Chapter 1

1. Chapter 1 What is a Cell? By Benjamin Lewin

2. 1.1 Introduction • Cells arise only from preexisting cells. • Every cell has genetic information whose expression enables it to produce all its components. • The plasma membrane consists of a lipid bilayer that separates the cell from its environment.

3. 1.2 Life began as a self-replicating structure • The first living cell was a self-replicating entity surrounded by a membrane.

4. 1.3 A prokaryotic cell consists of a single compartment • The plasma membrane of a prokaryote surrounds a single compartment. • The entire compartment has the same aqueous environment. • Genetic material occupies a compact area within the cell. • Bacteria and archaea are both prokaryotes but differ in some structural features.

5. 1.4 Prokaryotes are adapted for growth under many diverse conditions • Prokaryotes adapted to many extreme environmental conditions • This highlights the variations that are possible in constructing living cells.

6. 1.5 A eukaryotic cell contains many membrane-delimited compartments • The plasma membrane of a eukaryotic cell surrounds the cytoplasm.

7. 1.5 A eukaryotic cell contains many membrane-delimited compartments • Within the cytoplasm there are individual compartments, each surrounded by a membrane. • The nucleus is often the largest compartment within the cytoplasm • It contains the genetic material.

8. 1.6 Membranes allow the cytoplasm to maintain compartments with distinct environments • Organelles that are surrounded by membranes can maintain internal milieus that are different from the surrounding cytosol.

9. 1.7The nucleus contains the genetic material and is surrounded by an envelope • The nucleus is the largest organelle in the cell. • It is bounded by an envelope consisting of a double membrane.

10. 1.7The nucleus contains the genetic material and is surrounded by an envelope • Genetic material is concentrated in one part of the nucleus. • Nuclear pores provide the means for transport across the envelope for large molecules to enter or leave the nucleus.

11. 1.8The plasma membrane allows a cell to maintain homeostasis • Hydrophilic molecules cannot pass across a lipid bilayer. • The plasma membrane is more permeable to water than to ions.

12. 1.8 The plasma membrane allows a cell to maintain homeostasis • Osmotic pressure is created by ionic differences between the two sides of a membrane. • The plasma membrane has specific systems for transporting ions and other solutes into or out of the cell.

13. 1.8 The plasma membrane allows a cell to maintain homeostasis • The transport systems allow the cell to maintain a constant internal environment that is different from the external milieu. • Ion channels are proteinaceous structures embedded in membranes. • They allow ions to cross the membrane while remaining in an aqueous environment.

14. 1.9 Cells within cells • Organelles bounded by envelopes probably originated by endosymbiosis of prokaryotic cells.

15. 1.10 DNA is the cellular hereditary material, but there are other forms of hereditary information • DNA carries the genetic information that codes for the sequences of all the proteins of the cell. • Information can also be carried in cellular structures that are inherited.

16. 1.11 Cells require mechanismsto repair damage to DNA • The genetic material is continually damaged by: • environmental forces • errors made by cellular systems • Repair systems to minimize damage to DNA are essential for the survival of all living cells.

17. 1.12 Mitochondria are energy factories • All living cells have a means of converting energy supplied by the environment into the common intermediate of ATP.

18. 1.13 Chloroplasts power plant cells • Plastids are membrane-bounded organelles in plant cells. • They can develop into chloroplasts and other specialized forms.

19. 1.14 Organelles require mechanisms for specific localization of proteins • All organelles import proteins from the cytosol.

20. 1.15 Proteins are transported to and through membranes • Proteins are transported into organelles through receptor complexes embedded in the organelle’s membrane.

21. 1.15 Proteins are transported to and through membranes • Proteins are released into the cytosol after synthesis. • For the endoplasmic reticulum, proteins are transferred into the receptor complex on the ER membrane during synthesis. • Proteins then associate with the nucleus, or an organelle, such as: • Mitochondria • Chloroplasts

22. 1.16 Protein trafficking moves proteins through the ER and Golgi apparatus • All proteins that are localized in the • ER • Golgi apparatus • plasma membrane initially associate with the ER during synthesis. • Proteins are transported from one compartment to another by membranous vesicles.

23. 1.16 Protein trafficking moves proteins through the ER and Golgi apparatus • The vesicles bud from one membrane surface and fuse with the next. • Proteins are transported into the cell from the exterior by vesicular transport in the reverse direction.

24. 1.17 Protein folding and unfolding is an essential feature of all cells • Protein conformation is a consequence of primary sequence. • But often it cannot be achieved by spontaneous folding. • It requires assistance from chaperones.

25. 1.18 The shape of a eukaryotic cell is determined by its cytoskeleton • The eukaryotic cell cytoskeleton is an internal framework of filaments, including: • Microtubules • Actin filaments • Intermediate filaments • It provides an organizing template for many activities, including anchoring organelles in place.

26. 1.19 Localization of cell structures is important • Localization of certain structures at specific positions in a cell may be part of its hereditary information. • Positional effects are important in early development.

27. 1.20 Signal transduction pathways execute predefined responses • Events on the outside of the cell can trigger actions inside the cell by using receptor proteins embedded in the membrane. • A receptor spans the membrane and has domains on both the exterior and interior.

28. 1.20 Signal transduction pathways execute predefined responses • The receptor is activated when a ligand binds to the exterior domain. • Ligand binding causes a change in the structure or function of the interior domain.

29. 1.21 All organisms have cells that can grow and divide • The simplest form of division is shown by some organelles where the membrane is pinched inward.

30. 1.21 All organisms have cells that can grow and divide • Bacteria often divide by growing a rigid septum across the cell as an extension of the cell wall. • During mitosis, eukaryotic cells are extensively reorganized. • They form the specialized structure of the spindle. • It partitions the chromosomes to daughter cells.

31. 1.22 Differentiation creates specialized cell types, including terminally differentiated cells • A multicellular organism consists of many different cell types that are specialized for specific functions.

32. 1.22 Differentiation creates specialized cell types, including terminally differentiated cells • Many differentiated cells have lost the ability to divide and/or to give rise to cells of different types. • Stem cells have the potential to divide to generate the many different types of cells required to make: • an organism • or a tissue of an organism