Cellular Structures 

Overview: The Importance of Cells

• All organisms are made of cells.
  • Many organisms are single-celled.
  • Even in multicellular organisms, the cell is the basic unit of structure and function.
• The cell is the simplest collection of matter that can live.
• All cells are related by their descent from earlier cells.

Concept 6.1 To study cells, biologists use microscopes and the tools of biochemistry

• The discovery and early study of cells progressed with the invention of microscopes in 1590 and their improvement in the 17th century.
• In a light microscope (LM), visible light passes through the specimen and then through glass lenses.
  • The lenses refract light such that the image is magnified into the eye or onto a video screen.
• Microscopes vary in magnification and resolving power.
  • Magnification is the ratio of an object’s image to its real size.
  • Resolving power is a measure of image clarity.
  • It is the minimum distance two points can be separated and still be distinguished as two separate points.
  • Resolution is limited by the shortest wavelength of the radiation used for imaging.
• The minimum resolution of a light microscope is about 200 nanometers (nm), the size of a small bacterium.
• Light microscopes can magnify effectively to about 1,000 times the size of the actual specimen.
  • At higher magnifications, the image blurs.
• Techniques developed in the 20th century have enhanced contrast and enabled particular cell components to be stained or labeled so they stand out.
• While a light microscope can resolve individual cells, it cannot resolve much of the internal anatomy, especially the organelles.
• To resolve smaller structures, we use an electron microscope (EM), which focuses a beam of electrons through the specimen or onto its surface.
  • Because resolution is inversely related to wavelength used, electron microscopes (whose electron beams have shorter wavelengths than visible light) have finer resolution.
  • Theoretically, the resolution of a modern EM could reach 0.002 nanometer (nm), but the practical limit is closer to about 2 nm.
• Transmission electron microscopes (TEMs) are used mainly to study the internal ultrastructure of cells.
  • A TEM aims an electron beam through a thin section of the specimen.
  • The image is focused and magnified by electromagnets.
  • To enhance contrast, the thin sections are stained with atoms of heavy metals.
• Scanning electron microscopes (SEMs) are useful for studying surface structures.
  • The sample surface is covered with a thin film of gold.
  • The beam excites electrons on the surface of the sample.
  • These secondary electrons are collected and focused on a screen.
  • The result is an image of the topography of the specimen.
  • The SEM has great depth of field, resulting in an image that seems three-dimensional.
• Electron microscopes reveal organelles that are impossible to resolve with the light microscope.
  • However, electron microscopes can only be used on dead cells.
• Light microscopes do not have as high a resolution, but they can be used to study live cells.
• Microscopes are major tools in cytology, the study of cell structures.
• Cytology combined with biochemistry, the study of molecules and chemical processes in metabolism, to produce modern cell biology.

Cell biologists can isolate organelles to study their functions.

• The goal of cell fractionation is to separate the major organelles of the cells so their individual functions can be studied.
• This process is driven by an ultracentrifuge, a machine that can spin at up to 130,000 revolutions per minute and apply forces of more than 1 million times gravity (1,000,000 g).
• Fractionation begins with homogenization, gently disrupting the cell.
• The homogenate is spun in a centrifuge to separate heavier pieces into the pellet while lighter particles remain in the supernatant.
  • As the process is repeated at higher speeds and for longer durations, smaller and smaller organelles can be collected in subsequent pellets.
• Cell fractionation prepares isolates of specific cell components.
• This enables the functions of these organelles to be determined, especially by the reactions or processes catalyzed by their proteins.
  • For example, one cellular fraction was enriched in enzymes that function in cellular respiration.
  • Electron microscopy revealed that this fraction is rich in mitochondria.
  • This evidence helped cell biologists determine that mitochondria are the site of cellular respiration.
• Cytology and biochemistry complement each other in correlating cellular structure and function.

Concept 6.2 Eukaryotic cells have internal membranes that compartmentalize their functions

Prokaryotic and eukaryotic cells differ in size and complexity.

