1. Membranes are thin, flexible sheets of lipid and protein.

2. Most cells also contain a number of internal membranes.

3. The boundaries of plant cells are obvious because their plasma membranes are surrounded by heavy cell walls, made of cellulose and other polymers.

4. The plasma membrane controls passage of material in and out of the cells.

1. The metric units used for measuring cells are a micrometer (µm), which is a thousandth part of a millimeter, or 10-6 meter.

2. A nanometer (nm) is a thousandth of a micrometer, or 10-9 meter.

3. A tenth of a nanometer is an Angstrom unit, or 10-8 meter, used for expressing the dimensions of atoms.

1. Because the ratio of surface to volume decreases as the size of the cube increases, a cell with a large volume could not exchange matter with its surroundings fast enough to support metabolism.

2. Only small cells have a favorable surface/volume ratio.

3. Some large cells effectively increase their surface area by having internal membranes that make connections to the outside.

4. Some cells increase communication between their insides and outsides by actively stirring their contents and move materials around.

1. Human eyes lack sufficient resolving power, which is the ability to see two adjacent points as separate points (Figure 6.5). Higher quality microscopes have a higher power of resolution.

2. Magnification makes an object appear larger, but does not provide any additional structural detail (Figure 6.6).

3. In eyes as in microscopes, light is focused by a lens. The limit of resolution of a lens is the minimum distance, d, between two points that the lens can resolve. This limit depends on the numerical aperture (NA) of the lens (Figure 6.7).

4. Electromagnetic radiation, including visible light, is an oscillating wave with varying wavelength (Figure 6.8).

5. In the visible spectrum, red light has the longest wavelength and violet light the shortest. Infrared and ultraviolet radiation are just beyond the visible limits.

6. If light has wavelength l (lambda), then: limit of resolution = d = 0.61 l/NA.

7. The resolving power of a microscope is limited by the wavelength of the radiation it uses. For a light microscope, the practical limit of resolution is approximately equal to the wavelength of the light it focuses (Figure 6.9).

1. Microscopy relies on creating contrast with stains, reagents that bind strongly to certain cellular materials and make them stand out in contrast to others.

2. Before stains can be applied, the tissue must be preserved with a fixative such as formaldehyde.

3. Fixatives stabilize the macromolecules of cell structure in their natural positions, preserving native structures so they do not deteriorate during staining.

4. Fixatives can also produce unnatural structures called artifacts.

5. Microscopists who are interested in viewing living (unstained, unfixed) specimens often turn to phase contrast microscopes and interference microscopes. These instruments depend upon the fact that even colorless cellular structures bend and interfere with light waves passing through them (Figure 6.10).

1. In 1931, the electron microscope was invented. It uses a beam of electrons with far shorter wavelength than visible light and thus far greater resolving power.

2. Transmission electron microscopy (TEM) employs an electron beam that passes through the specimen and has a theoretical resolving power about 100 times greater than light microscopy. TEM can resolve structures as small as 2—3 nm (Figure 6.11).

3. Electron microscopy not only brought a clearer understanding of larger structures, but also revealed many previously unknown structures such as internal membranes (Figure 6.12).

4. Negative staining is a technique of electron microscopy that shows the shapes of small particles by surrounding them with a puddle of a heavy-metal stain. The portions not covered by the stain stand out in contrast as light areas.

5. Scanning electron microscopes (SEM) move an electron beam over the metal-coated surface of an object and detect the secondary electrons that are knocked out of the specimen. SEMs show three-dimensional forms of surface structures beautifully (Figure 6.13).

1. structures made of internal membranes which may be permanently or temporarily connected to one another,

2. filaments and tubules made of specific proteins that form the cytoskeleton, a complex that holds a cell in shape and makes it move,

3. large complexes of enzymes and other proteins, sometimes including nucleic acids, that perform certain metabolic functions.

1. The nuclear envelope divides a cell into two large compartments. The nucleoplasm inside the nuclear envelope and the cytoplasm outside the nuclear envelope.

2. The cytosol is the aqueous fluid of the cytoplasm outside of membrane-bounded structures.

1. The nuclear envelope forms the boundary of the nucleus with two membranes spaced about 5—15 nm apart, except where they are fused to form a pore or channel.

2. Each pore is about 50 nm in diameter and is surrounded by an octagonal complex of protein granules (Figure 6.18).

3. The nucleolus is the site in the nucleus where the ribonucleic acid (RNA) is synthesized (Figure 6.19).

1. The most prominent part of the system is the endoplasmic reticulum (ER), which is an extensive series of membranes called smooth ER or rough ER depending on their appearance in electron micrographs.

