human bio week 2

Overview of Cell Structure

• Cells typically have a general structure that includes:

  • Plasma Membrane: This membrane surrounds the entire cell, acting as a barrier between the internal and external environments.

  • Nucleus: Most cells contain a nucleus that houses the DNA (genetic material).

  • Cytoplasm: This is the fluid outside the nucleus, containing organelles essential for the cell's function and survival.

Function of the Plasma Membrane

• The primary role of the plasma membrane is to separate the internal cell environment from the external one, maintaining homeostasis by regulating the ion concentrations in the cell.

• Inside the cell, there is a high concentration of potassium (K) and proteins, while outside, sodium (Na) and chloride (Cl) ions are more concentrated.

• The plasma membrane plays a crucial role in keeping these environments stable.

Phospholipid Bilayer

• The main component of the plasma membrane is the phospholipid bilayer:

  • Hydrophilic Head: This part has an affinity for water (hydrophilic).

  • Hydrophobic Tails: The tails repel water (hydrophobic).

• In an aqueous environment, hydrophobic tails point inward, and hydrophilic heads face outward, creating a barrier that allows the membrane to be selectively permeable.

Selective Permeability

• A selectively permeable membrane allows certain molecules to pass while restricting others. Here’s how different types of substances interact with the membrane:

  • Small Nonpolar Lipophilic Molecules:

    • Molecules like oxygen, carbon dioxide, nitrogen, fatty acids, and steroid hormones can easily diffuse across the membrane because they can dissolve in the lipid bilayer due to their fat-loving properties.

  • Small Uncharged Polar Molecules:

    • Examples include water, urea, glycerol, and ethanol. These can also pass through due to their uncharged nature, but they are larger than nonpolar molecules.

  • Large Uncharged Polar Molecules:

    • Molecules such as amino acids, glucose, lactate, and nucleotides cannot pass through the membrane because they are physically too large.

  • Charged Molecules and Ions:

    • Ions like potassium, sodium, and chloride cannot pass through the membrane as they are repelled by the hydrophobic environment of the lipid bilayer.

Embedded Structures in the Plasma Membrane

• In addition to the phospholipid bilayer, the plasma membrane contains various structures:

  • Integral Membrane Proteins:

    • These proteins span the membrane and play key roles in controlling the movement of substances. For example, aquaporins facilitate the movement of water across the membrane.

  • Cholesterol:

    • Cholesterol molecules are embedded within the membrane, associating with hydrophobic tails. They contribute to the membrane's fluidity and stability while also preventing the passage of polar and charged molecules, thereby enhancing the barrier functions of the membrane.