膜的構造與功能
細胞膜具有選擇通透性 允許有些物質通過, 有些不行
細胞膜 ( 約 6~10nm) 細胞膜是由液態的雙層磷脂分子構成, 鑲嵌不規則分布的蛋白質分子, 所有的組成分子都是動態的, 可以個別移動 組成成分 : 磷脂質 蛋白質 醣類 膽固醇
1895 年歐夫頓 根據脂溶性物質通過細胞膜的速度較非脂溶性物質快 提出細胞膜是由脂質組成的假說 Hydrophilic head Hydrophobic tail WATER WATER
1917 年 - 藍謬爾把溶在苯中的磷脂質加入水中製造人造膜 1925 年 哥特和葛蘭德測量紅血球分離出的卵磷脂含量, 發現為雙層組成的概念 1935 年 戴布森和丹尼利提出三明治模型 (1950 的電子顯微鏡構造圖支持此假說 )
1972, 辛格和尼克森 推測膜蛋白是散佈的, 而且個別的嵌在磷脂雙層之中 此稱為流體鑲嵌模型 Hydrophobic region of protein Phospholipid bilayer Hydrophobic region of protein
研究細胞膜的利器 冷凍碎裂法 提供了令人信服的視覺證據 APPLICATION A cell membrane can be split into its two layers, revealing the ultrastructure of the membrane s interior. TECHNIQUE Extracellular A cell is frozen and fractured with a knife. layer The fracture plane often follows the hydrophobic interior of a membrane, splitting the phospholipid bilayer into two separated layers. The membrane proteins go wholly with one of the layers. Knife Proteins RESULTS Plasma Cytoplasmic membrane layer These SEMs show membrane proteins (the bumps ) in the two layers, demonstrating that proteins are embedded in the phospholipid bilayer. Extracellular layer Cytoplasmic layer
細胞膜上的磷脂 可移動的 Lateral movement (~10 7 times per second) Flip-flop (~ once per month) (a) Movement of phospholipids
細胞膜上的蛋白質 可漂移 EXPERIMENT Researchers labeled the plasma mambrane proteins of a mouse cell and a human cell with two different markers and fused the cells. Using a microscope, they observed the markers on the hybrid cell. RESULTS Membrane proteins + Mouse cell Human cell Hybrid cell Mixed proteins after 1 hour CONCLUSION The mixing of the mouse and human membrane proteins indicates that at least some membrane proteins move sideways within the plane of the plasma membrane.
磷質的碳氫長鏈尾端 影響膜的流動性 Fluid Viscous Unsaturated hydrocarbon tails with kinks Saturated hydro- Carbon tails (b) Membrane fluidity
膽固醇 在不同溫度下影響膜的流動性 Cholesterol (c) Cholesterol within the animal cell membrane
Fibers of extracellular matrix (ECM) EXTRACELLULAR SIDE OF MEMBRANE Carbohydrate Glycolipid Microfilaments of cytoskeleton Cholesterol Peripheral protein Integral protein CYTOPLASMIC SIDE OF MEMBRANE
主體 ( 整合 ) 蛋白 一部份進入膜的疏水性中心 兩端為疏水性特性 EXTRACELLULAR SIDE N-terminus C-terminus a Helix CYTOPLASMIC SIDE
六種膜蛋白的主要功能 (a) (b) (c) Transport. (left) A protein that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. (right) Other transport proteins shuttle a substance from one side to the other by changing shape. Some of these proteins hydrolyze ATP as an energy ssource to actively pump substances across the membrane. Enzymatic activity. A protein built into the membrane may be an enzyme with its active site exposed to substances in the adjacent solution. In some cases, several enzymes in a membrane are organized as a team that carries out sequential steps of a metabolic pathway. Signal transduction. A membrane protein may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone. The external messenger (signal) may cause a conformational change in the protein (receptor) that relays the message to the inside of the cell. Enzymes Signal ATP Receptor
(d) Cell-cell recognition. Some glyco-proteins serve as identification tags that are specifically recognized by other cells. Glycoprotein (e) Intercellular joining. Membrane proteins of adjacent cells may hook together in various kinds of junctions, such as gap junctions or tight junctions (see Figure 6.31). (f) Attachment to the cytoskeleton and extracellular matrix (ECM). Microfilaments or other elements of the cytoskeleton may be bonded to membrane proteins, a function that helps maintain cell shape and stabilizes the location of certain membrane proteins. Proteins that adhere to the ECM can coordinate extracellular and intracellular changes (see Figure 6.29).
