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A-Level Biology > Unit 3 - Substance Exchange > Flashcards

Unit 3 - Substance Exchange Flashcards

1
Q

what is the physical breakdown of food?

A

food is ‘physically’ broken down into smaller pieces
increasing its surface area
by chewing, stomach churning & bile emulsification

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2
Q

what is the chemical digestion of food?

A

by enzymes
hydrolysing covalent bonds in large, insoluble molecules to form small, soluble molecules

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3
Q

describe the digestion of polysaccharides

A

polysaccharides digested by carbohydrases that hydrolyse the glycosidic bonds
1. salivary amylase produced in salivary glands digests starch into maltose
2. pancreatic amylase produced in pancreas digests starch into maltose

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4
Q

describe the digestion of disaccharides

A

disaccharides are digested by membrane-bound disaccharidases found in the csm of epithelial cells
1. maltase - maltose –> 2x alpha glucose
2. sucrase - sucrose –> alpha glucose + fructose
3. lactase - lactose –> alpha glucose + galactose

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5
Q

what category of enzymes are proteins digested by?

A

proteases that hydrolyse peptide bonds

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6
Q
  1. what is the function of endopeptidases?
A

they hydrolyse peptide bonds in the central region of a polypeptide
which forms shorter peptide chains
e.g. pepsin produced in the stomach

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7
Q
  1. where are exopeptidases produced & what is their function?
A

they are produced in pancreas & ileum
they hydrolyse peptide bonds at the ends of polypeptides on the terminal amino acids
which forms dipeptides & single amino acids

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8
Q
  1. where are dipeptidases found & what is their function?
A

they are bound in csm of epithelial cells lining the ileum
they hydrolyse peptide bonds b/x 2 amino acids of a dipeptide

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9
Q

what happens to lipids before digestion?

A

emulsification - lipids are split into tiny droplets by bile salts (produced in liver & stored in gall bladder)
increases surface area of lipids so lipase can work faster so hydrolysis is faster
then the tiny droplets are converted into micelles, which carry fatty acids & monoglycerides to epithelial cells

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10
Q

how are triglycerides digested?

A

by lipases which hydrolyse ester bonds
triglycerides –> monoglycerides + fatty acids

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11
Q

describe the absorption of triglycerides

A
  1. micelles contain bile salts, fatty acids & monoglycerides
    they make fatty acids more soluble in water
  2. micelles carry fatty acids & monoglycerides to epithelial cells lining the ileum.
  3. micelles break down, releasing monoglycerides & fatty acids, which are non-polar so can simply diffuse across the csm into epithelial cells
  4. triglycerides reform in ser & in the golgi apparatus, they associate with cholesterol & lipoproteins to form chylomicrons
  5. vesicles containing chylomicrons move out of epithelial cells by exocytosis & enter lymphatic capillaries called lacteals
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12
Q

how is the ileum adapted for the absorption of the products of digestion?

A

absorption of digested food (glucose, aas, fatty acids & glycerol move into the blood by simple diffusion, facilitated diffusion & some active transport)
ileum surface is covered in millions of tiny villi, which increases the surface area for a higher rate of dif./fac. dif./at
ileum is very long, which increases surface area & time for absorption to happen

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13
Q

how is a villus adapted for the absorption of the products of digestion?

A

csm of epithelial cells is highly folded into many microvilli
- increased surface area for insertion of membrane proteins: many carrier & channel proteins for fac. dif. & co-transport, many carrier proteins for at
- increased sa for higher rate of absorption

epithelial cells are very thin
- short diffusion distance so faster diffusion/absorption

blood supply & capillaries close to surface
- moving blood maintains a steep concentration gradient for faster diffusion/absorption

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14
Q

how are glucose & aas absorbed?

