Cell Composition 70-90% water Organic chemistry key to the construction of cells is inherently linked to the properties of water vs. organic compounds Consider 4 groups of monomers (a single, repeated ‘building block’): –Sugars –Fatty Acids –Nucleotides –Amino Acids Polysaccharides Lipids Nucleic Acids Proteins Macromolecules

Cell Construction OK – using the building blocks we have described, let’s make a microbe…

Cell sizes and shape Most cells are between 0.1 and 5  m in diameter Several shapes are common: –Rod or bacilli –Spherical or cocci –Spiral –Other forms – including square, sheathed, stalked, filamentous, star, spindle, lobed, pleomorphic forms

Microbes on the head of a pin, false color SEM images, from j. Rogers, µm20 µm0.5 µm

Figure 27.3 The most common shapes of prokaryotes

Prokaryote Structure Cell wall membrane Nuclear material Membrane is critical part of how food and waste are transported - Selectively permeable Phospholipid layer Transport proteins

Eubacteria vs. Archaebacteria Bacterial cell structure Archaeal cell structure Difference?? Let’s look more closely at the membrane, though only 8 nm thick, it is the principle difference between these 2 groups of microbes

Archaea vs bacteria membranes Principle difference between these two is the membrane In archaea, lipids are unique  they have ether linkages instead of ester linkages

Cell Membranes The membrane separates the internal part of the cell from the external  that these environments remain separate, but under CONTROLLED contact is a key to life Membrane Components: Phospholipid bilayer Hopanoids, which provide additional structural stability (similar to sterols (cholesterols) which provide rigidity to eukaryote cells) Proteins – direct transport between outside and inside the cell

Membrane function SELECTIVELY PERMEABLE –Passive diffusion  Gases (O 2, N 2, CO 2, ethanol, H 2 O freely diffuse through layer –Osmosis  because solute concentration inside the cell are generally higher (10 mM inside the cell), water activity is lower inside, H 2 O comes in – increased water results in turgor pressure (~75psi) –Protein-mediated transport  selective and directional transport across the membrane by uniporters and channel proteins, these facilitate diffusion – still following a gradient and does not require an energy expenditure from the cell

Membrane function 2 Active transport  proteins that function to move solutes against a gradient, this requires energy Uniport, Symport, and Antiport proteins guide directional transport of ions/molecules across membrane – different versions can be quite selective (single substance or class of substances) as to what they carry

Membrane and metabolism As the membrane is the focus of gradients, this is where electron transport reactions occur which serve to power the cell in different ways Many enzymes important to metabolic activity are membrane bound

H + gradients across the membrane Proton Motive Force (PMF) is what drives ATP production in the cell

Figure 5.21

Membrane functions (other) In addition to directing ion/molecule transport and providing the locus for energy production, membranes are also involved in: –Phospholipid & protein synthesis for membrane –Nucleoid division in replication –Base for flagella –Waste removal –Endospore formation Though very small, the membrane is critical to cell function  Lysis involves the rupture of this membrane and spells certain death for the organism

Cell Wall Cell wall structure is also chemically quite different between bacteria and archaea Almost all microbes have a cell wall – mycoplasma bacteria do not Bacteria have peptidoglycan, archaea use proteins or pseudomurein The cell wall serves to provide additional rigidity to the cell in order to help withstand the turgor pressure developed through osmosis and define the cell shape as well as being part of the defense mechanisms

Cell wall structure Two distinct groups of bacteria with very different cell walls –Gram negative has an outer lipid membrane (different from the inner, or plasma membrane) –Gram positive lacks the outer membrane but has a thicker peptidogycan layer

Peptidoglycan layer This layer is responsible for the rigidity of the cell wall, composed of N-Acetylglucosamine (NAG) and N- acetylmuramic (NAM) acids and a small group of amino acids. Glysine chains held together with peptide bonds between amino acids to form a sheet

Outer membrane – Gram (-) Lipid bilayer ~7 nm thick made of phospholipids, lipopolysaccharides, and proteins LPS (lipopolysaccharides) can get thick and is generally a part that is specifically toxic (aka an endotoxin) LPS layers are of potential enviornmental importance as a locus of chelators and electron shuttles Porins are proteins that are basically soluble to ions and molecules, making the outer layer effectively more porous than the inner membrane, though they can act as a sort of sieve

External features Glycocalyx (aka capsule – tightly bound and adhering to cell wall, or slime layer – more unorganized and loosely bound) – helps bacteria adhere to surfaces as well as provides defense against viruses Flagella – ‘tail’ that allows movement by rotating and acting as a propeller Pili – thin protein tubes for adhesion (colonization) and adhering to surfaces

Inside the cell Cytoplasm – everything inside the membrane Nucleoid – DNA of the organism – it is not contained by a nuclear membrane (as eukaryote cell) Ribosomes – made of ribosomal RNA and protein  these are responsible for making proteins Vacuoles or vesicles – spaces in the cytoplasm that can store solids or gases Organelles – structures specifically for photosynthesis – a membrane system

Ribosomes RNA is a single stranded nucleic acid –mRNA- messanger RNA – copies information from DNA and carries it to the ribosomes –tRNA – transfer RNA – transfers specific amino acids to the ribosomes –rRNA – ribosomal RNA – with proteins, assembles ribosomal subunits