Membrane Transport (8/14 rev) Plasma membranes are selectively permeable  some molecules pass through membrane; some don’t Types of Membrane Transport Passive processes – No cellular energy (ATP) required – Substance moves down the concentration gradient Active processes – Energy (ATP) required – Occurs only in living cell membranes Lab 3-Cell Transport1

Membrane Lipids 75% phospholipids (lipid bilayer) – Phosphate heads: polar and hydrophilic – Fatty acid tails: nonpolar and hydrophobic 5% glycolipids – Lipids with polar sugar groups on outer membrane surface 20% cholesterol – Increases membrane stability 2

Lab 3-Cell Transport Membrane Proteins Allow communication with environment Most specialized membrane functions Two types: – Integral proteins – Peripheral proteins 3

Lab 3-Cell Transport PLAY Animation: Transport Proteins Membrane Proteins Integral proteins – Firmly inserted into membrane; most cross entire membrane – Have hydrophobic and hydrophilic regions Phosphate heads: hydrophilic Fatty acid tails: hydrophobic – Function as transport proteins (channels and carriers), enzymes, or receptors 4

Lab 3-Cell Transport Animation: Structural Proteins PLAY Animation: Receptor Proteins PLAY Membrane Proteins Peripheral proteins – Loosely attached to integral proteins – Include filaments on intracellular surface for membrane support – Function as enzymes, for shape change during cell division and muscle contraction, cell-to-cell connections and communication 5

Lab 3-Cell Transport Figure 3.4a Membrane proteins perform many tasks. A protein that crosses the membrane may provide a hydrophilic channel across membrane that is selective for a particular solute. Some transport proteins (right) hydrolyze ATP as an energy source to actively pump substances across the membrane. Transport 6

Lab 3-Cell Transport Figure 3.4b Membrane proteins perform many tasks. A membrane protein exposed to the outside of the cell may have a binding site that fits shape of a specific chemical messenger, such as a hormone. When bound, chemical messenger may cause a change in shape in protein that initiates a chain of chemical reactions in cell. Receptors for signal transduction Signal Receptor 7

Lab 3-Cell Transport Figure 3.4c Membrane proteins perform many tasks. Attachment to the cytoskeleton and extracellular matrix Elements of cytoskeleton (cell's internal supports) and extracellular matrix (fibers and other substances outside cell) may anchor to membrane proteins, which helps maintain cell shape and fix location of certain membrane proteins. Others play a role in cell movement or bind adjacent cells together. 8

Lab 3-Cell Transport Figure 3.4f Membrane proteins perform many tasks. Some glycoproteins (proteins bonded to short chains of sugars) serve as identification tags that are specifically recognized by other cells. Cell-cell recognition Glycoprotein 9

Cell Transport Passive Processes occur because of kinetic energy of molecules Substances move down the concentration gradient —from an area of high concentration to an area of lower concentration o Simple diffusion — o movement of solute throughout a solution o Facilitated diffusion— o Carrier-mediated-- moves through PM through a carrier protein o Channel-mediated -moves through PM via a water channel Osmosis—movement of the solvent (water) – Filtration Usually across capillary walls Lab 3-Cell Transport10

Lab 3-Cell Transport Passive Processes: Facilitated Diffusion Certain lipophobic molecules (e.g., glucose, amino acids, and ions) transported passively by – Binding to protein carriers – Moving through water-filled channels Carrier-Mediated Facilitated Diffusion Transport specific polar molecules (e.g., sugars and amino acids) too large for channels Binding of these polar molecules causes shape change in carrier first then movement across membrane 11

Osmosis Movement of solvent (water) across a selectively permeable membrane Water diffuses through plasma membranes down it’s concentration gradient : – Through lipid bilayer – Through water channels called aquaporins Water concentration is determined by solute concentration because solute particles displace water molecules Water moves by osmosis until hydrostatic pressure (back pressure of water on membrane) and osmotic pressure (tendency of water to move into cell by osmosis) equalize osmosis occurs until equilibrium is reached Lab 3-Cell Transport12

Importance of Osmosis When osmosis occurs, water enters or leaves a cell Change in cell volume can disrupt cell function Lab 3-Cell Transport13

Active Transport Energy to do work comes from splitting the bonds within the ATP molecule OR from the energy formed in the ionic gradients created by the “pumps” used by the splitting of the ATP bonds to move substances – Think of water being pumped uphill by an energy source; as some of the water leaks back downhill, it can carry substances with it; it co-transports substances Examples: – Vesicular transport—endocytosis and exocytosis Lab 3-Cell Transport14

– Types of Endocytosis: Phagocytosis—cell engulfs a relatively large or solid material i.e. bacteria Pinocytosis—cell engulfs a small amount of extracellular fluid containing dissolved molecules – Allows cell to “taste” extracellular fluids Receptor-mediated endocytosis—plasma membrane proteins which only bind certain substances Lab 3-Cell Transport15

Tonicity-- ability of a solution to cause a cell to shrink or swell Isotonic: solution with same solute concentration as that of cytosol Hypertonic: solution having greater solute concentration than that of surrounding cytosol Hypotonic: solution having lesser solute concentration than that of surrounding cytosol Lab 3-Cell Transport16

LAB: TO DO 1. Describe the phospholipid bilayer and how it functions as a semipermeable membrane. 2. Describe process of diffusion and perform diffusion and dialysis experiments. 3. Describe process of osmosis and how it can affect the cell. Perform the osmosis experiment. 4. Describe process of filtration (as it pertains to function of human cells and tissues). Perform filtration experiment. Lab 3-Cell Transport17

Lab 3-Cell Transport18

Structure & Function of Cells19 Tonicity of ECF Tonicity  relative concentration of solutes in 2 liquids Because water can diffuse across PM, ability of cell to control its volume is dependent upon tonicity of the ECF. Isotonic: ECF has same solute concentration as ICF (intracellular fluid). Homeostasis ensures that ECF solute concentration remains relatively constant. Hypertonic: When ECF concentration is higher than intracellular fluid, water diffuses out of cell and cell shrinks. This impairs normal function and may lead to cell death.

Hypertonic When fluid outside cell has a greater concentration of dissolved substance in it than fluid inside cell. eg: Outside water concentration (98.5%) is lower than water inside cell (99.1%). Osmosis causes water to diffuse from high to low concentration  water moves out of cell  cell shrinks Cell fluid is 99.1% water and 0.9% NaCl Fluid outside cell contains 98.5% water and 1.5% NaCl Water

Hypotonic fluid outside cell has a lower concentration of dissolved substance and higher concentration of water than fluid inside cell  water diffuses into cell until cell bursts. Cell fluid is 99.1% water and 0.9% NaCl Cell Fluid outside cell contains 100% water and 0% NaCl Water Hemolysis

Structure & Function of Cells22 Variations in Tonicity-a Summary Isotonic: extracellular and intracellular concentration equal Hypotonic: extracellular concentration less than intracellular Hypertonic: extracellular concentration more than intracellular