Relationship between the size of organism and its SA:V
-
Smaller organisms tend to have a higher SA:V than larger organisms
E.g. hippo vs. mouse
This can be calculated and proven mathematically:
Relationship between SA:V (and thus the size of an organism) and metabolic rate
-
Rate of heat loss / heat lost per unit body mass increases as SA:V increases
i.e. more heat lost per unit body mass in smaller animals with a high SA:V
So they need a higher metabolic rate / faster respiration
To generate enough heat to maintain a constant body temperature i.e. replace lost heat
Adaptations to facilitate exchange as this ratio reduces in larger organisms include
changes to body shape and the development of systems
-
Larger organisms need a specialised surface / organ for gaseous exchange e.g. lungs
Because they have a smaller SA:V and a long diffusion pathway (and skin is waterproof / gas
tight)
As well as having a high demand for oxygen and to remove carbon dioxide
Adaptations of gas exchange surfaces shown by gas exchange…
Across the body surface of a single-celled organism
-
-
Thin, flat shape
- Large SA(:V)
- Short diffusion pathway/distance (all parts of cell are a small distance away from
exchange surfaces)
For rapid diffusion e.g. oxygen / carbon dioxide
Adaptations of gas exchange surfaces shown by gas exchange…
In the tracheal system of an insect
1. Air moves through spiracles (pores) on the surface of the insect
2. Air moves through tracheae
3. Gas exchange at tracheoles directly to/from cells
- Oxygen diffuses down conc. gradient to respiring cell
- Carbon dioxide diffuses down conc. gradient from respiring cells
- Adaptations: lots of thin, branching tracheoles → short diffusion pathway and SA(:V) → rapid
diffusion
- Note: rhythmic abdominal movements increase the efficiency of gas exchange by increasing the
amount of air/oxygen entering → maintains greater concentration gradient for diffusion
Adaptations of gas exchange surfaces shown by gas exchange…
Across the gills of fish
-
Counter current flow
- Blood flows through lamellae and water flows over lamellae in opposite directions
- Always a higher concentration of oxygen in water than the blood it is near
- Hence, a concentration gradient of oxygen between the water and blood is maintained
along the whole length of lamellae (/gill plate) → equilibrium not met
- Maximising diffusion of oxygen
-
-
-
Note: if the current was parallel, equilibrium would be met, so a concentration
gradient wouldn’t be maintained and oxygen wouldn’t diffuse into the blood
along the whole gill plate
Each gill is made of lots of gill filaments (thin plates) which are covered in many lamellae → gill
filaments provide a large surface area, lamellae increase surface area even more
Vast network of capillaries on lamellae → remove oxygen to maintain a concentration gradient
Thin/flattened epithelium → shorter diffusion pathway between water and blood
Adaptations of gas exchange surfaces shown by gas exchange…
By the leaves of dicotyledonous plants
-
-
Process of gas exchange in leaves
- Carbon dioxide / oxygen diffuse through the stomata
- Stomata opened by guard cells
- Carbon dioxide / oxygen diffuse into mesophyll layer into air spaces
- Carbon dioxide / oxygen diffuse down concentration gradient
Adaptations
- Lots of stomata (small pores) that are close together
- Large surface area for gas exchange / unimpaired movement of gases / gases do
not have to pass through cells to reach mesophyll
- Interconnecting air space in mesophyll layers (exchange surface)
- Gases come into contact with mesophyll cells
- Mesophyll cells have a large surface area
- Rapid diffusion of gases
- Thin
- Short diffusion pathways
Structural and functional compromises between the opposing needs for efficient gas
exchange and the limitation of water loss shown by:
Xerophytic plants
-
Thick waxy cuticle
- Increases diffusion distance → less evaporation
Stomata in pits/grooves
- ‘Trap’ water vapour → water potential gradient decreased → less evaporation
Rolled leaves
- ‘Trap’ water vapour → water potential gradient decreased → less evaporation
Spindles/needles
- Reduces surface area to volume ratio
Hairs
- ‘Trap’ water vapour → water potential gradient decreased → less evaporation
Structural and functional compromises between the opposing needs for efficient gas
exchange and the limitation of water loss shown by:
Terrestrial insects
-
Thick waxy cuticle
- Increases diffusion distance → less evaporation
Spiracles can open and close
- Open to allow oxygen in, close when water loss too much
The gross structure of the human gas exchange system limited to the alveoli, bronchioles,
bronchi, trachea and lungs
-
Trachea
Splits into two bronchi
Each bronchus branches into smaller tubes called bronchioles
Bronchioles end in air sacs called alveoli
Ventilation and exchange of gases in lungs
-
-
How does gas exchange occur in the alveoli?
- Oxygen diffuses from alveoli
- Down its concentration gradient
- Across the alveolar epithelium
- Across the capillary endothelium
- Into the blood (in haemoglobin)
- Carbon dioxide diffuses from capillary
- Down its concentration gradient
- Across the capillary endothelium
- Across the alveolar epithelium
- Into the alveoli
Why is ventilation needed?
