Homeostasis: Maintaining Body Temperature

Key Definitions:

  • Ectotherm: regulates body temp. via external sources e.g. the sun
  • Endotherm: can generate heat internally to regulate body temp.  e.g. metabolism in the liver.
  • Negative feedback: brings a reversal of any change of conditions back to the optimal conditions.




• Need less food as less is used in respiration, so can go for long periods of time without food and more energy obtained from food can be used for growth.


• At greater risk of predation as less active in cool environments and may need to warm up in the mornings before they can become active.
• May be incapable of activity in winter months so need to build up large energy store so can survive without eating.
Ectotherms change their behaviour or physiology to react to environmental temperature changes:
• Expose body to sun to enable more heat to be absorbed
• Orientate body towards sun so larger S.A for heat absorption (or away from sun so less heat absorbed)
• Hide in burrow reducing heat absorption
• Alter body shape to expose smaller/greater S.A to sun
• Increase rate of breathing so more water evaporates
welcome and be happy
• Fairly constant body temp. despite any external temp. changes
•  Can be active when it’s cold i.e. night/morning/winter and so can live in colder parts of the planet


• Large proportion of food intake used to maintain body temp, so more food is needed, and less food is used for growth.

Physiological adaptations:

• Water can evaporate from lungs/nose/mouth or from the skin when the sweat glands produce sweat
• Hairs on skin can be raised/lowered to trap greater/smaller insulating air layer
• Vasodilation/vasoconstriction in arterioles leading to skin to increase/decrease radiation of heat at skin surface
• Liver cells can alter their rate of metabolism
• Skeletal muscles can contract spontaneously ti generate heat via respiration in muscle cells (shivering)

Behavioural adaptations:

• Move into shade/hide in burrow
• Orientate body towards/away from the sun

 Remain inactive and spread out limbs to increase S.A, or roll into a ball to decrease S.A


Body temperature in endotherms is controlled by negative feedback:


The body also contains peripheral temperature receptors located at the extremities so there is a quicker reaction to external temp. changes as core body temperature may take some time to decrease enough for the hyperthalomous to detect a significant change.

Diet and Food Production

Balanced Diet: a diet which contains all the nutrients required for health in the appropriate proportions.


  • Obesity is when a person is 20% or heavier than the recommended weight for their height. Obesity is caused through malnutrition, which is where a person’s diet is unbalanced. In this case of malnutrition, the person consumes too much energy and the excess is deposited as fat in the adipose tissues which can impair health. The conditions most commonly associated with obesity are cancer, cardiovascular disease, type 2 diabetes, gallstones, osteoarthritis and high blood pressure (hypertension).


  • High blood cholesterol is linked with coronary heart disease. Although it’s found in cell membranes, the skin and is used to make steroid sex hormones and bile, above 5.2mmol/dm3 is harmful. Cholesterol is insoluble but is carried by HDL’S and LDL’s. Lipoproteins are a combination of lipid, cholesterol and protein.
    •  HDL’s contain unsaturated fat and tend to carry cholesterol from the body tissues back to the liver where it’s used in cell metabolism to make bile or is broken down, meaning high levels of HDL’s are associated with reducing blood cholesterol levels. They reduce deposition of cholesterol in the artery walls (called atherosclerosis) and may help to remove the depositions already there.
    • LDL’s contain saturated fats. They tend to carry cholesterol to body tissue from the liver. The concentration of LDL’s in the blood increases when a diet contains too much saturated fat and cholesterol. High LDL levels result in deposition in the artery walls.
      • Saturated fats decrease the activity of LDL receptors in body tissue so as the concentration of blood LDL’s rises; less is removed resulting in higher LDL concentrations and more deposition. Polyunsaturated and monounsaturated fats increase the activity of LDL receptors so more is removed from the blood.
    • Coronary heart disease: a condition in which the coronary arteries narrow from an accumulation of plaque (atherosclerosis) and cause a decrease in blood flow.
  • Humans depend on plants for all food as they are the basis of all food chains. This is because humans eat both plants directly, and herbivores who eat plants.
  • Making food production more efficient:
    • Plants:
      • Improve growth rate, increase yield, reduce crop loss due to diseases/pests, easier harvest by standardising crop size, and improve response to fertilisers.
    • Animals:
      • Improve growth rate, increase productivity, and increase resistance to disease
    • Selective breeding:
      • Isolation → artificial selection → inbreeding/line breeding
      • A pair of animals displaying desirable characteristics is allowed to reproduce. The offspring are sorted and those with the best combinations of characteristics are allowed to reproduce. Over many generations this exaggerated the desired characteristic.
      • Selecting which animals can breed or which seeds to be sown is known as applying selection pressure.
    • Pesticides, fungicides and fertilisers increase plant yield as they are less likely to die from insects eating them or a fungal disease, and the fertilisers provide all the minerals the plant needs to grow so it can grow faster. Antibiotics can increase food production of animals as fewer animals die from disease so a higher yield is achieved.


