Transport in Cells (Uptake of Nutrients)

Transport in Cells

(Uptake of Nutrients)

Introduction:

• The plasma membrane is a selectively permeable barrier between the cell and the extracellular environment.
• Its permeability properties ensure that essential molecules such as ions, glucose, amino acids, and lipids readily enter the cell, metabolic intermediates remain in the cell, and waste compounds leave the cell.
• In short, the selective permeability of the plasma membrane allows the cell to maintain a constant internal environment.
• Movement of virtually all molecules and ions across cellular membranes is mediated by selective membrane transport proteins embedded in the phospholipid bilayer.
• Different cell types require different mixtures of low-molecular-weight compounds.
• So, the plasma membrane of each cell type contains a specific set of transport proteins that allow only certain ions and molecules to cross.
• Organelles within the cell often have a different internal environment from that of the surrounding cytosol, and organelle membranes contain specific transport proteins that maintain this difference.
• The phospholipid bilayer, the basic structural unit of biomembranes, is essentially impermeable to most water-soluble molecules, ions, and water itself.

Fig: Uptake of Nutrients

• In order to support its’ activities, a cell must bring in nutrients from the external environment across the cell membrane.
• In bacteria and archaea, several different transport mechanisms exist.

Passive transport:

• It is a spontaneous, unaided movement of solute particles along their concentration gradient, electric gradient or pressure gradient.
• The particles move from the higher concentration to the lower, till they equilibrate.
• Permeability depends on lipid solubility and molecular size.
• Membrane is highly permeable to gases such as carbondioxide(CO₂), nitrogen (N₂) and oxygen (O₂), small hydrophobic molecules and small uncharged polar molecules such as ethanol, steroids and urea, slightly permeable to water and impermeable to ions and to large polar molecules.

Passive Diffusion:

• Passive or simple diffusion allows for the passage across the cell membrane of simple molecules and gases, such as CO2, O2, and H2O.
• This process does not require energy and the rate of entry is proportional to the solubility of the solute in hydrophobic cores of the membrane.
• Solutes are transported by concentration gradient where they diffused from higher to lower concentrations.
• As more of the substance is transported into the cell the concentration gradient decreases, slowing the rate of diffusion.

         Osmosis:

• Osmosis is the spontaneous net movement of solvent molecules through a selectively permeable membrane into a region of higher solute concentration, in the direction that tends to equalize the solute concentrations on the two sides.

Fig: Omosis

Facilitated Diffusion:

• Facilitated diffusion also involves the use of a concentration gradient.
• It requires mediation of specific integral membrane protein for facilitating the movement of the solutes known as carrier proteins (sometimes called permeases).
• These proteins are embedded within the cell membrane and provide a channel or pore across the membrane barrier, allowing for the passage of larger molecules.
• If the concentration gradient dissipates, the passage of molecules into the cell stops.
• Each carrier protein typically exhibits specificity, only transporting in a particular type of molecule or closely related molecules.
• Movement of particles occurs along their concentration gradient and is energy independent process.
• As it is a carrier-mediated process, equilibrium is attained more rapidly than in passive diffusion.
• Structurally similar solutes can competitively inhibit the entry of original solutes.

Fig: Facilitated diffusion

• The solute binding site is exposed toward exterior of the cell.
• When a solute binds to this site, a conformation change is induced in the protein which permits the solute to be transferred to the cytosolic side of the membrane.
• Release of the solute on this side triggers the return of protein to its original conformation.
• This process is reversible which says that solute could be extruded from the cell by reversal of the above events.
• Direction of movement depends on the relative concentration of solutes on two sides of the membrane.
• Fate of transport also depends on the percent saturation of the carrier protein.
• It raises maximum when all the carrier proteins are fully saturated with the solute.
• Thus, systems subjected to facilitated diffusion are saturable.
• The symport systems as well as the antiport systems are known to be involved infaciliated transport; few examples are glucose transporters, aquaporins etc.

Glucose transporters:

• Glucose transporters are a wide group of membrane proteins that transport glucose across the cell membrane by facilitated diffusion.
• They shuttle between two conformational states, one in which substrate-binding site faces outward and another in which the binding site faces inward.
• 14 GLUTS are encoded by the human genome.
• First, open up on the one side and binds to glucose when fixed, the complex changes the configuration and opens up at the inner side releasing the glucose.
• These transporters display a tissue-specific pattern of expression.

Fig: Model of uniport transport by GLUT1

Model of uniport transport by GLUT1.

• In one conformation, the glucose-binding site faces outward; in the other, the binding site faces inward.
• Binding of glucose to the outward-facing site (step 1 ) triggers a conformational change in the transporter that results in the binding site’s facing inward toward the cytosol (step 2 ).
• Glucose then is released to the inside of the cell (step 3 ).
• Finally, the transporter undergoes the reverse conformational change, regenerating the outward-facing binding site (step 4 ).

