3. substance exchange

summarise the process of lipid digestion:

• starts in mouth and stomach, but most takes place in duodenum and ileum

• bile salts (not an enzyme!) emulsify lipids

• then lipase hydrolyses ester bonds

describe the digestion of lipids:

• bile salts neutralise stomach acid and emulsify large lipid globules → smaller lipid droplets

• this increases SA for lipase to act on - triglycerides → fatty acids and monoglycerides

• bile salts surround FA chains and monoglycerides to form micelles, making the FA chains more water soluble

describe the process of absorption of triglycerides:

• micelles contain bile salts, fatty acids and monoglycerides

• micelles make fatty acids more water soluble and carry fatty acids to epithelial lining of ileum

• micelles are broken down close to epithelial cells to release fatty acids and monoglycerides

• micelles maintain a higher concentration of fatty acids to the lining of the ileum

• fatty acids and monoglycerides diffuse into the epithelial cells that line the ileum

• triglycerides reform inside the cell’s ER

• triglycerides are packaged in the Golgi w/ cholesterol and proteins to form chylomicrons for transport

• chylomicrons are released from the epithelial cells by exocytosis into lacteals (lymphatic vessels in the villi)

• chylomicrons are transported via lymph vessels in the lymphatic system to the blood

where is lipase produced?

pancreas

where are bile salts produced and stored? how are they secreted into the duodenum and ileum?

• produced in liver, stored in gall bladder

• secreted into duodenum and ileum via bile duct

why can fatty acids diffuse across the epithelial lining?

non charged/non polar

explain the advantages of lipid droplet and micelle formation:

• droplets increase SA for lipase action

• ∴ faster hydrolysis of triglycerides

• micelles carry fatty acids and glycerol to intestinal epithelial cell

what is digestion?

the process in which large insoluble molecules are hydrolysed by enzymes to produce smaller molecules that can be absorbed and assimilated

give and explain 2 examples of mechanical digestion:

• larger food molecules broken down into smaller pieces by teeth - allows ingestion and a large SA:V for chemical digestion

• food churned by muscles in stomach wall

what does chemical digestion involve?

• HCl

• bile

• digestive enzymes !

what is the duodenum?

first part of the SI

what is the ileum?

last part of the SI

describe the process of starch digestion:

• glycosidic bonds in starch (polysaccharide) hydrolysed by amylase → maltose (disaccharide)

• glycosidic bonds in maltose hydrolysed by maltase - membrane bound - → glucose (monosaccharide)

what does it mean for disaccharidases to be membrane bound?

positioned on membrane surface of epithelial cells in ileum

give 2 examples of disaccharidases (other than maltase) and where they are produced:

both produced in ileum:

• sucrase

• lactase

what is sucrase? what does it hydrolyse?

• membrane bound disaccharidase

• hydroluses sucrose → glucose + fructose

what is lactase? what does it hydrolyse?

• membrane bound disaccharidase

• hydrolyses lactose → glucose + galactose

where are carbohydrases produced?

• salivary glands

• pancreas

• SI

describe the process of protein digestion:

• peptide bonds hydrolysed

• endopeptidases break internal peptide bonds

• exopeptidases break terminal peptide bonds

• membrane bound dipeptidases break dipeptides to AAs

what are endopeptidases? give 3 examples:

• peptidases that act to hydrolyse internal peptide bonds, forming multiple peptide fragments

• e.g. trypsin, chymotrypsin, pepsin

what are exopeptidases?

peptidases that act to hydrolyse terminal peptide bonds, removing single AAs from proteins and so forming a single peptide fragment and multiple AAs

what are dipeptidases?

• type of exopeptidase that works specifically on dipeptides

• act to separate the 2 AAs that make up a dipeptide by hydrolysing the peptide bond between them

• membrane bound to the cell surface membrane of epithelial cells

where are peptidases produced?

• (act in stomach) stomach

• (act in duodenum and ileum) pancreas and epithelial cells

describe the absorption of glucose/AAs in the bloodstream:

co transport - how glucose/AAs are absorbed from epithelium into bloodstream:

• Na⁺ ions are actively transported out of the epithelial cell into the blood

• this lowers the conc of Na⁺ ions in the epithelial cell, creating a conc gradient of Na⁺ ions for Na⁺ to enter from gut

• glucose enters by facilitated diffusion w/ Na⁺ - this is co-transport

• there is now a higher conc of glucose inside the epithelial cell than the blood

• glucose moves from cell to blood by facilitated diffusion

• (there is no glucose buildup in the blood as the blood flows and carries away absorbed glucose)

how is the surface of the ileum adapted to absorb digested substances?