  • Methods of preventing food spoilage;
    • Cooking – denatures enzymes and other proteins killing microorganisms
    • Pasteurising – heat then rapid cooling kills microorganisms
    • Drying/salting/coating in sugar all dehydrate microorganisms as water will leave their cells by osmosis
    • Smoking – smoke contains antibacterial chemicals and the food develops a hardened, dry outer surface.
    • Pickling – an acidic pH kills microorganisms by denaturing enzymes and other proteins.
    • Irradiation – ionising radiation disrupts the DNA structure of microorganisms so they are killed.
    • Cooling/freezing – although microorganisms aren’t killed, the activity of their enzymes is slowed so their metabolism/growth/reproduction is also slow.
  • Using microorganisms to make food (e.g. cheese/yoghurt/bread/alcohol/single cell protein (Quorn – a mycoprotein)
    • Advantages:
      • Can be much faster than animal/plant production
      • Production can be increased/decreased according to demand
      • No animal welfare issues
      • Good source of protein for vegetarians
      • Protein contains no animal fat or cholesterol
      • SCP could be combined with removal of waste products, as they grow on almost any organic substrate e.g. paper or whey
    • Disadvantages:
      • People might not want to eat fungi or food grown on waste
      • Need to isolate protein from the microorganisms in the fermenter
      • Protein needs purification so it’s not contaminated
      • Need to control infection – the microorganisms grow at the same temperatures as harmful pathogens
      • Palatability – the protein has a different taste/texture to meat

How are proteins synthesied from DNA?

The video above is a really good animation to help you visualise what is happening 100 trillion times per second in your body. The process can be split into two main parts; transcription and translation, which are summarised below.



  1. DNA-helicase unzips the DNA, exposing it.
  2. The exposed base sequence is used as a template for free RNA nucleotides. Activated RNA nucleotides (ones with 2 extra phosphoryl groups)  temporarily hydrogen bond onto the template strand of DNA (leaving the coding strand unchanged). RNA polymerase catalyses this reaction, and the extra phosphoryl groups are released, producing energy for the bonding of adjacent nucleotides.
  3. The mRNA, which is a copy of the coding strand (with T replaced by U), passes out of the nucleus, via a pore in the nuclear envelope, to a ribosome.



  1. The mRNA molecule binds with a ribosome. Two codons are attatched to the smaller subunit of the ribosome, and are therefore exposed to the larger subunit. The first codon is always AUG, and so a tRNA molecule with anticodon UAC and amino acid mathionine hydrogen bonds to the codon.
  2. A second tRNA molecule with a different amino acid and complementary anticodon binds to the second codon.
  3. The two adjacent amino acids are joined together by a peptide bond, the reaction being catalysed by an enzyme in the small ribosomal subunit.
  4. The ribosome moves along the mRNA, and a third tRNA molecule brings another amino acid, whoch forms a peptide bond with the dipeptide. The first tRNA is then released to bring another amino acid to the ribosome.
  5. The polypeptide chain continues to grow in this way until a stop codon is reached. The stop codon works because there is no tRNA for the codons UAA, UAC or UGA.
  6. The polypeptide is released and assumes its secondary and tertiary structure.

