Transport across Cell Membranes

• Membranes:

  • eukaryotic cells contain membrane-bound organelles and all cells including prokaryotic, are surrounded by a plasma membrane
  • due to the fact organelles are surrounded by membranes, they can all maintain their own conditions. The pH, enzymes and other substances can all be different in each organelle. Therefore, the cell is compartmentalised and many different reactions can take place
  • there may be a single phospholipid bilayer (cell membrane, vesicles, vacuole, ER) or they may be double (nucleus, mitochondria, chloroplasts)
• Functions of plasma membranes:

  the structure of plasma membranes is closely related to their functions;

  • to control the transport of substances into and out of the cell or organelles. Membranes allow certain molecules to pass through, but not others (they're partially permeable)

  Plant and prokaryotic cells are also surrounded by a cell wall, and though this is many times thicker than the plasma membrane it is freely permeable - all molecules can move through it, because it contains large pores
• Functions of plasma membranes:

  • to act as a receptor site to recognise chemicals which need to enter the cell or organelle
  • to separate off the cell from the environment, and the different reactions of the cell from each other by forming the organelles (compartmentalisation). Different concentrations can be maintained on either side of the membrane
• Cell membranes:

  • membranes are made up of phospholipids. In water, these form a two-layered structure, called a bilayer (water can pass through slowly), where the hydrophobic tails all point inward towards each other, and the hydrophilic heads point out into the water
  • in a cell the membranes separate the watery cytoplasms from the water outside

  membranes are partially permeable; generally, the smaller and less polar a molecule, the easier and faster it will diffuse across a cell membrane
• Molecules across membranes:

  • small, non-polar, lipid soluble molecules, such as oxygen and carbon dioxide, rapidly diffuse across a phospholipid bilayer
  • small, polar molecules, such as water and urea, also diffuse across, but much more slowly
  • charged particles (ions), such as Na+ and K+, are unlikely to diffuse straight across the bilayer, even if they are very small
  • large, polar, water-soluble molecules are highly unlikely to diffuse straight across the bilayer as well

  therefore, the membrane needs many other structures to allow these substances to pass through in a controlled way
• Intrinsic proteins:

  intrinsic proteins span the whole membrane and are used for transport, there are 2 types;

  • carrier proteins - these are used for large molecules (glucose)
  • channel proteins - these are used for charged particles (such as ions)
• Extrinsic proteins:

  extrinsic proteins sit on the inner or outer surface, and do not go the whole way through the membrane

  • provide structural support, binding cells together
  • form recognition sites (also glycoproteins)
  • act as receptor sites on cell surface for chemicals (hormones) to bind to - how cells detect chemicals from other cells
  • membrane-bound enzymes (maltase) in the membranes of epithelial cells in the small intestine
• Glycoproteins:

  • carbohydrate chains attached to proteins in the membrane. These glycoproteins act as receptors for hormones and neurotransmitters or antigens
• Glycolipids:

  • a carbohydrate bonded to a phospholipid in the membrane. These act as recognition sites. They also form hydrogen bonds with the water surrounding the membrane which helps to maintain the stability of the membrane and helps cell attach to one another, so forming tissues

  Collectively, the glycolipids and glycoproteins are called the glycocalyx
• Cholesterol:

  • cholesterol molecules are a type of lipid that sit between the hydrophobic tails in the phospholipid bilayer. They restrict the movement of molecules in the membrane: the more cholesterol, the less fluid and more rigid the membrane is. This is especially important at body temperature, when a membrane would become too fluid and break apart without cholesterol
• The fluid-mosaic model:

  the fluid-mosaic model is used to describe membrane structure;

  • fluid refers to the fact that all the different molecules, such as the phospholipids can move around
  • mosaic is due to the proteins that are embedded throughout the bilayer

  the fluidity means that cells are able to change their shape (e.g. phagocytes). The ease with which they do this depends on the number of phospholipids with unsaturated fatty acids in the phospholipid
• Membrane and temperature:

  a change in temperature will have an effect on the permeability of the membrane

  • at temperatures below 0°C, there is little kinetic energy, so the phospholipids are packed closely together and cannot move very much. The channel and carrier proteins deform which increases the permeability. Ice shards may also form and pierce the membrane (increasing permeability)
• Membrane and temperature:

  between 0°C and 45°C, the phospholipids can move around, and the membrane is partially permeable

  • above 45°C, the phospholipid bilayer starts to break down (melt) and become more permeable. The membrane proteins deform with high temperatures (denatures proteins). This increases the permeability
• RP4 - Membrane permeability:

