A. How We Study Cells-Microscopes
1. Microscopes provide windows to the world of the cell.
a. The early study of cells progressed with the invention of microscopes in the 17th century.
b. In a light microscope visible light passes through the specimen and then through glass lenses.
1. Light microscopes can magnify effectively to about 1,000 times the size of the actual specimen.
2. The minimum resolution of a light microscope is about 2 microns, the size of a small bacterium. (Fig. 7.1) (Textbook Activity 7A)
3. While a light microscope can resolve individual cells, it cannot resolve much of the internal anatomy.
c. To resolve smaller structures we use an electron microscope, which focuses a beam of electrons through the specimen or onto its surface.
1. The resolution of a modern EM is about 2 nm.
2. Transmission electron microscopes (TEM) are used mainly to study the internal ultrastructure of cells. (Fig. 7.2)
a. A TEM aims an electron beam through a thin section of the specimen.
b. The image is focused and magnified by electromagnets.
3. Scanning electron microscopes (SEM) are useful for studying surface structures.
d. Electron microscopes can only be used on dead cells.
e. Light microscopes do not have as high a resolution, but they can be used to study live cells.
B. View of the Cell-2 general cell types
1. Prokaryotic and eukaryotic cells differ in size and complexity.
a. Bacteria are prokaryotic cells; all other cells are eukaryotic.
b. Structures common to all cells
1. All cells are surrounded by a plasma membrane.
2. The semifluid substance within the membrane is the cytosol.
3. All cells contain genes in the form of DNA.
4. All cells also have ribosomes, tiny organelles that make proteins using the instructions contained in genes.
c. Differences between prokaryotic and eukaryotic cells (Textbook Activity 7B)
1. A major difference between prokaryotic and eukaryotic cells is the location of chromosomes. (Fig. 7.4, 7.7)
a. In a eukaryotic cell, chromosomes are contained in a membrane-enclosed organelle, the nucleus.
b. In a prokaryotic cell, the DNA is concentrated in the nucleoid region, without a membrane separating it from the rest of the cell.
2. Within the cytoplasm of a eukaryotic cell is a variety of membrane-bound organelles of specialized form and function.
a. These membrane-bound organelles are absent in prokaryotes.
3. Eukaryotic cells are generally much bigger than prokaryotic cells.
a. Most bacteria are 1-10 microns in diameter.
b. Eukaryotic cells are 10-100 microns in diameter. (Textbook Activity 7C)
d. Metabolic requirements set an upper limit to the size of a single cell.
1. As a cell increases in size its volume increases faster than its surface area. (Fig. 7.5)
2. Smaller objects have a greater ratio of surface area to volume.
3. The plasma membrane functions as a selective barrier that allows the passage of oxygen, nutrients, and wastes for the whole volume of the cell.
4. Rates of chemical exchange may be inadequate to maintain a cell with a very large cytoplasm.
5. The need for a surface sufficiently large to accommodate the volume explains the microscopic size of most cells.
6. Larger organisms do not generally have larger cells than smaller organisms - simply more cells.
C. The Nucleus and Ribosomes
1. The nucleus contains a eukaryotic cell�s genetic library.
a. The nucleus contains most of the genes in a eukaryotic cell.
1. Some genes are located in mitochondria and chloroplasts.
b. The nucleus averages about 5 microns in diameter.
c. The nucleus is separated from the cytoplasm by a double membrane.
d. Nuclear pores allow large macromolecules and particles to pass through. (Fig. 7.7, 7.9)
e. Within the nucleus, the DNA and associated proteins are organized into fibrous material, chromatin. (Fig. 7.7)
f. In the nucleus is a region of densely stained fibers and granules adjoining chromatin, the nucleolus.
1. In the nucleolus, ribosomal components are synthesized and assembled with proteins to form ribosomal subunits.
2. The subunits pass through the nuclear pores to the cytoplasm where they combine to form ribosomes.
2. Ribosomes build a cell�s proteins.
a. Ribosomes are composed of RNA and protein.
b. A ribosome consists of two subunits that combine to carry out protein synthesis. (Fig. 7.10)
c. Some ribosomes, free ribosomes, are suspended in the cytosol and synthesize proteins that function within the cytosol.
d. Other ribosomes, bound ribosomes, are attached to the outside of the endoplasmic reticulum.
1. These synthesize proteins that are either inserted into membranes or are for export from the cell.
D. The Endomembrane System
1. Introduction
a. A eukaryotic cell has extensive and elaborate internal membranes, which partition the cell into compartments. (Fig. 7.7)
b. The general structure of a biological membrane is a double layer of phospholipids with other lipids and diverse proteins.
c. These membranes are either in direct contact or connected via transfer of vesicles, sacs of membrane.
d. The barriers created by membranes provide different local environments that facilitate specific metabolic functions.
e. Each type of membrane has a unique combination of lipids and proteins for its specific functions.
f. The endomembrane system includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, and the plasma membrane.