- Maintains an oxygen concentration gradient
- Brings in air containing higher concentration of oxygen
- Removes air with lower concentration of oxygen
The essential features of the alveolar epithelium as a surface over which gas exchange
takes place
-
Squamous epithelium = thin/one cell thick
- Short diffusion pathway → fast diffusion
Large surface area to volume ratio
- Fast diffusion
Permeable
Good blood supply from network of capillaries
- Maintains concentration gradient
Elastic tissue allows it to recoil after expansion
Surfactant
How are the lungs adapted for efficient/rapid gas exchange?
-
Many alveoli/capillaries
- Large surface area → fast diffusion
Alveoli/capillary walls are thin / short distance between alveoli and blood
- Short diffusion distance → fast diffusion
Ventilation/circulation
- Maintains concentration gradient → fast diffusion
Mechanism of breathing including: role of the diaphragm antagonistic interaction
between external and internal intercostal muscles, in bringing about pressure changes
in thoracic cavity
-
-
Breathing in (inspiration)
- External intercostal muscles contract, internal intercostal muscles relax (antagonistic)
- Moving ribcage up and out
- Diaphragm muscles contract → flatten/move down diaphragm
- Increasing volume in thoracic cavity / chest
- Decreasing pressure in thoracic cavity
- Atmospheric pressure higher than pressure in lungs
- Air moves down pressure gradient into lungs
- (Active process)
Breathing out (expiration)
- Internal intercostal muscles contract, external intercostal muscles relax (antagonistic)
- Moving ribcage down and in
- Diaphragm relaxes, moves upwards
- Decreasing volume in thoracic cavity
- Increasing pressure in thoracic cavity
- Atmospheric pressure lower than pressure in lungs
- Air moves down pressure gradient out of lungs
- (Passive process)
Interpret information relating to the effects of lung disease on gas exchange and/or
ventilation
- Lung diseases affect ventilation and gas exchange in the lungs – lung function. Exams might ask
-
-
you to interpret results from the kind of tests doctors carry out to investigate lung function, for
example:
- Tidal volume → volume of air in each breath
- Ventilation rate → number of breaths per minute
- Forced expiratory volume (FEV) → maximum volume of air that can be breathed out in 1
second
- Forced vital capacity (FVC) → maximum volume of air possible to breathe forcefully out
of lungs after a deep breath in
Some examples of lung diseases and their effects
- Fibrosis
- Scar tissue in lungs → scar tissue is thicker and less elastic than normal
- Diffusion distance increased → rate of diffusion decreased
- Faster ventilation rate to get enough oxygen into lungs/blood
- Lungs can expand and recoil less → can’t hold as much air
- Reduced tidal volume
- Reduced forced vital capacity
- Asthma
- Asthma = inflamed bronchi
- Asthma attack: smooth muscle lining bronchioles contracts
- Constriction of airways – narrow diameter → airflow in/out of lungs reduced
- FEV reduced
- Less oxygen enters alveoli / blood
Reduce rate of gas exchange in alveoli → less oxygen diffuse into blood → cells receive less
oxygen → rate of aerobic respiration reduced → less energy released → fatigue, weakness etc.
Example exam question
Forced expiration volume (FEV1) is the volume of air a person can breathe out in 1 second.
Emphysema is a lung disease which results in a reduction in FEV1. Emphysema is mainly caused by longterm cigarette smoking.
Scientists investigated the effects of ageing and long-term cigarette smoking on FEV1 and on the
development of emphysema.
Figure 7 shows their results:
(a) Scientists determined the mean FEV1 value of 25-year-olds in the population.
Suggest two precautions that should have been taken to ensure that this mean FEV1 value was
reliable. (2 marks)
✓ Large sample size
✓ Individuals chosen at random
✓ Are healthy
✓ Equal number of males and females (accept: same sex)
✓ Repeat readings
(b) Explain the importance of determining a mean FEV1 value of 25-year-olds in this investigation. (2
marks)
✓ (For) comparison
✓ To see the effect of age/emphysema/smoking
OR
Takes into account outliers / anomalous results
(c) The mean FEV1 value of non-smokers decreases after the age of 30. Use your knowledge of
ventilation to suggest why. (1 mark)
✓ Internal intercostal muscle(s) less effective
✓ Less elasticity (of lung tissue)
(d) One of the severe disabilities that results from emphysema is that walking upstairs becomes
difficult.
Explain how a low FEV1 value could cause this disability. (3 marks)
✓ Less carbon dioxide removed (accept: carbon dioxide increases)
✓ Less oxygen (uptake/in blood)
✓ Less (aerobic) respiration/ ATP
OR
(More) anaerobic respiration
Example exam question
Emphysema is a disease that affects the alveoli of the lungs and leads to the loss of elastic tissue. The
photographs show sections through alveoli of healthy lung tissue and lung tissue from a person with
emphysema. Both photographs are at the same magnification.