Collagen Vs. Haemoglobin

Here are the similarities and differences between the structure and function of haemoglobin (as an example of a globular protein) and collagen (as an example of a fibrous protein):

  • Haemoglobin’s quaternary structure is four polypeptide subunits (2 alpha and 2 beta chains) whereas Collagen’s structure is 3 polypeptide chains wound around each other like rope.
  • Haemoglobin has a prosthetic group – each chain contains a haem group (Fe2+) but collagen has no prosthetic group.
  • Haemoglobin is made of a wide range of amino acid constituents in its primary structure whereas approx 35% of collagen’s primary structure is just one type of amino acid – glycine.
  • Much of a Haemoglobin molecule is wound into alpha helix structures but collagen’s molecule mainly consists of left-handed helix structures.
  • Haemoglobin’s function is as a transport molecule and carries oxygen around the body. In contrast collagen’s function is as a structural molecule and to give strength to many cells including artery walls, tendons, bones, and cartilage. This is why it is important that Haemoglobin is soluble (so it can travel around the body in the blood) and Collagen is insoluble (wouldn’t be able to provide support otherwise as would always dissolve).  Haemoglobin’s shape is important so that red blood cells become biconcave disks which can easily travel around the body and bind with oxygen.
  • Haemoglobin is a globular protein meaning its 3D feature is to roll up into balls, yet collagen is a fibrous protein so its 3D feature is to form fibres.
  • Haemoglobin molecules to not bond with each other. However, collagen molecules form covalent bonds between molecules called cross-links which are staggered along the collagen molecules, both increasing strength and forming a fibril. Many fibrils joined together make a collagen fibre.



Everything You Need To Know About Nucleic Acids

These are my AS Biology revision notes on nucleic acids – it’s everything you need to know if you’re following OCR’s syllabus 🙂

  • DNA is a polynucleotide, usually double-stranded, made up of nucleotides containing the bases adenine, thymine, cytosine and guanine. It’s stable and acts as an information store as the bases act as a coded sequence.
  • RNA is a polynucleotide, usually single-stranded, made up of nucleotides containing the bases adenine, uracil, cytosine and guanine.
  • Almost all DNA in a eukaryotic cell is found in the nucleus where it acts as an information store. RNA is found in three different forms needed to read and translate the information to produce the various proteins in an organism.
  • The monomer of all nucleic acids is called a nucleotide. Each nucleotide is made from one phosphate group, one sugar molecule and one organic nitrogenous base. These three subunits are joined by condensation reactions resulting in covalent bonds.

  • The phosphate group is always the same. The sugar molecule is a five carbon sugar – either ribose or deoxyribose.
  • A condensation reaction between the phosphate group of one nucleotide and the sugar of another nucleotide, forming a long chain of nucleotides. This repeating sugar-phosphate chain is the ‘backbone’ of the molecule and the organic bases project out from this backbone.
  • Chains of nucleotides are called nucleic acids. Only nucleotides carrying the same sugar bind together so if the sugar is ribose it’s RNA, if it’s deoxyribose then it’s DNA.
  • The organic bases are either purines (larger) or pyrimidines (smaller).The purines are adenine and guanine. The pyrimidines are thymine, cytosine and uracil.
  • Uric acid is produced when excess purines are broken down in the liver. It’s insoluble at low temperatures and forms crystals that are deposited in joints at the extremities causing gout.
  • The chain of nucleotide monomers is called a polynucleotide. When two polynucleotides come together a DNA molecule is formed. Hydrogen bonds between the base pairs in opposite chains strengthening the molecule. This is vital as it carries the instructions to make an organism.

