COURSEMO生物组的Daisy老师为大家总结了AS生物考前必过的5个知识点！如下：Unit4Cellmembrane and transport
Cell signalling
Main stages in the process of cell signalling leading to specific responses：
- Secretion of specific chemicals (ligands) from cells
- Transport of ligands to target cells
- Binding of ligands to cell surface receptors on target cells
Unit 6Nucleic acid and protein synthesis
semi-conservative replication of DNA
① The DNA double helix unwindsas the hydrogen bondsbetween the bases break.
（This is catalysed by the enzyme helicase at replication fork）
② The separated strands each provides a templatefor creating a new strand of DNA
③ Free activated nucleotides pair up with their complementary nucleotides on the template strand by forming hydrogen bonds in between（A with T，C with G）
④ DNA polymeraselinkstogether the phosphate and deoxyribose groupsof adjacent nucleotides in one direction, from the 5’end to the 3’end
⑤ The strand that is made continuously is called the leading strand（5’to 3’）
⑥ The other that can not be made in such a continous way, is called the lagging strand（3’to 5’）
⑦ On the lagging strand, the DNA polymerase can only make this strand in a series of smallchunks called Okazaki fragments
⑧ Finally, the enzyme DNA ligaseseals all the fragments of the lagging strand by joining thesugar-phosphate backbones
⑨ Then each pair of double strands winds up into a double helix
⑩ This is called semi-conservative replicationin which one strand of each new double helix comes from the parent DNA and one is newly synthesized strand
（i.e. half the original molecule is conserved）
Unit 7Transport in plants
Translocation
① Loading：
- H+are pumped from companion cells to the leaf cell by H+pump actively（driven by ATP）, creating a diffusion gradient for H+
- H+ diffuse back to the companion cell through the co-transporter proteins bringing sucrose with them
- High concentration sucrose diffuse into S.T.E from companion cell through plasmodesmata
② Mass flow（of phloem sap）：
- concentration of sucrose in S.T.E↑
- water potential↓
- water moves to S.T.E by osmosis
- building up of a hydrostatic pressure
- assimilates transported by mass flow froma region of high hydrostatic pressure to a region of low hydrostatic pressure
③ Unloading：
Sucrose are used for metabolism, storage（starch）and growth（cellulose）
Unit 8Transport in mammals
the role of red blood cells in transporting oxygen and carbon dioxide
– In respiring tissue：pO2↓pCO2↑
- ~ 5 % CO2dissolved in plasma
- ~10 % CO2binds to the NH2terminal of haemoglobin (Hb) to form carbamino-
haemoglobin (HbCO2)
- ~ 85 % CO2combines with H2O to form carbonic acid (H2CO3) which then dissociates into
hydrogen ions (H+) and hydrogencarbonate ions (HCO3-) catalysed by carbonic anhydrase ;
Oxyhaemoglobin (HbO8) combine with H+to form haemoglobinic acid (HHb) and release O2
* The haemoglobin absorbs the H+ ions and acts as a buffer to maintain the intracellular pH
* chloride(Cl-)shift:
the movement of chloride ions into red blood cells from blood plasma, to balance the movement of
hydrogencarbonate ions(HCO3-)into the plasma from the red blood cells
* H+ cannot diffuse out of red blood cells
–In lungs：pO2↑pCO2↓
When the red blood cell reaches the lungs, HCO3-is pumped back into the cell and the entire process is reversed
- carbaminohaemoglobin (HbCO2) dissociates to form CO2 and Hb
- HHb pick up O2to release H+;
HHb + 4O2 → HbO8 + H+
H+combine with HCO3-to form H2CO3;
H2CO3dissociates to form CO2and H2O catalyzed by carbonic anhydrase ;
CO2diffuses into alveolus down a concentration gradient
Unit 11 Immunity
the hybridoma method for the production of monoclonal antibodies
① small mammalis injectedwith sepcific antigen
② immune response occurs (over several weeks)
③ plasma cells are extracted from spleen ;
④ plasma cells are fused withcancercells (to form hybridomas) ;
⑤ hybridoma cells are separated (into wells) to produce clones
⑥ screenfor hybridomas producing desired(monoclonal) antibodies /Mabs
