KA

Cellular Transport and Bioenergetics

Concentration Gradients and Transport

  • Definition: A concentration gradient exists when there is a difference in the amount of a substance (solute) in one area versus another, often across a membrane.

    • It describes a situation where there are regions of high concentration of molecules and regions of low concentration of molecules.

  • Relates to Transport: Concentration gradients are fundamental to transport mechanisms, particularly passive transport.

  • Movement Principle: Substances move from an area of high concentration to an area of low concentration.

  • Visual Representation: Imagine a container divided by a membrane. A concentration gradient is present if the amount of solute on one side of the membrane is different from the amount on the other side.

  • Magnitude of Gradient:

    • More extreme/bigger discrepancy: A larger difference in substance amount between sides.

    • Smaller: A smaller difference.

    • Dynamic equilibrium: When the rates of movement in both directions are equal, resulting in no net movement, even though individual molecules are still moving randomly.

  • Molecular Motion: As long as the temperature is not at absolute zero (0 ext{ K} or -273.15^{ ext{o}} ext{C}), molecules are in constant, random motion due to molecular vibration and bouncing.

  • Net Movement:

    • No Concentration Gradient: If there is an equal distribution of material, there is no net movement in any one direction. Random wiggling occurs equally from one side to the other.

    • With Concentration Gradient: If there is an unequal distribution, random molecular motion will result in a net movement of substances passively from the area of high concentration to the area of lower concentration.

Passive Transport: Osmosis

  • Definition: Osmosis is a specific type of passive transport that refers to the diffusion of water across a selectively permeable membrane.

    • Water moves from an area of high water concentration to an area of low water concentration.

  • Water-Solute Concentration Relationship:

    • Water is the solvent of life, and its concentration is inversely related to the concentration of solutes.

    • As solute concentration increases, the concentration of free water ( ext{H}_2 ext{O} molecules) decreases.

    • Conversely, where there is low solute concentration, there is high water concentration.

  • Example Scenario (Cell):

    • Consider a cell with a membrane. If there's high solute X outside the cell and low solute X inside.

    • If the membrane is permeable to both solute and water: Both substances will move from high to low concentration.

      • Solute X moves from outside (high) to inside (low).

      • Water moves from inside (high water/low solute) to outside (low water/high solute).

  • Tonicity: Describes the relative solute concentration of a solution compared to another solution, typically a cell's cytoplasm.

    • Hypotonic Solution: A solution with a lower solute concentration (and thus higher water concentration) than the cell it surrounds.

      • Scenario: Cell in a hypotonic solution. Outside the cell: high water, low solute. Inside the cell: relatively lower water, relatively higher solute.

      • Water movement (if membrane is permeable to water only): Water will move into the cell (from high water concentration outside to lower water concentration inside).

    • Hypertonic Solution: A solution with a higher solute concentration (and thus lower water concentration) than the cell it surrounds.

      • Scenario: Cell in a hypertonic solution. The extracellular fluid would be described as hypertonic relative to the cell, meaning it has a relatively higher concentration of solute.

  • Osmosis Mechanism: Osmosis primarily occurs via facilitated diffusion.

    • Water ( ext{H}_2 ext{O}) is a polar molecule, meaning it typically requires channel proteins (like aquaporins) to efficiently cross the nonpolar lipid bilayer of the cell membrane.

ATP: The Energy Currency Molecule

  • ATP (Adenosine Triphosphate): The primary energy currency molecule for cells.

  • Molecular Structure: ATP is a type of nucleotide.

    • It consists of:

      • Adenine (a nitrogenous base)

      • Ribose (a five-carbon sugar)

      • Three negatively charged phosphate groups.

    • It is specifically a triphosphate adenine ribonucleotide.

  • Energy Storage in ATP: The high energy potential of ATP comes from the electrostatic repulsion between the three negatively charged phosphate groups.

    • These groups repel each other, and the covalent bonds holding them together require significant energy to maintain against this repulsion.

  • Energy Release: ATP Hydrolysis:

    • When the terminal phosphate group is broken off from ATP, the energy stored in that bond is released.

    • This is a chemical reaction called hydrolysis (meaning