Using the evidence given above and your own knowledge, explain why a person with emphysema is
unable to do vigorous exercise. (4 marks)
✓
✓
✓
✓
✓
✓
✓
Not enough O2
For increased respiration / for ATP needed for exercise
Reference to decreased surface area of alveoli / longer diffusion pathway
Less gas exchange / diffusion / less oxygen passes into the blood
OR
Reference to decreased elasticity / reduced elastic recoil
Meaning breathing becomes more difficult / lungs do not empty
Students should be able to: interpret data relating to the effects of pollution and
smoking on the incidence of lung disease AND analyse and interpret data associated
with specific risk factors and the incidence of lung disease AND recognise correlations
and causal relationships AND evaluate the way in which experimental data led to
statutory restrictions on the sources of risk factors
Example exam question
Scientists investigated the link between pollution from vehicle exhausts and the number of cases of
asthma. Between 1976 and 1996, the scientists recorded changes in the following
•
•
the concentration in the air of substances from vehicle exhausts
the number of cases of asthma
The graph shows their results
(a) Between which years on the graph was there
(a) (i) a positive correlation between the number of cases of asthma and the concentration in the
air of substances from vehicle exhausts? (1 mark)
✓ 1976 – 1980
(a) (ii) a negative correlation between the number of cases of asthma and the concentration in the
air of substances from vehicle exhausts? (1 mark)
✓ 1980 – 1996
(b) The scientists concluded that substances in the air from vehicle exhausts did not cause the
increase in asthma between 1976 and 1980. Explain why. (3 marks)
✓ Correlation does not necessarily mean that there is a causal relationship
✓ May be some other factor / named factor
✓ Associated with vehicles and asthma / producing rise in both
✓ (After 1980) asthma continues to rise but exhaust concentration falls / negative correlation
(after 1980)
During digestion, large biological molecules are hydrolysed to smaller molecules that
can be absorbed across cell membranes
-
Large biological molecules in food e.g. starch / proteins too big to be absorbed across cell
membranes
Digestion breaks them into smaller molecules e.g. glucose / amino acids → absorbed from the
gut to the blood
Digestion in mammals of carbohydrates by amylases and membrane-bound
disaccharidases
Digestion of starch (polysaccharide)
-
Amylase hydrolyses starch to maltose (polysaccharide to disaccharide)
- Amylase produced by salivary glands, released into mouth
- Amylase produced by pancreas, released into small intestine
Membrane bound maltase (attached to epithelial cells lining the ileum of the small intestine) →
hydrolyse maltose to glucose (disaccharide to monosaccharide)
Hydrolysis of glycosidic bond
Digestion of disaccharides
-
-
Membrane bound disaccharidases, e.g. maltase, sucrose, lactase (attached to epithelial cells
lining the ileum of the small intestine) → hydrolyse disaccharide to x2 named monosaccharides
- E.g. maltase – maltose → glucose + glucose
- E.g. sucrase – sucrose → fructose + glucose
- E.g. lactase – lactose → galactase + glucose
Hydrolysis of glycosidic bond
Digestion in mammals of lipids by lipase, including the action of bile salts
-
Bile salts produced by the liver
Bile salts emulsify lipid to smaller lipid droplets
- Increasing surface area (to volume ratio) of lipids speeds up action of lipases
Lipase made in the pancreas, released to small intestine
Lipase hydrolyses lipids → monoglycerides + fatty acids
Breaking ester bond
Monoglycerides, fatty acids and bile salts stick together to form micelles
Digestion in mammals of proteins by endopeptidases, exopeptidases and membranebound dipeptidases
-
Endopeptidases
- Hydrolyse peptide bonds within a protein / between amino acids in the central region
- Breaking protein into two or more smaller peptides
Exopeptidases
- Hydrolyse peptide bonds at the ends of protein molecules
- Removing a single amino acid
-
Dipeptidases (type of exopeptidase)
- Often membrane bound in ileum
- Hydrolyse peptide bond between a dipeptide
- = 2 amino acids
Example exam question
Suggest and explain why the combined actions of endopeptidases and exopeptidases are more
efficient than exopeptidases on their own. (2 marks)
✓ Endopeptidases hydrolyse internal peptide bonds
OR
Exopeptidases remove amino acids / hydrolyse bonds at ends
✓ More ends or increase in surface area (for exopeptidases)
Mechanisms for the absorption of the products of digestion by cells lining the ileum of
mammals, to include co-transport mechanisms for the absorption of amino acids and of
monosaccharides
1. Sodium ions actively transported out of epithelial cells lining the ileum, into the blood, by the
sodium-potassium pump. Creating a concentration gradient of sodium (higher conc. of sodium
in lumen than epithelial cell)
2. Sodium ions and glucose move by facilitated diffusion into the epithelial cell from the lumen, via
a co-transporter protein
3. Creating a concentration gradient of glucose – higher conc. of glucose in epithelial cell than
blood
4. Glucose moves out of cell into blood by facilitated diffusion through a protein channel
Example exam question
Figure 1 shows the co-transport mechanism for the absorption of amino acids into the blood by a cell
lining the ileum.
