AS/A Level Biology Formulae and Key Points


Cell Size Conversion Numbers

10,000μm = 1cm

1,000μm = 1mm

1,000nm = 1μm

With a light microscope you can see organelles up to a

diameter of 200nm/0.2μm (bacterium, mitochondrion,

animal and plant cells)

With an electron microscope you can see organelles up

to 0.5nm (ribosomes, cell membrane thickness, DNA


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Magnification Equations

Magnification = 

Size of image /

Actual size of specimen

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Nucleotide Basics

1. A 5-carbon sugar molecule (Deoxyribose in DNA and ribose in RNA)

2. A phosphate group

3. 1 of 4 Nitrogen Bases (Adenine, Cytosine, Guanine and Thymine (Uracil in RNA)

1 and 2 bond to form phospahte backbone for DNA double helix

One phosphate backbone looking upside down compared to other - chemically in opposite directions

1st phosphate connected to 1st sugar molecule's 5th carbon; underneath 2nd phosphate connected to 2nd sugar molcecule's 3rd carbon and so on (opposite on other side)

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Common Disaccharide Compositions

  • Sucrose = glucose + fructose
  • Lactose = glucose + galactose
  • Maltose = glucose + glucose

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Standard Deviation Formula


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Respiratory Quotient Formula

A respiratory quotient is used to measure what substrate is being used in respiration or to see whether or not anaerobic respiration is occurring

In regular, human respiration the amount of oxygen (O₂) taken in and the amount of carbon dioxide (CO₂) released is the same

So it has a ratio of 1:1; however if this ratio is different then we are able to see what other subtrates may or may not be being respired

The formula for working out a respiratory quotient is:

Respiratory Quotient = moles or molecules of carbon dioxide given out/moles or molecules of oxygen taken in

Or more bascially:

R = CO₂/O

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  • Xylem vessels are dead, hollow cells
  • Dead because they contain lignin which disintegrates the contents of the cell
  • Lignin makes cell wall impermeable so they are basically waterproof
  • Also makes them strong to prevent them from collapsing
  • Wide lumen to transport large volumes of water
  • Also have non-lignified "holes" called pits which allow water to flow laterally (horitontally) from one xylem vessel to another 
  • Water at the top of the xylem vessels is removed and goes into the mesophyll cells, down the waater potential gradient
  • This reduces the hydrostatic pressure that the liquid is exerting so the pressure at the top of the xylem vessel becomes greater than the pressure at the bottom
  • This pushes the water up the tube
  • They also push water up from the bottom by actively pumping minerals from the cells surrounding the xylem into the xylem itself
  • This changes the concentration gradient which causes more water to be drawn into the xylem by osmosis
  • Moving water from the soil to the air at the top of the xylem is a passive process
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  • Phloem sieve tube elements are living tube-like cells that are connected end to end 
  • End end cell walls have little holes in them and are called sieve plates
  • There is a little bit of cytoplasm in a layer next to the cell wall
  • Sieve tube elements do not have nuclei or any organelles so that there is more space for the solutes to move
  • The cell walls are made of cellulose so that the solutes can move horizontally from element to element as well as up and down (similar to transport in xylem)
  • Next to each sieve tube element there is a companion cell
  • These have nuclei and many other organelles (mitochondria, ribosomes, etc)
  • So because the sieve tube elements do not have any organelles the companion cells control the movement of solutes and provide ATP for this active process
  • Parenchyma surround the phloem and provide support and turgidity
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Movement in phloem

  • Movement in the phloem is a process called translocation and requires energy to alter pressure so is considered an active process
  • Sucrose is loaded into the phloem at a source (usually a photosynthesising leaf) by a protein carrier
  • This is done by pumping hydrogen ions out of the companion cells creating a high concentration of hydrogen ions outside the companion cells
  • The protein carrier has two sites; one for sucrose and one for a hydrogen ion; so the sucrose and the hydrogen ions are moved by the carrier protein into the companion cells
  • The hydrogen ions are then left in the companion cells while the sucrose is moved into the phloem through the plasmodesmata that connect the companion cells and the phloem sieve tube elements
  • Moving the sucrose into the sieve tube elements lowers the water potential in them so water moves into the phloem from the xylem by osmosis
  • The sucrose is them transported to a sink (e.g. a root or a shoot tip)
  • Not a lot is known about how sucrose is unloaded at the sink but it is likely that it moves out of the phloem by diffusion 
  • The sucrose is then converted into another sugar to decrease the concentration of it and maintain a concentration gradient (e.g. using the enzyme invertase to convert sucrose to glucose and fructose)
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Interphase and Prophase

