1. Lesson Overview
7.4 Homeostasis and Cells

2. The Cell as an Organism
Multicellular and unicellular organisms must achieve homeostasis, relatively constant internal physical and chemical conditions.

3. The Cell as an Organism
In terms of their numbers, unicellular organisms dominate life on Earth.
Unicellular organisms include both prokaryotes and eukaryotes.
Prokaryotes, especially bacteria, are remarkably adaptable and live almost everywhere—in the soil, on leaves, in the ocean, in the air, and even within the human body.
Many eukaryotes also spend their lives as single cells.
Some types of algae, which contain chloroplasts and are found in oceans, lakes, and streams around the world, are single celled.
Yeasts, or unicellular fungi, are also widespread. Yeasts play an important role in breaking down complex nutrients, which makes them available for other organisms.
Whether a prokaryote or a eukaryote, homeostasis is an issue for each unicellular organism.
Every unicellular organism needs to find sources of energy or food, to keep concentrations of water and minerals within certain levels, and to respond quickly to changes in its environment.

4. Multicellular Life
The cells of multicellular organisms are interdependent.
Cells in a multicellular organism become specialized for particular tasks and communicate with one another in order to maintain homeostasis.
and like the members of a successful baseball team, they work together.
In baseball, players take on a particular role, such as pitcher, catcher, infielder, or outfielder. Messages and signals are sent and understood by teammates and coaches to play the game effectively.
The cells of multicellular organisms are specialized, with different cell types playing different roles.
Some cells are specialized to move, others to react to the environment, and still others to produce substances that the organism needs.
No matter what the role, each specialized cell contributes to the overall homeostasis of the organism.
Pollen grains are highly specialized cells that are tiny and light, with thick cell walls to protect the cell’s contents.
Pine pollen grains have two tiny wings that enable the slightest breeze to carry them great distances.
Particles of dust, smoke, and bacteria are part of even the cleanest air.
Specialized animal cells act like street sweepers to keep the particles out of the lungs.
These cells are full of mitochondria, which provide a steady supply of the ATP that powers the cilia on their upper surfaces.

5. Levels of Organization
The specialized cells of multicellular organisms are organized into tissues, then into organs, and finally into organ systems.

6. Cell Membrane Head Tail Carbohydrate chain Phospholipids Cholesterol
Protein Channel

7. Cellular Communication
Cells use chemical signals to control the rate of activity and action of the cell.
Allows the cell to hold together with other cells and send signals to one another.
Cells in a large organism communicate by means of chemical signals that are passed from one cell to another.
These cellular signals can speed up or slow down the activities of the cells that receive them, and can cause a cell to change what it is doing.
Some cells form connections, or cellular junctions, to neighboring cells.
Some junctions hold cells firmly together.
Other junctions allow small molecules carrying chemical messages to pass directly from one cell to the next.
To respond to one of these chemical signals, a cell must have a receptor to which the signaling molecule can bind. Sometimes these receptors are on the cell membrane, although the receptors for certain types of signals are inside the cytoplasm.
The chemical signals sent by various types of cells can cause important changes in cellular activity. For example, such junctions enable the cells of the heart muscle to contract in a coordinated fashion.