AS生物考前5个必过知识点！

 <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

<semi-conservative replication of DNA>

① The DNA double helix unwinds as the hydrogen bonds between the bases break.

（This is catalysed by the enzyme helicase at replication fork）

② The separated strands each provides a template for 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 polymerase links together the phosphate and deoxyribose groups of 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 small chunks called Okazaki fragments  

⑧ Finally, the enzyme DNA ligase seals all the fragments of the lagging strand by joining the sugar-phosphate backbones

⑨ Then each pair of double strands winds up into a double helix

⑩ This is called semi-conservative replication in 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）

<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 from a region of high hydrostatic pressure to a region of low hydrostatic pressure

③ Unloading：

Sucrose are used for metabolism, storage（starch）and growth（cellulose）

<the role of red blood cells in transporting oxygen and carbon dioxide>

– In respiring tissue：pO2 ↓ pCO2 ↑

• ~ 5 % CO2 dissolved in plasma

• ~10 % CO2 binds to the NH2 terminal of haemoglobin (Hb) to form carbamino-

haemoglobin (HbCO2)

• ~ 85 % CO2 combines 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 O2 to release H+ ;

         HHb + 4O2 → HbO8 + H+

H+ combine with HCO3- to form H2CO3 ;

H2CO3 dissociates to form CO2 and H2O catalyzed by carbonic anhydrase ;

CO2 diffuses into alveolus down a concentration gradient

<the hybridoma method for the production of monoclonal antibodies>

① small mammal is injected with sepcific antigen

② immune response occurs (over several weeks)

③ plasma cells are extracted from spleen ;

④ plasma cells are fused with cancer cells (to form hybridomas) ;

⑤ hybridoma cells are separated (into wells) to produce clones

⑥ screen for hybridomas producing desired (monoclonal) antibodies /Mabs