• All cells are surrounded by a plasma membrane.
• The semifluid substance within the membrane is the cytosol, containing the organelles.
• All cells contain chromosomes that have genes in the form of DNA.
• All cells also have ribosomes, tiny organelles that make proteins using the instructions contained in genes.
• A major difference between prokaryotic and eukaryotic cells is the location of chromosomes.
• In a eukaryotic cell, chromosomes are contained in a membrane-enclosed organelle, the nucleus.
• In a prokaryotic cell, the DNA is concentrated in the nucleoid without a membrane separating it from the rest of the cell.
• In eukaryote cells, the chromosomes are contained within a membranous nuclear envelope.
• The region between the nucleus and the plasma membrane is the cytoplasm.
  • All the material within the plasma membrane of a prokaryotic cell is cytoplasm.
• Within the cytoplasm of a eukaryotic cell are a variety of membrane-bound organelles of specialized form and function.
  • These membrane-bound organelles are absent in prokaryotes.
• Eukaryotic cells are generally much bigger than prokaryotic cells.
• The logistics of carrying out metabolism set limits on cell size.
  • At the lower limit, the smallest bacteria, mycoplasmas, are between 0.1 to 1.0 micron.
  • Most bacteria are 1–10 microns in diameter.
  • Eukaryotic cells are typically 10–100 microns in diameter.
• Metabolic requirements also set an upper limit to the size of a single cell.
• As a cell increases in size, its volume increases faster than its surface area.
  • Smaller objects have a greater ratio of surface area to volume.
• The plasma membrane functions as a selective barrier that allows the passage of oxygen, nutrients, and wastes for the whole volume of the cell.
• The volume of cytoplasm determines the need for this exchange.
• Rates of chemical exchange across the plasma membrane may be inadequate to maintain a cell with a very large cytoplasm.
• The need for a surface sufficiently large to accommodate the volume explains the microscopic size of most cells.
• Larger organisms do not generally have larger cells than smaller organisms—simply more cells.
• Cells that exchange a lot of material with their surroundings, such as intestinal cells, may have long, thin projections from the cell surface called microvilli. Microvilli increase surface area without significantly increasing cell volume.

Internal membranes compartmentalize the functions of a eukaryotic cell.

• A eukaryotic cell has extensive and elaborate internal membranes, which partition the cell into compartments.
• These membranes also participate directly in metabolism, as many enzymes are built into membranes.
• The compartments created by membranes provide different local environments that facilitate specific metabolic functions, allowing several incompatible processes to go on simultaneously in a cell.
• The general structure of a biological membrane is a double layer of phospholipids.
• Other lipids and diverse proteins are embedded in the lipid bilayer or attached to its surface.
• Each type of membrane has a unique combination of lipids and proteins for its specific functions.
• For example, enzymes embedded in the membranes of mitochondria function in cellular respiration.

Concept 6.3 The eukaryotic cell’s genetic instructions are housed in the nucleus and carried out by the ribosomes

• The nucleus contains most of the genes in a eukaryotic cell.
  • Additional genes are located in mitochondria and chloroplasts.
• The nucleus averages about 5 microns in diameter.
• The nucleus is separated from the cytoplasm by a double membrane called the nuclear envelope.
  • The two membranes of the nuclear envelope are separated by 20–40 nm.
  • The envelope is perforated by pores that are about 100 nm in diameter.
  • At the lip of each pore, the inner and outer membranes of the nuclear envelope are fused to form a continuous membrane.
  • A protein structure called a pore complex lines each pore, regulating the passage of certain large macromolecules and particles.
• The nuclear side of the envelope is lined by the nuclear lamina, a network of protein filaments that maintains the shape of the nucleus.
• There is evidence that a framework of fibers called the nuclear matrix extends through the nuclear interior.
• Within the nucleus, the DNA and associated proteins are organized into discrete units called chromosomes, structures that carry the genetic information.
• Each chromosome is made up of fibrous material called chromatin, a complex of proteins and DNA.
  • Stained chromatin appears through light microscopes and electron microscopes as a diffuse mass.
• As the cell prepares to divide, the chromatin fibers coil up and condense, becoming thick enough to be recognized as the familiar chromosomes.
• Each eukaryotic species has a characteristic number of chromosomes.
  • A typical human cell has 46 chromosomes.
  • A human sex cell (egg or sperm) has only 23 chromosomes.
• In the nucleus is a region of densely stained fibers and granules adjoining chromatin, the nucleolus.
  • In the nucleolus, ribosomal RNA (rRNA) is synthesized and assembled with proteins from the cytoplasm to form ribosomal subunits.
  • The subunits pass through the nuclear pores to the cytoplasm, where they combine to form ribosomes.
• The nucleus directs protein synthesis by synthesizing messenger RNA (mRNA).
  • The mRNA travels to the cytoplasm through the nuclear pores and combines with ribosomes to translate its genetic message into the primary structure of a specific polypeptide.

