Cell structure and function

All organisms are made up of cells. Most cells are small and cannot be seen with the naked eye. There are many reasons for the small size of cells. A small cell can exchange matter with its environment more quickly than a large cell. Because cells are small, the transport of substances to different parts of the cell is faster, which means that cellular metabolism is also more efficient. Small cells can also divide more quickly. They can form complex entities such as tissues and organs.

A large number of organelles are located inside the cell. In these organelles, a huge number of complex functions take place. Even the simplest of organisms produce hundreds of different proteins that are involved in metabolism and reproduction.

Plant, fungal and animal cells can look very different when viewed under a microscope. The shape, size and function of cells can vary greatly. However, all cells have a similar basic structure.

DNA controls the function of all living cells. DNA is divided into chromosomes, which contain genes that determine the structure of the products that control cell function. In protein synthesis, the information contained in the DNA is transferred to a messenger RNA molecule (transcription) and the information in the messenger RNA is used to make proteins at the ribosomes (translation). All cells produce RNA and proteins in the same way.

Cells can also reproduce and divide. A cell can produce copies of itself by mitosis. Cells in sexually reproducing organisms can also divide by meiosis, in which the number of chromosomes in a cell is halved.

Eukaryotic cells have similar features to prokaryotic cells. However, they have structures and properties that the more primitive prokaryotes lack. In eukaryotic cells, DNA is stored in chromosomes inside a nucleus enclosed by a double membrane.

Additionally, eukaryotic cells contain membranous cell organelles, which are not found in prokaryotic cells. Such organelles include energy-producing mitochondria and chloroplasts. Plant and fungal cells, as well as those of many protists, are surrounded by a cell wall, whereas animal cells lack this structure.

Animal, fungal and plant cells share many of the same structures and organelles. On the other hand, all cells of an individual do not contain the same organelles and structures. For example, a red blood cell has no mitochondria at all, but a liver cell may have thousands of them.

The nucleus controls the function of the eukaryotic cell

Mitochondria and chloroplasts are involved in energy metabolism

Mitochondrion

The mitochondrion can be described as the powerhouse of the cell, as it is where many of the cell's energy metabolism reactions take place. These reactions include cellular respiration and the oxidation of fatty acids. The mitochondrion converts the chemical energy contained in organic molecules into a form that the cell can use.

The mitochondrion is a tiny organelle, about one micrometre long. It is surrounded by two membranes, and the space around the inner membrane is called the mitochondrial matrix. The inner membrane of the mitochondrion is where the citric acid cycle and electron transfer chain reactions of cellular respiration take place. These reactions are described in more detail later in book. The inner membrane of the mitochondrion has a folded structure.

The mitochondrion is thought to have originated from a bacterium that lived in symbiosis with the cell. This endosymbiosis theory is supported by the fact that mitochondria have their own DNA and ribosomes.

Chloroplast

The chloroplast is an organelle that is found only in photosynthetic eukaryotes (i.e. plants and algae). Chloroplasts use photosynthesis to convert solar radiation energy into chemical energy. In photosynthesis, the energy from sunlight is used to produce organic compounds, such as sugar, from carbon dioxide and water. At the same time, oxygen is released into the environment.

Like the mitochondrion, the chloroplast is surrounded by two membranes (A and B in the image below), but the chloroplast also contains membrane vesicles (thylakoids, C in the image below), which form the photosynthetic membranes. Photosynthetic reactions take place on the surface of these membranes.

Like mitochondria, chloroplasts also have their own DNA and ribosomes. This is why they are thought to have evolved from cyanobacteria that lived in close association with nucleated cells (endosymbiosis).

Many organelles have membranous structures

In addition to the nuclei, mitochondria and chloroplasts, cells contain many other membranous structures that are necessary for the transport and secretion of substances. Membranous organelles are typical for eukaryotes. Bacteria and archaea may instead have membranous structures formed from the cell membrane which are used for energy production.

The Golgi apparatus can be described as the freight hub of the cell. It is where proteins are sorted for transport and secretion. In the Golgi apparatus, proteins can also be modified, for example by attaching sugar components to them. The Golgi apparatus is closely associated with the endoplasmic reticulum, and small membrane vesicles are constantly moving between them.

In plant cells, the Golgi apparatus is sometimes called the dictyosome. However, its function in both plant and animal cells is broadly similar.

Lysosome

The lysosome can be called the cell's recycling centre, as it is where unnecessary proteins and contaminants are broken down. The pH inside the lysosome is clearly acidic. This, together with various enzymes, promotes the breaking down of substances.

The phagocytes of the immune defence system destroy harmful bacteria in the acidic lysosome. Lysosomes are only found in animal cells. In plant cells, the role of lysosomes is played by vacuoles.

Peroxisome

The peroxisome is a membranous structure that is responsible for breaking down fatty acids, among other things. As a by-product, they produce reactive hydrogen peroxide, from which the peroxisomes get their name. Peroxisomes are found in plants, animals and almost all other eukaryotes.

Vacuole

The vacuole is an organelle found in both plant and fungal cells. It is a membranous, fluid-filled vesicle that can fill almost the entire cell, especially in old cells. Plant cell growth is therefore largely based on an increase in the volume of the vacuole. The vacuole stores nutrients, salts and waste products and breaks down some macromolecules. In a plant cell, the vacuole performs similar functions to the lysosome of an animal cell.

Many cellular structures are made of proteins

Proteasome

The proteasome is a structure made up of several proteins. Its function is to break down unnecessary proteins. The proteasome recognises proteins that are marked for breakdown and cuts them into smaller parts.

The cell membrane and the cell wall separate the cell from its environment

Cells need different structures to retain their shape and prevent them from breaking down. All cells are surrounded by a cell membrane, which separates the cell from its environment. The cell membrane also regulates the flow of substances in and out of the cell.

In plant and fungal cells, and in many bacteria, the cell is surrounded by a cell wall. The cell wall consists mainly of cellulose in plants, of chitin in fungi and of peptidoglycan or murein in bacteria. Animal cells do not have a cell wall.

In animal tissues, cells are bound together by an extracellular matrix (ECM). It contains substances such as collagen (a filamentous protein), long sugar chains and associated proteins. The extracellular matrix is often directly attached to the cell membranes.

A good example of the importance of the extracellular matrix is scurvy, which is a disease caused by a lack of vitamin C. Vitamin C is needed to produce collagen. If collagen is not produced, the skin and muscles start to sag and bleed. This condition is called scurvy. When the importance of vitamin C was not known, scurvy was a common condition during long sea voyages. Gradually, it was discovered that onions helped sailors survive these voyages without developing scurvy. Gradually, people learned to stock up on vegetables, root vegetables and citrus juice that could keep well during the voyage and protect sailors against scurvy.