Move-In Day! Transporting Things into Cells

The plasma membrane of a human cell is selectively permeable, meaning it allows certain molecules to pass through while restricting others. The types of molecules that are typically transported into a human cell through the plasma membrane can be categorized into several groups:

1. Small, non-polar molecules: These molecules, such as oxygen and carbon dioxide, can diffuse directly through the lipid bilayer of the plasma membrane without the need for channels or transport proteins. Their movement is driven by the concentration gradient.
2. Small, polar molecules: Water is a small polar molecule that can diffuse through the plasma membrane via specialized channels called aquaporins. These channels facilitate the movement of water molecules across the membrane, allowing for rapid diffusion.
3. Large, polar molecules and ions: Large polar molecules and ions typically require transport proteins to cross the plasma membrane. This includes ions like sodium (Na+), potassium (K+), calcium (Ca2+), and chloride (Cl-) ions, as well as molecules like glucose and amino acids. These molecules traverse the membrane through either facilitated diffusion or active transport.
   • Facilitated diffusion: In facilitated diffusion, molecules move down their concentration gradient with the aid of transport proteins such as channels or carrier proteins. Channels provide a hydrophilic pore for ions to pass through, while carrier proteins undergo a conformational change to shuttle molecules across the membrane.
   • Active transport: Active transport requires the input of energy, usually in the form of ATP, to move molecules against their concentration gradient. This process is carried out by pumps, such as the sodium-potassium pump, which actively transports sodium ions out of the cell and potassium ions into the cell.
4. Large molecules and particles: Some molecules and particles are too large to cross the plasma membrane through diffusion or transport proteins. In these cases, the cell can engulf these substances through processes such as phagocytosis, endocytosis, and pinocytosis.
   • Phagocytosis: Phagocytosis is the process by which cells engulf large particles such as bacteria, dead cells, or other foreign particles. The plasma membrane extends around the particle, forming a phagosome, which is then internalized into the cell where it can be broken down by lysosomes.
   • Endocytosis: Endocytosis is a general term for the process by which cells internalize extracellular molecules and particles by forming vesicles derived from the plasma membrane. This includes phagocytosis, as well as other forms such as receptor-mediated endocytosis, where specific molecules bind to receptors on the cell surface and are internalized in vesicles.
   • Pinocytosis: Pinocytosis, also known as “cell drinking,” involves the non-selective uptake of small droplets of extracellular fluid containing dissolved solutes. The plasma membrane invaginates to form small vesicles containing the fluid, which are then internalized into the cell.

Next Topic: Pinocytosis: Who? What? Why?

Source: ChatGPT response prompted and edited by Joel Graff.

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