A

when there is a greater concentration of glucose/aas in the ileum than in the blood, these molecules can move down the concentration gradient into the blood by fac. dif.

when there is a greater concentration of glucose/aas in the blood than in the ileum, all molecules are transported against their concentration gradient by co-transport, which is allowed by active transport

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15
Q

describe the process of co-transport

A
  1. (3) sodium ions are actively transported from the epithelial cell into the blood by the Na+/K+ pump (carrier protein that requires ATP hydrolysis)
  2. this lowers the concentration of Na+ in the epithelial cell & creates a concentration/diffusion gradient for Na+ from ileum into the epithelial cell
  3. Na+ ions move into the epithelial cell from the ileum by fac. dif. & carries a glucose/aa with it by co-transport
  4. glucose/aa moves into the blood by fac. dif. down a concentration gradient using a glucose or aa channel protein
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16
Q

as the size of the organism increases, what is the effect on sa:v ratio?

A

decreases

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17
Q

what is fick’s law?

A

rate of diffusion is proportional to sa x conc. gradient/diffusion distance

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18
Q

how does sa increase the rate of gas exchange?

A

folds & branches
more membrane area over which exchange can happen

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19
Q

how does short diffusion distance increase the rate of gas exchange?

A

surface is often 1 cell thick so rapid gas exchange e.g. squamous epithelium & capillary endothelium

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20
Q

how is a steep diffusion gradient maintained?

A

ventilation & mass flow of air or water
rich blood supply by dense capillary network

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21
Q

what does the tracheal system consist of?

A

1- pores = spiracles
opened & closed by valves & regulate exchange of air & water

2- trachea(e) tubes supported by chitin to prevent collapse

3- smaller tracheoles increase sa
dead-end tubes

4- tracheoles extend throughout body tissues of the insect so oxygen is brought directly to respiring tissues/muscle fibres

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22
Q

how are gases exchanged in the tracheal system?

A

along a diffusion gradient (passive)
mass transport/ventilation
ends of tracheoles filled with water

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23
Q

describe the movement of gas along the diffusion gradient in tracheal system

A

when cells are respiring oxygen is used up so conc, towards the ends of the tracheoles decreases = creates a diffusion gradient
O2 diffuses from atmosphere to tracheoles to muscles
when cells respire CO2 is produced - creates a diffusion gradient so CO2 diffuses from tracheoles to atmosphere

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24
Q

describe the movement of gases by mass transport/ventilation in tracheal system

A

contraction of abdominal muscles in insects squeeze trachea
so mass movement of air in & out
maintains concentration gradient of O2 & CO2

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25
Q

how are tracheoles adapted for efficient gas exchange?

A

highly branched - increases sa
thin walls - short diffusion distance
permeable to oxygen
muscle cells around tracheoles respire anaerobically & produce lactic acid
which lowers the water potential of muscle cells so water carrying dissolved oxygen moves from tracheoles into muscle cells via osmosis
final diffusion pathway is in air rather than liquid, so it is faster

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26
Q

how do insects lose water & how do they limit this water loss?

A

water evaporates from the surface of insects’ bodies via spiracles (exoskeleton is waterproof)
thin, permeable surface with large sa for efficient gas exchange - but = water loss
adaptations:
1- spiracles can be closed by valves to reduce water loss
2- hairs around spiracles reduce water potential gradient
3- waxy waterproof layer covers exoskeleton of chitin
4- lower sa:v - minimise area over which water is lost

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27
Q

describe the structure & demands of fish

A

covered in scales & mucous so gas impermeable
quite large so small sa:v
high O2 demands to supply muscles for swimming

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28
Q

describe the structure of the gills

A

located behind the head
gill filaments stacked in a pile - supported by gill arches
at right angles - gill lamellae

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29
Q

how is water forced over the gills?

A

operculum - bony flap that acts as a valve to allow one way flow of water over the gills & is a tough protective layer
pathway:
water taken in through mouth, forced over gills & out through operculum

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29
Q

what are the features of the lungs?

A

ribcage - protects & supports lungs
trachea
bronchi
bronchioles
alveoli

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29
Q

what is the counter current exchange principle?

