Microbial Cell Structure and Function PDF

 Septa Septa occur at generally regular intervals along a length of a hypha Perforations allow cytoplasm to flow from one cell to another When a cell is damaged, a Woronin body or coagulated cytoplasm serves a plug to prevent l...

 Septa Septa occur at generally regular intervals along a length of a hypha Perforations allow cytoplasm to flow from one cell to another When a cell is damaged, a Woronin body or coagulated cytoplasm serves a plug to prevent loss of cytoplasm Coenocytic fungi are more susceptible to cellular damage Membrane-Bound Organelles of Eucaryotic Cells Nucleus Rough Endoplasmic Reticulum (RER) Smooth Endoplasmic Reticulum (SER) Golgi Apparatus Vacuoles Mitochondria The Cell Wall and Glycocalyx (Capsule and S-layers) When present, cell wall is chemically simpler than procaryotic cell wall and lacks peptidoglycan. Fungi: Chitin (polysaccharide) Yeasts: Glucan and mannan (polysaccharides)  The wall structure directly affects wall function and interactions with the environment including immune recognition by plants and animals. Fibrous and gel-like carbohydrate polymers form a tensile and robust core scaffold to which a variety of proteins and other superficial components are added that together make strong, but flexible, and chemically diverse cell walls. In most fungal species the inner cell wall consists of a core of covalently attached branched β-(1,3) glucan and chitin. β-(1,3) Glucan and chitin form intrachain hydrogen bonds and can assemble into fibrous microfibrils. The outer layers of fungi vary much more than the inner skeletal layer. The inner walls of many fungal spores and so-called black yeasts contain complex amorphous (non-crystalline solid) polymerized phenolic compounds called melanins, which also add protection— particularly from oxidants and some exoenzymes.  o Chitin - straight chain polymers of β-1,4-linked N- acetylglucosamine residues; Chitosan is de-acetylated chitin o Glucan - polymers of β-1,3-linked glucose residues with short β-1,6-linked side chains o Cellulose - β-1,4-linked glucans o Matrix polymers - Glucouronic acids - Mannoproteins - mannose attached to protein This general arrangement places the structural elements of the cell wall close to the membrane to provide mechanical support, and places the gel-like or hydrophobic polymers to the outside where they can protect the load-bearing elements from degradative enzymes in the environment or act as adhesins to anchor the cell to substrata. In A. fumigatus, the branched β-(1,3)(1,6) glucan is covalently bound to a linear β-(1,3)(1,4) glucan with a [3Glcβ1-4Glcβ1] repeating unit. The Cell Membrane Similar to procaryotic cell membranes, but: - Have different membrane proteins - Contain carbohydrates that are important for cell-cell recognition and serve as sites for bacterial attachment. - Contain sterols which increase resistance to osmotic lysis. The peripheral proteins (extrinsic) are partially embedded on one side of the membrane, whereas integral (transmembrane- intrinsic) proteins extend from one side through the membrane to the other side. Glycolipids, also a component of membranes, project into the extracellular space. Functionally, these may protect, insulate, and serve as receptor- binding sites. The lipid and protein compositions of membranes vary from cell type to cell type as well as within the various intracellular compartments that are defined by intracellular membranes. The functions of membrane proteins differ depending on their composition and location in the plasma membrane. They can: = Provide structural support = Transport molecules across the membrane = Act as enzyme regulators to control chemical reactions  Cytoplasmic Organelles Plasma membrane - phospholipid bilayer Involved in uptake of nutrients Differs in that it contains ergosterol - Site of action for certain antifungal drugs - Oomycota contain plant-like sterols Ergosterol is the major sterol found in the membranes of fungi, in contrast to the cholesterol found in the membranes of animals and phytosterols in plants. Hopanoid (sterol-like) is found in bacterial membrane. - Affects the cell membrane's fluidity and serves signaling.  Cholesterol phytosterol hopanoid bb Movement across eucaryotic cell membranes: Simple diffusion, facilitated diffusion, osmosis, and active transport. Group translocation does not occur. The Cytoplasm: - Many enzymes are sequestered in organelles. - Contains the cytoskeleton: A complex network of thread and tube-like structures, which provides support, shape, and movement. 1. Microfilaments: Smallest fibers – Actin Actin filaments (microfilaments) 2. Intermediate filaments: Medium sized fibers Anchor organelles (nucleus) and hold cytoskeleton in place. Abundant in cells with high mechanical stress. 3. Microtubules: Largest fibers - made of tubulin proteins. Work in cell division, moving chromosomes Flagella and ciliary movement. Flagella - Projections used for locomotion. - Enclosed by plasma membrane and contain cytoplasm. - Consist of 9 pairs of microtubules in a ring, with 2 single microtubules in center of ring (9 + 2). Flagella: Long whip-like projections. Eucaryotic flagella move in wavelike manner, unlike procaryotic flagella. The Nucleus Structure Envelope: Double nuclear membrane. Large nuclear pores DNA (genetic material) is combined with histones and exists in two forms: Chromatin (Loose, threadlike DNA. Most of cell life) Chromosomes (Tightly packaged DNA. Found only during cell division) Nucleolus: Dense region where ribosomes are made Unique features of fungal nucleus Membrane remains intact during mitosis Functions - House and protect cell’s genetic information (DNA). - Ribosome synthesis - The nucleus and the mitochondria represent unique membrane enclosed organelles given that both are composed of two lipid bilayers. The nuclear membrane is most often referred to as the nucleolemma and is composed of closely associated inner and outer lipid bilayers. The space between the inner and outer nuclear membranes is referred to as the perinuclear space. The inner and outer nuclear membranes are also connected at thousands of locations via multiprotein complexes that generate pores in the nuclear membrane called nuclear pore complexes. The nuclear pores are through which RNA and proteins are transported. Ribosomes - The site of protein synthesis (translation). - Found in all eucaryotic and procaryotic cells. - Made up of protein and ribosomal RNA (rRNA). - May be found free in the cytoplasm or associated with the rough endoplasmic reticulum (RER). - Eucaryotic ribosomes (80S) are larger and more dense than procaryotic ribosomes (70S). - Eucaryotic ribosomes have two subunits: Small subunit: 40S Large subunit: 60S - Mitochondria and chloroplasts have 70S ribosomes that are similar to procaryotic ribosomes. The Endoplasmic Reticulum (ER) “Network within the cell” ER is continuous with plasma membrane and outer nucleus membrane. Two types of ER: Rough Endoplasmic Reticulum (RER) Smooth Endoplasmic Reticulum (SER) Rough Endoplasmic Reticulum (RER) - Flat, interconnected, rough membrane sacs - ―Rough‖: Outer walls are covered with ribosomes. Ribosomes: Protein making ―machines‖. May exist free in cytoplasm or attached to ER. RER Functions: - Synthesis and modification of proteins. - Synthesis of cell and organelle membranes. - Packaging, and transport of proteins that are secreted from the cell.