2. The endoplasmic reticulum manufactures membranes and performs many other biosynthetic functions.
a. Introduction (Fig. 7.7)
1. The endoplasmic reticulum (ER) accounts for half the membranes in a eukaryotic cell.
2. The ER includes membranous tubules and internal, fluid-filled spaces.
3. The ER membrane is continuous with the nuclear envelope and the space between the two membranes of the nuclear envelope.
4. There are two connected regions of ER that differ in structure and function. (Fig. 7.11)
a. Smooth ER looks smooth because it lacks ribosomes.
b. Rough ER looks rough because ribosomes (bound ribosomes) are attached to the outside.
b. Smooth ER
1. Enzymes in the smooth ER synthesize lipids, including oils, phospholipids, and steroids.
2. Metabolic functions
a. Enzymes in the smooth ER also catalyze a key step in the mobilization of glucose from stored glycogen in the liver.
b. Other enzymes in the smooth ER of the liver help detoxify drugs and poisons.
c. ER in muscle cells has enzymes that participate in muscle contraction.
c. Rough ER
1. As a polypeptide is synthesized by the ribosome, it is threaded into the ER space through a pore formed by a protein in the ER membrane.
2. These secretory proteins are packaged in transport vesicles that carry them to their next stage.
3. The Golgi apparatus finishes, sorts, and ships cell products. (Fig. 7.12)
a. Many transport vesicles from the ER travel to the Golgi apparatus for modification of their contents.
b. The Golgi apparatus consists of flattened membranous sacs - cisternae.
1. The membrane of each cisterna separates its internal space from the cytosol.
2. One side of the Golgi, the cis side, receives material by fusing with vesicles, while the other side, the trans side, buds off vesicles that travel to other sites.
c. During their transit from the cis to the trans pole, products from the ER are modified to reach their final state.
1. This includes modifications of the carbohydrate portion of glycoproteins.
d. The Golgi can also manufactures its own macromolecules, including noncellulose polysaccharides that are incorporated into plant cell walls.
e. During processing material is moved from cisterna to cisterna, each with its own set of enzymes.
f. Finally, the Golgi tags, sorts, and packages materials into transport vesicles.
4. Lysosomes are digestive compartments. (Fig. 7.13)
a. The lysosome is a membrane-bound sac of digestive enzymes that break down macromolecules.
b. Lysosomal enzymes can hydrolyze proteins, fats, polysaccharides, and nucleic acids.
c. While rupturing one or a few lysosomes has little impact on a cell, massive leakage from lysosomes can destroy an cell by autodigestion
d. The lysosomes create a space where the cell can digest macromolecules safely.
e. The lysosomal enzymes (and membrane) are synthesized by rough ER and then transferred to the Golgi. (Fig. 7.14)
f. Lysosomes can fuse with food vacuoles, formed when a food item is brought into the cell by phagocytosis.
1. As the polymers are digested, their monomers pass out to the cytosol to become nutrients of the cell.
g. Lysosomes can also fuse with another organelle or part of the cytosol. (Fig. 7.13)
5. Vacuoles (larger than vesicles) are membrane-bound sacs with varied functions.
a. Food vacuoles, from phagocytosis, fuse with lysosomes.
b. Contractile vacuoles, found in freshwater protists, pump excess water out of the cell.
c. Central vacuoles are found in many mature plant cells.
1. The functions of the central vacuole include stockpiling proteins or inorganic ions, depositing metabolic byproducts, storing pigments. (Fig. 7.15) (Textbook Activity 7F)
E. Other Membranous Organelles
1. Mitochondria and chloroplasts are the organelles that convert energy to forms that cells can use for work.
a. Mitochondria generate ATP (an energy carrying molecule) from the breakdown of sugars, fats, and other fuels.
b. Chloroplasts, found in plants and algae, are the sites of photosynthesis.
1. They convert the sun's energy to chemical energy and use it to synthesize organic compounds.
c. Both organelles have some ribosomes small quantities of DNA that direct the synthesis of the polypeptides produced by these internal ribosomes.
d. Mitochondria and chloroplasts grow and reproduce as semiautonomous organelles.
2. Mitochondria
a. There may be one very large mitochondrion or hundreds to thousands of individual mitochondria.
1. The number of mitochondria is correlated with how much energy the cell needs.
b. A typical mitochondrion is 1-10 microns long.
c. Mitochondria have a smooth outer membrane and highly folded inner membranes, the cristae. (Fig. 7.17)
1. This creates a fluid-filled space between them.
2. The cristae present ample surface area for the enzymes that synthesize ATP.
3. The inner membrane encloses the mitochondrial matrix, a fluid-filled space with DNA, ribosomes, and enzymes.
3. Chloroplasts (Fig. 7.18, 7.8)
a. Chloroplasts gain their color from high levels of the green pigment chlorophyll.
b. Chloroplasts measure about 2 microns x 5 microns and are found in leaves and other green structures of plants and in eukaryotic algae.
c. The processes in the chloroplast are separated from the cytosol by two membranes.