The addition of a respiratory inhibitor stops the absorption of amino acids. Use figure 1 to explain why.
(3 marks)
✓ No/less ATP produced
OR
No active transport
✓ Sodium (ions) not moved (into/out of cell) / sodium ions increase in cell
✓ No diffusion gradient for sodium (to move into cell with amino acid)
OR
No concentration gradient for sodium (to move into cell with amino acid);
Mechanisms for the absorption of the products of digestion by cells lining the ileum of
mammals, to include the role of micelles in the absorption of lipids
-
Monoglycerides and fatty acids diffuse out of micelles (in lumen) into epithelial cell
- Because lipid soluble
Monoglycerides and triglycerides recombine to triglycerides which aggregate into globules
Globules coated with proteins to form chylomicrons
Leave via exocytosis and enter lymphatic vessels
Return to blood circulation
Design and carry out investigations into the effect of a pH or bile salts on the rate of
reaction catalysed by a digestive enzyme
Example exam question:
Students investigated the digestion of lipids in milk by lipase. They set up three test tubes.
-
In tube A, milk was incubated with lipase only
In tube B, milk was incubated with lipase and bile salts
In tube C, milk was incubated with bile salts only
The results are shown in the table.
(a) The pH changed in test tube A. Explain why. (2 marks)
✓ Production of fatty acids
✓ (Fatty) acids (produced) cause fall in pH
(b) The pH did not fall below a value of 6.5 in tube A. Suggest one reason why. (1 mark)
✓ Substrate/lipids all used up
✓ Equilibrium reached
✓ (pH) denatures enzymes
(c) The rate at which the pH fell in tube A was different from the rate at which the pH fell in tube B.
Explain why pH fell at a different rate. (2 marks)
✓ Bile salts produce many small lipid droplets/emulsifies lipids
(d) Explain why test tube C set up. (1 mark)
✓ To show that lipase has to be present for pH to change/reaction to take place / to show
that bile salts do not digest lipids
Using Visking tubing models to investigate the absorption of the products of digestion
Mass transport
-
In large multicellular organisms, mass transport systems needed to carry substances between
exchange surfaces and rest of body and between parts of body
- Most cells too far away from exchange surfaces / each other for diffusion alone to
maintain composition of tissue fluid within suitable metabolic range
- Mass transport maintains final diffusion gradients bringing substances to and from cells
- Mass transport helps maintain relatively stable immediate environment of cells that is
tissue fluid
The circulatory system
-
The general pattern of blood circulation in a mammal – names only required of coronary arteries
and of the blood vessels entering/leaving the heart, lungs and kidneys
-
-
-
Closed double circulatory system – two circuits
- Blood passes through heart twice for each complete circulation of body
- Pulmonary circulation
- Deoxygenated blood in right side of heart pumped to lungs → oxygenated blood
returns to left side of heart
- Systemic circulation
- Oxygenated blood in left side of heart pumped to tissues / organs of body →
deoxygenated blood returns to right side
- Important for mammals because
- Prevents mixing of oxygenated and deoxygenated blood → so blood pumped to
body is fully saturated with oxygen → efficient delivery of oxygen and glucose for
respiration
- Blood can be pumped at a higher pressure (after being lower from lings) →
substances taken to and removed from body cells quicker and more efficiently
Coronary arteries
- Deliver oxygenated blood to cardiac muscle
Blood vessels entering and leaving heart
- Aorta – takes oxygenated blood from heart → respiring tissues
- Vena cava – takes deoxygenated blood from respiring tissues → heart
- Pulmonary artery and pulmonary vein (see below)
Blood vessels entering and leaving lungs
- Pulmonary artery – takes deoxygenated blood from the heart → lungs
- Pulmonary vein – takes oxygenated blood from the lungs →heart
Blood vessels entering and leaving kidneys
- Renal arteries – take deoxygenated blood → kidneys
- Renal veins – take deoxygenated blood to the vena cava from the kidneys
Gross structure of the human heart
-
How the structure of the heart relates to its function
- Atrioventricular valves
- Prevent backflow of blood from ventricles to atria
- Semi lunar valves
- Prevent backflow of blood from arteries to ventricles
- Left has a thicker muscular wall
- Generates higher blood pressure
- For oxygenated blood has to travel greater distance around the body
- Right has thinner muscular wall
-
Generates lower blood pressure
For deoxygenated blood to travel a small distance to the lungs where high
pressure would damage alveoli
The structure of arteries, arterioles and veins in relation to their function
-
-
-
Arteries – carry blood from heart to rest of body at high pressure
- Thick smooth muscle layer
- Contract pushing blood along
- Control/maintain blood flow/pressure
- Elastic tissue layer
- Stretch as ventricle contracts (when under high pressure) and recoil as ventricle
relaxes (when under low pressure)
- Reduces pressure surges / even out blood pressure and maintain high pressure
- Thick wall
- Withstands high pressure and prevents artery bursting
- Smooth (and thin) endothelium
- Reduces friction
- Narrow lumen
- Increases and maintains high blood pressure
Arterioles – division of arteries to smaller vessels which can direct blood to different capillaries /
areas
- Note: their structure in relation to their function is similar to that of arteries, but…
- Thicker muscle layer than arteries
- Constricts (contracts) to reduce blood flow by narrowing lumen
- Dilates (relaxes) to increase blood flow by enlarging lumen
- Thinner elastic later as lower pressure surges
Veins – carry blood back to heart under lower pressure
- Wider lumen than arteries
- Very little elastic and muscle tissue
- Valves
- Prevent backflow of blood
- Contraction of skeletal muscles squeezes veins, maintaining blood flow
Example exam question
Figure 2 shows how the blood pressure changes as blood travels from the aorta to the capillaries.
