Unit 3.3 - organisms exchange surfaces with their environ

Question 1

what is digestion?

Answer 1

• large molecules are broken down into smaller molecules which can move across all membranes, this means they can be easily absorbed from the gut into the blood to be transported around the body for use.
• large polymers are broken down into monomers using hydrolysis reactions

Question 2

what is amylase?

Answer 2

• its a digestive enzyme that catalyses the conversion of starch (a polysaccharide) into the smaller sugar maltose (a disaccharide). this involves the hydrolysis of the glycosidic bonds in starch
• its produced by the salivary glands and by the pancreas

Question 3

what are membrane-bound disaccharide?

Answer 3

• they are enzymes that are attached to the cell membrane of epithelial cells lining the ileum. they help break down disaccharides into monosaccharides
• monosaccharides can be transported across the cell membrane of ileum epithelial cells via specific transporter proteins

Question 4

what is lipase?

Answer 4

• an enzyme that catalyses the breakdown of lipids into monoglycerides and fatty acids. this involves the hydrolysis of the ester bonds in lipids
• they are made in the pancreas and work in the small intestine

Question 5

what are bile salts?

Answer 5

• they are important in the process of lipid digestion. several small lipid droplets have a bigger SA than a single larger droplet, so the formation of small droplets greatly increases the SA of lipid that is available for lipases to work on
• once the lipid has been broken down, the monoglycerides and fatty acids stick out with bile salts to form tiny structures called micelles

Question 6

how are proteins broken down?

Answer 6

• by a combination of different proteases (peptidases) they are enzymes that catalyse the conversion of proteins into amino acids by hydrolysing the peptide bonds between amino acids

Question 7

what are endopeptidases?

Answer 7

• they act to hydrolyse peptide bonds between amino acids in the central region of a protein molecule forming a series of peptide molecules.

Question 8

what are exopeptidases?

Answer 8

• they act to hydrolyse the peptide bonds on the terminal amino acids of the peptide molecules formed by endopeptidases. they progressively release dipeptides and single amino acids.

Question 9

how are mononsaccharides absorbed across cell membranes?

Answer 9

• glucose and galactose are absorbed by active transport with sodium ions via co-transporter protein,
• fructose is absorbed via facilitated diffusion through another transporter protein

Question 10

how are monoglycerides and fatty acids absorbed across cell membranes?

Answer 10

• 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 aren’t taken up across the epithelium. monoglycerides and fatty acids are lipid-soluble so can diffuse directly across the epithelial cell membrane

Question 11

how are amino acids absorbed across cell membranes?

Answer 11

• they.re absorbed via co-transport. sodium ions are actively transported out of the ileum-epithelial cells into the blood. Na+ 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

Question 12

in single-celled organisms how do substances diffuse?

Answer 12

• how quickly substances diffuse depends on the organisms SA to volume ratio
• they can diffuse directly into or out of the cell-surface membrane. the diffusion rate is quick due to the small distances

Question 13

in multi-celled organisms how do substances diffuse?

Answer 13

• diffusion across the outer membrane is too slow as a) some cells are to deep within the body so there is a big distance between them and the outside environment or
  b) larger animals have a lower SA to volume ratio so its harder to exchange enough substances to supply a large volume of animal through a small outer surface

Question 14

what do multicellular organisms need?

Answer 14

• specialised exchange organs
• an efficient system to carry substances to and from their cells this is mass transport. in mammals, it refers to the circulatory system which uses blood to carry glucose, O2, hormones and waste around the body
• in plants mass transport involves the transport of water and solutes in the xylem and phloem

Question 15

how does size affect the rate of heat loss?

Answer 15

• if an organism has a large volume, its SA is smaller. this makes it harder for it to lose heat from its body.
• if and organism is small, its SA is larger so heat is lost more easily. this means smaller organisms need a relatively high metabolic rate in order to generate enough heat to stay warm

Question 16

how does shape affect the rate of heat loss?

Answer 16

• animals with a compact shape have a small SA to their volume which minimises heat loss from their surface
• animals with a less compact shape have a larger SA to their volume which increases heat loss from their surface. whether an animal is compact or not depends on the temp of its environment

Question 17

what other adaptions aside from shape and size do animals have?

