Cells are the little building blocks of life. It takes millions of cells to form a single tissue, let alone an organ. Scientists aren't quite sure how many cells are in the human body (there are far too many to count), but a recent estimation has suggested that the average person has 37,000,000,000,000 cells. That's 37 trillion!
Fitting 37 trillion cells into one person means that they must be tiny. You can only identify individual cells under a light microscope. If you want to look inside the cells, you need to use a powerful type of microscope called an electron microscope. So, what will you see? Lots of tiny structures and systems carrying out various functions to keep the cell alive! These are cell organelles, and we will learn their meaning, their functions, as well as identify them in a plant cell organelles and animal cell organelles diagrams. Time to zoom in and take a closer look...
The Meaning of Cell Organelles
Let's start with the definition of cell organelles.
Organelles are specialised parts of cells that carry out a specific function.
Cells can be analogous to our bodies. We have lots of organs that carry out different functions. In a way, so do cells. Organelles act like mini-organs, each carrying out a different role in the cell, but all working together to keep the cell alive.
Prokaryotes and Eukaryotes
All life consists of either prokaryotic or eukaryotic cells. The differences between the two types of cells are summarised in this table.
Table 1: Main differences between prokaryotic and eukaryotic cells.
| Differences | Prokaryotes | Eukaryotes |
| Genetic information | No nucleus, circular DNA bundled together in the nucleoid region | A membrane-bound nucleus containing linear DNA |
| Membrane-bound organelles | Absent | Present |
| Size | Smaller | Larger |
| Complexity | Simple | More complex |
| Examples | Bacteria, archaea | Animals, plants, fungi, protists |
Prokaryotes are much smaller and simpler than eukaryotic cells, so they lack membrane-bound organelles.
A List of Cell Organelles
There are many types of cell organelles. Where are they found - animal, plant, or prokaryotic cells? You will notice that eukaryotic plant and animal cells share five organelles, with plant cells containing three extra unique organelles. Prokaryotes have a different set of organelles altogether.
Other than the ones mentioned here, additional organelles related to prokaryotes will not be discussed.
Table 2: Summary of where the different organelles can be found among animal, plant, and prokaryotes' cells.
| Organelles | Animals | Plants | Prokaryotes |
| Cytoplasm | ✔ | ✔ | ✔ |
| Nucleus | ✔ | ✔ | ✖ |
| Cell membrane | ✔ | ✔ | ✔ |
| Mitochondria | ✔ | ✔ | ✖ |
| Ribosomes | ✔ | ✔ | ✔ |
| Cell wall | ✖ | ✔ | ✔ |
| Chloroplasts | ✖ | ✔ | ✖ |
| Permanent vacuole | ✖ | ✔ | ✖ |
Bacterial cells, or prokaryotic cells, are much smaller than eukaryotic cells. While they do include some components similar to eukaryotes, due to their function and size, they have many differences. They do contain a cell wall that encloses the cytoplasm and cell membrane. However, they lack a membrane-bound nucleus; instead, their genetic material is a single circular molecule of DNA referred to as a prokaryotic chromosome.
Besides the single circular chromosome, prokaryotes usually have extra molecules of DNA called plasmids.
A plasmid is a small ring of DNA that can be transferred between cells.
Cell Organelles: Functions
Large eukaryotic, multicellular organisms may contain hundreds of different types of cells. Some cells are highly specialised to carry out particular functions for the animal or plant.
Specialised cells include blood cells, muscle cells, neurons (nerve cells) and gametes (reproductive cells).
No matter the function of the cells, they all have the same basic features.
A brief overview of the functions of the prokaryotic organelles:
- Nucleoid: region of the cell containing DNA (not an organelle)
- Ribosome: site of protein synthesis
- Cell wall: provides structure and protection
- Cell membrane: separates the cell from the outside environment
- Plasmid: a ring of DNA that can be transferred between cells (not an organelle)
Cytoplasm
The inside of every cell is filled with a jelly-like material called the cytoplasm. It contains dissolved salts and nutrients. Various chemical reactions take place in this semi-fluid mixture.
The cytoplasm isn't an organelle. However, the true cell organelles are suspended within it.
Nucleus
The nucleus is the largest organelle. It contains chromosomes which carry the cell's genetic material. These genes determine what proteins can be made. The nucleus controls the activities of the cell.
Red blood cells don't have a nucleus. The only function of these cells is to transport haemoglobin around the body. They have omitted their nuclei to maximise storage space for haemoglobin and allow these blood cells to squeeze through capillaries.
The lack of a nucleus means that red blood cells cannot synthesise proteins, so they cannot repair themselves. As a result, they have a very short lifespan of just 120 days.
Cell Membrane
Every cell has a cell membrane: a thin layer that forms a boundary between the cell's cytoplasm and the outside world. The cell membrane is not a normal barrier - it can control what chemicals enter and leave the cell. So, the membrane is considered to be partially permeable.
