4.1 Comparing Prokaryotic and Eukaryotic Cells: An Overview

• Prokaryotes and eukaryotes both contain nucleic acids, proteins, lipids, and carbohydrates.

• The distinguish characteristics of prokaryotes are:

  • Typically their DNA is enclosed within a membrane and is usually a singular, circularly arranged chromosome.

  • Their DNA is not associated with histones; other proteins are associated with DNA.

  • They generally lack organelles.

  • Their cell walls almost always contain the complex polysaccharide  peptidoglycan.

  • They usually divide by binary fusion, where DNA is copied, and the cell slips into two cells. This involved fewer structures and processes than eukaryotic cell division.

• Eukaryotes have the following distinguishing characteristics:

  • Their DNA is found in the cell’s nucleus, which is separated from the cytoplasm by a nuclear membrane, and the DNA is found in multiple chromosomes.

  • Their DNA is consistently associated with chromosome proteins called histones and with nonhistones.

  • They have a number of membrane-enclosed organelles, including mitochondria, endoplasmic reticulum, Golgi complex, lysine’s, and sometimes chloroplasts.

  • Their cell walls, when present, are chemically simple.

  • Cell division usually involves mitosis, in which chromosomes replicate and an identical set is distributed into each of two nuclei. Division of the cytoplasm and other organelles follows so that the two cells produced are identical to each other.

4.2 The Size, Shape, and Arrangement of Bacterial Cells

• Bacteria may be spherical-shaped coccus,rod-shaped bacillus, and spiral.

• Cocci are usually round but can be oval, elongated, or flattened on one side.

  • Cocci that remain in pairs after dividing are called diplococci; those that divide and remain attached in chainlike patterns are called streptococci.

  • Those that divide in two planes and remain in groups of four are known as tetrads.

  • Those that divide in three planes and remain attached in cube-like groups of eight are called sarcinae.

  • Those that divide in multiple planes and form grape-like clusters or broad sheets are called staphylococci.

• Most bacilli appear as single rods, called single bacilli.

  • Diplobacilli appear in pairs after division, and streptobacilli occur in chains.

• The shape of a bacterium is determined by heredity.

• Genetically, most bacteria are monomorphic; that is, that maintain a single shape.

• Pleomorphic means they can have many shapes, not just one.

4.3 Structures External to the Cell Wall

• Glycocalyx is the general term used for substances that surround cells.

• If the substance is organized and is firmly attached to the cell wall, the glycocalyx is described as a capsule.

• If a substance is unorganized and only loosely attached to the cell wall, the Glycocalyx is described as a slime layer.

• A glycocalyx that helps cells in a biofilm attach to their target environment and to each other is called an extracellular polymeric substance (EPS).

• Some bacterial cells have flagella, which are long filamentous appendages that propel bacteria.

• Bacteria that lack flagella are referred to as atrocious.

• The flagella protein called H antigen is useful for distinguishing among serovars, or variations within a species, of gram-negative bacteria.

• Archaea share similarities with bacterial flagella and pili.

• Fin rise can occur at the poles of the bacterial cell or can be evenly distributed over the entire surface of the cell.

• Pili are usually longer than fimbriae and number only one or two per cell.

4.4 The Cell Wall

• The cell wall of the bacterial cell is a complex, semi grid structure responsible for the shape of the cell.

• The bacterial cell is composed of the macromolecular network called peptidoglycan.

• Adjacent rows are linked by polypeptides (the peptide portion of peptidoglycan).

• In most gram-positive bacteria, the cell wall consists of many layers of peptidoglycan, forming a thick, rigid structure.

• Gram-negative cell walls contain only a thin layer of peptidoglycan.

  • Part of the permeability of the outer membrane is due to proteins in the membrane, called porins.

  • The lipopolysaccharide of the outer membrane is a large, complex molecule that contains lipid and carbohydrates and consists of three components: lipid A, a core polysaccharide and an O polysaccharide.

• Lipid A is a lipid portion of the LPS and is embedded in the top layer of the outer membrane.

• The core polysaccharide is attached to lipid A and contains unusual sugars. Its role is structural—to provide stability.

• The O polysaccharide extends outward from the core polysaccharide and is composed of sugar molecules.

4.5 Structures Internal to the Cell Wall

• The plasma (cytoplasmic) membrane (or inner membrane) is a thin structure lying inside the cell wall and enclosing the cytoplasm of the cell.

• Proteins attached to carbohydrates are called glycoproteins, and lipids attached to carbohydrates are called glycolipids.

• Selective permeability indicates that certain molecules and ions are allowed to pass through the membrane but others are stopped.

• When viewed with an electron microscope, bacterial plasma membranes often appear to contain one or more large, irregular folds called mesosomes.

• Simple diffusion is the net (overall) movement of molecules or ions from an area of high concentration to an area of low concentration.

• In facilitated diffusion, integral membrane proteins function as channels or carriers that facilitate the movement of ions or large molecules across the plasma membrane.

