What Does The Plasma Membrane Do The cell

What Does The Plasma Membrane Do? § The cell plasma membrane separates the cell contents from the external environment. § The membrane acts as a gatekeeper, regulating the passage of molecules into and out of the cell. § Cell membranes are complex structures that contain many different components. Copyright © 2009 Pearson Education Inc.

What Is The Structure Of The Plasma Membrane? § Plasma membranes are “fluid mosaics”. • Membranes are composed of a double lipid layer that is highly fluid without breaking. • Proteins are embedded in this double lipid layer and give the membrane its mosaic character. Copyright © 2009 Pearson Education Inc.

What Is The Structure Of The Plasma Membrane? § The plasma membrane is a fluid mosaic. extracellular fluid (outside) binding site phospholipid bilayer carbohydrate cholesterol phospholipid receptor protein transport protein filaments recognition protein cytoplasm (inside) Copyright © 2009 Pearson Education Inc. Fig. 3 -1

What Is The Structure Of The Plasma Membrane? § The phospholipid bilayer is the fluid portion of the membrane. § Phospholipid molecules have a polar head group and a pair of nonpolar tails. CH 3 CH 2 CH 2 – CH 3 O + H 3 C N CH 2 O P O CH 2 O CH 3 O CH 2 CH HC O C CH 2 CH 2 CH O H 2 C O C CH 2 CH 2 CH 2 CH 2 CH 3 tails (hydrophobic) head (hydrophilic) Copyright © 2009 Pearson Education Inc. Fig. 3 -2

What Is The Structure Of The Plasma Membrane? § The head groups are hydrophilic and point toward the aqueous environment on both bilayer sides of the membranes. § The tail groups are hydrophobic and point away from the aqueous environments and toward each other. Copyright © 2009 Pearson Education Inc. extracellular fluid (watery environment) phospholipid hydrophilic heads hydrophobic tails hydrophilic heads cytoplasm (watery environment) Fig. 3 -3

• The phospholipid bilayer’s flexible, fluid membrane allows for cellular shape changes • Individual phospholipid molecules are not bonded to one another • Some of the phospholipids have unsaturated fatty acids, whose double bonds introduce “kinks” into their “tails” • Membranes become more fluid at high temperatures (more movement) and less fluid at low temperatures (less movement) • Cell membranes of organisms living in low temperatures tend to be more unsaturated (more kinks help them maintain fluidity) Copyright © 2009 Pearson Education Inc.

more unsaturated greater fluidity Copyright © 2009 Pearson Education Inc. more saturated less fluidity

What Is The Structure Of The Plasma Membrane? § A mosaic of proteins is embedded in the membrane. • Some of the membrane proteins are anchored to a network of protein filaments within the cytoplasm. • Other proteins are free to move around in the lipid matrix. • Many proteins have carbohydrate groups that stick outside of the cell. Copyright © 2009 Pearson Education Inc.

How Does The Plasma Membrane Play Its Gatekeeper Role? § The phospholipid bilayer blocks the passage of most molecules. § There are three types of membrane proteins —transport proteins, receptor proteins, and recognition proteins. Copyright © 2009 Pearson Education Inc.

How Does The Plasma Membrane Play Its Gatekeeper Role? § Membrane proteins • Transport proteins: allow the movement of water-soluble molecules through the plasma membrane by forming channels or by carrying them across • Receptor proteins: possess a binding site on the outer surface for binding specific chemicals that may alter overall cell function • Recognition proteins: with sugar groups attached to the exterior of the cell, are used by the immune system to identify cells as belonging to “self” Copyright © 2009 Pearson Education Inc.

What Is The Structure Of The Plasma Membrane? § The plasma membrane is a fluid mosaic. extracellular fluid (outside) binding site phospholipid bilayer carbohydrate cholesterol phospholipid receptor protein transport protein filaments recognition protein cytoplasm (inside) Copyright © 2009 Pearson Education Inc. Fig. 3 -1

What Is Diffusion? § Characteristics of molecules in fluids • Concentration: the number of molecules in a given unit of volume • Gradient: a physical difference between two regions of space that causes molecules to move from one region to another Copyright © 2009 Pearson Education Inc.

What Is Diffusion? § Molecules in fluids move in response to gradients. • Diffusion: the movement of molecules from regions of high molecular concentration to low molecular concentration § A drop of dye in water illustrates diffusion. Copyright © 2009 Pearson Education Inc.

What Is Diffusion? § Diffusion of a dye in water 1 A drop of dye is placed in water 2 The dye molecules diffuse into the water; the water molecules diffuse into the dye 3 Both dye molecules and water molecules are evenly dispersed drop of dye water molecule Copyright © 2009 Pearson Education Inc. Fig. 3 -4

What Is Diffusion? § Summing up: the principles of diffusion • Diffusion is the movement of molecules down a gradient from high concentration to low concentration. • The greater the concentration gradient, the faster the rate of diffusion. • If no other processes intervene, diffusion will continue until the concentration gradient is eliminated. Copyright © 2009 Pearson Education Inc.

