CELL OVERVIEW

A cell is no different from an highly organised and efficient modern city. Let’s look at an Eukaryotic cell and see if we can see the similarities. Typically around 6-100uM in diameter, a Eukaryotic cell comprises of several small functional units called organelles, each specialising in a particular job which co-operate and co-ordinate their activities to help this city thrive.

Cell membrane : The City wall. The city is surrounded on all sides by this highly guarded wall. At the gates are the watch towers, called the channels, made of proteins which check the identity of every molecule entering and exiting the cell, not everything can pass through it. The cell membrane is mainly made up of lipids and proteins. Some small molecules  can easily pass through the membrane especially if they are small lipid molecules, similar to the ones that constitute the membrane. Plant cells, fungi and certain bacteria are further protected by another stronger structure called the cell wall. Unlike cell membrane they are made of cellulose.
Cytoplasm: The hustling and bustling streets of the cell. Observe through a powerful electron microscope and you might be vaguely able to make out some rushing around like what we see in our cities during rush hour. These are molecules like ATPs, glucose, oligosaccarides, amino acids, RNAs to carry out various activities required for sustaining the cell. Some of them simply diffuse across and other more important ones take the cell’s railroad called the microtubules and the actins.

Nucleus: This is the administrative centre of the cell, where all the major decisions for running the city smoothly are made. Designing, planning and execution of all the laws and strategies are done in accordance to the constitution called the DNA, which contains the complete information to run any given cell city. Only some of the rules are used to run a given type of the city (called the cell type). The nucleus is also surrounded by a wall called nuclear membrane and which is heavily guarded. Only the molecules that the nucleus requires or asks for can enter into it. The sentries here are proteins called porins who check for special ID (tags) on each of the molecules awaiting an entrance. The information in the rule book to be conveyed to the rest of the cell has to be converted to RNA (a language understood by other components in cell) which is then translated to proteins, which are the actual executioners of the information conveyed by the DNA. Apart from DNA, nucleus has one more component called the nucleolus.

Nucleolus: The assembly of the components of an important machine of the cell, the Ribosome, happens here. Why so much care and importance for just this one organelle? Maybe because ribosome is one of the most important machines in the city, which helps in making proteins.

Ribosome:  One of the finest and efficient machine you will ever see- the ribosome. These are responsible for the production of proteins from amino acids (the building blocks of proteins). The mRNAs (messenger RNA, which carry the message/information from DNA) come here and the Ribosome recruits RNA called transfer RNAs (tRNA) which bring in the relevant amino acids to help manufacture proteins (translating the message from DNA). The platform for ribosome to manufacture its products is provided by the endoplasmic reticulum(ER).

Endoplasmic Reticulum (ER): The industrial park. Here is where the protein-making ribosomes are also located. ER is a chain of industries involved in processing and storage of proteins. From the ribosome the proteins move into the ER, where they are helped to assume their final structure (structure of the protein is essential for its function) and some proteins get processed further. During processing various changes like glycosylation (attaching sugar units to protein) is carried out, that are required for the proteins to do their respective jobs. Of course only proteins with tags and the glucose units are allowed inside. Structurally ER is a huge membranous network and hence the name. There are two types of ER: smooth ER and rough ER. Rough ER is so called because it is studded with ribosome on its surface and hence mainly work as processing units and smooth ER (without ribosome) is mainly for storing proteins. Some cells of your body like the muscle cells have a special type of ER called the sarcoplasmic reticulum which acts as reservoir for calcium ion.

Mitochondria: The PowerStation. The functioning of the city requires power supply which is manufactured here inside the mitochondria. Mitochondria have more roles to play in the cell apart from energy generation. Some cells in  plants have an additional solar power plant called the chloroplast that uses the sunlight to produce energy (ATP and NADPH) and which in turn is used for producing sugars.

Golgi apparatus: The Postal service. Golgi bodies can process lipids and proteins from ER and sort them to various regions (called “vesicular transport” in case of proteins from ER). Like the post offices, golgi adds address (labels) to the proteins and lipids which are then transported by the cells transportation system comprised of microtubules and actins to their destinations on time.

Lysosomes: The recycling plant. The cell is maintained spic and span by these. They eat up the old organelles, the bacteria that are engulfed by the cell and other cell debris. Lysosomes are loaded with acids(pH 4.5, cell’s pH is 7.5) and enzymes which act as cleaners. Apart from cleaning, they can also help the cell in digesting the ingested food (similar to what your stomach does). There is one more type of recycling plant called the peroxisomes which are very similar to lysosomes. These are the detoxifying units where the toxic peroxides are got rid of.

Centrosomes: Meet the architects of the city. Also called the microtubule organizing centre, it is the region from which microtubules are manufactured for connecting the various regions of the cell. Centrosomes not only help in connecting the regions of cell through the railroads but also look after the maintenance of the structure of the cell. They are also the junction where all railroads converge. Apart from this, centrosomes are vital during cell division (mitosis and meiosis). They form string like structures called mitotic spindles that help in pulling apart and hence distributing the chromosomes (Condensed DNA) equally to the daughter cells.

Vacuoles: Lastly the warehouses, where the food particles are stored. Even engulfed bacteria to be later thrown out are kept segregated here. They also serve as water towers and also play a role in exporting things outside the cell. Their exact usage depends on the type of cell concerned.

A prokaryotic cell on the other hand doesn’t have such distinct delineation of its components and all the required machinery, DNA, RNA, Proteins and sugars are suspended freely in the cytoplasm. One can consider them to be a very primitive tribal society where each member carries out her/ his duties without much categorising or organising