Biology Chapter: Passive Transport

Study Notes

Passive Transport

Passive transport is a fundamental biological process that involves the movement of molecules across the cell membrane without the expenditure of cellular energy, which makes it distinct from active transport mechanisms.
This form of transport is driven by the principles of diffusion and osmosis, relying on the natural kinetic energy of molecules. Molecules will spontaneously move from regions of higher concentration, where they are more densely packed, to areas of lower concentration, where they are less crowded.
The ultimate objective of passive transport is to achieve homeostasis, thereby equalizing concentrations on both sides of the membrane and thus maintaining cellular function and stability.

Passive transport is the movement of molecules across the cell membrane without the need for energy.
Passive transport relies on the natural movement of molecules from areas of high concentration to areas of low concentration.
The goal of passive transport is to reach equilibrium.

Simple Diffusion

Simple diffusion is the movement of a substance from an area of higher concentration to an area of lower concentration, and it is a fundamental process underlying many biological and chemical phenomena. This passive transport mechanism does not require energy input, relying instead on the intrinsic kinetic energy of molecules. Simple diffusion occurs until an equilibrium is reached, where the concentrations of the substance become equal throughout the medium.
Factors affecting diffusion in a liquid medium include:
- Temperature: Increasing temperature generally enhances the motion of molecules, leading to a faster rate of diffusion. Higher thermal energy increases molecular kinetic energy, which can help overcome intermolecular forces.
- Pressure: Changes in pressure can influence how close molecules are together; in higher pressure environments, molecules may move more closely, modifying the rate of diffusion.
- Solvent density: The density of the solvent can also impact diffusion rates, as more dense solvents provide more resistance to the movement of solute particles.
- Concentration gradient: A steep concentration gradient can drive faster diffusion, as molecules move from areas of high concentration to those of low concentration.
- Solute solubility: The ability of a solute to dissolve in a solvent directly affects its diffusion; highly soluble substances will diffuse easily, while less soluble substances may move more slowly.

Osmosis

Osmosis is the movement of solvent molecules from a solution of low concentration to a solution of high concentration through a semi-permeable membrane, which is a selective barrier that allows certain molecules or ions to pass through while obstructing others. This unique property is crucial in biological systems, especially in cellular processes where the cell membrane functions similarly. The mechanism of osmosis plays a vital role in maintaining homeostasis within cells, as it regulates the internal environment by controlling the flow of water and solutes.

Tonicity

Isotonic solution: Same concentration of solutes inside and outside the cell.
Hypertonic solution: Higher solute concentration outside the cell than inside.
Hypotonic solution: Higher solute concentration inside the cell than outside.

Facilitated Diffusion

Pores and channels in cell membranes enable the transport of ions and water, crucial for cellular communication and nutrient uptake. Pores allow small molecules to pass freely, whereas channels are selective. Their regulation is vital for maintaining cellular homeostasis by controlling the flow of essential compounds in response to changing conditions.

Facilitated diffusion is the transport of substances across a biological membrane from an area of higher concentration to an area of lower concentration with the help of a transport molecule.