What Is Osmosis? § Osmosis: the diffusion of water molecules from a high water concentration to a low water concentration across a biological membrane § Pure water has the highest water concentration. § The addition of dissolved solutes to pure water reduces the number of water molecules and thus lowers the water concentration. Copyright © 2009 Pearson Education Inc.

What Is Osmosis? § Osmotic water flow across a membrane takes place across selectively permeable membranes that allow water to pass, but not certain small impermeable molecules, such as sugars. Copyright © 2009 Pearson Education Inc.

What Is Osmosis? § A selectively permeable membrane water selectively permeable membrane Water molecule: can fit through the pore sugar Sugar with water molecules clustered around it: cannot fit through the pore Copyright © 2009 Pearson Education Inc. pore Fig. 3 -5

What Is Osmosis? § Example of osmosis: a water-permeable bag that is impermeable to sugar and has sugar inside of it • Water flows into the bag, down a water concentration gradient. • The bag swells and eventually bursts from the additional water. Copyright © 2009 Pearson Education Inc.

What Is Osmosis? § Osmosis selectively permeable membrane sugar molecule water molecule Copyright © 2009 Pearson Education Inc. Bag bursts Water flows in Fig. 3 -6

What Is Osmosis? § Summing up: the principles of osmosis • Osmosis is the diffusion of water across a selectively permeable membrane. • Dissolved substances reduce the concentration of water molecules in a solution. • Water moves across a membrane down its concentration gradient from a high concentration of water molecules to a low concentration of water molecules. Copyright © 2009 Pearson Education Inc.

How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane? § There are two kinds of transport across cell membranes: • Passive transport • Energy-requiring transport Copyright © 2009 Pearson Education Inc.

How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane? Copyright © 2009 Pearson Education Inc.

How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane? § Passive transport: movement of molecules across a membrane, down a concentration gradient, without the use of energy § Simple diffusion: the transfer of gases, water, and lipid-soluble substances—such as ethyl alcohol and vitamin A—across the phospholipid bilayer Copyright © 2009 Pearson Education Inc.

How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane? § Simple diffusion through the phospholipid bilayer (extracellular fluid) lipid-soluble molecules and O 2, CO 2, and H 2 O O 2 (cytoplasm) (a) Simple diffusion through the phospholipid bilayer Copyright © 2009 Pearson Education Inc. Fig. 3 -7 a

How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane? § Examples of passive transport (continued) • Facilitated diffusion: the diffusion of watersoluble molecules through a channel or carrier protein down a concentration gradient Copyright © 2009 Pearson Education Inc.

How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane? § Facilitated diffusion via a channel protein Cl– H 2 O, ions Proteins form a hydrophilic channel Cl– Cl channel protein (cytoplasm) – Cl– (b) Facilitated diffusion through a channel protein Copyright © 2009 Pearson Education Inc. Fig. 3 -7 b

How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane? § Facilitated diffusion via a carrier protein amino acids, sugars, small proteins carrier 1 A carrier protein 2 A molecule has a binding site enters the for a molecule binding site (extracellular fluid) 3 The carrier protein changes 4 The carrier protein shape, transporting the resumes its original shape molecule across the membrane (cytoplasm) (c) Facilitated diffusion through a carrier protein Copyright © 2009 Pearson Education Inc. Fig. 3 -7 c

How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane? § Examples of passive transport (continued) • Water crosses membranes in response to molecular concentration differences on each side of the membrane. § Isotonic extracellular fluid: the same molecular concentration outside the cell as inside the cell • There is equal movement of water across the cell membrane in each direction under this condition; there is no net water movement. Copyright © 2009 Pearson Education Inc.

How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane? § Isotonic solution 10 micrometers (a) Isotonic solution has the same salt concentration as the cytoplasm Copyright © 2009 Pearson Education Inc. Equal movement of water into and out of cells Fig. 3 -8 a

How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane? § Hypertonic extracellular fluid: the molecular concentration outside the cell is greater than the molecular concentration inside the cell • Net water movement out of the cell; the cell shrinks. Copyright © 2009 Pearson Education Inc.

How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane? § Hypertonic solution (b) Hypertonic solution has a higher salt concentration than the cytoplasm Copyright © 2009 Pearson Education Inc. Net water movement out of cells; cells shrivel Fig. 3 -8 b

How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane? § Hypotonic extracellular fluid: the molecular concentration outside the cell is less than the molecular concentration inside the cell • Net water movement into the cell; the cell swells. Copyright © 2009 Pearson Education Inc.

