Cell Membrane Structure and Function PDF

Structure of the Cell Cell membrane Prepared by Dr. Ahmed A. Morsi Assist. Prof. of Histology & Cell biology 1 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....

Structure of the Cell Cell membrane Prepared by Dr. Ahmed A. Morsi Assist. Prof. of Histology & Cell biology 1 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 2  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: 3  Structure of the cell Cell Cytoplasm Nucleus Organelles Inclusions 4 The Cell 5 I. Cytoplasmic organelles II. Cytoplasmic inclusions 1. Living 1.Non-living 2. Permanent structure 2.Transient 3. Essential component of the 3.Not vital for the cell cell 4.Metabolically inert 4. Metabolically active 5.Examples: CHO, pigment, 5. Examples: mitochondria lipids 6  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 7  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 8  Molecular structure of cell membrane Lipid component Protein component Carbohydrate component Phospholipid bilayer Cholesterol Glycopro- Glycolipids teins Integral proteins Peripheral proteins 9 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. 10  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. 11 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 12 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 13  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. 14 Endocytosis 15  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. 16  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 17 Exocytosis 18  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. 19 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 20  Reference: Junqueira’s Basic Histology: Text and atlas, 13th Edition, 2013. Anthony L. Mescher