Transport Across Membranes Study Notes

Transport Across Membranes: Overcoming the Permeability Barrier

• Introduction to Membranes
  • Membranes are selectively permeable or semipermeable, which allows for selective transport and accumulation of substances in cells and organelles.
  • This selective nature is critical for maintaining homeostasis, enabling the concentration of various substances within cells at levels different from their surrounding environment.

Cellular Homeostasis

• Homeostasis is essential for cell functions, primarily achieved through selective transport mechanisms that involve the movement of:
  • Ions
  • Small organic molecules (referred to as metabolites, which are components of metabolic pathways).

Types of Diffusion

Simple Diffusion vs. Osmosis

• Definition: When two solutions are separated by a selectively permeable membrane (permeable to water but not to solutes), water will move toward the area of higher solute concentration.
• This process is known as osmosis.
  • In most cells, water tends to move inward.
• Osmolarity: Refers to the total solute concentrations inside versus outside of the cell.
  • If the solute concentration is higher outside the cell, the solution is termed hypertonic.
  • Conversely, if the solute concentration is lower outside the cell, the solution is classified as hypotonic.

Cell Responses to Osmolarity

• Animal Cells:

  • In an isotonic solution (equal solute concentration), animal cells maintain their shape.
  • In a hypertonic solution, animal cells shrivel due to water loss.
  • In a hypotonic solution, animal cells may swell and eventually burst (lyse) due to excess water influx.

• Plant Cells:

  • Possess cell walls which prevent bursting in hypotonic solutions, instead becoming turgid due to turgor pressure.
  • In a hypertonic solution, plant cells undergo plasmolysis, where the plasma membrane separates from the cell wall.

• Example:

  • Place a cell in 20% sucrose solution (hypertonic): The cell shrinks (plasmolyzed).
  • Return it to distilled water (hypotonic): The cell inflates (becomes turgid).

Cells Without Cell Walls

• Cells lacking cell walls manage osmolarity by actively pumping out inorganic ions, reducing their intracellular osmolarity to minimize concentration differences with their surroundings.

Transport Proteins and Mechanisms

• Due to the hydrophobic nature of the lipid bilayer, hydrophilic solutes require special mechanisms for transport.
• Integral Membrane Transport Proteins (MTPs):
  • Belong to the Major Intrinsic Protein Family and facilitate the transport of solutes across membranes.

Categories of Membrane Transport

1. Passive Transport

• Energy Requirement: No energy required.
• Examples:
  • Simple diffusion: Movement of small molecules like gases (O₂, CO₂) across the membrane without proteins.
  • Facilitated diffusion (channel): Specific proteins are used to assist the movement of larger or polar molecules down their concentration gradients.

2. Active Transport

• Energy Requirement: Energy is required (usually from ATP hydrolysis).
• Examples:
  • Primary active transport: Directly coupled to an exergonic chemical reaction (usually ATP hydrolysis).
  • Secondary active transport: Depends on the electrochemical gradient established by primary active transport; includes symport and antiport mechanisms.

Comparison of Transport Methods

Table 8-1: Summary of Diffusion and Transport Types

Carrier Proteins and Transport

• Carrier Proteins: Bind solutes on one side of the membrane, undergo conformational changes, and release the solute on the opposite side.
• Alternating Conformation Model: Suggests that carrier proteins are allosteric and can transition between two states to facilitate transport.

Competitive Inhibition of Carrier Proteins

• Description: Occurs when structurally related molecules inhibit carrier proteins. Example: Glucose transport can be inhibited by other monosaccharides (like mannose and galactose).

Phosphorylation of Glucose

• Process: Immediate phosphorylation of glucose upon entering the cell helps maintain a low intracellular glucose concentration, effectively locking it inside since phosphorylated glucose cannot bind to the carrier protein.

Carrier Protein Types

Uniport vs. Coupled Transport

• Uniport: Transport of a single solute (uniporter).
• Coupled Transport: Involves simultaneous transport of two solutes.
  • Symport: Both solutes are transported in the same direction.
  • Antiport: Solutes are moved in opposite directions.

Channel Proteins

• Channel proteins form hydrophilic channels through the membrane to allow solutes to pass more easily across.
  • Types of channels include:
  1. Ion channels: Allow rapid passage of specific ions, characterized by high selectivity (e.g. Na⁺, K⁺, Ca²⁺).
  2. Porins: Larger and less specific than ion channels, allowing passage of many solutes up to a certain weight (e.g. 600 Da).
  3. Aquaporins: Specifically facilitate water movement, critical in certain tissues such as kidney cells and plant root cells.

Active Transport Mechanisms

• Functions of Active Transport:
  • Coupling endergonic transport to an exergonic reaction, typically ATP hydrolysis.
  • Key functions include:
  1. Uptake of essential nutrients.
  2. Removal of wastes.
  3. Maintenance of ion concentrations within cells that differ from their surroundings.

Primary Active Transport

• Defined as direct transport of solute molecules coupled to an energy-releasing reaction (ATP hydrolysis), facilitated by transport ATPases (or ATPase pumps).

Secondary Active Transport

• Involves coupled transport of two solute molecules. The favorable movement of one solute down its gradient drives the transport of the second solute against its gradient.

ATPase Transport Types

Direct Active Transport Mechanisms

• Four types of transport ATPases:
  1. P-type ATPases: Transport various ions, including Na⁺ and K⁺.
  2. V-type ATPases: Found in vacuoles and vesicles.
  3. F-type ATPases: Located in organelles such as mitochondria and chloroplasts.
  4. ABC transporters: Large family involved in various transport processes across organisms.

Medical Significance of ABC-Type ATPases

• ABC Transporters: Pump drugs out of cells, granting resistance to certain medications.
  • Example: MDR (multidrug resistance) transport protein pumps hydrophobic drugs, reducing their efficacy.

Cystic Fibrosis Transporter**

• CFTR is associated with cystic fibrosis and exhibits properties similar to ABC transporters but functions as an ion channel without ATP dependency.

Bacteriorhodopsin Proton Pump

• Functions by using light energy for ATP synthesis by creating an electrochemical gradient of protons across membranes in archaea.

Summary of Transport Processes

• Different transport methods include:
  • Passive transport (osmosis, simple diffusion)
  • Facilitated transport
  • Active transport (via protein pumps)
  • Endocytosis and exocytosis (vesicular transport)