Levels of organisation (cell → tissue → organ → system → organism) - Biology IGCSE Study Notes

Imagine you're building with LEGOs. You don't just dump all the pieces together, right? You start with tiny bricks, then connect them to make bigger sections, and eventually, you have a whole model. Living things are organised in a very similar way, from the smallest 'building blocks' to the complete living being.

Here's how it works, like a set of increasing steps:

• Cell: This is the absolute smallest unit of life. Think of it as a single LEGO brick. It's the basic building block that can carry out all the processes needed for life. For example, a muscle cell helps you move, and a nerve cell helps you think.
• Tissue: Now, imagine you take a bunch of the same kind of LEGO bricks and stick them together to make a wall. That's like a tissue! A tissue is a group of similar cells that work together to do a specific job. For example, all your muscle cells working together form muscle tissue.
• Organ: Next, you take different types of LEGO walls (different tissues) and combine them to make a bigger, more complex part, like a door or a window. An organ is made of several different tissues working together to perform a major function. Your heart, lungs, and stomach are all organs.
• Organ System: What if you put all the doors, windows, and walls together to make a whole room? That's an organ system! An organ system is a group of different organs that work together to carry out a big, important task for the body. For example, your digestive system includes your stomach, intestines, and liver, all working to process food.
• Organism: Finally, you put all the rooms together to make a complete LEGO house. An organism is a complete living thing, made up of many organ systems working together. You, a tree, a dog, or even a tiny bacterium are all organisms.

Let's use the example of your stomach to see these levels in action. Your stomach is a super important organ that helps digest your food.

• Cell: Inside your stomach, there are many different types of cells. For instance, there are parietal cells (say: pah-RYE-uh-tahl) which are tiny individual units that make stomach acid. There are also chief cells that make enzymes (special proteins that break down food).
• Tissue: Millions of these parietal cells group together to form a type of glandular tissue (tissue that makes and releases substances). Other cells form muscle tissue in the stomach wall, which helps churn the food. There's also connective tissue holding everything together and epithelial tissue (a protective lining) on the inside surface.
• Organ: All these different tissues – the glandular tissue making acid, the muscle tissue churning food, the connective tissue, and the epithelial lining – work together to form your stomach. The stomach's main job is to mix food with digestive juices and start breaking it down.
• Organ System: Your stomach doesn't work alone! It's part of the digestive system. This system also includes your mouth, oesophagus (food pipe), small intestine, large intestine, liver, and pancreas. All these organs work together to take the food you eat, break it down, absorb nutrients, and get rid of waste.
• Organism: Finally, your digestive system, along with all your other systems (like your circulatory system, respiratory system, nervous system, etc.), makes up you! You are a complete organism, a living being capable of eating, growing, moving, and thinking, all thanks to these amazing levels of organisation working in harmony.