The difference between eukaryotic organisms and prokaryotic organisms

Eukaryotic organisms, also known as eukaryotes, include animals, plants, fungi and protoctists. Eukaryotic organisms are normally multicellular and are defined as being made up of eukaryotic cells. Eukaryotic cells contain organelles that are surrounded by a membrane; for example, an organelle known as a nucleus. The few eukaryotic cells that are found in eukaryotic organisms but do not have a nucleus are usually dead; for example, mature red blood cells in humans that don’t live for very long (this particular cell will be discussed more in a later topic). Eukaryotic cells have other features that prokaryotic cells do not; one of these is their size, which is much larger at >10 μm, whereas prokaryotic cells measure at <5 μm.

Prokaryotic organisms, also known as prokaryotes, include bacteria, such as E. coli. Prokaryotic organisms contain prokaryotic cells and are normally unicellular (a single-celled organism). Prokaryotic cells do not contain membrane-bound organelles, such as a nucleus or mitochondria.

Over the next few chapters we will go into detail about the different eukaryotic and prokaryotic organisms and the cells that they are made up of.

Animals

Animals (including humans) are multicellular eukaryotic organisms (this means that they are made up of a number of different eukaryotic cells). These organisms usually have nervous coordination and are able to move from one place to another. As previously mentioned, the variety of species in the animal kingdom is huge. Some examples of different types of animal organisms are:

• Sponges
• Worms
• Insects
• Fish
• Reptiles
• Amphibians
• Birds
• Mammals (includes humans)

Of these, fish, reptiles, amphibians, birds and mammals are known as vertebrates. This means that they have a vertebral column (back bone). The other species that lack this feature are known as invertebrates

A general diagram of a ‘typical’ animal cell is below:

The cell membrane forms the outer covering of the animal cell to separate its contents from the environment. The cell’s membrane is semipermeable; meaning it controls what chemicals can pass into and out of the cell. You can think of the cell membrane as the cell’s skin to help you remember; just as our skin absorbs and secretes certain chemicals, so does the cell membrane – we will discuss this particular process in more detail in a later topic.

The cytoplasm is surrounded by the cell membrane and is the living material that makes up the cell. It is a gel-like substance between the cell membrane and nucleus that is made up of many different structures, such as organelles, vesicles and other various particles.

An organelle is a structure that performs specific functions within a cell. There are a number of organelles within animal cells, the largest of which is called the nucleus. The nucleus is made up of a surrounding nuclear envelope which is also known as the nuclear membrane. The nucleus contains chromosomes (there are 46 of these in human cells) which carry the genetic material (the genes) and therefore control the activities of the cell (we will discuss more about genes later in the course). It also contains the nucleolus which contains protein and ribonucleic acid (RNA) – again, this will be discussed in more detail later in the course. As mentioned before, most eukaryotic cells have a nucleus but the ones that do not are usually dead.

The endoplasmic reticulum (ER) is an organelle which is a network of tubular membranes that are normally continuous with the outer nuclear membrane (the outside of the nucleus). The ER is responsible for the synthesis and modification and also assists in the transport of materials into and out of the cell.

The Golgi apparatus (also known as the Golgi complex) in an animal cell, is an organelle located close to the ER. It is responsible for receiving proteins and lipids (meaning fats) from the ER and concentrating them into droplets known as vesicles. Depending on the contents of these vesicles, these are then either transported to other organelles in the cell, known as lysosomes, the cell membrane or out of the cell entirely.

A lysosome is an organelle that is attached to the cell membrane and varies in shape, size and number according to the particular cell it helps operate. Lysosomes contain proteins called enzymes (we will discuss these further in a later topic of this course) and are known to assist in degrading material from outside of a cell and also degrading material from inside of a cell that no longer has any significant use.

If you look at the animal cell diagram you will see some dark blue particles floating around in the cytoplasm. These are ribosomes. Ribosomes are sometimes found free like this or can be attached to the ER. They consist of RNA and associated proteins.

Another organelle that is found in the cytoplasm is the mitochondrion (pl. mitochondria). Mitochondria are organelles that are normally found in large quantities within most cells, especially muscle and nerve cells. This organelle is an essential component in the process of cell respiration. Mitochondria therefore help to release energy that the cell can use.

Animal cells can also contain a number of vacuoles. It is known as a storage centre for minerals and nutrients that the cell does not yet require; it can also contain waste products that shouldn’t be in the cytoplasm.

Not all animal cells will include all the features depicted in the diagram; this is just to give you a general idea of the structure of a ‘typical’ animal cell before we go into further detail in the other topics of this course.

