Key biological principles

Summary of all the A2 biological principles from the textbook

  • Created by: gemma-t96
  • Created on: 03-05-14 15:33

The role of ATP

Adenosine triphospgate is most important energy transfer molecule within cells.

Consists of adenine (organic base) ribose (5C sugar) and 3 phosphate groups. 3rd phospate group is losely bonded so is easily removed. When it is removed ADP is formed, the phosphate group becomes hydrated, forming bonds with surround water molecules. A lot of energy's released as bonds form between water and phosphate group. 

Energy can be used to drive-energy requiring reactions in the cell. ATPase is enzyme used in the breakdown of ATP-ADP.

ATP is created from ADP by the additionof inorganic phospgate. The addition is known is phosphorylation. To make ATP phosphate must be seperated from water molecules. ATP in water is higher in energy than ADP & phosphate ions in water.

The formation of ATP seperates the phosphate & water. The phosphate and water can be brought together in energy-yielding reaction each time energy is needed for reactions within the cell. This way ATP transfers energy around the cell. 

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Metabolic Pathways

Reactions in cells can involve simple molecules linking to form more complex molecules. Reactions can also involve the breakdown of complex molecules into simpler molecules 

Reactions rarely occur in a single step. There are generally a series of smaller reactions, controlled by specific enzymes, forming metabolic pathways.

Allows the rate of the oerall reacton to be controlled, each steps controlled by a specific enzyme. Rangeof intermediate products can be produced, which could be useful as end- products or take part in other reactions

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Why does photosynthesis take place inside chloropl

Thylakoids & LDR

Formation of ATP occurs in LDR as a result of oxidation & reduction reactions. They involve the transfer of electrons between electron carrier molecules. Electron carriers are found within thylakoid membranes in the chloroplast. Positioning within the membrane creates an ETC, allowing electrons to pass from EC to it's neighbour 

Stroma in LIR

ATP molecules formed act as energy carriers within the cel, allowing small amounts of energy o be transferred & used when needed. Chemical potential energy in ATP in water is used in the LIR's In the fixing of CO2 forming organic molecules. The reactions depend on collision of substrates and the appropriate enzymes. 
High conc must be mainaited. Low conc would reduce rate of reaction & efficiency of photosynthesis.

Taking place within chloroplast stroma means substrates & enzymes can be at concentrations that allow them to be catalysed quickly  

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The Carbon Cycle


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What is the difference between a virus and a bacte


Prokaryotic cells, they do not have a nuclues or membrane bound organelles. Reproduce asexually by binary fission, after replication of their DNA they divide into 2 identical cells.


Small organic particles, strand of nucelic acid encolsed within a protein coat. Single or double stranded DNA

Virus attaches to the host cell. Virus inserts nucleic acid which then replicates, viral protein coats are synthesised and causes new virus particles to form. Finally virus particles are released due to cell lysis (splitting/burst the cell) 

Retro virus contains RNA not DNA

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Complementary protein shapes

Immune response relies on recognition of specific protein antigens by T helper & B cells using receptors with the complimentary shape. 

Specific shapes of the binding sites on antibodies are crucial to their function. 

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Protein synthesis revisited


RNA polymerase attaches to DNA. Hbonds between base pairs break. DNA molecule unwinds. RNA nucelotides with complementary to those on template strand of DNA pair up & form mRNA. mRNA then leaves through nuclear envelope. mRNA attaches to ribosome usually on ER


mRNA attaches to smaller subunit. 2 mRNA codons face 2 binding sites of larger subunit. At one side of tRNA molecule is triplet base sequence (anticodon) 3 bases of anticodon complement mRNA codon.
Complementary anticodons = UUU and UUC. Free AA attach to correct tRNA molecules.
First codon exposed = AUG and binds to UAC. Peptide bonds form between 2 AA.

Nature of genetic code: Triplet code = 3 bases. Non-overlapping, no base of 1 triplet contributes to the next. Start & stop codes. Degenerative = several triplets code for same AA. 

mRNA splicing: mRNA often edited between 2 stages. Non-coding introns removed. Remainder is expressed (exons) several proteins formed from 1 length of mRNA if spliced in different ways.

