How do lipids affect cell membranes?

Hint: The cell wall may be a biological membrane that separates the inside of all cells from the surface environment which protects the cell from its environment. The cell wall consists of a lipid bilayer, including cholesterols that sit between phospholipids to take care of their fluidity at various temperatures.

Complete answer:
Lipids fulfil three general functions. First, due to their relatively reduced state, lipids are used for energy storage, principally as triacylglycerol esters and steryl esters, in lipid droplets. These function primarily as anhydrous reservoirs for the efficient storage of caloric reserves and as caches of carboxylic acid and sterol components that are needed for membrane biogenesis.
Second, the matrix of cellular membranes is made by polar lipids, which contains a hydrophobic and a hydrophilic portion. The propensity of the hydrophobic moieties to self-associate (entropically driven by water), and therefore the tendency of the hydrophilic moieties to interact with aqueous environments and with one another , is the physical basis of the spontaneous formation of membranes.
 These fundamentals of amphipathic lipids may be a property that enabled the primary cells to segregate their internal constituents from the external environment. This same principle is recapitulated within the cell to supply discrete organelles.
This compartmentalization enables segregation of specific chemical reactions for the needs of increased biochemical efficiency and restricted dissemination of reaction products. In addition to the barrier function, lipids provide membranes with the potential for budding, tubulation, fission and fusion, characteristics that are essential for cellular division , biological reproduction and intracellular membrane trafficking.
Lipids also allow particular proteins in membranes to aggregate, et al. to disperse. Finally, lipids can act as first and second messengers in signal transduction and molecular recognition processes. The degradation of amphipathic polar lipids allows for bipartite signalling phenomena, which may be transmitted within a membrane by hydrophobic portions of the molecule and also propagated through the cytosol by soluble portions of the molecule.

Note:
These fundamentals of amphipathic lipids may be a property that enabled the primary cells to segregate their internal constituents from the external environment. This same principle is recapitulated within the cell to supply discrete organelles.