Plants

Plants are also multicellular organisms. The main distinguishing features of plant cells is that they contain a large central vacuole, chloroplasts (which carry out the process of photosynthesis) and a cell wall.  Below, a diagram has been provided of a ‘typical’ plant cell.

As you may have already noticed, there are a few features and organelles in plant cells which we have also seen in animal cells. These include a cell membrane, cytoplasm, nucleus, nucleolus, nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosome, mitochondria and a vacuole. Again, plant cells are multicellular organisms. The most distinctive feature of a fully grown plant cell is a permanent vacuole. A plant cell vacuole is different to an animal cell vacuole in that plant cells have one large central vacuole, whereas animal cells generally have more than one vacuole which are much smaller in size.

A plant cell’s vacuole contains cell sap (a watery liquid) which is a mixture of amino acids, mineral salts, sugars and waste substances. As well as holding materials and waste products, the large central vacuole provides support and structure for the growing plant as it helps to maintain pressure within the plant cell. The outermost layer of a plant cell is the cellulose cell wall that encloses the cell membrane (this is a feature that is not found in animal cells). It is a non-living material that is made of mostly cellulose (a carbohydrate). It is a tough material that helps the cell keep its shape – unlike animal cells whose shapes tend to vary and so they do not have a cellulose cell wall. The toughness of the cell wall is essential. This is because as plant cells absorb water, internal pressure is produced which pushes against other cells in the plant to provide support. A cell wall is freely permeable as it contains large holes in it; this means that it does not constrict any materials from travelling in or out of the cell.

The plant cells of the green parts of plants contain organelles known as chloroplasts (you can see these in the diagram on the previous page). Chloroplasts contain an important biomolecule known as chlorophyll. Chlorophyll is vital in the process of photosynthesis (more about the details of this process in Topic 2 of this course). The plant cells that make up the flowers, roots and woody stems of a plant do not contain chloroplasts and therefore do not undergo the process of photosynthesis.

The diagram illustrates the typical structure of a plant cell (you must remember that not all plant cells contain the same structure as illustrated in the plant cell diagram; for example, the cells of plant roots do not contain chloroplasts).

Some examples of plants include:

• Flowering plants, such as roses
• Herbaceous legumes, such as peas or beans

Fungi

The bodies of fungi are normally made from thread-like structures known as hyphae which contain many nuclei. The thread-like structures of hyphae are organised into a network called mycelium. Take a look at the diagram below for an illustration of this:

Fungi are not able to carry out photosynthesis as their cells never contain chloroplasts; instead, the hyphae of the fungi feed on dead, and sometimes living, material by spreading and absorbing nutrients from plant or animal matter around them. They secrete digestive enzymes (this process is known as extracellular secretion) onto the dead or living material to break it down so that the nutrients can be easily absorbed by fungi cells. When an organism obtains their food in this way, it is known as saprotrophic nutrition. Fungi are known as important decomposers because they have the ability to recycle and break down dead organisms and waste products.

Fungi have cell walls that are similar to what you may see in a diagram of a plant cell; however, fungi cell walls are not made of cellulose and are instead composed of a different chemical called chitin.

Chitin is the same material that makes up the outside skeleton of insects.

Fungi can be split into two groups: multicellular fungi and unicellular fungi. A few examples of these have been provided below:

Mushrooms and toadstools

Mushrooms and toadstools are fruiting bodies that are products of the reproduction of the organism ‘fungi’. They are from the same family and are both multicellular; the main difference between the two is that mushrooms are edible whereas toadstools are poisonous – some toadstools have a distinctive colour to their caps to warn animals that they are poisonous. In mushrooms and toadstools, the hyphae are located under the soil; they gain their nutrients from absorbing nutrients from soil.

Moulds

Moulds feed on dead, and sometimes even on living, material by absorbing their nutrients. Moulds are found wherever these nutrients are present, for example on decaying food or in soil etc. They undergo the process of saprotrophic nutrition like mushrooms and toadstools; however, they do not have the fruiting body. Look at the image on the next page which shows an example of bread that has become mouldy.

This image shows a close-up of mouldy bread. The type of mould that is growing here is ‘pin mould’ from the Mucor genus. Mould travels by spores that are carried through the air and land on available nutrients. In the instance to the left, the mould spores have landed on a slice of bread and have then grown into a mycelium of hyphae. The hypha will continue to grow like this until the entire surface of the food is covered. Notice the dark spots on the picture to the left; these are structures that produce mould spores for reproduction.

