A Tour of the CellThe 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.A. Overview of the Cell1. 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.1- 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.B. The cell Membrane- The main macromolecules in membranes are lipids and proteins, but carbohydrates are also important.- The most abundant lipids are phospholipids, they are amphipathic molecules having both hydrophobic regions and hydrophilic regions.- The arrangement of phospholipids and proteins in biological membranes is described by the fluid mosaic model.1. Membranes are fluid.- Membrane molecules are held in place by relatively weak hydrophobic interactions.- Membrane fluidity is also influenced by its components. Membranes rich in unsaturated fatty acidsare more fluid that those dominated by saturated fatty acids because the kinks in the unsaturated fatty acid tails at the locations of the double bonds prevent tight packing.- The steroid cholesterol is wedged between phospholipid molecules in the plasma membrane of animal cells. Cholesterol acts as a “temperature buffer” for the membrane, resisting changes in membrane fluidity as temperature changes.- To work properly with active enzymes and appropriate permeability, membranes must be about as fluid as salad oil.2. Membranes are mosaics of structure and function.- A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer.- Proteins determine most of the membrane’s specific functions.- The plasma membrane and the membranes of the various organelles each have unique collections of proteins.- There are two major populations of membrane proteins.- Peripheral proteins are not embedded in the lipid bilayer at all. Instead, they are loosely bound to the surface of the protein, often connected to integral proteins.- Integral proteins penetrate the hydrophobic core of the lipid bilayer, often completely spanning the membrane (as transmembrane proteins). The hydrophobic regions embedded in the membrane’s core consist of stretches of nonpolar amino acids, often coiled into alpha helices. Where integral proteins are in contact with the aqueous environment, they have hydrophilic regions of amino acids.- On the cytoplasmic side of the membrane, some membrane proteins connect to the cytoskeleton.- On the exterior side of the membrane, some membrane proteins attach to the fibers of the extracellular matrix.- The proteins of the plasma membrane have six major functions:21. Transport of specific solutes into or out of cells.2. Enzymatic activity, sometimes catalyzing one of a number of steps of a metabolic pathway.3. Signal transduction, relaying hormonal messages to the cell.4. Cell-cell recognition, allowing other proteins to attach two adjacent cells together.5. Intercellular joining of adjacent cells with gap or tight junctions.6. Attachment to the cytoskeleton and extracellular matrix, maintaining cell shape and stabilizingthe location of certain membrane proteins3. 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.A. Membrane Structure-