Biology: The Nature and Variety of Living Organisms

Updated: 5 days ago

What is a 'Living Organism?'

You might think there is an easy answer to this question. Humans, animals and plants are all living things, right? Yes... but it’s not quite as simple as that.

You are alive as you read this. If you have a pet at home, they are alive. The flowers and trees outside your window are alive too, but what about a wooden chair? It once belonged to a tree, so is it a living organism? How about the clouds as they move across the sky or rain as it falls? What about a fire that dances while it burns - are these things alive?

You see, the answer to the question, 'what is a ‘living organism?' is a little more complicated than you might originally think. We cover everything you need to know about living organisms in the Biology IGCSE course and corresponding STEM Semester-End Revision Courses.

Characteristics of living organisms

To be considered a living organism, the 'thing' in question needs to share a few common characteristics. You will come to know these as 'Mrs Gren' – a helpful acronym to make them easier to remember.

Movement - An action by an organism causing a change of position or place

Respiration - The chemical reactions in cells that break down nutrient molecules and release energy

Sensitivity - The ability to detect and respond to changes in the environment

Growth - A permanent increase in size

Reproduction - The processes that make more of the same kind of organism

Excretion - Removal from organisms of toxic materials and substances in excess of requirements

Nutrition - Taking in of materials for energy, growth and development

Applying the Characteristics

Now that we have a better understanding of the characteristics of a living organism, we can apply them to determine what is and what is not a living organism.

Let’s take our wooden chair as an example. If we looked at the wood under a microscope, we might see traces of the cells that used to make up the living tree it came from but the wood itself is no longer alive. Once it was removed from the tree, it lost the ability to grow, breathe, absorb nutrition or reproduce.

Fire is a tricky one. It appears to be very much alive, but if we apply the characteristics above, we are able to determine that it is not considered a living organism. Living things are made up of cells and as they grow, they create new cells. Fire does not have cells. Fire seems to reproduce but no information is passed on. No DNA is carrying information from one generation to another. Fire remains the same every time. It may grow bigger or shrink smaller, hotter or cooler, it may even move differently, but that's all because of its current conditions, not because of information it inherited in DNA. Fire may appear to be breathing but it is unable to form proteins from the oxygen and hydrogen and carbon it "feeds on." It simply destroys proteins. Living organisms also require nutrients and water, and are gifted with incredible ways of finding and using these things in their environments. They can also sense and respond to threats. Fire is unable to do any of these things.

Examples of Living Organisms

Now that we know what the characteristics of a living organism are, what are some examples?

Plants are complex, multicellular organisms. As we discussed in a recent blog post, "How Do Plants 'Eat'?" they use the sun's energy to create food, in a process called photosynthesis. This, among other criteria, qualifies them as living organisms.

People and animals are also multicellular organisms. They don't create their own food internally, rather, they eat other plants or animals to get the energy they need to survive. They move, reproduce and grow, excrete and respire. They also detect and adapt to threats in their environment. If we look at the checklist, its clear that animals and humans are living organisms.

Fungi is considered a living organism and can be single-celled or multicellular. An example of a fungus is a mushroom. They 'eat' by releasing enzymes onto their food which breaks it down, so that they can absorb it.

Bacteria are considered living organisms even though they are so small that we have to use a microscope to see them. Most bacteria feed off other organisms. Bacteria can be good or bad. The bad ones cause disease while the good ones can help make delicious food like yogurt.

Protoctists are also tiny microscopic single-celled organisms. Some of them 'eat' just like plants do while others are more like animal cells. Some protoctists can cause diseases, for example Plasmodium causes Malaria.

Viruses can be even smaller than bacteria or protoctists and they always cause diseases. The Corona Virus is an example we are all too familiar with right now.

Pathogens are living organisms that cause diseases. Fungi, bacteria and protoctists can be pathogens, and viruses are always pathogens.

Why does it all matter?

The Cambridge IGCSE Biology course offered at ME Education allows our students to make sense of the living world around them. It’s not just information to be learned for an exam, it provides students with a real understanding of how the world works.

They also learn to develop an appreciation and respect for their environments and the delicate balance within which we all operate.

To learn more about our approach to education, or to talk to one of our admissions consultants about reserving a spot for your child, contact us at info@meeducationhk.com.


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"I've just found out that I passed my exams in Biology! Thank you so much for Ms Sonia's help. I do believe that my lessons with her helped me a great deal. I am so happy to continue my lessons with ME Education!"

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