International A Level Biology / Cell structure

The ultrastructure of an animal cell revision notes

Edexcel International A Level Biology (YBI11), chapter 7: Cell structure, specification point 3.3. Free lesson.

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An animal cell’s ultrastructure is the fine detail of its organelles, seen using an electron microscope. Knowing what each structure looks like and how many membranes it has helps you identify and compare them.

What ultrastructure and organelles mean

A light microscope shows only limited detail of an animal cell. An electron microscope shows the ultrastructure, which is the fine detail of structures inside the cell. An organelle is a structure inside a cell with a particular function. Most organelles are surrounded by a membrane, but some are not. Ribosomes and centrioles have no membrane. A molecule such as an enzyme is not an organelle.

How to recognise the nine structures

Learn each structure by its appearance. The nucleus is usually the largest and is surrounded by an envelope of two membranes. Gaps in the envelope are called nuclear pores. Inside it is the nucleolus, a dense region with no membrane around it. The rough ER (rough endoplasmic reticulum) is made of flattened sacs. Ribosomes on its outer surface give it a rough appearance. Ribosomes are tiny structures found both attached to rough ER and free in the cytoplasm. The smooth ER is a network of tubes with no ribosomes. The Golgi apparatus is a stack of curved, flattened sacs.…

How to sort the structures by membranes

On an electron micrograph, a membrane appears as a thin dark line. Counting these lines gives you a quick way to sort the structures. Structures with two membranes show two close lines with a narrow, pale gap between them. Two membranes: nucleus and mitochondrion. One membrane: rough ER, smooth ER, Golgi apparatus and lysosome. No membrane: ribosomes, centrioles and nucleolus. Vesicles also have one membrane, so they can look easy to confuse with lysosomes.

How rough ER differs from smooth ER

Rough and smooth ER are parts of one membrane network. The names describe the ER’s surface, not a different kind of membrane. The ribosomes are the key difference: rough ER has them, while smooth ER does not. Rough ER is mostly flattened sacs, called cisternae. It continues from the outer membrane of the nuclear envelope. Smooth ER is mostly a network of tubes and joins on to the rough ER.

Which structures contain DNA

The nucleolus is not a small nucleus and does not have its own membrane. It is a dense region inside the nucleus. If asked which organelle contains the nucleolus, give the answer nucleus. The nucleus contains DNA in the chromatin. Each mitochondrion also has its own small loop of circular DNA.