Cell membranes and transport
The need to control movementWaste products need to be removedNutrients need to be receivedRequirements for respirationOxygen, Carbon dioxide
PhospholipidsPolar structureHydrophobic tails (water hating)Hydrophilic head (water liking)
Cell membranePhosophlipid bylayer visible at x100 000 (7nm)Fluid mosaic ie the phospholipids are free to
move around.Hydrophobic tails point inwards in the cellSome proteins float between the two membrane
layers, which can assist with active transport Cholesterol helps regulate the rigidity and
fluidity of this layer.
ProteinsTransport proteins act as a hydrophylic
pathway through the membrane.Glycolipids and glycoproteins have a
carbohydrate chain attached. They can act as receptors for particular chemicals and hormones.
Movement in and out of cellsDiffusion- the spreading of molecules from a
high concentration to low concentration (down gradient) until evenly distributed.
Factors affecting rate of diffusion:Steepness of gradientTemperature (kinetic energy)Surface area for movementMolecule sizeFacilitated diffusion (hydrophilic protein
channels) is used for large molecules.
Osmosis Diffusion of water molecules across a semi
permeable membraneWater moves from regions of high water
potential to low water potential. The presence of solutes on one side of the membrane affects the water potential
Solute plus solvent = SolutionAdding solute to water lowers the water
potential.Pure water has a water potential of zeroAdding some solute will give the water a
negative value for water potential(The amount that a solute will do this is
called solute potential)
Osmosis in plant cellsPlasmolysis, Turgidity
Answer SAQ 4.3Page 56
Pressure potential
Answer SAQ 4.4 and 4.5
Active transportActive transport involves movement of ions
UP the concentration gradient. Transport proteins form ‘kissing gates’ which open and close to move substances. This process uses energy (ATP)
Bulk transportSome processes exsist to move substances on
a larger (non molecular scale)Phagocytosis involves the cell ‘eating up’
substances by engulfing them through the membrane. This is carried out by cells called phagocytes, which typically exsist in the immune system.
Pinoscytosis is the similar proccess where cells may take in liquids.
Removing substances from the cells in this way is called exocytosis
Exchange surfacesIn many instances WHOLE ORGANISMS
need to be involved in the transfer of substances.
Lungs are one such example in gas exchange. Alveoli create a structure with a huge surface area where blood is passed very closely to gasses from the outside environment. (total lung surface area of an adult is 70m2)
Lungs allow CO2 from blood to diffuse down concentration gradient into lungs, and O2 down concentration gradient into blood.
Plant rootsPlant root hair cells give the plant an
enormous surface area for the uptake of mineral ions.
Plants can reduce their water potential trough transpiration, which causes the roots to draw up more water from the soil by osmosis.
Plants may use facilitated diffusion with water movement to gain mineral ions.
Active transport will more often be used when the concentration of ions is much higher in the root than the soil.
Answer SAQ 4.7