The Cell

What is a cell?
This may sound like a stupid question. However the answer to this question, will allow us to better understand all living things around us. The simple definition for a cell is the fundamental unit of life. A cell is a living system, capable of metabolism, which is carrying out of biochemical reactions essential for the normal functioning of the cells. There all are kinds of cells, bacteria cell, animal cells, plant cells etc. Animal cells can be further divided into skeletal cells, muscle cells, epithelium and the list goes on.

Cells are classified according to their cell structure. Scientists classify them as prokaryotes and eukaryotes. Prokaryote is subdivided into bacteria (or eubacteria) and archaea (or archaebacteria). Eukaryotes usually refer to animal cells. Virus is NOT a cell. Refer to the diagram below for the comparison of the size of various cells and virus.

What is a prokaryote?
To answer this question, would mean asking the question what does an organism has which confer the name prokaryote? Prokaryote is the most abundant organism on Earth, subdivided into bacteria (or eubacteria) and archaea (or archaebacteria). The size of prokaryote is about 1-10um in diameter. Structurally, a prokaryote is like a zodiac boat with oars and the side (refer to diagram below). Typical structure of a prokaryote comprises:

• Cell/Plasma membrane, which is a lipid bilayer (2 layers of oil), with embedded proteins, which control entry and exit of molecules into the cell (Imagine a zodiac boat with taps at the side to regulate in and out of seawater).
• Cell wall (much like an amour shield) to protect the cell from any environmental changes.
• Pilli(oars at the side of the zodiac boat), which assist in attaching cell surfaces
• Flagella(motor of the zodiac boat), literally rotates to propel the bacteria
• Nucleoid, a single, circular compacted chromosome, attached to cell membrane
• Ribosomes, macromolecules, organic compounds and ions: for cellular metabolism

Compared to eukaryotes, prokaryotes are much simpler in structure. Prokaryotes reproduce by asexual reproduction known as binary fission, NOT mitosis. Recall, mitosis is nuclear division. Prokaryotes do NOT have nucleus. During binary fission, the chromosome replicates, and the two copies pulled apart as the cell grows.

What are bacteria?
Bacteria are prokaryotes which can be found almost everywhere, in soil, water and even in our body. They are different from archaea mainly in their biochemistry. Example of a bacterium beneficial to humans is Lactobacillus casei strain Shirota, the bacteria found in Yakult. Some bacteria is also capable of forming spores, one notorious example is the anthrax spores, formed by Bacillus anthracis

What are archaea?
Archaea are prokaryotes which mostly thrive in environments deemed as extreme to human beings, e.g. hot springs, acidic/alkaline soil and extreme saline water.

What is a eukaryote?
Eukaryotes refer to cells which have membrane-bound organelles (small organs within the cells) with specialized metabolic functions. They are about 10-100um in diameter. Eukaryotes are divided into 4 kingdoms, animals, plants, fungi and protists (algae and protozoa). Note that protists have a rigid cell wall.

Typical structure of a eukaryote compriseses:

• Convoluted plasma membrane, to increase surface area
• Cytoskeleton controls shape and movement of cells. Include microtubules, made of tubulin, and microfilaments made of actin
• Specialised organelles, e.g. rough endoplasmic reticulum, and golgi apparatus. Their individual functions will be elaborated in upcoming posts.

Eukaryotes undergo mitosis. Below is a video clip of mitosis in a plant cell, as view under a light microscope.

What is a virus?
As mentioned earlier on, a virus is NOT a cell. They are just DNA or RNA (ribonucleic acid) surrounded by protein and occasionally other macromolecular components. They depend on cell’s functions for protein synthesis. In the extracellular(outside the cell) state, a virus is known as a virion which is metabolically inert. Upon entering the cell, its intracellular(inside the cell) state is initiated and virus replication occurs. A virus only has a simplest structure, yet capable of harming us. How certain virus harm us would be described in future postings.