Cell Membranes and Cell Signalling

• Osmosis investigation

  - Pieces of onion or potato can be placed in sugar or salt solutions with different sugar concentrations (different water potentials)

  - Water will move into or out of the cells depending on the relative water potential gradient of the cell and solution

  - As the plant cells gain or lose water, they will increase or decrease in mass

  - By measuring the changes in mass, you can find where there is no change in mass in the potato/onion cell to find the sugar concentration of the plant cell.
• Plasmolysis


  - Too much water is lost from the plant cell and the protoplast pulls away completely from the cell wall, with the space left being filled with the solution of the lower water potential
• Turgid

  - An excess of water moves into a plant cell and the cell swells. The cell wall is rigid, however, and this prevents the cell from bursting. This results in turgidity with is used by plants to give them structure.

  - This occurs when an a plant cell is placed in a solution with a higher water potential than it
• Crenation


  - Too much water moves out of an animal cell and the cell shrivels and 'puckers' (crenation).

  - This occurs when an animal cell is placed in a solution with a lower water potential than it
• Cytolysis

  - Too much water moves into an animal cell and the cell-surface membrane cannot withstand the increased pressure. This causes the cell to burst (cytolysis).

  - This occurs when an animal cell is placed in a solution with a higher water potential than it.
• Hydrostatic pressure


  The pressure of water against the walls of its container (measured in kpa).
• Water potential of pure water

  0�� (impure water will have a negative 𝛙)
• Water potential


  The pressure exerted by water molecules as they collide with a membrane/container, which is measured in pressure pascals (Pa) or kilopascals (kPa) with the symbol for water pressure being �� (psi)
• Osmosis


  Diffusion of water across a partially permeable membrane. It does not require energy.
• Exocytosis


  - The reverse of endocytosis/Bulk transport out of the cell

  1. Vesicles, formed by the golgi apparatus, move towards and fuse with with the cell surface membrane

  2. The contents are then released outside of the cell
• Pinocytosis

  The endocytosis of liquids
• Phagocytosis


  The endocytosis of solids
• Endocytosis


  - The bulk transport of material into cells

  1. The cell-surface membrane bends upwards when it comes into contact with the transport material

  2. The membrane enfolds the material until the membrane fuses, forming a vesicle

  3. The vesicle pinches off and moves into the cytoplasm for further processing
• Bulk transport

  - Another form of active transport that transports large molecules such as enzymes or hormones or whole cells like bacteria that cannot move through channel or carrier proteins

  - Energy is required (ATP) for the movement of vesicles along the cytoskeleton, the changing of cell shape and the fusion of cell membranes as vesicles either form or meet the cell surface membrane
• Process of Active transport

  - This process requires energy and carrier proteins, with carrier proteins acting as pumps

  1. The molecule or ion to be transported binds to receptors in the channel of the carrier protein outside of the cell

  2. On the cell inside, ATP binds to the carrier protein and is hydrolysed into ADP and phosphate

  3. Binding of the phosphate molecule to the carrier protein causes it to change shape, opening up the cell inside

  4. The molecule/ion is released into the cell inside

  5. The phosphate is released from the carrier protein and recombines with ADP to form ATP

  6. The carrier protein returns to its original shape

  - This process is selective (specific carrier proteins for specific substances)
• Process of facilitated diffusion

  - A protein channel specific to one molecule or ion transports said molecule/ion down a concentration gradient

  - Facilitated diffusion can also involve carrier proteins which change shape when a specific molecule binds to them.

  - Facilitated diffusion does not require external energy

  - The rate of facilitated diffusion is dependent on temperature, concentration gradient, membrane surface area and thickness and the number of channel proteins present
• Facilitated diffusion


  Diffusion across a membrane through protein (channel or carrier) proteins
• Factors affecting the rate of diffusion across membranes

  - Surface area (the larger the area of an exchange surface, the greater the rate of diffusion

  - Thickness of membrane (the thinner the exchange surface, the higher the rate of diffusion)
• Diffusion across membranes

  - Diffusion across membranes involves passing through the phospholipid bilayer, which can only happen if the membrane is permeable to the particles

  - Non-polar particles such as oxygen can diffuse freely down a concentration gradient

  - The hydrophobic interior of the membrane repels charged ions so they cannot pass easily through.

  - Polar molecules with partial positive and negative charges, such as water, can pass through but very slowly, although smaller polar molecules can pass through much more easily than larger ones
• Partially permeable membrane

  A membrane that allows some substances to cross but not others
• Simple diffusion


  Diffusion that doesn't involve a direct input of energy or assistance by carrier proteins.
• Factors affecting the rate of diffusion

  Temperature

  - The higher the temperature, the higher the rate of diffusion. This is because particles have more kinetic energy at higher temperatures and so can move faster

  
  

  Concentration gradient (difference)

  - The greater the difference in concentration between two regions the faster the rate of diffusion because the overall movement from higher to lower concentration will be lower
• Diffusion

  - The net movement of particles from a region of higher concentration to a region of lower concentration

  - It is a passive process and will continue until an equilibrium is reached between the two areas

  - Diffusion happens because the particles in a gas/liquid have kinetic energy. This random movement and an unequal distribution of particles will eventually become an equal distribution where the particles continue moving, but the movements are equal in both directions

  - Diffusion is fast over short distances but slow over long distances as there is more room for collisions between particles, slowing down both particles in the collision

  - Because of this, cells are generally microscopic as the rate of diffusion across a cell would be too slow and ATP would be diffused to slowly to sustain it
• Active transport

