jimtrue.com : school : BSC2010 : CH 08: Membrane Structure & Function

Posted by Jim True on February 17, 2004 6:44 AM. Last Updated October 22, 2006 9:23 PM

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CH 08: Membrane Structure & Function

Membrane Structure

Cell membrane structure was first suggested to be constructed mainly of lipids back in 1895.
This was because substances dissolved in lipids were rapidly absorbed by cells.
Biological membranes are composed of an array of both lipids and proteins.
The variety of both lipids and proteins in membranes are amphipathic ("amphi" -- on both sides) molecules. Both hydrophilic and hydrophobic sections.
Amphipathic molecules have both hydrophilic and hydrophobic regions.
The currently accepted model, i.e., a physical representation of structure, for the biological membranes is the Fluid Mosaic Model.
Fluid Mosaic Model -- Cell (plasma) membrane (as well as most membrane bound organelles) is a double layer of phospholipids, with proteins embedded in and across the layer. in a Geometric array (a mosaic). Phospholipid bi-layer.
Original model was the Davson-Danielli model with proteins outside the phopholipid layers on both sides. Fluid Mosaic shows proteins are embedded inside and through the phospholipid layers with their own amphipathic regions.
The fluid part of the model comes from the fact that with few exceptions, the phospholipid and most protein molecules are not "locked in place" in the membrane.
Instead, they move freely within the layer in which they are found (it is extremely rare for molecules to "flip-flop" across membrane layers (outer to inner).
This allows membranes to function properly.
Molecules move about much like molecules in a soap bubble film.
At maximum fluidity, membranes have a flow consistent with that of vegatable oil.
The degree of fluidity is dependent on temperature. At certain low temperatures, the membranes may actually solidify, depending on the lipids present. See this effect in "frostbite" -- ice crystals form in the cytoplasm and damage cell membranes. Membrane fluidity, based on saturated or unsaturated hydrocarbon tails (unsaturated hydrocarbon tails have kinks and are more fluid; saturated pack tightly together and are more viscous. Cholesterol packs between the tightly packed saturated lipids allowing movement.
It has been found that phospholipids with unsaturated "tails" remain liquid to lower temperatures because it is hard to compact the molecules together.
Cholesterol, present in animal cells, plays an important role in stabilizing the membrane structure and also preventing solidification.
Its presence also prevents packing together of the membrane at lower temperatures.
Components of Biological Membranes:
- Lipids -- As mentioned, the largest number of molecules in the membrane are phospholipids in two layers called the phospholipid bilayer.
  - Their hydrophilic "heads" are turned outwards, while the hydrophobic "tails" extend inwards.
  - Cholesterol -- found in either layer, helping to stabilize the layers;
  - Glycolipids -- (a carbohydrate bonded to a lipid) -- located ONLY on the outer membrane (in contact with the external environment), and have roles in cell communication and adhesion.
- Proteins - Several Categories:
  - Integral -- these are proteins which are firmly anchored in the membrane and cannot move (ie like a rock in a stream).
  - Transmembrane -- Integral proteins that extend through both membrane layers (some multiple times). ("Trans" means "across").
  - Peripheral (Membrane) -- Associated with one layer only. These are not embedded in the membrane layer, but are on the surface, often associated with integral proteins. ("peripheral" means "on the edge"). Can be attached to one end of a transmembrane protein. May or may not be integral; if attached to a integral protein, they also will be integral.
  - Integral proteins typically have hydrophilic ends that are exposed to the aqueous environment and a hydrophobic interior region (amphipathic).
  - Glycoproteins -- (a carbohydrate bonded to a protein) -- similar to glyolipids, in that these are located on the outer membrane (external only), in contact with the outer environment, and have roles in cell communcation and adhesion.
  - The transmembrane and integral proteins have a wide variety of roles:
    - Transport -- channels for substances moving in/out of the cell.
    - Cell Communications -- sending and receiving signals.
    - Physical Connections -- hold cells together.
    - Anchors -- for cytoskeletal components; proteins embedded in the membrane that provide the 'tent poles' for the cytoskeletal tent.
    - Combination -- varied functions.
The outer and inner surfaces of membranes have distinct differences in terms of molecular orientation and certain structures, e.g., glycolipids and glycoproteins are restricted to the "outer face".
Membrane construction itself takes places within the ER.