A

blood flows through the gill lamellae in the opposite direction to water flowing over the gills
so blood with high O2 conc. meets water, which has a max conc. of O2 so O2 diffuses into the blood
blood with low conc. of O2 meets water that has most O2 removed so O2 still diffuses into the blood
therefore
a diffusion gradient is maintained across the whole length of the lamellae

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30
Q

describe the bronchioles

A

branching sub-divisions of the bronchi
smooth muscle walls lined with epithelial cells so can constrict to control air flow to alveoli

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30
Q

describe the trachea

A

flexible airway
supported by C-shaped cartilage
which prevents trachea from collapsing when air pressure decreases when breathing in
tracheal walls are made of muscle, lined with ciliated epithelium & goblet cells that secrete mucous

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30
Q

why is the volume of of O2 absorbed & CO2 removed large in mammals?

A

they have a large volume of living cells
they maintain a high body temperature which is related to their high metabolic & respiratory rates
so evolved lungs

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31
Q

describe the bronchi

A

2 divisions of the trachea, each leading to a lung
cartilage rings, ciliated epithelium, goblet cells

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32
Q

describe the alveoli

A

tiny air sacs
lined with epithelium
collagen & elastic fibres b/w alveoli so they can stretch & fill when breathing in & spring back when breathing out to expel CO2-rich air
alveolar membrane is the gas exchange surface

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33
Q

describe the mechanism of breathing

A

inspiration:
when the air pressure of the atmosphere is greater than that inside the lungs, air moves in down the pressure gradient
active by muscle contraction
expiration:
when the air pressure of the lungs is greater than that of the atmosphere, air moves out, the pressure gradient is reversed

pressure changes are due to change in volume of the thoracic cavity due to internal & external intercostal muscles & diaphragm

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34
Q

what happens to the body on inspiration?

A

external intercostal muscles contract
internal relax
ribcage moves up & out
diaphragm muscles contract so diaphragm moves down & flattens
volume in thorax increases
so pressure in thorax decreases
air moves in down the pressure gradient

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35
Q

what happens to the body on expiration?

A

external intercostal muscles relax
internal contract
ribcage moves down & in
diaphragm muscles relax so diaphragm moves up
volume in thorax decreases
so pressure in thorax increases
air moves out down the pressure gradient

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36
Q

what is the formula for pulmonary ventilation (volume of air exchanged per unit time dm^3min^-1)?

A

tidal volume (dm^3) x ventilation rate (min^-1)

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37
Q

define tidal volume

A

volume of air exchanged during normal breathing

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38
Q

define vital capacity

A

max. volume of air exchanged from full inspiration to full expiration

39
Q

define residual volume

A

volume of air that cannot be expelled by forced expiration
cartilage supports tubes & deflated alveoli

40
Q

why is gas exchange important in plants?

A

to facilitate respiration & photosynthesis
at times gas produced in one process can be used for the other, but not sufficient to meet the demands of the plant

41
Q

when can gases from one process be used for the other?

A

see notes

42
Q

describe the upper epidermis

A

few or no chloroplasts - transparent so light can pass through to palisade cells
upper surface has thick, waxy, waterproof cuticle - protection & reduces water loss

43
Q

describe palisade mesophyll cells

A

absorb light for ps
many chloroplasts (that can move up & down)

44
Q

describe spongy mesophyll cells

A

make movement of gases needed for ps more efficient
air spaces b/w cells

45
Q

describe lower epidermis

A

guard cells (contain chloroplasts) & stomata
stomata control rate of gas exchange

46
Q

how do stomata work?

A

located on underside of leaves bc it is cooler so less transpiration
each stoma surrounded by guard cells that open & close the pore - regulate diameter, have chloroplasts for atp production –> active transport
control rate of gas exchange
important bc terrestrial plants lose water by evaporation/evapotranspiration down water potential gradient
conflict b/w gas exchange & water loss

47
Q

how does a stoma open?

A

active K+ ion movement into guard cell using atp from ps
lowers water potential of cytoplasm
water moves into guard cell by osmosis down water potential gradient
cell becomes turgid
stoma opens

48
Q

how does a stoma close?

