Lecture 3 4 Biological Transport PDF

Transport across the plasma membrane  Plasma membrane acts as a barrier The lipid bilayer of membrane forms a barrier to the passage of various polar molecules. Therefore, the concentrations of various solutes in both sides of the lipid bilayer are different from each other. By generating...

Transport across the plasma membrane  Plasma membrane acts as a barrier The lipid bilayer of membrane forms a barrier to the passage of various polar molecules. Therefore, the concentrations of various solutes in both sides of the lipid bilayer are different from each other. By generating ionic concentration differences across the lipid bilayer, potential energy is stored in the form of electrochemical gradients, which can be used to:- (1) generate high energy molecules, e.g. ATP, in certain membrane-enclosed organelles, such as mitochondria and chloroplasts. (2) drive various transport processes (3) convey electrical signals in electrically excitable cells Ion concentrations within the cell are different from those outside Few molecules cross the membrane by passive diffusion Permeabilities of various biomolecules Dependent on their polarity (solubility in lipid bilayer) and molecular sizes, different molecules have different permeabilities. Non-polar (lipid soluble) molecules: can cross the lipid bilayer rapidly. E.g. steroid hormones, oxygen, cholesterol. Polar, uncharged molecules: only small molecules (e.g. urea, water, ethanol) can pass but not bigger molecules (e.g. glucose). Charged molecules: cannot pass through the lipid bilayer.  Passive transport vs. Active transport If a molecule is transported from a high concentration area to a low concentration area without consumption of extra-energy, it is called passive transport (or facilitated diffusion). If a molecule is transported from a low concentration area to a high concentration area, it is called active transport. Additional energy source is required for this process.  Driving force in passive transport In passive transport, if the molecule is uncharged, the only driving force is the concentration gradient. If the molecule carries net charge, both its concentration gradient and the electrical potential difference across the membrane (membrane potential) are the driving forces of passive transport. The net driving force due to the concentration gradient and the membrane potential is named electrochemical gradient. Solutes cross membranes by passive or active transport Two major classes of transport proteins 1. Transporters: which have moving parts for the transport of the protein. 2. Channel proteins: which have a hydrophilic pore that allows the passive movement of small inorganic molecules. All channel proteins allow passive transport only. Some transporter proteins allow passive transport and some allow active transport. Transport through channel protein is usually faster than that of the transport protein. - usually transport amino acids or - allows specific ions (Na+, K+, sugars or other molecules Ca2+, Cl-) to pass through - bind the molecule on one side, - a pore that a specific ion can change shape, and release it on pass through the other side - passive (facilitated) or active transport - passive transport only Kinetics of simple diffusion compared to carrier-mediated diffusion Active membrane transport Coupled transport Ion Channels  2 Na+/glucose symporter electrochemical Na+ drives import of glucose Glucose uptake in the gut - 3 Na+ are pumped out - 2 K+ are pumped in - Ouabain is an inhibitor as it competes with K+ for its extracellular binding site. Na+-K+ pump is an ATPase that operates as an antiporter - 3 Na+ are pumped out - 2 K+ are pumped in - Ouabain is an inhibitor as it competes with K+ for its extracellular binding site. The Na+-K+ pump is important for the osmotic balance and cell volume regulation of the cells Marcomolecules Charged metabolites (DNA, Proteins) (amino acids, nucleotides) Na+-K+ ATPase has a direct role in regulating cell volume Aquaporins So how does the water get in and out? Low intracellular level of Ca2+ ions are maintained by the Ca2+ ion pumps Extracellular level: 10-3 M Cytosolic level: 10-7 M Intracellular storage: sacroplasmic reticulum of muscle cells. One calcium pump is an ATPase. Another pump is an Na+ antiporter. Ion Channels and Electrical Properties of Membranes ⚫ More than one hundred types of ion channels have been described. A single nerve cell can contain > 10 kinds of ion channels, located in different regions of its plasma membrane. ⚫ They allow ions to pass from a high concentration area to a low concentration area. The rate of ion flux increases proportionally with the ion concentration difference until it saturates at its maximum level. ⚫ Different ion channels have different ion selectivity. Voltage- gated Na+ channel is 10 times more permeable to Na+ than K+, whereas K+ channel is 100 times more permeable to K+ than Na+. How are channels selective? K+ Channel selectivity ⚫ The Na+ channel works as a size filter that allows Na+, which has a smaller ionic radius than K+, to pass. ⚫ In contrast, charged ion needs to shed off their associated water molecules in order to pass the narrowest part of the K+ channel. The selectivity of K+ channel is restricted from the interactions between K+ ions and polar amino acid side chains lining the pore. These interactions displace the water molecules bound to K+ and allow dehydrated K+ to pass. The K+ leak channel is only selective to K+ (10,000- fold), but not to Na+ ⚫ The diameters of the K and Na ions are 0.133 nm and 0.095 nm, respectively. Gated ion channels respond to different kinds of stimuli  Voltage-gated ion channels These voltage-gated cation (Na+, K+, Ca2+) channels all belong to the same protein family. Each channel is composed of four subunits (K+) or domain (Na+, Ca2+) and each domain contains six transmembrane - helices and an anti-parallel  sheet which lines the pore. The side chains of the -sheet determine the ion selectivity of the channel. Open when the membrane is depolarized Passive ion movement is the largest contributor to membrane potential in animal cells K+ leak channels allow K+ to flow