1. The Discovery of Cells
Microscope observations of organisms led to the discovery of the basic characteristics common to all living things.
Scientists first discovered cells in the 1600s using crude microscopes.
Observations made by scientists using more powerful microscopes in the 1800s led to the formation of the cell theory.

2. Looking at Cells Cells vary greatly in their size and shape.
A cell’s shape reflects its function. Cell size is limited by a cell’s surface area-to-volume ratio.
Cells can be branched, flat, round, or rectangular.

3. Looking at Cells, continued
All substances that enter or leave a cell must cross the surface of the cell.
A cell’s ability to move substances across its surface can be estimated by finding its surface area-to-volume ratio.
Cells with greater surface area-to-volume ratios can exchange substances more efficiently.

4. Relationship between Surface Area and Volume

5. Looking at Cells, continued
When comparing cells of the same shape, small cells have greater surface area-to-volume ratios than large cells.
So, small cells function more efficiently than large cells.

6. Cell Features
All cells share common structural features, including a cell membrane, cytoplasm, ribosomes, and DNA.
The cell membrane is the outer layer that covers a cell’s surface and acts as a barrier between the outside environment and the inside of the cell.
The cytoplasm is the region of the cell within the cell membrane. The cytoplasm includes the fluid inside the cell called the cytosol.

7. Cell Features, continued
A ribosome is a cellular structure that makes proteins.
The DNA of a cell provides instructions for making proteins, regulates cellular activities, and enables cells to reproduce.

8. Features of Prokaryotic and Eukaryotic Cells

9. Cell Features, continued
Features of Prokaryotic Cells
A prokaryote is an organism made of a single prokaryotic cell.
Prokaryotic cells do not have a nucleus or other internal compartments. The genetic material of a prokaryotic cell is a single loop of DNA.
For millions of years, prokaryotes were the only organisms on Earth.

10. Cell Features, continued
Features of Eukaryotic Cells
A eukaryote is an organism made up of one or more eukaryotic cells. All multicellular organisms are made of eukaryotic cells.
The DNA of a eukaryotic cell is found in an internal compartment of the cell called the nucleus.
All eukaryotic cells have membrane-bound organelles. An organelle is a small structure found in the cytoplasm that carries out specific activities inside the cell.

11. Cell Features, continued
Each organelle in a eukaryotic cell performs distinct functions.
The complex organization of eukaryotic cells enables them to carry out more specialized functions than prokaryotic cells.

12. Comparing Prokaryotes and Eukaryotes

13. Summary
Microscope observations of organisms led to the discovery of the basic characteristics common to all living things.
A cell’s shape reflects its function. Cell size is limited by a cell’s surface area-to-volume ratio.
The complex organization of eukaryotic cells enable them to carry out more specialized functions than prokaryotic cells.