CELLS 2.3 EUKARYOTIC CELLS 2 THINGS TO COVER Structure of membranes Hydrophobic & hydrophilic properties of membranes Membrane proteins: structure & function Diffusion & osmosis Passive transport: simple & facilitated diffusion Active transport Vesicles, endocytosis & exocytosis J WERBA IB BIOLOGY
3 2.4.1 MEMBRANE STRUCTURE J WERBA IB BIOLOGY 4 2.4.2 PHOSPHOLIPIDS Consist of a polar head (hydrophilic) made from glycerol and phosphate Consist of two non-polar fatty acid tails (hydrophobic)
Phospholipids arrange in a bilayer Hydrophobic tail regions face inwards The two hydrophilic head regions associate with the cytosolic and extracellular environments respectively J WERBA IB BIOLOGY 5 2.4.2 PHOSPHOLIPIDS Phospholipids are held together by hydrophobic interactions (weak associations) Hydrophilic / hydrophobic layers restrict entry and exit
of substances Phospholipids allow for membrane fluidity / flexibility (important for functionality) Fluidity allows for the breaking / remaking of membranes (exocytosis / endocytosis) J WERBA IB BIOLOGY 6 2.4.3 MEMBRANE PROTEINS Remember: TRACIE
J WERBA IB BIOLOGY 7 2.4.3 MEMBRANE PROTEINS Remember: TRACIE Transport: Protein channels and protein pumps (eg. Na+/K+ Pump) Receptors: Peptide-based hormones (insulin, glucagon, etc.) Anchorage: Cytoskeleton attachments and extracellular matrix Cell recognition: MHC proteins and antigens
Intercellular joinings: Tight junctions and plasmodesmata Enzymatic activity: Metabolic pathways (e.g. ETC in cellular respiration) J WERBA IB BIOLOGY 8 2.4.4 DIFFUSION Command term = DEFINE The net movement of particles from a region of high concentration to a region of low concentration, along the gradient, across a semi-permeable membrane J WERBA IB BIOLOGY
9 2.4.4 OSMOSIS Command term = DEFINE The net movement of water molecules from a region of low solute concentration to a region of high solute concentration , along the gradient, across a semipermeable membrane J WERBA IB BIOLOGY 10 2.4.4
OSMOSIS Command term = DEFINE J WERBA IB BIOLOGY 11 2.4.5 PASSIVE TRANSPORT Does not require energy Movement of substances is along the concentration gradient (high low) 2 types of passive transport:
Simple diffusion (inc. osmosis) Facilitated diffusion J WERBA IB BIOLOGY 12 2.4.5 PASSIVE TRANSPORT Simple diffusion: Small, non-polar molecules can freely diffuse across the membrane eg. O2, CO2, H2, H2O, glycerol
Facilitated diffusion: Larger, polar substances cannot freely diffuse Require the transport proteins to facilitate their movement eg. glucose, sucrose, Cl-, Na+, K+ J WERBA IB BIOLOGY 13 2.4.6 ACTIVE TRANSPORT Does require energy Movement of substances is against the concentration gradient (low high)
Will only occur in the presence of ATP eg. Na+/K+ Pump vesicular transport J WERBA IB BIOLOGY 14 2.4.6 ACTIVE TRANSPORT They use the energy from ATP to translocate the molecules against the gradient
The hydrolysis of ATP to ADP causes a the protein pump to change shape, resulting in the forced movement of the substance Protein pumps are specific for a given molecule J WERBA IB BIOLOGY 15 2.4.7 VESICULAR TRANSPORT Proteins destined for secretion are directed to the endoplasmic reticulum The protein is transferred to the golgi apparatus via a
vesicle, which forms from the budding of the membrane The protein moves via vesicles from one side of the golgi to the other and may be modified along the way (eg. glycosylated, truncated, etc.) The protein is transferred via a vesicle to the plasma membrane, whereby it is either immediately released or stored for a delayed release. J WERBA IB BIOLOGY 16 2.4.7 VESICULAR TRANSPORT
J WERBA IB BIOLOGY 17 2.4.7 VESICULAR TRANSPORT Vesicular transport is possible b/c: Membrane has some fluidity Small amounts can be added or removed without tearing the membrane (endo- & exo- cytosis) Membranes of all organisms are the same J WERBA IB BIOLOGY 18
2.4.8 VESICULAR TRANSPORT Exocytosis Rough ER produces proteins intended for export Golgi apparatus prepares substances for exocytosis Transport vesicle formed from a section of membrane from the Golgi apparatus This membrane then joins the cell surface membrane Exocytosis requires energy! J WERBA IB BIOLOGY
19 2.4.8 VESICULAR TRANSPORT Endocytosis Transport vesicle formed from a section of the cell membrane This process requires energy too! Used to take up substances that are too large and/or highly polar and cant enter on their own Requires recognition of the
substance by a membrane receptor protein J WERBA IB BIOLOGY 20 2.4.8 VESICULAR TRANSPORT Endocytosis Two types of endocytosis : Pinocytosis when the substance is fluid cell-drinking Phagocytosis when the substance is solid
cell-eating J WERBA IB BIOLOGY 21 SUMMARY of CELLULAR TRANSPORT no Concentration gradient down no down
no down yes against if possible yes against if possible ATP required Diffusion Facilitated diffusion Osmosis Active transport
with carrier proteins Endocytosis J WERBA IB BIOLOGY 2.4 22 2.4 MEMBRANES Sample question Q: What do diffusion and osmosis have in common? A: Both are forms of passive transport
Both move substances along a concentration gradient Both transport substances across a semi-permeable membrane J WERBA IB BIOLOGY 23 2.4 MEMBRANES Sample question Describe the movement of water across membranes.  Osmosis / passive transport Moves from regions of low solute concentration to
high solute concentration or high water concentration to low water concentration passes through protein channels (or aquaporins) in a semi-permeable membrane J WERBA IB BIOLOGY 24 2.4 MEMBRANES Sample question Draw a labelled diagram to show the fluid mosaic structure of a plasma membrane, indicating the hydrophilic and hydrophobic regions. 
J WERBA IB BIOLOGY 25 2.4 MEMBRANES Sample question DISTINGUISH between active and passive movements of materials across plasma membranes, using named examples.  Hint: Use a table!
Passive Active Diffusion / osmosis / facilitated diffusion active transport / ion pumps / exocytosis / pinocytosis / phagocytosis a second passive method (from above) a second active method; (from above) does not require energy requires energy/ATP; down concentration gradient
against concentration gradient no pumps needed requires protein pumps oxygen across alveoli / other example glucose absorption in ileum / other example J WERBA IB BIOLOGY 26
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