Aquaporins (AQP):

• They are integral membrane proteins that serve as channels in the transfer of water, and in some cases, small solutes across the membrane.
• The cell membranes of a variety of different bacteria, fungi, animal and plant cells contain aquaporins through which water can flow more rapidly into and out of the cell than by diffusing through the phospholipid bilayer.

Fig: Aquaporins (AQP)

Ion channels:

• Ion channels are protein molecules that span across the cell membrane allowing the passage of ions from one side of the membrane to the other.
• They have an aqueous pore, which becomes accessible to ions after a conformational change in the protein structure that causes the ion channel to open.

Fig: Ion channels

Ligand-gated ion channels:

• Ligand-gated ion channels, also commonly referred to as ionotropic receptors.
• They are a group of transmembrane ion-channel proteins which open to allow ions such as Na⁺, K⁺, Ca²⁺, and/or Cl⁻ to pass through the membrane.
• In response to the binding of a chemical messenger, such as a neurotransmitter.

Fig: Ligand-gated ion channels

G protein–coupled receptor:

• G protein-coupled receptor (GPCR), also called the seven-transmembrane receptor or heptahelical receptor.
• This protein is located in the cell membrane that binds extracellular substances and transmits signals from these substances to an intracellular molecule called a G protein (guanine nucleotide-binding protein).
• G-protein-coupled receptors (GPCRs) are the largest and most diverse group of membrane receptors in eukaryotes.

Fig: G protein-coupled receptor (GPCR)

Voltage–gated ion channels

• G protein-coupled receptors are a class of transmembrane proteins that form ion channels that are activated by changes in the electrical membrane potential near the channel.
• The membrane potential alters the conformation of the channel proteins, regulating their opening and closing.

Fig: Voltage-gated ion channels

Ionophores:

• An ionophore is a chemical species that reversibly binds ions.
• Many ionophores are lipid-soluble entities that transport ions across a cell membrane.
• Ionophore means “ion carrier” as these compounds catalyze ion transport across hydrophobic membranes or lipid bilayers found in the living cells or synthetic vesicles (liposomes).
  Example:
  • Gramicidin A (H⁺, Na⁺, K⁺)
  • 2,4-Dinitrophenol (DNP) (H⁺)
  • Ionomycin (Ca²⁺)
  • Lasalocid (K⁺, Na⁺, Ca²⁺, Mg²⁺)
  • Monensin (Na⁺, H⁺)

Fig: Ionophores

Active Transport

• Many types of nutrient uptake require that a cell be able to transport substances against a concentration gradient (i.e. with a higher concentration inside the cell than outside).
• In order to do this, a cell must utilize metabolic energy for the transport of the substance through carrier proteins embedded in the membrane. This is known as active transport.
• All types of active transport utilize carrier proteins.
• Actively drives molecules across the cell membrane from a region of lower concentration to a region of higher concentration.
• It requires energy input.

Fig: Active Transport

Primary active transport

• Primary active transport involves the use of chemical energy, such as ATP, to drive transport.
• One example is the ABC system, which utilizes ATP Binding Cassette transporters.
• Each ABC transporter is composed of three different components:
• membrane-spanning proteins that form a pore across the cell membrane (i.e. carrier protein),
• an ATP binding region that hydrolyzes ATP, providing the energy for the passage across the membrane, and
• a substrate-binding protein, a peripheral protein that binds to the appropriate substance to be transporter and ferries it to the membrane-spanning proteins.

Fig: ABC system

• In gram-negative bacteria, the substrate-binding protein is located in the cell’s periplasm, while in gram-positive bacteria the substrate-binding protein is attached to the outside of the cell membrane.

Sodium–Potassium Pump

• The sodium-potassium pump is found in many cell (plasma) membranes.
• Powered by ATP, the pump moves sodium and potassium ions in opposite directions, each against its concentration gradient.
• In a single cycle of the pump, three sodium ions are extruded from and two potassium ions are imported into the cell.

Fig: Sodium–Potassium Pump

Secondary active transport

• Secondary active transport utilizes energy from a proton motive force (PMF).
• A PMF is an ion gradient that develops when the cell transports electrons during energy-conserving processes.
• Positively charged protons accumulate along the outside of the negatively charged cell, creating a proton gradient between the outside of the cell and the inside.

Fig: Secondary active transport

Different types of transport events:

• There are three different types of transport events for simple transport: Uniport, Symport, and Antiport and each mechanism utilizes a different protein porter. 