A

active K+ ion movement stops
K+ gates open so K+ moves out of guard cell
water follows, moving out of guard cell by osmosis down water potential gradient
cells become flaccid
stoma closes

49
Q

describe the closed double circulatory system in mammals

A

closed - blood is contained in vessels

double - blood flows through heart twice per complete circuit
separation of oxygenated & deoxygenated blood
can regulate blood flow & pressure - high P to body

50
Q

what are the names of the blood vessels going to & from the kidneys?

A

to: renal artery
from: renal vein

51
Q

label a diagram of the heart

A

see notes booklet

52
Q

what is the function of the coronary arteries?

A

supply cardiac muscle with oxygen-rich blood

53
Q

what is coronary heart disease caused by & what might it cause?

A

atheroma - build up of fatty deposits within coronary artery endothelium
decreased rate of blood flow
decreased oxygen supply
decreased rate of aerobic respiration
so lactic acid buildup, causing pain (angina)
cells may die
lack of atp
heart attack

54
Q

explain the difference in thickness of the muscle of the atria, LV & RV walls

A

atria have thinner walls than ventricles
bc low force of contraction, forcing blood a short distance

LV wall is thicker than RV wall
bc LV pumps blood a greater distance (around the body)
so more force & higher pressure needed with each contraction
RV only needs to pump blood a shorter distance than LV - to lungs

55
Q

what is the function of valves?

A

to prevent backflow of blood to ensure blood flow is only one-way

56
Q

where are the atrioventricular valves & describe their activity?

A

b/w atria & ventricles
open when pressure in atria > pressure in ventricles
closed when pressure in ventricles > pressure in stria

57
Q

where are the semilunar valves & describe their activity?

A

b/w ventricles & arteries
open when pressure in ventricles > pressure in arteries
closed when pressure in arteries > pressure in ventricles

58
Q

what maintains unidirectional flow of blood during the cardiac cycle?

A

pressure & volume changes
valve movements

59
Q

what are the 3 steps in the cardiac cycle?

A
  1. atrial systole
  2. ventricular systole
  3. ventricular diastole
60
Q

what happens in atrial systole?

A

atria contract
pressure in atria increases above pressure in ventricles
AV valves open & blood is forced from atria into ventricles
SLVs shut to prevent backflow of blood into ventricles

61
Q

what happens in ventricular systole?

A

ventricles contract
pressure in ventricles increases quickly
AV valves shut bc pressure in ventricles > pressure in atria
pressure in ventricles increases above pressure in arteries so SLVs open
blood is forced into arteries
so ventricular blood vol. decreases

62
Q

what happens in ventricular diastole?

A

ventricles relax
so pressure in ventricles decreases quickly
pressure in ventricles decreases below pressure in arteries so SLV shut
pressure in ventricles decreases below pressure in atria so AV valves open
ventricles fill passively with blood
atria fill from veins
heart completely relaxed

63
Q

define cardiac output & what is its formula?

A

the volume of blood ejected by the heart per minute

cardiac output = stroke volume (vol. ejected per heartbeat) x heart rate (bpm)

cm^3min^-1

64
Q

how does the structure of arteries relate to their function?

A

function: transport high pressure blood away from the heart
blood is oxygenated except pulmonary (& umbilical) artery

thick smooth muscular wall to withstand high pressure
smooth to reduce friction

lots of elastic tissue to allow stretch & recoil for smooth blood flow

narrow lumen relative to diameter

capable of vasoconstriction

impermeable

no valves except SLV

has pulse

65
Q

how does the structure of arterioles relate to their function?

A

function: to transport blood from arteries to capillaries
muscular for vasoconstriction to redistribute blood, but less so than arteries

66
Q

how does the structure of capillaries relate to their function?

A

function: site of substance exchange
connects arterioles to venules
blood is under decreasing pressure

no muscular wall or elastic tissue
large lumen relative to diameter to allow red blood cells to fit
cannot vasoconstrict
very permeable to allow substance exchange
no valves

67
Q

what is the function of venules?

