3 Cell membrane structure_4fc82e062a3b11daa9c21d67c64663a9.pdf

Structure of the Cell Cell membrane Prepared by Dr. Ahmed A. Morsi Assist. Prof. of Histology & Cell biology 0  Learning objectives By the end of this lecture, you will be able to:- 1. Outline the structure of the cell 2. Classify cyt...

Structure of the Cell Cell membrane Prepared by Dr. Ahmed A. Morsi Assist. Prof. of Histology & Cell biology 0  Learning objectives By the end of this lecture, you will be able to:- 1. Outline the structure of the cell 2. Classify cytoplasmic organelles 3. Recognize the structure & function of cell membrane 0  The cell The structural & functional unit Size: variable 4-150 um Shape: variable rounded, oval, flat, cubical columnar, polygonal Function: secretion, absorption, sensation, contraction…… Structure: 0  Structure of the cell Cell Inclusions Cytoplasm Nucleus Organelles 0 The Cell 0 I. Cytoplasmic organelles II. Cytoplasmic inclusions 2. Living 1.Non-living 3. Permanent structure 2.Transient 4. Essential component of the 3.Not vital for the cell cell 4.Metabolically inert 5. Metabolically active 5.Examples: CHO, pigment, 6. Examples: mitochondria lipids 0  I. Cytoplasmic organelles a)Membranous organelles b)Non-membranous organelles Surrounded by membrane Not surrounded by membrane Contain enzymes No enzymes Examples: Examples 1.Cell membrane 1.Ribosomes 2.Mitochondria 2.Cytoskeleton 3.Endoplasmic reticulum a) Filaments 4.Golgi apparatus b) Microtubules 5.Lysosomes 3. Proteasome 6.Peroxisome 0  Cell membrane: Definition: The outermost covering of the cell that determines its boundary, separating cytoplasm from the extracellular fluid. Light microscope (LM): can be localized by special stains only. Electron microscope (EM): Trilamellar membrane i.e appear as 2 dark lines with a light one inbetween 0  Molecular structure of cell membrane Lipid component Protein component Carbohydrate component Phospholipid bilayer Cholesterol Glycolipids Glycopro- teins Integral proteins Peripheral proteins 0  Molecular structure: I. Lipid component: a) Phospholipid molecules: arranged in 2 layers. Each molecule is composed of Polar head: hydrophilic, directed outward Non polar tail: hydrophobic, so directed inward so, facing each other in the center. b) Cholesterol molecule: inserted among hydrophobic tails. 0  II. Protein component: a) Peripheral protein: small sized, found on both surfaces of cell membrane. b) Integral protein: Embedded within phospholipid bilayer spanning the entire thickness. 0 III. Carbohydrate component Located only on the outer surface of the cell membrane Formed of: 1.Glycoproteins: sugar (oligosaccharides) chains linked to proteins. 2.Glycolipids: sugar (oligosaccharides) chains linked to phospholipids 0  Cell coat (glycocalyx): It is composed of the glycoproteins and glycolipid molecules present on the external surface of the plasma (cell) membrane. By EM: appear as fuzzy material on outer side of the cell membrane Has several functions 0  Functions of cell membrane I. Transport of materials 1. Passive diffusion:unassisted(no need for carrier protein) movement of small, substances down their concentration gradient. 2. Facilitated diffusion: assisted (transported by carrier protein) movement of ions and small molecules down their concentration gradient. 3. Active transport: occurs against concentration gradient e.g. Na/ k pump. 4. Bulk (vesicular) transport: the cell takes up or releases large molecules through the formation of vesicles. It includes both endocytosis & exocytosis. 0 Endocytosis 0  Endocytosis The process by which the cell engulf extracellular substances by enclosing them in a small membrane vesicle or sac. Once the sac is formed, it detaches from the plasma membrane and sink into cytoplasm. Fate of the formed vesicle: fusion with a lysosome and its subsequent digestion. 0  Forms of endocytosis 1-Phagocytosis (cell eating): if the cell engulfs solid particles e.g bacteria. 2-Pinocytosis (cell drinking): if the cell engulf fluid droplets e.g dissolved protein or fat. 3- Receptor-mediated endocytosis The cell membrane receptor proteins bind only with certain substances e.g hormones 0 Exocytosis 0  Exocytosis The process by which substances are extruded out of the cell. It is the means by which cells actively secrete hormones, mucus, and other cell products. The product released into small vesicle or sac. Vesicles migrates to cell membrane and fuses with it and then ruptures , spilling the sac contents out of the cells. 0  II. Functions of cell coat: 1. Cell protection & stabilization 2. Cell identity (recognition): role of sugar chain of glycoproteins & glycolipids. 3. Cell –cell interaction 4. Act as receptors for certain pathogens III. Cell membrane modifications: see later 0  Reference: Junqueira’s Basic Histology: Text and atlas, 13th Edition, 2013. Anthony L. Mescher