細胞辨識 碳水化合物 用來辨識鄰近細胞
醣類 : 附在膜外的蛋白質或磷脂質上 胚胎發育過程中, 不同細胞表面具有不同的醣類, 有相同或相似的醣類之細胞會發育成特定的織和器官 藉由細胞膜表面的醣蛋白完成辨識自我和非我, 是免疫系統中對抗抗原的憑藉
膜蛋白及脂質 在內質網及高基氏體合成 Secretory protein Glycolipid Golgi 2 apparatus ER Transmembrane glycoproteins 1 Vesicle Secreted protein 3 4 Plasma membrane: Cytoplasmic face Extracellular face Transmembrane glycoprotein Membrane glycolipid
疏水性分子 通過細胞膜的速度快 極性分子 通過細胞膜的速度慢
擴散作用 滲透作用 主動運輸 : 需耗能 胞吐及胞吞
擴散作用 (a) Diffusion of one solute. The membrane has pores large enough for molecules of dye to pass through. Random movement of dye molecules will cause some to pass through the pores; this will happen more often on the side with more molecules. The dye diffuses from where it is more concentrated to where it is less concentrated (called diffusing down a concentration gradient). This leads to a dynamic equilibrium: The solute molecules continue to cross the membrane, but at equal rates in both directions. Molecules of dye Membrane (cross section) Net diffusion Net diffusion Equilibrium
(b) Diffusion of two solutes. Solutions of two different dyes are separated by a membrane that is permeable to both. Each dye diffuses down its own concentration gradient. There will be a net diffusion of the purple dye toward the left, even though the total solute concentration was initially greater on the left side. Net diffusion Net diffusion Equilibrium Net diffusion Net diffusion Equilibrium
擴散作用的種類
滲透作用 水分子通過選擇性辦透膜的現象
Lower concentration of solute (sugar) Higher concentration of sugar Same concentration of sugar Selectively permeable membrane: sugar molecules cannot pass through pores, but water molecules can More free water molecules (higher concentration) Osmosis Water molecules cluster around sugar molecules Fewer free water molecules (lower concentration) Water moves from an area of higher free water concentration to an area of lower free water concentration
高張高滲透壓高濃度 等張等滲透壓等濃度 低張低滲透壓低濃度
滲透壓 - 液體自外界滲入的壓力, 水自 滲透壓低處往滲透壓高處滲透 吸水膨脹脫水萎縮不變
如果沒有細胞壁的情況下 (a) Animal cell. An animal cell fares best in an isotonic environment unless it has special adaptations to offset the osmotic uptake or loss of water. Hypotonic solution Isotonic solution Hypertonic solution H 2 O H 2 O H 2 O H 2 O Lysed Normal Shriveled
動物若生活於低張或高張環境下 必須有適應性的滲透壓調節方式 (a) Animal cell. An animal cell fares best in an isotonic environment unless it has special adaptations to offset the osmotic uptake or loss of water. Hypotonic solution Isotonic solution Hypertonic solution H 2 O H 2 O H 2 O H 2 O Lysed Normal Shriveled
有細胞壁的情況下 (b) Plant cell. Plant cells are turgid (firm) and generally healthiest in a hypotonic environment, where the uptake of water is eventually balanced by the elastic wall pushing back on the cell. H 2 O H 2 O H 2 O H 2 O Turgid (normal) Flaccid Plasmolyzed
通道蛋白 EXTRACELLULAR FLUID (a) Channel protein Solute A channel protein (purple) has a channel through which water molecules or a specific solute can pass. CYTOPLASM Figure 7.15
載體蛋白 Carrier protein Solute Figure 7.15 (b) A carrier protein alternates between two conformations, moving a solute across the membrane as the shape of the protein changes. The protein can transport the solute in either direction, with the net movement being down the concentration gradient of the solute.
