CHAPTER 58 THE STRUCTURE MEMBRANE AND STUCTURE FUNCTION

CHAPTER 58 THE STRUCTURE MEMBRANE AND STUCTURE FUNCTION OF MACROMOLECULES AND FUNCTION How things get into and out of the cell

Section A: Membrane Structure

Introduction v The plasma membrane separates the living cell from its nonliving surroundings. v This thin barrier, 8 nm thick, controls traffic into and out of the cell. v Like other membranes, the plasma membrane is selectively permeable, allowing some substances to cross more easily than others. v The main macromolecules in membranes are lipids and proteins, but include some carbohydrates. v The most abundant lipids are phospholipids. v Phospholipids and most other membrane constituents are amphipathic molecules. v Amphipathic molecules have both hydrophobic regions and hydrophilic regions. v The phospholipids and proteins in membranes create a unique physical environment, described by the fluid mosaic model. v A membrane is a fluid structure with proteins embedded or attached to a double layer of phospholipids.

v The molecules in the bilayer ﻃﺒﻘﺔ ﻣﺰﺩﻭﺟﺔ are arranged as hydrophobic fatty acid tails are sheltered ﻣﺤﻤﻴﺔ from water while the hydrophilic phosphate groups interact with water. v Membrane proteins are amphipathic, with hydrophobic and hydrophilic regions. Fig. 8. 1 b v If at the surface, the hydrophilic regions would be in contact with water. v In this fluid mosaic model, the hydrophilic regions of proteins and phospholipids are in contact with water and the hydrophobic regions are in a nonaqueous ﻻ ﻣﺎﺋﻰ environment. Fig. 8. 2 b

Cell membrane is fluid • Most of the lipids and some proteins can drift laterally in the plane of the membrane, but rarely flip-flop from one layer to the other. • The lateral movements of phospholipids are rapid, about 2 µm/second. • Many larger membrane proteins move more slowly but do drift. • As temperatures cool, membranes switch from a fluid state to a solid state as the phospholipids are more closely packed. • Membranes rich in unsaturated fatty acids are more fluid than those dominated by saturated fatty acids because the kinks in the unsaturated fatty acid tails prevent tight packing. Fig. 8. 4 a & b • To work properly with permeability, membrane must be fluid, about as fluid as oil.

Membranes are mosaics of structure and function • A membrane is a collage ﺗﻉ of different proteins embedded ـﺱ in the fluid matrix of the lipid bilayer. CD, Unit 2 ch. 8 Fig. 8. 6, Page 142

A)- The plasma membrane has a unique collection ﺗﺠﻤﻊ ﻣﻤـﺰ of proteins. • There are two populations of membrane proteins. 1. Peripheral proteins ﻃﺮﻓﻰ are not embedded in the lipid bilayer at all. Instead, they are loosely bounded to the surface of the protein, often connected to the other population of membrane proteins. 2. Integral proteins ( ﺗﺩ )ﻧﻣﺞ penetrate the hydrophobic core of the lipid bilayer, often completely spanning the membrane (a transmembrane protein). Integral proteins • Where they contact the core, they have hydrophobic regions with nonpolar amino acids, often coiled into alpha helices. • Where they are in contact with the aqueous environment, they have hydrophilic regions of amino acids. Peripheral proteins Fig. 8. 7, Page 143

• The proteins in the plasma membrane may provide a variety of major cell functions. Fig. 8. 9, Page 144 CD, Unit 1 ch. 5

B)- Membrane carbohydrates are important for cell-cell recognition • The membrane plays the key role in cell-cell recognition. • Cell-cell recognition is the ability of a cell to distinguish one type of neighboring ﺍﻟﻤﺠﺎﻭﺭﺓ cell from another. • It is the basis for rejection of foreign cells by the immune system. • Cells recognize other cells by recognizing the surface molecules, often carbohydrates, on the plasma membrane. • Membrane carbohydrates may be covalently bonded either to lipids, forming glycolipids, or, to proteins, forming glycoproteins. • The four human blood groups (A, B, AB, and O) differ in the external carbohydrates on red blood cells.

Functions of cell membrane (Plasma membrane) 1 - Selective permeability ﺍﻟﻨﻔﺎﺫﻳﺔ ﺍﻹﺧﺘﻴﺎﺭﻳﺔ v A steady traffic ﺍﻟﻌﺒﻮﺭ ﺍﻟﻤﻨﺘﻄﻢ of small molecules and ions moves across the plasma membrane in both directions. v For example, sugars, amino acids, and other nutrients enter a muscle cell and metabolic waste products leave it. v The cell absorbs O 2 and expels CO 2. v It also regulates concentrations of inorganic ions, like Na +, K+, Ca 2+, and Cl-, by passing them across the membrane. v However, substances do not move across the barrier indiscriminately ﻋﺸﻮﺍﺋﻴﺎ as membrane is selectively permeable. v Hydrophobic molecules, like hydrocarbons, CO 2, and O 2, can dissolve in the lipid bilayer and cross easily. v Ions and polar molecules pass through with difficulty. v This includes small molecules, like water, and larger critical molecules, like glucose and other sugars. v Proteins can assist and regulate ﻳﺴﺎﻋﺪ ﻭ ﻳﻨﻈﻢ the transport of ions and polar molecules.