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Understanding the chemistry of respiration

Controlled by enzymes. 4 important types of reaction in Krebs cycle

Phosphorylation reactions - add phosphate

Decarboxylation reactions - break off carbon dioxide

Redox reactions - substrates are oxidised & reduced

Oxidised - electrons lost from substrate molecule & molecule that accepts electrons is reduced,

Dehydrogenation - molecule loses H, also oxidised, molecule gains hydrogen when it is reduced  

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Internal conditions must be maintianed within narrow range of cells' optimum conditions.

Partly achieved by maintaining stable conditions within blood = rise in tissue fluid. Blood concentration of glucose, ions and CO2 must be kept within narrow limits. Water potential pH and temp of bloos are tightly regulated.

Role of Negative Feedback
Receptors are used to detect deviations from the norm. Receptors connect to control mechanism, which turns effectors on/off to bring it back to normal

Deviation from the norm results in a change in the opposite direction, back to the norm. This process is known as negative feedback. Corrections mean that the actual value fluctuates in a narrow range around the norm.  

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How transcription factors work

Transcription initiation complex binds to section of DNA adjacent to gene being to be transcribed. This section's called the promoter region. Some factors are present in cells; others synthesised only in a particular type of cell or at a particular stage of development. Most are created in an inactive form. Turn active by action of hormones, growth factors or other regulatory molecules. 
Gene remains switched off until all factors are present.

Transcription of a gene can be prevented by protein repressor molecules attaching to DNA of promoter region. Blocking attachment sites for the factors, preventing formation of complex.
Protein repressor molecules can attach to the transcription factor themseles, preventing them forming the complex. Repressor molecules may actually be inactive factors.
Activator molecules in the cell stimulate the binding of the complex.

Genes are switched on by successful formation and attachment of the transcription initiation complex to the promoter region

Genes remain off by failure of the complex to form & attach to the promoter region. Due to absence of protein factors or the action of repressor molecules.  

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Why is there a potential difference

Uneven distribution of ions across the cell surface membrane is acheived by the action of sodium potassium pumps in the cell surface membrane of the axon. Pumps act against the conc gradients of the 2 ions, driven by energy supplied by the hydrolosis of ATP. Organic anions are large and stay within the cell, chloride move out to help balance change across membrane.

Resting Potential
Sodium potassium pumps establish conc, when there is no difference in charge between inside & outside of membrane potassium ions diffuse out of neurone down potassium gradient. K ions pass through K channels, outside of membrane + inside is -
Membrane permeable to K but virtually impermeable to Na. Difference in charge caused by diffusion of K causes potential difference across the membrane = resting potential

Why is axon resting potential -70? 
2 forces involved in movement of potassium ions. Result from conc gradient generated by NA/K pump. Electrical gradient due to difference in charge on 2 sides of membrane resulting from K diff
K ions diffuse out due to conc gradient. More diffuse out = larger potential difference across mem/ Increased - charged inside cell & attracts K back into membrane
Around -70 electrical gradient balances chemical gradient. No net movement of K, maintaining potential difference at -70. Electrochemical equilibrium for K is in place & membrane polarised. 

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Comparing nervous and hormonal coordination

NS not only means by which activites of the body are coordinated. 

Many hormones are produced steadily over long periods of time to control long term changes in the body such as growth & sexual development. 

Adrenaline is more short term in its action but takes longer than the NS to produce a response

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Coordination of Plants

Plant growth regulator/ plant growth substances: chemicals produced in the plant in very low concentrations & transported to where thy cause a response. 

Chemical identified as the auxin Indoleactic acid (IAA) it stimulates growth, response is the result of cell elongation. 
Increased conc of auxi on shaded side increased cell elongation, reduced conc on illuminated side inhibited cell elongation. As a result shoots grow towards the light.

Cholodny-Went model - explains growth curvature resulting from unequal distribution of auxin due to lateral transport. Criticised due to small sample sizes but still keep the basis of it. 

Auxins synthesised in actively growing plant tissues e.g. shoot tips. Auxins actively transported away from sites where they bring range of responses through effect of CE. By binding with receptors on plasma memrane in zone of elongation, auxins produce 2nd messenger signal molecules = changes in gene expression. Increased potential difference across membrane enhances uptake of ions into cell = uptake of H2O by osmosis = elongation

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