Cellular Membranes: Structure and Function

Study Notes

Cellular membranes are composed primarily of lipids and proteins.
The fundamental structure of a cellular membrane is a phospholipid bilayer, a double layer of phospholipid molecules.
Phospholipids have hydrophilic heads that face the aqueous environments inside and outside the cell, and hydrophobic tails that associate with each other in the interior of the membrane.
Membrane fluidity is influenced by temperature and the types of lipids present.
The presence of cholesterol within the membrane helps maintain fluidity over a wider range of temperatures.
Membranes are dynamic and constantly changing, with lipids and proteins moving laterally within the bilayer.
Proteins embedded within the membrane perform a variety of functions, including transport, enzymatic activity, signal transduction, cell-cell recognition, and attachment to the cytoskeleton.

The phospholipid bilayer acts as a barrier, restricting the passage of most polar molecules and ions.
The selective permeability of a membrane is due to the hydrophobic nature of its interior and the presence of membrane proteins that facilitate the transport of specific molecules.
The membrane allows for the passage of small, nonpolar molecules like oxygen and carbon dioxide by simple diffusion, moving down their concentration gradient.
The membrane is less permeable to larger and polar molecules, which require transport proteins to cross.

Passive transport does not require cellular energy for movement.
Diffusion is the spontaneous movement of a substance from a region of higher concentration to a region of lower concentration, driven by entropy.
Simple diffusion occurs across a membrane directly, without the assistance of transport proteins.
Facilitated diffusion utilizes transport proteins to help specific molecules move across the membrane.

Active transport requires energy to move molecules against their concentration gradients, from a region of lower concentration to a region of higher concentration.
It is crucial for maintaining cellular gradients and importing nutrients.
ATP is the most common energy source for active transport.
Transport proteins play a critical role, specifically binding to the molecule being transported and changing shape to facilitate movement across the membrane.

Exocytosis releases large molecules from the cell by fusing a vesicle containing the molecules with the plasma membrane.
This is how hormones, neurotransmitters, and other signaling molecules are secreted from cells.
Endocytosis allows the cell to take in large molecules by forming a vesicle from the plasma membrane.
Phagocytosis is a type of endocytosis where the cell engulfs solid particles.
Pinocytosis is another type where the cell takes in fluid.
Receptor-mediated endocytosis is highly specific, using receptors on the plasma membrane to bind and internalize specific molecules.