Cell Membranes and Transport

Cell Membrane and Phospholipids

Phospholipid Bilayer

• The cell membrane, also known as the plasma membrane, is crucial for cells' selective permeability.

• Composed of a phospholipid bilayer.

• Phospholipids are unique molecules with both:

  • A polar region (hydrophilic head).

  • A non-polar region (hydrophobic tails).

Polarity and Non-Polarity

• Polar molecules:

  • Have a positive and negative end, like a battery.

  • Many carbon-based molecules in the body are polar (e.g., glucose).

• Non-polar molecules:

  • Lipid-based or steroid-based.

  • Includes the tails of phospholipids.

• Phospholipids:

  • Have a polar head and non-polar tails.

Cell Membrane Structure

• The cell membrane is a phospholipid bilayer.

• Consists of two layers of phospholipids:

  • Heads face outwards (extracellular fluid).

  • Heads face inwards (intracellular fluid/cytosol).

  • Tails project from the heads, facing each other.

• Extracellular fluid: fluid outside the cell

• Intracellular fluid/cytosol: fluid inside the cell

Water and Molecular Interactions

• Water is a polar molecule (H2O).

• H2O Composition: Two hydrogen atoms and one oxygen atom.

• Electronegativity: Oxygen is more electronegative; it pulls electrons in covalent bonds closer.

• Covalent Bonds: Sharing of electrons between atoms. In H2O, oxygen attracts electrons more strongly, creating partial negative charges (\delta^-) on oxygen and partial positive charges (\delta^+) on hydrogen.

• Water Molecule Charge: Overall neutral but with partial charges. Water molecules form hydrogen bonds with each other due to polarity.

• Polar Interactions: Polar molecules interact favorably with other polar molecules.

• Non-Polar Interactions: Non-polar molecules interact favorably with other non-polar molecules.

• Polar and Non-Polar Repulsion: Polar and non-polar molecules do not mix well.

Phospholipid Bilayer Orientation

• Polar heads face the intracellular and extracellular fluids (both water-based).

• Non-polar tails face away from water, creating a hydrophobic core within the bilayer.

Selective Permeability

• The phospholipid bilayer makes the cell membrane selectively permeable.

  • Polar molecules cannot easily cross without channels.

  • Non-polar molecules can pass through without channels (e.g., lipids, steroids).

Channels and Transport

• Channels: Allow polar molecules and ions to cross the cell membrane.

• Example: Sodium channel allowing sodium (Na^+) to move into the cell down its electrochemical gradient.

• Channel Composition: Proteins that span the phospholipid bilayer.

• Selective Permeability: Dependent on the type of channels present in the cell membrane.

• Example: A cell with only sodium channels is permeable only to sodium.

• Sodium Concentration: High outside the cell, low inside the cell.

Passive Transport: Facilitated Diffusion

• Sodium moves into the cell via a sodium channel down its electrochemical gradient passively.

• Potassium moves out of the cell via a potassium channel down its electrochemical gradient passively.

• Facilitated Diffusion: Utilizes protein channels or transporters for movement down the electrochemical gradient; therefore, no energy is needed.

Simple Diffusion

• Super small non-polar molecules (e.g., oxygen, water) can diffuse across the membrane without a channel passively.

• Simple Diffusion: Movement of molecule directly across the phospholipid bilayer.

Diffusion Direction

• Simple and facilitated diffusion can occur bidirectionally (into or out of the cell).

• Example: Carbon dioxide (CO_2) diffuses out of the cell down its concentration gradient (or partial pressure).

Channel Types

• Leakage Channels: Always open, allowing continuous movement of specific ions (e.g., sodium, potassium).

• Gated Channels: Open or close in response to specific stimuli.

Ligand-Gated Channels

• Ligand: A signaling molecule that binds to the channel receptor (e.g., neurotransmitter, hormone).

• Mechanism: Ligand binding opens the channel, allowing specific ions to pass through.

• Selectivity: The ligand itself does not pass through the channel; it only opens it.

• Regulation: Allows cells to regulate when and what enters the cell.

Voltage-Gated Channels

• Activation: Open in response to changes in voltage (electrical potential) across the cell membrane.

• Sensitivity: Proteins are sensitive to voltage changes.

• Example: If the inside of the cell becomes more positive, a voltage-gated sodium channel may open, allowing sodium influx.

• Description: Should be specific. For example, voltage-gated sodium/potassium channel.

Mechano-Gated Channels

• Mechanical Stimulation: Open in response to physical changes, pressure, or manipulation of the cell.

Summary of Channel Types

• Leakage Channels: Always Open.

• Gated Channels:

  • Ligand-Gated: Open by ligand binding.

  • Voltage-Gated: Open by voltage change.

  • Mechanically Gated: Open by physical stimulus.

Gradient and Passive Movement

• If a channel is used, the transported entity still moves passively down its chemical or electrochemical gradient.

• Glucose Channels: Facilitate passive movement of glucose into or out of a cell.