members of the connexin gene family in humans. Each can, if made in a population of cells, generate complete, working gap junction channels of 12 identical connexins. However, not all connexin isoforms are compatible. A cell with connexin 43 cannot form gap junctions with another cell that makes connexin 50 (Figure 2.7). However, a connexon made from connexin 43 is able to form a good working channel if it meets a connexon made from connexin 46, and a connexon made from connexin 46 is in turn compatible with connexin 50.
Anchoring junctions bind cells tightly together and are found in tissues such as the skin and heart that are subjected to mechanical stress. They are described later (page 229).
Answer to thought question: This can only be speculation, but one obvious effect of cells having incompatible connexin isoforms is that the intracellular route for cell–cell communication is lost. Consider a tissue whose cells make connexin 43 that lies adjacent to another tissue whose cells use connexin 50. The cells of each tissue can coordinate their activity by passing signals via gap junctions, for example by intracellular messengers (page 166) or as voltage changes. However, the signals will remain private to each tissue and will not be shared with the neighboring tissue. The two tissues can still communicate when necessary, for instance by transmitter chemicals that the cells release into the extracellular medium (see Chapters 10 and 11).
SUMMARY
1 Membranes are made of phospholipids, protein, and cholesterol.
2 Cells are bounded by membranes, while cell functions are compartmentalized into membrane‐bound organelles.
3 Solutes with a significant solubility in hydrophobic solvents can pass across biological membranes by simple diffusion. Charged molecules cannot.
4 The nucleus and mitochondrion are bounded by double‐membrane envelopes. In the case of the nucleus, this is perforated by nuclear pores. Both organelles contain DNA.
5 Mitochondria produce the energy currency ATP.
6 Peroxisomes carry out a number of reactions, including the destruction of hydrogen peroxide.
7 The ER is a major site of protein synthesis. Cell stimulation can often cause calcium ions stored in the ER to be released into the cytosol.
8 The Golgi apparatus is concerned with the modification of proteins after they have been synthesized.
9 Lysosomes contain powerful degradative enzymes that degrade material.
10 Organelles form membrane contact sites with other organelles and the plasma membrane to allow information flow.
11 Tight junctions prevent the passage of extracellular water or solute between the cells of an epithelium.
12 Gap junctions allow solute and electrical current to pass from the cytosol of one cell to the cytosol of its neighbor.
13 Anchoring junctions form a strong physical link between cells.
FURTHER READING
1 Balda, M. S. & Matter, K. (2008). Tight junctions at a glance. Journal of Cell Science 121: 3677–3682.
2 Duchen, M. R. (2004). Mitochondria in health and disease: perspectives on a new mitochondrial biology. Molecular Aspects of Medicine 25: 365–451.
3 Dundr, M. & Misteli, T. (2001). Functional architecture in the cell nucleus. Biochemical Journal 356: 297–310.
4 Gall, J. G. & McIntosh, J. R. (2001). Landmark Papers in Cell Biology. New York: Cold Spring Harbor Laboratory Press. 544 pp.
5 Lamond, A. I. & Earnshaw, W. C. (1998). Structure and function in the nucleus. Science 280: 547–553.
6 Platt, F. M. et al. (2018). Lysosomal storage diseases. Nature Reviews Disease Primers 4 (27).
7 Short, B. & Barr, F. A. (2000). The Golgi apparatus. Current Biology 10: R583–585.
8 van der Klei, I. & Veenhuis, M. (2002). Peroxisomes: flexible and dynamic organelles. Current Opinion in Cell Biology 14: 500–505.
9 Wu, H, Carvalho, P, & Voeltz, G. K. 2018). Here, there, and everywhere: the importance of ER membrane contact sites. Science 361 (6401): eaan5835.
1 2.1 Theme: Membranesthe membrane or set of membranes surrounding the ERthe membrane or set of membranes surrounding the Golgi apparatusthe membrane or set of membranes surrounding the lysosomethe membrane or set of membranes surrounding the mitochondrionthe membrane or set of membranes surrounding the peroxisomethe plasma membraneFrom the above list, select the membrane that best fits the descriptions below.a double layer comprising an inner and an outer membraneis continuous with a membrane surrounding the nucleusforms a set of flattened sacks called cisternaeoften contains connexons as integral membrane proteinssurrounds a space containing calcium ions at a much higher concentration than in the cytosol. This store of calcium ions can be released into the cytosol upon cell stimulation. (Other such releasable calcium stores may exist in cells, but this is the largest.)
2 2.2 Theme: Organelles in eukaryotic cells endoplasmic reticulumGolgi apparatuslysosomemitochondrionnucleusperoxisomeFrom the above list of organelles, select the organelle described by each of the descriptions below.a major site of protein synthesiscontains many powerful digestive enzymescontains small circular chromosomescontains the enzyme catalasefilled with chromatinmade up of flattened sacks called cisternaemost of the cell's ATP is made hereusually found at the cell center
3 2.3 Theme: Transport across membranescan move from the cytosol of one cell to the cytosol of a neighboring cell, crossing the lipid bilayer component of each cell's plasma membrane as it does socannot cross lipid bilayers, but can move from the cytosol of one cell to the cytosol of a neighboring cell via gap junctionscannot move to a neighboring cell either by crossing the lipid bilayer component of the plasma membrane or via gap junctionsAbove we list three different possible constraints on the movement of a cytosolic solute. For each of the molecules below, state which of the three conditions apply.an RNA molecule of Mr = 10 000. As we will describe later, in Chapter 5, RNA molecules bear many negative chargesinositol trisphosphate, a small charged molecule of Mr = 649K+ (atomic weight = 39)nitric oxide (NO) (Mr = 30)
Review question covering chapters 1 and 2: Some basic components of the eukaryotic cell
Identify each of the cellular components below from the figure above.
1 cytosol
2 internal membranes
3 mitochondrion
4 nucleus
5 plasma membrane
Why might it be useful for the genome to code for many connexin isoforms, some of which are incompatible with each other?
SECTION 2
THE MOLECULAR BIOLOGY OF THE