2.1: Cell Structure: Subcellular Components

All living organisms are composed of cells. Unicellular organisms (like bacteria) are made of one cell. Multicellular organisms (like humans) are made of many cells. Topic 2.1 explores the structure and components that make these cells function.

Vocab List

Written Explanation

Cell Types:

The two classifications of cells are prokaryotic and eukaryotic. Prokaryotic cells are simple. All prokaryotic organisms are unicellular, and they contain no organelles (specialized components with membranes). Eukaryotic cells differ in several aspects. Eukaryotic cells can be either unicellular or multicellular, and they all contain organelles.

Diagram showing a eukaryotic cell and a prokaryotic cell

Cell Structure:

All cells share a few common traits. Firstly, they all have cell membranes, which surround and protect the inside material of the cell. All of this inside material is collectively called the cytoplasm, and the watery fluid in the cytoplasm is called the cytosol.

In addition to having cell membranes, only plant cells have cell walls, which surround the membrane and provide additional protection and rigidity.

Diagram showing the cell membrane, cytoplasm, cytosol, and cell wall (plant cell only) of various cells

Next, all cells have cytoskeletons, which are structures that help cells maintain their shape and hold their organelles in place. There are several types of cytoskeleton structures, but the most important two are cilia and flagella. Cilia are small “hair” like protrusions from the cell that help certain cells move. Flagella are long tails that help cells move (present in sperm cells). Most human cells have neither of these two cytoskeleton structures but still have others which hold organelles and the cell as a whole.

Diagram showing the cytoskeleton of cells, including cilia and flagella

Furthermore, all cells contain ribosomes (which are not organelles because they don't have membranes). Ribosomes synthesize (produce) protein molecules. These ribosomes read information from RNA and translate it into a polypeptide/protein.

Diagram showing ribosomes in cells

Organelles:

An organelle is a component of a cell which has a specialized task. Organelles are surrounded by membranes which are similar in composition to cell membranes. Only eukaryotic cells have organelles.

The nucleus of a cell stores genetic material (in the form of DNA). It is roughly spherical, and is surrounded by a membrane called the nuclear envelope. The nuclear envelope has channels/holes, called nuclear pores, in its surface. These pores allow information (in the form of RNA) and enzymes to enter and exit the nucleus. In addition to storing genetic information, the nucleus also contains a nucleolus, which produces ribosomes (which exit the cell via nuclear pores).

Diagram of a nucleus with the nuclear envelope, nuclear pores, nucleolus, genetic information, and exiting ribosomes labeled

The endoplasmic reticulum (ER) is an organelle that surrounds the nucleus and is the center of production in the cell. The ER comes in two varieties (each cell has one of each). The rough ER is covered in ribosomes (this is where its name comes from). The ribosomes on its surface produce proteins. The smooth ER produces lipids and breaks down certain molecules.

Diagram of the smooth and rough endoplasmic reticulum

The golgi apparatus (aka golgi body or complex) is an organelle responsible for the packaging and processing of material in the cell. Most of this material (proteins and lipids) was originally produced in the endoplasmic reticulum. The majority of the proteins going through the golgi apparatus are later exported from the cell via transport vesicles.

Diagram of the golgi apparatus with vesicles

Lysosomes are organelles that contain enzymes (proteins) that are capable of breaking down biological molecules. Lysosomes are as such involved in the breakdown of old cells, organelles, food molecules, and more. This can either be by popping and releasing the enzymes within it, or by "consuming" a substance.

Diagram of a lysosome in action

Vacuoles are organelles that contain a fluid or absorbed substance. They are similar to lysosomes, but just contain different substances. Plant cells have one large central vacuole, in addition to their other vacuoles, which stores water. The large central vacuole keeps the plant cell in tension.

Diagram of a plant cell with a large central vacoule and several smaller vacoules in an animal cell

Vesicles are organelles that are very similar in structure and function to vacuoles. They also store absorbed material. However, vesicles are more temporary and serve the purpose of transporting material between the outside of the cell and various organelles inside the cell.

Diagram of a vesicle forming on the outside of a cell and transporting its material inside the cell

As you might already know, the mitochondria is the powerhouse of the cell. In other words, the mitochondria produces energy that the cell uses by converting food molecules into energy. Just like the inside of a cell is called the cytosol, the inside of a mitochondria is called the matrix.

Diagram of a mitochondria

The chloroplast is also an organelle that produces energy for cells. Unlike the mitochondria, the chloroplast gets its energy from the sun by the process of photosynthesis. The inside of a chloroplast is called the stroma.

Diagram of a chloroplast