Selective Permeability CO 2 Nucleus O 2 The cell is able to take up particular molecules and exclude others

2 - Passive transport ﺍﻹﻧﺘﻘﺎﻝ ﺍﻟﺴﻠﺒﻰ is diffusion across a membrane • Diffusion: ﺍﻹﻧﺘﺸﺎﺭ Is the tendency ﺇﺳﺘﻌﺪﺍﺩ of molecules of any substance to spread out ﻟﻺﻧﺘﺸﺎﺭ in the available space randomly ﻋﺸﻮﺍﺋﻴﺎ. • For example, a permeable membrane ﻏﺸﺎﺀ ﻣﻨﻔﺰ separating a solution with dye molecules from pure water, dye molecules will cross the barrier randomly. • The dye will cross the membrane until both solutions have equal concentrations of the dye (dynamic equilibrium )ﺍﻟﺘﻌﺎﺩﻝ ﺍﻟﺪﻳﻨﺎﻣﻴﻜﻰ. Fig. 8. 10 a, Page 145

• A substance will diffuse from where it is more concentrated to where it is less concentrated, down its concentration gradient ﻣﻧﺤﺪﺭ ﺍﻟﺘﺮﻛﻴﺰ. Fig. 8. 10 b, Page 145 • Passive transport: ﺍﻹﻧﺘﻘﺎﻝ ﺍﻟﺴﻠﺒﻰ is the diffusion of a substance across a biological membrane which requires no energy from the cell to make it happen. • The concentration gradient represents potential energy and drives diffusion. CD Unit 2, Chapter 8.

2. Osmosis is the passive transport of water • Differences in concentration of dissolved materials in two solutions can lead to the movement of ions from one to the other. • The solution with the higher concentration of solutes is hypertonic. • The solution with the lower concentration of solutes is hypotonic. • Solutions with equal solute concentrations are isotonic. • Osmosis: Is a passive transport in which water diffuses across a selectively permeable membrane from the hypotonic solution to the hypertonic solution until the solutions become isotonic. Fig. 8. 11, page 146

Principal of water movement Osmosis Isotonic Osmosis Selectively permeable membrane Low conc. of sugar hypotonic High conc. of sugar hypertonic

Passive transport of water (Osmosis) • Hypertonic solution: ﻋﺎﻟﻰ ﺍﻟﺘﺮﻛﻴﺰ contains high concentration of solute ﺫﺍﺏ molecules. • Hypotonic solution: ﻣﻨﺨﻔﺾ ﺍﻟﺘﺮﻛﻴﺰ contains low concentration of solute molecules. • Isotonic solution: ﻣﺘﻌﺎﺩﻝ contains equal concentrations of solute molecules Biological Membrane H 2 O Hypertonic Hypotonic

Osmoregulation ﺍﻟﺘﻮﺍﺯﻥ ﺍﻷﺴﻤﻮﺯﻯ • The cell in a hypertonic environment will loose water, shrivel ﺗﻨﻜﻤﺶ , and die. • A cell in a hypotonic solution will gain ﺗﺴﺤﺐ water, swell, and burst. • Organisms without rigid walls have osmotic problems in either a hypertonic or hypotonic environment and must have adaptations for osmoregulation to maintain ﻟﻠﺤﻔﺎﻅ ﻋﻠﻰ their internal environment. • Example, Paramecium have a specialized organelle (the contractile vacuole), that functions as a pump to force ﺗﻘﺰﻑ water out of the cell. Fig. 8. 13 & 8. 12, Page 147 Fig. 8. 12, Page 147

Specific proteins facilitate ﺗﻫﻞ passive transport • Many polar molecules and ions diffuse passively through the lipid bilayer with the help of transport proteins (gated channels )ﻗﻨﻮﺍﺕ ﻭﺑﺔ. • The passive movement of molecules down its concentration gradient via a transport protein is called facilitated diffusion. • Many transport proteins simply provide channels allowing a specific molecule or ion to cross the membrane. • Fig. 8. 14 a & b, Page 148 Other transport proteins translocate ﻳﻨﻘﻞ the molecules across the membrane as the protein changes shape (active transport) CD U 2 Ch. 8