A

function: connect capillaries to veins

68
Q

how does the structure of veins relate to their function?

A

function: to transport lower pressure blood back to the heart
blood is deoxygenated except in pulmonary (& umbilical) vein

thinner muscular wall & less elastic tissue
wider lumen relative to diameter
incapable of vasoconstriction
impermeable
valves that prevent backflow of blood

69
Q

what is tissue fluid?

A

water liquid derived from blood plasma that bathes all cells of the body
it forms the immediate environment of all cells & supplies them with substances

70
Q

what does tissue fluid contain?

A

supplies cells with: glucose, AAs, O2, CO2, FAs, lipids, ions (Na+, Cl-, K-)

receives from cells & tissues: waste materials inc. CO2 from respiration & urea from deamination

71
Q

what is tissue fluid formed from?

A

blood plasma, which is continually forced from capillaries via pores b/w capillary endothelial cells

72
Q

describe the formation & reabsorption of tissue fluid

A
  1. hydrostatic pressure of the blood is higher at the arterial end
  2. so water & small soluble molecules are forced out of the capillary to form TF
  3. RBCs & plasma proteins are too big so remain in capillary
  4. this decreases the water potential of the capillary as blood flows to the venous end
  5. 90% water is reabsorbed by osmosis down the water potential gradient into the venous end of the capillary
  6. excess TF is drained into the lymph, into the lymphatic system & later returns to circulatory system
73
Q

why does high blood pressure lead to an accumulation of TF?

A

high blood pressure = high hydrostatic pressure
which increases outwards pressure from arterial end of capillary
so more tissue fluid is formed

74
Q

table of comparison b/w blood, TF & lymph

A

see notes booklet

75
Q

what are haemoglobins?

A

group of chemically similar protein molecules found in a wide variety of organisms
Hb has a quaternary structure that has evolved to make it efficient at loading & unloading O2

76
Q

what is the role of haemoglobin?

A

to transport O2

77
Q

what is the process by which Hb binds with O2 called & where does it happen?

A

loading/association
happens at gas exchange surface = alveolar epithelium

78
Q

what is the process by which Hb releases its O2 called & where does it happen?

A

unloading/dissociation
happens at respiring tissue e.g. muscle

79
Q

what does Hb change under different conditions & why?

A

its affinity (chemical attraction) for O2
bc its shape changes in the presence of specific substances e.g. CO2, lactic acid or temp. (increasing these factors decreases affinity)

80
Q

Hb: what happens at the gas exchange surface?

A

high O2 conc.
low CO2 conc.
Hb has high affinity for O2
so O2 associates w Hb

81
Q

Hb: what happens at respiring tissues?

A

low O2 conc.
high CO2 conc.
Hb has low affinity for O2
so O2 readily dissociates from Hb

82
Q

what is the oxyhaemoglobin dissociation curve?

A

graph of the relationship b/w the partial pressure of O2 vs saturation of Hb with O2

83
Q

describe & explain the shape of the oxyhaemoglobin dissociation curve

A

sigmoid shape

  1. shallow at start
    loading the first O2 molecule onto Hb is difficult
  2. steeper
    once 1st O2 is bonded, Hb changes shape, exposing the 2nd haem binding site so it is easier for the 2nd & 3rd O2 molecule to bind = positive cooperativity of O2 binding
  3. binding the 4th O2 molecule is more difficult bc there is a decreased chance of O2 colliding with the ‘empty’ haem group
84
Q

what happens at the steepest part of the O2 dissociation curve?

A

as RBCs & oxyHb enter tissues, partial pressure of O2 decreases rapidly bc O2 is used up in respiration

over the steepest part of the curve a small decrease in ppO2 causes a big drop in % saturation of Hb

ie. oxyHb rapidly loses affinity so O2 rapidly dissociates & offloads lots of O2 into tissues that need it

85
Q

describe & explain the O2 dissociation curve at high ppO2

A

high % saturation
bc in alveoli in lungs, Hb is 98% saturated w O2
bc Hb has a high affinity for O2 so readily associates

86
Q

what is the Bohr effect?

