1. Cells: The Working Units of Life4
Cells: The Working Units 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. 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.
See Chapters 2 and 3

5. 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
VIDEO 4.1 Amoeba with visible organelles
VIDEO 4.2 Newt epithelial cells
VIDEO 4.3 Cell Visualization: Membranes, hormones, and receptors

6. Concept 4.5 Extracellular Structures Allow Cells to Communicate with the External Environment
Plant cell wall—semi-rigid structure outside the plasma membrane The fibrous component is the polysaccharide cellulose. The gel-like matrix contains cross-linked polysaccharides and proteins.
See Figure 2.10

7. Figure 4.15 The Plant Cell Wall

8. The plant cell wall has three major roles:
Concept 4.5 Extracellular Structures Allow Cells to Communicate with the External Environment
The plant cell wall has three major roles:
Provides support for the cell and limits volume by remaining rigid
Acts as a barrier to infection
Contributes to form during growth and development

9. Concept 4.5 Extracellular Structures Allow Cells to Communicate with the External Environment
Adjacent plant cells are connected by plasma membrane-lined channels called plasmodesmata. These channels allow movement of water, ions, small molecules, hormones, and some RNA and proteins.
VIDEO 4.14 Cell walls and stomatal complexes in Tradescantia
See Figure 4.7

10. Concept 4.5 Extracellular Structures Allow Cells to Communicate with the External Environment
Many animal cells are surrounded by an extracellular matrix. The fibrous component is the protein collagen. The gel-like matrix consists of proteoglycans. A third group of proteins links the collagen and the matrix together.

11. Figure 4.17 Cell Membrane Proteins Interact with the Extracellular Matrix

12. Concept 4.5 Extracellular Structures Allow Cells to Communicate with the External Environment
Cell junctions are specialized structures that protrude from adjacent cells and “glue” them together—seen often in epithelial cells: • Tight junctions • Desmosomes • Gap junctions

13. Concept 4.5 Extracellular Structures Allow Cells to Communicate with the External Environment
Tight junctions prevent substances from moving through spaces between cells. Desmosomes hold cells together but allow materials to move in the matrix. Gap junctions are channels that run between membrane pores in adjacent cells, allowing substances to pass between the cells.

14. Figure 4.18 Junctions Link Animal Cells (Part 1)

15. Figure 4.18 Junctions Link Animal Cells (Part 2)

16. Figure 4.18 Junctions Link Animal Cells (Part 3)

17. Figure 4.18 Junctions Link Animal Cells (Part 4)

18. Chapter 5 Cell Membranes and Signaling
Key Concepts
5.1 Biological Membranes Have a Common Structure and Are Fluid
5.2 Some Substances Can Cross the Membrane by Diffusion
5.3 Some Substances Require Energy to Cross the Membrane

19. Chapter 5 Cell Membranes and Signaling
5.4 Large Molecules Cross the Membrane via Vesicles
5.5 The Membrane Plays a Key Role in a Cell’s Response to Environmental Signals
5.6 Signal Transduction Allows the Cell to Respond to Its Environment

20. Figure 5.1 Membrane Molecular Structure

21. Figure 8.7 The structure of a transmembrane protein

22. Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid
Membranes may differ in lipid composition as there are many types of phospholipids.
Phospholipids may differ in:
Fatty acid chain length
Degree of saturation
Kinds of polar groups present

23. Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid
Two important factors in membrane fluidity:
Lipid composition—types of fatty acids can increase or decrease fluidity
Temperature—membrane fluidity decreases in colder conditions
APPLY THE CONCEPT Biological membranes have a common structure and are fluid
INTERACTIVE TUTORIAL 5.1 Lipid Bilayer: Temperature Effects on Composition

24. Figure 5.2 Rapid Diffusion of Membrane Proteins

25. Concept 5.2 Some Substances Can Cross the Membrane by Diffusion
Biological membranes allow some substances, and not others, to pass. This is known as selective permeability.
Two processes of transport:
Passive transport does not require metabolic energy.
Active transport requires input of metabolic energy.

26. Concept 5.2 Some Substances Can Cross the Membrane by Diffusion
Osmosis is the diffusion of water across membranes.
It depends on the concentration of solute molecules on either side of the membrane.
Water passes through special membrane channels.

27. Concept 5.2 Some Substances Can Cross the Membrane by Diffusion
When comparing two solutions separated by a membrane:
A hypertonic solution has a higher solute concentration.
Isotonic solutions have equal solute concentrations.
A hypotonic solution has a lower solute concentration.

