Please enable JavaScript.
Coggle requires JavaScript to display documents.
Chapter 7 Membrane Structure
and Function
image, Synthesis of Membrane…
Chapter 7 Membrane Structure
and Function
*Fluid Mosaics of Lipids and Proteins (some carbohydrates) *
- Proteins are not random, associated with long-lasting, specialized patches, that carry out function
- Most membrane lipids and some membrane proteins can shift sideways through the membrane
- Some membrane proteins are immobile by their attachment to the cytoskeleton or the extracellular matrix
- Some membrane proteins move in a directed manner along cytoskeletal fibers by motor proteins
Phospholipids: primary lipid that make up the membrane
- Amphipathic Molecule - hydrophilic region (loves water), hydrophobic region (hates water), polar and nonpolar regions
-
Extracellular Matrix: a non-cellular network of proteins and polysaccharides that surrounds and supports cells and tissues.
Cytoskeleton: a network of protein filaments and tubules in the cytoplasm of many cells that provides the cell with its shape, internal organization, and allows for movement. It is made up of three main components: actin filaments, microtubules, and intermediate filaments.
Fluid Mosaic Model: the membrane is a mosaic of protein molecules bobbing ina fluid bilayer of phospholipids
Integral Proteins
- Penetrate the hydrophobic interior of the lipid bilayer
- Extend only partway into the hydrophobic interior
- Majority of the integral proteins are transmembrane proteins
Transmembrane proteins:
- consists of 1 or more stretches of nonpolar amino acids, typically 20-30 in length coiled into a (alpha) helices
- some have 1 or more hydrophilic channels that allow passage through membrane
- hydrophilic parts are exposed to the aqueous solution on either side of the membrane
-
Transport Proteins: may provide a hydrophilic channel across that is selective for a particular solute, or shuttle a substance from one side to the other by changing shape
- Some of these proteins require hydrolyze ATP as an energy source to actively pump substances across the membrane
Enzymatic activity: protein built into the membrane may be an enzyme with its active site exposed to substances in adj solution. (some organized and work as a team)Signal transduction: A membrane protein (receptor) may have binding site with a specific shape that fits the shape of a chemical messenger (Ex: Hormone) Ext Messenger may change shape of protein, allowing it to relay the msg to the inside of the cellCell-Cell recognition: Some glycoproteins serve as ID tags that are specifically recognized by membrane proteins of other cells (short lived)Intercellular joining: membrane proteins of adjacent cells may hook together in various kinds of junctions (long-lasting)Attachment to Cytoskeleton/ECM: Microfilaments or elements of cytoskeleton may be (noncovalently bound?) to membrane proteins, helps to stabilize the location of certain membrane proteins
Transport Proteins
Enzyme Activity & Signal Transduction
Cell-Cell Recog & Intercellular Joining
Role of Membrane Carbohydrates in Cell-Cell Recog
- cell's ability to distinguish btw one another is crucial to function, also basis for rejection of foreign cells by immune system, important line of defense in vertebrate animals
- Cells recog is by binding to other molecules, often containing carbohydrates, on the Extracellular surface
- These are normally short, branched chains of fewer than 15 sugar units
- Glycolipids - membrane carbs covalently bonded to lipids
- Glycoproteins - membrane carbs covalently bonded to proteins
Attachment Proteins
Peripheral Proteins:
- Not embedded in lipid bilayer
- loosely bound to the surface of membrane, often exposed to parts of integral proteins
Membranes fluidity dependent on temperature
- As Temp Decreases, membrane remains fluid until the phospholipids settle into a closely packed arrangement and membrance solidifies
- Speed of membrane solidification dependent on what lipid type they are made of.
- Unsaturated hydrocarbons are not able to pack together closely (kinks in tails, more fluid)
- Saturated hydrocarbons pack together, so solidify faster
- Cholesterol in Animal Cell Membranes reduces fluidity at moderate temperatures by reducing phospholipid movement, but at low temperatures it hinders solidification by disrupting the regular packing of phospholipids. considered a "fluidity buffer"
-
Membrane Evolution:
the ability to change the lipid composition of cell membranes in response to changing temperatures has evolved in organisms that live where temps vary
- Plants tolerant of extreme cold, Ex: Winter Wheat
- Bacteria and Archaea
Selective Permeability:
it allows some substances to cross more easily than others
- Small, nonpolar, and uncharged molecules easily pass through, EX: O2, CO2,
Transport Proteins:
allow ions and a variety of polar molecules through the cell membrane. Specific to the substance it moves
- Carrier Proteins: hold on to their passengers and change shape in a way to shuttle them across the membrane.
