In the middle of the 19th century, the botanist Matthias Schleiden and the physiologist Theodor Schwann developed the so-called cell theory. This states that all organisms – plants, animals, fungi – are made up of microscopic cells. These cells work together in a highly coordinated manner, but they are functional units that can be separated in many ways. Plant and fungal cells have a relatively firm shell, whereas animal cells are only surrounded by a membrane, which in principle consists of fat. This membrane represents the outer boundary of the cell, but at the same time fulfills a number of important functions.
The Cell Membrane
One of the most important tasks of this cell membrane is the perception of information about the outside world and its transmission inside the cell. For this purpose, every cell carries receptors in completely different compositions. Receptors are proteins that are located at least partly on the outside of the cell membrane or protrude from it. Substances that are outside of the cell can now bind to this extracellular part of the protein. Our taste buds, for example, contain such receptors that can bind sugar or proteins and form the basis for our sense of taste.
Hormones and neurotransmitters are also registered by receptors and cause cells to react to these signals. In our embryonic development there are innumerable signal substances which cause the initially indistinguishable stem cells to migrate in a certain direction, to divide and / or to develop into certain cells of the body (we call this process differentiation). But even in the already fully differentiated tissues of the adult body, signal substances from the outside constantly splash onto the cells. Without these signal substances it would not be possible for each individual cell type of the organism to exercise its specific functions. Almost all of these signaling substances have an effect on the genome of the cell and ensure that exactly the right genes are activated and other genes are not. It would be fatal if the cells in our eyes or in our stomach read the same genes from their genome as the cells that are responsible for the ossification of our skeleton.
In addition to signal perception, the cell membrane fulfills a number of other functions. It clearly separates the interior of the cell from the space between cells. These two different compartments are composed very differently and the cell membrane regulates which substances should come from inside to outside and which substances should come from outside to inside. The cell membrane maintaining this order or dynamically change it depending on the circumstances is crucial for cell survival and function. Due to the uneven distribution of charged particles, a certain electrical potential difference is generated on the cell membrane. In response to a certain stimulus, this membrane potential can change extremely quickly. In our nervous system, for example, this is the basis for communication between nerve cells with one another or with a muscle.
The cell membrane is made up of a double layer of phospholipids, fat-like molecules that have water-attracting sections (hydrophilic), which are arranged in the direction of the cell exterior or interior, and water-repellent (hydrophobic) sections, which form the inside of the approximately 6 nm thick membrane. Proteins are embedded in the cell membrane and serve as channels for certain substances or register or anchor substances to the membrane. The cytoskeleton, which protrudes through the entire cell, is of fundamental importance for stability but also flexibility. This cytoskeleton can be remodeled rapidly, which allows movement in the cells.
Some Important Cell Organelles
The cell is filled with an aqueous solution, the cytoplasm. But the cell interior is not a homogeneous area, but is divided into further compartments, which in turn are enclosed by membranes. Just like the organs of an organism, the cell also has cell organelles. In the following, four important organelles of an animal cell are presented: the mitochondria, the endoplasmic reticulum, the Golgi apparatus and the lysosomes. Some authors also count the cell nucleus among the organelles; In this article, however, it has its own section because of its special importance.
Cellular respiration is the basis for how we (and other animals) obtain energy from the sugars made by plants. It refers to a sequence of chemical reactions that take place in our cells and in which which sugar and oxygen are converted into carbon dioxide and water. The majority of these reactions take place in the mitochondria, which are therefore sometimes referred to as the power plants of the cell. We now assume that mitochondria used to be independent bacterial cells and were then taken up by larger cells. This is supported by the fact that mitochondria still have residues of their own DNA and still use them. Since mitochondria are inherited only from the mother and not from the father, they are sometimes used to reconstruct female family trees. We also know some diseases that are related to defects in the mitochondrial DNA and that can be passed on from mothers to their children.
The endoplasmic reticulum can be thought of as a membrane system that runs through large parts of the cell in lobes. So called rough endoplasmic reticulum is characterized by a high density of Ribosomes sitting on it, synthesizing proteins. Some proteins are synthesized directly through the membrane so that when a membrane vesicle is constricted, the protein is contained in this vesicle. Vesicles can subsequently fuse with the cell membrane, in order for the cell to releases proteins into the extracellular space. In this way, for example, digestive proteins can be released into the gastrointestinal tract.
In contrast, some substances have to be digested, i.e. broken down, within our cells. This is done by lysosomes, which are tied off by another stack of membranes, the Golgi apparatus. Membrane vesicles constricted from there must first mature for a while, with the pH value in their interior is steadily decreasing. When the environment within this vesicle is really acidic and hostile, the lysosome is fully matured and can properly fulfill its function as a decomposer vesicle. All material that was registered by the cell as no longer needed is sent to such lysosomes and broken down there.
The Cell Nucleus
The heart of every animal, plant or fungal cell is the cell nucleus. Even more: the presence of a real cell nucleus (Greek: genuine: eu, nucleus: karyon) is the defining criterion according to which these three clades of living beings are grouped together with the name eukaryotes. Opposite this group are the prokaryotes, which simplified can also be referred to as “bacteria“. In the bacterial cell, the DNA is naked, so to speak, i.e. it is not surrounded by a special shell.
In contrast to this, the cell nucleus in us animals, as in our green relatives in the higher plants, is surrounded by a double membrane. Inside is the chromatin, the DNA strands that make up the genome. In response to certain signals, RNA transcripts are generated from certain areas of this DNA. These single-stranded RNA transcripts are then transported out of the nucleus. To do this, the cell nucleus has so-called nuclear pores, i.e. channels through which the RNA reaches the cytoplasm. Because in the cytoplasm are the ribosomes that read the RNA and put together a protein that exactly corresponds to the blueprint coded there.