Chapter 4

Solute transport through cell membraneLearning OutcomeRepresent and relate the basic processes,Describe the principles and mechanisms involved,Illustrate the concepts underlying the topicsolute transportTopics:Overview of the cell membrane and its functionsActive transport vs passive diffusionType of transport processes:Simple transportGroup translocationATP Binding Casette SystemOverview of the cell membraneA.k.a cytoplasmic membrane or plasma membrane. It lies internal to the cell wall and encloses the cytoplasm of the bacterium.

A. Structure and CompositionLike all biological membranes in nature, the bacterial cytoplasmic membrane is composed of:phospholipidand protein molecules

The phospholipid bilayer is arranged so that thepolar endsof the molecules (the phosphate and glycerol portion of the phospholipid that is soluble in water) form theoutermost and innermost surfaceof the membrane while thenon-polar ends(the fatty acid portions of the phospholipids that are insoluble in water) form thecenterof the membrane.

B. FunctionsThe cytoplasmic membrane is aselectively permeable membrane that determines what goes in and out of the organism . All other molecules require carrier molecules to transport them through the membrane.Materials move across the bacterial cytoplasmic membrane by passive diffusion and active transport.

Figure 4.14b

The Plasma MembranePhospholipid bilayerPeripheral proteinsIntegral proteinsTransmembraneProteinsFigure 4.14a

The Plasma MembraneMovement of Substances Across MembranesPassive Transport:Simple DiffusionFacilitated DiffusionOsmosis

Active Transport:AntiportersSymportersATP-binding cassette (ABC) systemGrouptranslocation

1.Passive DiffusionPassive diffusion is the net movement of gases or small uncharge polar molecules across a phospholipid bilayer membrane from an area ofhigher concentration to an area of lower concentration.Figure 4.17a

a) Simple diffusion: Movement of a solute from an area of high concentration to an area of low concentrationb. OsmosisThe diffusion ofwateracross a membrane from an area ofhigher water concentration(lower solute concentration)to lower water concentration(higher solute concentration).Osmosis is powered by the potential energy of a concentration gradient and does not require the expenditure of metabolic energyFigure 4.18ce

The Principle of Osmosisc.Facilitated DiffusionTransport of substances across a membrane by transport proteinsFacilitated diffusion is powered by the potential energy of a concentration gradient and does not require the expenditure of metabolic energy.

Facilitated Diffusion: Carrier protein molecules aid in the movement of substances through cell membrane from high to low concentrationFigure 4.17b-c

Facilitated diffusion: Solute combines with a transporter protein in the membraneFigure 4.17d

Through lipid layerAquaporins (water channels)Movement of Materials across MembranesActive transport: Requires a transporter protein and ATPGroup translocation: Requires a transporter protein and PEP2.Active TransportActive transportis a process whereby the cell usesboth transport proteins and metabolic energyto transport substances across the membraneagainst the concentration gradient. Active transport allows cells to accumulate needed substances even when the concentration is lower outside.The energy is provided byproton motive force,the hydrolysis of ATP, orthe breakdown of some other high-energy compoundsuch as phosphoenolpyruvate (PEP).

Proton motive force is an energy gradient resulting from hydrogen ions (protons) moving across the membrane from greater to lesser hydrogen ion concentration. ATP is the form of energy cells most commonly use to do cellular work. PEP is one of the intermediate high-energy phosphate compounds produced during glycolysis.

Active Transport: Carrier Protein molecules aid in movement of molecules against a concentration gradientFor the majority of substances a cell needs for metabolism to cross the cytoplasmic membrane, specifictransport proteins(carrier proteins) are required.This is because the concentration of nutrients in most natural environments is typically quite low. Transport proteins allow cells to accumulate nutrients from even a space environment.Transport proteins involved in active transport include antiporters, symporters, the proteins of the ATP-binding cassette (ABC) system, and the proteins involved in group translocation.

AntiporterAntiporters are transport proteins that simultaneously transport two substances across the membrane in opposite directionsOne against the concentration gradient and one with the concentration gradient.Metabolic energy is required for this type of transport

SymporterSymportersare transport proteins thatsimultaneously transport two substances across the membrane in the same direction.Symporters use the potential energy of electrochemical gradients from protons (H+), that is, proton motive force to co-transport ions, glucose, and amino acids against their concentration gradient

ATP-binding cassette (ABC) systemAn example of an ATP-dependent active transport found in various gram-negative bacteria is theATP-binding cassette (ABC) system. This involves substrate-specific binding proteins located in the bacterial periplasm, the gel-like substance between the bacterial cell wall and cytoplasmic membraneStep 1The periplasmic-binding protein picks up the substance to be transported and carries it to a membrane-spanning transport protein

Step 2The molecule to be transported across the membrane enters the transporter protein system and a molecule of ATP enters the ATP binding site of the ATP-hydrolyzing protein.

Step 3Energy provided by the hydrolysis of ATP into ADP, phosphate, and energy moves the molecule across the membrane

Step 4The carrier protein releases the molecule being transported and the transporter system is ready to be used again.

GrouptranslocationIt is another form of active transport that can occur in prokaryotes. In this case, a substance is chemically altered during its transport across a membrane so that once inside, the cytoplasmic membrane becomes impermeable to that substance and it remains within the cell.

Step 1When bacteria use the process of group translocation to transport glucose across their membrane, a high-energy phosphate group from phosphoenolpyruvate (PEP) is transferred to the glucose molecule to form glucose-6-phosphate.

Step 2A high-energy phosphate group from PEP is transferred to the glucose molecule to form glucose-6-phosphate.

Step 3The glucose-6-phosphate is transported across the membrane.

Step 4Once the glucose has been converted to glucose-6-phosphate and transported across the membrane, it can no longer be transported back out.