Cell biology: Introduction to cell

Cell biology is the study of cell structure and function, and it revolves around the concept that the cell is the fundamental unit of life.

Some organisms have only one cell, while others are organized into cooperative groups with huge numbers of cells.

On the whole, cell biology focuses on the structure and function of a cell, from the most general properties shared by all cells, to the unique, highly intricate functions particular to specialized cells.

What is cell?

• A cell is the smallest unit that is typically considered alive and is a fundamental unit of life.
• All living organisms are composed of cells, from just one (unicellular) to many trillions (multicellular).
• Cells are the basic building blocks of all living things.
• The human body is composed of trillions of cells. They provide structure for the body, take in nutrients from food, convert those nutrients into energy, and carry out specialized functions.
• Cells also contain the body’s hereditary material and can make copies of themselves.

Characteristics of Cells

Following are the various essential characteristics of cells:

• They are the basic structural building blocks of living things.
• Based on their complexity, two types of cells can be distinguished: prokaryotic and eukaryotic. Prokaryotic cells are simpler than eukaryotic cells. The main difference is that prokaryotic cells do not have a nucleus and their DNA is not encased in a nuclear membrane separating them from the cytoplasm.
• They contain genetic information stored in the form of DNA.
• They are separated from their environment by a plasmatic membrane, which at the same time allows them to communicate with the outside world.
• They synthesize proteins through ribosomes.
• They have a functioning metabolism with biomolecules.
• They have organelles suspended in an aqueous medium.
• They are building blocks from which various organisms form organs or tissues.

Discovery of Cells

Did you know that more than 330 years ago there was no knowledge of cells? This is because they were too small for the naked eye. The discovery of the microscope made it possible to observe cells and even study them in detail. Discovery of cells is one of the remarkable advancements in the field of science. It helps us know that all the organisms are made up of cells, and these cells help in carrying out various life processes. The structure and functions of cells helped us to understand life in a better way.

Who discovered cells?

• Robert Hooke discovered the cell in 1665. Robert Hooke observed a piece of bottle cork under a compound microscope and noticed minuscule structures that reminded him of small rooms. Consequently, he named these “rooms” as cells. However, his compound microscope had limited magnification, and hence, he could not see any details in the structure. Owing to this limitation, Hooke concluded that these were non-living entities.
• Later Anton Van Leeuwenhoek observed cells under another compound microscope with higher magnification. This time, he had noted that the cells exhibited some form of movement (motility). As a result, Leeuwenhoek concluded that these microscopic entities were “alive.” Eventually, after a host of other observations, these entities were named as animalcules.
• In 1883, Robert Brown, a Scottish botanist, provided the very first insights into the cell structure. He was able to describe the nucleus present in the cells of orchids.

Cell theory

Cell theory, fundamental scientific theory of biology, according to which cells are held to be the basic units of all living tissues.

Formulation of the Cell Theory

In 1838, Theodor Schwann and Matthias Jakob Schleiden were enjoying after-dinner coffee and talking about their studies on cells.

It has been suggested that when Schwann heard Matthias Schleiden describe plant cells with nuclei, he was struck by the similarity of these plant cells to animal cells he had observed in tissues.

The two scientists went immediately to Schwann’s lab to look at his slides. Schwann published his book on animal and plant cells (Schwann 1839) the next year, a treatise devoid of acknowledgments of anyone else’s contribution, including that of Schleiden (1838). He summarized his observations into three conclusions about cells and those are the cell theory.

1. The cell is the fundamental unit of structure, physiology, and organization in living things.
2. The cell retains a dual existence as a distinct entity and a building block in the construction of organisms.
3. Cells form by free-cell formation, similar to the formation of crystals (spontaneous generation).

Therefore, the three important points of the modified cell theory are as follows:

• The cell is the basic functional and structural unit of all living organisms.
• All living organisms are made up of cells.
• All cells arise from pre-existing cells.

 Modern Cell Theory

Modern cell theory is a widely accepted explanation of how cells and living things are related and is a foundation of biology. Modern cell theory has seven main principles:

• All organisms are made of one or more cells
• Cells are the basic unit of structure in all living things
• New cells arise from existing cells through cellular division
• All energy flow occurs within the cell
• Cells contain genetic material passed to daughter cell during cell division.
• All cells are similar in their chemical composition.
• Activities of organisms are a result of combined action of individual cell.