Answer 17

1. animals with a high SA to volume ratio tend to lose more water as it evaporates from their surface eg small desert mammals have kidney structure adaptions so they produce less urine
2. small mammals in cold places need to eat a big amount of high energy foods eg seeds to support their metabolic rates or have thick layers of fur/they hibernate
3. larger organisms living in hot regions find it hard to keep cool as their heat loss is slow. eg hippos spend much of their day in the water a behavioural adaption

Question 18

describe the structure of the respiratory system?

Answer 18

• as you breathe in, air enters the trachea which splits into two bronchi, one bronchus leading to each lung.
• each bronchus then branches off into smaller tubes called bronchioles which end in small ‘air sacs’ called alveoli
• the ribcage, diaphragm and intercostal muscles work to move air in and out

Question 19

describe inspiration (breathing in)?

Answer 19

• the external intercostal and diaphragm muscles contract
• this causes the ribcage to move upwards and outwards and the diaphragm to flatten increasing the volume of the thoracic cavity and decreasing the lung pressure
• air will flow from and area of high pressure to low ie down the trachea to the lungs
• its an active process and requires energy

Question 20

describe expiration? (breathing out)

Answer 20

• the external intercostal and diaphragm muscles relax
• the ribcage moves downwards and inwards and the diaphragm becomes curved again, the volume of the thoracic cavity decreases causing the air pressure to increase
• air is forced down the pressure gradient and out of the lungs
• its a passive process and doesn’t require energy

Question 21

what is forced expiration?

Answer 21

• when you blow candles out
• the external muscles relax and the internal intercostal muscles contract, pulling the ribcage further down and in. the movement of the intercostal muscles is said to antagonistic

Question 22

how does O2 and CO2 diffuse in the alveoli?

Answer 22

• O2 diffuses out of the alveoli, across the alveolar epithelium and the capillary epithelium and into the haemoglobin in the blood
• CO2 diffuses into the alveoli from the blood and is breathed out

Question 23

what features do alveoli have to increase their rate of diffusion?

Answer 23

• a thin exchange surface - the alveolar epithelium in only one cell thick, its made from squamous epithelium which decreases the diffusion path
• a large SA - lots of alveoli
• cells produce surfactant which reduces surface tension of fluid in the alveoli which prevents the alveoli from deflating when we exhale

Question 24

describe a fish’s gills? and adaptations?

Answer 24

• made up of lots of thin plates called filaments which gives a big surface area for exchange of gases
• the filaments are covered with lots of tiny structures called lamellae, which increase the SA more
• also have lots of capillaries and a thin surface layer of cells to speed up diffusion

Question 25

what is counter current flow?

Answer 25

• blood flows through the lamellae in one direction and water flows over in the other
• this system maintains a large concentration gradient between water and the blood. the concentration of oxygen in the water is always higher than in the blood, so as much O2 diffuses from the water into the blood

Question 26

what are an insects air-filled pipes called?

Answer 26

• tracheae which they use for gas exchange. air moves into the tracheae (down the conc grad) through pores on the surface called spiracles
• tracheae branch off into smaller tracheoles which have thin, permeable walls and go to each cell. this means oxygen diffuses directly into respiring cells as the insects respiratory system doesn’t transport O2.

Question 27

how does carbon dioxide move from cells in insects?

Answer 27

• moves down its conc gradient towards the spiracles to be released
• insects use rhythmic abdominal movements to move air in and out of the spiracles

Question 28

describe the mesophyll cells in plants?

Answer 28

• its the main gas exchange surface, and have a large surface area
• they are inside the leaf where gases move in and out of through special pores called stomata
• stomata can open to allow gases to exchange and close if a plant is losing too much water
• guard cells control the opening and closing of stomata

Question 29

what happens when insects are losing too much water?

Answer 29

• they close their spiracles using muscles. they also have a waterproof, waxy cuticle all over their body and tiny hairs around their spiracles which reduce evaporation.
• they also have a small SA:VOL ratio to reduce area over which water is lost

Question 30

what happens when plants are losing too much water?

