Cell Organelles

Plasma Membrane

This is by far my favorite part of a cell! The plasma membrane separates the living cell from its surroundings. It is a flexible boundary composed of a phospholipid bilayer which is composed of a hydrophobic (water-fearing) area sandwiched between two hydrophilic (water-loving) areas. This is due to the fact that phospholipids are made of a glycerol head (negatively charged due to an additional phosphate group) and two uncharged fatty acid tails. If you remember from A Little Bit of Chemistry, water mixes well with polar molecules and does not mix well with nonpolar molecules. The hydrophobic tails force the heads outward and sandwich together, forming the plasma membrane. In my opinion, this is the coolest thing in the world. But that's not all. Membrane proteins are embedded in the bilayer that form channels to help charged particles enter the cell. Aquaporins are proteins meant to allow water to pass into the cell, and they help with osmoregulation, a process I will discuss in a future page. Plant cells additionally have cell walls around their plasma membranes that surround the cell on all sides. They form a boundary between the cell and the outside world, and give the cell shape, support, and protection.

Nucleus

The nucleus of the cell is the house of its DNA. It consists of the nucleus envelope (a double membrane surrounding the nucleus), a nucleolus (an organelle that synthesizes ribosomes), and chromatin (DNA when it is just a soup floating around the nucleus before cellular reproduction).

Endoplasmic Reticulum

The Endoplasmic Reticulum, or ER, is a network of membranous channels that extend through the cytoplasm. It functions in synthesizing the secretory elements of the cell, and provides intracellular (within the cell) storage and transport. It consists of the Rough and Smooth ER. The Rough ER is attached to the nucleus. This organelle is responsible for protein synthesis, modification, and packaging. It ships its created proteins to the Golgi Apparatus. The diagram on the left is a depiction of how proteins are produced by the Rough ER. It's really amazing! The Smooth ER created lipids and carbohydrates. It is attached to the Rough ER. The reason the Smooth ER is known as smooth is because it does not have ribosomes attached to it, while the Rough ER does. These ribosomes give the Rough ER a bumpy appearance. 

Ribosomes

Ribosomes are organelles that specialize in protein synthesis. Many are attached to the Rough ER, but some are free-floating in the cytoplasm (the jelly-like substance in a cell where all of the organelles are housed). Ribosomes are sent RNA from the nucleus. They translate this RNA into a protein. In the section above, you can see a picture of the Rough ER and a ribosome working together to produce and ship a protein. 

Lysosomes and Vacuoles

Lysosomes are vesicles that contain digestive enzymes. They remove damaged organelles or pathogens in cells, and are produced by the Rough ER and processed by the Golgi Apparatus. On the left, I have drawn a diagram of how lysosomes work - it's also really cool. Vacuoles are large vesicles that function in the general maintenance of a cell. Contractile vacuoles contract to squeeze water. Lysosomes digest pathogens and damaged organelles. In plant cells, there is a large vacuole that stores water and minerals and helps the cell maintain its shape. Vacuoles can also house poisons. What for, I have no idea. They might be out to eat children for all I know.

Mitochondria

The mitochondria is the powerhouse of the cell 🤩. I know that sentence, and the mitochondria itself, is pretty much a meme at this point. It's actually a really interesting organelle that performs cellular respiration - an advanced process that is not easy to learn or to do. Isn't it so cool that cells evolved things like this? Well, there is a theory called Endosymbiosis that mitochondria and chloroplasts were prokaryotes that were absorbed into cells. It is based on the fact that mitochondria and chloroplasts both have tiny bits of DNA in them. My biology teacher thinks that this is a little ridiculous, but there you have it. Either way, the mitochondria has a double membrane with inner membrane folds called cristae that enclose important metabolic enzymes. This organelles produces 95% of the ATP required to run the cell. So without it, you would die 😱.

Chloroplasts

Chloroplasts are organelles found in plant cells that perform photosynthesis. They convert energy from sunlight, water, and carbon dioxide into glucose and oxygen. They consist of stacks of thylakoids called grana (singular: granum) that perform the light reactions in photosynthesis. Grana are located within the inner membrane of the chloroplast. The rest of the chloroplast is filled with stroma, a thick fluid that performs the Calvin cycle in the process of photosynthesis. I will explain all this in depth in the photosynthesis page. 

Cytoskeleton

The cytoskeleton is an assortment of proteins organized in fine filaments or slender tubes. These proteins give strength and support, and allow for the movement of cellular structures and materials throughout the cell. There are three kinds of cytoskeleton fibers: microtubules, intermediate filaments, and microfilaments. I have drawn each of them to the right.

Cilia and Flagella

Cilia are short extensions that are made of microtubule doublets in a 9 x 2 array that function in the movement of material all throughout the cell's surface. They basically look like tiny hairs sticking out of the cell. Flagella is a long, whiplike structure that cells may have that allows them to move in their environment. A picture of a flagella has been drawn by me to the left (passive sentence structure!)

Extracellular Fluid

Animal cells produce and secrete extracellular fluid, which functions in the support and regulation of a cell. It helps hold cells and tissues together and products and supports the plasma membrane. It attaches through to the plasma membrane via glycoproteins that bind to membrane proteins called integrins, which also bind to the cytoskeleton.

Cell Junctions and Plasmodesmata

There are three types of cell junctions: tight junctions, anchoring junctions, and gap junctions. Tight junctions prevent fluid from moving across a layer of cells. Anchoring junctions hold cells together, and gap junctions allow for the movement of particles between cells. Plasmodesmata are holes in plant cell walls that allow for communication between cells. Just imagine a plant being like "Send a message to Gary. Tell him I need more water."

Golgi Apparatus

Ok guys... I may or may not have forgotten about this one.... so here it is. The Golgi Apparatus consists of stacks of flattened membranes called cisternae that contain chambers. It stores, alters, packages, and ships secretory products and lysosomal enzymes from the Rough ER. When the Rough ER sends a protein to the Golgi Apparatus, it receives it on its receiving end and pushes the protein through itself, where it changes and packages it before shipping it off to where it needs to go in the cell.