Topic 3B - More Exchange and Transport Systems - complete

- large biological molecules (e.g. starch, proteins) found in food are too bid to cross cell membranes
- this means they can't be abosrbed from the gut into the blood

- during digestion, these large molecules are broken down into smaller molecules (e.g. glucose, amino acids), which can move across cell membranes
- this means they can be easily absorbed from the gut into the blood, to be transported around the body for use by the body cells

- most large biological molecules are polymers, which can be broken down into smaller molecules (monomers) using hydrolysis reactions
- hydrolysis reactions break bonds by adding water

- during hydrolysis, carbohydrates are broken down into disaccharieds and then monosaccharides
- fats are broken down into fatty acids and monoglycerides
- proteins are broken down into amino acids

- a variety of different enzymes are produced by specialised cells in the digestive system of mammals
- these enzymes are then released into the gut to mix with food

- since enzymes only work with the specific subrates, different enzymes are needed to catalyse the breakdown of different food molecules

- amylase is a digestive enzyme that catalyses the conversion of starch into the smaller sugar maltose
- this involves the hydrolysis of the glycosidic bonds in starch

- amylase is produced by the salivary glands (which release amylase into the mouth) and also by the pancreas (which releases amylase into the small intestine)

- membrane-bound disaccharides are enzymes that are attached to the cell membranes of epithelial cells lining the ileum (the final part of the small intestine)
- they help to break down disaccharides into monosacchairdes
- again this involves the hydrolysis of glycosidic bonds

- monosaccharides can be transported across the cell membrane of the ileum epithelial cells via specific transporter proteins

- lipase enzymes catalyse the breakdown of lipids into monoglycerides and fatty acids
- this involves the hydrolysis of the ester bonds in lipids

- lipases are made in the pancreas
- they work in the small intestine

- bile salts are produced by the liver and emulsify lipids, this means they cause the lipids to form small droplets

- bile salts are really important in the process of lipid digestion
- several small lipid droplets have a bigger surface area than a single large droplet (for the same volume of lipid) 
- so the formation of small droplets greatly increases the surface area of lipid that's available for lipases to work on 

- once the lipid has been broken down, the monoglycerides and fatty acids stick with the bile salts to form tiny structures called micelles

- proteins are broken down by a combination of different proteases (or peptidases)
- these are enzymes that catalyse the conversion of proteins into amino acids by hyrdolysing the peptide bonds between amino acids

Endopeptidases
- endopeptidases act to hydrolyse peptide bonds within a protein
- trypsin an chymotrypsin are two examples of endopeptidases
- they're synthesised in the pancreas and secreted into the small intestine
- pepsin is another endopeptidase
- its released into the stomach by cells in the stomach lining
- pepsin only works in acidic conditions which are provided by the hydrochloric acid  in the stomach

Exopeptidases
- exopeptidases act to hydrolyse peptide bonds at the ends of protein molecules
- they remove single amino acids from proteins
- dipeptidases are exopeptidases that work specifically on dipeptides
- they act to separate the two amino acids that make up a dipeptide by hydrolysing the peptide bond between them
- dipeptidases are often located in the cell-surface membrane of epithelial cells in the small intestine

- the products of digestion are absorbed across the ileum epithelium into the bloodstream

Monosaccharides
- glucose is absorbed by active transport with sodium ions via a co-transporter protein
~ galactose is absorbed in the same way using the same co-transporter protein
- fructose is absorbed via facilitated diffusion through a different transporter protein

Monoglycerides and Fatty Acids
- micelles help to move monoglycerides and fatty acids towards the epithelium
- because micelles constantly break up and reform they can 'release' monoglycerides and fatty acids, allowing them to be absorbed
- whole micelles are not taken up across the epithelium
- monoglycerides and fatty acids are lipid-soluble, so can diffuse directly across the epithelial cell membrane

Amino Acids
- amino acids are absorbed via co-transport, in a similar way to glucose and galactose
- sodium ions are actively transported out of the ileum epithelial cells into the blood
- this creates a sodium ion concentration gradient
- sodium ions can then diffuse from the lumen of the ileum into the epithelial cells through sodium-dependent transporter proteins, carrying the amino acids with them

- red blood cells contain haemoglobin (Hb)
- haemoglobin is a large protein with a quarternary structure 
~ its made up of four polypeptide chains
- each chain has a haem group, which contains an iron ion and gives haemoglobin its red colour
- haemoglobin has a high affinity for oxygen & each molecule can carry four oxygen molecules
- in the lungs, oxygen joins to haemoglobin in red blood cells to form oxyhaemoglobin
- this is a reversible reaction
~ when oxygen leaves oxyhaemoglobin (dissociates from it) near the body cells, it turns back to haemoglobin

