1. Cell StructureSection 2 Key Ideas What does the cytoskeleton do? How does DNA direct activity in the cytoplasm? What organelles are involved in protein production? What are vesicles and vacuoles? How does the cell get energy?

2. Cell StructureSection 2 The Framework of the Cell Eukaryotic cells have an intricate network of protein fibers called the cytoskeleton which provides the interior framework of the cell. The cytoskeleton helps the cell move, keep its shape, and organize its parts. There are three different kinds of cytoskeleton fibers: microfilaments, microtubules, and intermediate fibers.

3. Cell StructureSection 2 Cytoskeleton A B C D Click to animate the image.

4. Cell StructureSection 2 Directing Cellular Activity DNA contains instructions for making proteins which control most of the activity of the cell. The DNA of eukaryotic cells is stored in the nucleus. DNA instructions are copied as RNA messages, which leave the nucleus. In the cytoplasm, ribosomes use the RNA messages to assemble proteins.

5. Cell StructureSection 2 Directing Cellular Activity, continued Nucleus A double membrane called the nuclear envelope surrounds the nucleus. Nuclear pores located on the nuclear envelope act as channels to allow certain molecules to move in and out of the nucleus. The nucleolus is a structure within the nucleus where ribosome parts are made. These ribosome parts are transported out of the nucleus into the cytoplasm where they are assembled to form a complete ribosome.

6. Cell StructureSection 2 The Nucleus A B C D Click to animate the image.

7. Cell StructureSection 2 Directing Cellular Activity, continued Ribosomes Each ribosome in a cell is made of RNA and many different proteins. Ribosomes that are suspended in the cytosol are called “free” ribosomes. Free ribosomes make proteins that remain inside the cell.

8. Cell StructureSection 2 Directing Cellular Activity, continued Ribosomes Ribosomes that are attached to the membrane of another organelle are called “bound” ribosomes. Bound ribosomes make proteins that are exported from the cell. Ribosomes can switch between being bound or free, depending on what proteins the cell needs to make.

9. Cell StructureSection 2 Protein Processing Some proteins that a cell manufactures are needed outside the cell that makes them. Proteins that are sent outside the cell are packaged in vesicles. Vesicles are small, membrane-bound sacs that enclose the proteins and keep them separate from the rest of the cytoplasm. The endoplasmic reticulum and Golgi apparatus are organelles involved in preparing proteins for extracellular export.

10. Cell StructureSection 2 Protein Processing, continued Endoplasmic Reticulum The endoplasmic reticulum, or ER, is an extensive system of internal membranes that moves proteins and other substances through the cell. The membranes of the ER are connected to the outer membrane of the nuclear envelope. The endoplasmic reticulum is divided into two portions: rough ER and smooth ER.

11. Cell StructureSection 2 Endoplasmic Reticulum (ER)

12. Cell StructureSection 2 Protein Processing, continued Endoplasmic Reticulum The portion of the ER with attached ribosomes is called rough ER because it has a rough appearance when viewed with an electron microscope. The portion of the ER with no attached ribosomes is called smooth ER because it has a smooth appearance when viewed with an electron microscope. The ribosomes on the rough ER make proteins that are packaged into vesicles. Enzymes of the smooth ER make lipids and break down toxic substances.

13. Cell StructureSection 2 Protein Processing, continued Golgi Apparatus The Golgi apparatus is a set of flattened, membrane- bound sacs. The Golgi apparatus helps modify, sort, and package cell products for distribution.

14. Cell StructureSection 2 Golgi apparatus

15. Cell StructureSection 2 Protein Processing, continued Making and Exporting Proteins The ribosomes located on the rough ER make proteins which then cross into the membranes of the ER. The ER membrane then pinches off and forms a vesicle around the proteins. Vesicles transport the proteins from the rough ER to the Golgi apparatus, where they are modified by enzymes and repackaged in new vesicles. These new vesicles transport the modified proteins to the cell membrane to be released outside the cell.

16. Cell StructureSection 2

17. Cell StructureSection 2 Storage and Maintenance Lysosomes Vesicles help maintain homeostasis by storing and releasing a variety of substances as the cell needs them. A lysosome is a vesicle produced by the Golgi apparatus that contains enzymes that break down large molecules. Lysosomes recycle old or damaged organelles and digest food particles to provide nutrients for the cell.

18. Cell StructureSection 2 Storage and Maintenance, continued Vacuoles A vacuole is a fluid-filled vesicle found in the cytoplasm of many plant cells. Plant cells contain a large compartment called the central vacuole, which stores water, ions, nutrients, and wastes. When water fills the central vacuole, the cell becomes rigid, allowing the plant to stand up. When the vacuole loses water, the cell shrinks, and the plant wilts.

19. Cell StructureSection 2 Storage and Maintenance, continued Other Vacuoles Some protists have contractile vacuoles which pump excess water out of the cell in order to control the concentration of salts and other substances. A food vacuole is another type of vacuole. It is formed when the cell membrane surrounds food particles outside the cell and pinches off to form a vesicle inside the cell.

20. Cell StructureSection 2 Energy Production Cells need a constant source of energy. The energy for cellular functions is produced by chemical reactions that occur in the mitochondria and chloroplasts. In both organelles, chemical reactions produce adenosine triphosphate (ATP), the form of energy that fuels almost all cell processes.

21. Cell StructureSection 2 Energy Production, continued Chloroplasts A chloroplast is an organelle found in plant and algae cells that uses light energy to make carbohydrates from carbon dioxide and water. Chloroplasts are surrounded by two membranes and have several stacks of flattened sacs where energy production takes place. Plant cells may have several chloroplasts.

22. Cell StructureSection 2 Energy Production, continued Mitochondria Mitochondria are cell organelles that use energy from organic compounds to make ATP. Most of the ATP needed by a cell is produced inside mitochondria. Both animal and plant cells contain mitochondria. A smooth outer membrane and a folded inner membrane surround a mitochondrion. ATP is produced by enzymes on the folds of the inner membrane.

23. Cell StructureSection 2 Mitochondrion

24. Cell StructureSection 2 Summary The cytoskeleton helps the cell move, keep its shape, and organize its parts DNA instructions are copied as RNA messages, which leave the nucleus. In the cytoplasm, ribosomes use the RNA messages to assemble proteins. The endoplasmic reticulum and Golgi apparatus are organelles involved in preparing proteins for extracellular export.

25. Cell StructureSection 2 Summary, continued Vesicles help maintain homeostasis by storing and releasing a variety of substances as the cell needs them. The energy for cellular functions is produced by chemical reactions that occur in the mitochondria and chloroplasts.