Cell membranes and transport

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