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

Macrom

olecules

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, http://people.westminstercollege.edu/faculty/jrogers/V%20prokaryotes.ppt#298,3,Slide 3

100 µm 20 µm 0.5 µm

Figure 27.3 The most common shapes of prokaryotes

http://people.westminstercollege.edu/faculty/jrogers/V%20prokaryotes.ppt#298,3,Slide

Prokaryote Structure

Cell wall

membrane

Nuclear material

Membrane is critical part of how food and waste are transported - Selectively permeable

Phospholipid layerTransport 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 (O2, N2, CO2, ethanol, H2O freely diffuse through layer

– Osmosis because solute concentration inside the cell are generally higher (10 mM inside the cell), water activity is lower inside, H2O 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