Molecules, Blood and Gas Exchange

Revision notes for OCR Human Biology Exam. 

  • Created by: Parmz
  • Created on: 26-03-13 17:36

Types of blood cell

Erythrocytes = red blood cells, biconcave discs in shape meanin they have a relatively large surface area-to-volume ratio to speed up gas exchange. Transport carbon dioxide and oxygen and packed with a protein called haemoglobin in the cytoplasm. Mature red blood cells have no nucleus so there's more room for haemoglobin, and they are small and flexible so can be flattened against capillary walls reducing the distance they have to diffuse across = speeding up gas exchange

(Leucocytes = white blood cells) Neutrophils= lobed nucleus and small granules in the cytoplasm, they engulf microoraganisms by phagocytosis

Lymphocytes = dark-stained nucleus and clear cytoplasm.

B: produce antibodies

T:cell destruction 

Monocytes = largest type, bean shaped nucleus, clear cytoplasm. Become macrophages,engulfing microorganisms and other forgein material. 

Platelets = fragments of megakaryocytes (giant cells). They are involved in blood clotting

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Calculating magnification

Magnification = size of structutre in the picture/real size of picture

Real size = size of structure in the picture/magnification 

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The plasma membrane

Membranes = phospholipids (which form the bulk of the membrane) + proteins (scattered around in the membrane) 

Phospholipids = glycerol molecule + 2 fatty acid chains .. The phosphate group is hydrophillic because it has a charge and is soluble in water. The fatty acid chains are made of hydrocarbons and are hydrophobic and do not have a charge and are insoluble. 

They form a double bilayer:

  • Fatty acid 'tails' hydrophobic, pack away from the water
  • Hydrophillic 'heads' arrange themselves on the outside of the membrance facing the water


  • intrinsic proteins = large proteins that span the bilayer
  • extrinsic proteins = smaller proteins on one side of the bilayer 
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Plasma membrane cont.

proteins + carbohydrate chains = glycoproteins

phospholipids + carbohydrate chains = glycolipids 

Cholesterol = is a steroid which helps keep the membrane stable 

Fluid-mosaic model = moving molecules made up of small pieces. 

Membranes around and within cells: 

On cells surface: Membrane separates the cell from its environment and from other cells. Glycoproteins on outside which are important for cell-to-cell signaling, binding of ions or other chemicals to these receptors can trigger reactions e.g. switching on/off of genes in nucleus, blocking these receptors is one way drugs can have an effect.

Membrane inside separate the cell into compartments. 

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Eukaryotic Cells

How organelles work together to modifiy proteins such as antibodies:

Nucleus -> Ribosomes -> Rough Endoplasmic Reticulum -> Vesicles -> Golgi Apparatus -> cell surface membrane -> energy provided from ATP mitochondria -> lysosomes 

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Haemoglobin: structure and formation of proteins

Amino acod: Haemoglobin is a polymer and the monomers are amino acids. Amino acids have a carboxylic acid group and an amino group. Two amino acids joined together form a dipeptide = Hydrogen removed from one amino acid, oxygen and hydrogen atom are removed from carboxylic of other amino acid. Water molecule produced forming condensation reaction = peptide bond ... Many amino acids can join together to form a polypeptide. 

Proteins have four structures: Primary, secondary, tertiary and quaternary. 

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The role of haemoglobin

Fibrous proteins: polypeptides join together to form long fibres or sheets. They are strong and insoluble in water and tend to have structural functions e.g. keratin in hair. 

Globular proteins: They are roughly spherical or globular in shape and are soluble in water and have biochemical functions e.g. enzymes are globular proteins and haemoglobin. 

Denaturation: The tertiary structure of a globular protein is held together mostly by fairly weak bonds e.g. hydrogen bonds. When the temperature increases the molecule vibrates more, if it vibrates too much the bonds break and the shape changes. = denatured ... most proteins denature at 45 degrees. 

Hydrogen bonds depend on a very weak attraction between slightly positively charged hydrogen and slightly negatively charged oxygen. Ionic bonds also depend on tiny charges like this. As a result bonds can be broken if the pH changes.

pH measures the concentration of hydrogen ions, which are postively charged. As the concentration of them surrounding the protein increases or decreases, this affects the charges holding the protein together. As weak bonds break = protein becomes denatured.

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The role of haemoglobin cont.

Hameoglobin molecule is made of four polypeptide chains, each with a haem (a prosthetic group) group attached to it. In the middle of each haem group there's an iron ion, which can associate with one oxygen molecule. Haemoglobin has 4 haem groups = 4 oxygen molcules can be carried. As the first haem group combines with an oxygen molecule, the shape of the haemoglobin molecule changes slightly exposing the next haem group making it easier to pick up more oxygen. 

Beta thalassaemia: where haemoglobin is in the beta chains which are shorter than normal = haemoglobin does not carry as much oxygen as normal. 

