2. Cells – The building blocks of life
Contents of the chapter
2.1 Is cell biology necessary?
Watch a video of a self-illuminating mouse and discuss what you think about cells and the importance of studying cells?
Do you understand how the self-illuminating mouse was created?
Key words: cell biology, opportunies, threats.
2.2 The kingdom of cells
Each human is an individual. These individuals join together to form families. Families form villages, towns and cities with their own schools, factories, roads, power plants and landfills. On a larger level, the cities and towns form states and the global community, with its own rules and challenges.
Similarly, one can think that cells form tissues, organs, bodies and individuals with their own qualities, joys and sorrows.
Cells are the basic building blocks of all living things.
The original cell you were built of was a fertilized ovum (egg cell). Cells can differentiate from similar stem cells and form cells of different tissues and organs.
We waste cells continuously. Tens of millions of cells are lost every day in the form of dandruff and dead skin cells. Similarly, during one male ejaculation, 200 million sperm cells are lost. Baby girls are born with 1-2 million follicles (immature eggs) in their ovaries, though most often only a few of these cells are fertilized during their life span.
Although a large number of cells are destroyed, more cells are also generated at every moment. The younger the child, the faster their cells are formed. For example, think about the rate at which bone fractures heal. A fractured collarbone of a newborn infant heals in a few days. A similar injury takes a few weeks to heal for a child in elementary school. For a person in secondary school, the healing process can take six weeks. For the elderly, the injury can take several months to heal.
2.3 The structure of a cell
The plant cell has a cell wall, chloroplasts, plastids, and a central vacuole. None of these structures are found in animal cells.
A plant cell.
In all cells, the cell membrane separates the cells from their environment. All cells are filled is cytoplasm, which contains many different organelles.
The cell membrane has a structure that consists of two layers. The cell membrane protects the cell from foreign substances, bacteria and viruses. It is selectively permeable, which means that it regulates which substances can enter the cell and which are removed from the cell.
Small water molecules are able to permeate the porous cell membrane. Larger molecules, such as glucose, require a specific transport protein to facilitate their movement through the cell membrane. The outer surface of the cell membrane has sensors that allow the cell to take in or block substances from entering.
Cytoplasm contains membrane-bound organelles and the endoplasmic reticulum, which is a system of flat cavities or cisterns that produces and transports substances.
Mitochondria are essential organelles. They break down glucose to release the energy stored in it during a process called cell respiration. Cells also contain various other organelles, such as lysosomes, which digest worn out organelles and food particles, as well as viruses or bacteria.
The genes found in the chromosomes control the function of the whole cell. In plant, animal and fungal cells, the chromosomes are located inside a nucleus. Cells with a nucleus are called eukaryotic cells.
Bacteria do not have a nucleus. Instead, bacteria carry their only chromosomes in their cytoplasm. This means that bacteria are prokaryotic organisms.
An animal cell.
2.4 The parts of a cell
| Structure | Form | Function | Which organisms? |
|---|---|---|---|
| Nucleus | Surrounded by a membrane, contains chromosomes. | Stores the genome and interprets DNA. | All eukaryotes: animals, plants, fungi, and protists. |
| Cell membrane | Double membrane. | Separates the cell from its environment, regulates the movement of substances into and out of the cell. | All organisms. |
| Cell wall | Cellulose (plants), chitin (fungi), murein (bacteria). | Protects and supports the cell. | Plants, fungi, many protists, most bacteria and archaea. |
| Mitochondrion. | Organelle surrounded by two double membranes. | Cell respiration and energy metabolism. | All eukaryotes. |
| Chloroplast | Organelle surrounded by two double membranes, contains photosynthetic membranes. | Energy production through photosynthesis. | Algae, plants, some bacteria. |
| Endoplasmic reticulum | Membraneous pouches and blisters. | The movement and modification of substances. | All eukaryotes. |
| Lysosome | Membraneous blister | Breaks down unnecessary substances. | Animals. |
| Vacuole | Membraneous blister | Stores fluids, breaks down unnecessary substances. | Plants, fungi. |
2.5 Each cell of yours contains about two metres of DNA
A human body contains about 100 trillion cells (100 000 000 000 000 000). However, we all began as the result of two merged sex cells.
The ovum is the largest human cell. It has a diameter of about 0.1 mm. Except for the red blood cells, all cell nuclei contain two meters of deoxyribonucleic acid molecule, or DNA.
Human cell DNA is packaged in 23 chromosome pairs, 46 chromosomes in total. This is true for all human cells except gametes, which have 23 chromosomes.
The chromosome is made up of DNA (a chromatin chain) and its excipients. Certain proteins (yellow balls in the image on the right) help to pack these two meters of DNA into a smaller space.
A particular section of DNA forms a gene, or a 'genetic factor'. Genes are able to direct a cell to produce specific proteins. Proteins, in turn, guide our development and metabolism. Each DNA strand of a chromosome contains thousands of genes, smaller chromosomes have fewer than the larger ones. However, most of the DNA consists of ‘non-coding DNA’, which does not provide instructions for making proteins. The identity of regulatory elements and other functional regions in non-coding DNA is not completely understood. However, the human genome has been fairly well mapped.
DNA can be thought of as the "cookbook of life", as it contains instructions for an individual's growth, development and vital functions. DNA even passes these instructions on to the next generation in gametes. Some scientists think that we are just a suitcase for our genes (DNA) to be passed onto future generations.
It's mind-boggling to think that we all have received our own DNA from a chain of generations that has lasted for millions of years. This chain of germ cells is called the germline. Changes in the cells of the germline are passed on to the next generations.
As the cells divide in mitosis, the DNA molecule is copied and divides into daughter cells identical to parent cells. On the other hand, when germ cells are formed, the cells divide differently in meiosis, whereby only one of each pair of chromosomes, or a total of 23 chromosomes, ends up in daughter cells.
2.6 Extra: The structure of DNA
The importance of DNA in determining individual traits and transmitting genetic information from one generation to the next was not discovered until about 70 years ago. Of course, even before this discovery, people knew that certain traits could be inherited. In fact, many family names come from hereditary traits, such as Armstrong, Fairfax and Blunt (fair-haired).
DNA is a molecule composed of two chains that coil around each other. The structure of a DNA molecule is often described as a zipper or a double helix.
The sugar phosphate backbone is an important structural component of DNA. There are four nitrogenous bases found in DNA that are called:
- Guanine.
- Adenine.
- Thymine.
- Cytosine.
The order of these bases in the DNA molecule determines the structure of the gene and the effects its has on the organism as a whole.
For example, if the the bases that regulate growth hormone secretion are of a certain order, an exceptionally large amount of growth hormone is secreted. If the order is different, less growth hormone is secreted. This affects the size of the individual.
2.7 Videos about cells
The invention of the microscope led to the discovery of cells. Now, about 500 years wiser, we are even able to repair diseased cells. The cells were simultaneously found in plants and animals. It was discovered that all organisms are made up of cells. The following videos explain the history of how cells were discovered: