WHAT’S IN A CELL?

loveforlove(67)

Published in

#steemstem

Words

1866

Reading

9 min

Listen

Play

Are you aware that you started life as a stem cell? You were once as small a embryo as a point of a pin, that’s about nine months before you were born.

An embryo consists of just eight very special cells. They may not look like much, but they are stem cells – they have the potential to develop into any of the different cells that make up your body. Given time, these cells will divide and then differentiate into particular cell types, such as nerve, muscle, bone or blood.

Just how these cells divide and grow into a complete, healthy organism is the central puzzle of biology. We know that the key lies in the genes – tiny sections of DNA that code for making proteins – but we are still a long way from understanding how genes interact to control the growth and development and repair throughout an organism ‘s life.

^Pixabay

When we do unravel these mysteries, the potential benefits are enormous. Although there is a lot of research still to do, stem cells could be used to regenerate tissue that normally can’t regenerate itself after injury or damage. Stem cells could be used to bridge spinal cord injuries to bring feeling and function back into limbs, or they could be used to regenerate heart muscle after a heart attack. There have already been some successful attempts to restore sight in people with damaged retinas. Transplanting stem cells in sheets over the damaged part of the retina can allow the eye to reclaim some of its function.^source

THE COMPLEXITY OF CELLS

There is a sharp contrast in the structure of a human cell as compared to that of animal cells that you would have seen with a light microscope. The first time you saw these little bags with dots you were probably disappointed and it is difficult to believe that something that looks like this can be so complex.

But all of your body cells are complex and you have about 50 million million (or 50 trillion) of them. Many are actively dividing. Cells from the lining of the digestive system, the blood and the skin are dying constantly and need to be replaced.

In this chapter, I will start to look at the complex internal structure of basic cell types. I will also investigate the techniques that have helped us learn more about the relationship between the structure of cells and their function.

THE CELL THEORY

Following the invention and refinement of the microscope came the cell theory, a general acceptance that all living things are made of cells. Modern cell theory has three central ideas:

The cell is the smallest independent unit of life.
The cell is the basic living unit of all organisms – all organisms are made up of one or more cells.
Cells arise from other cells by cell division. They cannot arise spontaneously.

As we study biology in more detail, we will discover that there are exceptions to every rule. In the case of cell theory, that exception is the virus. Viruses do not have a cellular structure or organisation, and whether they are actually living organisms is a subject of debate.^source

^{The cells of eukaryotes (left) and prokaryotes (right). Mortadelo2005, Public Domain}

USING CELL STRUCTURE TO CLASSIFY ORGANISMS

Organisms can be classified on the basis of the internal organisation of their individual cells. With the exception of viruses, all organisms are either prokaryotic or eukaryotic. Prokaryotic cells are relatively simple, they have no separate nucleus and show little organisation. Bacteria are prokaryotes. In contrast, eukaryotic cells are larger and show much more internal organisation. Animals, plants, fungi and protoctists are all eukaryotes. The main differences between prokaryotes and eukaryotes are tabulated below.^source

	Prokaryotes	Eukaryotes
Organisms	Bacteria	Plants, animals, protoctists, fungi
Diameter of cells	0.1-10 μm	10-100 μm
Site of genetic material	DNA in cytoplasm	DNA inside distinct nucleus
Organisation of genetic material	DNA is circular; no histone proteins; DNA does not condense at cell division	DNA is linear; attached to histone proteins; condenses into visible chromosomes before cell division
Internal structure	Few organelles	Many organelles with complex membrane systems
Cell walls	Always present	present in plants and fungi and some protoctists; never in animals
Flagella	Have simple flagella	have modified cilia that consist of microtubules in a distinctive ‘9+2’ arrangement

^source

PROKARYOTES – THE FIRST ORGANISMS?

A few billion years ago, the first living organisms to evolve on Earth were probably prokaryotes. The term literally means ‘before the nucleus’ because the genetic material (DNA) of these organisms is not enclosed by membrane, and therefore they do not have a true nucleus.

^{Structure of a typical prokaryotic cell. Mariana Ruiz Villarreal, LadyofHats, Public Domain}

It is tempting to think of prokaryotes as inferior to eukaryotes, but in some ways they have achieved greater success. They have been on Earth more than twice as long as eukaryotes, they are present in greater numbers (there are more bacteria living on your skin than there are people on Earth) and they occupy an enormous number of different habitats. Some bacteria, for example, are able to live in volcanic springs at temperatures as high as 90 °C.^source

How small is small?

Cells and the molecules they contain are very small. When we study them, we must think about measurements that are minute beyond our imagination. It is easy, for example, to develop a mental block when confronted by the statement ‘The nanometre is 10-⁹m.’

The two units commonly used to describe microscopic objects are the micrometre (μm) (commonly called the micron) and the nanometre (nm). Starting with a familiar unit, the millimetre (mm), one thousandth of one millimetre is known as a micrometre (μm). The micrometre is used to describe cells and organelles. An average animal cell is 30 to 50 μm across; the nucleus has a diameter of about 10μm. Plant cells can reach 150 μm or more in length.

