What is a cell?

Right now your body is doing a million things at once. It’s sending electrical impulses, pumping blood, filtering urine, digesting food, making protein, storing fat, and that’s just the stuff you’re not thinking about! You can do all this because you are made of cells — tiny units of life that are like specialized factories, full of machinery designed to accomplish the business of life. Cells make up every living thing, from blue whales to the archaebacteria that live inside volcanos. Just like the organisms they make up, cells can come in all shapes and sizes. Nerve cells in giant squids can reach up to 12m [39 ft] in length, while human eggs (the largest human cells) are about 0.1mm across. Plant cells have protective walls made of cellulose (which also makes up the strings in celery that make it so hard to eat) while fungal cell walls are made from the same stuff as lobster shells. However, despite this vast range in size, shape, and function, all these little factories have the same basic machinery.

There are two main types of cells, prokaryotic and eukaryotic. Prokaryotes are cells that do not have membrane bound nuclei, whereas eukaryotes do. The rest of our discussion will strictly be on eukaryotes. Think about what a factory needs in order to function effectively. At its most basic, a factory needs a building, a product, and a way to make that product. All cells have membranes (the building), DNA (the various blueprints), and ribosomes (the production line), and so are able to make proteins (the product - let’s say we’re making toys). This article will focus on eukaryotes, since they are the cell type that contains organelles.

What’s found inside a cell

An organelle (think of it as a cell’s internal organ) is a membrane bound structure found within a cell. Just like cells have membranes to hold everything in, these mini-organs are also bound in a double layer of phospholipids to insulate their little compartments within the larger cells. You can think of organelles as smaller rooms within the factory, with specialized conditions to help these rooms carry out their specific task (like a break room stocked with goodies or a research room with cool gadgets and a special air filter). These organelles are found in the cytoplasm, a viscous liquid found within the cell membrane that houses the organelles and is the location of most of the action happening in a cell.

Image: Diagram of a cell highlighting the membrane bound organelles.

Nucleus

Our DNA has the blueprints for every protein in our body, all packaged into a neat double helix. The processes to transform DNA into proteins are known as transcription and translation, and happen in different compartments within the cell. The first step, transcription, happens in the nucleus, which holds our DNA. A membrane called the nuclear envelope surrounds the nucleus, and its job is to create a room within the cell to both protect the genetic information and to house all the molecules that are involved in processing and protecting that info. This membrane is actually a set of two lipid bilayers, so there are four sheets of lipids separating the inside of the nucleus from the cytoplasm. The space between the two bilayers is known as the perinuclear space.

Though part of the function of the nucleus is to separate the DNA from the rest of the cell, molecules must still be able to move in and out (e.g., RNA). Proteins channels known as nuclear pores form holes in the nuclear envelope. The nucleus itself is filled with liquid (called nucleoplasm) and is similar in structure and function to cytoplasm. It is here within the nucleoplasm where chromosomes (tightly packed strands of DNA containing all our blueprints) are found.

Image: This illustrtion shows a close up of the nucleus, highlighting structures specific to the nucleus.

A nucleus has interesting implications for how a cell responds to its environment. Thanks to the added protection of the nuclear envelope, the DNA is a little bit more secure from enzymes, pathogens, and potentially harmful products of fat and protein metabolism. Since this is the only permanent copy of the instructions the cell has, it is very important to keep the DNA in good condition. If the DNA was not sequestered away, it would be vulnerable to damage by the aforementioned dangers, which would then lead to defective protein production. Imagine a giant hole or coffee stain in the blueprint for your toy - all of a sudden you don’t have either enough or the right information to make a critical piece of the toy.

Mitochondria

Just like a factory can’t run without electricity, a cell can’t run without energy. ATP (adenosine triphosphate) is the energy currency of the cell, and is produced in a process known as cellular respiration. Though the process begins in the cytoplasm, the bulk of the energy produced comes from later steps that take place in the mitochondria.

Like we saw with the nuclear envelope, there are actually two lipid bilayers that separate the mitochondrial contents from the cytoplasm. We refer to them as the inner and outer mitochondrial membranes. If we cross both membranes we end up in the matrix, where pyruvate is sent after it is created from the breakdown of glucose (this is step 1 of cellular respiration, known as glycolysis).The space between the two membranes is called the intermembrane space, and it has a low pH (is acidic) because the electron transport chain embedded in the inner membrane pumps protons (H+) into it. Energy to make ATP comes from protons moving back into the matrix down their gradient from the intermembrane space.