The rise and fall in blood pressure in the aorta is greater than in the small arteries. Suggest why. (3
marks)
✓ Aorta is close/directly linked to the heart/ventricle / pressure is higher
✓ Aorta has elastic tissue
✓ Aorta has stretch/recoil
Structure of capillaries and the importance of capillary beds as exchange surfaces
-
Capillaries allow the efficient exchange of gases and nutrients between blood and tissue fluid
Capillary wall is a thin layer (one cell thick) of squamous endothelial cells
- Short diffusion pathway → rapid diffusion
Capillary bed is made of a large network of (branched) capillaries (which are all thin)
- Increase surface area (to volume ratio) → rapid diffusion
Narrow lumen
- Reduces flow rate so more time for diffusion / exchange
Capillaries permeate tissues (no cell is far away from capillary)
- Short diffusion pathway
Pores in walls between cells
- Allows substances to escape e.g. white blood cells to deal with infections
The formation of tissue fluid and its return to the circulatory system
-
-
-
Tissue fluid – the fluid surrounding cells / tissues
- Provides respiring cells with e.g. water / oxygen / glucose / amino acids
- Enables (waste) substances to move back into the blood e.g. urea, lactic acid, carbon
dioxide
The formation of tissue fluid – at / nearest arteriole end of capillaries (start)…
- Higher blood / hydrostatic pressure inside capillaries (due to contraction of left ventricle)
than tissue fluid (net outward pressure/force)
- Forces fluid / water out of capillaries (into spaces around cells)
- Large plasma proteins remain in capillary (too large to leave capillaries)
The return of tissue fluid to the circulatory system - towards venule end of capillaries (end) …
- Hydrostatic pressure reduces as fluid leaves capillary (also due to friction)
- (Due to water loss,) an increasing concentration of plasma proteins (too large to leave
capillaries) lowers the water potential in the capillary below the water potential of the
tissue fluid
- Water (re-)enters the capillaries from the tissue fluid by osmosis down a water potential
gradient
- Excess water taken up by lymph system (lymph capillaries) and is returned to the
circulatory system (through veins in the neck)
-
Application examples: causes of accumulation of tissue fluid
- Low concentration of protein in blood plasma can lead to an accumulation of tissue fluid
- Water potential in capillary not as low so water potential gradient is reduced
- More tissue fluid formed at arteriole end
- Less / no water absorbed into blood capillary by osmosis
- High blood pressure can lead to an accumulation of tissue fluid
- High blood pressure = high hydrostatic pressure
- Increases outward pressure from arterial end of capillary / reduces inward
pressure at venule end of capillary
- So more tissue fluid formed / less tissue fluid is reabsorbed
- And the lymph system is not able to drain tissues fast enough
Pressure and volume changes and associated valve movements during the cardiac cycle
that maintain a unidirectional flow of blood
-
-
-
Atrial systole
- Atria contract → decreasing volume and increasing pressure inside atria
- Atrioventricular valves forced open
- When pressure inside atria > pressure inside ventricles, atrioventricular valves
open
- Blood pushed into ventricles
- (note: semilunar valves are shut)
Ventricular systole
- Ventricles contract from the bottom up → decreasing volume and increasing pressure
inside ventricles
- Semilunar valves forced open
- When pressure inside ventricles > pressure inside arteries
- Atrioventricular valves shut
- When pressure inside ventricles > pressure inside atria
- Blood pushed out of heart through arteries
Diastole
- Atria and ventricles relax → increasing volume and decreasing pressure inside chambers
- Blood from veins fills atria (increasing pressure inside atria slightly) and flows passively to
ventricles
- Atrioventricular valves open
- When pressure inside atria > pressure inside ventricles blood flows passively to
ventricles
-
-
Semilunar valves shut
- When pressure inside arteries > pressure inside ventricles
Note: the purpose of valves shutting is to prevent back flow into (named chamber / vein) to
maintain unidirectional flow of blood through the heart
Maths skill: Use and rearrange the equation cardiac output = stroke volume x heart rate
when given two measures
-
-
Can be rearranged to make 3 equations
- Cardiac output = stroke volume x heart rate
- Stroke volume = cardiac output / heart rate
- Heart rate = cardiac output / stroke volume
Cardiac output = amount of blood pumped out of the heart per minute
