Cell Membranes and Movement of Substances

• fluid mosaic model

  fluid - phospholipids can move around in each layer meaning the membrane is flexible and can change shape

  mosaic - membrane is filled with protein molecules and the arrangement of the proteins varies
• • membranes act as barriers between internal and external contents of the cell and organelles or the external environment
  • can separate one part of an organelle from another
  • location for chemical reactions, e.g. stages in respiration
  • involved in cell signalling
• as phospholipids contain a hydrophobic and a hydrophilic region, they arrange themselves like this in water

  • the hydrophilic region interacts with water
  • the hydrophobic region is buried away in the centre away from the water

  
• because there is a hydrophobic centre, hydrophobic molecules can easily pass through the membrane (steroid hormones). however the hydrophobic centre prevents hydrophilic molecules from passing through.

  • hydrophilic molecules are polar (charged) and cannot easily pass through the non polar region
  • although water molecules are polar, they can pass through the membrane as they are extremely small and takes place at a slow rate
  • by packing the spaces in between the phospholipids with cholesterol it reduces the movement of water soluble chemicals across the membrane
• cholesterol

  • it has a has a polar, hydrophilic group at one end which attracts the phospholipid head groups. the rest is non polar and hydrophobic which attracts the hydrophobic fatty acid tails on the phospholipids.
  • cholesterol interacts with the phospholipids it increases the strength of the cell surface membrane, makes it more stable and less likely to become damaged
  • cholesterol reduces sideways movement of molecules and phospholipids which helps control the fluidity which prevents it from coming too fluid under warm conditions and too rigid under cold conditions
• membrane proteins

  • intrinsic (channel proteins and carrier proteins)
  • extrinsic (glycoproteins)
• 2 types of intrinsic proteins

  • protein channels - channel running through the centre, lined with hydrophilic amino acids and filled with water molecules. Allows for water soluble molecules and ions to diffuse through
  • carrier proteins - change shape or position in order to transfer the molecule from one side of the membrane to the other. Allows for sugars or amino acids to diffuse through the membrane
• intrinsic proteins are fully embedded into the surface of the membrane from one side to the other. they have hydrophobic amino acids on the outside of the protein and these amino acids interact with the hydrophobic tails

  extrinsic proteins do not span the membrane and they are found on either side of the membrane, or can be attached to intrinsic proteins
• extrinsic proteins

  • structural role within a membrane
  • act as enzymes
  • receptors (e.g for hormones)
• many membrane proteins contain a carb molecule attached (glycoprotein)

  • allow for cells to attach to each other to form tissue (e.g. nervous tissue)
  • role in the immune system (present antigens to T cells)
  • receptors for hormones
• glycolipids are used when cells come in contact with each other

  • glycolipids on the surface of one cell can be recognised by another cell which can determine whether cells come into contact
  • act as antigens (e.g. determining your blood group)
• diffusion is the net movement of particles from a region go high concentration to a region of lower concentration. down a concentration gradient.

  • net movement - overall movement
  • concentration of the gas will be the same both inside and outside of the cell. equilibrium has been reached, no net movement of particles
• diffusion is a passive process as it does not require metabolic energy
• factors affecting diffusion

  • concentration gradient - greater the concentration gradient the greater the rate of diffusion
  • temperature - in warmer conditions the particles have more kinetic energy so diffuse faster than in colder conditions (not a problem for mammals as they maintain a constant temperature)
  • surface area - quicker if its a large surface area
  • distance - greater the distance the slower the rate of diffusion
• facilitated diffusion is used so hydrophilic molecules can diffuse across the membrane without interacting with the hydrophobic centre

  • hydrophilic molecules (ions and polar molecules) cannot diffuse across the membrane. water is an exception polar molecule because it is small so can diffuse across the membrane
• 2 types of intrinsic membrane proteins that span the membrane from one side to another

  • carrier proteins (sugars and amino acids) - binding site for a specific chemical and when the molecule binds it causes the tertiary structure of the carrier protein to change shape. it brings the chemical across the membrane where it is released
  • protein channel (water soluble molecules and ions) - proteins with a central channel so hydrophilic substances can pass from one side to another. selective for chemicals that can pass through meaning only certain chemicals can pass through each type of channel
• metabolic energy is not required for facilitated diffusion
• in active transport carrier proteins in the membrane transport a chemical from a region of lower concentration to a region of higher concentration. against a concentration gradient
• active transport requires metabolic energy, its an active process
• 1. molecule or ion to be transported attaches to a receptor site on the carrier protein, takes place on the side of the membrane where the chemical is at a lower concentration
  2. ATP molecule binds to the carrier protein
  3. ATP molecule undergoes hydrolysis = ADP + phosphate
  4. phosphate attaches to the carrier protein and causes a shape change
  5. shape change allows the carrier protein to transport the molecule or ions to the other side where it is released
  6. phosphate leaves the carrier protein causing it to return to its original shape. the ADP and phosphate will reform to ATP during respiration
• • a lot of ATP is needed so lots of mitochondria will be found in cells that carry out active transport
  • carrier proteins used in active transport are specific so each carrier protein will transport one type of molecule or ion
• osmosis is the diffusion of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane.

  • at some point bath sides of the membrane will have the same water potential so reached equilibrium and osmosis has stopped
  • water molecules will still move across the membrane but the same number will move in either direction so the net movement will be 0
• pure water has the highest water potential and that is because pure water has the highest concentration of water molecules - 0kpa

  • this means the water potential of any solution must be less then 0
  • a more concentrated solution has a lower water potential than a less concentrated solution
• osmosis is the diffusion of water molecules and does nor require any metabolic energy (energy released through respiration)
• water potential - tendency of water molecules to move from one area to another

  • measure of pressure
• animal cells - the cytoplasm of animal cells has a number of solutes dissolved in water, this means there is a negative water potential
• animal cells - red blood cells are surrounded by plasma, the water potential of the plasma is the same as the cytoplasm water potential. as the water potential is the same inside and outside of the red blood cell, osmosis is not taking place.

  • water molecules are still moving across the membrane, but the inwards rate is the same as the outwards rate. no net movement
  • if a red blood cell is placed in pure water, water molecules will move into the red blood cell by osmosis. the pressure inside the cell will increase (hydrostatic pressure) and the cell membrane is not strong enough so will burst
• plant cells - the vacuole is filled with sugars and mineral ions. between the vacuole and cell wall there is the protoplast (cell membrane, cytoplasm and organelles) because of the solutes the interior of the cell has a low water potential
• plant cell - if the plant cell was placed in pure water osmosis will occur into the cell, so the hydrostatic pressure increases the protoplast pushes against the cell wall. as the cell wall is made from cellulose the [lant becomes firm (doesn't burst, turgid)
• plant cell - if the plant cell was placed in concentrated glucose solution, water would move out of the cell by osmosis. this causes the protoplast to pull away from the cell wall. the plant cell is now plasmolysed and when this happens the space between the cell membrane and the cell wall is filled with the solution surrounding the plant