration than the intracellular fluid.
Water will move into the cell from the hypotonic solution, causing it to swell.

In animal cells (e.g., human red blood cells), swelling can lead to bursting (lysis) if too much water enters.

Plant cells maintain their structure due to the cell wall. Water influx increases turgor pressure, helping the plants remain upright and preventing wilting. A turgid plant cell is one that has absorbed water and is maintained by internal pressure against the cell wall.

A hypertonic solution has a lower water concentration and higher solute concentration than the intracellular fluid.
Water will move out of the cell, leading to a decrease in size.

Animal cells will shrink (crenate) and take on a shriveled appearance, resembling a raisin.

Plant cells also maintain original size but experience cytoplasm shrinkage; the membrane pulls away from the cell wall, a state termed plasmolysis, leading to significant wilting due to lack of turgor pressure.