• villi and microvilli - increase SA for absorption

• mitochondria - release ATP for active transport

• carrier proteins - allow active transport/facilitated diffusion into epithelial cells

• network of capillaries and peristalsis of muscles maintain diffusion gradient

what are microvilli?

highly folded cell-surface-membrane

explain the function of ATP hydrolase:

• (hydrolyses ATP → ADP + Pi) release E

• allows ions to be moved against a conc gradient/allows active transport of ions

describe and explain 4 structural features of capillaries that make them adapted for the exchange of substances between the blood and surrounding tissues (4):

any 4 from:

• capillary wall permeable (has fenestrations) so molecules can pass through

• capillary wall is 1 cell thick so reduces diffusion distance

• flattened (endothelial) cells so reduces diffusion distance

• small diameter/narrow so large SA:V/short diffusion distance

• narrow lumen so reduces flow rate, giving more time for diffusion

why does blood flow through the lungs at a lower pressure?

• prevents damage to capillaries around the alveoli

• reduces speed of blood flow - more time for gas exchange

why is oxygenated blood pumped out of the blood at a higher pressure?

to ensure blood reaches all respiring cells

what is the role of the arteries?

to carry blood away from the heart

describe and explain the structure and function of arteries:

• muscle withstands high pressure and control blood flow

• elastic stretches and recoils to maintain blood pressure

• smooth endothelium reduces friction

• protein coat withstands high pressure

what is the role of the veins?

to return blood to the heart

describe and explain the structure and function of veins:

• thin muscle layer - constriction and dilation cannot control blood flow

• wide lumen - blood at low pressure

• thin outer wall - does not need to w/stand high pressure

• valves - prevent backflow

  

what is the function of capillaries?

link arterioles to veins - site of substance exchange

describe and explain the structure and function of capillaries:

• wall 1 cell thick, flattened endothelial cells and small diameter - short diffusion distance

• small diameter - larger SA:V

• narrow lumen reduces flow rate - more diffusion time

• fenestrations allow larger molecules to pass through

• numerous and highly branched - large SA:V

  

what is the role of arterioles?

to reduce blood flow from arteries to capillaries

describe the structure and function of arterioles:

• relatively thicker muscle layer than arteries

• relatively thinner elastic layer than arteries

• muscle layer contraction allows constriction of lumen - restricts blood flow into capillaries

• elastic layer thickness due to lower blood pressure

compare veins and arteries:

• veins have valves whereas arteries do not

• veins have a wider lumen than arteries

• arteries have more elastic tissue than veins

• arteries have a thicker wall than veins

• arteries have more muscular tissue than veins

why do all blood vessels have a smooth endothelium?

to reduce friction

what is diastole? what occurs during this process?

both atria and ventricles relaxed:

• blood enters heart through vena cava and pulmonary vein

• atrial pressure increases as blood is entering the atria

• this causes the atrioventricular valve to open as the pressure in the atria is higher than the pressure in the ventricles

what is atrial systole? what occurs during this process?

atria contract (and ventricles relax):

• contraction of atria causes atrial pressure to increase further

• atrioventricular valves open

• semi lunar valves still closed because pressure in arteries > pressure in ventricles

• blood is being pumped from the atria to the ventricles

what is ventricular systole? what occurs during this process?

ventricles contract (and atria relax):

• there is a short delay before the ventricles can contract because ventricles need to fill up completely before contracting

• atrioventricular valves close as pressure in ventricles > pressure in atria

• semi lunar valves open as pressure in ventricles > pressure in arteries

• blood moves from ventricles to pulmonary artery/aorta

when does blood start flowing into the aorta? how do you know?

• at point A, the ventricles are contracting (and AV valves are shut), forcing blood into the aorta

• this can be seen by pressure increase in ventricles, meaning SL valve must open as ventricle pressure > aorta pressure

why is ventricular volume decreasing at point B? how do you know?

• ventricles contract, reducing the volume of the chamber

• can also be seen by pressure increase in ventricles

are the SL valves open or closed at point C? how do you know?

