Cell Structure

Prokaryotic and eukaryotic cell structures

The definition of prokaryote roughly translates from Greek as: 'without kernel' meaning 'without nucleus'. Hence, prokaryotes never have a nucleus. Prokaryotes are usually unicellular, which means that bacteria, for example, are only made up of one single cell. There are, however, exceptions to that rule where the organism is unicellular but has a nucleus, so it's a eukaryote. Yeast is one example.

On the other hand, eukaryote in Greek translates to “true nucleus”. This means that all eukaryotes have a nucleus. Except for yeast, eukaryotes are multicellular as they can be made up of millions of cells. Humans, for example, are eukaryotes, and so are plants and animals. In terms of cell structure, eukaryotes and prokaryotes share some traits but are different in others. The following table shows the similarities and differences while also giving us a general overview of the cell structures we will be discussing in this article.

Table 1. Features of prokaryotic and eukaryotic cells.

Fig. 1 - An example of prokaryotic cells

Fig. 2 - An animal cell

Human Cell Structure and Function

The structure of a human cell, as for any cell, is tightly linked to its function. Overall, all cells have the same basic functions: they give structure to the organs or organisms they are part of, they turn food into usable nutrients and energy and carry out specialised functions. It is for those specialised functions that human (and other animal cells) have distinct shapes and adaptations.

Structures within a cell

There are many cell organelles, the presence and abundance of each organelle will depend on whether an organism is prokaryotic or eukaryotic, and the cell type and function.

Cell membrane

Both eukaryotic and prokaryotic cells contain cell membranes that are made up of a phospholipid bilayer (as seen below). The phospholipids (red in the figure) are made up of heads and tails. Heads are hydrophilic (water-loving) and face into the extracellular medium, while the tails are hydrophobic (do not like water) and face inwards.

The cell membrane separates the cellular contents from the surrounding medium. The cell membrane is a single membrane.

Fig. 3 - Phospholipid bilayer of the plasma membrane

If there are two lipid bilayers on the membrane, we call this a double membrane (Figure 4).

Most organelles have single membranes, except the nucleus and the mitochondria, which have double membranes. In addition, cell membranes have different proteins and sugar-bound proteins (glycoproteins) embedded in the phospholipid bilayer. These membrane-bound proteins have different functions, for example, facilitating communication with other cells (cell signalling) or allowing specific substances to enter or leave the cell.

Fig. 4 - Structural differences between single and double membranes

Regardless of the structural differences, these membranes provide compartmentalization, separating the individual contents that these membranes surround. One good way to understand compartmentalization is to imagine walls of a house that separate the interior of the house from the external environment.

Cytosol (matrix)

The cytosol is a jelly-like liquid within the cell and supports the function of all the cells' organelles. When you refer to the whole contents of the cell, including the organelles, you would call it the cytoplasm. The cytosol consists of water and molecules such as ions, proteins, and enzymes (proteins that catalyse a chemical reaction). Various processes take place in the cytosol, such as the translation of RNA into proteins, also known as protein synthesis.

Flagellum

Though flagella can both be found in prokaryotic and eukaryotic cells, they have a different molecular build. They are, however, used for the same purpose: motility.

Fig. 5 - A sperm cell. The long appendage is an example of a eukaryotic flagellum.

Ribosomes

Ribosomes are small protein-RNA complexes. You can either find them in the cytosol, mitochondria or membrane-bound (rough endoplasmic reticulum). Their main function is to produce proteins during translation. The ribosomes of prokaryotes and eukaryotes have different sizes, with prokaryotes having smaller 70S ribosomes and eukaryotes having 80S.

Fig. 6 - Ribosome during transcription

Eukaryotic cell structure

Eukaryotic cell structure is much more complex than prokaryotic. Prokaryotes are also single-celled, so they can not "create" specialized structures. For example, in the human body, eukaryotic cells form tissues, organs and organ systems (e.g. cardiovascular system).

Here are some structures unique to eukaryotic cells.

Nucleus and nucleolus

The nucleus contains most of a cell's genetic material and has its own double membrane called the nuclear membrane. The nuclear membrane is covered in ribosomes and has nuclear pores throughout. The biggest part of the eukaryotic cell's genetic material is stored in the nucleus (different in prokaryotic cells) as chromatin. Chromatin is a structure where special proteins called histones package the long DNA strands to fit inside the nucleus. Inside the nucleus is another structure called the nucleolus that synthesizes rRNA and assembles ribosomal subunits, which are both needed for protein synthesis.

