Day 2 of 30
Cells, tissues, organs, systems
Lessons are AI-drafted and being reviewed and improved over time.
Learning Objective
Compare cells, tissues, organs, and organ systems, and identify the four basic tissue types and what each does.
How many cells make up your body? Scientists estimate tens of trillions, and they are not all alike. Your body has more than 200 different types of cells, each shaped for a job. Yesterday you saw the levels of organization. Today you zoom in on the lowest ones.
A cell is wrapped in a cell membrane that controls what enters and leaves. Inside, the nucleus holds DNA, the instructions for building and running the cell. The cytoplasm is the jelly-like fluid that fills the cell, and mitochondria are the parts that release energy from food. Cells use these parts differently. A muscle cell packs in many mitochondria because it needs lots of energy, while a mature red blood cell gives up its nucleus to make room for carrying oxygen.
Cells of the same kind that work together form a tissue. The human body has four basic types. Epithelial tissue covers surfaces and lines the inside of body cavities and organs, like your outer skin and the inside of your intestines. Connective tissue supports and connects other tissues. Bone, cartilage, fat, and even blood are connective tissues. Muscle tissue contracts to produce movement. Nervous tissue sends electrical signals through the brain, spinal cord, and nerves.
An organ is made of two or more tissue types working together on a shared job. Take the stomach. Epithelial tissue lines the inside and makes mucus and digestive juices. Layers of muscle tissue in the wall squeeze and churn food. Connective tissue holds the layers together, and nervous tissue helps control how much the stomach moves and releases. No single tissue could do all of this on its own. Finally, organs that cooperate form an organ system. The stomach, small intestine, liver, and other organs make up the digestive system, which you will study later in the unit.
Be careful with two common mistakes. A tissue is not just a layer of cells; it is a group of similar cells doing the same job. And blood is a tissue, a fluid connective tissue, while the heart and the skin are organs just like the stomach. Whenever you meet a new body part, ask yourself which level it belongs to and what it is made of.
The basic unit of structure and function in living things.
The part of a cell that holds the DNA and directs the cell.
Cell parts that release energy from food for the cell to use.
A group of similar cells that work together to do one job.
Tissue that covers body surfaces and lines organs and cavities.
A structure made of two or more tissue types that work together on a job.
Instructions
Materials Needed
- Zip-top bag
- Clear hair gel
- Grape or small ball
- Dried beans
- Sticky notes
Name the four basic tissue types and give a job for each.
What is the job of the nucleus, and what is the job of the mitochondria?
Why can the stomach be called an organ and not just a tissue?
Which tissue type is blood? Explain why this may seem surprising.
A muscle cell has many more mitochondria than a skin cell. Predict why.
Teaching Tip
Students often call any layer of cells a tissue and any body part an organ. Have them name the specific tissues inside one organ, such as the stomach, to make the difference between the levels concrete.
Go Deeper
Research one specialized cell, such as a neuron, a red blood cell, or a muscle cell, and explain how its shape fits its function.
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