• ventricles relaxed and refilling, so pulmonary artery/aorta pressure > ventricle pressure

• so SL valves must be closed

at Q in the diagram above there is a small increase in pressure and in rate of blood flow in the aorta - explain how this happens and its importance (2)

at P in the diagram above, the pressure in the left ventricle is increasing. at this time, the rate of blood flow has not yet started to increase in the aorta - use evidence from the diagram above to explain why (2)

• semi-lunar valves closed

• because pressure in aorta higher than pressure in the ventricle

how do we calculate heart rate from a graph?

• calculate length of each cardiac cycle (in this e.g. = 0.36)

• divide 60 by length of cardiac cycle to find bpm (in this e.g. = 167)

give the eqn for cardiac output:

cardiac output = stroke volume x heart rate

what can you conclude from the appearance of valves in the image above about heart muscle activity and movement between:

1. ventricles and arteries? (2)

2. atria and ventricles? (2)

1. ventricles and arteries:

• ventricles relaxed

• no backflow into ventricles

2. atria and ventricles:

• atria contracted

• blood movement from atria into ventricles

which blood vessel carries blood at the lowest pressure?

• capillary

• pulmonary vein

• renal vein

• vena cava

vena cava

what is haemoglobin?

• globular protein found in RBCs

• quaternary structure made of 4 polypeptide chains

• each polypeptide has a haem group containing a Fe²⁺

what is loading/associating?

binding of Hb w/ O₂

what is unloading/disassociating?

Hb releasing O₂

draw the shape of the oxyhaemoglobin dissociation curve:

how is O₂ loaded in terms of pressure?

loaded in regions w/ high partial pressure and unloaded in regions of low partial pressure

explain what happens at the first stage of the oxygen dissociation curve:

• it is difficult for the first O₂ molecule to bind

• at low O₂ concs, little O₂ binds to Hb

• so the gradient of the curve is shallow initially

explain the second stage of the oxygen dissociation curve:

• binding of the first O₂ changes the shape of Hb, making it easier for O₂ to bind to the other subunits

• the binding of the first O₂ molecule allows more O₂ to bind/greater saturation w/ O₂/cooperative binding

• so the gradient of the curve steepens

explain the last stage of the oxygen dissociation curve:

• after binding of the third O₂ molecule, the gradient of the curve decreases/flattens

• it is harder to bind the fourth O₂ molecule because most of the binding sites are occupied

• so it is less likely that an O₂ molecule will find an empty site to bind to

what is the Bohr effect?

affinity of Hb changing due to changes in CO₂ conc

explain the Bohr effect in terms of the alveoli:

• CO₂ conc low/O₂ conc high

• so O₂ affinity increases

• O₂ less readily unloaded by Hb

• so dissociation curve shifts to the left

explain the Bohr effect in terms of respiring tissues:

• CO₂ conc high/O₂ conc low

• O₂ affinity decreases

• O₂ readily unloaded/released by Hb

• so dissociation curve shifts right

explain the release of O₂ into respiring tissues in terms of pH:

• in respiring tissues, CO₂ is produced, which decreases the pH

• lower pH changes the quaternary structure of Hb into one w/ a lower affinity for O₂

• so Hb releases its O₂ more readily into the respiring tissues → more O₂ for respiration

explain the loading of O₂ from the alveoli in terms of pH:

• in the alveoli, there is a low CO₂ conc, which increases the pH

• this higher pH changes the quaternary structure of Hb into one w a higher affinity for O₂

• Hb loads O₂ from the alveoli more readily

  

describe and explain the Bohr effect for foetal Hb:

• curve shifts to left - even at same PP, Hb has a higher affinity for O₂

• foetal Hb more saturated

• as foetus cannot inhale/exhale

• so requires higher affinity in order to load O₂

  

describe and explain the Bohr effect for llama Hb (lives in high altitudes):

• lives in high altitudes where PP of O₂ is lower

• so curve shifts left - llama Hb more saturated

• so requires higher affinity in order to load O₂

describe and explain the Bohr effect for earthworm Hb (lives in environment w/ depleted O₂):

• lives in environment w/ depleted O₂ so PP of O₂ is lower

• so curve shifts left - earthworm Hb more saturated

• so requires higher affinity in order to load O₂

describe and explain the Bohr effect for dove Hb (has a faster metabolism):

• faster metabolism so curve shifts right

• Hb has a lower affinity for O₂

• offloads at same PP

• ensures respiring tissue has an abundant supply of O₂

why may some organisms have a dissociation curve which is to the right of ours?

• very active so have a high O₂ demand

• so have Hb w/ a lower affinity for O₂ than human Hb

why may some organisms have a dissociation curve which is to the left of ours?

