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 • 4.4 The Cytoskeleton Provides Strength and Movement • 4.5 Extracellular Structures Provide Support and Protection For Cells and Tissues

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

4. 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.

5. 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—all the way back to the evolution of the first living cells.

6. 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 per unit of time. • The surface area of a cell determines the amount of substances that can enter or leave the cell.

7. Figure 4.1 The Scale of Life

8. Figure 4.2 Why Cells Are Small

9. Concept 4.1 Cells Provide Compartments for Biochemical Reactions • As cells grow larger, metabolic activity and need for resources and rate of waste production increases faster than surface area. • Some large cells increase surface area by folds in the cell membrane.

10. Concept 4.1 Cells Provide Compartments for Biochemical Reactions To see 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 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. • Properties of the cell-free extract are the same as those inside the cell. • Cell structures and macromolecules can be separated according to size in a centrifuge.

13. Figure 4.4 Centrifugation

14. Concept 4.1 Cells Provide Compartments for Biochemical Reactions • The cell membrane: • A selectively permeable barrier that allows cells to maintain a stable internal environment (homeostasis) • Important in communication and receiving signals • Often has proteins for binding and adhering to adjacent cells

15. Concept 4.1 Cells Provide Compartments for Biochemical Reactions • Two types of cells: • Prokaryotes have no membrane-enclosed compartments. • Eukaryotes have membrane-enclosed compartments called organelles, such as the nucleus.

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

17. Figure 4.5 A Prokaryotic Cell

18. Concept 4.2 Prokaryotic Cells Do Not Have a Nucleus • Most prokaryotes have a rigid cell wall outside the cell membrane. • Bacterial 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.

19. Concept 4.2 Prokaryotic Cells Do Not Have a Nucleus Some bacteria, including cyanobacteria, have an internal membrane system that contains molecules needed for photosynthesis.

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 cell membrane or outer membrane spins each flagellum and drives the cell.

21. Figure 4.6 Prokaryotic Flagella (Part 1)

22. Figure 4.6 Prokaryotic Flagella (Part 2)

23. Concept 4.2 Prokaryotic Cells Do Not Have a Nucleus Cytoskeleton: Some rod-shaped bacteria have a network of helical actin-like protein structures to help maintain their shape.

24. Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments • Eukaryotic cells have a cell membrane, cytoplasm, and ribosomes, as well as membrane-enclosed compartments called organelles. • Each organelle plays a specific role in the cell.

25. Figure 4.7 Eukaryotic Cells (Part 1)

26. Figure 4.7 Eukaryotic Cells (Part 2)

27. Figure 4.7 Eukaryotic Cells (Part 3)

28. Figure 4.7 Eukaryotic Cells (Part 4)

29. Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments • Ribosomes translate the nucleotide sequence of a messenger RNA molecule into a polypeptide. • They occur in both prokaryotic and eukaryotic cells and consist of one large and one small subunit. • Each subunit consists of ribosomal RNA (rRNA) bound to smaller protein molecules.

30. Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments • 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 prokaryotes, ribosomes float freely in the cytoplasm.

31. Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments • The nucleus is usually the largest organelle: • Location of DNA and DNA replication • Site where DNA is transcribed to RNA • Contains the nucleolus, where assembly of ribosomes from RNA and proteins begins

32. 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. • The outer membrane of the envelope is continuous with the endoplasmic reticulum.

33. 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 cell membrane.

34. Figure 4.8 The Endomembrane System

35. 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)

36. Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments • Rough endoplasmic reticulum (RER) has ribosomes attached to its outer surface. • Newly made proteins enter the RER lumen where they are chemically modified and tagged for delivery to specific locations. • The proteins are transported in vesicles that pinch off from the ER. • All secreted proteins and most membrane proteins pass through the RER.

37. Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments Polypeptides are transported into the RER lumen as they are being synthesized. In the lumen they are folded into their tertiary structures. Many are linked to carbohydrate groups, becoming glycoproteins. Many glycoproteins are important in recognition and interactions between cells.

38. Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments • Smooth endoplasmic reticulum (SER)—more tubular, no ribosomes • Chemically modifies small molecules such as drugs and pesticides • Site of glycogen degradation in animal cells • Site of synthesis of lipids and steroids • Stores calcium ions, which trigger many cell responses

39. Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments • Golgi apparatus: flattened sacs (cisternae) and small membrane-enclosed vesicles. • Receives proteins from the RER and can further modify them • Concentrates, packages, and sorts proteins • Adds carbohydrates to proteins • Site of polysaccharide synthesis for plant cell walls

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

41. Figure 4.8 The Endomembrane System

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

43. Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments • Macromolecules may enter the cell by phagocytosis—part of the cell 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.

44. Figure 4.9 Lysosomes Isolate Digestive Enzymes from the Cytoplasm

45. Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments • Phagocytes are cells specialized to take in materials and break them down. • Autophagy is the programmed destruction of cell components. Cells break down their own materials, and even entire organelles, within lysosomes. • Lysosomal storage diseases occur when lysosomes fail to digest cell components.

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

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

48. 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. • Chloroplast:contains chlorophyll; site of photosynthesis • Photosynthesis converts light energy into chemical energy (anabolic process).

49. Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments Chloroplasts have two membranes, plus internal membranes called thylakoids. Granum—a stack of thylakoids; light energy is converted to chemical energy on these membranes. Stroma—aqueous matrix around grana; contains ribosomes and DNA; carbohydrates are synthesized here.

50. Concept 4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments • Other plastids: • Chromoplasts make and store red, yellow, and orange pigments, especially in flowers and fruits.