Image: An illustration showing the various parts of the mitochondria.

Chloroplasts

Chloroplasts are found only in plants and photosynthetic algae. (Humans and other animals do not have chloroplasts.) The chloroplast's job is to carry out a process called photosynthesis.

In photosynthesis, light energy is collected and used to build sugars from carbon dioxide. The sugars produced in photosynthesis may be used by the plant cell, or may be consumed by animals that eat the plant, such as humans. The energy contained in these sugars is harvested through a process called cellular respiration, which happens in the mitochondria of both plant and animal cells.

Chloroplasts are disc-shaped organelles found in the cytosol of a cell. They have outer and inner membranes with an intermembrane space between them. If you passed through the two layers of membrane and reached the space in the center, you’d find that it contained membrane discs known as thylakoids, arranged in interconnected stacks called grana (singular, granum).

Image: Diagram of a chloroplast, showing the outer membrane, inner membrane, intermembrane space, stroma, and thylakoids arranged in stacks called grana.

The plasma membrane

Both prokaryotic and eukaryotic cells have a plasma membrane, a double layer of lipids that separates the cell interior from the outside environment. This double layer consists largely of specialized lipids called phospholipids.
A phospholipid is made up of a hydrophilic, water-loving, phosphate head, along with two hydrophobic, water-fearing, fatty acid tails. Phospholipids spontaneously arrange themselves in a double-layered structure with their hydrophobic tails pointing inward and their hydrophilic heads facing outward. This energetically favorable two-layer structure, called a phospholipid bilayer, is found in many biological membranes.
The plasma membrane is the border between the interior and exterior of a cell. As such, it controls passage of various molecules—including sugars, amino acids, ions, and water—into and out of the cell. How easily these molecules can cross the membrane depends on their size and polarity. Some small, nonpolar molecules, such as oxygen, can pass directly through the phospholipid portion of the membrane. Larger and more polar, hydrophilic, molecules, such as amino acids, must instead cross the membrane by way of protein channels, a process that is often regulated by the cell.

Image: The plasma membrane shows the phospholipid bilayer, embedded proteins, and cholesterol molecules. The membrane separates the extracellular space, outside of the cell, from the cytosol inside the cell. The plasma membrane limits the exchange of materials between a cell and its environment.

The cytoplasm

The part of the cell referred to as cytoplasm is slightly different in eukaryotes and prokaryotes. In eukaryotic cells, which have a nucleus, the cytoplasm is everything between the plasma membrane and the nuclear envelope. In prokaryotes, which lack a nucleus, cytoplasm simply means everything found inside the plasma membrane.

One major component of the cytoplasm in both prokaryotes and eukaryotes is the gel-like cytosol, a water-based solution that contains ions, small molecules, and macromolecules. In eukaryotes, the cytoplasm also includes membrane-bound organelles, which are suspended in the cytosol. The cytoskeleton, a network of fibers that supports the cell and gives it shape, is also part of the cytoplasm and helps to organize cellular components.

Even though the cytosol is mostly water, it has a semi-solid, Jello-like consistency because of the many proteins suspended in it. The cytosol contains a rich broth of macromolecules and smaller organic molecules, including glucose and other simple sugars, polysaccharides, amino acids, nucleic acids, and fatty acids. Ions of sodium, potassium, calcium, and other elements are also found in the cytosol. Many metabolic reactions, including protein synthesis, take place in this part of the cell.

Cytoskeleton

Within the cytoplasm there is network of protein fibers known as the cytoskeleton. This structure is responsible for both cell movement and stability. The major components of the cytoskeleton are microtubules, intermediate filaments, and microfilaments.

Images: Elements of the cytoskeleton include microtubules (a), microfilaments (b), and intermediate fibers (c). These structures work together in cell structure and motility.

oqa3zPG31Uk

Content taken or modified from Khan Academy: https://www.khanacademy.org/science/biology/structure-of-a-cell/tour-of-organelles/a/chloroplasts-and-mitochondria

All Khan Academy Content is made available to Users in accordance with the Creative Commons Attribution – CC-BY-NC-SA 4.0 License (ttps://creativecommons.org/licenses/by-nc-sa/4.0/) , All Khan Academy content is available for free at www.khanacademy.org.