IPMAT = Interphase, Prophase, Metaphase, Anaphase and Telophase

Uses diploid cells which have 46 chromosomes grouped into 23 pairs

  • Interphase: 
    • DNA is not arranged properly and is loosely coiled (known as chromatin)
    • Centrioles duplicate and DNA is replicated
  • Prophase:
    • Chromatin condenses and coils up on itself to form thick strands of DNA wrapped around proteins (aka chromosomes)
    • Duplicates stay attached to originals in pairs
    • Each strand is called a chromatid and they are connected in the centre by a centromere 
    • Nuclear envelope disintegrates 
    • Centrioles peel away from the nucleus and begin moving to opposite ends of the cell
    • As they do this they leave behind long protein ropes called microtubules going from one centriole to another
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Metaphase, Anaphase, Telophase and Cytokinesis

  • Metaphase:
    • Can take up to 20 mins
    • Chromosomes attach to microtubules at their centromeres
    • Pulled from one end of the cell to the other by motor protein called dynein until they are arranged down the exact middle of the cell
  • Anaphase:
    • Motor proteins begin pulling so hard on the centromere that the chromatids separate at the centromere and are pulled to opposite ends of the cell
  • Telophase:
    • Nuclear membrane reforms 
    • Nuclear envelope reforms around chromosomes 
    • Chromosomes relax back into chromatin
    • Centrioles move back to their places near the nuceli
    • A crease between the two new cells called cleavage forms
  • Cytokinesis:
    • Nuclei move away from one another splitting the cell down the middle into two, new, identical cells
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Infectious Diseases Info


Pathogen(s):Vibrio Cholerae

Transmission: Food-borne, water-borne

Site of action: Wall of small intestine

Method of diagnosis: microscopial analysis of faeces

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Infectious Diseases Info #2


Pathogen(s): Human Immunodeficiency Virus

Transmission: Semen and vaginal fluids during intercourse, infected blood or blood products, contaminated hypodermic syringes, mother to fetus through placenta, mother to infant in breast milk

Site of action: T-helper lymphocytes, macrophages, brain cells

Method of diagnosis: Blood test for antibodies to HIV

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Infectious Diseases Info #3


Pathogen(s): Plasmodium falciparum, P. vivax, P. ovale, P. malariae

Transmission: Insect vector (female Anopheles mosquito)

Site of action: Liver, red blood cells, brain

Method of diagnosis: microscopial analysis of blood

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Infectious Diseases Info #4

Tuberculosis (TB)

Pathogen(s): Mycobacterium tuberculosis, M. bovis

Transmission: Airborne droplets, via unpasteurised milk

Site of action: Primary infection in lungs, secondary infections in lymph nodes, bones and gut

Method of diagnosis: Microscopial examination of sputum for bacteria, chest X-ray

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The Calvin Cycle

  • The Calvin cycle is a series of reactions that occur in the light-independent stage of photosynthesis
  • These reactions can occur in the light or dark which is why they are called light-independent
  • The reactions occur in the stoma of the chloroplast
  • Carbon dioxide is combined with a 5-carbon sugar called ribulose biphosphate (RuBP)
  • This forms a 6-carbon sugar in a reaction called carbon fixation by the enzyme ribulose biphosphate carboxylase (rubisco)
  • This 6-carbon sugar is very unstable so is split into two 3-carbon sugars
  • These 3-carbon sugars are then converted into triose phosphates using the energy from ATP and the hydrogen from reduced NADP
  • Most of this triose phosphate is used to regenerate the RuBP 
  • But some of it is used to produce 6-carbon sugars from which complex carbohydrates, amino acids and other substances are made
  • For every twelve 3-carbon sugars
    • 2 form one 6-carbon sugar (6 carbons)
    • 10 form six 5-carbon sugars (RuBP) (30 carbons)
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