Smooth ER: Definition, Functions & Structure

Definition of the Smooth ER

The smooth endoplasmic reticulum, or smooth ER, is an organelle found in both animal cells and plant cells. An organelle is a sub-unit within a cell that has a specialized function.

The main function of the smooth ER is to make cellular products like hormones and lipids. It also distributes those products throughout the cell and to places in the organism. The smooth ER also regulates and releases calcium ions and processes toxins. It is described as ‘smooth’ to distinguish it from rough ER, which has ribosomes for protein synthesis on its surface.

Each organelle in the cytoplasm of a cell is responsible for performing a certain function. Rough and smooth ER each manufacture different products for the cell.

Smooth ER is Like a Factory

You can think of the smooth ER as a factory manufacturing many of the products that a cell needs to function. Exactly what it makes depends on the type of cell. Imagine two factories in two different locations. One is in the middle of farm country, where corn grows as far as the eye can see. The other is on the coast of the ocean where fishing is the main industry. The factory in farmland manufactures corn syrup to be used in a number of packaged foods. The ocean-side factory manufactures canned fish products. Each factory makes and distributes the type of product that makes sense for its location.

Smooth ER also makes or distributes products according to its location. In muscle cells, it releases calcium ions because they are required for muscle contraction. In cells of the endocrine system, which is responsible for regulating hormones, smooth ER makes steroid hormones.

The Structure of Smooth ER

Smooth ER consists of a long network of a folded, tube-like structure. You can think of it like a pipeline with a lot of twists and turns. The interior of the smooth ER is called the lumen, which is enclosed by a phospholipid membrane, just like the membrane that encloses the entire cell.

The number of smooth ER units in a cell depends on the type of cell and what its manufacturing needs are. These units are situated in the cytoplasm, the gel-like substance inside a cell, and are sometimes connected to a unit of rough endoplasmic reticulum. Smooth and rough ER are often attached to each other, but can be distinguished by shape (smooth ER is tubular, while rough ER is flatter) and the presence or absence of ribosomes on the surface.

The Functions of Smooth ER

Here is a slightly more in-depth look at some of the main functions of smooth ER.

Manufacturing

Smooth ER is largely a manufacturing organelle, making and distributing the molecules that a particular type of cell needs. Most smooth ER in both plant and animal cells make lipids, or fatty acids, and so cells involved in lipid manufacturing contain many units of this organelle. As an example, animal endocrine cells that use cholesterol to make steroid hormones have a lot of units of smooth ER. In these cells, the smooth ER manufactures hormones.