Alevel AS生物考前5个必过知识点！

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：Unit4 Cellmembrane 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 6 Nucleic 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 7 Transport 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