  Ribosomes build a cell’s proteins.

• Ribosomes, containing rRNA and protein, are the organelles that carry out protein synthesis.
  • Cell types that synthesize large quantities of proteins (e.g., pancreas cells) have large numbers of ribosomes and prominent nucleoli.
• Some ribosomes, free ribosomes, are suspended in the cytosol and synthesize proteins that function within the cytosol.
• Other ribosomes, bound ribosomes, are attached to the outside of the endoplasmic reticulum or nuclear envelope.
  • These synthesize proteins that are either included in membranes or exported from the cell.
• Ribosomes can shift between roles depending on the polypeptides they are synthesizing.

Concept 6.4 The endomembrane system regulates protein traffic and performs metabolic functions in the cell

• Many of the internal membranes in a eukaryotic cell are part of the endomembrane system.
• These membranes are either directly continuous or connected via transfer of vesicles, sacs of membrane.
  • In spite of these connections, these membranes are diverse in function and structure.
  • The thickness, molecular composition and types of chemical reactions carried out by proteins in a given membrane may be modified several times during a membrane’s life.
• The endomembrane system includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, and the plasma membrane.

The endoplasmic reticulum manufactures membranes and performs many other biosynthetic functions.

• The endoplasmic reticulum (ER) accounts for half the membranes in a eukaryotic cell.
• The ER includes membranous tubules and internal, fluid-filled spaces called cisternae.
• The ER membrane is continuous with the nuclear envelope, and the cisternal space of the ER is continuous with the space between the two membranes of the nuclear envelope.
• There are two connected regions of ER that differ in structure and function.
  • Smooth ER looks smooth because it lacks ribosomes.
  • Rough ER looks rough because ribosomes (bound ribosomes) are attached to the outside, including the outside of the nuclear envelope.
  • The smooth ER is rich in enzymes and plays a role in a variety of metabolic processes.
  • Enzymes of smooth ER synthesize lipids, including oils, phospholipids, and steroids.
  • These include the sex hormones of vertebrates and adrenal steroids.
  • In the smooth ER of the liver, enzymes help detoxify poisons and drugs such as alcohol and barbiturates.
    • Frequent use of these drugs leads to the proliferation of smooth ER in liver cells, increasing the rate of detoxification.
    • This increases tolerance to the target and other drugs, so higher doses are required to achieve the same effect.
  • Smooth ER stores calcium ions.
    • Muscle cells have a specialized smooth ER that pumps calcium ions from the cytosol and stores them in its cisternal space.
    • When a nerve impulse stimulates a muscle cell, calcium ions rush from the ER into the cytosol, triggering contraction.
    • Enzymes then pump the calcium back, readying the cell for the next stimulation.
• Rough ER is especially abundant in cells that secrete proteins.
  • As a polypeptide is synthesized on a ribosome attached to rough ER, it is threaded into the cisternal space through a pore formed by a protein complex in the ER membrane.
  • As it enters the cisternal space, the new protein folds into its native conformation.
  • Most secretory polypeptides are glycoproteins, proteins to which a carbohydrate is attached.
  • Secretory proteins are packaged in transport vesicles that carry them to their next stage.
• Rough ER is also a membrane factory.
  • Membrane-bound proteins are synthesized directly into the membrane.
  • Enzymes in the rough ER also synthesize phospholipids from precursors in the cytosol.
  • As the ER membrane expands, membrane can be transferred as transport vesicles to other components of the endomembrane system.