主動運輸 : 鈉 - 鉀幫浦 1 Cytoplasmic Na + binds to the sodium-potassium pump. EXTRACELLULAR FLUID [Na + ] high [K + ] low Na + Na + 2 Na+ binding stimulates phosphorylation by ATP. Na + Na + Na + Na + CYTOPLASM [Na + ] low [K + ] high P ADP ATP Na + Na + Na + 3 K + is released and Na + K + 4 Phosphorylation causes the sites are receptive again; protein to change its conformation, the cycle repeats. K + P expelling Na + to the outside. K + K + 5 Loss of the phosphate restores the protein s original conformation. K + K + P P i 6 Extracellular K + binds to the protein, triggering release of the Phosphate group. Figure 7.16
Passive transport. Substances diffuse spontaneously down their concentration gradients, crossing a membrane with no expenditure of energy by the cell. The rate of diffusion can be greatly increased by transport proteins in the membrane. Active transport. Some transport proteins act as pumps, moving substances across a membrane against their concentration gradients. Energy for this work is usually supplied by ATP. Diffusion. Hydrophobic molecules and (at a slow rate) very small uncharged polar molecules can diffuse through the lipid bilayer. Facilitated diffusion. Many hydrophilic substances diffuse through membranes with the assistance of transport proteins, either channel or carrier proteins. ATP
產電性幫浦 ATP + + EXTRACELLULAR FLUID H + H + Proton pump H + + H + CYTOPLASM + + H + H +
同向運輸 + ATP H + + H + H + Proton pump H + + Sucrose-H + cotransporter + H + Diffusion of H + H + H + + + Sucrose
胞吞作用 In phagocytosis, a cell engulfs a particle by Wrapping pseudopodia around it and packaging it within a membraneenclosed sac large enough to be classified as a vacuole. The particle is digested after the vacuole fuses with a lysosome containing hydrolytic enzymes. In pinocytosis, the cell gulps droplets of extracellular fluid into tiny vesicles. It is not the fluid itself that is needed by the cell, but the molecules dissolved in the droplet. Because any and all included solutes are taken into the cell, pinocytosis is nonspecific in the substances it transports. EXTRACELLULAR CYTOPLASM FLUID Pseudopodium Food or other particle PHAGOCYTOSIS Plasma membrane Food vacuole PINOCYTOSIS Vesicle Bacterium Food vacuole 1 µm Pseudopodium of amoeba An amoeba engulfing a bacterium via phagocytosis (TEM). 0.5 µm Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM).
Receptor-mediated endocytosis enables the cell to acquire bulk quantities of specific substances, even though those substances may not be very concentrated in the extracellular fluid. Embedded in the membrane are proteins with specific receptor sites exposed to the extracellular fluid. The receptor proteins are usually already clustered in regions of the membrane called coated pits, which are lined on their cytoplasmic side by a fuzzy layer of coat proteins. Extracellular substances (ligands) bind to these receptors. When binding occurs, the coated pit forms a vesicle containing the ligand molecules. Notice that there are relatively more bound molecules (purple) inside the vesicle, other molecules (green) are also present. After this ingested material is liberated from the vesicle, the receptors are recycled to the plasma membrane by the same vesicle. Receptor RECEPTOR-MEDIATED ENDOCYTOSIS Coat protein Ligand Coat protein Coated pit Coated vesicle A coated pit and a coated vesicle formed during receptormediated endocytosis (TEMs). Plasma membrane 0.25 µm