Fig: Uniport, Symport, and Antiport

• Uniporters transport a single substance across the membrane, either in or out.
• Symporters transport two substances across the membrane at the same time, typically a proton paired with another molecule.
• Antiporters transport two substances across the membrane as well, but in opposite directions. As one substance enters the cell, the other substance is transported out.

Fig: Examples of Uniport, Symport, and Antiport

ATP:

• Like with sodium and potassium in the neuron, the hydrogen ions are “pumped,” using ATP, across the mitochondrial inner membrane (against the H+ gradient).
• They then diffuse across the membrane through a protein channel (an enzyme called ATP Synthase).
• The enzyme uses the movement of the H+ to create ATP from a precursor called ADP.

Fig: Chemiosmosis

Group Translocation

• Group translocation is a distinct type of active transport, using energy from an energy-rich organic compound that is not ATP.
• Group translocation also differs from both simple transport and ABC transporters in that the substance being transported is chemically modified in the process.
• A mechanism utilized by bacteria to transport a compound into their cell by first allowing the compound to bind with protein on the cell surface followed by altering its chemical structure during its passage across the
  membrane.
• One of the best-studied examples of group translocation is the phosphoenolpyruvate: sugar phosphotransferase system (PTS), which uses energy from the high-energy molecule phosphoenolpyruvate (PEP) to transport sugars into the cell.
• Phosphate is transferred from the PEP to the incoming sugar during the process of transportation.

Fig: Group translocation

Iron Uptake

• Iron is required by microbes for the function of their cytochromes and enzymes, resulting in it being a growth-limiting micronutrient.
• However, little free iron is available in environments, due to its insolubility.
• Many bacteria have evolved siderophores, organic molecules that chelate or bind ferric iron with high affinity.
• Siderophores are released by the organism to the surrounding environment, whereby they bind any available ferric iron.
• The iron-siderophore complex is then bound by a specific receptor on the outside of the cell, allowing the iron to be transported into the cell.

Fig: Iron Uptake by bacteria

Endocytosis

• Endocytosis – the process of taking liquids or fairly large molecules into a cell by engulfing them in a membrane.
• The cell membrane makes a pocket around the substance.

Fig: Endocytosis

Types of Endocytosis:

There are three types of endocytosis: phagocytosis, pinocytosis, and receptor-mediated endocytosis.

Fig: Types of endocytosis (phagocytosis, pinocytosis, and receptor-mediated endocytosis)

Phagocytosis:

• Phagocytosis – the word literally means “cell eating.” 
• The cell’s plasma membrane surrounds a macromolecule or even an entire cell from the extracellular environment and buds off to form a food vacuole or phagosome.
• It is a special type of endocytosis which plays a major role in your immune system.
• White blood cells find foreign materials, such as bacteria, engulf them and destroy them. 
  • They are your body’s enforcers.

Fig: Phagocytosis

Pinocytosis

• In cellular biology, pinocytosis, known as fluid endocytosis and bulk phase pinocytosis,
• It is a mode of endocytosis in which small particles suspended in extracellular fluid are brought into the cell through an invagination of the cell membrane.
• Resulting in a suspension of the particles within a small vesicle.

Fig: Pinocytosis

Receptor–mediated endocytosis

• Receptor-mediated endocytosis is a process through which bulk amounts of specific molecules can be imported into a cell after binding to cell surface receptors.
• Also called clathrin-mediated endocytosis, is a process by which cells absorb metabolites, hormones, proteins – and in some cases viruses.
• By the inward budding of the plasma membrane (invagination).

Fig: Receptor-mediated endocytosis

• The pocket breaks off inside the cell and forms a vesicle.
• The vesicle then fuses with a lysosome. 
• Lysosomal enzymes break down the vesicle membrane and the vesicle’s contents are release into the cell.

Fig: Break down of vesicle

Exocytosis:

• Exocytosis is the opposite of endocytosis.
• It is the release of substances out of a cell by the fusion of a vesicle with the cell membrane.

Fig: Exocytosis

• Note how the vesicle pinches off from the Golgi Apparatus.
• Remember that the Golgi Apparatus is the cell’s “storage, packing, and shipping center.”

Fig: Golgi Apparatus

• The cell forms a vesicle around material that needs to be removed or secreted.
• The vesicle is transported to the cell membrane.
• The vesicle membrane fuses with the cell membrane and releases the contents.

Fig: Exocytosis by cell.

Exocytosis in Neuron

• The neurotransmitter is stored in vesicles in the terminus of the axon of the neuron.
• When the action potential arrives, the vesicles, via exocytosis, release, their neurotransmitter (for example, acetylcholine) into the synaptic cleft.
• Membrane receptors on the neuron on the other side of the synaptic cleft stimulate is stimulated by the neurotransmitter to “fire” – transferring the action potential from one neuron to the other.

Fig: Exocytosis in Neuron