1. Passive transport is the cellular process of moving molecules and other substances across membranes. Learn about diffusion, facilitated diffusion, filtration, and osmosis.
Passive transport differs from active transport in that it does not involve any chemical energy. Rather, passive transport relies on the innate permeability of the cell membrane and its component proteins and lipids.
Kinds of Passive Transport
(a) Diffusion – the process by which molecules spread from areas of high concentratiion, to areas of low concentration.
Facilitated diffusion is the carrier-mediated transport of large molecules through the cell membrane using transport proteins embedded within the cell membrane.These molecules would otherwise not be able to breach the cell membrane, but the transport proteins effectively "transport" them through.
This process is still diffusion, however, so the concept of the molecules moving from a higher concentration (outside the cell) to a lower concentration (inside the cell) without utilizing any chemical energy still applies.
The exchange of oxygen and carbon dioxide in the alveoli of the lungs is an example of facilitated diffusion.
(b) Osmosis is the movement of water molecules from an area of high concentration to an area of low concentration.
Cell membranes are completely permeable to water, therefore, the environment the cell is exposed to can have a dramatic effect on the cell.
–Hypertonic solution – a solution with more solute (dissolved substances) than water.
–Hypotonic solution - a solution with with less solute and more water.
–Isotonic Solutions – contain the same concentration of solute as an another solution.
When a cell is placed in an isotonic solution, the water diffuses into and out of the cell at the same rate.
The fluid that surrounds the body cells is isotonic.
–Cytolysis or Osmotic lysis – the bursting or rupturing of cell membrane when the cell can no longer contain the excessive inflow of water (or extracellular fluid).
The degeneration or dissolution of cell caused by the disruption of cell membrane.
–Turgor Pressure also called turgidity – is the main pressure of the cell contents against the cell wall in plant cells.
Turgor is a force exerted outward on a plant cell wall by the water contained in the cell. This force gives the plant rigidity, and may help to keep it erect.
Turgor can result in the bursting of a cell.
2. Active Transport. When cells must move materials in an opposite direction against a concentration gradient.
It requires Energy.
Proteins or Pumps are found in the cell membrane transport molecules across the membrane.
(a) Endocytosis is the process by which cells ingest materials.
The cellular membrane folds around the desired materials outside the cell.
The ingested particle is trapped within a pouch, vacuole or inside the cytoplasm.
Often enzymes from lysosomes are then used to digest the molecules absorbed by this process.
Endocyctosis can be split up into two main types: pinocytosis and phagocytosis.
· Phagocytosis – "cell eating" – extensions off cytoplasm surround a particle and package it within a food vacuole and then the cell engulfs it.
· Pinocytosis – the process of taking up liquid from the surrounding environment. Tiny pockets form along the membrane, fill with liquid, and pinch off.
(b) Exocytosis is the process by which cells excrete waste and other large molecules from the protoplasm.
· Protoplasm is the living contents of a cell that are surrounded by a plasma membrane.
Protoplasm is composed of a mixture of small molecules such as ions, amino acids, monosaccharides and water, and macromolecules such as nucleic acids, proteins, lipids and polysaccharides.