- Define and explain Levels of Organisation in your own words
- Use key terms such as organ accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
A human body contains around 30 trillion cells, yet they work together as one coordinated whole. In this chapter you will build the hierarchy from cells to organ systems, follow food through the digestive system, explore the heart's double circulation, and meet the enzymes that make digestion possible.
This lesson focuses on Levels of Organisation: build the hierarchy from cells to tissues, organs and organ systems.
Build the hierarchy from cells to tissues, organs and organ systems.
Key ideas
The hierarchy of organisation
Cells with a similar structure and function form a tissue, such as muscle tissue. Different tissues combine into an organ, such as the stomach, which contains muscle, glandular and epithelial tissue. Organs work together in organ systems, such as the digestive system. Plants are organised the same way: mesophyll tissue forms part of a leaf, which belongs to the shoot system.
The heart and double circulation
The heart is a double pump made of muscle. The right side pumps deoxygenated blood to the lungs, where it picks up oxygen; the left side pumps oxygenated blood around the body. Valves prevent backflow. This double circulation keeps high pressure on the body side, so oxygen reaches tissues quickly, and lets blood be re-oxygenated every circuit.
Key term — organ: A group of different tissues working together as a structure with a specific job, such as the stomach or a leaf.
Put these in order from smallest to largest: organ, cell, organ system, tissue.
Cell → tissue → organ → organ system.
Answer: Cell → tissue → organ → organ system.
- Mixing up arteries and veins Arteries carry blood away from the heart (usually oxygenated); veins carry blood towards the heart (usually deoxygenated). The direction relative to the heart defines them, not the oxygen content.
- Saying enzymes are used up or killed Enzymes are not used up in reactions, and they are not alive so they cannot be killed. High temperatures denature them by changing the shape of the active site.
Practice
Amylase breaks down starch (carbohydrate); protease breaks down protein; lipase breaks down lipids (fats).
Any two of: large surface area; wall one cell thick for a short diffusion distance; rich blood supply to maintain a concentration gradient; many mitochondria for active transport.
The left ventricle pumps blood all around the body, which needs much higher pressure than pumping blood the short distance to the lungs, so its muscle wall is thicker.
Valves prevent the backflow of blood, keeping it moving in one direction through the heart and veins.
Quick check
Which of these best defines "organ"?
A student counts 21 pulse beats in 15 seconds after exercise. Calculate the heart rate.
- Levels of Organisation: build the hierarchy from cells to tissues, organs and organ systems.
- The hierarchy of organisation: Cells with a similar structure and function form a tissue, such as muscle tissue.
- tissue: A group of similar cells working together to carry out a particular function, such as muscle tissue or xylem tissue.
- Watch out for: mixing up arteries and veins