Ritu's Biology Hub

Continued from Part 2

Nutrient Uptake

Diffusion – The net movement of molecules down their concentration gradient by random thermal motion

1. Passive Diffusion – Often called simple diffusion, is a process in which molecules from a region of higher concentration to one of lower concentration. Only very small molecules like H₂O and CO₂ (non-polar) can move across by passive diffusion. Characteristics:
   • Does not involve the use of carrier proteins
   • Along the concentration gradient
   • No metabolic energy is required
   • If concentration gradient disappears, then net inward movement ceases
   • Reversible movement
   • No specificity as there are no carrier protein involved
   • Shows saturation
   • Slow process

2. Facilitated Diffusion – Diffusion involving carrier proteins (permeases). Rate of diffusion increases with the concentration gradient. Major intrinsic proteins (MIPs) facilitate diffusion of small polar molecules. Glycerol is transported by facilitated diffusion in bacteria. Characteristics:
   • Involves the use of permeases
   • Along the concentration gradient
   • No metabolic energy is required
   • If concentration gradient disappears, then net inward movement ceases
   • Reversible movement
   • Permeases show high specificity
   • Shows saturation

3. Osmosis – Movement of solvent molecules through a selectively permeable membrane from a region of higher to lower concentration
   • Isotonic solution – When the concentration of both the solution on either sides of a semipermeable membrane are same
   • Hypotonic solution – When external environment of the membrane has higher concentration than the internal environment of the cell
   • Hypertonic solution – When external environment of the membrane has lower concentration than the internal environment of the cell
   • Plasmolysis – The shrinkage of a cell when the water is drawn out as and when placed in a hypertonic solution

4. Active Transport – Transport of solute molecules to higher concentration with the input of metabolic energy. Characteristics:
   • Involves the use of permeases
   • Against concentration gradient
   • Metabolic energy is required
   • Shows saturation
   • Permeases shows specificity
   • Irreversible movement

5. Group Translocation – A process in which a molecule is chemically modified as it is brought into the cell. It is a type of active transport since it utilizes metabolic energy during uptake of the molecule. One well known example is the PTS system (Phosphoenolpyruvate:sugar phosphotransferase system). In this system, when a sugar is being taken up, it gets phosphorylated by using PEP as the phosphate donor yielding pyruvate. Bacteria that possess this system include Escherichia, Salmonella, Staphylococcus, Clostridium Most aerobes except Bacillus lack PTS system.

6. Iron Uptake – Iron acts as a very essential micronutrient to microbes for use in cytochromes and many enzymes. Iron uptake becomes extremely difficult due to its insoluble properties which leaves little free iron for utilization. Hence, several bacteria and fungi secrete siderophores. These are low molecular weight organic molecules that can complex with ferric iron and supply it to the cell. 3 molecules of siderophores complex with 1 Fe atom via 6 coordinate bonds. Could be –

• Hydroxamates – eg., Ferrichrome (produced by fungi)
• Phenolate-Catecholate – eg., Enterobactin (a catecholate synthesized by coli)

Microorganisms secrete siderophores when iron is scare in the medium. Once the iron-siderophore complex reaches the cell surface it binds to a siderophore receptor protein. Then either the iron is released into the cell or the whole complex is transported by an ABC transporter. After the iron enters the cell, it is reduced to ferrous state.

ABC Transporters

ATP Binding Cassette transporters are a type of active transport systems. They are observed in all the three domains of life. They contain two hydrophobic transmembrane domains each associated with an ATP binding domain on the cytoplasmic side. The transmembrane domains form a pore in the membrane that allow substances to be transported across the cell membrane. The ATP binding domains bind and hydrolyze ATP to ADP that provides energy for the transport against a concentration gradient. ABC transporters employ special substrate binding proteins present in the periplasmic space in gram negative bacteria or attached to the membrane lipids on the external face of gram positive bacteria. Several superbugs are able to expel drugs out of the cell by ABC transporters.

More on Active Transport

Besides using energy from ATP, cells can also harness energy of the proton gradient that is established during Electron transport chain (ETC). One example is the uptake of lactose by lactose permease in E.coli. This permease is a single protein that takes in a molecule of lactose along with a proton. This kind of transport is known as symport. When lactose and proton bind to their respective sites on the permease on the outward facing conformation, the transporter changes conformation, now facing inward. This reduces the binding affinity of the molecules to their sites and consequently are released in the cytoplasm. E.coli uses this energy in proton gradient also to take up amino acids and organic acids like succinate and malate.

Proton gradient can also power active transport indirectly through a sodium ion gradient. In E.coli, a sodium ion moves out of the cell as a proton moves in. This sodium ion gradient is then used to take up amino acids and sugars. A sodium ion attaches to a carrier protein. The carrier protein then binds to the substrate of interest and orients towards the interior of the cell. Both of them dissociate subsequently. E.coli uses this form of transport to transfer melibiose and glutamate. It is also present in eukaryotes.