Transport across cell membranes

• what is compartmentalization?

  conditions in certain areas that are different to other areas
• function of cholesterol in cell membrane?

  regulates the membranes fluidity by providing the mechanical stability and flexibility
• Why does E.Coli not have any cholesterol but can maintain a stable shape?

  The cell is unable to change shape as the cell has a cell wall which is made out of murein.
• Why might a Red Blood cell have a higher percentage of cholesterol?

  Free in the blood, helps to maintain the shape of the RBC which is needed for it to be adapted to the function
• What is the relationship between absorbance and temperature

  As temperature increases, so does absorbance. This is because...

  - As temperature increases, the molecules of the membrane gain more kinetic energy

  AND

  any one of the following:

  - molecules move at a faster rate and become less tightly packed together

  - bilayer starts to break apart

  - volume of water inside the cell expands so putting pressure on the membrane

  - 3D structure of membrane proteins changes/ membrane proteins begin to denature
• Effect of temperature on membrane permeability

  An increase in temperature will increase the kinetic energy of molecules. This means that if a cell is heated the phospholipids will vibrate more, creating larger gaps for molecules to diffuse through. At a certain temperature the proteins will start to denature. This will leave big gaps in the membrane resulting in an increase in the volume of pigment/ molecule diffusing out of the cell.
• Explain why the potato cylinder in distilled water increased in length

  The potato has a more negative water potential , so water enters by osmosis causing the potato cylinder to extend in length
• What does it mean where the line crosses the x-axis on a calibration curve

  Water potential of sucrose solution same as potato tissue (the solution is isotonic and there is no net movement of water)
• How to calculate % change in mass

  New Mass - Original Mass / Original Mass x 100
• What factors affect the rate of osmosis

  The water potential gradient — the higher the water potential gradient, the faster the rate of osmosis. As osmosis takes place, the difference in water potential on either side of the membrane decreases, so the rate of osmosis levels off over time
• How does water move through osmosis

  From a region of higher (less negative) water potential (-20kPa) to a region of lower (more negative) water potential (-30kPa)
• Water potential

  Relative tendency of water to move from one place to another
• Osmosis

  Movement of water across a selectively permeable membrane from an area of high water potential to an area of low water potential - down a concentration gradient - passive
• How is Co-transport used to transport glucose and sodium ions?

  Sodium ions are actively transported out of the ileum epithelial cells into the blood

  By the Sodium-Potassium pump

  This creates a gradient of sodium ions between the lumen of the ileum and the interior of the epithelial cells

  Via sodium-glucose co-transporter proteins

  The co-transporter proteins transport glucose into the cells along with the sodium ions
• Co-Transport

  Two molecules are transported together

  The carrier protein binds two molecules at a time
• Active Transport

  Movement against the concentration gradient from a low concentration to a high concentration using carrier proteins requiring ATP hydrolysis
• Channel Proteins vs Carrier Proteins

  - Channel proteins are an open 'pore' that can allow polar/hydrophillic molecules to pass through

  - Carrier proteins change shape, channel proteins do not

  - Active transport requires carrier proteins - a site of ATP hydrolysis

  - Carrier proteins can be active or passive/channel proteins are always passive

  - Carrier proteins can transport up or down the concentration gradient/channel proteins always transport down the concentration gradient
• How do molecules move through carrier proteins

  Large molecules across the membrane, which attach and the protein changes shape - the binding of the molecule changes tertiary structure - which releases the molecule on the other side of the membrane
• How do molecules move through channel proteins

  Charged particles diffuse through, no changing shape, the particles go through a simple tunnel to cross the cell membrane
• Facilitated Diffusion

  Larger, polar (or water soluble) molecules through channel/carrier proteins.
• Simple diffusion

  The net movement of non-polar particles from a high concentration to a low concentration until equilibrium is reached, it is passive and goes down a concentration gradient.
• What is the centre of the bilayer (hydrophobic or hydrophilic)

  Hydrophobic, so it doesn't allow water-soluble substances (such as ions) to pass through it
• How are phospholipids arranged?

  The head is hydrophilic - attracting water so it faces out towards the water. The tail is hydrophobic - it repels water so it faces inwards away from the water.
• Compare co-transport and facilitated diffusion

  Co-transport only involves protein carriers, whilst facilitated diffusion involves protein carriers and protein channels

  Co-transport requires ATP, facilitated diffusion does not require ATP (passive)

  Co-transport and facilitated diffusion both involve protein carriers

  Facilitated diffusion takes place down a concentration gradient whereas active transport can occur against a concentration gradient.
• How can a cell be adapted for rapid transport?

  Membrane might be folded --> increases surface area

  Large number of protein channels AND carriers --> for facilitated diffusion

  Large number of protein carriers --> for active transport
• How does the pH affect membrane permeability?

  - The cell membrane contains carrier proteins and channel proteins

  - At extreme pHs, proteins begin to become denatured causing them to change shape and change the tertiary structure of the membrane proteins

  - The proteins can no longer control the movement of substances in and out of the cell which increases the membrane's permeability
• What is the function of the cell membrane?

  Acts as barrier / separates the internal contents of the cell from the external environment

  Regulates transport of substances into and out of cell

  Contains receptors for chemical signalling

  Contains antigens used for self recognition

  Releases chemicals for cell signalling
• What is the role of glycolipids and glycoproteins?

  They form receptors on the cell surface membrane, which are able to receive signals, such as hormones, from neighbouring cells or from the environment

  - Enzyme

  - Antigens/cell signals/cell recognition proteins

  - Receptors (e.g., for hormones)

  - Carriers (for active transport)

  - Channel proteins (for facilitated diffusion)
• What is the role of cholesterol?

  Cholesterol molecules fit between the phospholipids, binding to the hydrophobic tails of the phospholipids, causing them to pack more closely together. This restricts the movement of the phospholipids, making the membrane less fluid and more rigid.
• What is the function of phospholipids

  - To form (water) impermeable barrier to water-soluble substances OR to form a selectively/semi-permeable barrier/membrane OR to allow non-polar molecules through

  - To allow the cell to maintain different concentrations on either side (of the membrane)

  - Makes the membrane self-sealing OR enable the membrane to fuse with other membranes OR enables the membrane to form vesicles

  - Gives (the membrane) flexibility/fluidity
• Describe what is meant by the fluid-mosaic model

  Fluid : The phospholipid bilayer in which individual phospholipids are able to move around - the membrane has a flexible shape.

  Mosaic : Extrinsic and Intrinsic proteins are embedded
• is the membrane selectively permeable?

  yes
• role of atp

  hydrolysed to provide energy for active transport and carrier proteins
• is each protein generic?

  each protein is selective and specific to the molecule they allow to enter
• how are the epithelial cells adapted?

  microvilli increase the surface area for the co-transporter proteins
• co transport of glucose and sodium ions

  • sodium ions move by active transport out of the epithelium and into the blood
  • this lowers the sodium ion concentration and creates a sodium ion concentration gradient between the ileum and the epithelium cells
  • sodium ions then move back into the epithelium carrying glucose via facilitated diffusion using a co-transporter protein
  • glucose can then move out of the epithelium and into the blood via facilitated diffusion as the glucose concentration inside the cell has increased