1 Cells and Membrane Structure

The cellular basis of life

Cell theory describes the shared cellular basis of life. It states that all living organisms consist of one or more cells, that the cell is the basic unit of structure and function in living things, and that new cells arise from preexisting cells.

These principles apply across very different forms of life. A bacterium is a single cell that carries out the functions of an organism. In a multicellular organism, many cells work together, often with specialized structures and roles.

Takeaway: Cells are both the fundamental building blocks of living things and the source of new cells.

Two patterns of cell organization

All cells have a plasma membrane, cytoplasm, DNA, and ribosomes. A key difference is how their internal structures are organized.

Prokaryotic cells have DNA in a nucleoid region, which is not enclosed by a nuclear membrane. They generally lack membrane-bound organelles. Bacteria and Archaea are prokaryotes, though they are distinct groups and differ in features such as membrane chemistry.

Eukaryotic cells have DNA enclosed in a nucleus and contain membrane-bound organelles. Animals, plants, fungi, and protists are eukaryotes. Mitochondria, the endoplasmic reticulum, and the Golgi apparatus are examples of organelles found in eukaryotic cells.

A bacterial cell may have a cell wall outside its plasma membrane, with DNA in the nucleoid and ribosomes in the cytoplasm. A plant cell also has a cell wall, as well as a nucleus and other membrane-bound organelles. Animal cells do not have cell walls. These are broad patterns; not every cell in a group has identical structures.

Takeaway: The presence or absence of a nucleus and membrane-bound organelles distinguishes the two major patterns of cell organization.

How membranes are built

The plasma membrane forms the cell’s boundary. Its basic framework is a phospholipid bilayer, made of two layers of phospholipid molecules arranged tail-to-tail.

Each phospholipid is amphipathic: its phosphate-containing head is hydrophilic, or attracted to water, while its fatty-acid tails are hydrophobic, or avoid water. In a watery environment, the heads face the fluid inside and outside the cell, and the tails point inward, away from water. This arrangement creates a stable boundary.

The bilayer is not the membrane’s only component. Membranes also contain proteins and, depending on the membrane and organism, other lipids and carbohydrates. In the fluid mosaic model, lipids and many proteins can move sideways within the membrane, making it flexible and dynamic. Some proteins span the bilayer, while others attach to one surface. Proteins contribute specific membrane functions, and carbohydrate chains on some lipids and proteins can help cells interact and be recognized.

Membrane composition can vary. Cholesterol is an important component of animal-cell membranes, while other organisms and cellular membranes have different lipid mixtures. The two sides of a membrane may also differ in composition; this is called membrane asymmetry.

Takeaway: The membrane combines an organized lipid boundary with mobile proteins and other components.

A boundary that regulates passage

The hydrophobic interior of the bilayer makes it difficult for most ions and water-soluble molecules to cross without assistance. Small, nonpolar molecules can pass through more readily, while many ions and polar substances cross with help from membrane proteins.

This selective permeability lets a cell regulate what crosses its boundary and maintain an internal environment that differs from its surroundings. The plasma membrane, for example, separates the cytoplasm from surrounding fluid while still allowing particular substances to pass.

Takeaway: A membrane is not an impenetrable wall; its structure makes passage easier for some substances than for others.