Answer 30

• they are usually kept open during the day to allow gaseous exchange. water enters the guard cells, making them turgid, which opens the stomatal pore,.
• if the plant starts to get dehydrated, the guard cells lose water and become flaccid which closes the pore

Question 31

what are xerophytes?

Answer 31

• plants that are adapted for life in warm, dry or windy habitats where water loss is a problem
  eg they may have waxy, waterproof cuticles on leaves and stems to reduce evaporation

Question 32

what other adaptations can xerophytes have?

Answer 32

• stomata sunk in pits that trap moist air, reducing the conc gradient of water between leaf and the air. this reduces the amount of water diffusing out of the leaf and evaporating away
• a layer of hairs on the epidermis to trap moist air round the stomata
• curled leaves with stoma inside protecting the from wind
• a reduced number of stomata so there are fewer places for water to escape

Question 33

what are the two circuits in the heart?

Answer 33

• one takes blood from the heart to the lungs then back to the heart
• the other loop takes blood around the body
• the heart has its own blood supply, left and right coronary arteries

Question 34

describe arteries?

Answer 34

• they carry blood from the heart to the body. their walls are thick and muscular and have elastic tissue to stretch and recoil as the heart beats, this helps maintain the high pressure
• the inner lining (endothelium) is folded, allowing the artery to stretch which allows the artery to stretch and maintains the high pressure
• all arteries carry oxygenated blood except for the pulmonary arteries, which take deoxygenated blood to the lungs

Question 35

describe veins?

Answer 35

• takes blood back to the heart under low pressure. they have a wider lumen, with little elastic or muscle tissue. they contain valves to stop the blood flowing backwards
• blood flow through veins is helped by muscle contractions, all veins carry deoxyenated blood, except for the pulmonary vein which may carry oxygenated blood to he heart from the lungs

Question 36

what do arteries divide into?

Answer 36

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

Question 37

what does the right and left side of the heart do?

Answer 37

• right pumps deoxygenated blood to the lungs and left pumps oxygenated blood to the whole body

Question 38

what do the ventricles do?

Answer 38

• the left ventricle of the heart has thicker, more muscular walls than the right ventricles as it needs to contract to pump blood all around the body. the right side only needs to pump blood to the lungs, which are nearby
• they have thicker walls than the atria as they have to push blood out of the heart whereas the atria just need to push blood a short distance into the ventricles

Question 39

what do the AV valves do?

Answer 39

• the atrioventricular valves link the atria to the ventricles and stop blood flowing back into the atria when the ventricles contract
• the cords attach the AV valves to the ventricles to stop them being forced up into the atria when the ventricles contract

Question 40

what do the SL valves do?

Answer 40

• the semilunar valves link the ventricles to the pulmonary artery and aorta, and stop blood flowing back into the heart after the ventricles contract

Question 41

how do valves open?

Answer 41

• the valves only open one way, whether they’re opened 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 the front of the valve its forced shut.
• this means blood only flows in one direction through the heart

Question 42

what is the first step to the cardiac cycle?

Answer 42

1. blood enters atria and ventricles from pulmonary veins and vena cava.
   - semi lunar valves closed, left and right AV valves open.
   - ventricles relax to allow blood to enter from atria

Question 43

what is the second step in the cardiac cycle?

Answer 43

• atria contract to push remaining blood into ventricles
• semi lunar valve closed, left and right AV valves open
• blood pumped from atria to ventricles as ventricles relax

Question 44

what is the third step in the cardiac cycle?

Answer 44

3. blood pumped into pulmonary arteries and aorta
   - semi lunar valves open, left and right Av valves closed,
   - ventricles contract and walls thicken as blood is pushed away from heart into arteries and aorta

Question 45

what does the higher pressure in the vena cava and pulmonary artery do?

Answer 45

• it starts to increase 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, this allows blood to flow passively into the ventricles from the atria. the atria contract and it starts again

Question 46

what is haemoglobin Hb?

Answer 46

• red blood cells contain haemoglobin, a large protein with a quaternary structure. Each chain has a haem group, which contains an iron ion and gives Hb its red colour
• it has a high affinity for oxygen, each molecule can carry 4 oxygen molecules

Question 47

what happens in the lungs to oxygen?

