The cell is the most basic and elementary form of life. Every living thing is comprised of or is a cell; therefore the study of cellular biology is fundamental to understanding biology in general. Scientists have discovered that there are in fact two basic types of cells: Prokaryotes and Eukaryotes. Prokaryotic cells have no nucleus and no membrane bound organelles. What this means is that they have not a whole lot of organization within them. However eukaryotic cells do have a nucleus and membrane bound organelles. This will mean more to you after reading this article. This article will deal mainly with eukaryotes as these are the ones whom have membrane bound organelles which can be easily identified and studied.
So what do a lot of these words mean, organelle, membrane, nucleus? Well an organelle is a lot like an organ in our body, only organs are made of tissues and tissues are made of cells. An organelle is a particle, or grouping of macromolecules and polymers, in which have one distinct function. So then what is a membrane? A membrane is basically a barrier in which separates what’s inside a cell or organelle, from what is outside of a cell or organelle. These are made of lipids, mainly phospholipids, which are generally hydrophobic, meaning they don’t like water, so they are excellent barriers as cells generally are in wet environments. All cells have a plasma membrane surrounding them which is made up of the same thing as well as receptor proteins in which are embedded in the membrane in which allows cells to be able to detect external signaling molecules, for example hormones. Animal cells only have these plasma membranes but plant cells have cell walls. Cell walls are made up of mainly cellulose, which is a polypeptide used in plant cells for structure. These Cell walls are used as barriers as well to separate the inside of the cell from the outside. Another function of these membranes is to run intracellular junctions which allow macromolecules and other material pass from one cell to another. It is important to realize that inside the cell membranes enclose cytoplasm. This cytoplasm exists between the plasma membrane and the organelles within the cell (except the nucleus). This is a gel like substance that holds the cells shape and harbors many of the cells activities.
So then if you know what organelles and membranes are, what are nuclei? Nuclei are organelles within the cell and are the most important of them all. These basically run the cell as they are the place where DNA (Deoxyribonucleic Acid) and RNA (Ribonucleic Acid) are kept and created. It is encased by a double membrane called the nuclear envelope, this regulates incoming and outgoing of molecules within the nucleus. DNA and RNA are the genetic materials in which all living organisms have, these tell the cell how to work. Think of the DNA as the information and the RNA as the messenger. DNA copies itself onto RNA, and then the RNA is sent through pores in the nuclear envelope to Ribosomes. The Nucleolus is the place within the middle of the cell where RNA is actually created.
Ribosomes are little factories within cells that carry out protein synthesis. These receive RNA, which tells them which polypeptides to manufacture, and then they put together amino acid chain combinations according to the RNA. These then release them into the Endoplasmic Reticulum for transport. These are composed of 1/3 proteins and 2/3 RNA. There are two kinds of ribosomes, free and bound ribosomes. Free ribosomes aren’t attached to membranes; these float in another organelle or sit in the cytoplasm. Whereas bound ribosomes are attached or held in place to a membrane mainly in the endoplasmic reticulum or the nuclear envelope.
The Endoplasmic Reticulum exists just outside the Nuclear envelope and accounts for more than half of the total membrane in many eukaryotic cells; it has many functions within the cell. There are also two areas of this, the smooth and rough endoplasmic reticulum. The smooth endoplasmic reticulum is closer to the nucleus and is called smooth as it does not contain that many ribosomes. Its functions include the synthesis of lipids, the metabolizing of carbohydrates, detoxation of poisons, and the storage of calcium. The rough endoplasmic reticulum contains many ribosomes and its functions include: the bound ribosomes secrete glycoproteins, distribution of transport vesicles (proteins surrounded by membranes), and are membrane factories. So when the lipids are synthesized in the smooth endoplasmic reticulum, they are then assembled to membranes in the rough endoplasmic reticulum.
The Golgi apparatus consists of membrane sacs called cristenea. This receives the transport vesicles from the Endoplasmic Reticulum. The membranes of these vesicles fuse with the Golgi and the macromolecule travels through the Golgi where it may be modified if needed. It then is once again surrounded by a membrane and sent away in a vesicle wherever it is needed in the cell.
Lysosomes are the digestive compartments of a cell. Some types of cells can engulf other cells by a process called phagocytosis; forming a food vacuole containing the organelles of the other cell. The lysosome will then fuses with the food vacuole and spills its digestive acids inside the vacuole. These can also use enzymes to recycle the cells own organelles and macromolecules through a process called autography.
Mitochondria are an animal cell's engine so to speak. These will burn monosaccharides or sugars for energy in a process called cellular respiration. These also are known for having their own DNA within them. There are usually several of these throughout the cell and are made up of an outer membrane and a folded up inner membrane which makes convolutions called cristea. A plant cell uses light energy and converts it to a chemical fuel, this is a process known as photosynthesis in which is done in plant cells engines called Chloroplasts. These are also where we find the green pigment in many plants as it contains a green chemical known as chlorophyll. These have two membranes separated by the intermembrane space. Within these chloroplasts is a fluid called stroma, this contains an interconnected system of sacs, these sacks are called thylakoids. These are in stacks called granum, and it is here that photosynthesis actually occurs.