Membranes and Transport - Lecture 8 PDF

# Membranes ## Cell Membrane - Image of two cells with their plasma membranes in close contact. - The image is a microscopic visualization of the cell membrane, with Cell 1 and Cell 2 labeled. - Both cell membranes appear as a wavy line, with some structures, identified by a "C," visible within th...

# Membranes ## Cell Membrane - Image of two cells with their plasma membranes in close contact. - The image is a microscopic visualization of the cell membrane, with Cell 1 and Cell 2 labeled. - Both cell membranes appear as a wavy line, with some structures, identified by a "C," visible within them. ## Membrane Structure - The _fluid mosaic model_ is a model of the membrane structure that includes phospholipids and globular proteins. - _Phospholipids_ are arranged in a bilayer. - _Globular proteins_ are inserted in the lipid bilayer. ## Phospholipids - Phospholipid structure consists of: - _Glycerol_ - a 3-carbon polyalcohol acting as a backbone for the phospholipid. - Two _fatty acids_ attached to the glycerol. - A _phosphate group_ attached to the glycerol. - The _fatty acids_ are nonpolar chains of _carbon_ and _hydrogen_. - Their nonpolar characteristic makes them _hydrophobic_ ("water-fearing"). - The _phosphate group_ is polar and _hydrophilic_ ("water-loving"). - Image of phospholipid structure with polar head and nonpolar tail. ## Membrane proteins - Membrane proteins perform various functions: - _Transporters_ - _Enzymes_ - _Cell surface receptors_ - _Cell surface identity markers_ - _Cell-to-cell adhesion proteins_ - _Attachments to the cytoskeleton_ - Image of various types of membrane proteins. - Image of a _transporter_ protein helping move molecules across the membrane. - Image of an _enzyme_ protein catalyzing a reaction. - Image of a _cell surface receptor_ protein receiving a signal from the outside. - Image of a _cell surface identity marker_ protein identifying the cell. - Image of a _cell-to-cell adhesion_ protein attaching two cells together. - Image of an _attachment to the cytoskeleton_ protein connecting the membrane to the cytoskeleton. ### Peripheral membrane proteins: - Anchored to a phospholipid in one layer of the membrane. - Possess nonpolar regions that are inserted in the lipid bilayer. - Free to move throughout one layer of the bilayer. - Image of a protein anchored to a phospholipid. ### Integral membrane proteins: - Spans the lipid bilayer (transmembrane proteins). - Nonpolar regions of the protein are embedded in the interior of the bilayer. - Polar regions of the protein protrude from both sides of the bilayer. - Image of a transmembrane protein. - Integral proteins possess at least one _transmembrane domain_. - Region of the protein containing hydrophobic amino acids. - Spans the lipid bilayer. - Image of two types of integral membrane proteins. ## Passive Transport - Passive transport is movement of molecules through the membrane in which: - No energy is required. - Molecules move in response to a _concentration gradient_. - Diffusion is movement of molecules from _high concentration to low concentration_. - Image of a beaker with a concentration gradient. - The beaker has a high concentration of red dots in the bottom part and a low concentration in the top part. - The dots are scattering from a high concentration to the low concentration. - _Selective permeability_ is determined by the presence of integral membrane proteins that allow the cell to be selective about what passes through the membrane. - _Channel proteins_ allow polar molecules to pass through with a polar interior. - _Carrier proteins_ facilitate the passage of a specific molecule by binding to it. - _Channel proteins_ include: - _Ion channels_ allow the passage of ions (charged atoms or molecules) which are associated with water. - _Gated channels_ are opened or closed in response to a stimulus, which may be chemical or electrical. - _Carrier proteins_ bind to the molecule that they transport across the membrane. - _Facilitated diffusion_ is movement of a molecule from high to low concentration with the help of a carrier protein. - This type of diffusion is specific. - It is passive. - It saturates when all carriers are occupied. - In an aqueous solution: - Water is the _solvent_. - Dissolved substances are the _solutes_. - _Osmosis_ is the movement of water from an area of high to low concentration of water. - It is also movement of water toward an area of _high solute concentration_. - Image of a U-shaped tube divided by a semipermeable membrane. - The left part of the tube has a high concentration of urea molecules and a low concentration of water molecules. - The right part of the tube has a low concentration of urea molecules and a high concentration of water molecules. - Water molecules are moving from the right side of the tube to the left side of the tube. - When two solutions have different osmotic concentrations: - The _hypertonic solution_ has a higher solute concentration. - The _hypotonic solution_ has a lower solute concentration. ## Active Transport - Active transport: - Requires energy - ATP is used directly or indirectly to fuel active transport. - Moves substances from _low to high concentration_. - Requires the use of carrier proteins. - Carrier proteins used in active transport include: - _Uniporters_ - move one molecule at a time. - _Co-transport_ - move two molecules in the same direction. - _Counter-transport_ - move two molecules in opposite directions. - _Primary transport_ - Sodium-potassium (Na+-K+) pump. - An active transport mechanism. - Uses an antiporter to move 3 Na+ out of the cell and 2 K+ into the cell. - ATP energy is used to change the conformation of the carrier protein. - The affinity of the carrier protein for either Na+ or K+ changes so the ions can be carried across the membrane. - Image of a sodium-potassium pump. - The image shows the steps of the pump: carrier binding to sodium, protein phosphorylation, conformational change and sodium release, potassium binding, and protein dephosphorylation. - _Secondary transport_ - Coupled transport. - Uses the energy released when a molecule moves by diffusion to supply energy to active transport of a different molecule. - A _symporter_ is used. - Glucose-Na+ symporter captures the energy from Na+ diffusion to move glucose against a concentration gradient. - Image of a glucose-sodium symporter. - The image shows the role of the sodium-potassium pump in providing the energy for the symporter. ## Bulk Transport - Bulk transport of substances is accomplished by: - _Endocytosis_ - movement of substances into the cell. - _Exocytosis_ - movement of materials out of the cell. - _Endocytosis_ occurs when the plasma membrane envelops food particles and liquids. - _Phagocytosis_ is when the cell takes in particulate matter. - _Pinocytosis_ is when the cell takes in only fluid. - _Receptor-mediated endocytosis_ is when specific molecules are taken in after they bind to a receptor. - Image of phagocytosis, pinocytosis, and receptor-mediated endocytosis. - _Exocytosis_ occurs when material is discharged from the cell. - Vesicles in the cytoplasm fuse with the cell membrane and release their contents to the exterior of the cell. - It is used in plants to export cell wall material. - It is used in animals to secrete hormones, neurotransmitters, and digestive enzymes. - Image of exocytosis.