Cell Structure/Function

The cytosol is the jelly-like fluid that is mostly made of water that is found in all cells. It is the fluid that everything inside the cell moves around in. It is where a lot of cell processes take place and holds things in place inside of the cell. 

When considering all of the contents of the cell within the membrane but outside the nucleus, so the cytosol and everything inside of it, you are discussing the cytoplasm.

The cytoskeleton consists of threadlike proteins of three major types: microtubules, intermediate filaments, and microfilaments. They each vary in their individual function, but the overall job of the cytoskeleton is to provide structure and support, along with helping with transport and motility.

Centrioles are composed of microtubules, with a bound pair of them surrounded by pericentriolar material known as the centrosome (and which is also known as a microtubule organizing center or MTOC). These are responsible for the formation of spindle fibers and the movement of chromosomes during cell division.

The nucleus is surrounded by a double membraned nuclear envelope with pores that control what can go in and out of the nucleus. Its major function is that it stores the cell's DNA (typically in the form of chromatin) and is where processes involving the DNA, such as replication or transcription, take place.

Inside of the nucleus is the nucleolus, which is responsible for the creation of ribosomes.

Ribosomes are found in all cells and are composed of two subunits, with each being made of proteins and ribosomal RNA (rRNA). They are responsible for polypeptide synthesis - the creation of proteins by joining amino acids into a chain during translation.

The Rough ER is a membrane network with ribosomes embedded in it. It is found right outside the nucleus and is also responsible for polypeptide synthesis, due to the ribosomes; it also functions in intracellular transport of proteins. The proteins made by the Rough ER will be exported from the cell or used in the cell membrane.

The Smooth ER is a bare membrane network (the lack of ribosomes resulting in the difference in appearance). It is also responsible for intracellular transport, but has no role in making proteins. Instead, it is responsible for lipid synthesis and detoxification. 

When the Rough ER sends its proteins, it is sending them to the Golgi. This is a group of flattened membrane sacs in the cytoplasm. It received vesicles, which are little membrane sacs, from the rough ER. It then processes and modifies whatever it receives and sends it on its way. Due to its job of “shipping and receiving” the Golgi is often thought of as the mailman of the cell.

Vacuoles are membrane sacs found in the cytoplasm – they’re kind of like larger vesicles. Their job is to store things – they can quarantine potentially harmful materials, store molecules or waste for the cell, and in animal cells in particular, they play a useful role in exo and endocytosis. 

In plant cells, there is a special, large vacuole known as the Central Vacuole which plays a major role in maintaining turgor pressure - the force with which the membrane pushes against the cell. Think of how a water balloon gets harder as you fill it with water - it's basically like that as the vacuole fills with water.

Fairly similar to a vacuole in structure is the lysosome. This is another membrane sac, however, this one contains hydrolytic enzymes. If you break down the name lysosome, “lyse” means to break down or destroy something. 

The lysosome is responsible for intracellular digestion and the recycling of materials. For example, if a protein is no longer working, the lysosome can break it down so the amino acids can be reused. This organelle is also responsible for apoptosis. This is programmed cell death and is useful for the body dealing with infected or dysfunctional cells, and some specific apoptosis is also a part of normal human development, such as in the apoptosis of embryonic hand cells in order to form fingers.

In order to do most cell processes, you need a certain amount of energy, which leads us to the mitochondria (mitochondrion = singular). 

Mitchondrion have a double membrane, with the inner membrane having a number of folds – these are called cristae and increase the surface area allowing for more reactions. The part inside of the innermost membrane is called the matrix, while the area between the two membranes is the intermembrane space. The mitochondria are the site of ATP synthesis during aerobic respiration. meaning it produces energy for the cell.

The chloroplast, an endosymbiont similar to the mitochondria, have a double membrane with internal stacks (grana) of membrane discs known as thylakoids. Within the thylakoids are pigments that take in light and use it for the process of photosynthesis, making food for the cell.

These are found only in plant and algal cells, for the most part. There are some cool animals, like the leaf slug, that steal chloroplasts from algae that they eat.

If you think of things that you use energy for, one of the things you probably first thing of involves movement of some kind. Cells are capable of moving as well. They use protrusions known as cilia or flagella. 

The cilia are smaller, hair-like protrusions, while the flagella is a longer, whip-like protrusion that almost looks like a tail. They are responsible for movement – the cilia move things across the cell’s surface – for example, there are cilia in the respiratory tract that, through a rhythmic beating motion, keep dust and dirt away from the lungs. Flagella are responsible for moving the entire cell, such as in sperm cells which allows them to move around and reach the egg cell. Flagella are powered by a very complex series of thousands of proteins all working together in what is known as the “flagellar motor”.

Not found in animal cells, but found in plants, fungi, and prokaryotes, the cell wall is a rigid external outer layer made of complex carbohydrates. In plants, it is made of the polysaccharide, cellulose. It provides structure/support, mechanical strength, and prevents excess water uptake in the cell. They act as a barrier for some substances, preventing them from entering the internal environment of the cell.