Biology, Answering the Big Questions of Life/organelles

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Animal cell


Eukaryotic cells contain many substructures called organelles. These little organs perform different functions inside the cell.



The largest organelle is the nucleus. This is where the information used to run the cell is found.

A dark spot inside a nucleus is called a nucleolus and this is thought to be the location where ribosomal RNA (ribonucleic acid) is made. Some cells have nucleoli and some cells do not.

The nucleus is surrounded by a double membrane. The membrane contains holes called nuclear pores that are used to transport large objects ( such as ribosomes) out of the nucleus.

The endomembrane system[edit]

Endomembrane system diagram en.svg

The nucleus works together with a group of organelles that make up what is called the endomembrane system. They are the factories which build the cell.

Ribosomes are small structures found in the cytoplasm which make proteins. When proteins must be inserted into a membrane, or when they must be secreted, then the ribosome will attach to a structure called the rough endoplasmic reticulum where the protein will be made. ( Reticulum means a net.)

The rough endoplasmic reticulum is rough because of the ribosomes on its surface. Proteins made here will travel in small bubbles called vesicles to another organelle called the Golgi apparatus where they are sorted, assembled into larger structures, and sent to different parts of the cell.

The Golgi makes lysosomes which contain digestive enzymes for breaking down food. It also assembles structures that will end up in the plasma membrane.

The smooth endoplasmic reticulum does not contain ribosomes. It is thought to be the site of many chemical processes including the synthesis of lipids.


Small round bubbles found in a cell are called vesicles. Some vesicles are simply molecules in transit such as from the Endoplasmic reticulum (ER) to the Golgi, or from the Golgi to the plasma membrane.

Others have special functions such as lysosomes which contain digestive fluids or peroxisomes that contain chemicals to breakdown harmful oxidants in the cell.

Secretory vesicles are used to store chemicals that will be secreted from the cell. A vesicle that is filled with water is called a vacuole. Plants for example have large vacuoles in the center of the cell.



The mitochondria is the powerhouse of the cell. It produces high energy molecules called ATP that are used throughout the cell to drive chemical reactions. Mitochondria are the site of aerobic respiration where sugar is broken down in the presence of oxygen to make energy.