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Anatomy & physiology of cells
Huan Ma(马欢),PhD Department of Physiology Room 515, Block C, Research Building School of Medicine, Zijingang Campus Tel:
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ahmad ata
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Structure of cell membrane:
Fluid Mosaic Model (Singer & Nicholson, 1972)
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Net flux accounts for solute movements in both directions.
Note: the partition between the two compartments is a membrane that allows this solute to move through it. Net flux accounts for solute movements in both directions.
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Simple Diffusion 单纯扩散 Relative to the concentration gradient
movement is DOWN the concentration gradient ONLY (higher concentration to lower concentration) Rate of diffusion depends on The concentration gradient Charge on the molecule Size Lipid solubility Temperature
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In simple diffusion, flux rate is limited only by the concentration gradient. In carrier- mediated transport, the number of available carriers places an upper limit on the flux rate.
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Characteristics of ion channels
Specificity Gating
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Three types of passive, non-coupled transport through integral membrane proteins
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facilitated diffusion, solutes move in the direction predicted
membrane In both simple and facilitated diffusion, solutes move in the direction predicted by the concentration gradient. In active transport, solutes move opposite to the direction predicted by the concentration gradient.
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Active transport 主动转运 Primary active transport 原发性主动转运
Secondary active transport 继发性主动转运
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Primary Active Transport
making direct use of energy derived from ATP to transport the ions across the cell membrane
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The transported solute binds to the protein
as it is phosphorylated (ATP expense).
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Extracelluar (mmol/L) Intracellular (mmol/L)
Concentration gradient of Na+ and K+ Extracelluar (mmol/L) Intracellular (mmol/L) Na K
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Here, in the operation of the Na+-K+-ATPase, also known
as the “sodium pump,” each ATP hydrolysis moves three sodium ions out of, and two potassium ions into, the cell.
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Na+-K+ pump (Na+ pump, Na+-K+ ATPase)
electrogenic pump
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Physiological role of Na+-K+ pump
Maintaining the Na+ and K+ gradients across the cell membrane Partly responsible for establishing a negative electrical potential inside the cell Controlling cell volume Providing energy for secondary active transport
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Other primary active transport
Primary active transport of calcium Primary active transport of hydrogen ions etc.
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Secondary Active Transport
The ion gradients established by primary active transport permits the transport of other substances against their concentration gradients
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Secondary active transport uses the energy in
an ion gradient to move a second solute.
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Cotransport Countertransport
the ion and the second solute cross the membrane in the same direction (e.g. Na+-glucose, Na+-amino acid cotransport) Countertransport the ion and the second solute move in opposite directions (e.g. Na+-Ca2+, Na+-H+ exchange)
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Cotransporters
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Exchangers
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Ion gradients, channels, and transporters in a typical cell
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Solvent + Solute = Solution
Osmosis(渗透) Solvent + Solute = Solution Here, water is the solvent. The addition of solute lowers the water concentration. Addition of more solute would increase the solute concentration and further reduce the water concentration.
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Begin: The partition between the compartments is permeable to water and to the solute. After diffusional equilibrium has occurred: Movement of water and solutes has equalized solute and water concentrations on both sides of the partition.
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Begin: The partition between the compartments is permeable to water only. After diffusional equilibrium has occurred: Movement of water only has equalized solute concentration.
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Role of Na-K pump in maintaining cell volume
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Response to cell shrinking
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Response to cell swelling
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Endocytosis and Exocytosis 入胞与出胞
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Alternative functions
of endocytosis: Transcellular transport 2. Endosomal processing 3. Recycling the membrane 4. Destroying engulfed materials
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Endocytosis
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Exocytosis
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Two pathways of exocytosis
Constitutive exocytosis pathway -- Many soluble proteins are continually secreted from the cell by the constitutive secretory pathway Regulated exocytosis pathway -- Selected proteins in the trans Golgi network are diverted into secretory vesicles, where the proteins are concentrated and stored until an extracellular signal stimulates their secretion
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Epithelial Transport
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Glands
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Summary Diffusion: solute moves down its concentration gradient:
simple diffusion: small (e.g., oxygen, carbon dioxide) lipid soluble (e.g., steroids) facilitated diffusion: requires transporter (e.g., glucose)
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Active transport: solute moves against its concentration gradient:
primary active transport: ATP directly consumed (e.g., Na+ -K+ ATPase) secondary active transport: energy of ion gradient (usually Na+) used to move second solute (e.g., nutrient absorption in gut) Exo- and endo- cytosis: large scale movements of molecules
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