The History of Cell Biology Timeline

Below is a timeline of some of the key events in the development of cell theory and cell biology.

1595 – Jansen is credited with the first compound microscope. 
1655 – Hooke described ‘cells’ in cork.
1674 – Leeuwenhoek discovered protozoa. He saw bacteria some nine years later.
1833 – Brown described the cell nucleus in cells of the orchid.
1838 – Schleiden and Schwann proposed cell theory.
1840 – Albrecht von Roelliker realized that sperm cells and egg cells are also cells.
1856 – N. Pringsheim observed how a sperm cell penetrated an egg cell.
1858 – Rudolf Virchow (physician, pathologist, and anthropologist) expounds his famous conclusion: omnis cellula e cellula, that is, cells develop only from existing cells (cells come from preexisting cells).
1857 – Kolliker described mitochondria.
1879 – Flemming described chromosome behavior during mitosis.
1883 – Germ cells are haploid, chromosome theory of heredity.
1898 – Golgi described the Golgi apparatus.
1938 – Behrens used differential centrifugation to separate nuclei from cytoplasm.
1939 – Siemens produced the first commercial transmission electron microscope.
1952 – Gey and coworkers established a continuous human cell line.
1955 – Eagle systematically defined the nutritional needs of animal cells in culture.
1957 – Meselson, Stahl, and Vinograd developed density gradient centrifugation in cesium chloride solutions for separating nucleic acids.
1965 – Ham introduced a defined serum-free medium. Cambridge Instruments produced the first commercial scanning electron microscope.
1976 – Sato and colleagues publish papers showing that different cell lines require different mixtures of hormones and growth factors in serum-free media.
1981 – Transgenic mice and fruit flies are produced. Mouse embryonic stem cell line established.
1995 – Tsien identifies a mutant of GFP with enhanced spectral properties.
1998 – Mice are cloned from somatic cells.
1999 – Hamilton and Baulcombe discovered siRNA as part of post-transcriptional gene silencing (PTGS) in plants.
2006 – Factors required to create induced pluripotent stem cells are identified, allowing stem cells to be created from differentiated cells.
2009 – Single cell sequencing makes its debut, allowing insight into transcriptomics at the resolution of individual cells.

2012 – CRISR gene editing is developed, allowing precise RNA-targetted genome engineering.

Types of cell

Cells are similar to factories with different laborer and departments that work towards a common objective. Various types of cells perform different functions. Based on cellular structure, there are two types of cells:

• Prokaryotes
• Eukaryotes

Prokaryotic Cells

1. Prokaryotic cells have no nucleus. Instead, some prokaryotes such as bacteria have a region within the cell where the genetic material is freely suspended. This region is called the nucleoid.
2. They all are single-celled microorganisms. Examples include archaea, bacteria, and cyanobacteria.
3. The cell size ranges from 0.1 to 0.5 µm in diameter.
4. The hereditary material can either be DNA or RNA.
5. Prokaryotes generally reproduce by binary fission. It is one kind of asexual reproduction. They are also known to use conjugation.

Among prokaryotes we will discuss about Bacteria in short-

Bacteria

Bacteria are single-celled organisms that are pretty much everywhere: in the ground, in the ocean, on your hands and in your gut. While some are harmful, most are not — and some are even beneficial to human health. In many cases, humans live in symbiosis with bacteria, maintaining a mutually beneficial relationship without even knowing it.

-Bacteria are single-celled organisms with a unique internal structure. Humans and other multicellular organisms are eukaryotes, which means our cells have distinct nuclei bound with a membrane. Bacteria are prokaryotes, meaning they don’t have organized nuclei or any other membrane-bound organelles.

Bacterial DNA floats freely within bacterial cells in a twisted, thread-like mass called the nucleoid. Some also have separate, circular pieces of DNA called plasmids. According to the Microbiology Society, plasmids often contain genes that give bacteria a survival edge, such as genes conveying antibiotic resistance.

-The cytoplasm of some bacteria may also have little pockets, called inclusions, where nutrients are stored for lean times. Photosynthetic bacteria, which generate energy from sunlight, may have structures called chromatophores spread throughout their cytoplasm. These chromatophores hold pigments used in photosynthesis.

