All living things are made from one or more cells. A cell is the simplest unit of life and they are responsible for keeping an organism alive and functioning. This introduction to cells is the starting point for the area of biology that studies the various types of cells and how they work. There is a massive variety of different types of cells but they all have some common characteristics. Almost every different type of cell contains genetic material, a membrane and cytoplasm. Cells also have many other features such as organelles and ribosomes that perform specific functions. Many different organisms on the tree of life contain only one cell and are known as single-celled or unicellular organisms. Their single cell performs all the necessary functions to keep the organism alive. All species of bacteria and archaea are single-celled organisms. On the other hand, large organisms like humans are made from many trillions of cells that work together to keep the organism alive. The most basic categorisation of Earth’s organisms is determined by different types of cells. All cells can be divided into one of two classifications: prokaryotic cells and eukaryotic cells. Prokaryotic cells are found in bacteria and archaea. Eukaryotic cells are found in organisms from the domain Eukaryota which includes animals, plants, fungi and protists. This introduction to cells will take you through the basic structure of cells, the difference between prokaryotic and eukaryotic cells and you will learn about organelles. STRUCTURE OF A CELL Every cell is different but there is a basic structure that is common to all cells. A cell is essentially genetic material in a gel-like substance surrounded by a membrane. The genetic material of cells is found as molecules called DNA. The DNA of a cell holds all the information that a cell needs to keep itself alive. A DNA molecule contains a code that can be translated by a cell and tells it how to perform different tasks. A gene is a specific segment of a DNA molecule and each gene tells a cell how to perform one specific task. The gel-like substance that the genetic material is found in is called the cytoplasm. The cytoplasm fills a cell and gives it it’s shape. The cytoplasm also allows for different materials to move around the cell. All cells have other structures in their cytoplasm that help the cell stay alive. The cytoplasm of all cells is surrounded by a membrane called the plasma membrane. The plasma membrane separates the cell from the outside world and keeps the contents of the cell together. The plasma membrane provides a barrier that substances have to pass through before they can enter or exit a cell. EUKARYOTIC CELLS VS. PROKARYOTIC CELLS The main difference between prokaryotic cells and eukaryotic cells is the presence of a nucleus and organelles. Prokaryotic cells do not have either a nucleus or organelles. The word prokaryotic can be translated to mean ‘before nucleus’. Eukaryotic cells have both a nucleus and a range of different organelles. The nucleus is a structure found in eukaryotic cells that contains the cell’s DNA. Organelles are cellular ‘factories’ that perform important functions such as building different molecules of life, removing wastes and breaking down sugars. Having organelles makes eukaryotic cells much more efficient at completing important cellular functions. Because they are more efficient, eukaryotic cells can grow much larger than prokaryotic cells. For a cellular structure to be considered an organelle it must be surrounded by a membrane just as the nucleus is. Prokaryotic cells contain various structures that help with certain functions, such as ribosomes, but these structures are not encapsulated by membranes and are therefore not considered organelles. Eukaryotic cells have evolved into multicellular organisms. By specializing into different types of cells, they are able to perform functions even more efficiently and are able to keep large, multicellular organisms alive. ORGANELLES Organelles are a common feature of eukaryotic cells. A wide range of different organelles has evolved over millions of years to perform various roles within cells. Some are widespread across most of the Eukaryota domain. Others are less common and only found in one or two groups of eukaryotes. Important organelles include the nucleus, mitochondria, chloroplasts, and the endoplasmic reticulum. Mitochondria are involved in the process of cellular respiration where sugar is broken down and converted into cellular energy. Chloroplasts are found in the cells of plants and other photosynthetic organisms. Inside chloroplasts are where plant cells are able to use energy from the sun to create sugars from carbon dioxide and water. The endoplasmic reticulum is a network of membranes that are attached to the membrane of the nucleus. The endoplasmic reticulum is involved with many important tasks such as producing proteins and breaking down fats and carbohydrates.