  The movement of substances across a cell membrane that requires the use of energy by the cell
• Passive transport


  The movement of substances across a cell membrane without the use of energy by the cell
• Investigating cell membrane permeability

  - Beetroot cells contain a red pigment (betalain) that gives them their colour and this makes them useful for investigating the effects of temperature and organic solvents on membrane permeability

  - To investigate the effect of temperature on the permeability of cell membranes, a student carried out the following procedure:

  1. five small pieces of beetroot were cut using a cork borer

  2. The beetroot pieces were thoroughly washed in running water and then they were placed in 100 ml of distilled water in a water bath

  3. The water bath's temperature was increased in 10 degree intervals

  4. Samples of the water containing the beetroot were taken five minutes after each temperature was reached and the absorbance of each sample was measured using a colorimeter using a blue filter

  5. The experiment was done three times, each time with a set of fresh beetroot pieces and a man was calculated for each temperature
• Factors affecting membrane structure

  Temperature

  - Phospholipids in a cell membrane are constantly moving

  - When temperature is increased, the phospholipids will have more kinetic energy and will move more, making the membrane more fluid and causing it to begin to lose its structure. If temperature continues to increase, the cell will break down completely

  - This loss of structure increases the permeability of the membrane, making it easier for particles to cross it

  - Carrier and channel proteins in the membrane will be denatured at higher temperatures and, as these proteins are involved in transport across the membrane, membrane permeability will be affected

  
  

  Solvents

  - Water, a polar solvent, is essential in the formation of the phospholipid bilayer as the non-polar tails of the phospholipids are orientated away from the water, forming a bilayer with a hydrophobic core. The charged phosphate heads interact with water, helping them to keep the bilayer intact.

  - Many organic solvents are less polar than water, for example alcohols, or are non-polar, e.g. benzene. Organic solvents will dissolve membranes, disrupting cells. This is why alcohols are used in antiseptic wipes as the alcohols dissolve bacteria membranes in the wound, killing them and reducing the risk of infection

  - Pure or very strong alcohol solutions are toxic as they destroy cells in the body whereas less concentrated solutions of alcohols, such as alcoholic drinks, will not dissolve membranes but will still cause damage. These non-polar alcohol molecules can enter the cell membrane and the presence of these molecules between the phospholipids disrupts the membrane

  - When the membrane is disrupted, it becomes more fluid and permeable. Some cells need intact cell membranes for specific functions, e.g. nerve impulse transmission by neurones, and when their membranes are disrupted, they no longer function as normal (this is why you get drunk).
• Site of chemical reactions

  - Like enzymes, proteins in the membranes forming organelles, or present within organelles, have to be in particular positions for chemical reactions to take place.

  - For example, the electron carriers and the enzyme ATP synthase have to be in the correct positions within the cristae (inner membrane of mitochondrion) for the production of ATP in respiration.

  -The enzymes of photosynthesis are found on the membrane stacks within chloroplasts
• Cholesterol


  - A lipid with a hydrophilic end and a hydrophobic end, much like a phospholipid, that regulates the fluidity of membranes

  - cholesterol molecules are positions between phospholipids in the membrane bilayer, with the hydrophilic end interacting with the heads and the hydrophobic end interacting with the tails, pulling them together. In this way cholesterol adds stability to membranes without making them too rigid. The cholesterol molecules prevent the membranes becoming too solid by stopping the phospholipid molecules from grouping too closely and crystalising.
• Extrinsic proteins


  - Present in one side of the bilayer

  - They normally have hydrophilic R-groups on their outer surfaces and interact with the polar heads of the phospholipids or with intrinsic proteins

  - They can be present in either layer and some move between layers
• Glycoplipids


  - They are similar to glycoproteins

  - They are lipids with attached carbohydrate chains

  - They are called cell markers or antigens and can be recognised by the cells of the immune system as self or non-self
• Cell signalling

  - When the chemical binds to the receptor, it elicits a response from the cell

  - This may cause a direct response or set off a cascade of events inside the cell

  - Examples include receptors for neurotransmitters such as acetylcholine at nerve cell synapses where the binding of neurotransmitters triggers or prevents an impulse in the next neurone and receptors for peptide hormones, including insulin and glucagon, which affect the uptake and storage of glucose by cells
• Glycoproteins

  - Embedded in the cell-surface membrane with attached carbohydrate (sugar) chains of varying lengths and shapes

  - They play a role in cell adhesion and as receptors for chemical receptors
• Carrier proteins


  - Proteins with an important role in both passive and active transport

  - This often involves the shape of the protein changing
• Channel proteins


  - Provide a hydrophilic channel that allows passive movement of polar molecules and ions down a concentration gradient through membranes.

  - They are held in position between the hydrophobic membrane core and hydrophobic R-groups on the protein's surface
• Types of intrinsic proteins

  - Channel proteins

  - Carrier proteins

  - Glycoproteins
• Intrinsic proteins


  Transmembrane proteins that are embedded through both layers of a membrane. They have amino acids with hydrophobic R-groups on their external surfaces which interact with the hydrophobic core of the membrane, keeping them in place.
• Types of membrane proteins

  Extrinsic (peripheral) and intrinsic (integral)
• Fluid Mosaic model


  Structural model of the plasma membrane where phospholipid molecules are able to move freely within a lipid bilayer (fluid). Furthermore, proteins in the cell surface membrane vary in size, shape and position like mosaic tiles (mosaic).
• Who proposed the Fluid Mosaic model?

  Singer and Nicolson (1972)