A TOUR OF THE CELL Prokaryotic and eukaryotic

A TOUR OF THE CELL

Prokaryotic and eukaryotic cells differ in size and complexity Ø All cells are surrounded by a plasma membrane , contain chromosomes , and have ribosomes Ø In a eukaryotic cell, chromosomes are contained in a membrane-enclosed organelle, the nucleus. Ø In a prokaryotic cell, the DNA is concentrated in the nucleoid without a membrane separating it from the rest of the cell.

Ø Within the cytoplasm of a eukaryotic cell is a variety of membrane-bounded organelles of specialized form and function. Ø These membrane-bounded organelles are absent in prokaryotes. Ø Eukaryotic cells are generally much bigger than prokaryotic cells.

Ø A eukaryotic cell has extensive and elaborate internal membranes, which partition the cell into compartments. Ø These membranes also participate in metabolism as many enzymes are built into membranes.

The Nucleus Ø The nucleus contains most of the genes in a eukaryotic cell. Ø Some genes are located in mitochondria and chloroplasts.

Ribosomes build a cell’s proteins Ø Ribosomes contain r. RNA and protein. Ø A ribosome is composed of two subunits that combine to carry out protein synthesis. Ø Some ribosomes, free ribosomes, are suspended in the cytosol Ø Other ribosomes, bound ribosomes, are attached to the outside of the endoplasmic reticulum.

The Endomembrane System Ø 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 Ø 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. Ø Smooth ER looks smooth because it lacks ribosomes.

Ø Rough ER looks rough because ribosomes (bound ribosomes) are attached to the outside, including the outside of the nuclear envelope. Ø The smooth ER is rich in enzymes and plays a role in a variety of metabolic processes. Ø The smooth ER also catalyzes a key step in the mobilization of glucose from stored glycogen in the liver. Ø Other enzymes in the smooth ER of the liver help detoxify drugs and poisons.

Ø Rough ER is especially abundant in those cells that secrete proteins. Ø Rough ER is also a membrane factory. Ø Enzymes in the rough ER also synthesize phospholipids from precursors in the cytosol.

The Golgi apparatus finishes, sorts, and ships cell products Ø The Golgi apparatus is especially extensive in cells specialized for secretion. Ø the Golgi tags, sorts, and packages materials into transport vesicles.

Lysosomes are digestive compartments Ø Lysosomal enzymes can hydrolyze proteins, fats, polysaccharides, and nucleic acids. l l Lysosomes can also fuse with another organelle or part of the cytosol. This recycling, or autophagy, renews the cell. • The lysosomes play a critical role in the programmed destruction of cells in multicellular organisms. l This process allows reconstruction during the developmental process.

Vacuoles have diverse functions in cell maintenance Ø Food vacuoles , Contractile vacuoles , and Central vacuoles

Mitochondria and chloroplasts are the main energy transformers of cells Ø Mitochondria are the sites of cellular respiration, generating ATP from the catabolism of sugars, fats, and other fuels in the presence of oxygen. Ø Chloroplasts, found in plants and eukaryotic algae, are the sites of photosynthesis. Ø Mitochondria and chloroplasts grow and reproduce as semiautonomous organelles.

Mitochondria and chloroplasts are mobile and move around the cell along tracks in the cytoskeleton Ø Endosymbiotic Origin : proposed ancestors of mitochondria were aerobic heterotrophic prokaryotes AND for chloroplast were photosynthetic prokaryotes. Ø The ancestors gained entry and eventually became mutually beneficial Ø