-Bacteria come in five basic shapes: spherical, cylindrical, comma-shaped, corkscrew and spiral. The scientific names for these shapes are cocci (round), bacilli (cylindrical), vibrios (comma-shaped), spirochaetes (corkscrew) and spirilla (spiral). The shapes and configurations of bacteria are often reflected in their names. For example, the milk-curdling Lactobacillus acidophilus are bacilli, and pneumonia-causing Streptococcus pneumoniae are a chain of cocci.

Eukaryotic Cells

1. Eukaryotic cells are characterized by a true nucleus.
2. The size of the cells ranges between 10–100 µm in diameter.
3. Present plasma membrane. The plasma membrane is responsible for monitoring the transport of nutrients and electrolytes in and out of the cells. It is also responsible for cell to cell communication.
4. They reproduce sexually as well as asexually.
5. There are some contrasting features between plant and animal cells. For eg., the plant cell contains chloroplast, central vacuoles, and other plastids, whereas the animal cells do not.

Plant cell

Within eukaryotic cells, we can distinguish different types. The plant cell is a type of eukaryotic cell that forms the plant tissue of organisms from the kingdom Plantae.

The main function of the plant cell is photosynthesis, a chemical process in which plants use light energy to synthesize organic substances and then release oxygen. Unlike the animal cells, plant cells consist of a cell wall, which gives them rigidity and shape, and chloroplasts, which are used to carry out photosynthesis.

Let us take a closer look at some other main characteristics of these cells:

• The plant cell has few similarities with the animal cell. For example, both are eukaryotic cells, have a differentiated nucleus, contain genetic hereditary information (DNA), a membrane and cytoplasm.
• Plant organisms require support from the central vacuole, which fills with water to create swelling and firmness. In many plant cells, the central vacuole is so large that it occupies almost the entire space of the cell, pushing and squeezing the other components toward the cell membrane.
• They are also characterized by containing chloroplasts with chlorophyll, pigments that capture sunlight to perform photosynthesis. They also give plants their green color.
• They have glyoxysomes, vesicles that are useful in germination to obtain carbohydrates from the fats in the seeds.
• Since they have a cell wall made of cellulose, they need plasmodesmata to communicate between cells.

Animal cell

The animal cell is a type of eukaryotic cell that forms the tissue of animals. It is responsible for the functions of nutrition and reproduction. As mentioned earlier, both the animal cell and the plant cell have defined nuclei. They differ in that they do not have the same organelles and serve different needs.

Animal cells are characterized by the fact that they do not have a cell wall, which allows them to take on different forms. They are found only in animals and humans.

• The vacuole of the animal cell is very numerous and tiny, unlike that of plant cells. The vacuole stores food and nutrients needed for the plant to survive.
• Each animal cell consists of three important parts: the cell membrane, the cytoplasm, and the nucleus, which in turn consist of other parts that are vital for the cell to perform its function.
• These cells can differentiate, i.e., specialize in a particular function, e.g., epithelial cells (protect the skin, cavities, and organs), bone cells (form bone), etc.
• The animal cell also does not have a cellulose cell wall. This structure is located on the outside of the plant cell and provides it with additional protection against water loss and external attacks.
• Animal cells do not have chloroplasts as they receive their food from the blood that transport food and nutrients over and does not need to make food.

Functions of cell

Structure and Support

You know a house is made of bricks. Similarly, an organism is made up of cells. Though there are certain cells such as collenchyma and sclerenchyma are present for offering structural support however in general too, all cells generally provide the structural basis of all organisms.

Growth

In complex organisms such as humans, the tissues grow by simple multiplication of cells. Hence, cells are responsible for the growth of the organism. The entire thing takes place via a process of mitosis.

Transport

Cells import the nutrients that are used in the different chemical process which take place inside them. As a result of these processes, a waste product is produced. Cells then work to get rid of this waste. In this manner, the small molecules like the such as oxygen, carbon dioxide, and ethanol pass through the cell membrane by diffusion. This method is known as passive transport. On the other hand, the larger molecules like the proteins and polysaccharides, go in and out of the cell via active transport.

Energy Production

Organisms need energy, to perform different chemical reactions. In plants, the energy comes from the process of photosynthesis while in the animals the energy comes via respiration.

Metabolism

Cell is responsible for metabolism that includes all the chemical reactions that take place inside an organism to keep it alive.

Reproduction

A cell helps in reproduction by the processes of mitosis (in more evolved organisms) and meiosis.