Cell Biology: Membrane Transport Processes

Study Notes

Membrane transport requires energy to move substances against the concentration gradient
Active transport utilizes carrier proteins with two binding sites: one for the molecule to be moved and another for ATP, which fuels the pumping action
Coupled transport involves coupling the movement of ions (e.g., H⁺ or Na⁺) with other molecules, moving them down the ion's concentration gradient

These processes involve changes in membrane shape to transport large molecules in or out of the cell
Exocytosis: a process by which cells release substances from the inside to the outside through vesicles fusing with the plasma membrane
Endocytosis: a process by which cells take in substances from the outside through different mechanisms:

Specific receptor molecules attract substances, inducing the formation of a coated pit
When enough molecules gather, the pit pinches off, forming a coated vesicle within the cytoplasm

Cells engulf large particles, like bacteria or foreign bodies, by surrounding them with the plasma membrane, forming an enclosed vesicle (phagosome)

Cell membranes are selectively permeable, meaning some materials pass freely, some are blocked, and some require energy for transport
Diffusion: movement of a substance from a region of high concentration to a region of low concentration, driven by the concentration gradient

Does not require energy
The cell has no direct control over simple diffusion, although factors influence its rate:
- Temperature: Higher temperature increases the rate of diffusion
- Molecule size: Smaller molecules diffuse more easily
- Concentration: Higher initial concentration increases the rate of diffusion
Materials that can move through membranes by simple diffusion include:
- Water (H₂O)
- Carbon dioxide (CO₂)
- Oxygen (O₂)
- Small lipid-soluble molecules (e.g., alcohol)

Play vital roles within the cell membrane
Transport Proteins: Act as carriers with specific binding sites for molecules, triggering a shape change to facilitate transport across the membrane. This process often requires energy (ATP)
Recognition Proteins: Glycoproteins and glycolipids act as surface receptors for cell recognition and identification, crucial for the immune system's ability to distinguish between self and non-self
Enzymatic Proteins: Many enzymes embedded in membranes, often aligned to act as an assembly line for metabolic pathways, attracting molecules to the membrane surface

Composed of a phospholipid bilayer and associated proteins
Contains unique cell recognition markers, enabling the cell to recognize itself and other cells in the organism, playing a critical role in the immune system's defense mechanisms