Diabetes: When glucose levels are high, glucose attachs to haemoglobin in the red blood cells = glycosylated haemoglobin which picks up oxygen easily but is not good at releasing oxygen to respiring tissues as normal = damage to certain part of body e.g. eyes

Sickle cell anaemia: Haemoglobin is normal in alpha chains but beta chains have amino acid valine present instead of amino acid glutamic acid. Haemoglobin is long and stiff which causes the red blood cells to become misshapen and makes it difficult to pass down the capillaries properly so tissues become short of oxygen and its very painful. 

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Is a polar molecule because it has areas of positive and negative charges. Water molecules are attracted to each other the area of positive charge on the hydrogen atom attracts to the area of negative charge of the oxygen atom of another water molecule. These forces of attraction are hydrogen bonds, so water molecules 'stick' together. 

Water molecules form a 'shell' around ions and many other molecules that have slight charges on their surface causing them to dissolve. Water is a universal solvent because so many chemicals will dissolve in it making it an excellent transport medium too. 

Blood plasma is the liquid part of the blood made up of various things such as: proteins e.g. fibrinogen, antibodies, albumin, ions e.g. sodium, potassium, calcium and chloride ions, hormones e.g. insulin, oestrogen, dissolved food substances e.g. amino acids, glucose, glycerol, Oxygen, waste products e.g. urea and carbon dioxide and heat. 

Serum, tissue fluid and lymph:

blood plasma - fibrinogen = serum 

Tissue fluid is formed when blood passes through the capillaries. 

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Serum, Tissue fluid and lymph cont.

Capillary walls are permable to everything in the blood except most blood cell and the large plasma proteins. At the arterial end of the capillaries the bloods under enough pressure so smaller components of the blood plasma can be squeezes out = allowing the exchange of matierals between blood and tissues. 

 Some of this tissue fluid returns to the blood capillaries at the venule end. The rest of the tissue fluid drains into blind-ended lymphatic capillaries = lymph. The lymph capillaries drain into larger lymph vessels. The fluid moves through these lymph vessels also contain valves to prevent backflow. Eventually the lymph returns to the bloodstream at a vein in the neck region. 

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Water potential and diffusion

Diffusion: passive process ... small, lipid-soluble molecules can diffuse across the plasma membrane through the phospholipid bilayer. 

Faciliated diffusion: Using protein channels that are permantely open, the protein channel is lined with hydrophillic amino acids and water. 

Molecules can also diffuse through the membrane by binding to carrier proteins. The molecule binds to the carrier protein, which causes the protein to change shape and release the molecule on the other side of the membrane. No additional energy is used = passive process

One example: diffusion of glucose into the red blood cells through carrier proteins. 

Osmosis: Water potential is the tedency of a solution to gain or lose water. Pure water has the highest possible water potential of zero. Adding solutes to water decreases the water potential i.e. it makes the water potential more negative. 

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Keeping the osmotic balance

The concentration of electrolytes is responsible for maintaing a water potential balance in plasma and cells. Electrolytes are ions with a positive or negative charge. A test for electrolytes includes the measurement of sodium, potassium, chloride and bicarbonate ions but other plasma ions can also be tested for in plasma such as calcium, magnesium and phosphate. 

Electrolytes are measured by a process known as potentiometry. 

Monitoring these levels is necessary for the diagnosis and management of many conditions such as diabetes and kidney disease.

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Active Transport

When substances move against their concentration gradient they are moved by active transport:

  • Molecule gets actively transported in
  • Shape change of active transport protein requires ATP. The shape change does not allow the molecule to go the 'wrong way'
  • Active transport pump is shaped so that the molecule it carries fits on one side of the membrane only.

Sodium-potassium pump = a carrier protein that uses ATP energy to transport sodium ions out of the cell and potassium ions into the cell. 


  • Vesicle moves towards membrane
  • Vesicle fuses with membrane
  • Molecules released to outside of cell
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Active Transport Cont.


  • Molecules move to membrane surface
  • Membrane invaginates and forms a vesicle around the molecules
  • Vesicle moves into cytoplasm 

Chloestrol is taken into cells by endocytosis. Substances such as enzymes and antibodies are secreted from cells exocytosis. 

  • Transported through blood attached to proteins
  • Transported through bloodstream in the form of lipoprotein particles (low-density lipoprotein/ LDL)
  • LDLs bind to a specific protein in the cell membrane
  • LDL taken in by endocytosis
  • Cholestrol released for use by the cell 
  • Receptor protein returns to the cell surface membrane to be used again. 
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Blood Glucose

Monosaccharides: e.g. glucose, the kind of glucose found in human blood is alpha glucose. 

  • Used as a respiratory substrate 
  • Easily broken down by cells during cellular respiration 
  • Small and compact 
  • Energy released is used to make ATP
  • Soluble so its easy to transport in blood plasma

Disaccharides: e.g. 2 glucose = maltose, glucose + fructose = sucrose

Two monosaccharides can be joined together to make a disaccharide. The two molecules join together by a condensation reaction which makes a glycosidic link (the bond made). 

Polysaccharides: e.g. many alpha glucose molecules = glycogen 

Formed when many monosaccharides are joined together by condensation reactions. 