Yeasts

Another group of fungi are known as yeasts. There are over a thousand different species of yeast that have been discovered. They live everywhere, in water, soil and even in the air on dust particles. Yeasts are unicellular organisms. An example of a yeast is the powder that is used for baking; for instance, when baking bread.  Certain types of microscopic fungi can cause disease; they are pathogenic. We will discuss this in more detail in the ‘Pathogens’ chapter of this topic.

Protoctists

Protoctists are mostly microscopic, unicellular organisms. They are a mixed group of organisms that don’t fit into the kingdoms of animals, plants or fungi.

Some protoctists are known as protozoa, for example Amoeba. Amoebae live in pond water and are a single-celled organisms; this means that their whole body is only made up of one cell. Amoebae have similar features to animal cells.

Other protoctists are known as algae and are more like plants. They have chloroplasts and carry out the process of photosynthesis, for example chlorella. The majority of algae are unicellular; however, seaweed, as an example, is a multicellular organism.

As previously mentioned, some protoctists are pathogenic. An example of a pathogenic prototcist is Plasmodium which is responsible for causing malaria.

Prokaryotic organisms (bacteria)

Prokaryotic organisms include bacteria. Bacteria are small, microscopic, unicellular organisms that have a cell wall, cell membrane, cytoplasm and plasmids. They lack a membrane-bound nucleus and instead contain a circular chromosome of DNA known as a nucleoid. Certain species of bacteria cells also contain pili or flagellums. Take a look at the diagram below which illustrates the ‘typical’ features of this type of bacteria cell.

Bacteria are much smaller in size than animal, plant or protoctist cells; they also have a much simpler structure. To put it in perspective, an animal cell is usually 10–50 μm in diameter, whereas a bacterium is 1–5 μm in length and a bacterium’s actual volume can be much less than this.

Bacteria cells include a cell wall which surrounds the cell membrane to offer further protection. A bacterium’s cell wall is composed of a mix of complex chemicals made of polysaccharides and proteins. This is different to plant cells which have a cellulose cell wall and fungi which have cell walls made of chitin. To offer further protection, certain species of bacteria have a capsule (or slime layer) which is a layer covering the outside of the cell wall.
Like we have seen in other types of cells, bacteria cells also have a cell membrane which surrounds the cytoplasm.

In the cytoplasm of a bacterium cell is the nucleoid. The nucleoid is the cell’s genetic material contained in a single chromosome which is loose in the cytoplasm.

In certain bacteria, but not in the type that is depicted in the previous diagram, they contain a form of chlorophyll in their cytoplasm. As mentioned previously, chlorophyll is vital in the process of photosynthesis. Some species have this feature and can therefore carry out photosynthesis. However, other species of bacteria (like the one depicted in the diagram) obtain their nutrients by feeding on other dead or living organisms; similar to fungi, these types of bacteria are known as important decomposers as they help break down dead organisms and other waste products.

Ribosomes in bacteria cells make up to 30% of a bacterium’s total weight. They are particles that float around in the cytoplasm. Ribosomes consist of RNA and associated proteins; they are the site of protein synthesis – we will go into more detail about this later in the course.

Another feature which is commonly found in a bacterium’s cytoplasm is plasmids – these are not depicted in the diagram provided. Plasmids are small molecules of DNA that have very important uses in genetic engineering. Plasmids are present in around three-quarters of all known species of bacteria.

Pili (sing. pilus) are hair-like structures found on the surface of many different species of bacteria. They have two main purposes. One of these is the exchange of genetic material between other bacteria cells through conjugation; in other words, they conjugate (attach) in order to pass on genetic material and reproduce. The other purpose of pili is the ability for pathogenic bacteria to attach to their host (the host is another type of cell – we will discuss this in detail shortly).

A flagellum (pl. flagella) is a thread-like structure that can be found on specific types of cells, e.g. protozoa, bacteria and spermatozoa cells as an example. A cell can have just one and sometimes even more than one flagellum. The primary role of flagella is locomotion; however, they also have a sensory function which helps them detect chemical and temperature changes outside of the cell. The eukaryotic cells and prokaryotic cells that have a flagellum differ. The movement of a eukaryote’s flagella is in a whip-like manner; whereas the movement of a prokaryote’s flagella is similar to the movement of a propeller (it rotates in a clockwise or counterclockwise direction). Bacteria that can swim have a flagellum that helps propel them through water. However, not all bacteria have a flagellum and so they cannot move by themselves.

Examples of different types of bacteria include Lactobacillus bulgaricus which is a type of rod-shaped bacterium that is not dangerous to humans and is used in the production of yoghurt from milk. In contrast, the spherical bacterium pneumococcus is a dangerous bacterium that acts as the pathogen causing pneumonia.