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

Active transport is the pumping ضَخ 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). Fig. 8.14b, Page 148

Outside the cell Inside the cell 1)- Small molecules (Ions ) Outside the cell 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. Na Na Na Na Protein molecule Na Na ATP Cellular membrane Na High conc. of K+ High conc. of Na+ Low conc. of K+ Low conc. of Na+ 1ATP T. protein 2 Inside the cell 3

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, while active transport requires an investment of energy to move molecules against their concentration gradient.

Some ion-pumps generate voltage جُهد 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.

Large molecules are transported by Exocytosis and endocytosis 2)- Large molecules (macromolecules) Large molecules are transported by Exocytosis and endocytosis 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 يُفرع 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

A)- Phagocytosis: Called “cellular eating”. The cell engulfs تـَبْلَع a particle by extending pseudopodia أقدام كاذبة around it and packaging it تـُغلفها in a large vacuole. The contents of the vacuole are digested when the vacuole fuses with a lysosome. Fig. 8.19a, Page 152

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. Fig. 8.19b, 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 تـُستـَحث when extracellular substances bind to special receptors مُستقبـِلات 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. Fig. 8.19c, Page 152

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