Transport across Cell Membrane: Passive, Active & Bulk transport

• Transport across the cell membrane is essential for cells to exchange materials with their environment and maintain homeostasis.

Types of transport mechanisms

1. Passive transport:

• Passive transport does not require energy input from the cell, as it relies on the natural movement of molecules from an area of higher concentration to an area of lower concentration (down the concentration gradient).

There are three types of passive transport:

A. Simple diffusion:

• Small, nonpolar molecules (such as oxygen and carbon dioxide) and lipid-soluble substances can pass directly through the phospholipid bilayer of the cell membrane.

• This process is driven by the concentration gradient until equilibrium is reached.

B. Facilitated diffusion:

• Some larger or polar molecules (such as glucose and amino acids) cannot pass through the cell membrane directly.

• Instead, they move down their concentration gradient with the help of transport proteins, such as channel proteins and carrier proteins, which are embedded in the cell membrane.

C. Osmosis:

• This is the movement of water molecules across a selectively permeable membrane from an area of higher water concentration (lower solute concentration) to an area of lower water concentration (higher solute concentration).

• Osmosis is crucial for maintaining the correct balance of water and solutes within cells.

2. Active transport:

• Active transport requires energy input from the cell, usually in the form of ATP (adenosine triphosphate), to move substances against their concentration gradient (from an area of lower concentration to an area of higher concentration).

• There are two types of active transport:

a. Primary active transport:

• This process uses energy directly from the hydrolysis of ATP to pump molecules against their concentration gradient.

• A well-known example is the sodium-potassium pump, which moves sodium ions out of the cell and potassium ions into the cell, maintaining the cell's electrochemical gradient.

b. Secondary active transport (co-transport):

• This process uses the energy stored in the electrochemical gradient created by primary active transport to move other molecules against their concentration gradient.

• There are two types of co-transport:

  1. symport (both molecules move in the same direction)

  2. antiport (molecules move in opposite directions).

• An example of secondary active transport is the glucose-sodium symporter, which transports glucose into the cell along with sodium ions.

3. Bulk transport:

• In addition to the above mechanisms, cells can move large particles or large quantities of substances across the cell membrane through bulk transport, which involves the formation of vesicles.

There are two types of bulk transport:

a. Endocytosis:

• The process of engulfing large particles or large volumes of extracellular fluid into the cell by folding the cell membrane inward and forming a vesicle.

• There are three types of endocytosis:

  1. phagocytosis (cell eating, engulfing large particles)

  2. pinocytosis (cell drinking, engulfing small particles or liquid)

  3. receptor-mediated endocytosis (specific molecules bind to receptors on the cell membrane, triggering vesicle formation).

b. Exocytosis:

• The process of expelling large particles or large volumes of substances from the cell by fusing a vesicle with the cell membrane and releasing its contents into the extracellular environment.

• Exocytosis is essential for the secretion of hormones, neurotransmitters, and waste products.