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  • Stored in the liver and muscle cells
  • Insoluble so it does not affect the water potential of cells
  • Compact, so a lot of glucose can be stored in a small space
  • Long and branched so there are a lot of 'ends' where glucose can be released quickly. 

One glucose residue in a chain can from a glycosidic link with three different glucose molecules = branched molecule. 

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  • Fats, oils and cholesterol
  • non-polar (no charge) and hydrophobic = insoluble 

Fat and oils = triglycerides ... made up of a glycerol molecule joined to 3 fatty acids

Fatty acid = hydrocarbon chain with a carboxylic acid group attached. 

3 fatty acids join onto a glycerol molecule via condensation, they form ester bonds = triglyceride. 

Fatty acids: 


  • single carbon-carbon bonds
  • maximum number of hydrogen atoms in chain
  • higher melting point
  • tend to make fats which are solid at room temperature. 
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Lipids Cont.


  • Double carbon-carbon bonds
  • Fewer hydrogen atoms in chain 
  • Lower melting point
  • Make up oils which are liquid at room temperature

Uses of lipids in the body: 

  • Triglycerides: energy storage and insoluble so ideal to be stored in body fat (adipose tissue)
  • Adipose tissue: heat insulation and protects delicate organs from damage
  • Fat soluble vitamins A and D stored in live globules in liver cells. 


  • A steriod
  • A lipid keeps cell membrane fluid
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Lipids Cont 2.


  • Polar molecules
  • Form a bilayer that is important in cell membranes
  • Contain glycerol and 2 fatty acids and a phosphate group 

Transport of lipids: 

  • Insoluble 
  • When the triglycerides in the adipose tissue are broken down, they form glyercol and fatty acids.
  • Glycerol can be dissolved in blood plasma
  • Fatty acids combine with plasma proteins and are carried in the blood as small globules
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How blood clots

  • Tissues are damaged and exposed to air 
  • Collagen fibres are exposed and platelets stick to them 
  • Platelets release a chemical to make other platelets sticky and form a plug by clumping
  • Calcium ions are need for this process
  • The white blood cells release thromboplastin along with the platelets
  • Which catalyses prothrombin into thrombin. 
  • Which hydrolyses fibrinogen into small units to form long, insoluble fibres of fibrin
  • This creates a mesh over the wound 
  • Blood cells become trapped in the mesh and form a blood clot
  • The clot dries to form a scab
  • Preventing further blood loss and pathogens getting inside

Enzyme = A protein that catalyses a specific reaction in the body e.g. thrombin - by reducing activation energy needed for the reaction to occur.

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How enzymes lower activation energy

  • Enzymes each have a particular active site
  • This is exactly the right shape for one specific substrate to fit
  • When the substrate is in the active site it forms an enzyme-substrate complex
  • Because they fit together the enzyme is able to exert forces on the substrate
  • Lowering the activation energy required

(Lock and key) 

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Factors affecting enzyme activity

The effect of enzyme concentration:

  • Enzyme concentration low = reaction rate is low
  • More enzyme added = increase of reaction rate

When the enzyme concentration is low, only a few of the substrate molecules can form an enzyme-substrate complex. The rest of the substrates are 'waiting' for an active site to become avaliable. If you add more enzyme molecules, there are more active sites which can be filled, so more enzyme-substrate complexes can be formed and the product is made quicker. 

The effect of substrate concentration: 

If the amount of enzyme is kept the same, but only a low concentration of substrate is avaliable = low reaction rate. This is because there are some active sites still avaliable .. more substrate added = increase of reaction rate. However, when the substrate concentration is very high, there is a constantly high rate of reaction, because all active sites are filled and now some substrate molcules are 'waiting' for an active site. 

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Giving blood

Storing blood: 

  • Store in a plastic bag
  • Stored overnight 
  • Correct conditions - 4 degrees
  • Important that blood does not clot
  • Buffer solution added to keep pH same

The effect of temperature on enzyme activity:

  • Low temperature = enzyme and substrate do not have enough kinetic energy so do not collide enough = slow rate
  • Increase temperature = more kinetic energy, more collisions = faster reaction
  • Optimum temperature = If the temperature goes past this the enzyme begins to vibrate more breaking its weak bonds and changing the shape causing it to denatured = slowing the reaction down

pH: more acid a solution is = more hydrogen ions .. causing weak bonds to break and change the enzymes shape = denatured 

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Enzyme co-factors

Co-factors are substances that are needed for an enzyme-controlled reaction to occur.

e.g. calcium ions for blood clotting

  • Present in blood plasma
  • Released by damaged platelets 
  • When bloods stored they need to be removed to stop clotting 
  • Sodium citrate may be added to remove the calcium ions. 
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Screening blood

  • When a person donates blood you must check their health 
  • Donated blood will be screened for a number of infections including HIV and Hepatitis C
  • Blood is tested to find out the blood group

If a person has been exposed to these viruses, their blood will contain antibodies to the virus. The blood is tested by adding a small drop of blood to the antigens from these viruses, if the blood contains the right antibody, it will attach to the antigen. 

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