Passive Transport

Passive Transport

1. Diffusion 2.Osmosis 3.Facilitated Diffusion

Diffusion

Molecules in a solution or a gas move about randomly and collide. This random movement is called Brownian motion.

When the molecules of a solute move from a high concentration area to a low concentration area this is called diffusion.

Diffusion is affected by concentration, temperature, and pressure

The increase in any of the above factors will result in the increase of the rate of diffusion

Osmosis

Osmosis is the diffusion of water molecules through a selectively permeable membrane.

The cells are bathed in isotonic solution in which the solute concentration outside the cell is equal to that inside of the cell

One of the blood’s major functions is to keep the internal body environment in a state of isotonic balance, this is called homeostasis.

A hypotonic solution is a solution that has a lower concentration of solute, it results in the swelling of cells

A hypertonic solution is a solution that has a high concentration of solute, it results in the shrinking of cells

Facilitated Diffusion

This is done through carrier protein molecules located in the cell membrane, without the expenditure of cell energy.

How this is done, is not well understood, however scientists believe that the protein carrier speeds the passage of molecules already moving across the cell membrane

Active Transport

The use of the cells’ own energy is called active transport

About 30 – 40 % of the total body energy is used for active transport

Active transport includes exocytosis (for large molecules to be transported outside the cell) and endocytosis, which is further classified into pinocytosis (for dissolved molecules) and phagocytosis (for solid particles)

Active Transport

Active transport

Exocytosis Endocytosis

Pinocytosis Phagocytosis

The Importance of Cell surface to Volume RatioThe rate at which these substances cross this membrane depends of the surface area. The surface area to volume ratio of a cell is therefore very important.

1×1×1 cm cube has a surface area of 6×1×1 cm, or 6 cm² with a volume of 1 cm³. 2×2×2 cm cube has a surface area of 6×2×2 cm, or 24 cm² with a volume of 8 cm³. So, the volume has increased by 8 times (1 to 8) but the surface area has increased by just four times (6 to 24).