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www.soran.edu.iq Cell and Molecular Biology Behrouz Mahmoudi Cell Membrane Structure and Functions 1

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www.soran.edu.iq Membrane Function and structure Membranes organize the chemical activities of cells. The outer plasma membrane forms a boundary between a living cell and its surroundings Exhibits selective permeability Controls traffic of molecules in and out Internal membranes provide structural order for metabolism - Form the cell's organelles - Compartmentalize chemical reactions 2

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www.soran.edu.iq Fluid Mosaic Model of the PM A membrane is a mosaic Proteins and other molecules are embedded in a framework of phospholipids A membrane is fluid Most protein and phospholipid molecules can move laterally 3

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www.soran.edu.iq Macromolecules found in Membranes Lipids Phospholipid bilayer Cholesterol Proteins Transport proteins Receptor proteins Recognition proteins Carbohydrates Oligosaccharides on glycoproteins 4

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Functions of Membrane Macromolecules Lipids Phospholipid bilayer Forms boundary to isolate cell contents from environment Restricts passage of hydrophilic substances across the membrane Cholesterol Increases bilayer strength, flexibility Reduces membrane fluidity Reduces permeability to water- soluble substances 6

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www.soran.edu.iq Functions of Membrane Macromolecules Proteins Transport proteins Regulate movement of water soluble substances Channel proteins have pores that allow passage of ions and small water-soluble molecules Carrier proteins bind to molecules and change shape for delivery across membrane 7

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www.soran.edu.iq Functions of Membrane Macromolecules Proteins Receptor proteins Docking site for molecules outside the cell Trigger internal cellular response Recognition proteins Identification tags Oligosaccharides aid in cell-cell recognition Cell-surface attachment sites 8

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www.soran.edu.iq Fluid Mosaic Model of Membrane Structure Within the phospholipid bilayer are many different types of embedded proteins and cholesterol molecules whose presence spawned the term mosaic. From scanning electron microscope images, it was observed that the embedded molecules can move sideways throughout the membrane, meaning the membrane is not solid, but more like a fluid. 9

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www.soran.edu.iq Passive Transport A process that does not require energy to move molecules from a HIGH to LOW concentration DDiffusion FFacilitated Diffusion OOsmosis 10

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www.soran.edu.iq Diffusion is the movement of small particles across a selectively permeable membrane like the cell membrane until equilibrium is reached. These particles move from an area of high concentration to an area of low concentration. outside of cell inside of cell 11

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www.soran.edu.iq Osmosis is the diffusion of water through a selectively permeable membrane like the cell membrane Water diffuses across a membrane from an area of high concentration to an area of low concentration. Semi-permeable membrane is permeable to water, but not to sugar 12

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www.soran.edu.iq Facilitated Diffusion is the movement of larger molecules like glucose through the cell membrane larger molecules must be helped Proteins in the cell membrane form channels for large molecules to pass through Proteins that form channels (pores) are called protein channels outside of cell inside of cell Glucose molecules 13

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www.soran.edu.iq Hypertonic Solutions: contain a high concentration of solute relative to another solution (e.g. the cell's cytoplasm). When a cell is placed in a hypertonic solution, the water diffuses out of the cell, causing the cell to shrivel. Hypotonic Solutions: contain a low concentration of solute relative to another solution (e.g. the cell's cytoplasm). When a cell is placed in a hypotonic solution, the water diffuses into the cell, causing the cell to swell and possibly explode. Isotonic Solutions: contain the same concentration of solute as another solution (e.g. the cell's cytoplasm). When a cell is placed in an isotonic solution, the water diffuses into and out of the cell at the same rate. The fluid that surrounds the body cells is isotonic. 15

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www.soran.edu.iq Active Transport Active transport is the movement of molecules from LOW to HIGH concentration. Energy is required as molecules must be pumped against the concentration gradient. Proteins that work as pumps are called protein pumps. Ex: Body cells must pump carbon dioxide out into the surrounding blood vessels to be carried to the lungs for exhale. Blood vessels are high in carbon dioxide compared to the cells, so energy is required to move the carbon dioxide across the cell membrane from LOW to HIGH concentration. outside of cell inside of cell Carbon Dioxide molecules 17

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www.soran.edu.iq Endocytosis Active process for movement of large molecules and organisms Substance is taken in by vesicle formed from cell membrane Phagocytosis: solid substance in vesicle Pinocytosis: liquid droplets in vesicle; The uptake of extracellular fluids such as hormones and enzymes by the body cells Receptor-Mediated Endocytosis: incoming substance binds to receptor Iron Uptake, Cholesterol Uptake, 20

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www.soran.edu.iq Exocytosis Substance is expelled after being enclosed in a vesicle within the cell Used to move large molecules out of the cell 22