2. Rough ER is covered with ribosomes and smooth ER has no ribosomes attached.

3. Rough ER is a stack of membranes that lie parallel to one another and to the nuclear envelope. The ER membranes separate a space within themselves, the lumen, from the surrounding cytoplasm (Figure 6.20).

4. Ribosomes attached to the ER membranes on their cytoplasmic face synthesize proteins that pass through the membranes into the ER lumen, where they may be chemically modified.

5. Smooth ER membranes form tubules and other irregular shapes (Figure 6.21). The smooth ER of liver cells contains enzymes that alter and detoxify drugs and other foreign materials.

6. Around 1900, Camillo Golgi described structures that readily took up a silver stain in the nerve cells of barn owls. These structures came to be called the Golgi complex.

7. The Golgi complex is a stack of distinctive, flattened membranous sacs connected to branching tubules, and associated with ER membranes (Figure 6.2).

8. Golgi complexes are the packaging, sorting, and exporting centers of the cell.

1. Vesicles are small, rounded sacs that are bounded by single membranes. Some vesicles transport substances from one compartment to another, such as from the ER to the Golgi. Other vesicles contain proteins or substances that have been packaged in the Golgi for export from the cell.

2. Animal cells contain distinctive vesicles called lysosomes that are packed with many hydrolytic enzymes.

3. Peroxisomes are small vesicles containing specific enzymes, including oxidases that break down amino acids and fats.

4. Plant cells often have a large, prominent central vacuole whose fluid contents, called cell sap, is quite different from the surrounding cytoplasm.

5. The membrane of the vacuole is called the tonoplast. Proteins embedded in the tonoplast regulate the passage of ions and molecules in and out of the vacuole.

1. Mitochondria are the organelles that carry out the principal chemical reactions in which energy is extracted from food (Figure 6.23).

2. Mitochondria are made of two membranes. The inner membrane divides the organelle into two internal compartments and is thrown into many crosswise folds called cristae.

3. The inner membrane contains many of the enzymes and other proteins that carry out the reactions of energy metabolism. The cristae greatly increase the surface area of the inner membrane, thereby increasing the rate at which metabolism can be carried out.

4. Chloroplasts are organelles found in eucaryotes that conduct photosynthesis. Chloroplasts capture light energy and store it in chemical forms.

5. The thylakoid membrane (one of three membranes found in chloroplasts) contains chlorophyll and many of the proteins that carry out photosynthesis.

6. Chloroplasts are one of a variety of small plant organelles called plastids (Figure 6.24).

1. The glycocalyx is made of oligosaccharides that stick to one another. The glycocalyx helps bind neighboring cells together.

2. The glycocalyx also serves as identification between cells, enabling them to recognize one another and interact appropriately.

1. Desmosomes are one group of adhering junctions that hold adjacent cells together like spot welds (Figure 6.28).

2. Half-desmosomes form in adjacent cells and the two halves join across the intercellular space through a complex of fibrils and granules.

3. Spot desmosomes are found inside each cell. They are interconnected by part of the cytoskeleton and probably help distribute stresses applied to a part of the tissue through a series of linked cells to the tissue as a whole.

4. Tight junctions, another type of adhering junction, are marked by a network of extraordinary ridgelike structures, where the membranes of the two cells are held intimately together by connecting proteins.

5. Where tight junctions are present, materials must pass through the cells, and cannot go around them in the extracellular fluid.

1. Gap junctions are prime examples of communicating junctions. Gap junctions are made of protein pores that span the membranes of adjacent cells.

2. Molecules smaller than about 1000 daltons can flow through gap junctions.

3. Gap junctions convey ions that stimulate contraction in heart muscle cells, thereby causing the heart to contract in a coordinated way.

4. The channels in gap junctions are controlled by the concentration of calcium ions around them.

5. Synapses are regions where cells can send signals to one another.

6. Gap junctions are also called electrical synapses. Gap junctions have very low electrical resistance and provide conduits for electrical communication between cells.

7. Chemical synapses are used by cells in the nervous system to transmit information to other cells by means of molecules called neurotransmitters.

1. These communicating junctions are channels left at the time of cell division and wall formation.

2. Plasmodesmata serve as conduits for the exchange of molecules and ions among cells.

1. Most bacterial cells have rigid walls just outside a plasma membrane.

2. Bacterial cytoplasm is rich in free-floating ribosomes concentrated in the outer regions of the cell.

3. The center of bacterial cells is largely occupied by the nucleoid or nucleoids.

4. In some bacteria, the plasma membrane has mesosomes that extend into the interior of the cell and connect to the nucleoid (Figure 6.30). Mesosomes separate the nucleoids of a dividing cell equally into the daughter cells.