1. Homeostasis and Cells
Read the lesson title aloud to students.

2. Learning Objectives
Explain how unicellular organisms maintain homeostasis.
Explain how multicellular organisms maintain homeostasis.
Click to show each of the learning objectives.
Ask: What do you recall about homeostasis?
Answer: Homeostasis is the maintenance of relatively constant internal physical and chemical conditions.
Ask: What are some things a human body must maintain to be balanced and function well?
Answer: body temperature, energy levels, water levels, hormone levels (other answers possible)
Ask: What things do students do to help the body maintain balance?
Answer: eat well, sleep, drink water, wear appropriate clothing for the weather (other answers possible)
Transition to prokaryotes.
Ask: Now imagine your entire body was one single cell. How would you maintain homeostasis?
For the answer, see the next slide.

3. The Cell as an Organism
Tell students: Single-celled organisms, like this freshwater protozoan, must be able to carry out all of the functions necessary for life. To maintain homeostasis, unicellular organisms grow, respond to the environment, transform energy, and reproduce.
Ask: How often during the day do you encounter single-celled organisms?
Answer: Many students may not realize that they come in contact with millions of bacteria every time they brush their teeth, eat a piece of fruit, touch a doorknob, or take a breath. Our bodies are hosts to trillions of microorganisms.
Ask: What pros and cons likely exist for maintaining homeostasis as a unicellular organism?
Answer: Pros: simpler to maintain one cell than many; all processes take place in one location; no need to communicate with other parts; requires fewer resources
Cons: vulnerable to any sudden change in the environment; if something goes wrong, the organism could die because no other cells exist; if necessary resources cannot be found in the immediate surroundings, they may be hard to locate

4. Multicellular Life Player Team
Explain that just as a baseball team is made up of different kinds of players, multicellular organisms are made up of many different kinds of cells.
Click to highlight a cell as “player” and an organism as “team.”
Tell students: Each cell is specialized for different functions. The cells of multicellular organisms become specialized for particular tasks and communicate with one another to maintain homeostasis.
Tell students: The baseball players hold different positions and therefore carry out different functions. However, outside of their uniform number, they are all still people and the positions do not require a certain kind of person. But for cells it is different.
Ask: For a multicellular organism like a person or a tree, do you think all of the cells look pretty much the same, like the baseball players do?
Answer: No. Cells come in many different types, and although each has similar parts internally, their overall structure and functions can vary widely.
Player
Team

5. Specialized Animal Cells
These cells are human
trachea epithelium.
They line the upper air passages and sweep mucus, debris, and bacteria out of
the lungs.
Ask: What main features do you notice about these cells?
Answer: They have cilia hairs. They are tall.
Tell students: These cells also have high concentrations of mitochondria.
Ask: How are these cells specialized for the role they play?
Answer: A trachea epithelium cell has cilia that clear debris. The mitochondria are necessary to provide energy to move the cilia. By being long and slender, the cilia are like brooms sweeping away the unwanted things in the trachea.
Write other types of cells on the board and ask students to draw and explain what specialized shape and features the cells might have. If students need hints, provide the function.
Answers:
Cell types/most likely shape/function:
Fat/store extra energy/shaped like a potato or sac
Red blood cells/travel throughout body/shaped like discs or saucers for easy movement, smooth edges
Muscle cells/long, with the ability to stretch like a rubber band or oar/to move bones
Nerve cells/wire like, very long/sending and receiving messages

6. Specialized Plant Cells
Tell students: This image is of pine pollen. Pollen grains are tiny and light, despite tough walls to protect the cells inside. In addition, pine pollen grains have two tiny wings that enable them to float in the slightest breeze. Pine trees release millions of pollen grains like these to scatter in the wind, land on seed cones, and begin the essential work of starting a new generation.
Ask: Some pollen grains have little barbs all around the outside. What might be the function of this structure?
Answer: to stick to things it lands on
Tell students: Plants have many other specialized cells just like animals.
Ask: What kind of specialized cells might plants need?
Sample answers: Root cells would be specialized to absorb water. Leaf cells would be specialized to carry out photosynthesis and withstand heat and sunlight. Plants also need cells to transport materials up and down the organism.

7. Levels of Organization
Ask: How is a grocery store organized?
Answer: The store is split into departments, each department is broken down into aisles, each aisle is broken down into shelf sections, which are broken down to a single product.
Tell students: A multicellular organism is organized in a similar way.
Click through the levels of organization on the slide.
Click to reveal the “cell” label. Explain that the first level of organization is the cell.
Click to reveal the “tissue” label. Tell students that many cells working together are called tissues.
Click to reveal the “organ” label. Tell students that tissues working together are called organs.
Click to reveal the “organ system”’ label. Explain that an organ system is many organs working together.
Explain that the entire organism is like the whole store.
Cell
Tissue
Organ
Organ system

8. Cellular Communication
Tell students that cells in a large organism communicate by means of chemical signals that are passed from one cell to another. To respond to one of these chemical signals, a cell must have a receptor to which the signaling molecule can bind.
Explain that in biology, a receptor is a structure that responds to a stimulus. In cellular communication, it is a molecule to which a signaling molecule can bind, producing a change that can affect cell activity. In physiology, it is a structure in the nervous system that detects and responds to an environmental stimulus, such as light.
Tell students that some junctions, like the one seen in brown in this micrograph of capillary cells in the gas bladder of a toadfish, hold cells together in tight formations.

9. Review
Group students and have each group develop a series of true/false statements about homeostasis. Have each group read a statement aloud to the class. Ask the rest of the class to decide if the statement is true or false.