  • The two DNA strands run parallel to each other as the space between is taken up by the nitrogenous bases projecting inwards. The term antiparallel is used as the strands run in opposite directions to each other – the sugars are pointing in opposite directions.
  • The chains are the same distances apart as the bases pair up in a specific way. If there is a purine on one chain, opposite there will be a pyrimidines on the other chain. Adenine always pairs with Thymine (or uracil in RNA) and Guanine always pairs with Cytosine. Hydrogen bonds form between the bases. Base pairing rules apply due to the different structure of the bases. The base pairing is described as complementary, so A is complementary to T, and G is complementary to C.
  • In a complete DNA molecule, the antiparallel chains twist to form the final structure called a double helix.
  • When a cell divides, the new cell must receive a full copy of the DNA for that organism and this must occur precisely. DNA replication takes place in the interphase of the cell cycle and is the process that creates identical sister chromatids.
  • In order to make a new copy of a DNA molecule, the double helix is untwisted and the hydrogen bonds between the bases are broken apart, exposing the bases.  Free DNA nucleotides are hydrogen-bonded onto the exposed bases according to the base-pairing rules. Covalent bonds are formed between the phosphate of one nucleotide and the sugar of the next to seal the backbone.  This continues along the whole molecule producing two new DNA molecules, both an exact copy of the original due to the base pairing rules.
  • This process of DNA replication is described as semi-conservative replication as each new DNA molecule consists of one conserved strand and one newly built strand
  • The sequence of bases is information storage – it’s in the form of a code to build proteins. As the molecules are long a lot of information can be stored. The base pairing means complementary strands of information can be replicated. The double helix gives the molecule stability. Hydrogen bonds allow easy ‘unzipping’ for copying and reading information.
  • RNA is structurally different from DNA as the sugar molecule in the nucleotides is ribose instead of deoxyribose. The nitrogenous base uracil is found instead of the organic base thymine.  The polynucleotide chain is usually single-stranded. There are three forms of the RNA molecule.
  • Base-pairing rules mean molecules of RNA can be made so they are complementary to molecules of DNA. This is because exposed nucleotides can have free RNA nucleotides hydrogen-bonded to them and then the sugar-phosphate backbone is sealed to form a chain of RNA nucleotides.

  • Copying the genetic code of the DNA base sequence is called transcription.
  • Messenger RNA (mRNA): made as a strand complementary to one strand of a DNA molecule (template molecule) making it a copy of the other DNA strand (coding strand) of the double-helix.
  • Ribosomal RNA (rRNA): found in ribosomes.
  • Transfer RNA (tRNA): carries amino acids to the ribosomes where they are bonded together to form polynucleotides.


  • A gene is a length of DNA that codes for one (or more) polypeptides. Each gene occupies a specific place (locus) on a chromosome. Different versions of the same gene are called alleles.
  • The sequence of bases in DNA code for the sequence of amino acids for a particular protein molecule. This gene can be exposed by splitting the hydrogen bonds that hold the double helix together in that region. RNA nucleotides form the complementary strand mRNA which is a copy of the gene. The mRNA peels away from the DNA and leaves the nucleus through a nuclear pore before attaching to a ribosome. tRNA molecules bring amino acids to the ribosome in the correct order according to the base sequence on the mRNA. The amino acids are joined together by peptide bonds to give a protein with a specific primary structure (which gives rise to the secondary and tertiary structures).

The Transport of Carbon Dioxide

  • Respired carbon dioxide has to be removed:
    • 5% Dissolves as a gas in the plasma
    • 10% Combines with Haemoglobin (Carbaminohaemoglobin)
    • 85% Dissolved in the form of hydrogen carbonate ions HCO3
  • Carbonic acid is produced using the enzyme carbonic anhydrase:
    • CO2 + H2O  →  H2CO3
  • Carbonic acid in solution produces hydrogen ions and hydrogen carbonate ions:
    • H2CO3 → HCO3 + H+
  • Chloride shift:
    • The negatively charged hydrogen carbonate ions diffuse out of the red blood cells. To balance the charge, Cl ions move in.
  • Hydrogen ions cause the red blood cell to become acidic. To control this, haemoglobin takes up the hydrogen ions to form haemoglobinic acid. The haemoglobin acts as a buffer.
  • The Bohr Effect:
    • The hydrogen ions compete for the space on the haemoglobin originally taken up by oxygen.
    • The hydrogen ions displace the oxygen making the oxyhaemoglobin dissociate faster.
    • More CO2 → more H+ ions → more freely O2 dissociates from oxyhaemoglobin.