  • we can investigate the effect of a variable on membrane permeability by measuring the amount of pigment leaking from beetroot cells under different conditions

  common variables used are temperature or the use of ethanol

  • ethanol would dissolve phospholipids in the membranes, making them more permeable so more pigment would leave the cells
  • high temperatures would increase membrane fluidity and denature transport proteins in the membranes, making them more permeable so more pigment would leave the cells
• RP4 - Membrane permeability:

  • the same volume of water/ethanol solution should be added to each tube - if too much, the concentration of pigment in the solution would be lower and so would appear lighter/more light would pass through. This allows comparison of results
  • water baths can be used to set specific temperatures
  • we can measure the amount of pigment that has leaked out of the cells by using a colorimeter - this measures the absorbance of light by a solution
• RP4 - Membrane permeability:

  • the more pigment that leaks out, the darker the solution will be, so more light will be absorbed, so there will be a higher absorbance reading on the colorimeter
  • when using a colorimeter, you must remember to zero/calibrate the colorimeter with pure water between each reading
• Transport across membranes:

  • there are 3 basic mechanisms: diffusion, osmosis and active transport
  • diffusion and osmosis are passive processes (do not require energy from ATP and therefore do not need respiration to occur). This is not the same as requiring no energy. They will still use energy from the solution they're in
  • active transport is an active process (requires the cell to provide energy from the hydrolysis of ATP). If respiration cannot occur in a cell, there isn't sufficient ATP for active transport to occur
• Simple diffusion:

  • diffusion is the net movement of particles/molecules from an area of high concentration to an area of low concentration
  • the difference in concentration between 2 areas is called a concentration gradient
  • diffusion is the movement of molecules down a concentration gradient
  • this is a passive process - it doesn't require ATP
• Simple diffusion:

  • as a result of diffusion, substances tend to reach a dynamic equilibrium where they are evenly spread, although still randomly moving through the solution
  • this process involves the movement of molecules straight across the phospholipid bilayer
  • the molecules that move via this method are small, lipid-soluble and/or uncharged (e.g. oxygen molecules, because they can fit between gaps in the bilayer)
• Limiting factors in diffusion:

  • surface area - the greater the surface area of the membrane over which diffusion occurs the more space there is for the particles to move and the greater the rate of diffusion
  • diffusion pathway - the thinner the exchange surface the faster the rate of diffusion (shorter diffusion pathway)
  • concentration gradient - the steeper the gradient the faster the rate of diffusion
  • temperature - the higher the temperature the faster the rate of diffusion as particles have more kinetic energy
  • type of molecule - non-polar molecules generally diffuse faster than polar ones
• Increasing diffusion rates:

  to increase diffusion rate, we need to;

  • maximise the surface area
  • maximise the concentration difference
  • minimise the thickness of the exchange surface
• Facilitated diffusion:

  facilitated diffusion is when large and charged/polar/hydrophilic molecules move through the membrane bound proteins down their concentration gradient, rather than between the phospholipids. It is still a passive process

  • large molecules move through carrier proteins. The molecule attaches, the carrier protein changes shape, and it is released on the other side
  • charged molecules, such as ions, move through hydrophilic channel proteins - these create a water-filled pore in the membrane to allow specific substances to pass through
• Facilitated diffusion:

  • both types of transport protein are specific - they only let a specific type of molecule bind and move through
  • as there are a limited number of transport proteins in membranes, this can limit the rate of facilitated diffusion. Cells that require a lot of facilitated diffusion will have lots of carrier and channel proteins in their membranes to prevent them becoming saturated
  • some molecules may move via both simple and facilitated diffusion depending on conditions and location in the body
• Osmosis:

  • osmosis is a special type of diffusion, involving water molecules only
  • they can move between the phospholipids or use protein channels called aquaporins
  • osmosis is the diffusion of water molecules from a region where it has a higher water potential to a region where it has a lower water potential through a partially permeable membrane
• Water potential:

  • water potential is represented by ψ and is measured in units of pressure, usually kilopascals (kPa)
  • under standard conditions of temperature and pressure, for pure water ψ = 0
  • the addition of a solute to pure water will lower its potential. The water potential of a solution must be less than zero (negative value)
  • the more solute that is added, the lower its water potential (more negative)
• Water potential:

  • the more solute that is added, the lower its water potential (more negative)
  • this is because water molecules bind to the solute molecules via hydrogen bonds, reducing the number of water molecules that that are free to diffuse quickly through the solution. Bound water molecules diffuse much more slowly. Therefore, when referring to water potential we are mainly concerned with free water molecules
• Osmosis in animal cells:

  hypertonic solution (a solution with a lower water potential/higher solute concentration than inside the cell)