Answer 47

• it joins to haemoglobin to form oxyhaemoglobin (dissociates from it) near the body cells and turns back to haemoglobin.
• Hb + 4O2 -> HbO8
• haemoglobin has many chemically similar types found in different organisms all of which carry out the same function

Question 48

what is the partial pressure of oxygen (pO2)?

Answer 48

• a measure of oxygen concentration. the greater the concentration of dissolved oxygen in cells, the higher the partial pressure similar with CO2 (pCO2).
• oxygen loads onto haemoglobin to form oxyhaemoglobin where there is a high pO2
• oxyhaemoglobin unloads its oxygen where there’s a lower pO2

Question 49

describe the pO2 of alveoli?

Answer 49

• oxygen enters blood capillaries at the alveoli in the lungs. they have a high pO2 so oxygen loads onto haemoglobin to form oxyhaemoglobin. when cells respire, they use up oxygen, which leaves the pO2. RBCs deliver oxyhaemoglobin to respiring tissues, where it unloads O2. the Hb returns to the lungs to pick up more O2

Question 50

what do different types of organisms have?

Answer 50

• different types of haemoglobin with different oxygen transporting capacities. having a particular type of Hb is an adaption that helps the organism to survive in an environment
• organisms that live in places with a low conc of O2 have Hb with a higher affinity of O2 than human Hb (curve to the left)

Question 51

what do organisms that are active and have a high oxygen demand?

Answer 51

• have haemoglobin with a lower affinity of oxygen than human haemoglobin the curve is to the right of human one

Question 52

describe an atheroma?

Answer 52

• if damage occurs to the endothelium, white blood cells and lipids form blood clots together under the lining to form fatty streaks. over time, more WBCs, lipids and connective tissue build up to form a fibrous plaque called an atheroma.
• this plaque blocks part of the lumen of the artery and restricts blood flow, which causes BP to increase

Question 53

describe an aneurysm?

Answer 53

• they narrow arteries so when blood travels through a weakened artery at high pressure, it ma push the inner layers of the artery through the outer elastic layer to form an aneurysm which may burst causing a haemorrhage.

Question 54

describe thrombosis?

Answer 54

• an atheroma plaque can rupture the endothelium and leave a rough surface. platelets and fibrin accumulate there and form a thrombus which can cause a blockage or become dislodged and block a blood vessel elsewhere in the body.
• debris from the rupture can cause another clot to form further down the artery

Question 55

describe myocardial infarction?

Answer 55

• if a coronary artery becomes blocked, an area of the heart muscle will be cut off from its blood supply, receiving no oxygen -> myocardial infarction.
• a heart attack can cause damage and death of the heart muscle, symptoms include pain, shortness of breath and sweating`

Question 56

how does cholesterol and poor diet act as a risk factor?

Answer 56

• high cholesterol increases risk of diseases as its one of the main constituents of the fatty deposits that form atheroma’s which cause high BP and blood clots.
• this could block the coronary arteries leading to heart attacks. a diet high in saturated fat is associated with cholesterol

Question 57

how does cigarette smoking act as a risk factor?

Answer 57

• nictoine increases risk of high BP and carbon monoxide combines with oxygen and reduces the amount of O2 transported in the blood -> less to heart muscle
• smoking decreases the amount of antioxidants which are important for protecting cells from damage, fewer cells means cell damage in the coronary artery walls is more likely

Question 58

how high blood pressure act as a risk factor?

Answer 58

• High BP increases risk of damage to artery walls which increases risk of atheroma formation. these can cause clots which could block flow of blood to the heart muscle -> heart attack.

Question 59

what is the problem with some risk factors?

Answer 59

• they cant be controlled, such as having a genetic predisposition to coronary heart disease or high BP as a result of having another condition

Question 60

what does phloem do?

Answer 60

• transports solutes round plants. its made from cells arranged in tubes. translocation is the movement of solutes eg amino acids and sucrose to where they’re needed.
• the solutes are sometimes called assimilates, its an energy requiring process

Question 61

what are sieve tube elements?

