Transport and membrane CAQ

Transport and Cell membrane Created Tags Describe how the carrier protein works June 13 , 2025 132 AM T ranspor t and Cell membrane 1

Notes molecule to be transported is complementary to binding site of carrier protein and will bind to binding site of carrier protein This induced a conformational change on the carrier protein and hence molecule is taken into carrier protein and released to other side of plasma membrane Channel proteins are constantly open the rate of transport of molecules across plasma membrane with increasing concentration of molecules The rate of transport increases with increasing concentration However, it is limited by the number of transport proteins embedded in cell membrane Note that rate of transport by carrier protein is lower than channel protein due to limited number if channel protein being saturated with ions T ranspor t and Cell membrane 2

Significance of transport proteins To transport charged, hydrophilic and polar molecules that are unable to transverse the hydrophobic core of phospholipid bilayer and hence interior of channels are lined with hydrophilic residues for transport of these molecules across cell membrane Why is the rate of transport (that involves ATP hydrolysis) expected to stay constant, even if the concentration difference changes? Notes  ATP provides energy: The process uses energy from ATP hydrolysis (breaking ATP into ADP  Pi) to power the transport. This energy input is fixed and doesnʼt depend on how much more or less concentrated the molecules are outside/inside the cell. T ranspor t and Cell membrane 3

 Works against the gradient: This type of transport (e.g., active transport) moves molecules against their natural concentration gradient (from low to high concentration). Since the energy comes from ATP (not the gradient), the rate stays steady regardless of how big the concentration difference is.  Bulk transport exception: For processes like endocytosis/exocytosis (where large amounts are moved at once), the concentration gradient doesnʼt matter because the cell "decides" to move a set amount Passive Transport: Rate depends on gradient (e.g., diffusion). Active Transport: Rate depends only on ATP availability (fixed speed). Bulk Transport: Gradient irrelevant; cell decides how much to move. Features of active transport Requires ATP to provide energy for reaction to occur Requires a carrier protein Note that channel protein is used in facilitated diffusion as no confrontational change is needed to open the gate of protein Carrier protein has specific binding sites Binding causes conformational changes in carrier proteins Function of hydrocarbon tails in phospholipid bilayer of cell membrane Provides a hydrophobic core which acts as a selective barrier, restricting movement of charged ions, polar or hydrophilic molecules across membrane into cell Describing and explaining the process of endocytosis Endocytosis is a process of bulk transport which involves bringing in macromolecules or molecules in large quantity into a cell and ATP is needed for this due to the rearrangement of cytoskeleton that causes infolding of cell membranes , forming an invagination of cell surface membrane forming T ranspor t and Cell membrane 4

endosome/vesicle contains proteins formed within the cell from the infolding of cell surface membrane Substance taken up by cell via phagocytosis which involves the rearrangement of cytoskeleton forming pseudopodia which extend outwards to engulf the large insoluble macromolecule and solids. The ends of pseuodopia fuse, pinching off the cell surface membrane to form vesicles contains the foreign material A lysosomes then fuse with phagosome forming phagolysosome amd the hydrolytic enzymes in lysosome digest the contents into soluble products Useful products absorbed into cytoplasm while unwanted products are released out the cell via exocytosis There is also pinocytosis which is to bring in tiny vesicle of liquid when a small area of the plasma membrane invaginates For transport of molecules in large quantity against concentration gradient, receptor-mediated endocytosis occurs where specific ligands bind to receptor proteins in membrane causing invagination of membrane Role of membrane Due to the hydrophobic core of phospholipid bilayer, membranes serve as a selectively permeable barrier to movement of ions, polar, charged and large molecules this is for when question ask role of membranes WITHIN cells and not cell surface membrane: Within a cell, membrane allows compartmentalisation to occur allowing formation of unique environments for specialised activities such as enzyme reactions in lysosomes where an optimal pH is required, For establishment of proton gradients within specialised organelles such as mitochondria and chloroplasts for chemiosmosis and formation of ATP allow localisation of proteins of related function together so that sequential biochemical processes are facilitated for example enzymes and proteins are T ranspor t and Cell membrane 5

grouped together into photosystems II and I on thylakoid membranes of chloroplasts facilitating electron energy transfer Increase surface area for chemical reactions such as highly folded cristae of mitochondria increases surface area for chemical reactions such as highly folded cristae of mitochondria increases surface area for insertion of electron transport carriers and ATP synthase complexes for oxidative phosphorylation to take place Site for protein synthesis at rough endoplasmic reticulum where translation takes place at ribosomes at RER membrane Allows storage of molecules within a separate environment to prevent degradation by other enzymes such as starch in amyloplasts Transport of ATP and H+ ion Notes How a phospholipid bilayer is formed T ranspor t and Cell membrane 6

Notes Describe structure of a phospholipid molecule + bonds Consists of one polar, hydrophilic, charged phosphate head held by ionic & hydrogen bonds between phosphate head and aqueous medium of cytoplasm and external environment and two hydrophobic, non-polar fatty acid tails which are held by hydrophobic interactions between fatty acid tails Phosphate heads faces outwards which is the aqueous medium and fatty acid tails faces inwards to shield itself from the aqueous environment forming a phospholipid bilayer How a molecule travels across membrane by simple diffusion Diffuse directly across the phospholipid bilayer by interacting with hydrophobic fatty acids via hydrophobic interactions How structure of cell membrane and aquaphorin channels facilitates movement of particles /water Two layers of amphitatic phospholipid molecules with hydrophilic phosphate heads facing the aqueous extracellular and intracellular medium of the cell and hydrophobic fatty acids tails facing inwards away from the aqueous medium forms hydrophobic core of the bilayer, adjacent phospholipid held by weak hydrophobic interactions between fatty acids tails Hydrophobic core restricts the free movement of hydrophilic,polar and charged substances across the membrane T ranspor t and Cell membrane 7

Fluid nature of membrane due to lateral movement of phospholipids contributes to slight permeability to water Aquaphorin channels have hydrophilic amino acid residues lining the pore or channel allowing passage of polar, hydrophilic and charged molecules down/against the concentration gradient and also have hydrophobic a,into acid residues forming hydrophobic interactions with the fatty acids tails to embed in the membrane describing exocytosis secretory vesicles containing protein substances bud off from cisternae at trans face of Golgi body and move towards the cell surface membrane The membrane of the secretory vesicles fuses with CSM amd moved by microtubules to CSM ATP required Releasing protein out of cell via exocytosis Explain why a CSM is described as a fluid mosai