Cell membranes are made of molecules called phospholipids. They look a bit like tadpoles. The 'head' is hydrophilic (water-loving) and the 'tail' is hydrophobic (water-repellent).
Every cell membrane is made of two layers of phospholipids. The hydrophobic tails meet in the centre, whilst the hydrophilic heads interact with the cytoplasm or the external environment. This structure helps to separate the contents of the cell from the rest of the world.
Mitochondria
Mitochondria are sausage-shaped organelles that carry out respiration and release energy in the cytoplasm.
Mitochondria are nicknamed 'the powerhouse of the cell', which is undoubtedly true. Cells that require energy, such as muscle or nerve cells, have extra mitochondria.
Ribosomes
These tiny organelles are the site of protein synthesis.
Ribosomes are incredibly abundant within cells. Large eukaryotic cells can contain up to ten million ribosomes.
In the much smaller E. coli cells, the 15,000 ribosomes make up 25% of the cell's mass.
Chloroplasts (Plant Cells Only)
These organelles are only found in some plant cells. Chloroplasts are the site of photosynthesis in plants and algae, where light energy is converted into chemical energy (i.e. food).
Chloroplasts get their green colour from a pigment called chlorophyll. This pigment absorbs light energy for photosynthesis.
It's easy to determine what parts of a plant will have chloroplasts in its cells. Leaves and green stems will. Flowers, roots and woody stems will not.
Cell Wall (Plant Cells Only)
A cell wall is a layer of non-living cellulose found outside the cell membrane of plant cells. It helps the cell keep a fixed shape. The cell wall is freely porous and does not act as a barrier to water or other dissolved substances.
Cellulose is a tough, rigid, complex carbohydrate made from over 3000 glucose molecules. Humans are unable to digest cellulose.
Permanent Vacuole (Plant Cells Only)
Mature plant cells often have a large vacuole filled with cell sap in the centre of the cell, surrounded by a membrane. This helps the plant cell to keep its shape.
Cell sap stores dissolved sugars, mineral ions and other solutes.
Plant vacuoles are referred to as permanent vacuoles. This is because animal cells can contain vacuoles but are only small and temporary.
Earlier, we likened individual cell organelles to different parts of our bodies. Which organelles could represent the brain and stomach?
Animal Cell Organelles Diagram
An animal cell consists of several organelles that all play a role in its generic structure. They come in all shapes and sizes but are generally smaller and more irregular in shape than plant cells.
Animal cells can come in oval, round, rod, concave, and even rectangle shapes due to a lack of a rigid cell wall. The shape is usually conducive to its function in the body.
They share many organelles with plant cells because they are both eukaryotes. This means animal cells have a membrane-bound nucleus to encapsulate the genetic material. They also have several other cell organelles within a cell membrane that helps the animal cell to carry out its function and maintain normal functions of the body.
Plant Cell Organelles Diagram
Plant cells are exactly that. They are cells from photosynthetic eukaryotes - mainly green plants. As mentioned above, plant cells tend to run bigger than animal cells; they come in much more uniform sizes and tend to be rectangular in shape. Although eukaryotic cells share many of the same components, plant cells have specific structural organelles which are not found in animal cells, such as a cell wall, permanent vacuole and chloroplasts. These all play a vital role in maintaining the function of plants.
Cell Organelles - Key takeaways
Cell organelles are specialised structures within cells that carry out a specific function. They are so small that they can only be seen using an electron microscope.
There are two kinds of cells: prokaryotic and eukaryotic. Prokaryotic cells are small, simple, and lack membrane-bound organelles (including the nucleus). Eukaryotic cells are larger, more complex, and contain a nucleus and other membrane-bound organelles.
Animal cells contain the cytoplasm, the nucleus, the cell membrane, mitochondria, and ribosomes.
Plant cells contain the same organelles as animal cells but also chloroplasts, cell walls and a permanent vacuole.
1. Carl Zimmer, How Many Cells Are In Your Body?, National Geographic, 2013
2. John P. Rafferty, Fast Facts about the Cell Membrane, Britannica, 2022
3. Kara Rogers, Ribosome, Britannica, 2016
4. Ken Campbell, Blood cells - Part two - Red blood cells, Nursing Times, 2005
5. Melissa Petruzzello, Cellulose, Britannica, 2022
6. Melissa Petruzzello, Chloroplast, Britannica, 2021
7. Merriam-Webster, Organelle Definition & Meaning, 2022
8. Neil Campbell, Biology: A Global Approach Eleventh Edition, 2018
9. Pearson, Edexcel International GCSE (9 - 1) Science Double Award, 2017
10. Sylvie Tremblay, Specialized Cells: Definition, Types & Examples, Sciencing, 2019
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