The Plasma Membrane. 5.1 The Nature of the Plasma Membrane. Phospholipid Bilayer. The Plasma Membrane

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1 5.1 The Nature of the Plasma Membrane The Plasma Membrane Four principal components in animals Phospholipid bilayer Molecules of cholesterol interspersed within the bilayer. Membrane proteins embedded within integral or transmembrane lie on the surface peripheral Glycocalyx short carbohydrate chains on the cell surface function in cell adhesion binding sites on proteins. The Plasma Membrane phospholipids cytoskeleton Phospholipid bilayer: a double layer of phospholipid molecules whose hydrophilic heads face outward, and whose hydrophobic tails point inward, toward each other. cholesterol peripheral protein Cholesterol molecules that act as a patching substance and that help the cell maintain an optimal level of fluidity. proteins integral protein glycocalyx Proteins, which are integral, meaning bound to the hydrophobic interior of the membrane, or peripheral, meaning not bound in this way. cell exterior cell interior Glycocalyx: sugar chains that attach to proteins and phospholipids, serving as protein binding sites and as cell lubrication and adhesion molecules. Phospholipid Bilayer Composed of two fatty acid chains linked to a charged phosphate group. fatty acid chains (tails) hydrophobic non-polar cannot form hydrogen bonds with water repel polar (hydrophilic) molecules allow non-polar (hydrophobic) molecules to pass through phosphate group (head) hydrophilic polar can form hydrogen bonds with water Figure 5.1 1

2 The Phospholipid Bilayer Phospholipid Bilayer (a) Phospholipid molecule polar head nonpolar tails (b) Phospholipid bilayer hydrophobic molecules pass through freely hydrophilic molecules do not pass through freely watery extracellular fluid hydrophilic hydrophobic hydrophilic watery cytosol Spontaneously arrange themselves into bilayers two layers of phospholipids fatty acid tails of each layer point inward (avoiding water) phosphate heads point outward (hydrogen bonding with it). due to watery (aqueous) environment on either side of the membrane. Figure 5.2 Phospholipid Bilayer Phospholipid Bilayer Cholesterol molecules interspersed between phospholipid molecules in the plasma membrane perform two functions: 1. They act as a patching material that helps keep some small molecules from moving through the membrane. 2. They keep the membrane at an optimal level of fluidity. Plasma membrane proteins integral bound to the hydrophobic interior of the phospholipid bilayer. peripheral lie on either side of the membrane but are not bound to its hydrophobic interior often bound to other integral proteins 2

3 Membrane Protein Functions In animal cells, membrane protein molecules perform four functions: 1. structural support Connect to cytoskeleton 2. cell identification serve as external recognition proteins that interact with immune system cells 3. Communication serve as external receptors for signaling molecules 4. Transport provide channels for the movement of compounds into and out of the cell The Plasma Membrane (a) Structural support (b) Recognition (c) Communication (d) Transport Membrane proteins can provide structural support, often when attached to parts of the cell s scaffolding or cytoskeleton. Binding sites on some proteins can serve to identify the cell to other cells, such as those of the immune system. Receptor proteins, protruding out from the plasma membrane, can be the point of contact for signals sent to the cell via traveling molecules, such as hormones. Proteins can serve as channels through which materials can pass in and out of the cell. Figure 5.3 Plasma Membrane Plasma Membrane Described by a conceptualization called the fluid-mosaic model views the membrane as a fluid, phospholipid bilayer that has a mosaic of proteins either fixed within it or capable of moving laterally across it. 3

4 Fluid Mosaic Model Fluid Mosaic Model 5.2 Diffusion, Gradients, and Osmosis Diffusion, Gradients, and Osmosis Diffusion, Gradients, and Osmosis Diffusion movement of molecules or ions from a region of their higher concentration to a region of lower concentration. Concentration gradient defines the difference between the highest and lowest concentrations of a solute within a given medium. compounds move from higher to lower concentrations down their concentration gradients due to Brownian movement 4

5 Diffusion, Gradients, and Osmosis Diffusion, Gradients, and Osmosis (a) Dye is dropped in (b) Diffusion begins (c) Dye is evenly distributed Moving against concentration gradients lower to a higher concentration requires energy water molecules dye molecules Figure 5.4 Diffusion, Gradients, and Osmosis Diffusion, Gradients, and Osmosis A semipermeable membrane is one that allows some compounds to pass through freely while blocking the passage of others. Osmosis net movement of water across a semipermeable membrane from an area of lower solute concentration to an area of higher solute concentration. 5