28. Figure Osmosis

29. Figure 5.3A Osmosis Can Modify the Shapes of Cells

30. Figure 5.3B Osmosis Can Modify the Shapes of Cells

31. Figure 5.3C Osmosis Can Modify the Shapes of Cells

32. Figure 8.12 The water balance of living cells

33. Figure 36.3 Water potential and water movement: a mechanical model

34. Figure 36.4 Water relations of plant cells

35. Concept 5.2 Some Substances Can Cross the Membrane by Diffusion
The concentration of solutes in the environment determines the direction of osmosis in all animal cells.
In other organisms, cell walls limit the volume that can be taken up.
Turgor pressure is the internal pressure against the cell wall—as it builds up, it prevents more water from entering.

36. Concept 5.2 Some Substances Can Cross the Membrane by Diffusion
Diffusion may be aided by channel proteins.
Channel proteins are integral membrane proteins that form channels across the membrane.
Substances can also bind to carrier proteins to speed up diffusion.
Both are forms of facilitated diffusion.

37. Concept 5.2 Some Substances Can Cross the Membrane by Diffusion
Ion channels are a type of channel protein—most are gated, and can be opened or closed to ion passage.
A gated channel opens when a stimulus causes the channel to change shape.
The stimulus may be a ligand, a chemical signal.

38. Concept 5.2 Some Substances Can Cross the Membrane by Diffusion
A ligand-gated channel responds to its ligand.
A voltage-gated channel opens or closes in response to a change in the voltage across the membrane.

39. Figure 5.4 A Ligand-Gated Channel Protein Opens in Response to a Stimulus

40. Figure 5.5 Aquaporins Increase Membrane Permeability to Water (Part 1)

41. Figure 5.6 A Carrier Protein Facilitates Diffusion (Part 1)

42. Concept 5.2 Some Substances Can Cross the Membrane by Diffusion
Transport by carrier proteins differs from simple diffusion, though both are driven by the concentration gradient.
The facilitated diffusion system can become saturated—when all of the carrier molecules are bound, the rate of diffusion reaches its maximum.
ANIMATED TUTORIAL 5.1 Passive Transport

43. Table 5.1 Membrane Transport Mechanisms

44. Concept 5.3 Some Substances Require Energy to Cross the Membrane
Two types of active transport:
Primary active transport involves hydrolysis of ATP for energy.
Secondary active transport uses the energy from an ion concentration gradient, or an electrical gradient.

45. Concept 5.3 Some Substances Require Energy to Cross the Membrane
The sodium–potassium (Na+–K+) pump is an integral membrane protein that pumps Na+ out of a cell and K+ in.
One molecule of ATP moves two K+ and three Na+ ions.

46. Figure 5.7 Primary Active Transport: The Sodium–Potassium Pump

47. Concept 5.3 Some Substances Require Energy to Cross the Membrane
Secondary active transport uses energy that is “regained,” by letting ions move across the membrane with their concentration gradients.
Secondary active transport may begin with passive diffusion of a few ions, or may involve a carrier protein that transports both a substance and ions.
ANIMATED TUTORIAL 5.2 Active Transport

48. Figure 8.16 Review: passive and active transport compared

49. Figure 5.8 Endocytosis and Exocytosis (Part 1)

50. Figure 5.8 Endocytosis and Exocytosis (Part 2)

51. Concept 5.4 Large Molecules Cross the Membrane via Vesicles
In phagocytosis (“cellular eating”), part of the membrane engulfs a large particle or cell.
A food vacuole (phagosome) forms and usually fuses with a lysosome, where contents are digested.
LINK Review the discussion of phagocytosis in Concept 4.3

52. Concept 5.4 Large Molecules Cross the Membrane via Vesicles
In pinocytosis (“cellular drinking”), vesicles also form.
The vesicles are smaller and bring in fluids and dissolved substances, as in the endothelium near blood vessels.
VIDEO 5.2 Pinocytosis and membrane ruffling in a mouse epithelial cell

53. Figure 5.9 Receptor-Mediated Endocytosis (Part 1)

54. Figure 5.9 Receptor-Mediated Endocytosis (Part 2)

55. Concept 5.4 Large Molecules Cross the Membrane via Vesicles
Exocytosis moves materials out of the cell in vesicles.
The vesicle membrane fuses with the plasma membrane and the contents are released into the cellular environment.
Exocytosis is important in the secretion of substances made in the cell.
ANIMATED TUTORIAL 5.3 Endocytosis and Exocytosis
VIDEO 5.4 Exocytosis of coccoliths in a marine golden alga, Pleurochrysis