Stroke volume = volume of blood pumped by the ventricles in each heart beat
Heart rate = number of beats per minute
Understanding the equation: the amount of blood pumped out of the heart per minute (cardiac
output) is the volume of blood pumped out of the heart in each beat (stroke volume), multiplied
by the number of beats per minute (heart rate)
Analyse and interpret data relating to pressure and volume changes during the cardiac
cycle
-
-
Calculating heart rate from cardiac cycle data
1. Note: one beat = one cardiac cycle
2. Find the length of one cardiac cycle (human average = 0.83 secs)
3. Heart rate in beats per minute = 60 seconds / length of one cardiac cycle in seconds
(human average = 72bpm)
Interpreting if valves are open or closed, when given data on pressure in different parts of the
heart throughout a cardiac cycle…
- Semilunar valve closed
- When pressure in aorta / pulmonary artery is higher than in ventricle → prevents
backflow of blood from arteries to ventricles
- Semilunar valve open
- When pressure in ventricle is higher than in aorta / pulmonary artery → blood
flows from ventricle to aorta
- Atrioventricular valve closed
- When pressure in atrium is higher than in ventricle → prevents backflow of blood
from ventricle to atrium
- Atrioventricular valve open
- When pressure higher in ventricle than atrium → blood flows from ventricle to
atrium
-
Example:
-
-
Blood starts flowing into the aorta at A because
- When pressure inside ventricles exceeds pressure inside atria
- Shuts atrioventricular valve and opens semilunar valve
- Blood forced into aorta
Ventricular volume is decreasing at B because
- In ventricular systole, the ventricles are contracting
- Therefore the volume inside the ventricles is decreasing
The semilunar valves are closed at C because
- Ventricles are relaxing
- Pressure is higher in pulmonary than aorta
- Forces semilunar valves shut
Cardiovascular disease and risk factors
-
-
-
Example of cardiovascular disease (conditions affecting structures or function of the heart)
- Coronary heart disease
Often associated with atherosclerosis and atheroma formation
How an atheroma can result in a heart attack
- Atheroma causes narrowing of coronary arteries
- Restricts blood flow to heart muscle supplying glucose, oxygen etc.
- Heart anaerobically respires → less ATP produced → not enough energy for heart to
contract → lactate produced → damages heart tissue / muscle
Risk factor: increases probability of getting disease
- Age
- Diet high in salt or saturated fat
- High consumption of alcohol
- Stressful lifestyle
- Smoking cigarettes
- Genetic factors
High blood pressure increases risk of damage to endothelium of artery wall which increases risk
of atheroma which can cause blood clots (thrombus)
Analyse and interpret data associated with specific risk factors and the incidence of
cardiovascular disease
-
Data interpretation questions
- Describe overall trend
- Positive / negative correlation
- Linear
- Describe most obvious trend
-
Manipulate data to support your statements
- Calculations
- Work out the difference from two points
- Work out how many times greater
- Work out percentage change
Exam question example:
High blood cholesterol levels, obesity and high blood pressure are factors that increase the risk of
cardiovascular disease. The graph below shows the percentage of people with CVD who have high
blood pressure or have high blood cholesterol or are obese for the period 1960 to 1990.
Using the information in the graph, describe the overall changes that have occurred in these risk
factors during this period:
✓
✓
✓
✓
✓
(risk due to) high blood pressure has fallen overall
(risk due to) high blood cholesterol has fallen overall
(risk due to) obesity has risen overall
Obesity was the lowest risk factor but is now the highest
Credit use of manipulated figures e.g. 17% drop for high blood pressure / 16% drop for high blood
cholesterol / 10.5% increase in obesity
Evaluate conflicting evidence associated with risk factors affecting cardiovascular
disease
-
Evaluating study design: things to consider
- Small sample size
- Take into account other risk factors (variable) that could have affected results
- Used similar groups e.g. age, gender
- Way in which info collected e.g. questionnaires may be unreliable as people lie or give
inaccurate information
- Results reproduced by other scientist by carrying out more studies and collecting more
results
Exam question example:
Studies of CVD patterns between different countries suggest that there is a link between CBD and diet.