- there are many chemically similar types of haemoglobin found in many different organisms, all of which carry out the same function
- as well as being found in all vertebrates, haemoglobin is found in earthworms, starfish, some insects, some plants and even in some bacteria

- the partial pressure of oxygen (pO₂) is a measure of oxygen concentration
- the greater the concentration of dissolved oxygen in cells, the higher the partial pressure

- similarly, the partial pressure of carbon dioxide is a measure of the concentration of carbon dioxide in a cell

- heamoglobin's affinity for oxygen varies depending on the partial pressure of oxygen:
"oxygen loads onto haemoglobin to form oxyhaemoglobin where there's a high partial pressure of oxygen. oxyheamoglobin unloads its oxygen where there's a lower partial pressure of oxygen"

- oxygen enters blood capillaries at the alveoli in the lungs
- alveoli have a high pO₂ so oxygen loads onto haemoglobin to form oxyhaemoglobin

- when cells respire, they use up oxygen which lowers the pO₂
- red blood cells deliver oxyhaemoglobin to respiring tissues, where it unloads its oxygen

- the haemoglobin then returns to the lungs to pick up more oxygen

- a dissociation curve shows how saturated the haemoglobin is with oxygen at any given partial pressure

- where pO₂ is high (e.g. in the lungs) haemoglobin has a high affinity for oxygen (i.e. it will readily combine with oxygen), so it has a high saturation of oxygen

- where pO₂ is low (e.g. in respiring tissues) haemoglobin has a low affinity for oxygen, which means it releases oxygen rather than combines with it
- that's why it has a low saturation of oxygen

- the graph is s-shaped because when haemoglobin combines with the first O₂ molecule, its shape alters in a way that makes it easier for other molecules to join too
- but as the Hb starts to become saturated, it gets harder for more oxygen molecules to join
- as a result, the curve has a steep bit in the middle where its really easy for oxygen molecules to join, and shallow bits at each end where its harder
- when the curve is steep, a small change in pO₂ causes a big change in the amount of oxygen carried by the Hb

- Hb gives up its oxygen more readily at higher pCO₂ 
- its a way of getting more oxygen to cells during activity

- when cells respire they produce carbon dioxide which raises the pCO₂ 

- this increases the rate of oxygen unloading (i.e. the rate at which oxyhaemoglobin dissociates to form haemoglobin and oxygen)
- so the dissociation curve 'shifts' right
- the saturation of blood with oxygen is lower for a given pO₂ meaning that more oxygen is being released

- this is called the Bohr effect

- different organisms have different types of haemoglobin with different oxygen transporting capacities
- having a particular type of haemoglobin is an adaptation that helps the organism to survive in a particular environment

- organisms that live in environments with a low concentration of oxygen have haemoglobin with a higher affinity for oxygen that human haemoglobin 
- the dissociation curve would be to the left of the human one

- organisms that are very active and have a high oxygen demand have haemoglobin with a lower affinity for oxygen than human haemoglobin
- the diisociation curve would be to the right of the human one

- multicellular organisms, like mammals, have a low surface area to volume ratio so they need a specialised transport system to carry raw materials from specialised exchange organs to their body cells (the circulatory system)

- the circulatory system is made up of the heart and blood vessels

- the heart pumps blood through blood vessels (arteries, arterioles, veins and capillaries) to reach different parts of the body

- blood transports respiratory gases, products of digestion, metabolic wastes and hormones round the body

- there are two circuits
- one circuit takes blood from the heart to the lungs, then back to the heart
- the other loop takes blood around the rest of the body

- the heart has its own blood supply which is the left and right coronary arteries

- arteries carry blood from the heart to the rest of the body
- their walls are thick and muscular and have elastic tissue to stretch and recoil as the heart beats, which helps maintain the high pressure
- the inner lining (endothelium) is folded, allowing the artery to stretch which also helps it to maintain high pressue
- all arteries carry oxygenated blood except for the pulmonary arteries, which take deoxygenated blood to the lungs

- arteries divide into smaller vessels called arterioles
- these form a network throughout the body
- blood is directed to different areas of demand in the body by muscles inside the arterioles, which contract to restrict the blood flow or relax to allow full blood flow

- veins take blood back to the heart under low pressure
- they have a wider lumen than equivalent arteries, with very little elastic or muscle tissue
- veins contain valves to stop the blood flowing backwards
- blood flow through the veins is helped by contraction of the body muscles surrounding them
- all veins carry deoxygenated blood (because oxygen has been used up by body cells), except for the pulmonary veins, which carry oxygenated blood to the heart from the lungs