Transport

Cell membranes are permeable (able to be penetrated) to certain molecules such as hydrocarbons, CO₂ and O₂, which cross with ease in either direction. Pass if they are not even there; certain lipid molecules cross easily as well because phospholipids are soluble in lipid solution.
Larger molecules and polar molecules, including water, may also cross membranes but must use specific transport protein channels to do so. Water through specific transport channels, and also based on its polarity can push phospholipids out of the way (like a bead curtain).
Because the cell can regulate the movement of many substances crossing in or out, it is referred to as semi- or selectively permeable.
Some substances can cross the membrane freely with no energy expenditure by the cell, but others may require the cell to use energy to bring them in or push them out.
Movement across the membrane in either direction is referred to as transport ("trans" - across, "port" - to carry).
Two categories of transport:
- Passive -- Transport requiring no energy output from the cell.
- Active -- Transport requires an energy expenditure by cell.

Passive Transport

Passive -- Several different types:
- Diffusion -- movement of molecules along a concentration gradient from a region of high to low concentration. Will continue until equilibrium is reached. Easy way to think of this is a ball on the hill (a gradient is a slope); the ball will roll from a high gradient to a low gradient. All molecules have potential and kinetic energy; if there are more at one area (higher concentration), they will naturally move to a area of lower concentration, equilibrium. At equilibrium, the molecules still have motion, but not directed motion to an area of lower concentration.
  - Diffusion occurs spontaneously because it decreases free energy!
  - Equilibrium -- even distribution of molecules. Molecules still move but randomly (depends on the heat of the system!).
  - Diffusion can involve gas < -- > gas, gas < -- > liquid, liquid < -- > liquid.
  - Diffusion rates will be affected by concentration gradient, temperature, molecular size (weight) and electrical charge on the molecules.
  - Two special types of diffusion across semi-permeable membranes:
    - Dialysis -- Diffusion of a solute across a semi-permeable membrane. related to the number of solute molecules on either side of the membrane.
    - Osmosis -- Diffusion of a solvent across a semi-permeable membrane. for cells, the solvent is ALWAYS water. for biology, osmosis is the movement of water across a semi-permeable membrane. Osmosis requires a semi-permeable membrane.
  - All cells possess, and exist in, an aqueous (H₂O) environment.
  - Osmosis is a function of both the relative concentration of H₂O on either side of the membrane, plus the osmotic membrane which is the tendency of water to move across a membrane against gravity.
  - All cells contain a certain amount of water plus solutes, as does their environment.
  - Rarely does the solute/water concentration in the cell equal that in the environment. No environment on earth that is pure water; all sorts of dissolved solutes. Deionized and distilled water have had all other solutes removed.
    - Isotonic -- solutions with equal concentrations of solute on either side of a semi-permeable membrane.
      - While water molecules will cross the membrane, there is no osmosis because the movement is random. Isotonic does not mean that we have 50%/50% solvent and solute. It means equal concentrations -- 0.5 % salt in the cell, 0.5% salt in the environment... doesn't have to be 0.5% of salt, can be any solute, or any combinations of solutes.
      - Dialysis will not always occur because there are limitations based on the size of the solutes; osmosis will ALWAYS occur, because water is soluble.
      - Isotonic, hypertonic and hypotonic do not have WATER in their definition because the concentration is based on the SOLUTE.
    - Hypertonic -- a solution with a HIGHER solute concentration on one side of a semi-permeable membrane. "hyper" -- "more than"
    - Hypotonic -- a solution with a LOWER solute concentration on one side of a semi-permeable membrane. "hypo" -- "less than, or under"
  - Hyper- and Hypotonic are relative opposites. If a solution on one side of a membrane is hypertonic, then the other side MUST be hypotonic.
  - If we consider equal volumes on either side of a membrane, a hypertonic solution contains few H2O molecules than an equal volume of a hypotonic solution. One side has less water than the other side even though it may have more volume (due to the hypotonic solution). Osmosis moves in the opposite direction of osmosis.
  - Thus, H2O will always move FROM a hypotonic TO a hypertonic solution by osmosis.
  - A solute will always move FROM a hypertonic TO a hypotonic solution by dialysis (IF the solute's molecules can cross the membrane)
  - Any cell not bound by a cell wall cannot withstand much water loss or gain. Thus, cells of any organisms lacking cell walls must be able to osmoregulate (series of methods and structures, will be covered in Biology II).
  - Osmoregulation is the maintenance of a cell's specific water/solute balance relative to the environment.
  - Cells without walls that swell excessively may rupture and becomes lysed (destroyed).
  - Cells with or without walls that lose water undergo plasmolysis (the breakdown of the cytoplasm) and may die because water is no longer present to mediate biochemical reactions.
  - Cells with cell walls that fill with water don't burst because the walls prevent it.
  - Instead, the cells are said to undergo turgor, (become turgid).
  - This is an advantage to multicellular organisms such as plants and fungi because turgid cells provide support and allow the plant to mushroom to remain upright against gravity.
- Facilitated Diffusion -- Many substances that are unable to pentrate the phospholipid bilayer may still diffuse across the membrance by passing through a carrier protein (transmembrane).
  - This includes water and other important substances.
  - In some cases, such as water, the transport proteins are channels for passage, so that large numbers of molecules can pass through in either direction.
  - In other cases, the proteins may physically change shape or open and close to allow substances across.
  - Facilitated diffusion rates will vary depending on the number of available carrier proteins and amount of substance to be diffused.
  - In some cases, rate can only increase until all channels are being used. Items must wait in line until a carrier protein is available.

