Membrane Transport

The plasma membrane is selectively permeable, allowing specific substances to pass while restricting others.
Phospholipid bilayer acts as a barrier to separate the internal environment of the cell from the external surroundings.
The structure ensures that:
- Essential molecules enter the cell
- Metabolic intermediates remain inside
- Waste products exit the cell

Living cells maintain gradients to sustain a relatively constant internal environment, which differs from the external environment.
Transmembrane gradient: A condition where the concentration of a solute is higher on one side of the membrane than on the other.
Ion electrochemical gradient: This involves both an electrical gradient and a chemical gradient, crucial for the function of cells.
- Example: Sodium has a more positive charge and a higher concentration outside the cell.

Diffusion: The tendency of molecules to spread out evenly into the available space.
- At dynamic equilibrium, molecules move across the membrane at equal rates in both directions, but the overall concentration will remain stable.
- Two types of diffusion are discussed:
- Passive diffusion: Movement occurs without the aid of a transport protein.
- Facilitated diffusion: Movement occurs with the help of transport proteins.
Examples of Diffusion:
- Single Solute Diffusion: Molecules of a dye passing through membrane pores where the net movement occurs from areas of high concentration to areas of low concentration until equilibrium is reached.
- Multiple Solute Diffusion: When solutions of different dyes are separated by a permeable membrane, each dye diffuses down its own concentration gradient.

There are two main categories for moving substances across membranes:
- Passive Transport: No energy is required, and substances move down their concentration gradient.
- Simple Diffusion: Movement through a membrane without transport proteins.
- Facilitated Diffusion: Movement through a membrane with the aid of transport proteins.
- Active Transport: Requires energy to move substances against their concentration gradient.

Osmosis is defined as the diffusion of free water across a selectively permeable membrane.
- Water movement occurs from regions of lower solute concentration to regions of higher solute concentration until equilibrium is established.
At a molecular level, osmosis is influenced by solute concentrations on either side of the membrane, with water moving accordingly.

Tonicity refers to the ability of a surrounding solution to influence the water balance of a cell.
- Isotonic solution: Equal concentrations of solute and water on both sides of the membrane.
- Hypertonic solution: Higher solute concentration, leading to water loss from the cell.
- Hypotonic solution: Lower solute concentration, leading to water influx into the cell.

Animal Cells: Thrive in isotonic environments unless adaptations are present to handle osmotic pressure.
Plant Cells: Prefer a hypotonic environment, which allows water uptake until the cell wall exerts counter pressure.

Paramecium caudatum possesses a contractile vacuole that pumps excess water out when in a hypotonic pondwater environment.
Plasmolysis in Elodea occurs when the plant cell loses water in a hypertonic saline solution, leading to the cell membrane detaching from the cell wall.

Transport Proteins: Integral membrane proteins that facilitate the movement of ions and hydrophilic molecules across membranes.
Two types of transport proteins:
- Channels: Form open passageways, allowing facilitated diffusion; often gated to regulate solute flow.
- Example: Aquaporins allow rapid water movement at a rate of $3 imes 10^9$ molecules per second.
- Transporters (Carriers): Change shape to transport solute across the membrane, serving organic molecule uptake pathways.

Glucose Transporter (GLUT1): A carrier protein involved in facilitated diffusion for glucose.
- Shows a rate of uptake inversely related to concentration gradients and is characterized by a maximal transport rate (Vmax).

Active Transport: Movement from regions of low concentration to high concentration, requiring energy.
- Primary Active Transport: Directly uses energy (e.g. ATP).
- Secondary Active Transport: Utilizes pre-existing gradients for solute transport.

Sodium-Potassium Pump: An antiporter that uses ATP to move Na+ out and K+ into the cell, maintaining the electrochemical gradient essential for cellular functions.
- Operates via a conformational change influenced by phosphorylation.

Electrogenic pumps create charge differences across membranes. The sodium-potassium pump acts as the major electrogenic pump in animal cells, exporting one net positive charge.

Ion pumps are crucial for maintaining electrochemical gradients, aiding in:
- Transport of nutrients (via symporters and antiporters)
- Energy production (using H+ gradients for ATP synthesis)
- Regulation of cellular volume through osmotic control
- Neuronal signaling and muscle contraction.

Used for the transport of large molecules such as proteins and polysaccharides:
- Exocytosis: Secretion of materials from a vesicle that fuses with the plasma membrane (e.g. hormones and digestive enzymes).
- Endocytosis: Importing substances into cells by forming vesicles; can also be receptor-mediated, pinocytosis, or phagocytosis.

Phagocytosis: Engulfing large particles or cells, notably performed by amoebas.
Pinocytosis: Involves the uptake of dissolved solutes, leading to the formation of pinocytic vesicles.
Receptor-Mediated Endocytosis: Involves the specific binding of solutes to receptors before internalization.

Cholesterol transported in low-density lipoproteins (LDLs). In familial hypercholesterolemia, defective or missing LDL receptors lead to cholesterol accumulation, contributing to atherosclerosis and cardiovascular diseases.