  • a concentrated solution will have a lower water potential than inside the cell, so water leaves the cell by osmosis down a water potential gradient. This causes the cell to shrink (or crenate)
• Osmosis in animal cells:

  hypotonic solution (a solution with higher water potential/lower solute concentration than inside the cell)

  • a dilute concentration or pure water will have a higher water potential than the cell and so water enters the cell by osmosis down a water potential gradient. The cell swells and eventually bursts (osmotic lysis)
• Osmosis in animal cells:

  Isotonic solution (a solution with the same water potential as inside the cell)

  • when placed in an isotonic solution, there is no net osmotic movement of water (there is no water potential gradient) and so the shape and size of the cell remains the same
• Osmosis in plant cells:

  as plant cells have strong cellulose walls, they are prevented from large changes in volume and the cell wall will prevent the cell from bursting by osmotic lysis. Plant cell walls are freely permeable
• Osmosis in plant cells:

  hypertonic solution (a solution with a lower water potential/higher solute concentration than inside the cell)

  • a concentrated solution will have lower water potential than the cell, so water leaves the cell cytoplasm by osmosis
  • this causes the vacuole to shrink
  • the cytoplasm shrinks away from the cell wall leaving gaps between the membrane and the cell wall
  • this is called plasmolysis
• Osmosis in plant cells:

  hypotonic solution (a solution with a higher water potential/lower solute concentration than inside the cell)

  • a dilute solution or pure water will have a higher water potential than the cell, so water enters the cell by osmosis
  • the cytoplasm and the vacuole gain some water, but because of the high tensile strength of the cellulose cell wall, the cell will not burst
  • the cell is said to be turgid
• Osmosis in plant cells:

  Isotonic solution (a solution with the same water potential as inside the cell)

  • when placed in an isotonic solution, there is no net osmotic movement of water (there is no water potential gradient) and so the shape and size of the cell and vacuole remains the same

  (water molecules will still move but there is no net movement)
• RP3 - Osmosis:

  • plant tissue is placed into solutions (often sucrose) with different water potentials and the change in mass of the plant tissue is recorded
  • this is often done with potato, but other root vegetables/plants/foods can be used
  • control variables - the tissues will be left in the solutions for a specific amount of time at a specific temperature. A specific volume of solution will also be used
• RP3 - Osmosis:

  • the start and final masses are recorded and used to find the change/difference in mass - this will be + for an increase or - for a loss in mass
  • as the starting length of the tissues can differ, a percentage change or ratio (final: initial mass) is calculated to allow us to compare results
  • as we have done calculations on our data, we describe it as processed data. The masses we have recorded would be described as the raw data
  • tissues need to be blotted dry before weighing to remove excess water that would otherwise increase the mass
• RP3 - Osmosis:

  • if mass is lost - WP of the solution was lower than inside the cell, so water left the cells by osmosis down a water potential gradient
  • if mass is gained - WP of the solution was higher than inside the cells, so water entered the cells by osmosis down a water potential gradient

  the results can be represented in a table or in a calibration curve (graph). We can use this graph to work out the concentration of sucrose inside the tissue (where the line of best fit crosses the x-axis - the change in mass is 0). We can then use this to find the WP inside the tissue
• Dilutions:

  • producing a single diluted solution from a stock solution;

  C1 x V1 = C2 x V2

  starting conc x starting volume = final conc x final volume

  • producing several diluted solutions from a stock solution;

  this is done by using a serial dilution/dilution series - you create a set of solutions that decrease in concentration by the same factor each time. It's useful when you need to create a very weak solution, as it means that you do not have to measure out very small volumes of liquid. The solution would need to be mixed thoroughly using a pipette between each dilution
• Active transport:

  • active transport is the movement of molecules into or out of a cell or organelle from a region of lower concentration to a region of higher concentration (against the concentration gradient) using ATP and carrier proteins
• Active transport:

  • there are carrier proteins spanning the plasma membrane
  • a specific molecule or ion will bind to a complimentary receptor on the carrier protein
  • ATP will bind to the protein on the inside of the cell, splitting into ADP and a phosphate
  • the energy released from the hydrolysis of ATP causes the proteins to change shape and the molecule/ion will be released onto the other side of the cell
• Limiting factors in active transport:

  • supply of ATP - less ATP = less active transport. A cell will need lots of mitochondria and a good oxygen supply so the mitochondria can perform aerobic respiration to produce ATP
  • number of carrier proteins in the membrane - lower number of carrier proteins means they can become saturated and the rate of active transport will plateau (rate stays constant - it won't drop). Microvilli increase the surface area of cell membranes so there is more space for carrier proteins in the membrane (in the small intestine where lots of active transport occurs)