CHAPTER 8 MEMBRANE STUCTURE AND FUNCTION How things

CHAPTER 8 MEMBRANE STUCTURE AND FUNCTION How things get into and out of the cell Active Transport ﺍﻟﻨﻘﻞ ﺍﻟﻨﺸﻂ 1

Active transport is the pumping ﺥ of solutes against their concentration gradients ﺍﻹﻧﺤﺪﺍﺭ ﺍﻟﺘﺮﻛﻴﺰﻯ • Some facilitated transport proteins can move solutes against their concentration gradient, from the side where they are less concentrated to the side where they are more concentrated. • This active transport requires metabolic energy via ATP. • Active transport is critical ﺑﺎﻟﻎ ﺍﻷﻬﻤﻴﺔ for a cell to maintain its internal concentrations of small molecules. • Active transport is performed by specific proteins embedded in the membranes (transport protein). 2 Fig. 8. 14 b, Page 148

1)- Small molecules (Ions ) • The sodium-potassium pump actively maintains the gradient of sodium (Na+) and potassium ions (K+) across the membrane. – The animal cell has higher concentrations of K+ and lower concentrations of Na+ inside the cell. – The sodium-potassium pump (T. protein) uses the energy of one ATP to pump 3 Na+ ions out and 2 K+ ions in. 1 ATP T. protein Low conc. + 2 of K High conc. + of Na High conc. + of K Low conc. 3 of Na+ Outside the cell Na Na Na Protein molecule ATP Cellular membrane Na Inside the cell 3

4 Fig. 8. 15, Page 149

Two roles of membrane protein Fig. 8. 16, Page 150 Both diffusion and facilitated diffusion are forms of passive transport of molecules down their concentration gradient, gradient while active transport requires an investment of energy to move molecules against their concentration gradient 5

Some ion-pumps generate voltage ﻫﺪ ﻛﻬﺮﺑﺎﺋﻰ across membranes • All cells maintain a voltage across their plasma membranes. – The cytoplasm of a cell is negative in charge compared to the extracellular fluid because of an unequal distribution of cations and anions on opposite sides of the membrane. – This voltage, the membrane potential ﺍﻟﺠﻬﺪ ﺍﻟﻐﺸﺎﺋﻰ , ranges from -50 to -200 millivolts. • The membrane potential acts like a battery. • The membrane potential favors the passive transport of cations into the cell and anions out of the cell. • Two combined forces, collectively called the electrochemical gradient ﺍﻹﻧﺤﺪﺍﺭ ﺍﻟﻜﻬﺮﻭﻛﻴﻤﻴﺎﺋﻰ , drive the diffusion of ions across a membrane. • Ions diffuse not simply down its concentration gradient, but diffuses down its electrochemical gradient. 6

2)- Large molecules (macromolecules) Large molecules are transported by Exocytosis and endocytosis v v 1. Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteins. Large molecules, such as polysaccharides and proteins, cross the membrane by vesicles ﺃﻮﻋﻴﺔ. Exocytosis ﺍﻹﺧﺮﺍﺝ ﺍﻟﺨﻠﻮﻯ : a transport vesicle budded from ﻳﻨﺸﺄ ﻣﻦ the Golgi apparatus is moved by the cytoskeleton to the plasma membrane. When the two membranes come in contact ﺗﻼﻣﺲ , the bilayers fuse ﻳﻨﺪﻣﺞ and spill ﻓﺮﻉ the contents to the outside. 2 - Endocytosis ﺍﻹﺩﺧﺎﻝ ﺍﻟﺨﻠﻮﻯ , a cell brings in macromolecules and particulate matter by forming new vesicles from the plasma membrane 7

A)- Phagocytosis: Called “cellular eating”. The cell engulfs ﺗﻉ a particle by extending pseudopodia ﺃﻘﺪﺍﻡ ﻛﺎﺫﺑﺔ around it and packaging it ﺗﻏﻠﻔﻬﺎ in a large vacuole. • The contents of the vacuole are digested when the vacuole fuses with a lysosome. Fig. 8. 19 a, Page 152 8

B)- Pinocytosis, ﺍﻟﺸﺮﺏ ﺍﻟﺨﻠﻮﻯ “cellular drinking”, a cell creates a vesicle around droplets ﻧﻘﺎﻁ of extracellular fluid ﺍﻟﺴﺎﺋﻞ ﺍﻟﻤﻮﺟﻮﺩ ﺧﺎﺭﺝ ﺍﻟﺨﻠﻴﺔ. – This is a non-specific process ﻋﻤﻠﻴﺔ ﻏﻴﺮ ﻣﺘﺨﺼﺼﺔ. 9 Fig. 8. 19 b, Page 152

C)- Receptor-mediated endocytosis: ﺍﻹﺩﺧﺎﻝ ﺍﻟﺨﻠﻮﻯ ﻋﻦ ﻃﺮﻳﻖ ﺍﻟﻤﺴﺘﻘﺒﻼﺕ ﺍﻟﻤﺘﺨﺼﺼﺔ It Is called (Selective eating) which very specific in what substances are being transported. • It is triggered ﺗﺳﺘﺣﺚ when extracellular substances bind to special receptors ﺳﺘﻘﺒﻻﺕ ﺧﺎﺻﺔ , on the membrane surface. This triggers the formation of a vesicle ﻭﻋﺎﺀ • It enables a cell to take large quantities of specific materials that may be in low concentrations in the environment. 10 Fig. 8. 19 c, Page 152

Things Transport Active Exocytosis Passive Large molecules Small Molecules/ions (Membrane) (T. protein) Facilitated diffusion (T. protein) Diffusion (Membrane) Endocytosis Phagocytosis Pinocytosis Receptor-mediated endocytosis Cellular eating Cellular drinking Selective eating Using specific receptors (coated vesicles) that bind to specific legends and engulf it. It is specific 11