Video: Introduction to cells

Ah, the cell, something we study over and over and over again every year of school. Organisms are built step-by-step from these structures, so it makes sense that understanding how the body functions would start with fundamental knowledge of the functions of the cell. And in this tutorial, we're going to quickly orient you to the structure and function of a typical cell.

Not all cells look like this in the body. They vary based on location and function. This typical cell is the one we'll use today to understand how they work. Each cell has three main parts: the cell membrane, the nucleus, and the cytoplasm.

The cell membrane, also known as plasma membrane, separates the extracellular and the intracellular environments. Substances are distributed differently between the two compartments and the cell membrane maintains that difference to ensure homeostasis. For example, proteins are higher inside the cell, contributing to the gel-like consistency of the intracellular fluid. With regard to ions, the intracellular fluid has a higher concentration of potassium and the extracellular fluid has a higher sodium concentration.

The cell membrane is very careful about what it lets into and out of the cell, making it selectively permeable. Some substances can cross freely, such as oxygen, while others need to use channels and carrier proteins.

Often cells have to expend energy to move substances using carrier proteins, or when they use vesicles by processes known as endocytosis and exocytosis. As we'll learn in later tutorials, exchange of substances between intra- and extracellular environments is important for cell function and communication.

Now let's journey inside the cell where we have the nucleus and cytoplasm.

Cytoplasm and cytosol are two confusing terms. We often use the two interchangeably; however, they are different. The cytosol is the intracellular fluid. The cytoplasm is the contents of the cell within the cell membrane, except for the nucleus. Therefore, the cytoplasm includes the cytosol along with the organelles.

The nucleus is considered separate from the cytoplasm and is usually the most prominent organelle of the cell. The nucleus is the cell's command center where most of the genetic material -- that's DNA -- is stored in the form of chromatin. This DNA guides the synthesis of proteins. Proteins are essential for a lot of things including cell functions that are carried out by the other organelles. They can be classified into those with a membrane and those without one. Let's start with non-membranous organelles of the cell.

Non-membranous organelles include the cytoskeleton, ribosomes, and proteosomes. The cytoskeleton forms a supportive framework for the cells. It's made up of protein filaments such as microfilaments, intermediate filaments, and microtubules. These cytoskeletal structures contribute to maintaining the cell's shape and structure, enable movement, and form the core of cellular extensions such as microvilli, cilia, and flagella.

The ribosomes are also non-membranous organelles. They are our protein factories. Free ribosomes are dispersed in the cytosol. Bound or fixed ribosomes are found on the surface of the nuclear envelope and the rough endoplasmic reticulum.

The third organelle is the proteosome. These small cylindrical organelles break down proteins that don't meet the minimum standards.

But who decides whether a protein is acceptable or not? This kind of quality control takes place in the rough endoplasmic reticulum. That brings us to the membranous organelles.

Membranous organelles include the nucleus, rough and smooth endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria, and peroxisomes. Remember how the cell membrane separates the contents of the cell from the outside environment? These organelles have membranes that separate what is inside the organelles from the cytosol.

Mitochondria are the power plants of the cell, using oxygen to generate energy in the form of ATP. Peroxisomes are detoxifying organelles. That process of detoxification generates harmful byproducts, which are quickly neutralized. The membrane protects the rest of the cell from harm.

The rest of the organelles work together as the endomembrane system of the cell. The rough endoplasmic reticulum folds and modifies the proteins synthesized by the ribosomes bound to its surface. The smooth endoplasmic reticulum has no ribosomes, synthesizes lipids and steroid hormones, and plays an important role in detoxification.

Proteins and lipids reach the Golgi apparatus via transport vesicles. The Golgi apparatus modifies, packages, and sorts them into vesicles that can have a few different destinations. Some leave the cell, some contribute to forming and repairing the cell membrane, others form lysosomes.

Lysosomes are our recycling crew that use enzymes to digest unneeded substances. They also break down damaged and worn-out organelles to recycle them.

Thus, within the protective cell membrane, all these organelles have their own part to play in this well-functioning tiny factory called the cell.

Now it's time to move on to each part in more detail. Check out all the cell study units and articles we have at Kenhub!