Transport Across Membranes: Overcoming the Permeability Barrier

Explore the differences and similarities between various transport mechanisms:
- Simple diffusion vs. facilitated diffusion
- Passive transport vs. active transport
- Direct active transport vs. indirect active transport
Discuss osmosis

Overcoming the permeability barrier of cell membranes is crucial for proper functioning of the cell.
Specific molecules and ions must be selectively moved into and out of the cell or organelle.
Membranes exhibit selective permeability or semipermeability, allowing only certain substances to pass through.

Common Solutes:
- Most substances that move across membranes include dissolved gases, ions, and small organic molecules, collectively known as solutes.
Transport Mechanisms:
- Solutes cross membranes by three primary mechanisms:
- Simple Diffusion:
  - Direct, unaided movement of solutes driven by concentration differences across the membrane.
  - Only a few molecules can cross membranes this way.
- Facilitated Diffusion (Passive Transport):
  - Most solutes use transport proteins (integral membrane proteins) to assist in crossing membranes.
  - Moves solutes to regions of lower concentration without using energy.
- Active Transport:
  - Transport proteins move solutes against their concentration gradient, requiring energy.
  - This energy is often sourced from the hydrolysis of ATP or by simultaneous transport of another solute down its gradient.

Erythrocyte Plasma Membrane:
- Oxygen gas crosses the lipid bilayer through simple diffusion.
- Erythrocytes take up oxygen in the lungs (high concentration) and release it in body tissues (low concentration).
- Question: Differentiate between the movement of charged molecules/ions vs. hydrophobic molecules across the membrane.

General Features of Transport Proteins:
- Large, integral membrane proteins with multiple transmembrane segments.
Types of Transport Proteins:
- Carrier Proteins (Transporters or Permeases):
- Bind solute molecules on one side and undergo a conformational change to release the solute on the opposite side.
- Channel Proteins:
- Form hydrophilic channels to create a passage route for solutes.

Carrier Proteins:
- Alternate between two conformational states during transport.
Transport Types by Carrier Proteins:
- Uniport:
- Transport of a single solute; the transporter is called a uniporter.
- Coupled Transport:
- Involves simultaneous transport of two solutes, which can be classified as:
  - Symport (Cotransport):
  - Both solutes are moved in the same direction across the membrane.
  - Antiport (Countertransport):
  - Solutes are moved in opposite directions across the membrane.
- Transporters facilitating these processes are called symporters and antiporters.

Classification of Active Transport:
- Measured based on energy sources and simultaneous transport of solutes:
- Direct Active Transport (Primary Active Transport):
- Accumulation of solute molecules on one side of the membrane is coupled directly to an exergonic chemical reaction, typically ATP hydrolysis.
- Transport proteins involved are referred to as transport ATPases or ATPase pumps.
- Indirect Active Transport:
- Depends on the simultaneous transport of two solutes:
  - A favorable movement of one solute down its gradient drives the unfavorable movement of another solute up its gradient.
  - Can occur as symport or antiport depending on the direction of solute movement.

Direct Active Transport Example:
- Transport of H+ ions using ATP directly.
Indirect Active Transport Example:
- Transport of S ions using ATP indirectly.

Definition of Osmosis:
- The diffusion of water across a selectively permeable membrane.
Mechanism:
- If two solutions are separated by a membrane permeable to water but not to solutes, water moves toward the region of higher solute concentration.
Osmotic Movement:
- Related to the relative osmolarity, or total solute concentrations, both inside and outside the cell.
- Solutions defined:
- Hypertonic Solution:
  - Higher solute concentration outside the cell.
- Hypotonic Solution:
  - Lower solute concentration outside the cell.
Comparison to Other Transport Mechanisms:
- Osmosis differs as it involves solvent (water) movement instead of solute movement, and can be considered a form of passive transport since water moves from high concentration (more water) to low concentration (less water).

Simple Diffusion vs. Osmosis:
- Simple Diffusion:
- Occurs when the membrane separating chambers has permeability to solute; solute moves from high to low concentration until equilibrium is reached.
- Osmosis:
- Occurs when the membrane is not permeable to solute; water moves from a region of low solute concentration (more water) to high solute concentration (less water) until solute concentrations equalize on both sides.

Cells can maintain their size using active transport mechanisms.
Human red blood cells manage to avoid shrinking or bursting by maintaining an intracellular environment that is isotonic with the extracellular environment (the blood).