- EX: Glucose carrier proteins transports glucose across 50k times faster than it could do on its own
- Channel proteins: function by having a hydrophilic channel that certain molecules or ions use a tunnel through the membrane
-EX: Aqauporins, channel protein that allows the passage of water molecules through the membrane
Aquaporins consist or four identical polypeptide subunits, each one forms a channel that water molecules pass through, single-file, allowing entry up to 3 billion per second.
Passive Transport:
the diffusion of a substance across a biological membrane because it requires no energy
Diffusion: the movement of particles of any substance so they spread out into the available space.
- when molecules use their own thermal energy, due to constant motion of particles
- is a spontaneous process, needing no input of energy, substances move down its own concentration gradient.
Simple Rule of Diffusion: In the absence of any other forces, a substance will diffuse from where it is more concentrated to where it is less concentrated
Concentration gradient: the region along which the density of a chemical substance increases or decreases
Osmosis:
the movement of water across a semi-permeable membrane from a region of lower concentration of solute to a region of higher concentration of solute.
Water Balance of Cells without cell wallsTonicity: the ability of a surrounding solution to cause a cell to gain or lose water
- dependent on its concentration of solutes that cannot cross the membrane relative to that inside the cell
- dependent on membrane permeability
Isotonic (iso means "same")
- no net movement of water across the plasma membrane
- water diffuses across the membrane, but at the same rate in both directions
- EX: seawater to marine invertebrates
Hypertonic (hyper means "more"):
- the cell will lose water, shrivel, and probable die
- EX: an increase in salinity of a lake can kill the animals there
Hypotonic (hypo means "less"):
- water will enter the cell faster than it leaves, and the cell will swell and lyse (burst) like a balloon
-EX: an
-
Plant Water Balance
- Turgid:* turgor pressure* - the cell wall will expand only so much before it exerts a back pressure on the cell. hypotonic solution and is a healthy state for most plants.
- Flaccid: limp, the plant wilts, there is no tendency for water to enter.
- Plasmolysis: in a hypertonic solution, it causes the plant to wilt and can lead to plant death.
Facilitated Diffusion: Passive Transport Aided by Proteins
- polar molecules and ions blocked by the lipid bilayer of the membrane diffuse passively with the help of transport proteins
Ion channels: channel proteins that help ions, some function as a gate
- Gated channels: open and close in response to a stimulus
- some respond to electrical stimulus, which allow a stream of potassium ions to leave the cell
- some respond to chemical stimulus, and open or close when a substance binds to the channel (not the transported substance)
Carrier Proteins:
- result in the net movement of substance down its concentration gradient, some seem to undergo a subtle change in shape
- Ex: Glucose Transporter
Osmoregulation:
- the control of solute concentrations and water balance
- EX: unicellular protist Paramecium caudatum lives in pond water
-
Active Transport:
- the use of energy to move solutes against their concentration gradients, across the plasma membrane from the side where they are less concentrated
- Active transport proteins are all Carrier Proteins
- ATP Hydrolysis supplies energy for most active transport.
- ATP Can power active transport when its terminal phosphate group is transferred directly and induce the protein to change its shape to translocates a solute bound to the membrane
- Ex: Sodium-potassium pump
-
All cells have voltages across their plasma membranes
- Voltage is electrical potential energy - a separation of opposite charges
- Membrane potential - the voltage across a membrane, ranges from about -50 to -200 millivolts (mV)
- the inside of cell is negative, favors transport cations into the cell and anions out of the cell which drives diffusion of ions
-
-
Bulk Transport
Exocytosis: the process where the cell secretes certain molecules by fusion of vesicles with the plasma membrane
-
Endocytosis: the process of when the cell takes in molecules and particulate matter by forming new vesicles from the plasma membrane
Pinocytosis: "Cellular drinking", the intake of liquids into the cell
Receptor-Mediated Endocytosis: a specialized type of pino that enables the cell to acquire bulk quantities of specific substances, even
-
Ex: Human Cells use RME's to take in cholesterol for membrane synthesis and synthesis of other steroids.
Phagocytosis: "Cellular eating", the intake of solids into the cell
-
Synthesis of Membrane Components
STEP 1: Secretory Proteins, membrane proteins and lipids are synthesized in the Endoplasmic Reticulum (ER)
- Within the ER, carbohydrates are added to the transmembrane proteins, making glycoproteins, materials are transported in vesicles to the Golgi apparatus
STEP 2: Inside the Golgi apparatus, glycoproteins undergo further carbohydrate modification, and lipids acquire carbohydrates, becoming glycolipids
STEP 3: The glcyoproteins, glycolipids, and secretory proteins are transported in vesicles to the plasma membrane
STEP 4: As vesicles fuse with the plasma membrane, the outside face of the vesicle becomes continous with the inside face of the plasma membrane. This release the secretory proteins from the cell, a process called exocytosis and positions the carbohydrates of membrane glycoproteins and glycolipids on the outside (extracellular) face of the plasma membrane.