1. Inside the innermost membrane is a fluid-filled space, the stroma, in which float membranous sacs, the thylakoids.
2. The stroma contains DNA, ribosomes, and enzymes for part of photosynthesis.
3. The thylakoids, flattened sacs, are stacked into grana and are critical for converting light to chemical energy. (Textbook Activity 7G)
F. The Cytoskeleton (Fig. 7.20)
1. The cytoskeleton functions in structural support, cell motility and regulation.
a. The cytoskeleton is a network of fibers extending throughout the cytoplasm.
b. The cytoskeleton organizes the structures and activities of the cell.
1. The cytoskeleton provides mechanical support and maintains shape of the cell.
2. The cytoskeleton provides anchorage for many organelles and cytosolic enzymes.
3. The cytoskeleton also plays a major role in cell motility.
a. This involves both changes in cell location and limited movements of parts of the cell.
c. The cytoskeleton is dynamic, dismantling in one part and reassembling in another to change cell shape.
d. There are three main types of fibers in the cytoskeleton: microtubules, microfilaments, and intermediate filaments.
2. Microtubules (Table 7.2)
a. Microtubules, the thickest fibers, are hollow rods.
b. Microtubule fibers are constructed of the globular protein tubulin, and they grow or shrink as more tubulin molecules are added or removed.
c. They move chromosomes during cell division.
d. They function as tracks that guide motor proteins carrying organelles to their destination. (Fig. 7.21)
e. Microtubules provide structural support for the cell by resisting compression to the cell.
f. Microtubules are the central structural supports in cilia and flagella.
1. Both can move unicellular and small multicellular organisms by propelling water past the organism.
2. If cilia and flagella are anchored in a large structure, they move fluid over a surface.
a. For example, cilia sweep mucus carrying trapped debris from the lungs.
3. There are usually just one or a few flagella per cell. (Fig. 7.23)
4. Cilia usually occur in large numbers on the cell surface.
5. Both cilia and flagella have a core of microtubules sheathed by the plasma membrane. (Fig. 7.24)
3. Microfilaments (Table 7.2)
a. Microfilaments, the thinnest class of the cytoskeletal fibers, are twisted double chains of actin subunits.
b. With other proteins, they form a three-dimensional network just inside the plasma membrane that supports the cell's shape. (Fig. 7.26)
c. Muscle contraction-in muscle cells, thousands of actin filaments are arranged parallel to one another. (Fig. 7.21)
1. Thicker filaments composed of myosin, interdigitate with the thinner actin fibers.
2. Myosin molecules walk along the actin filament, pulling stacks of actin fibers together and shortening the cell.
d. In other cells, these actin-myosin aggregates are less organized but still cause localized contraction.
1. A contracting belt of microfilaments divides the cytoplasm of animal cells during cell division.
2. Pseudopodia extend and contract through the reversible assembly and contraction of actin subunits into microfilaments.
4. Intermediate filaments (Table 7.2)
a. Intermediate filaments are built from a diverse class of subunits from a family of proteins called keratins.
b. Intermediate filaments are more permanent fixtures of the cytoskeleton than are the other two classes.
c. They reinforce cell shape and fix organelle location. (Fig. 7.26)
G. Cell Surfaces and Junctions
1. Plant cells are encased by cell walls.
a. The cell wall, found in prokaryotes, fungi, plants, and some protists, has multiple functions.
1. In plants, the cell wall protects the cell, maintains its shape, prevents excessive uptake of water and supports the plant against the force of gravity.
b. The thickness and chemical composition of cell walls differs from species to species and among cell types.
c. The basic design consists of microfibrils of cellulose embedded in a matrix of proteins and other polysaccharides. (Fig. 7.28)
2. Intercellular junctions help integrate cells into higher levels of structure and function.
a. Neighboring cells in tissues, organs, or organ systems often adhere, interact, and communicate through direct physical contact.
b. Plant cells are perforated with plasmodesmata, channels allowing cytosol to pass between cells. (Fig. 7.28)
b. Animals have 3 main types of intercellular links: tight junctions, desmosomes, and gap junctions. (Fig. 7.30)
1. In tight junctions, membranes of adjacent cells are fused, forming continuous belts around cells.
2. Desmosomes fasten cells together into strong sheets, much like rivets.
3. Gap junctions (or communicating junctions) provide cytoplasmic channels between adjacent cells.
a. Special membrane proteins surround these pores.
b. Salt ions, sugar, amino acids, and other small molecules can pass. (Textbook Activity 7I, J, K)