2. Nitrogen-containing group
Fatty acid tail
Nitrogen-containing group
Phosphate group
Figure 04.UN00a
Title:
A phospholipid
Caption:
A phospholipid.
斥水端
親水端

3. Figure 04.UN00b
Title:
A phospholipid
Caption:
A phospholipid bilayer.

4. Fluid mosaic model Figure 04.1 Title:
The plasma membrane is a fluid mosaic
Caption:
The plasma membrane is a bilayer of phospholipids in which various proteins are embedded. Many proteins have carbohydrates attached to them, forming glycoproteins. The wide variety of membrane proteins fall mostly into three categories: transport proteins, receptor proteins, and recognition proteins.
Fluid mosaic model

6. Diffusion Figure 04.2 Title: Diffusion of a dye in water Caption:

7. Figure 04.3a
Title:
Diffusion through the plasma membrane
Caption:
Simple diffusion through the phospholipid bilayer: Gases such as oxygen and carbon dioxide and lipid-soluble molecules can diffuse directly through the phospholipids.

11. Figure 04.3b
Title:
Diffusion through the plasma membrane
Caption:
Facilitated diffusion through a channel: Some molecules cannot pass through the bilayer on their own. Protein channels (pores) allow some water-soluble molecules, principally ions, to enter or exit the cell.

12. Figure 04.3c
Title:
Diffusion through the plasma membrane
Caption:
Facilitated diffusion through a carrier: Carrier proteins may bind a specific molecule and, as a result, change their own shape, passing the molecule through the middle of the protein to the other side of the membrane.

13. Figure 04.4
Title:
Osmosis
Caption:
(a) Membrane pores allow “free” water molecules to pass through, but sugar molecules are too large. “Bound” water molecules, attracted to the sugars by hydrogen bonds, are also kept from passing through the pore. (b) A membrane is differentially permeable to free water molecules (white dots) but not to larger molecules such as sugar (yellow hexagons) or water molecules held to the sugars by hydrogen bonds. If a bag made of such a membrane is filled with a sugar solution and suspended in pure water, free water molecules will diffuse down their concentration gradient from the high concentration of water outside the bag to the lower concentration of water inside the bag. The bag swells up as water enters. If the bag is weak enough, the increasing water pressure will cause it to burst.

14. Figure 04.4a
Title:
Osmosis
Caption:
Membrane pores allow “free” water molecules to pass through, but sugar molecules are too large. “Bound” water molecules, attracted to the sugars by hydrogen bonds, are also kept from passing through the pore.

15. Figure 04.4b
Title:
Osmosis
Caption:
A membrane is differentially permeable to free water molecules (white dots) but not to larger molecules such as sugar (yellow hexagons) or water molecules held to the sugars by hydrogen bonds. If a bag made of such a membrane is filled with a sugar solution and suspended in pure water, free water molecules will diffuse down their concentration gradient from the high concentration of water outside the bag to the lower concentration of water inside the bag. The bag swells up as water enters. If the bag is weak enough, the increasing water pressure will cause it to burst.

16. Figure 04.5
Title:
The effects of osmosis
Caption:
Red blood cells are normally suspended in the fluid environment of the blood and cannot regulate water flow across their plasma membranes. (a) If red blood cells are immersed in an isotonic salt solution, which has the same concentration of dissolved substances as the blood cells do, there is no net movement of water across the plasma membrane. The red blood cells keep their characteristic dimpled disk shape. (b) A hypertonic solution, with too much salt, causes water to leave the cells, shriveling them up. (c) A hypotonic solution, with less salt than is in the cells, causes water to enter, and the cells swell.

17. Figure 04.5a
Title:
Red blood cells in an isotonic salt solution
Caption:
(a) If red blood cells are immersed in an isotonic salt solution, which has the same concentration of dissolved substances as the blood cells do, there is no net movement of water across the plasma membrane. The red blood cells keep their characteristic dimpled disk shape.

18. Figure 04.5b
Title:
Red blood cells in a hypertonic salt solution
Caption:
(b) A hypertonic solution, with too much salt, causes water to leave the cells, shriveling them up.

19. Figure 04.5c
Title:
Red blood cells in an hypotonic salt solution
Caption:
(c) A hypotonic solution, with less salt than is in the cells, causes water to enter, and the cells swell.

21. Figure 04.6
Title:
Active transport
Caption:
Active transport uses cellular energy to move molecules across the plasma membrane, often against a concentration gradient. (a) A transport protein (blue) has an ATP binding site and a recognition site for the molecules to be transported, in this case calcium ions (Ca2+). (b) The transport protein binds ATP and Ca2+. (c) Energy from ATP changes the shape of the transport protein and moves the ion across the membrane. (d) The carrier releases the ion and the remnants of the ATP (ADP and P) and resumes its original configuration.

25. Figure 04.7
Title:
Three types of endocytosis
Caption:
(a) Pinocytosis: A dimple in the plasma membrane deepens and eventually pinches off as a fluid-filled vesicle, which contains a random sampling of the extracellular fluid. (b) Receptor-mediated endocytosis: Receptor proteins selectively bind molecules (for example, nutrients) in the extracellular fluid. The receptors migrate along the plasma membrane to dimpling sites (coated pit). The membrane dimples inward, carrying the receptor-captured molecule with it. The end of the coated pit buds off a coated vesicle into the cell’s cytoplasm. The vesicle contains both extracellular fluid and a high concentration of the molecules that bind to the receptors. (c) Phagocytosis: Extensions of the plasma membrane, called pseudopodia, encircle an extracellular particle (for example, food). The ends of the pseudopodia fuse, forming a large vesicle (a food vacuole) containing the engulfed particle.

26. Figure 04.8
Title:
Receptor-mediated endocytosis
Caption:
These electron micrographs illustrate the sequence of events in receptor-mediated endocytosis. (a) This type of endocytosis begins with a shallow depression in the plasma membrane, coated on the inside with a protein (dark, fuzzy substance in the micrographs) and bearing receptor proteins on the outside (not visible). (b,c) The pit deepens and (d) eventually pinches off as a coated vesicle. The protein coating is eventually recycled back to the plasma membrane.

27. Figure 04.9
Title:
Exocytosis
Caption:
Exocytosis is functionally the reverse of endocytosis. The material to be ejected from the cell is encapsulated into a membrane-bound vesicle that moves to the plasma membrane and fuses with it. As the vesicle opens to the outside, its contents leave by diffusion.

28. Figure 04.10a
Title:
Desmosome
Caption:
(a) Cells lining the small intestine are firmly attached to one another by desmosomes. Protein filaments bound to the inside surface of each desmosome extend into the cytoplasm and attach to other filaments inside the cell, strengthening the connection between cells.

29. Figure 04.10b
Title:
Tight junction
Caption:
(b) Leakage between cells of the urinary bladder is prevented by close-fitting tight junctions.

30. Figure 04.11a
Title:
Gap junction
Caption:
(a) Gap junctions, such as those between cells of the liver, contain cell-to-cell channels that interconnect the cytoplasm of adjacent cells.

31. Figure 04.11b
Title:
Plasmodesmata
Caption:
(b) Plant cells are interconnected by plasmodesmata, which pass through openings in the walls of adjacent plant cells.

32. Figure 04.12
Title:
Plant cell walls
Caption:
Each plant cell secretes cellulose and other carbohydrates to form its primary cell wall, just outside the plasma membrane. Many cells may then secrete additional cellulose and other polysaccharides beneath the primary wall, forming a secondary cell wall, which pushes the primary cell wall away from the plasma membrane. The middle lamella separates adjacent plant cells.