Biological Membranes and Transport

Overview of Biological Membranes and Transport

• The plasma membrane is a selectively permeable barrier.
• It controls the exchange of substances between a cell and its environment.
• A phospholipid bilayer with a hydrophobic core forms the membrane's basic structure, acting as a barrier.
• Membrane proteins aid in transport, cell signaling, and recognition processes.
• Cells maintain homeostasis by carefully controlling transport across their membranes.

Selective Permeability

• Not all molecules cross membranes equally easily.
• Nonpolar molecules like O₂, CO₂, and steroid hormones easily diffuse due to interactions with the hydrophobic interior.
• Small polar molecules like water and ethanol can cross, but at a slower rate due to interactions with hydrophobic tails.
• Large polar molecules like glucose need transport proteins to cross the hydrophobic core.
• Ions and charged molecules such as Na⁺, Cl⁻, and Ca²⁺ are highly impermeable due to their charge, needing specialized proteins.

Passive Transport

• Passive transport does not require energy.

Simple Diffusion

• Molecules move from high to low concentration, following the gradient until equilibrium.
• Equilibrium is dynamic with molecules moving in both directions, maintaining equal concentration.
• A steeper concentration gradient increases diffusion rate.
• Higher temperatures increase diffusion rates.
• Smaller molecules diffuse faster than larger ones.

Osmosis (Water Transport)

• Water moves across a selectively permeable membrane from an area of lower solute concentration to higher.
• Water moves to bind to solutes, reducing free water molecules on the solute-rich side.
• Osmosis causes volume changes, increasing volume on the side with higher solute and decreasing it on the other side.
• Equilibrium is reached when solute concentrations are equal.

Facilitated Diffusion

• Uses transport proteins, requiring no energy input.
• Diffusion is driven by the concentration gradient.
• Transport proteins shield substances from the hydrophobic core.
• Channel proteins are used by ions and small polar molecules to move through hydrophilic passageways.
• Carrier proteins bind certain target molecules, changing shape to move them across.
• Highly specific carrier proteins recognize just one type of molecule.

Active Transport

• Active transport requires energy.

Overview of Active Transport

• Substances are moved against the concentration gradient, from low to high concentration.
• It requires ATP input
• Active transport maintains ion gradients, nutrient uptake, and waste removal.

Primary Active Transport

• Uses ATP to directly power the transport.
• ATP donates a phosphate group to the transport protein.
• Phosphorylation leads to a change in the protein's shape.
• This shape change enables the protein to pump substances against their concentration gradient.

Sodium-Potassium Pump (Na+/K+ Pump)

• The sodium-potassium pump is a primary active transport system.
• It moves 3 Na⁺ out and 2 K⁺ into the cell, against their respective gradients.
• The first step occurs when 3 Na⁺ bind to the pump inside the cell.
• Secondly, ATP phosphorylates the pump, inducing a shape change.
• Thirdly, the pump releases 3 Na⁺ outside the cell, then 2 K⁺ bind to the pump from the outside.
• The process continues as phosphate is released, causing another change.
• Finally, the pump releases 2 K⁺ inside the cell.
• The result is high [Na⁺] outside and high [K⁺] inside, creating electrochemical gradients.

Secondary Active Transport

• Uses energy stored in ion gradients created by primary active transport.
• Symport involves two substances moving together in the same direction, like Na⁺-glucose co-transport.
• Antiport involves one substance moving in while another moves out, like the Na⁺-Ca²⁺ exchanger.

Bulk Transport

• Bulk Transport requires energy.

Endocytosis

• Membrane vesicles engulf materials.
• Phagocytosis involves engulfing large particles, for instance, immune cells consuming bacteria.
• Pinocytosis involves taking in extracellular fluid and dissolved solutes.
• Receptor-Mediated Endocytosis: surface receptors bind to specific molecules before engulfment.
• Defective LDL uptake results in familial hypercholesterolemia, leading to high blood cholesterol.

Exocytosis

• Vesicles fuse with the plasma membrane, releasing contents.
• Functions include hormone secretion, neurotransmitter release, and waste elimination.