Fig. 7 - Structure of the nucleus

Mitochondria

Mitochondria are often referred to as energy-producing cell's powerhouses and for a good reason - they make ATP which is essential for the cell to carry out its functions.

Fig. 8 - Structure of the mitochondrion

They are also one of the few cell organelles that have their own genetic material, mitochondrial DNA. Chloroplasts in plants are another example of an organelle with its own DNA.

Mitochondria have a double membrane just like the nucleus, but without any pores or ribosomes attached. Mitochondria produce a molecule called ATP which is the organism’s energy source. ATP is essential for all organ systems to function. For example, all our muscle movements require ATP.

Endoplasmic reticulum (ER)

There are two types of endoplasmic reticulum - the rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER).

Fig. 9 - The endomembrane system of the eukaryotic cell

The RER is a channel system that is directly connected to the nucleus. It is responsible for the synthesis of all proteins as well as the packaging of these proteins into vesicles that are then transported to the Golgi apparatus for further processing. For proteins to be synthesised, ribosomes are needed. These are directly attached to the RER, giving it a rough appearance.

In contrast, the SER synthesises different fats and stores calcium. The SER does not have any ribosomes and therefore has a smoother appearance.

Golgi apparatus

The Golgi apparatus is a vesicle system that bends around the RER on one side (also known as the cis side), the other side (trans side) faces towards the inside of the cell membrane. The Golgi apparatus receives the vesicles from the ER, processes the proteins and packages the processed proteins to be transported out of the cell for other uses. Furthermore, it synthesises lysosomes by loading them with enzymes. In plants, the Golgi apparatus also synthesises cellulose cell walls.

Fig. 10 - Structure of the Golgi apparatus

Lysosome

Lysosomes are membrane-bound organelles that are packed with specific digestive enzymes called lysozymes. Lysosomes break down all unwanted macromolecules (i.e. big molecules made up of a lot of parts) they are then recycled into new molecules. For example, a large protein would be broken down into its amino acids, and those can later be reassembled into a new protein.

Cytoskeleton

The cytoskeleton is like the bones of cells. It gives the cell its shape and keeps it from folding in on itself. All cells have a cytoskeleton, which is made up of different protein filaments: big microtubules, intermediate filaments, and actin filaments which are the smallest part of the cytoskeleton. The cytoskeleton is found in the cytoplasm near the cell membrane of a cell.

Plant cell structure

Plant cells are eukaryotic cells just like animal cells, but plant cells have specific organelles that are not found in animal cells. Plant cells, however, still have a nucleus, mitochondria, a cell membrane, Golgi apparatus, endoplasmic reticulum, ribosomes, cytosol, lysosomes and a cytoskeleton. They also have a central vacuole, chloroplasts, and a cell wall.

Fig. 11 - Structure of the plant cell

Vacuole

Vacuoles are large, permanent vacuoles mostly found in plant cells. A vacuole of a plant is a compartment that is filled with isotonic cell sap. It stores fluid that maintains turgor pressure and contains enzymes that digest chloroplasts in mesophyll cells.

Chloroplasts

Chloroplasts are organelles present in leaf mesophyll cells. Like mitochondria, they have their own DNA, termed chloroplast DNA. Chloroplasts are where photosynthesis takes place within the cell. They contain chlorophyll, which is

a pigment responsible for the green colour that is typically associated with leaves.

Fig. 12 - Structure of a chloroplast

Cell wall

The cell wall surrounds the cell membrane and, in plants, is made of a very sturdy material called cellulose. It protects the cells from bursting at high water potentials, makes it more rigid and gives plant cells a distinctive shape.

Prokaryotic cell structure

Prokaryotes are much simpler in structure and function than eukaryotes. Here are some of the features of these types of cells.

Plasmids

Plasmids are DNA rings that are commonly found in prokaryotic cells. In bacteria, these rings of DNA are separate from the rest of the chromosomal DNA. They can be transferred into other bacteria to share genetic information. Plasmids are often where the genetic advantages of bacteria originate, such as antibiotic resistance.

Capsule

A capsule is usually found in bacteria. Its sticky outer layer prevents the cell from drying out and helps bacteria, for example, stick together and stick to surfaces. It is made up of polysaccharides (sugars).