[BIO 11.1] E3 - Transport and the Plasma Membrane

• The plasma membrane, also known as the cell membrane, separates the cytosol from the external environment.
• The plasma membrane has the property of self-repair due to the amphipathic nature of the phospholipid molecule.
• The plasma membrane is composed of a phospholipid bilayer with a hydrophobic tail and a hydrophilic head.
• The plasma membrane has the property of fluidity caused by the differences in length and saturation of fatty acid tails. Temperature affects the fluidity of the membrane, thus, when a plant cell is heated, the cell membrane enters a more fluid state.
• The plasma membrane has the property of semi-permeability because only selected molecules can diffuse (selective permeability).
• Small uncharged molecules (O2 and CO2), nonpolar, or hydrophobic molecules can freely cross the plasma membrane via diffusion.
• Polar or hydrophilic molecules cannot cross the plasma membrane without special channels.
• Aquaporins help water molecules diffuse.
• The fluid mosaic model describes the cell membrane’s properties.
• The experiment demonstrating the plasma membrane’s self-repair and semi-permeability:

  1.) Top layer of alcohol - the exterior of the cell

  2.) Middle layer of oil - cell membrane

  3.) Bottom layer of water - the interior of the cell
• Property of Self-repair: When a syringe was used to puncture the oil, or when it was shaken, oil globules separated from the oil layer. However, the oil globules eventually reformed back with the oil layer.
• Property of Semi-permeability: Soy sauce droplets were dropped slowly into the test tube. These droplets stopped at the bottom of the oil layer. Upon adding more soy sauce, it eventually passed through the oil layer and into the water layer. This demonstrates passive transport areas separated by a semi-permeable membrane.
• Diffusion is the movement of solute particles from a region of high concentration to one with a low concentration.
• Osmosis is the special diffusion of water. It involves the movement of water particles from a region of low solute concentration to a high solute concentration. It can also be thought of as the movement from a region of high water concentration to low water concentration.
• Sugar beet was used to demonstrate the fluidity of the cell membrane. This was done by preparing three test tubes.
• 1.) When a cut of sugar beet is simply added to the water, little pigment is seen.

  2.) When a steamed cut of sugar beet is mixed with water, a concentrated amount of pigment leaks at the bottom. Steaming increased cell membrane fluidity, so some anthocyanin molecules freely passed through.

  3.) When a cut of sugar beet was added to water, and then subjected to a warm bath, the pigment spread to all of the water in the test tube. Anthocyanins themselves get excited by the warm bath, forcing them out of the cell.
• Tonicity is defined as a solution’s ability to induce a change in the water concentration of a cell. This is due to the effects of osmosis.
• In the experiment, the boat of Moses was examined for the effects of solutions with different tonicities. Sucrose solution was used as sucrose cannot penetrate the membrane as it is too big.
• Hypotonic Solution: The normal state of a plant cell, where the cells are called turgid. Turgidity lets a plant stay rigid and stable, thereby making it upright. Cell walls prevent outright bursting of the cell, so turgidity is safe and ideal.
• Isotonic Solution: The plant cell turns flaccid, losing its ability to remain upright.
• Hypertonic Solution: The plant cell becomes plasmolyzed. Plasmolyzation is when the central vacuole shrinks, thereby causing the cytoplasm to shrink. Due to this, the plant cell has a massive gap between its cytoplasm and cell wall, making it wilt. Completely white cells lack a protoplasm.
• In the experiment, red blood cells were examined for the effects of solutions with different tonicities. A saline solution was used, as its sodium ions cannot penetrate the membrane due to being polar.
• Isotonic Solution: An animal cell’s normal state, where it is shaped like a biconcave disk. This equates to being in a 0.9% saline solution. It is also called flaccid.
• Hypotonic Solution: The animal cell is considered lysed. The cell swells and threatens to burst.
• Hypertonic Solution: The animal cell is crenated, gradually shriveling, and losing its shape due to water going out.
• Protozoa have a special structure called a contractile vacuole. It contracts and expels excess water to ensure it does not suffer from the effects of hypotonicity.