freely in and out of the cell This always moves the cell towards “resting potential” Equilibrium potential is given by the Nernst equation A small imbalance of charge gives significant voltage Nerve impulse can travel at a speed of 100 meters/second and it is directional Voltage-gated channels are responsible for the generation of action potentials in neurons Myelination insulates the neurons and increases the speed and efficiency of action potential propagation in nerve cells. ⚫ Neurons is surrounded by supporting cells (glial cells). - Glial cells in CNS: oligodendrocytes. - Glial cells in PNS: Schwann cells. ⚫ These cells synthesize myelin sheath that wraps the neuron several times. The myelin sheath insulates the axonal membrane and stops ion flow/leakage of the membrane potentials. ⚫ The sheath is interrupted at nodes of Ranvier at regular spacing, where a high density of Na+ channels are located. The action potential jumps from one node to another node, thereby: 1. travel at faster speed. 2. save metabolic energy as the excitation is confined to a limited area. Transmitter-gated cation channels The neurotransmitters bind to ligand-gated channels and signal a transient opening of the channels, ion influx then creates a depolarization of membrane, which is transmitted along the dendrites as a membrane potential.  Transmitter-gated channels There are many transmitter-gated channels and they have different characteristics. 1. They have highly selective binding sites to the respective neurotransmitters. 2. They have different ion selectivity. 3. They are insensitive to membrane potential but their actions may initiate or inhibit the formation of action potentials. Excitatory neurotransmitters (acetylcholine, glutamate, serotonin, etc.): open cation channels and cause an influx of Na + ions (Depolarize the postsynaptic membrane and fire an action potential). Inhibitory neurotransmitters (GABA, glycine, etc.): open anion channels and cause an influx of Cl-. Influx of Cl- hyperpolarize the membrane and increase the threshold for the firing of action potential. Light-gated ion channels - Channelrhodopsins de Maleprade et al. Phys Rev. 2020 40 Light-gated ion channels - Channelrhodopsins From algae Requires the chromophore all-trans-retinal Responds to blue ~480 nm light Returns to dark conformation within ms of stimulation Conformational change in chromophore allows pore to open up to ~0.6 nm Allows various cations to pass Has been engineered for different wave/lengths specificity 41  Optogenetics Karl Deisseroth Lin et al., Nature, 2011 42  Cellular optogenetics Natural photoreceptors Engineerable photosensitive proteins 43 Optogenetics can be used to precisely and dynamically control localization CAT 44 Optogenetics can be used to precisely and dynamically control localization opto CAT 45 Optogenetics can be used to precisely and dynamically control localization opto CAT youtube.com 46 Cellular optogenetics can control… cell biology! Migration Signaling Toettcher et al., Cell., 2013 Yoo et al., Dev Cell., 2010 Morphogenesis Izquierdo et al., Nat. Comm., 2018  Various light-sensitive domains have been engineered into tools for cellular optogenetics Flavin ON/OFF PCB / Biliverdin ON/OFF 48 The Cytoskeleton Cytoskeleton: a complex network of protein filaments that extend throughout the cytoplasm. - Cytoskeleton is a dynamic network that reorganize continuously to help the cell change its shape, divide and respond to its environment. - Cell migration transportation of organelle, segregation of chromosomes, etc…  Actin There are three types of protein filaments: a. actin filaments b. intermediate filaments c. Microtubules Microtubules Intermediate Filaments Tissues and Organs are composed of Cells and Extracellular Matrix (ECM)  Main ways cells are bound together Some tissues have plenty ECM where cells are sparsely distributed within it, such as connective tissues. Some tissues are mainly composed of cells, such as the epithelial tissues. Types of junctions Tight junctions form a selective permeability barrier across epithelial cell sheets Tight junctions form barriers to the movement of water, solutes and cells from one body compartment to the other. To maintain a directional transcellular transport of solutes from one side of the cell to the other side of the cell. - 3 Na+ are pumped out - 2 K+ are pumped in - Ouabain is an inhibitor as it competes with K+ for its extracellular binding site. Tight junction consists of the transmembrane claudin and occludin proteins. The claudins are the main components of the sealing strands. The function of the occludin effects the permability. The specific makup of the tight junction proteins may make in permable to certain ions or small molecules. Four specific types of anchoring junctions have been defined in vertebrates: A. Adherens junction B. Desmosome C. Actin-linked cell matrix junction D. Hemidesomsome Adherens junctions and cell matrix junctions The actin filament bundles lie parallel to the plasma membrane and are linked to the cadherins through various intracellular attachment proteins such as catenins, vinculin, -actinin and plakoglobin. Hence, a transcellular actin network is formed. The contraction of this actin network, which depends on myosin motor proteins, is thought to mediate the folding of epithelial cell sheets, a fundamental process in morphogenesis. E-Cad N-Cad  Desmosomes Desmosomes are buttonlike junctions that connect the intermediate filaments of adjacent cells indirectly to form a continuous network throughout the tissue. The particular type of intermediate filaments attached to the desmosomes depends on the cell type: keratin filaments in epithelial cells and desmin filaments in heart muscle cells. Desmosomes  Cell-Matrix junctions A. Hemidesomsome B. Actin-linked cell matrix junction  Hemidesmosomes Instead of joining adjacent cells, they connect the basal surface of epithelial cells to the underlying basal lamina – a specialized mat of ECM at the interface between the epithelium and connective tissue. Integrins are involved in hemidesmosomes whereas cadherins are involved in the desmosomes. Actin-linked cell matrix junctions (Integrins)