2.9 Mechanisms of Transport

Public Transit... in Cells?

1. Active transport: This is the movement of substances across the cell membrane using energy from the cell. Active transport includes primary and secondary active transport.
2. Passive transport: This is the movement of substances across the cell membrane without requiring energy from the cell. Passive transport includes diffusion, osmosis, and facilitated diffusion.
3. Endocytosis: This is the process by which substances are taken into the cell by invagination of the cell membrane. There are several types of endocytosis, including phagocytosis, pinocytosis, and receptor-mediated endocytosis.
4. Exocytosis: This is the process by which substances are secreted from the cell by fusion of vesicles with the cell membrane.

Active Transport

• Primary active transport involves the direct transfer of molecules across the membrane using energy from ATP (adenosine triphosphate), the cell's energy currency. 

• Secondary active transport involves the transfer of molecules across the membrane using the energy stored in the concentration gradient of another substance. This is often accomplished by the action of transport proteins called cotransporters or exchangers. 

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Passive Transport

• Diffusion is the movement of molecules from an area of high concentration to an area of low concentration. It occurs due to the random thermal motion of the molecules, and no energy is required.
• Osmosis is the movement of water molecules across a semi-permeable membrane from an area of high water concentration (high osmotic pressure) to an area of low water concentration (low osmotic pressure). Like diffusion, osmosis occurs due to the random thermal motion of the water molecules and does not require energy.
• Facilitated diffusion is the movement of molecules across a membrane down a concentration gradient with the help of transport proteins. These proteins form channels or carriers through which the molecules can pass, but no energy is required.

Source: Jack Westin

Endocytosis

• Phagocytosis: This is the process by which a cell takes in solid particles, such as bacteria or cell debris, by enclosing them in a vesicle called a phagosome. Phagocytosis is carried out by specialized cells called phagocytes, which are found in tissues such as the skin and the immune system.

• Pinocytosis: This is the process by which a cell takes in liquids, such as extracellular fluid or dissolved substances, by enclosing them in small vesicles called pinocytotic vesicles. Pinocytosis is also known as "cell drinking" and is often referred to as a type of "non-specific" endocytosis.

• Receptor-mediated endocytosis: This is a specific type of endocytosis that involves the internalization of specific molecules or substances by binding to specific receptors on the cell surface. The receptors and the bound substances are then internalized in a vesicle called an endosome. Receptor-mediated endocytosis plays a key role in the uptake of large molecules, such as hormones and growth factors, as well as in the immune system.

Source: Jack Westin

Exocytosis

1. The substance to be secreted, such as a protein or lipid, is synthesized in the cell and then transported to a storage organelle called a vesicle.
2. The vesicle fuses with the cell membrane, and the substance is released into the extracellular space. This process is regulated by signaling pathways and specific proteins called SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors).

Source: ScienceFacts

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