Suggest why such studies may not prove the link between CVD and diet. (2 marks)
✓
✓
✓
✓
Other variables / uncontrolled variables affect CVD
Genetic differences (between national populations)
(Countries have) environmental / life style differences
Idea that data does not provide a causal link / mechanism
Recognise correlations and causal relationships
✓ Correlation – the relationship between two variables
✓ Causation – a change in one variable will directly cause a change in the other variable
✓ However, correlation does not imply causation. There may be another variable that causes both
of these variables to change
Haemoglobin
-
-
The haemoglobins are a group of chemically similar molecules found in many different
organisms
- Chemical structure may differ between organisms e.g. sequence of amino acids in the
primary structure
Found in red blood cells (erythrocytes)
- No nucleus – contain more haemoglobin
- Biconcave shape – increase surface area for rapid diffusion/absorption of oxygen
Structure
- Quaternary structured protein – made of 4 polypeptide chains
- Each polypeptide chain contains a Haem group containing an iron ion (Fe 2+) which
combines with oxygen
How oxygen is loaded, transported and unloaded in the blood
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Haemoglobin in red blood cells carries/transports oxygen (as oxyhaemoglobin)
- Haemoglobin can carry 4 oxygen molecules – one at each Haem group
In the lungs, at a high pO2, haemoglobin has a high affinity for oxygen → oxygen readily loads /
associates with haemoglobin
At respiring tissues, at a low pO2, oxygen readily unloads / dissociates from haemoglobin
- Also, concentration of CO2 is high, increasing the rate of unloading (Bohr effect – see
further on)
The loading, transport and unloading of oxygen can be seen in relation to the
oxyhaemoglobin dissociation curve
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At high pO2, haemoglobin is saturated with O2
At low pO2, haemoglobin is less saturated with O2
The cooperative nature of oxygen binding – why the graph is ‘s’ shaped
- Haemoglobin has a low affinity for oxygen as the 1 st oxygen molecule binds
- So from 0% saturation, an increase in pO2 results in a slow increase in saturation
(shallow gradient)
- After the 1st oxygen molecule binds, the shape of haemoglobin changes in a way that
makes it easier for the 2 nd and 3rd oxygen molecules to bind too i.e. haemoglobin has a
higher affinity for oxygen
- The rate of increase in % saturation increases (between approximately 25-75%
saturation) as pO2 further increases (steep gradient)
- After the 3rd molecule binds, and haemoglobin starts to become saturated, the shape of
haemoglobin changes in a way that makes it harder for other molecules to bind too
- At a high pO2, the rate increase in % saturation decreases
The effects of carbon dioxide concentration on the dissociation of oxyhaemoglobin –
the Bohr effect
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When rate of respiration is high e.g. during exercise → releases CO2
High pCO2 lowers pH and reduces haemoglobin’s affinity for oxygen as haemoglobin changes
shape
- Increases rate of oxygen unloading
Advantageous because provides more oxygen for muscles/tissues for aerobic respiration
Oxygen dissociation curve for haemoglobin shifts to the right
Organisms can be adapted to their environment by having different types of
haemoglobin with different oxygen transport properties → enables organism to
survive better in their environment
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Curve shifted left → haemoglobin has a higher affinity for oxygen
- More oxygen associates with haemoglobin more readily (in the lungs) at the lower pO 2
BUT dissociates less readily
- Advantageous to organisms such as those living in high altitudes, underground, or
foetuses
Curve shifted right → haemoglobin has a lower affinity for oxygen
- Oxygen dissociates from haemoglobin more readily to respiring cells at a higher pO2 BUT
associates less readily
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Advantageous to organisms such as those with a high rate of respiration (metabolic rate)
e.g. small / active organisms
Example exam question
The graph shows oxygen dissociation curves for the haemoglobin of a mother and her fetus.
(a) What is the difference in percentage saturation between the haemoglobin of the mother and
her fetus at a partial pressure of oxygen (pO2) at 4 kPa?
✓ 16
(b) The oxygen dissociation curve of the fetus is to the left of that for its mother. Explain the
advantage of this for the fetus. (2 marks)
✓ Higher affinity / loads more oxygen
✓ At low/same/high partial pressure
✓ Oxygen moves from mother to fetus
(c) After birth, fetal haemoglobin is replaced with adult haemoglobin. Use the graph to suggest the
advantage of this to the baby. (2 marks)
✓ Low affinity / oxygen dissociates
✓ (Oxygen) to respiring tissues/muscles/cells
(d) Hereditary persistence of fetal haemoglobin (HPFH) is a condition in which production of fetal
haemoglobin continues into adulthood. Adult haemoglobin is also produced.
