Cell Membrane Structure and Function

Cell Membrane Structure

• The cell membrane is composed of a phospholipid bilayer.
• The hydrophobic tails of phospholipids face inwards, forming the inside of the membrane.
• The hydrophilic heads face outwards, interacting with the intracellular and extracellular fluids.
• The membrane is impermeable to water-soluble substances like ions, glucose, and urea.
• The membrane is permeable to fat-soluble substances like oxygen, carbon dioxide, and alcohol.

Sphingolipids

• Sphingolipids are derived from the amino alcohol sphingosine.
• They contain both hydrophobic and hydrophilic groups.
• They are present in small amounts in cell membranes, especially in nerve cells.
• Functions of sphingolipids include protection from environmental factors, signal transmission, and adhesion sites for extracellular proteins.

Cholesterol

• Cholesterol has a steroid nucleus, making it highly fat-soluble.
• It is interspersed within the phospholipid bilayer.
• It helps regulate membrane permeability to water-soluble substances.
• Cholesterol affects membrane fluidity.

Cell Membrane Proteins

• There are two types of cell membrane proteins: integral and peripheral.
• Integral proteins span the entire membrane.
• Peripheral proteins are attached to one surface of the membrane.

Integral Protein Functions

• Form channels for water and water-soluble substances (especially ions).
• Act as carrier proteins for transporting substances across the lipid bilayer.
• Function as enzymes.
• Serve as receptors for molecules that cannot easily penetrate the membrane.

Peripheral Protein Functions

• Primarily function as enzymes or regulators of substance transport through membrane pores.

Membrane Carbohydrates (Glycocalyx)

• Attached to membrane proteins (glycoproteins) or lipids (glycolipids).
• Located exclusively on the external surface of the cell.
• Form a protective layer called the glycocalyx.

Glycocalyx Functions

• Contribute to the negative electrical charge of cells.
• Facilitate cell-to-cell adhesion.
• Act as receptors for hormones (e.g., insulin).
• Participate in immune reactions.

Transport of Substances Through Cell Membranes

• Extracellular fluid has high sodium and chloride concentrations, while intracellular fluid has high potassium and phosphate concentrations.
• Membrane proteins create alternative pathways through the cell membrane.

Lipid-Soluble Substances

• Move through the membrane directly.

Transport Proteins

• Facilitate the movement of substances across the membrane.
• Channel proteins allow free movement of water, ions, and molecules.
• Carrier proteins bind with transported molecules and undergo conformational changes to move substances across the membrane.

Membrane Transport Mechanisms

• Diffusion: Random molecular movement of substances down their concentration gradient.
  • Simple diffusion occurs through membrane openings or intermolecular spaces without carrier proteins.
  • Facilitated diffusion involves carrier proteins.
• Active Transport: Moves substances against their concentration gradient, requiring energy.

Diffusion

• Molecules and ions in body fluids are in constant motion.
• Diffusion is the random movement of molecules among one another in liquids or gases.

Diffusion Through the Cell Membrane

• Simple diffusion: Movement through membrane openings or intermolecular spaces without carrier proteins.
• Facilitated diffusion: Requires carrier proteins, but transport is still down the concentration gradient. Most glucose and amino acids are transported this way.

Facilitated Diffusion Characteristics

• Facilitated diffusion approaches a maximum rate (Vmax) as the concentration of the diffusing substance increases.

Osmosis

• Net movement of water across a membrane due to a water concentration difference.
• Occurs if:
  • There is a concentration difference of water.
  • The membrane is not permeable to the solute.

Tonicity

• The ability of an extracellular solution to make water move into or out of a cell by osmosis.
• Isotonic: Equal solute concentration compared to the cell interior.
• Hypotonic: Lower solute concentration than the cell, causing water to enter the cell and potentially lead to swelling.
• Hypertonic: Higher solute concentration than the cell, causing water to leave the cell and potentially lead to shrinking.

Active Transport

• Movement of solutes against their electrochemical gradient (uphill).
• Requires energy to move molecules or ions uphill.
• Crucial for maintaining cellular function.

Summary of Key Concepts

• The cell membrane is a complex structure that controls what enters and exits the cell.
• Phospholipids, cholesterol, and proteins contribute to the membrane's structure and function.
• Membrane transport mechanisms include diffusion, facilitated diffusion, and active transport.
• Osmosis is a special type of diffusion that involves the movement of water across a membrane.
• Tonicity describes the effect of a solution on the volume of a cell.
• Active transport is essential for maintaining the appropriate intracellular environment.