Answer 61

• they are living cells that form the tube for transporting solutes, they have no nucleus and few organelles so there’s a companion cell for each sieve tube element; which carry out living functions for sieve cells eg providing the energy needed for the active transport of solutes

Question 62

what are sources and sinks?

Answer 62

• the source for a solute is where its made (high conc there) for sucrose its usually the leaves where its made
• the sink is the area where its used up (low conc there) for sucrose the sinks are the other parts of the plant, especially the food storage organs and the meristems in the roots stem and leaves.

Question 63

what do enzymes do in translocation?

Answer 63

• they maintain a concentration gradient from the source to the sink by changing the solutes at the sink eg by breaking them into something else. this makes sure there’s always a lower concentration at the sink then at the source

Question 64

what is part 1 of the mass flow hypothesis theory?

Answer 64

1- sucrose is manufactured from the products of photosynthesis in cells with chloroplasts
2- the sucrose diffuses down a concentration gradient by facilitated diffusion from the photosynthesising cells into companion cells
3- H+ ions are actively transported from companion cells into the spaces within cell walls using ATP
4- these H+ ions diffuse down a conc gradient through carrier proteins into the sieve tube elements
5- the sucrose along with the H+ ions are transported in co-transport.

Question 65

what is part 2 of the mass flow hypothesis theory?

Answer 65

1. as mentioned before, sucrose produced by photosynthesising cells (source) is actively transported into the sieve tubes
2. this causes the sieve tubes to have a lower (more negative) water potential
3. as xylem has a higher water potential, water moves from the xylem into the sieve tubes by osmosis, creating a high hydrostatic pressure
4. at the sink, sucrose is used up or converted to starch
5. these cells have a low sucrose content and is actively transported into them from the sieve tubes lowering their water potential

Question 66

what is part 3 of the mass flow hypothesis theory?

Answer 66

• due to this lowered water potential, water moves into these respiring cells from the sieve tubes by osmosis
• the hydrostatic pressure of the sieve tubes is lowered
• as a result there is amass flow of sucrose solution down this hydrostatic gradient in the sieve tubes
• sucrose is actively transported out by companion cells into the sink cells

Question 67

how can aphids be used to prove the mass flow theory?

Answer 67

• pressure in the phloem can be investigated using aphids, they pierce the phloem, then their bodies are removed leaving the mouthparts behind, which allows the sap to flow out.
• the sap flows out quicker nearer the leaves than further down the stem - evidence there’s a pressure gradient

Question 68

what other two things support the mass flow theory?

Answer 68

• if a ring of bark which has phloem not xylem, is removed from a woody stem a bulge forms the above the ring. the fluid from the bulge has a higher concentration of sugars than the fluid from below the ring - evidence there is a downward flow of sugar
• if a metabolic inhibitor which stops ATP production is put into the phloem, then the translocation stops - evidence active transport is used

Question 69

what are the objections to the mass flow theory?

Answer 69

• sugar travels to many different sinks, not just to the one with the biggest water potential
• the sieve plates would create a barrier to mass flow. a lot of pressure would be needed for the solutes to get through at a measurable rate

Question 70

What is tissue fluid?

Answer 70

It’s the fluid that surrounds cells in tissues it’s made from small molecules that leave the blood plasma eg O2, H2O and nutrients. Cells take in O2 and nutrients from the fluid and release metabolic waste into it

Question 71

What happens to the excess tissue fluid?

Answer 71

It’s drained into the lymphatic system which transports this excess fluid from the tissues and dumps it back into the circulatory system

Question 72

what are the limitations to the tracheal system?

Answer 72

it relies mostly on diffusion to exchange gases. for it to be effective the diffusion pathway needs to be short which is why insects are small. as a result the length of the diffusion pathway limits the size that insects can attain.

Question 73

what are dipeptidases?

Answer 73

they hydrolyse the bond between the two amino acids of a dipeptide.

Question 74

what is the ileum?

Answer 74

it is adapted to the function of absorbing the products of digestion, the wall of the ileum is folded and possess finger-like projections.

Question 75

how do villi increase the efficiency of absorption?

