B2.1 Membrane and Membrane Transport

All cells possess a plasma membrane (also known as the cell membrane).
- Terms "plasma membrane" and "cell membrane" are synonymous.
- Structure:
- Composed of two layers of phospholipids, referred to as the lipid bilayer (the prefix "bi-" indicating two).

Control of Substances:
- The plasma membrane regulates what enters and exits the cell, ensuring proper compartmentalization for cellular function.
- This characteristic is known as being semi-permeable or selectively permeable (both terms refer to the same concept).

Molecule permeability across the membrane is influenced by:
- Size of the molecule: Larger molecules are less permeable and require alternate transport mechanisms.
- Charge of the molecule: Membrane is not permeable to polar or charged molecules due to hydrophobic phospholipid tails that repel these substances.
Molecules that can pass through:
- Small, non-polar molecules (e.g., oxygen) can easily diffuse across the membrane.

Molecules are constantly in motion, resulting in random collisions:
- More crowded molecules collide more frequently, leading to diffusion:
- Diffusion is defined as the passive movement of particles from an area of high concentration ( crowded) to an area of low concentration (less crowded).
- Passive Transport:
- No additional energy required for diffusion to occur.
Equilibrium:
- States when concentrations are equal on both sides of the membrane; diffusion continues until this point is reached.

Transport Mechanisms:
- Molecules that cannot diffuse must utilize proteins for transport.
- Proteins within the membrane:
- Peripheral Proteins: Positioned on the surface; do not span the membrane completely.
- Integral Proteins: Span across the membrane and include:
  - Channel Proteins: Facilitate transport by forming pores through which substances can pass.
  - Protein Pumps: Actively transport substances against their concentration gradient.

Aquaporins:
- Special channel proteins that facilitate the transport of water.
- Water movement across a membrane is called osmosis.
Osmosis:
- Defined as the net movement of water molecules across a semi-permeable membrane from low solute concentration (high water potential) to high solute concentration (low water potential); passive process, requiring no energy.

In scenarios where a solute (e.g., glucose) cannot cross a semi-permeable membrane, water will:
- Move across the membrane toward the area of higher solute concentration to achieve equilibrium.

Types include:
- Diffusion: Movement of gases and small non-polar molecules.
- Facilitated Diffusion:
- Passive movement of molecules from high concentration to low through channel proteins (e.g., glucose).
- Requires specific channel proteins for charged and polar molecules and substances that cannot diffuse through the bilayer alone.

Active Transport:
- Movement of molecules against their concentration gradient (from low to high concentration).
- Requires energy (ATP) to change the shape of protein pumps in the membrane.
- Specific protein pumps (e.g., glucose pump, sodium pump) operate in this capacity.

Membrane must be selectively permeable to maintain homeostasis:
- Molecules like oxygen and water are naturally permeable, while others must be regulated via channel proteins or pumps.

Glycoproteins:
- Proteins with carbohydrate chains on their surfaces promote cell recognition and adhesion.
Glycolipids:
- Lipids with carbohydrate chains; serve similar functions in recognition and are unique to eukaryotic cells.

Current model illustrating that cell membranes are both:
- Fluid: Components can move laterally within the layer.
- Mosaic: Composed of various proteins interspersed among the lipid bilayer.
Importance of fluidity and structure in relation to membrane function and molecule transport.