4.1 Cells as Life’s Fundamental Unit

2. 4.1 Cells as Life’s Fundamental Unit

3. Cells as Life’s Fundamental Unit • With the possible exception of viruses, every form of life on Earth either is a cell or is composed of cells.

4. Cells as Life’s Fundamental Unit • Cells come into existence only through the activity of other cells.

5. 4.2 Prokaryotic and Eukaryotic Cells

6. Prokaryotic and Eukaryotic Cells • All cells can be classified as prokaryotic or eukaryotic.

7. Prokaryotic and Eukaryotic Cells • Prokaryotic cells either are bacteria or another single-celled life-form called archaea.

8. Prokaryotic and Eukaryotic Cells • Setting bacteria and archaea aside, all other cells are eukaryotic.

9. Prokaryotic and Eukaryotic Cells • Eukaryotic cells have most of their DNA contained in a membrane-lined compartment, called the cell nucleus, whereas prokaryotic cells do not have a nucleus.

10. Prokaryotic and Eukaryotic Cells • Eukaryotic cells tend to be much larger than prokaryotic cells. They have more of the specialized internal structures called organelles than do prokaryotic cells.

11. Prokaryotic and Eukaryotic Cells • Many eukaryotes are multicelled organisms, whereas all prokaryotes are single-celled.

12. Eukaryotic cells Prokaryotic cells DNA spread through much of cell within membrane-bound nucleus Size much smaller much larger Organization often multicellular always single-celled Organelles many types of organelles only one type of organelle Figure 4.2

13. 4.3 The Eukaryotic Cell

14. The Eukaryotic Cell • There are five principal components to the eukaryotic cell: the nucleus, other organelles, the cytosol, the cytoskeleton, and the plasma membrane.

15. The Eukaryotic Cell • Organelles are “tiny organs” within the cell that carry out specialized functions, such as energy transfer and materials recycling.

16. The Animal Cell nuclear pores The smooth endoplasmic reticulum is the site of the production of lipid molecules such as estrogen and testosterone. The nucleus contains the cell’s primary complement of DNA. DNA nuclear envelope nucleolus free ribosomes plasma membrane cytoskeleton Mitochondria are the powerplant organelles that extract energy from food and put it into a form cells can use. lysosome The folds of the rough endoplasmic reticulum form a set of chambers within which proteins are processed. transport vesicle How are cell proteins sorted and shipped, so that they end up at the right location? Partly through the work of the Golgi complex. All the cell’s structures outside the nucleus are immersed in a jelly-like fluid called the cytosol. Composed mostly of water, the cytosol is a location for countless chemical reactions carried out within the cell. Figure 4.4

17. The Eukaryotic Cell • The cytosol is the jelly-like fluid outside the nucleus in which these organelles are immersed. • The cytosol should not be confused with the cytoplasm, which is the region of the cell inside the plasma membrane but outside the nucleus.

18. The Eukaryotic Cell • The cytoskeleton is a network of protein filaments. • It functions in cell structure, cell movement, and the transport of materials within the cell.

19. The Eukaryotic Cell • The plasma membrane is the outer lining of the cell. • A membrane can be defined as the flexible, chemically active outer lining of a cell or of its compartments.

20. The Eukaryotic Cell Components of eukaryotic cells nucleus other organelles cytosol cytoskeleton plasma membrane Figure 4.3

21. 4.4 A Tour of the Animal Cell’s Protein Production Path

22. Tour of the Animal Cell’s Protein Production Path • Information for the construction of proteins is contained in the DNA located in the cell nucleus.

23. The Protein Production Path • This information is copied onto a length of messenger RNA (mRNA) that departs the cell nucleus through its nuclear pores and goes to the sites of protein synthesis, structures called ribosomes, which lie in the cytoplasm.

24. The Protein Production Path • Many ribosomes that receive mRNA chains process only a short stretch of them before migrating to, and then embedding in, one of a series of sacs in a membrane network called the rough endoplasmic reticulum (RER).