How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane? § Hypotonic solution (c) Hypotonic solution has a lower salt concentration than the cytoplasm Copyright © 2009 Pearson Education Inc. Net water movement into cells; cells swell and burst Fig. 3 -8 c

The effects of osmosis on red blood cells Red blood cells in an isotonic solution Equal movement of water into and out of cells Copyright © 2009 Pearson Education Inc. Red blood cells in a hypertonic solution Net water movement out of cells; cells shrivel Red blood cells in a hypotonic solution Net water movement into cells; cells swell and burst

§ Osmosis across the plasma membrane plays an important role in the lives of cells § Water flows into plant cytoplasm because it is more concentrated than the extracellular fluid • Water flows into the vacuole because its contents are more concentrated than the cytoplasm • Water pressure within the vacuole is called turgor pressure Copyright © 2009 Pearson Education Inc.

• Water leaks in continuously because the cytosol is hypertonic to fresh water • Salts are pumped into the vacuoles, making them hypertonic to the cytosol • Water follows by osmosis and is then expelled by contraction Copyright © 2009 Pearson Education Inc.

cytoplasm Turgor pressure in plant cells central vacuole When water is plentiful, it fills the central vacuole, pushes the cytoplasm against the cell wall, and helps maintain the cell’s shape Water pressure supports the leaves of this impatiens plant Turgor pressure provides support cell wall plasma membrane When water is scarce, the central vacuole shrinks and the cell wall is unsupported Copyright © 2009 Pearson Education Inc. Deprived of the support from water, the plant wilts Loss of turgor pressure causes the plant to wilt

How Do Molecules Move Against A Concentration Gradient? § Energy-requiring transport processes • During active transport, the cell uses energy to move substances against a concentration gradient. • Membrane proteins regulate active transport. • Adenosine triphosphate (ATP) donates energy to the active transport processes. Copyright © 2009 Pearson Education Inc.

How Do Molecules Move Against A Concentration Gradient? § Active transport (extracellular fluid) 1 The transport protein binds both ATP and CA 2+ recognition site ATP binding ATP site Ca 2+ Copyright © 2009 Pearson Education Inc. 2 Energy from ATP changes the shape of the transport protein and moves the ion across the membrane 3 The protein releases the ion and the remnants of ATP (ADP and P) and closes ADP ATP P (cytoplasm) Fig. 3 -9

Energy-requiring transport § Energy-requiring transport includes active transport, endocytosis, and exocytosis • Cellular activities rely on the energy-expending activities of active transport, endocytosis, and exocytosis • Crucial to sustaining life • Crucial to maintaining concentration gradients, acquiring food, excreting wastes, and cell-to-cell communication Copyright © 2009 Pearson Education Inc.

How Do Molecules Move Against A Concentration Gradient? § During endocytosis, a portion of the plasma membrane engulfs the extracellular fluid or particle and pinches off into the cytoplasm as a membranous sac, called a vesicle. Copyright © 2009 Pearson Education Inc.

How Do Molecules Move Against A Concentration Gradient? § Phagocytosis: movement of solid material into a cell (extracellular fluid) food particle pseudopods 1 (cytoplasm) 2 vesicle containing the particle 3 1 The plasma membrane extends pseudopods toward an extracellular particle (food, for example). 2 The ends of the pseudopods fuse, encircling the particle. 3 A vesicle that contains the engulfed particle is formed. (b) Phagocytosis Copyright © 2009 Pearson Education Inc. Fig. 3 -10 b

An Amoeba engulfs a Paramecium Copyright © 2009 Pearson Education Inc. A white blood cell engulfs a disease-causing fungal cell

How Do Molecules Move Against A Concentration Gradient? § Exocytosis: moves material out of the cell, including the waste products of digestion and secreted materials, such as hormones • During exocytosis, a vesicle carrying material to be expelled moves to the cell surface, where the vesicle fuses with the plasma membrane. • Following fusion, the vesicle opens to the extracellular fluid and its contents diffuse out. Copyright © 2009 Pearson Education Inc.

How Do Molecules Move Against A Concentration Gradient? § Exocytosis (extracellular fluid) plasma membrane secreted material plasma membrane vesicle Material is enclosed in a vesicle that fuses with the plasma membrane, allowing its contents to diffuse out (cytoplasm) 0. 2 micrometer Copyright © 2009 Pearson Education Inc. Fig. 3 -11

How Do Molecules Move Against A Concentration Gradient? § Some plasma membranes are surrounded by cell walls. • Cell walls occur around the plasma membranes of plants, fungi, and some bacteria. • Cell walls provide support for the cells, making them capable of resisting gravity and blowing winds. Copyright © 2009 Pearson Education Inc.

How Do Molecules Move Against A Concentration Gradient? § Some plasma membranes are surrounded by cell walls (continued). • Cell walls are porous to small molecules, which can pass across these barriers to the plasma membrane. • Plasma membranes of these cells regulate the transport of molecules by the same processes as those that occur in other cells without cell walls. Copyright © 2009 Pearson Education Inc.