Cell Membrane and Transport Notes

Properties of Water

• Water is unique as it naturally exists in all three states: solid, liquid, and gas.
• Water's chemical composition is H2O (two hydrogen atoms and one oxygen atom).
• Water is an excellent solvent.
• Ice floats because it is less dense than liquid water.
• Water has a high heat capacity; an example is using a hot water bottle.

Hydrogen Bonds

• Hydrogen bonds are weak attractions between water molecules.
• A hydrogen bond forms between a hydrogen atom (attached to one molecule) and a more electronegative atom on another molecule.

Surface Tension

• Surface tension is the "skin" that forms on the surface of a liquid.
• It occurs because molecules at the surface are more attracted to each other (cohesion) than to the air above.

Cohesion and Adhesion

• Cohesion is when water molecules stick to each other.
• Adhesion is the attraction of water to another substance.

Hydrophobic and Hydrophilic Molecules

• Hydrophobic molecules are nonpolar and do not dissolve well in water.
• Hydrophilic molecules are polar, charged, and mix well with water.

Capillary Action

• Capillary action is when a liquid moves up a small space against gravity, due to adhesion and cohesion.

Cell Membrane

• The cell membrane is selectively permeable, controlling what passes through.
• Substances enter and leave cells via osmosis, diffusion, and active transport.
• The cell membrane surrounds all cells.
• Homeostasis is a stable internal condition.
• The membrane maintains homeostasis by controlling what enters and exits the cell.

Phospholipids

• Phospholipids make up cell membranes.
• Phospholipids have a water-loving (hydrophilic) head and water-fearing (hydrophobic) tail.

Cell Membrane Structure: Phospholipid Bilayer

• Phospholipids form a phospholipid bilayer.
• Hydrophilic heads face the water.
• Hydrophobic tails face away from water (inward).

Full Membrane Structure

• Phospholipids: The main component, forming the bilayer.
• Proteins: Embedded in the membrane, used for transport into and out of the cell.
• Carbohydrates: Attached to phospholipids or proteins, used for cell identification.
• Cholesterol: Located in the hydrophobic part of the membrane, regulating fluidity.

Transport Mechanisms

Diffusion

• Definition: Random movement of particles from high to low concentration until equal.
• Reason: Molecules in liquids and gases are constantly moving and bumping into each other, causing them to spread out.

Facilitated Diffusion

• A type of passive transport where molecules move across the membrane with the help of a protein.
• Facilitated diffusion uses proteins to help larger or charged molecules enter or exit the cell.

Active Transport

• Definition: Movement of molecules or ions in or out of a cell against the concentration gradient.
• Requires energy (e.g., ATP).

Osmosis

• Definition: Diffusion of water molecules from a high concentration to a low concentration.

Tonicity

• Hypertonic: If cells are in a concentrated solution, water moves out of the cell.
• Isotonic: If cells have the same concentration of water and ions inside and out, water moves in and out evenly.
• Hypotonic: If cells are in a dilute solution, water moves into the cells, causing them to swell.