Passive and Active Transport

Movement of Material through the Cell Membrane Each individual cell exists in a liquid environment. The presence of a liquid environment makes it easier for materials such as food, oxygen, and water to move into and out of the cell. There are several ways in which materials enter and leave the cell.

Two types of Passive Transport: Diffusion and Osmosis

Diffusion Diffusion is the process by which molecules of a substance move from areas of higher concentration (more particles) of that substance to areas of lower concentration (less particles). Examples: Food color in water Air freshener in house Dry erase cleaner

What determines whether diffusion occurs through a membrane? If two substances are present at unequal amounts on either side of the membrane, each substance will tend to move towards the area of lower concentration until equilibrium is reached (same amount of particles on both sides). The permeability of the membrane. (allows substances to diffuse across the membrane)

Note A membrane is said to be impermeable to those things that cannot pass across it. Biological membranes are semi-permeable (cell membrane). They are permeable to some substances and impermeable to others.

Facilitated Diffusion Many molecules are transported across a membrane in the direction of the lowest concentration by a carrier protein. For example: In red blood cells, a carrier protein in the cell membrane transports glucose from one side of the membrane to the other. The glucose transporter protein facilitates, or helps in the diffusion of glucose.

Osmosis The diffusion of water through a selectively permeable membrane from an area of high concentration of water to an area of low concentration. For example if you place a concentrated sugar solution on one side of a semi-permeable membrane and a dilute sugar solution on the other side. The result is the net movement of water through the membrane from the side with the dilute sugar solution to the side with the concentrated sugar solution.

Note If, however two solutions contain exactly the same amount of dissolved material, there is no osmotic pressure across the membrane separating them because the concentrations of dissolved materials are in equilibrium.

Normal blood cells will shrink if to much water leaves the cells due to osmosis. If to much water enters the cell during osmosis the red blood cell will swell. If the amount of water that enters the cell is not regulated , then eventually to much water will enter the cell and it will burst. Osmosis and Diffusion are forms of passive transport across the cell membrane because energy is not needed for these processes.

Active Transport Active transport is an energy-requiring process that enables materials to move across the membrane against the concentration difference. Think of a unicyclist going up hill, requires more energy then going down hill

There are two types of Active Transport Individual molecules are carried through the membrane-associated pumps in the cell membrane. Molecules include calcium, potassium, and sodium and require chemical energy.

2) Large amount of materials are transported through the movement of the cell membrane Endocytosis: is the process of taking material into the cell by means of infoldings or pockets of the cell membrane. Large molecules, clumps of food, and even whole cells can be taken up in this way. Phagocytosis: large particles taken into the cell by endocytosis. In phagocytosis, extension of cytoplasm surround and engulf large particles. Amoebas use this method to take in food.

Endocytosis

Pinocytosis: process of taking up liquid form the surrounding environment by cells. Tiny pockets form along the cell membrane, fill with liquid, and pinch off from vacuoles within the cell. Exocytosis: The removal of water by means of a contractile vacuole, along with removal of large material.

Exocytosis