Anatomy & physiology of cells Huan Ma（马欢），PhD Department of Physiology Room 515, Block C, Research Building School of Medicine, Zijingang Campus Email: mah@zju.edu.cn Tel: 88208068

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Structure of cell membrane: Fluid Mosaic Model (Singer & Nicholson, 1972)

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.

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

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.

Characteristics of ion channels Specificity Gating

Three types of passive, non-coupled transport through integral membrane proteins

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.

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

Primary Active Transport making direct use of energy derived from ATP to transport the ions across the cell membrane

The transported solute binds to the protein as it is phosphorylated (ATP expense).

Extracelluar (mmol/L) Intracellular (mmol/L) Concentration gradient of Na+ and K+ Extracelluar (mmol/L) Intracellular (mmol/L) Na+ 140.0 15.0 K+ 4.0 150.0

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.

Na+-K+ pump (Na+ pump, Na+-K+ ATPase) electrogenic pump

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

Other primary active transport Primary active transport of calcium Primary active transport of hydrogen ions etc.

Secondary Active Transport The ion gradients established by primary active transport permits the transport of other substances against their concentration gradients

Secondary active transport uses the energy in an ion gradient to move a second solute.

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)

Cotransporters

Exchangers

Ion gradients, channels, and transporters in a typical cell

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.

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.

Begin: The partition between the compartments is permeable to water only. After diffusional equilibrium has occurred: Movement of water only has equalized solute concentration.

Role of Na-K pump in maintaining cell volume

Response to cell shrinking

Response to cell swelling

Endocytosis and Exocytosis 入胞与出胞

Alternative functions of endocytosis: Transcellular transport 2. Endosomal processing 3. Recycling the membrane 4. Destroying engulfed materials

Endocytosis

Exocytosis

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

Epithelial Transport

Glands

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)

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