A

Hb has a reduced affinity for O2 in the presence of CO2 bc of conformational change in shape

the greater the conc. of CO2, the more readily the Hb releases its O2

the graph shifts right

this explains why Hb changes behaviour in different regions of the body (lungs vs tissues)

87
Q

explain how the Bohr effect impacts the O2 dissociation curve

A

as conc CO2 increases, affinity for bound O2 decreases & Hb is less saturated
so oxyHb offloads more readily

88
Q

define high affinity haemoglobin

A

Hb that loads/associates O2 more readily but dissociates less readily in tissues

89
Q

define low affinity haemoglobin

A

Hb that loads/associates O2 less readily (bc needs higher ppO2 in environment to saturate) but dissociates more readily in tissues

90
Q

why do species have different haemoglobins?

A
  1. environmental conditions: higher or lower O2 availability (high altitude has lower ppO2)
    can have low affinity Hb in O2-rich environments bc Hb can get v saturated
  2. SA:V body size & heat loss
    high SA:V = tissues need more O2 per gram
  3. metabolic demand - energy requirements

e.gs in notes booklet

91
Q

what is the role of phloem in plants?

A

to transport sucrose & AAs (organic assimilate) from source to sink
flow is bidirectional

92
Q

what is the structure of the phloem?

A

living cells

  1. sieve tube elements (STE) - elongated cells connected end-to-end, peripheral cytoplasm
  2. sieve plates connect cells - perforated w pores to let phloem sap flow
  3. companion cells connected to STE via plasmodesmata - have lots of mitochondria to provide lots of ATP for translocation
93
Q

what is translocation?

A

the active, mass flow of sucrose & AAs from source to sink

(bc of positive hydrostatic pressure)

94
Q

what are sources & sinks?

A

sources: cells where sucrose is made/released from storage into phloem
e.g. leaves

sinks: cells where sucrose is removed from phloem,
where it is needed for respiration, storage as starch or to make cellulose
e.g. roots & fruits

95
Q

describe the mass flow hypothesis for the mechanism of translocation

A
  1. companion cells actively load sucrose into STE at source
    by companion cells
  2. this lowers water potential of STE so water enters STE by osmosis down water potential gradient
  3. so hydrostatic pressure of STE increases
  4. sucrose moves by mass flow towards sink
  5. sucrose is actively unloaded at sink and used e.g. for respiration
  6. water leaves STE & mass flow of phloem sap occurs down pressure gradient
96
Q

what 3 experiments give evidence for translocation?

A

ringing experiments
tracer experiments
aphids

97
Q

describe ringing experiments & how do they give evidence for translocation?

A

bark & phloem removed from plant, not xylem

after time, the region above the ring swells

samples from the swollen region have high conc. of sucrose & AAs

non-photosynthetic tissue in region below ring withers & dies whilst these regions above the ring continue to grow

conclusion: phloem, not xylem, is responsible for translocation of sugars

98
Q

describe tracer experiments & how do they give evidence for translocation?

A

radioactive isotopes used to trace movement of substances

the 14C isotope makes radioactively labelled 14CO2

if plant is grown in atmosphere of 14CO2, 14CO2 is taken up by photosynthesis & turned into heavy sucrose (w 14C)

heavy sucrose can be traced using autoradiography

14C only in phloem & with time, it is shown to be in all areas of plant

conclusion: only phloem is responsible for translocation of sugars

99
Q

describe the role of aphids on proving translocation

A

aphids = insects that feed on plants
needle-like mouth penetrates STE & extracts contents of STE
the contents are analysed & identified as organic assimilate, which proves organic assimilate is transported in phloem by translocation

100
Q

what is the evidence for (6) & against (3) the mass flow hypothesis?

A

see table in notes booklet

A-Level Biology (6 decks)

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