3.2 - Concept of Movement of Substances Across a Plasma Membrane

Characteristics of Substances That Move Across a Plasma Membrane

Movement depends on molecule size, molecule polarity and ionic charge.
Lipid-soluble, nonpolar substances move through the phospholipid bilayer: fatty acids, glycerol, fat-soluble vitamins (A, D, E and K) and steroid compounds.
Small lipid-insoluble molecules and ions move with the aid of transport proteins:
- polar molecule: water;
- nonpolar molecules: oxygen and carbon dioxide;
- ions: $K^{+}$ , $N a^{+}$ , $C a^{2 +}$ and $M g^{2 +}$ .
Large lipid-insoluble molecules move with the aid of carrier proteins: glucose and amino acids.

Problem: Does the size of dissolved particles affect movement across a selectively permeable membrane?
Hypothesis: Small molecules diffuse through the membrane; large molecules do not.
Manipulated variable: molecule size. Responding variable: presence of molecules in the Visking tubing and beaker. Fixed variables: solution temperature and immersion time.
Fill Visking tubing with glucose solution and starch suspension; immerse it in distilled water for 30 minutes.
Test the tubing contents and beaker solution with iodine and Benedict’s tests.
Glucose molecules cross the Visking tubing because they are smaller than its pores; starch molecules do not because they are larger than its pores.
Visking tubing and the plasma membrane are selectively permeable.

Problem: How does time affect the level of sucrose solution in a simple osmometer?
Hypothesis: The level of sucrose solution rises with time.
Manipulated variable: time. Responding variable: rise in sucrose-solution level in the capillary tube. Fixed variable: sucrose-solution concentration.
Fill Visking tubing with 30% sucrose solution, attach it to a capillary tube and immerse it in distilled water.
Record the solution level every two minutes and plot solution level against time.
Water moves through the selectively permeable Visking tubing into the sucrose solution by osmosis, so the level rises.

Movement of molecules or ions from high concentration to low concentration, down the concentration gradient, until dynamic equilibrium is achieved.
May occur with or without a plasma membrane.
Fatty acids, glycerol, oxygen and carbon dioxide diffuse through the phospholipid bilayer.

Net random movement of water molecules from high water potential (low solute concentration) to low water potential (high solute concentration) through a selectively permeable membrane.
The membrane is permeable to water but impermeable to some solutes such as sucrose.

Ions and large lipid-insoluble molecules such as amino acids and glucose move with the aid of channel or carrier proteins.
Does not require energy; substances move down the concentration gradient until dynamic equilibrium is achieved.
Channel proteins form specific channels or canals for small solutes and specific ions.
Carrier proteins have specific sites that bind only to particular molecules.
Glucose binds to the specific site of its carrier protein; the protein changes shape, moves glucose across the membrane, releases it and returns to its original shape.

Moves molecules or ions against the concentration gradient.
Requires energy from ATP generated during cellular respiration.
Requires specific carrier proteins with specific binding sites; these carrier proteins are called pumps.
A carrier protein binds the substance and ATP; attachment of a phosphate group changes the protein’s shape and moves the substance across the membrane.
Active transport accumulates or removes molecules or ions in cells.

Three sodium ions bind to the carrier protein inside the cell.
ATP decomposes into ADP and phosphate; the phosphate binds to the carrier protein.
The phosphate bond supplies energy and changes the carrier protein’s shape; sodium ions move out of the cell.
Two potassium ions bind to the carrier protein outside the cell; the phosphate leaves.
Loss of phosphate restores the carrier protein’s original shape.
Potassium ions move into the cell.

Proton pumps occur in epithelial cells lining the stomach cavity.
Energy from ATP enables carrier proteins to move hydrogen ions into the extracellular fluid.
Hydrogen ions accumulate and cause acid production in the stomach cavity.

Both move substances across a selectively permeable membrane.
Passive transport: no energy required; follows the concentration gradient; continues until dynamic equilibrium.
Active transport: requires energy; moves against the concentration gradient; accumulates or removes molecules or ions.