25. The Protein Production Path • The polypeptide chains produced by the ribosomal “reading” of the mRNA sequences are dropped from ribosomes into the internal spaces of the RER. • There, the polypeptide chains fold up, thus becoming proteins, and undergo editing.

26. Tour of an Animal Cell Suggested Media Enhancement: Tour of an Animal Cell • To access this animation go to folder C_Animations_and_Video_Filesand open the BioFlix folder.

27. The Protein Production Path • Some ribosomes are not embedded in the RER but instead remain free-standing in the cytosol.

28. The Protein Production Path • Materials move from one structure to another in the cell via the endomembrane system. • Here a piece of membrane, with proteins or other materials inside, can bud off from one organelle, move through the cell, and then fuse with another membrane-lined structure.

29. The Protein Production Path • Membrane-lined structures that carry cellular materials are called transport vesicles.

30. The Protein Production Path • Once protein processing is finished in the rough ER, proteins undergoing processing move, via transport vesicles, to the Golgi complex. • They are processed further and marked for shipment to appropriate cellular locations.

31. The Golgi Complex Golgi complex 1. Transport vesicle from RER fuses with Golgi 2. Protein undergoes more processing in Golgi cisternae cisternal space vesicle Side chains are edited (sugars may be trimmed, phosphate groups added). to cytosol 3. Proteins are sorted and shipped… to plasma membrane for export out of cell Figure 4.9

32. 4.5 Cell Structures Outside the Protein Production Path

33. Cell Structures Outside the Protein Production Path • The smooth endoplasmic reticulum is a network of membranes that functions to synthesize lipids and to detoxify potentially harmful substances.

34. Lysosomes and Cellular Recycling • Lysosomes are organelles that break down worn-out cellular structures or foreign materials that come into the cell. • Once this digestion is completed, the lysosomes return the molecular components of these materials to the cytoplasm for further use.

35. lysosome worn-out organelle digestive enzymes 1. Lysosome fuses with worn-out organelle. 2. Organelle broken down. 5. Usable molecules recycled to make new organelles. 3. Small molecules returned to cytosol. 4. Waste molecules expelled from cell. Figure 4.10

36. Mitochondria and Energy • Mitochondria are organelles that function to extract energy from food and to transform this energy into a chemical form the cell can use, the molecule ATP.

37. Mitochondria and Energy Mitochondrion food oxygen outer membrane inner membrane water carbon dioxide ATP Figure 4.11

38. 4.6 The Cytoskeleton: Internal Scaffolding

39. The Cytoskeleton: Internal Scaffolding • Cells have within them a web of protein strands, called a cytoskeleton.

40. The Cytoskeleton: Internal Scaffolding • The cytoskeleton provides the cell with structure, facilitates the movement of materials inside the cell, and facilitates cell movement.

41. The Cytoskeleton: Internal Scaffolding • There are three principal types of cytoskeleton elements. • Ordered by size, going from smallest to largest in diameter, they are microfilaments, intermediate filaments, and microtubules.

42. (b) Intermediate filaments (c) Microtubules (a) Microfilaments (in red) 7 nm 10 nm 25 nm Main function: changes in cell shape Main function: maintenance of cell shape Main functions: maintenance of cell shape, movement of organelles, cell mobility (cilia and flagella) Figure 4.12

43. Microfilaments • Microfilaments are made of the protein actin.

44. Microfilaments • They help the cell move and capture prey by forming rapidly in the direction of movement and decomposing rapidly at their other end.

45. Microfilaments Figure 4.13

46. Intermediate Filaments • Intermediate filaments provide support and structure to the cell.

47. Microtubules • Microtubules play a structural role in cells and facilitate the movement of materials inside the cell by serving as transport “rails.”

48. Microtubules • Cilia and flagella are extensions of cells composed of microtubules.

49. Cilia • Cilia extend from cells in great numbers, serving to move the cell or to move material around the cell.

50. Flagella • By contrast, one—or at most a few—flagella extend from cells that have them.