When describing small cellular components and molecules, the useful unit is the nanometre (nm). A nanometre is one thousandth of a micrometre. As a rough guide, the light microscope reveals structures that can be measured in micrometres, but you need an electron microscope to see objects measured in nanometres. ^source

BACTERIA

Most bacteria are spherical or rod-shaped cells, and several micrometres long. Their rigid protective cell wall is made of peptidoglycan, a substance unique to bacteria. Beneath the cell wall is the plasma membrane, which is similar in structure to the membrane of eukaryotic cells. This completely encloses the contents of the cell.

Some types of bacteria, such as Neisseria meningitidis, which can cause meningitis, also have a capsule. This is a sticky coat outside the cell wall that prevents the bacterium from drying out, from being digested by host intestinal enzymes, or from being attacked by the host’s immune system.^source

The figure below shows the internal structure of a rod-shaped bacterium, Escherichia coli.

^{Scanning electron micrograph of an E. coli colony. CDC/Janice Haney Carr, Public Domain}

Inside the plasma membrane, the bacterial cell is a single cytoplasmic compartment that contains DNA, RNA, proteins and small molecules. Bacteria have a circular piece of DNA in a region of the cytoplasm known as the nucleoid. There are smaller rings of DNA, known as plasmids, elsewhere in the cytoplasm. Plasmids are of great interest to biologists because they often contain genes that code for antibiotic resistance, and can be used to carry genes between cells in genetic engineering. ^source

Bacteria feed by extracellular digestion. They release enzymes into the surrounding medium and absorb the resulting soluble products. Glycogen granules and lipid droplets often occur in the bacterial cytoplasm and provide a limited store of these polymers. Bacteria can synthesize a wide variety of enzymes, and some species are able to digest unlikely substances such as oil and plastic. Proteins are synthesized on ribosomes and cell respiration occurs on mesosomes, inner extensions of the plasma membrane.

Some bacteria are motile: they can swim. They have thin fibres called flagella (singular flagellum) that are corkscrew-shaped and that rotate, propelling the bacteria in different directions. Other fibres, called pili, enable the bacteria to perform a primitive form of sex. During conjugation, the pili become joined and form a channel to allow plasmids to be copied and transferred to other individuals. ^source

EUKARYOTES: ANIMAL CELLS

All species of animals, plants, fungi and protoctists are made of eukaryotic cells. The term eukaryote means ‘true nucleus’, because the DNA of eukaryotic cells is confined to a definite area inside the cell enclosed by a nuclear envelope.

^{Structure of an animal cell. Royroydeb - Own work, CC BY-SA 4.0}

Eukaryotic cells also have other organelles that form compartments. By being in a compartment, the chemicals involved in a particular process, such as respiration or photosynthesis, are kept separate from the rest of the cytoplasm. This allows the chemical reactions of the process to take place quickly and efficiently. This high degree of internal organisation is one of the reasons why eukaryotic cells are larger than prokaryotic cells. The fluid that occupies the space between organelles is the cytosol, a solution containing a complex mixture of enzymes, the products of digestion (amino acids, sugars, etc.) and waste materials.^source

INSIDE AN ANIMAL CELL

If you look at an animal cell under a light microscope, the right staining ind illumination techniques and a good quality microscope should allow you to see the nucleus, nucleolus, the chromosomes in a dividing cell, and even the Golgi body, mitochondria and food storage particles. But to make sure you see the inside of a cell in more intricate detail, you really need an electron microscope. I’ll discuss more on the electron microscope in my next article.

The electron micrograph shows the internal structure of an animal cell. You can see far more of the cell’s components – the endoplasmic reticulum (ER), the internal features of the mitochondria, the plasma membrane, lysosomes, ribosomes and cytoskeleton are all now visible. ^source

^{Structure of a typical animal cell. LadyofHats, Public Domain}

Below is a table that summarises the functions of some of the major organelles and structures in the eukaryotic cell.

Organelle	Occurrence	Size	Function
Nucleus	usually one per cell	10 μm	site of the nuclear material – the DNA
Nucleolus	inside nucleus	1-2 μm	manufacture of ribosomes
Mitochondrion	numerous in cytoplasm; up to 1 000 per cell	1-10 μm	aerobic respiration
Rough endoplasmic reticulum	continuous throughout cytoplasm	Extensive membrane network	isolation and transport of newly synthesized proteins
Smooth endoplasmic reticulum	usually small patches in cytoplasm	Variable	synthesis of some lipids and steroids
Ribosome	free in cytoplasm or attached to rough ER	20 nm	site of protein synthesis
Golgi body	free in cytoplasm	Variable	modification and synthesis of chemicals
Chloroplast	cytoplasm of some plant cells, e.g. mesophyll	4-10 μm	site of photosynthesis
Vacuole	usually large, single fluid-filled space in plant; smaller and more numerous in animals	up to 90% of volume of whole plant cell.	storage of salts, sugars and pigments; creates turgor pressure by interaction with cell wall
Plasma membrane	encloses the cytoplasm of all cells	7-10 μm	exchange and transport of materials into and out of the cell
cell wall	Surrounds all plant and fungal cells (but of different structure)	thickness varies	provides rigidity and strength

^source

In my next post, I’ll discuss vividly the difference between the operation of the light and electron microscope and also more on animal cell as viewed under a light microscope.