• live in areas w/ low O₂ conc/very inactive

• so have Hb w/ a higher affinity for O₂ than human haemoglobin

explain a property of iron ions that enables these ions to carry out their role in RBCs

• charged

• so binds w/ oxygen

can you label this heart?

yes

what is the significance of the double circulatory system?

• blood pressure decreases in the lungs

• single pump would slow blood flow to cells

• 2 pumps increase pressure before it circulates

• prevents oxygenated and deoxygenated blood mixing

which part of the body does the term ‘pulmonary’ refer to?

lungs

which part of the body does ‘renal’ refer to?

kidneys

which part of the body does ‘hepatic’ refer to?

liver

give 3 features of the atria:

• thin walled

• elastic, stretch, recoil

• do not need to contract as hard as ventricles

give 3 features of the ventricles:

• thicker muscular wall

• contracts strongly to pump blood some distance (either lungs or rest of body)

• bigger contraction = higher blood pressure

give 3 features of the cardiac muscle in general:

• thick muscular walls

• myogenic

• never fatigues

what is the function of the left side of the heart?

pumps blood to the body

does the right hand side of the heart have a higher or lower blood pressure? why?

lower blood pressure:

• to prevent damage to capillaries

• slows down blood flow → allows time for gas exchange

does the left side of the heart have a higher or lower blood pressure? why?

higher blood pressure to ensure blood reaches all cells

what is the function of the right side of the heart?

pumps blood to the lungs

how do valves work?

• open when pressure is higher behind the valve

• close when pressure is higher in front of the valve

• prevent backflow

  

what may cause an increase in pressure in the heart?

filling/contraction

give 2 safety precautions that should be followed when dissecting a heart (2)

give an overview of the heart dissection:

• before cutting - identify coronary arteries. run water into top of heart to see valves close, squeeze to see valves open

• cut down each side of heart - identify tendinous cords, examine thickness of walls, stick finger/needle through aorta/pulmonary arteries

give the ‘do’s of a biological drawing:

• include a title

• take proportions into account

• use single, clear, joined lines

• write labels horizontally

• include magnification and scale (where relevant)

give the ‘don’t’s of a biological drawing:

• shading

• sketchy lines

• arrowheads

• hanging lines

aortic valves removed from donors were stored in isotonic containing an antibiotic before being used in valve replacement surgery - explain why the valves were stored in a solution containing an antibiotic (1)

kills bacteria that could cause infection/disease

the scientists calculated the cardiac output of the volunteers before and after the exercise programme. in some volunteers, their cardiac output stayed the same, even though their resting heart rate decreased.

explain how their cardiac output could stay the same even when their resting heart rate had decreased (2)

• cardiac output = stroke volume x heart rate

• so stroke volume increases

what is tissue fluid? what does it contain?

• fluid which passes out of the blood and bathes the tissue cells

• contains mostly water, glucose, amino acids, fatty acids, ions and oxygen

• (virtually same composition as blood plasma, just w/o proteins and cells)

describe the process of tissue fluid formation:

• at the arterial end of the capillary, there is a high hydrostatic pressure due to contraction of the ventricles

• there is a Ψ gradient into the capillaries (due to the hydrophilic plasma proteins exerting an oncotic pressure), but the hydrostatic pressure is greater and so overcomes this

• ultrafiltration - water and dissolved substances, e.g. glucose forced out through the gaps (fenestrations) between the capillary endothelial cells, forming tissue fluid

• proteins and RBCs are too large to pass through so remain in the capillaries

describe the process of tissue fluid reabsorption:

• at the venule end of the capillary, the hydrostatic pressure is lower due to loss of water/fluid

• Ψ in capillaries is lower than in the tissue fluid due to proteins remaining in the blood

• water returns from tissue fluid to the venous end of the capillaries by osmosis down the Ψ gradient

• the lymphatic system collects any XS tissue fluid which returns to the circulatory system

summarise the composition of tissue fluid surrounding the arterial and venule end of the capillaries and that of the blood plasma at the arterial end:

what is mass transport?

movement of fluids down a pressure gradient

(can occur over long distances)

what is the transpiration stream?

movement of water and dissolved ions up a plant

why is transpiration necessary for a plant?

• provides water for photosynthesis (compromise between losing water and taking in CO₂)

• allows mineral ions to be transported from roots to leaves

• evaporation has a cooling effect on leaves, preventing enzymes denaturing

what is transpiration?

process by which water is lost from the leaves by evaporation and diffusion