• Osmosis is the net movement of water molecules across a selectively permeable membrane from an area with a high concentration of water molecules (low concentration of solute molecules) to an area of low concentration of water molecules (high concentration of solute molecules).

• Osmotic pressure is the pressure needed to stop the flow of water across the selectively permeable membrane.

• An isotonic solution is a medium in which the overall concentration of solutes equals that found inside a cell.

• A hypotonic solution outside the cell is a medium whose concentration of solutes is lower than that inside the cell.

• A hypertonic solution is a medium having a higher concentration of solutes than that inside the cell.

• In performing active transport, the cell uses energy in the form of ATP to move substances across the plasma membrane.

• For a prokaryotic cell, the term cytoplasm refers to the substance of the cell inside the plasma membrane.

• The nucleoid of a bacterial cell usually contains a single long, continuous, and frequently circularly arranged thread of double-stranded DNA called the bacterial chromosome.

• Bacteria often contain small, usually circular, double-stranded DNA molecules called plasmids.

4.6 Flagella and Cilia

• If the projections are few and are long in relation to the size of the cell, they are called flagella.

• If the projections are numerous and short, they are called cilia.

• Microtubules are long, hollow tubes made up of protein called tubules.

4.7 The Cell Wall and Glycocalyx

• Glycocalyx is a layer of material containing substantial amounts of sticky carbohydrates.

4.8 The Plasma (Cytoplasmic) Membrane

• The plasma (cytoplasmic) membrane of eukaryotic and prokaryotic cells is very similar in function and basic structure.

  • Eukaryotic membranes contain carbohydrates, which serve as attachment sites for bacteria and as receptor sites that assume the role in such functions as cell recognition.

• Endocytosis occurs when a segment of the plasma membrane surrounds a particle of a large molecule, encloses it, and brings it into the cell.

4.9 Cytoplasm

• The cytoplasm of eukaryotic cells encompasses the substance inside the plasma membrane and outside the nucleus.

• The cytoskeleton of eukaryotes consists of small rods and cylinders.

• The movement of eukaryotic cytoplasm from one part of the cell to another, which helps distribute nutrients and move the cell over a surface, is called cytoplasmic streaming.

4.10 Ribosomes

• Ribosomes are found free in the cytoplasm.

4.11 Organelles

• Organelles are structures with specific shapes and specialized functions and are characteristic of eukaryotic cells.

• The nucleus is usually spherical or oval, is frequently the largest structure in the cell, and contains almost all of the cell’s hereditary in the cell, and contains almost all of the cell’s hereditary information (DNA).

• The nucleus is surrounded by a double membrane called the nuclear envelope.

• Tiny channels in the membrane called nuclear pores allow the nucleus to communicate with the cytoplasm.

• Within the nuclear envelope are one or more spherical bodies called nucleoli.

• The nucleus also contains most of the cell’s DNA, which is combined with several proteins, including some basic proteins called histones.

• When the cell is not reproducing, the DNA and its associated proteins appear as a threadlike mass called chromatin.

• During nuclear division, the chromatin could form shorter and thicker rod-like bodies called chromosomes.

• Within the cytoplasm of eukaryotic cells is the endoplasmic reticulum, or ER, an extensive network of flattened membrane sacs or tubules called cisternae.

• The membrane of rough ER is continuous with the nuclear membrane and usually unfolds into a series of flattened sacs.

• Smooth ER extends from the rough ER to form a network of membrane tubules.

• The first step in the transport pathway is through an organelle called the Golgi complex.

• The transport vesicle fuses with a cistern of the Golgi complex, releasing proteins into the cistern.

• The proteins are modified and moved from one cistern to another via transfer vesicles that bud from the edges of the cisternae.

• Some of the processed proteins leave the cisternae in secretory  vesicles, which detach from the cistern and deliver the proteins to the plasma membrane, where they are discharged by exocytosis.

• Some processed proteins leave the cisternae in vesicles that are called storage vesicles.

• Lysosomes are formed from Golgi complexes and look like membrane-enclosed spheres.

• A vacuole is a space or cavity in the cytoplasm of a cell that is enclosed by a membrane called a tonoplast.

• Elongated, irregularly shaped organelles called mitochondria appear throughout the cytoplasm of most eukaryotic cells.

• The outer mitochondrial membrane is smooth, but the inner mitochondrial membrane is arranged in a series of folds called cristae.

• The center of the mitochondrion is a semi fluid substance called the matrix.

• A chloroplast is a double membrane-enclosed structure that contains both the pigment chlorophyll and the enzymes required for the light gathering phases of photosynthesis.

• The centrosomes, located near the nucleus, consist of two components: the peri estriol area area and centrioles.

4.12 The Evolution of Eukaryotes

• According to the endosymbiotic theory, larger bacterial cells lose their cell walls and engulfs smaller bacterial cells.

  • According to this theory, the ancestral eukaryote developed a rudimentary nucleus when the plasma membrane folded around the chromosome.