  The Golgi apparatus is the shipping and receiving center for cell products.

• Many transport vesicles from the ER travel to the Golgi apparatus for modification of their contents.
• The Golgi is a center of manufacturing, warehousing, sorting, and shipping.
• The Golgi apparatus is especially extensive in cells specialized for secretion.
• The Golgi apparatus consists of flattened membranous sacs—cisternae—looking like a stack of pita bread.
  • The membrane of each cisterna separates its internal space from the cytosol.
  • One side of the Golgi, the cis side, is located near the ER. The cis face receives material by fusing with transport vesicles from the ER.
  • The other side, the trans side, buds off vesicles that travel to other sites.
• During their transit from the cis to the trans side, products from the ER are usually modified.
• The Golgi can also manufacture its own macromolecules, including pectin and other noncellulose polysaccharides.
• The Golgi apparatus is a very dynamic structure.
  • According to the cisternal maturation model, the cisternae of the Golgi progress from the cis to the trans face, carrying and modifying their protein cargo as they move.
• Finally, the Golgi sorts and packages materials into transport vesicles.
  • Molecular identification tags are added to products to aid in sorting.
  • Products are tagged with identifiers such as phosphate groups. These act like ZIP codes on mailing labels to identify the product’s final destination.

  Lysosomes are digestive compartments.

• A lysosome is a membrane-bound sac of hydrolytic enzymes that an animal cell uses to digest macromolecules.
• Lysosomal enzymes can hydrolyze proteins, fats, polysaccharides, and nucleic acids.
• These enzymes work best at pH 5.
  • Proteins in the lysosomal membrane pump hydrogen ions from the cytosol into the lumen of the lysosomes.
  • Rupture of one or a few lysosomes has little impact on a cell because the lysosomal enzymes are not very active at the neutral pH of the cytosol.
  • However, massive rupture of many lysosomes can destroy a cell by autodigestion.
• Lysosomal enzymes and membrane are synthesized by rough ER and then transferred to the Golgi apparatus for further modification.
• Proteins on the inner surface of the lysosomal membrane are spared by digestion by their three-dimensional conformations, which protect vulnerable bonds from hydrolysis.
• Lysosomes carry out intracellular digestion in a variety of circumstances.
• Amoebas eat by engulfing smaller organisms by phagocytosis.
  • The food vacuole formed by phagocytosis fuses with a lysosome, whose enzymes digest the food.
  • As the polymers are digested, monomers pass to the cytosol to become nutrients for the cell.
• Lysosomes can play a role in recycling of the cell’s organelles and macromolecules.
  • This recycling, or autophagy, renews the cell.
  • During autophagy, a damaged organelle or region of cytosol becomes surrounded by membrane.
  • A lysosome fuses with the resulting vesicle, digesting the macromolecules and returning the organic monomers to the cytosol for reuse.
• The lysosomes play a critical role in the programmed destruction of cells in multicellular organisms.
  • This process plays an important role in development.
  • The hands of human embryos are webbed until lysosomes digest the cells in the tissue between the fingers.
  • This important process is called programmed cell death, or apoptosis.

  Vacuoles have diverse functions in cell maintenance.

• Vesicles and vacuoles (larger versions) are membrane-bound sacs with varied functions.
  • Food vacuoles are formed by phagocytosis and fuse with lysosomes.
  • Contractile vacuoles, found in freshwater protists, pump excess water out of the cell to maintain the appropriate concentration of salts.
  • A large central vacuole is found in many mature plant cells.
  • The membrane surrounding the central vacuole, the tonoplast, is selective in its transport of solutes into the central vacuole.
  • The functions of the central vacuole include stockpiling proteins or inorganic ions, disposing of metabolic byproducts, holding pigments, and storing defensive compounds that defend the plant against herbivores.
  • Because of the large vacuole, the cytosol occupies only a thin layer between the plasma membrane and the tonoplast. The presence of a large vacuole increases surface area to volume ratio for the cell.