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Lecture 5


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McMaster University
Lovaye Kajiura

Biology Week 5 – Chapter 5 Sept. 30, 2013 Membranes • separate living entities from nonliving entities • allow cell's internal contents to be different from the external environment • chemical reactions are more efficient since greater collisions between What functions are served by the PLASMA MEMBRANE? Fig. 5-1 The plasma membrane • serves as a boundary (separation of internal and external things) • selective barrier • increases reaction efficiency What are the NONPOLAR regions? • Hydrophobic fatty acid tails (water fearing) What are the POLAR regions? • Hydrophilic Fig E5-1 Tail kinks in phospholipids -Unsaturated fatty acids greater fluidity; more permeable -more saturated less fluidity Fig. E5 – 2 Caribou legs adapted to cold Table 5-1 Transport Across Membranes* Fig 5-6 Types of Diffusion Through Plasma Membrane* Fig 5-7 Effect of Solute Concentration on Osmosis* (compare solutions)(table) (plant vs. Animal cells) Hypotonic – Lower solute concentration than that in the cell Hypertonic – Greater solute concentration than that in the cell Isotonic – Equal solute concentration compared to that in the cell Fig. 5-8* What happens when PLANT cell are placed in different solutions? Hypotonic solution – move to area of great concentration – plant cells become TURGID (plump up cell; not going to burst) Isotonic solution – plant cell become FLACCID – in and out of cell wall Hypertonic solution – plant cell becomes PLAYSMOLYZED – pulling away from the cell wall What happens when ANIMAL cell are placed in different solutions? Isotonic solution – equal movement of water in and out of Animal cell Hypertonic solution – animal cell shrivels or CRENATES Hypotonic solution – animal cell swells or bursts or LYSES Home study task Find other examples of facilitated diffusion Fig. 5-10 Active Transport Fig. 5-11 Pinocytosis Fig. 5-12 Receptor mediated endocytosis – uptake of cholesterol from blood by liver cells Exocytosis – Exports macromolecules out of a cell • Crying – tear glands export salty solutions containing proteins Endocytosis – Imports macromolecules into a cell • Pinocytosis – Example Mammalian egg cells receives nutrients from nearby cells 5-14 Exocytosis HOW IS THE STRUCTURE OF THE CELL MEMBRANE RELATED TO ITS FUNCTION? • Proteins suspended in a double layer of phospholipids • phospholipids are responsible for isolating the cell's contents whereas proteins are responsible for selectively exchanging substances and communicating with the environment, controlling biochemical reactions associated with the cell membrane and forming connections between cells • Cell membranes preform several crucial functions ◦ selectively isolate the contents of membrane enclosed organelles from the surrounding cytosol and the cell's contents from the surrounding interstitial fluid ◦ they regulate the exchange of essential substances between the cell and the interstitial fluid or between membrane enclosed organelles and the surrounding cytosol ◦ they allow communication among the cells of multicellular organisms ◦ they create attachments within and between cells ◦ they regulate many biochemical reactions MEMBRANES ARE FLUID MOSAICS IN WHICH PROTEINS MOVE WITHIN LAYERS OF LIPIDS • knew that cell membranes consist primarily of proteins and lipids but did not know how these molecules create the structure of the membranes • 1972- Fluid Mosaic Model (J. Singer & G.L Nicholson) developed the fluid mosaic model of cell membranes which now known to be accurate • A fluid is any substance whose molecules can flow past one another; fluids include gases, liquids and also cell membranes, whose molecules move quite freely within the thin layer • Cell membranes consist of a fluid formed by a double layer of phospholipids (constantly shifting with respect to one another THE FLUID PHOSPHOLIPID BILAYER HELPS TO ISOLATE THE CELL'S CONTENTS • A phospholipid consists of two very different parts: a head that is polar and hydrophilic and a pair of fatty acid tails that are non-polar and hydrophobic. Membranes contain a variety of phospholipids with structure similar to shown in fig. 5-2 • arrangement of phospholipids in membranes arise from the fact that all cells are immersed in watery solutions; Example: single celled organisms may live in fresh water or in the ocean and the water also saturates the cell walls that surround plants cells. • Outer surfaces of animal plasma membranes are bathed in interstitial fluid a weakly salted liquid resembling blood without its cells or large proteins • Cytosol (fluid portion of the cytoplasm) is mostly water • In these watery surroundings phospholipids spontaneously arrange themselves into a double layer called a phospholipid bilayer. • Hydrogen bonds form between water and the hydrophilic phospholipid heads causing the heads to face the water on either side. • Phospholipid tails being hydrophobic cluster together forming a bilayer • Cell membranes are continually shifting; phospholipid molecules within each layer move relatively rapidly and larger proteins move slower • temperature above zero = constant motion; as temp. Increases their rate of motion increase; Phospholipid do not bond to one another so at body temp. Their constant random motion causes them to shift freely they generally maintain their orientation and only rarely flip between the two membrane layer • A flexible fluid nature of the bilayer is very important for membrane function; if cells were stiff they would break open and die • Also allows membrane – enclosed compartments to ferry substances into the cell, carry materials within they cell and expel them to the outside, merging their membranes maintain a consistency • All membranes contain cholesterol; a high cholesterol content reduces the permeability of the membrane to hydrophilic substances and small molecules that would other wise diffuse through it; allows the cell to exert more control over what substances enter and leave it • Few hydrophobic biological molecules can diffused directly through the phospholipid bilayer including hormones such as estrogen and testosterone • salts, amino acids and sugars are hydrophilic that is they are polar and water soluble; cant pass through the nonpolar, hydrophobic fatty acid tails of the phospholipid bilayer • Glycoproteins a protein which a carbohydrate is attached to • Membrane fluidity is strongly influenced by the relative amounts of saturated and unsaturated fatty acid tails in membrane phospholipids because unsaturated fatty acids have one or more double bonds causing them to kink ◦ Kinked tails occupy more space and cannot pack as tightly together ◦ Saturated fatty acid tails are straight and can pack more tightly ◦ Organisms such as plants, protists, bacteria and cold blooded animals actually change the compositions of their membranes in response to changes in temperature, incorporating more saturated fatty acids into their membranes as the temperature rises and vice versa when temperature drops. ◦ Ex. Caribou maintain a core temperature of 100 f degrees but allow the temperature of their lower legs to drop to about 32 f degree thus conserving body heat as they stand in winter snow. ◦ The membranes of cells near the chilly hooves have lots of unsaturated fatty acids whereas near the warmer trunk have more saturated fatty acid • Enzymes: proteins that promote chemical reactions that synthesize or break apart biological molecules ◦ located within cytoplasm; some span cell membranes and others are attached to membrane surfaces ◦ Ex. some help synthesize the extracellular matrix a web of protein and glycoprotein fibers that fill spaces between enzymes in cells lining the small intestine complete the digestion of carbohydrates and proteins • Receptor Proteins: bear dozens; that span their plasma membrane ◦ allow cells to respond to specific messenger molecules (such as hormones) carried
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