I figured that before I start posting pages regarding brain cancers and brain disorders, I might as well go back to the beginning and address what animal cells appear like under normal circumstances. I’ll talk about the cells’ organelles here and what each of them do.
What does an animal cell look like, and what are the structures within it?
Animal cells have come a long way since the amoebae: for one, most creatures on Earth
today are multi-cellular as opposed to amoebae, which are typically single-celled. In order to truly explain the marvel of the human body, I feel I must get in touch with my inner geek. To paraphrase the Borg from Star Trek (yes, I am a closet Trekkie), humans are like the Collective: we are billions of cells working as one. Depending on where they are located within the body, they each serve a specific function and they each serve the greater whole. Additionally, each cell has several structures within them known as organelles which each serve their own purpose: I’ll talk about them below.
What do each of the organelles do?
Animal cells (including human cells) are known as eukaryotic cells. What are eukaryotic cells, you ask? Well, the main difference between eukaryotic cells and prokaryotic cells (which I’ll address in Introduction to Bacteria) is that eukaryotic cells have a nucleus, whereas prokaryotic cells have a nucleotid. The cells’ DNA is contained within the nucleus and can often be condensed within the nucleolus (a nucleus within the nucleus, if you will). The edge of the nucleus is known as the nuclear envelope: this structure is not solid, instead it has small holes in it in order to allow the exchange of messenger RNA to and from the nucleus. More details about this process can be found in DNA and RNA: The Basics.
Ribosomes are the protein manufacturers of the cell. They are responsible for translating messenger RNA that arrives from the nucleus. They can be found either attached within the rough endoplasmic reticulum or floating freely. This structure is one of only a couple of structures that exist within both eukaryotic and prokaryotic cells. The ribosome is actually made up of two parts: a top subunit and a bottom subunit, which sandwich together when proteins are being created.
Mitochondria are only found within eukaryotic cells, and they are the energy producers of the cell. They take our dietary energy sources (sugars, fats and proteins) and convert them to ATP (adenosine tri-phosphate, our main energy source) through the citric acidi or Krebs cycle. This cycle requires oxygen to work: hence why we cannot survive without it in our air supply. There are folds within the mitochondria called cristae, which serve a similar function to villi in the small intestine or sulci and gyri within the grey matter of the brain: they increase surface area. Cristae, like the nuclear envelope, is selective in which molecules are allowed to pass through it.
The endoplasmic reticulum is made up of membranes, and there are two separate categories of ER: smooth endoplasmic reticulum and rough endoplasmic reticulum. The difference between the two is that the rough variety has ribosomes entangled within it and the smooth ER does not. The organ within the human body that contains the most rough ER is the liver, as this is the place where most of our protein creation takes place.
Rough Endoplasmic Reticulum
Protein formation may be the most well-known function of rough ER, however it is not the only function that it serves. Another key function that it serves it that it is a key factor in the production of our body’s enzymes.
Smooth Endoplasmic Reticulum
Since the smooth endoplasmic reticulum does not have ribosomes, it must serve some other purpose other than the production of proteins. In fact, it has several: it is involved in the metabolism of lipids (fat) and carbohydrates and it helps to get rid of our body’s toxins.
A lysosome is an organelle involved in waste disposal. This is essential for keeping the cells healthy, as it is involved in digesting foreign materials such as invading bacteria, recycling of cellular receptors and unneeded or damaged organelles. The recycling process of lysosomes is known as endocytosis.
Like the endoplasmic reticulum, the Golgi body is also made up of membranes. Products created in the ER such as digestive enzymes and hormones are then transferred to the Golgi body and these are modified there and then are sorted for secretion.
Centrioles are structures composed of tubulin and tend to be found mostly in eukaryotic cells. When they are seen in pairs, they are known as centrosomes. The tubules extend across the cell. They are responsible for maintaining the cells’ strength: it is similar to how the flexible beams hold a tent up when it is put together: without them the tent would just collapse.
The plasma membrane is the outer edge of the cell.
The cytoplasm is the gel-like fluid which fills the inside of the cell and its primary function is to act like a shock absorber for the organelles. Due to the cytoplasm of the cells being composed of mostly water, and the fact that the human body is composed of literally billions of cells, we require water at least once every three days in order to survive, because some of our water is lost every day through our sweat as well as through our urine and stool. It is also the meeting place for molecules that perform the metabolic reactions within our body and a place where calcium is transferred.
In the medical lab setting, the cytoplasm is very useful, as it is the secondary site (the primary being the nucleus) where diagnostic stains tend to like to hang around. In tissue biopsies, the H&E (haematoxylin and eosin stain) is used and the pink eosin stain tends to hang around the cytoplasm, whereas the purple haematoxylin attaches to the nuclei. The equivalent can be seen in the Gram stain when examining for bacteria: the cytoplasm tends to hold either the crystal violet-Grams iodine complex or the safranin (or neutral red) counterstain, making the bacteria appear either purple or reddish-pink under the microscope .