Answer 75

• they increase SA for diffusion
• they are thin walled, reducing the distance over which diffusion takes place
• they contain muscle and so are able to move which helps maintain a concentration gradient
• they have lots of blood vessels so blood can carry away absorbed molecules
• the epithelial cells lining the ileum possess microvilli which further increase SA

Question 76

what happens when monoglycerides and fatty acids are inside the epithelial cells?

Answer 76

they are transported to the endoplasmic reticulum where they are recombined to form triglycerides. after they pass through endoplasmic reticulum and golgi apparatus, the triglycerides associate with cholesterol and lipoproteins to form chylomicrons.

Question 77

what are chylomicrons?

Answer 77

they are special particles adapted for the transport of lipids, they move out of the epithelial cells by excoytosis . they enter lymphatic capillaries called lacteals that are found in the centre of each villus.

Question 78

what happens after chylomicrons enter lacteals?

Answer 78

they pass via the lymphatic vessels into the blood stream. the triglycerides in the chylomicrons are hydrolysed by the enzyme in the endothelial cells of blood capillaries from where they diffuse into cells.

Question 79

what does the shape of the haemoglobin molecule make difficult on the graph?

Answer 79

it makes it difficult for the first oxygen molecule to bind to one of the sites of its four polypeptide subunits because they’re closely united. so at low oxygen concentration, little oxygen binds to Hb. the gradient of the curve is shallow initially

Question 80

what does the binding of the first oxygen molecule do to the graph?

Answer 80

it changes the quaternary structure of the haemoglobin molecule, causing it to change shape. this change makes it easier for other subunits to bind to an oxygen molecule. basically, the binding of the first oxygen molecule induces the other subunits to bind to an oxygen molecule

Question 81

when does the gradient of the curve steepen?

Answer 81

it therefore takes a smaller increase in the partial pressure of oxygen to bind to the second oxygen molecule than it did to bind to the first one. this is known as positive cooperativity because binding of the first molecule makes binding of the second easier

Question 82

what happens when the third oxygen Hb binds?

Answer 82

in theory it is easier for Hb to bind the fourth oxygen molecule , in practice it is harder, this is due to probability. With majority of the binding sites are occupied, it is less likely that a single oxygen molecule will find an empty to bind to. the gradient of the curve reduces and the graph flattens

Question 83

what does it mean if the curve is further to the left?

Answer 83

the greater the affinity of haemoglobin to oxygen, so it loads oxygen readily but unloads it less easily

Question 84

what does it mean if the curve is further to the right?

Answer 84

the lower the affinity of haemoglobin for oxygen, so it loads oxygen less readily but unloads it more easily

Question 85

what is associating and dissociating?

Answer 85

• associating is the process by which haemoglobin binds with oxygen
• dissociating is the process by which haemoglobin releases its oxygen

Question 86

what does haemoglobin with a high affinity for oxygen do?

Answer 86

it takes up oxygen more readily but releases it less easily.

Question 87

what does haemoglobin with a low affinity for oxygen do?

Answer 87

it takes up oxygen less easily, but releases it more easily

Question 88

what is the bohr effect?

Answer 88

haemoglobin has a reduced affinity for oxygen in the presence of carbon dioxide. the greater the concentration of CO2, the more readily Hb releases its oxygen

Question 89

what happens to CO2 and O2 in the lungs?

Answer 89

to concentration of CO2 is low as it diffuses and is excreted from the organism. the affinity for oxygen is increases, which coupled with the high levels of O2 means that O2 is loaded onto Hb. the reduced carbon dioxide concentration has shifted the oxygen dissociation curve to the left.

Question 90

what happens to CO2 and O2 in muscles?

Answer 90

the concentration of CO2 is high. the affinity of haemoglobin for oxygen is reduced, which coupled with the low concentration of oxygen means that O2 is readily unloaded from the Hb into muscle cells. the increased CO2 concentration shifted the oxygen dissociation curve to the right

Question 91

what does dissolved CO2 do?

Answer 91

it is acidic and the low pH causes Hb to change shape.

Question 92

describe the process by which a more active tissue, unloads more oxygen

Answer 92

1. the higher the rate of respiration
2. the more CO2 the tissue produces
3. the lower the pH
4. the greater the haemoglobin shape change
5. the more readily oxygen is unloaded
6. the more oxygen is available for respiration

Question 93

what is the aorta, pulmonary vein, pulmonary artery and the vena cava do?