- arterioles branch into capillaries, which are the smallest of the blood vessels
- substances (e.g. glucose and oxygen) are exchanged between cells and capillaries, so they're adapted for efficient diffusion

- they're always found very near cells in exchange tissues e.g. alveoli in the lungs, so there's a short diffusion pathway

- their walls are only one cell thick, which also shortens the diffusion pathway

- there are a large number of capillaries, to increase surface area for exchange
- networks of capillaries in tissue are called capillary beds

- tissue fluid is the fluid that surrounds cells in tissues
- its made from small molecules that leave the blood plasma, e.g. oxygen, water and nutrients
- unlike blood, tissue fluid doesn't contain red blood cells or big proteins, because they're too large to be pushed out through the capillary walls
- cells take in oxygen and nutrients from the tissue fluid, and release metabolic waste into it
- in a capillary bed, substances move out of the capillaries, into the tissue fluid, by pressure filtration

- at the start of the capillary bed, nearest the arteries, the hydrostatic (liquid) pressue inside the capillaries is greater than the hydrostatic pressure in the tissue fluid

- this difference in hydrostatic pressure means an overall outward pressure forces fluid out of the capillaries and into the spaces around the cells, forming tissue fluid

- as fluid leaves, the hydrostatic pressure reduces in the capillaries - so the hydrostatic pressure is much lower and the venule end of the capillary bed (the end nearest the veins)

- due to the fluid loss, and an increasing concentration of plasma proteins (which dont leave the capillaries), the water potential at the venule end of the capillary bed is lower than the water potential in the tissue fluid

- this means that some water re-enters the capillaries from the tissue fluid at the venule end by osmosis

- any excess tissue fluid is drained into the lymphatic system, which is a network of tubules that acts like a drain, which transports this excess fluid from the tissues and dumps it back into the circulatory system

- the right side of the heart pumps deoxygenated blood to the lungs and the left side pumps oxygenated blood to the whole body

components of the heart (top to bottom, right then left)
- superior vena cava
- inferior vena cava
- right atrium
- semi-lunar valve
- right atrioventricular valve
- right venticle

- aorta
- pulmonary vein
- left atrium
- semi-lunar valve
- left atrioventricular valve
- cords (valve tendons)
- left ventricle

- the left ventricle of the heart has thicker, more muscular walls than the right ventricle, because it needs to contract powerfully to pump blood all the way round the body
- the right side only needs to get blood to the lungs, which are nearby

- the ventricles have thicker walls than the atria, because they have to push blood out of the heart whereas the atria just need to push blood a short distance into the ventricles

- the atrioventricular (AV) valves link the atria to the ventricles and stop blood flowing back into the atria when the ventricles contract

- the semi-lunar (SL) valves link the ventricles to the pulmonary artery and aorta, and stop blood flowing back into the heart after the ventricles contract

- the cords attach the AV valves the to ventricles to stop them being forced up into the atria when the ventricles contract

- the valves only open one way, whether they're open or closed depends on the relative pressure of the heart chambers
- if there's higher pressure behind a valve, its forced open, but if pressure is higher in front of the valve its forced shut
- this means blood only flows in one direction through the heart

- the cardiac cycle is an ongoing sequence of contraction and relaxation of the atria and ventricles that keeps blood continuously circulating round the body
- the volume of the atria and ventricles changes as they contract and relax
- pressure changes also occur, due to the changes in chamber volume (e.g. decreasing the volume of a chamber by contraction will increase the pressure in a chamber
1) the ventricles are relaxed
- the atria contract, decreasing the volume of the chambers and increasing the pressure inside the chambers
- this pushed the blood into the ventricles
- there's a slight increase in ventricular pressure and chamber volume as the ventricles receive the ejected blood from the contracting atria
2) the atria relax
- the ventricles contract (decreasing their volume), increasing their pressure
- the pressue becomes higher in the ventricles than the atria, which forces the AV valves shut to prevent back-flow
- the pressure in the ventricles is also higher than in the aorta and pulmonary artery, which forces open the SL valves and blood is forced out into these arteries
3) the ventricles and the atria both relax
- the higher pressure in the pulmonary artery and aorta closes the SL valves to prevent back flow into the ventricles
- blood returns to the heart and the atria fill again due to the higher pressure in the vena cava and pulmonary vein
- in turn this starts to increase the pressure of the atria
- as the ventricles continue to relax, their pressure falls below the pressure of the atria and so the AV valves open again
- this allows blood to flow passively into the ventricles from the atria
- the atria contract, and the whole process begins again