Cell Structure: Study Guide
All Cells have 3 Portions
Except for red blood cells, all cells in your body have 3 portions:
- The Plasma Membrane - It's a barrier that separates the inside of the cell from the outside. Think of it as the "skin" of the cell. It protects the cell from anything foreign from entering. It can also be called the "Cell Membrane."
- The Cytoplasm - It contains organelles (organelles are structures inside of the cell that have specific functions - mitochondria is an example; more on that later). The cytoplasm basically holds everything into place, it's the fluid inside of the cell.
- The Nucleus - It controls the cell's activities. It's the "brain" of the cell. It's also the largest organelle.
The Plasma Membrane
It is the cell's gatekeeper. It's made up of two layers of phospholipids, which is called the "Lipid Bilayer." The lipid bilayer is made up of, well, lipids, which are fats. They look like a wall of sperm cells, traveling in opposite directions. See the picture to the right for a visual.
Hydrophobic means that the tails do not like water, the heads are hydrophilic, which means they are always near the water.
There are proteins associated with the plasma membrane, as well.
- Integral proteins are embedded in the bilayer and can extend into the bilayer or completely through the bilayer.
- Peripheral proteins are loosely associated with the bilayer.
- These proteins are used for transporting substances across the bilayer. If the Plasma Membrane is the wall of the cell, the proteins are the gate, which will only allow certain things to pass through. They have other functions as well, such as binding substances to either side of the wall, enabling the cell to be recognized by immunity cells, and speeding up chemical reactions.
Note: Another term you should learn is selective permeability. This just means that the cell will only allow certain things to pass into it, hence, it's selective in what it allows in. Pretty self explanatory.
As stated before, the cytoplasm is the area between the plasma membrane and the nucleus, and it contains all of the organelles. Cytosol is the fluid, which consists of water and other molecules.
The cytoskeleton is made up of elements:
- Microfilaments - The "muscle" of the cell. They provide support to the cell shape and allow for cell movement. They supply support for microvilli.
- Microvilli - Fingerlike projections at increase the surface area of the cell for absorption.
- Intermediate Filaments - Hold organelles in place and attach cells to one another.
- Microtubules - They are the skeleton of the cell, they determine shape and movement of the cell.
Centrosomes are near the nucleus. They consist of 2 hollow cylinders called centrioles. This will all play a huge part in cell division later on.
Cilia and Flagella:
- Cilia are short projections that move material across the surface of a cell. They are important, for example, in cells that are in the lungs, because they help sweep mucus out of the lungs and keep the airway clear. Smokers burn off the cilia, which is why they have so many respiratory problems.
- Flagella is a single tail. It's simple to remember that only the sperm cell has a flagella, and they are easily recognizable by their tail. The flagella's purpose is the move the cell towards it's destination.
Ribosomes are made of RNA and proteins, and are associated with the rough endoplasmic reticulum (more on that in a moment). Some float freely throughout the cell as well. Regardless of their location, they are involved in creating new proteins.
Endoplasmic Reticulum - This is a large membrane system and comes in 2 forms:
- Rough ER (ER stands for Endoplasmic Reticulum): This contains ribosomes. Proteins are made here and the rough ER transports them to the plasma membrane or the membrane of other organelles.
- Smooth ER: Creates fatty acids and steroids. It also detoxifies the cell. Lastly, it is the storage site for ionized calcium, which is released for chemical reactions which signal for hormones or muscle contractions in muscle cells.
The Golgi Complex: Basically, this organelle tells proteins where to go. It processes proteins and sends them either to the plasma membrane or other organelles.
Lysosomes are organelles that digest material that has entered the cell from the outside, such as proteins and bacteria. The also digest worn-out organelles and can even digest the entire cell itself.
Two other digestive organelles are peroxisomes and proteasomes.
- Peroxisomes remove hydrogen atoms from various molecules, and in the liver, they can detoxify alcohol.
- Proteasomes break down faulty, damaged, or unneeded proteins into amino acids which are recycled throughout the cell.
Mitochondria - the main function is to produce ATP. (ATP transports chemical energy within a cell for metabolism.) They also contain their own DNA and ribosomes, so they can reproduce themselves and make their own new proteins.
The largest organelle of a cell is the nucleus. Most cells have only one, but skeletal muscle cells can have many. The nucleus is like a cell within a cell. It has a nuclear envelope, which surrounds it (think of it as it's own plasma membrane). On the nuclear envelop are nuclear pores which control what goes into and out of the nucleus (think of it as the integral proteins of the plasma membrane), and inside the nucleus is a nucleolus, which is made up of DNA, RNA, and protein, and the nucleolus creates ribosomes.
The nucleus contains all of the genetic material needed to control cell activities and create new cells.
- In a NON DIVIDING CELL, the genetic material is spread out in the form of chromatin.
- In a dividing cell, the genetic material is condensed into structures called chromosomes, which are involved in cell division (mitosis).
There you have it, that is the structure of a typical, human, cell. Please be sure, if you are a student, to READ YOUR BOOK. This article is intended to summarize the basics of what you need to know, however, nothing can replace the depth of your book, or even better, the help of a teacher. If you notice something that is missed, feel free to let me know in the comments, and if you need a hand, I'll try to help as best I can. Also, I'd highly recommend the coloring book you can probably get from your school bookstore (or in the link provided below) as it helps A LOT for the anatomy portion of your class. Good luck.