Answer 93

• aorta is connected to the left ventricle and carries oxygenated blood to all parts of the body except the lungs
• vena cava is connected to the right atrium and brings deoxygenated blood back from the tissues of the body except the lungs
• pulmonary vein is connected to the left atrium and brings oxygenated blood back from the lungs
• pulmonary artery is connected to the right ventricle and carries deoxygenated blood to the lungs

Question 94

describe the movement of water up the stem?

Answer 94

1. water evaporates from mesophyll cells due to heat from the sun leading to transpiration
2. water molecules form hydrogen bonds between one another and stick together, this is cohesion.
3. water forms a continuous, unbroken column across the mesophyll cells and down the xylem
4. as water evaporates into the air spaces beneath the stomata, more water is drawn up
5. a column of water is therefore pulled up the xylem, this is the transpiration pull
6. this pull puts the xylem under tension -> cohesion tension.

Question 95

how do gases diffuse in and out of tracheal system in insects?

Answer 95

1. along a diffusion gradient = when cells respire the conc of O2 is lower towards ends of tracheoles. this creates a gradient, that causes O2 to diffuse from atmosphere along tracheoles to cells. CO2 creates gradient in opposite direction, so it diffuses along tracheoles and out to atmosphere
2. mass transport = contraction of muscles can squeeze trachea enabling movements of air in and out
3. ends of tracheoles are filled with water. During periods of lots of activity, muscles cells around tracheoles respire and carry out anaerobic respiration. this produces lactate, which lowers water potential. water in the ends of the tracheoles decreases in volume and so draws air in. the final diffusion path is in a gas phase and so increases rate at which air is moved in but leads to greater water evaporation.

Question 96

describes a mammals’ closed, double circulatory system?

Answer 96

blood is confined to vessels and passes twice through the heart for each complete circuit of the body. this is because when blood is passed through lungs, its pressure is reduced. if it were to pass immediately ti rest of body its low pressure makes circulation slow

Question 97

describe arterioles?

Answer 97

1. muscle layer is thicker than arteries, which allows constriction of lumen of arteriole. this restricts flow of blood and controls movement into the capillaries that supply the tissue with blood
2. elastic layer thinner than arteries as pressure is lower

Question 98

what is the outward pressure of tissue fluid to move out of blood plasma resisted?

Answer 98

1. hydrostatic pressure of tissue fluid outside the capillary which resists outward movement of liquid
2. the lower water potential of the blood, due to plasma proteins, that cause water to move back into blood within capillaries

Question 99

describe how tissue fluid returns to circulatory system?

Answer 99

1. loss of tissue fluid from the capillaries reduces the hydrostatic pressure in them
2. as a result, by the time blood reaches the venous end of capillary network, its pressure is lower than that of tissue fluid outside it
3. therefore the tissue fluid is forced back into the capillaries by the higher hydrostatic pressure outside them
4. in addition, the plasma has lost water and still contains proteins. it therefore, has a lower water potential than the tissue fluid
5. as a result, water leaves the tissue by osmosis down a water potential gradient

Question 100

describe capillaries?

Answer 100

• their walls are mostly lining which is thin, so shortens diffusion pathway
• there’s many of them and highly branched, so increases SA
• narrow diameter so short pathway
• lumen is narrow so short pathway
• spaces between lining (endothelial) cells that allow white blood cells to escape to deal with infections

Question 101

when do the intercostal muscles contract?

Answer 101

• the internal intercostal muscles contract when expiration occurs
• the eternal intercostal muscles contract when inspiration occurs

Question 102

why is diffusion between the alveoli and blood fast?

Answer 102

• RBC’s are slowed as they pass through the pulmonary capillaries, allowing more time for diffusion
• distance between the alveolar air and RBC’s is reduced as RBC’s are flattened against capillary walls
• blood flow through the pulmonary capillaries maintains a gradient
• alveoli and pulmonary capillaries have large SA

Question 103

what are lipids firstly split up into?

Answer 103

they’re split into tiny droplets called micelles by bile salts, which are produced by the liver. this is emulsification.