Cells: The Working Units of Life

1. 4 Cells: The WorkingUnits of Life

2. Chapter 4 Cells: The Working Units of Life • Key Concepts • 4.1 Cells Provide Compartments for Biochemical Reactions • 4.2 Prokaryotic Cells Do Not Have a Nucleus • 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments

3. Chapter 4 Cells: The Working Units of Life • 4.4 The Cytoskeleton Provides Strength and Movement • 4.5 Extracellular Structures Allow Cells to Communicate with the External Environment

4. Chapter 4 Opening Question What do the characteristics of modern cells indicate about how the first cells originated?

5. Concept 4.1 Cells Provide Compartments for Biochemical Reactions • Cell theory was the first unifying theory of biology. • Cells are the fundamental units of life. • All organisms are composed of cells. • All cells come from preexisting cells.

6. Concept 4.1 Cells Provide Compartments for Biochemical Reactions • Important implications of cell theory: • Studying cell biology is the same as studying life. • Life is continuous.

7. Concept 4.1 Cells Provide Compartments for Biochemical Reactions • Most cells are tiny, in order to maintain a good surface area-to-volume ratio. • The volume of a cell determines its metabolic activity relative to time. • The surface area of a cell determines the number of substances that can enter or leave the cell.

8. Figure 4.1 The Scale of Life

9. Figure 4.2 Why Cells Are Small

10. Concept 4.1 Cells Provide Compartments for Biochemical Reactions • To visualize small cells, there are two types of microscopes: • Light microscopes—use glass lenses and light • Resolution = 0.2 μm • Electron microscopes—electromagnets focus an electron beam • Resolution = 2.0 nm

11. Figure 4.3 Microscopy

12. Concept 4.1 Cells Provide Compartments for Biochemical Reactions • Chemical analysis of cells involves breaking them open to make a cell-free extract. • The composition and chemical reactions of the extract can be examined. • The properties of the cell-free extract are the same as those inside the cell.

13. Figure 4.4 Centrifugation

14. Concept 4.1 Cells Provide Compartments for Biochemical Reactions • The plasma membrane: • Is a selectively permeable barrier that allows cells to maintain a constant internal environment • Is important in communication and receiving signals • Often has proteins for binding and adhering to adjacent cells

15. 4.1 CelConcept 4. Provide Compartments for Biochemical Reactions • Two types of cells: Prokaryotic and eukaryotic • Prokaryotes are without membrane-enclosed compartments. • Eukaryotes have membrane-enclosed compartments called organelles, such as the nucleus.

16. In-Text Art, Ch. 4, p. 59

17. Concept 4.2 Prokaryotic Cells Do Not Have a Nucleus • Prokaryotic cells: • Are enclosed by a plasma membrane • Have DNA located in the nucleoid • Therest of the cytoplasm consists of: • Cytosol (water and dissolved material) and suspended particles • Ribosomes—sites of protein synthesis

18. Figure 4.5 A Prokaryotic Cell

19. Concept 4.2 Prokaryotic Cells Do Not Have a Nucleus • Most prokaryotes have a rigid cell wall outside the plasma membrane. • Bacteria cell walls contain peptidoglycans. • Some bacteria have an additional outer membrane that is very permeable. • Other bacteria have a slimy layer of polysaccharides, called the capsule.

20. Concept 4.2 Prokaryotic Cells Do Not Have a Nucleus • Some prokaryotes swim by means of flagella, made of the protein flagellin. • A motor protein anchored to the plasma or outer membrane spins each flagellum and drives the cell. • Some rod-shaped bacteria have a network of actin-like protein structures to help maintain their shape.

21. Figure 4.6 Prokaryotic Flagella (Part 1)

22. Figure 4.6 Prokaryotic Flagella (Part 2)

23. Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments • Eukaryotic cells have a plasma membrane, cytoplasm, and ribosomes—and also membrane-enclosed compartments called organelles. • Each organelle plays a specific role in cell functioning.

24. Figure 4.7 Eukaryotic Cells (Part 1)

25. Figure 4.7 Eukaryotic Cells (Part 8)

26. Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments • Ribosomes—sites of protein synthesis: • They occur in both prokaryotic and eukaryotic cells and have similar structure—one larger and one smaller subunit. • Each subunit consists of ribosomal RNA (rRNA) bound to smaller protein molecules.

27. Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments • Ribosomes translate the nucelotide sequence of messenger RNA into a polypeptide chain. • Ribosomes are not membrane-bound organelles—in eukaryotes, they are free in the cytoplasm, attached to the endoplasmic reticulum, or inside mitochondria and chloroplasts. • In prokaryotic cells, ribosomes float freely in the cytoplasm.

28. Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments • The nucleus is usually the largest organelle. • It is the location of DNA and of DNA replication. • It is the site where DNA is transcribed to RNA. • It contains the nucleolus, where ribosomes begin to be assembled from RNA and proteins.

29. Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments • The nucleus is surrounded by two membranes that form the nuclear envelope. • Nuclear pores in the envelope control movement of molecules between nucleus and cytoplasm. • In the nucleus, DNA combines with proteins to form chromatin in long, thin threads called chromosomes.

30. Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments • The endomembrane system includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, and lysosomes. • Tiny, membrane-surrounded vesicles shuttle substances between the various components, as well as to the plasma membrane.

31. Figure 4.8 The Endomembrane System

32. Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments • Endoplasmic reticulum (ER)—network of interconnected membranes in the cytoplasm, with a large surface area • Two types of ER: • Rough endoplasmic reticulum (RER) • Smooth endoplasmic reticulum (SER)

33. Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments • Rough endoplasmic reticulum (RER) has ribosomes attached to begin protein synthesis. • Newly made proteins enter the RER lumen. • Once inside, proteins are chemically modified and tagged for delivery. • The RER participates in the transport. • All secreted proteins and most membrane proteins, including glycoproteins, which is important for recognition, pass through the RER.

34. Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments • Smooth endoplasmic reticulum (SER)—more tubular, no ribosomes • It chemically modifies small molecules such as drugs and pesticides. • It is the site of glycogen degradation in animal cells. • It is the site of synthesis of lipids and steroids.

35. Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments • The Golgi apparatus is composed of flattened sacs (cisternae) and small membrane-enclosed vesicles. • Receives proteins from the RER—can further modify them • Concentrates, packages, and sorts proteins • Adds carbohydrates to proteins • Site of polysaccharide synthesis in plant cells

36. Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments • The Golgi apparatus has three regions: • The cis region receives vesicles containing protein from the ER. • At the trans region, vesicles bud off from the Golgi apparatus and travel to the plasma membrane or to lysosomes. • The medial region lies in between the trans and cis regions.

37. Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments • Primary lysosomes originate from the Golgi apparatus. • They contain digestive enzymes, and are the site where macromolecules are hydrolyzed into monomers.

38. Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments • Macromolecules may enter the cell by phagocytosis—part of the plasma membrane encloses the material and a phagosome is formed. • Phagosomes then fuse with primary lysosomes to form secondary lysosomes. • Enzymes in the secondary lysosome hydrolyze the food molecules.

39. Figure 4.9 Lysosomes Isolate Digestive Enzymes from the Cytoplasm (Part 1)

40. Figure 4.9 Lysosomes Isolate Digestive Enzymes from the Cytoplasm (Part 2)

41. Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments • Phagocytes are cells that take materials into the cell and break them down. • Autophagy is the programmed destruction of cell components and lysosomes are where it occurs. • Lysosomal storage diseases occurs when lysosomes fail to digest the components.

42. Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments • In eukaryotes, molecules are first broken down in the cytosol. • The partially digested molecules enter the mitochondria—chemical energy is converted to energy-rich ATP. • Cells that require a lot of energy often have more mitochondria.

43. Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments • Mitochondria have two membranes: • Outer membrane—quite porous • Inner membrane—extensive folds called cristae, to increase surface area • The fluid-filled matrix inside the inner membrane contains enzymes, DNA, and ribosomes.

44. Figure 4.7 Eukaryotic Cells

45. Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments • Plant and algae cells contain plastids that can differentiate into organelles—some are used for storage. • A chloroplast contains chlorophyll and is the site of photosynthesis. • Photosynthesis converts light energy into chemical energy.

46. Figure 4.7 Eukaryotic Cells

47. Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments • Other organelles perform specialized functions. • Peroxisomes collect and break down toxic by-products of metabolism, such as H2O2, using specialized enzymes. • Glyoxysomes, found only in plants, are where lipids are converted to carbohydrates for growth.

48. Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments • A chloroplast is enclosed within two membranes, with a series of internal membranes called thylakoids. • A granum is a stack of thylakoids. • Light energy is converted to chemical energy on the thylakoid membranes. • Carbohydrate synthesis occurs in the stroma—the aqueous fluid surrounding the thylakoids.

49. Figure 4.7 Eukaryotic Cells

50. Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments • Vacuoles occur in some eukaryotes, but mainly in plants and fungi, and have several functions: • Storage of waste products and toxic compounds; some may deter herbivores • Structure for plant cells—water enters the vacuole by osmosis, creating turgor pressure