6 Diffusion, Gradients, and Osmosis Diffusion, Gradients, and Osmosis Because the plasma membrane is a semipermeable membrane, osmosis operates in connection with it. Osmosis is a major force in living things; it is responsible for much of the movement of fluids into and out of cells. (a) An aqueous solution divided by a semipermeable membrane has a solute in this case, salt poured into its right chamber. (b) As a result, though water continues to flow in both directions through the membrane, there is a net movement of water toward the side with the greater concentration of solutes in it. semipermeable membrane osmosis solute solvent (c) Why does this occur? Water molecules that are bonded to the sodium (Na + ) and chloride (Cl ) ions that make up salt are not free to pass through the membrane to the left chamber of the container. pure water water bound to salt ions Figure 5.5 Osmotic Imbalances Solute Concentration Osmotic imbalances condition where a solute concentration gradient is present on opposite sides of a cell membrane effects cell shape animal cells Lysis - break from taking in too much water Crenation - shriveling from losing water plant cells Turgor pressure - central vacuole swells, exerting pressure on cell wall» No lysis occurs Plasmolysis - central vacuole shrinks, when too much water is lost pulls cell membrane away from cell wall Cells gain or lose water relative to their surroundings due to solute concentration inside the cell as opposed to outside it 6

7 Solutions Solute Concentration Hypertonic A solution that has a higher concentration of solutes in it than does the cell s cytosol A cell will lose water to a surrounding hypertonic solution. Hypotonic A solution that has a lower concentration of solutes in it than does the cell s cytosol A cell will gain water from a surrounding hypotonic solution Solute Concentration Isotonic Equal solute concentration inside and outside the cell Water flow is balanced between the cell and its surroundings Solute Concentration (a) Hypertonic (b) Isotonic (c) Hypotonic surroundings surroundings surroundings Plasma Membranes and Diffusion H 2 O Animal cell: plasma membrane Plant cell: plasma membrane cell wall H 2 O H 2 O H 2 O H 2 O H 2 O Diffusion wilted turgid Net movement of Balanced water Net movement of water out of cell movement water into cell Figure 5.6 7

8 Moving Smaller Substances In and Out 5.3 Moving Smaller Substances In and Out Some compounds are able to cross the plasma membrane strictly through diffusion; others require diffusion and special protein channels; still others require protein channels and the expenditure of cellular energy. Transport Through the Plasma Membrane Two types of transport Active transport movement of molecules or ions across a cell membrane that requires the expenditure of energy. Passive transport movement of molecules or ions across a cell membrane that does not require the expenditure of energy. Types of Passive Transport Two types of passive transport simple diffusion As discussed previously facilitated diffusion Requires a membrane protein channel For either form of transport to bring about a net movement of materials into or out of a cell, a concentration gradient must exist. 8

9 Facilitated Diffusion Facilitated Diffusion Transport proteins function as channels For larger hydrophilic substances substances that, because of their size and electrical charge, cannot diffuse through the hydrophobic portion of the plasma membrane. outside cell plasma membrane inside cell glucose 1. The transport protein 2. Glucose binds 3. This binding causes 4. Glucose passes has a binding site for glucose that is open to the outside of the cell. to the binding site. the protein to change shape, exposing glucose to the inside of the cell. into the cell and the protein returns to its original shape. Figure 5.8 Active Transport Active Transport Cells cannot rely solely on passive transport to move substances across the plasma membrane. A cell may need to maintain a greater concentration of a given substance on one side of its membrane. Yet, passive transport equalizes concentrations of substances on both sides of the plasma membrane. Active transport Uses energy and protein channels Chemical pumps Moves compounds across the plasma membrane against their concentration gradients. 9

10 Active Transport One example of such transport is the pumping of glucose into cells that line the small intestines. Transport Through the Plasma Membrane Passive transport Active transport simple diffusion facilitated diffusion ATP Materials move down The passage of Molecules again move their concentration materials is aided both through a transport gradient through the by a concentration protein, but now energy phospholipid bilayer. gradient and by a must be expended to transport protein. move them against their concentration gradient. Figure 5.7 Getting the Big Stuff In and Out 5.4 Getting the Big Stuff In and Out Two processes for moving larger materials endocytosis Brings material in Exocytosis Sends material out 10

11 Exocytosis and Endocytosis Exocytosis Vesicles Used by endo- and exocytosis membrane-lined enclosures that alternately bud off from membranes or fuse with them. Exocytosis transport vesicle moves from the interior of the cell to the plasma membrane and fuses with it, at which point the contents of the vesicle are released to the environment outside the cell. Exocytosis Endocytosis (a) Exocytosis extracellular fluid protein (b) Micrograph of exocytosis Two types of endocytosis Pinocytosis Uptake of fluids Cellular drinking Phagocytosis Uptake of solids Cellular eating transport vesicle cytosol Figure

12 Endocytosis Endocytosis Pinocytosis Cell drinking movement of moderate-sized molecules into a cell by means of the creation of transport vesicles produced through an infolding or invagination of a portion of the plasma membrane. vesicles will bud off from the plasma membrane and travel deeper into the cell Phagocytosis when certain cells use pseudopodia or false feet to surround and engulf whole cells, fragments of them, or other large organic materials. Endocytosis (a) Pinocytosis receptors captured molecules coated pit vesicle (b) Phagocytosis Formation of a pinocytosis vesicle. bacterium (or food particles) pseudopodium vesicle A human macrophage (colored blue) uses phagocytosis to ingest an invading yeast cell. Figure