AP Biology Cell Membrane – Structure, Function & Fluid Mosaic Model

Cell Membrane Structure and Function: AP Biology Unit 2 Study Guide

Semipermeable membrane is a type of synthetic or biologic, polymeric membrane that allows certain molecules or ions to pass through it by osmosis. The rate of passage depends on the pressure, concentration, and temperature of the molecules or solutes on either side, as well as the permeability of the membrane to each solute. Depending on the membrane and the solute, permeability may depend on solute size, solubility, properties, or chemistry. How the membrane is constructed to be selective in its permeability will determine the rate and the permeability. Many natural and synthetic materials which are rather thick are also semipermeable. One example of this is the thin film on the inside of an egg.^[1]

What is the Fluid Mosaic Model of the Cell Membrane?

The fluid mosaic model describes the cell membrane as a tapestry of several types of molecules (phospholipids, cholesterols, and proteins) that are constantly moving. This fluidity allows the membrane to maintain its role as a dynamic barrier, regulating what enters and exits the cell.

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The Phospholipid Bilayer: Amphipathic Properties

The structural foundation of the plasma membrane is a double layer of phospholipids. Phospholipids are amphipathic molecules, meaning they possess both hydrophilic and hydrophobic regions:

• Hydrophilic Phosphate Heads: These polar, water-loving heads point outward, interacting with the aqueous environments inside and outside the cell.
• Hydrophobic Fatty Acid Tails: These nonpolar, water-fearing tails point inward, facing each other to form a secure, oily barrier that keeps water-soluble substances from crossing freely.

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Selective Permeability: What Can Cross the Membrane?

Because of its hydrophobic core, the cell membrane is selectively permeable. It controls the rate and ease with which different molecules pass through:

Molecule Type	Examples	How It Crosses the Membrane
Small, Nonpolar	$O_2$, $CO_2$, $N_2$	Passes freely via Simple Diffusion.
Small, Polar / Uncharged	$H_2O$, Urea	Passes slowly in small amounts (Water uses Aquaporins).
Large Polar / Charged Ions	Glucose, $Na^+$, $K^+$, $Cl^-$	Requires transport proteins (Facilitated Diffusion/Active Transport).

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Eukaryotic Cell Wall vs. Cell Membrane

While animal cells only have a plasma membrane, plants, fungi, and some protists have an outer cell wall.

• The cell membrane is flexible, selectively permeable, and composed primarily of lipids and proteins.
• The cell wall is rigid, permeable to most small molecules, and composed of structural carbohydrates (like cellulose in plants or chitin in fungi) to protect against osmotic pressure and provide structural support.

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