The plasma membrane that surrounds the cell can be thought of as the barrier between life and death, separating a live cell from its environment and controlling all inbound and outgoing traffic.

The plasma membrane, like other biological membranes, has selective permeability, which means that certain chemicals may pass it more easily than others. The capacity of the cell to distinguish in its chemical exchanges is essential to life, and this selectivity is enabled by the plasma membrane and its component molecules.

This chapter will teach you how cellular membranes govern the movement of substances, typically via the use of transport proteins.

• The term phospholipid refers to an amphipathic molecule, meaning it has both a hydrophilic (“water-loving”) region and a hydrophobic (“water-fearing”) region. Other types of membrane lipids are also amphipathic.

• A phospholipid bilayer can exist as a stable boundary between two aqueous compartments because the molecular arrangement shelters the hydrophobic tails of the phospholipids from water while exposing the hydrophilic heads to the water.

Membranes are mostly composed of lipids and proteins, however, carbohydrates play a significant role as well. Phospholipids are the most prevalent lipids in most membranes. Phospholipids' capacity to create membranes is built into their molecular structure.

Most membrane proteins, like membrane lipids, are amphipathic. These proteins may live in the phospholipid bilayer, with their hydrophilic portions protruding. This molecular orientation optimizes the interaction of proteins' hydrophilic regions with water in the cytosol and extracellular fluid.

However, the proteins are not dispersed randomly throughout the membrane. Protein groups are frequently found in long-lasting, specialized patches where they perform similar activities.

Researchers have discovered particular lipids in these patches and proposed calling them lipid rafts, although there is still debate about whether such structures exist in live cells or are a result of biochemical methods.

The fluid mosaic model, like other models, is always being improved as new research reveals more about membrane structure.

Other membrane proteins appear to be immobilized due to their connection to the cytoskeleton or extracellular matrix.

As the temperature drops, the phospholipids settle into a tightly packed configuration and the membrane solidifies, similar to how bacon fat solidifies to create lard as it cools, as shown in the image attached.

The temperature at which a membrane solidifies is determined by the type of lipids used. If the membrane is rich in phospholipids with unsaturated hydrocarbon tails, it will stay fluid as the temperature drops.

Unsaturated hydrocarbon tails cannot pack together as tightly as saturated hydrocarbon tails due to bends in the tails where double bonds are present.

Nonpolar substances, such as hydrocarbons, CO2, and O2, as well as lipids, are hydrophobic. As a result, they may all dissolve in the lipid bilayer of the membrane and readily cross it without the assistance of membrane proteins.

However, the hydrophobic core of the membrane prevents ions and polar molecules that are hydrophilic from passing directly through the membrane.

Polar molecules like glucose and other sugars move slowly through a lipid bilayer, and even water, a very tiny polar molecule, moves slowly in comparison to nonpolar molecules. A charged atom or molecule and its surrounding water shell are much less likely to enter the mem's hydrophobic core.

• The term Diffusion of one solute refers to the membrane that has pores large enough for molecules of dye to pass through. The random movement of dye molecules will cause some to pass through the pores; this will happen more often on the side with more dye molecules.

The dye diffuses from where it is more concentrated to where it is less concentrated (called diffusing down a concentration gradient).

This leads to a dynamic equilibrium: The solute molecules continue to cross the membrane, but at roughly equal rates in both directions.

A cell with flexible cell walls cannot withstand high water absorption or loss. If such a cell survives in isotonic conditions, the problem of water balance is immediately solved. Many marine creatures are isotonic with seawater.

Most terrestrial (land-dwelling) animals' cells are immersed in an extracellular fluid that is isotonic to the cells. In hypertonic or hypotonic settings, animals without stiff cell walls must rely on alternative adaptations for osmoregulation, solute concentration control, and water balance.

Paramecium, a unicellular eukaryote, dwells in pond water, which is hypotonic to the cell.

The plasma membrane of Paramecium is significantly less permeable to water than the membranes of most other cells, however, this simply delays the absorption of water, which is constantly entering the cell.

The Paramecium cell does not burst because it also has a contractile vacuole, an organelle that acts as a bilge pump, forcing the water out of the cell as quickly as it arrives by osmosis.