Active Transport

Active -- Whereas most of passive transport involves molecules moving down a gradient, in active transport, they typically move AGAINST the gradient.
- Move against the gradient requires the cell to use energy.
- Active transport can be used to move both needed and unwanted substances in/out of the cell.
  - Pumps -- Many active transport mechanisms are called "pumps" because molecules are pumped out or in against the concentration gradient. Pushed or pulled AGAINST the concentration gradient.
    - Some pumps can require a signficant amount of energy to maintain, but are critical to the cell's survival.
    - Sodium-Potassium (Na+/K+) Pump -- Extremely important to cell. Requires approximately 70% of cells total energy budget to run.
      - During each "pump", 3 Na+ ions are pumped out while 2 K+ ions are pumped in.
      - Thus, the cell maintains a high Na+ concentration outside the cell, and a high K+ concentration inside.
      - Outside and inside cell are negative ions are negatively charged molecules.
      - Because there are few positives (K+) inside and more (Na+) outside, there is an overall + charge outside the membrane surface and the - charge inside.
      - The membrane is referred to as polarized.
    - The imbalance of charge on either side of the membrance is an electrochemical gradient
    - With an electrochemical gradient, charged substances can not only diffuse along their concentration gradient, but along the electrochemical gradient, drawn by the attraction of opposite charges. Potassium's may
    - Such a process generates voltage across the membrane.
    - This voltage gradient represents a major source of potential energy for the cell that can be used to run other transport systems, including pumps.
    - Such pumps that generate voltage are referred to as electrogenic("genesis" -- creation)
    - A very important electrogenic pump we'll discuss at length in Chapters 9 and 10 is the proton pump
  - Co-Transport (Coupled transport) -- Where a needed molecule can be transported against its gradient by being coupled with a substance that is diffusing, eg., Glucose with Na+, sucrose with H+.
    - The diffusing substance is then pumped back across the membrane to maintain the concentration gradient.
  - Bulk Transport -- Transport of large quantities of material in solid (particles to entire cells) or liquid (quantity of solution) form.
    - Bulk transport involves restructuring of a region of the cell membrane.
      - Exocytosis -- Release of bulk material from a cell. A Vacuole or vesicle containing the material moves from cytoplasm to cell membrane and joins, releasing materials to outside of cell.
        Release of wastes and various products (secretions) from cell occurs in this way.
      - Endocytosis -- Involves the infolding and pinching off of a region of the cell membrane, enclosing a quantity of substance within.
        Phagocytosis -- Endocytosis of solids (phage means to eat)
        PinoCytosis -- endocytosis of dissolved substances (pino means to drink)
      - Both of these provide an intake of nutrients and raw materials for cell.
  - Receptor Mediated Endocytosis -- In this case, substances can be drawn into the cell only after coming into contact with specific receptor proteins.
    - These proteins receive a chemical signal causing the cell membrane to infold and enclose the substance within a vesicle.
    - the vesicle then divides into two different vesicles.
    - One vesicle, containing the receptor proteins, returns to the cell surface, maintaining the cell membrane.
    - The other holds the materials in the cytoplasm until a lysosome arrives and releases enzymes to break down the materials.

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