People with HPFH do not usually show symptoms. Suggest why. (1 marks)
✓ Enough adult Hb produced / enough oxygen released / idea that curves/affinities/Hb are
similar / more red blood cells produced
Xylem is the tissue that transports water in the stem and leaves of plants
The cohesion-tension theory of water transport in the xylem
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Cohesion tension theory: How water moves up the xylem against gravity via the transpiration
stream
Water evaporates from the leaves via the (open) stomata due to transpiration
Reducing water potential in the cell and increasing water potential gradient
Water drawn out of xylem
Creating tension
Cohesive forces between water molecules pull water up as a column
Water lost enters the roots via osmosis
Water is moving up, against gravity
Water is also cohesive so sticks to the edges of the column
Adaptations of the xylem (don’t specifically need to learn these)
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Elongated cells arranged end to end to form a continuous column
Hollow due to lignification so no cytoplasm/nucleus to slow water flow
End walls break down for flow
Thick cell walls with lignin
Rigid so less likely to collapse under low pressure
Waterproof preventing water loss
Pits allow lateral water movements
Narrow lumen increases height water can rise due to cohesion tension/ capillary action
Phloem is the tissue that transports organic substances in plants
The mass flow hypothesis for the mechanism of translocation in plants
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Translocation:
- Movement of solutes/ assimilates from source to sink/ one place to another
- E.g. sugars made from photosynthesis in the leaves are transported to the site of
respiration
At the source:
- High concentration of solute
- Active transport loads solutes from companion cells to sieve tubes of the phloem
- Lowering the water potential inside the sieve tubes
- Water enters sieve tubes by osmosis from xylem and companion cells
- Increasing pressure inside sieve tubes at the source end
At the sink:
- Low concentration of solute
- Solutes removed to be used up e.g. enzymes hydrolyse
- Increasing the water potential inside the sieve tubes
- Water leaves tubes via osmosis
- Lowering pressure inside sieve tubes
Mass flow:
- Pressure gradient from source to sink
- Pushes solutes from source to sink
- Solutes used or stored at the sink e.g. respiration
Adaptations of the phloem
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Sieve tube elements have no nucleus and few organelles
Companion cell for each sieve tube element to carry out the living functions for the sieve cells
i.e. ATP for active transport of solutes
The use of tracers and ringing experiments to investigate transport in plants
Interpret evidence from tracer and ringing experiments and to evaluate the evidence
for and against the mass flow hypothesis
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Use of tracers
- Supply plant with radioactive tracer such as 14C in CO2 to a photosynthesising leaf by
pumping the radioactive CO2 into a container surrounding the leaf
- 14C is incorporated into the organic substances produced by the leaf e.g. sugars via
photosynthesis
- Organic substances undergo translocation
- Autoradiography – plant killed and placed in a photographic film, film turns black where
the radioactive substance is present
- Identifies where radioactive substance has moved to and thus where the organic
substances have moved to via translocation from source to sink
- Can show this over time by taking autoradiographs at different times
Example exam question:
Scientists investigated the movement of organic substances in four plants, A, B, C and D. One leaf from
each plant was supplied carbon dioxide containing the radioactive isotope of carbon, 14C. Figure 1
shows one of these plants. Each plant was treated differently before it was supplied with the
radioactive isotope. A was ringed at position X, B at position Y, C at X and Y and D wasn’t ringed.
(a) All 4 plants were kept in bright light for one hour. Explain why. (1 mark)
✓ Rapid photosynthesis to produce radioactive sugars
(b) Figure 2 shows the distribution of 14C after one hour in one of these plants.
Which plant is shown in figure 2? Explain your answer. (3 marks)
✓ Plant A
✓ No 14C at top of plant (so ringed at X)
✓ 14C in roots (so not ringed at Y)
(c) Using the information give, explain the purpose of including plant D in the investigation. (1
mark)
✓ To compare distribution with an unringed plant / phloem present (accept: control if
qualified)
Example exam question:
One leaf on a young plant was supplied with carbon dioxide containing the radioactive isotope of
carbon. The plant was kept in bright light for one hour. The amount of radioactivity was then measured
at three places in the plant. The diagram shows the results. Only the treated leaf is shown.
(a) Suggest one explanation for the difference in the amount of radioactivity in the bud and the
roots. (2 marks)
✓ More carbohydrate transported to bud than root
✓ Carbohydrate needed for growth / high growth rate in bud
✓ Carbohydrate needed for respiration / to release energy/ATP
✓ Synthesis of molecules e.g….
(b) Suggest why some radioactivity remains in the leaf. (1 mark)
✓ Remains in organic molecules in lead / not enough time for removal
(c) Describe how a ringing experiment could be carried out to determine which tissue transports
the substances containing the radioactive carbon. (3 marks)
✓ Remove / kill phloem
✓ Named technique / method for measuring radioactivity e.g. use of radiography / analysis of
feeding aphids / Geiger counter
✓ If no radioactivity passes ring, transport is via phloem
Alternative experiment: Aphid
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Aphids pierce the phloem using mouthpiece
Releasing sap from plants
Flow of sap higher at leaves/source than further down/ sink
Evidence of a pressure gradient; higher pressure near source
Alternative experiment: Metabolic inhibitor
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Add a metabolic inhibitor to phloem
Translocation stops
Proves active transport is involved
As it requires ATP to move against a concentration gradient
Set up and use a potometer to investigate the effect of a named environmental
variable on the rate of transpiration
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Potometer estimates the transpiration rate by measuring water uptake
- Assume that water uptake is directly related to the water loss of the leaves
Method:
- Cut a shoot underwater
- To prevent air entering xylem → interrupt water flowing in column, stopping
transpiration
- Assemble potometer with capillary tube end submerged in a beaker of water
- Insert shoot underwater
- Ensure apparatus is watertight and airtight
- Dry leaves and allow time for the shoot to acclimatise
- Shut off tap to reservoir
- Remove the end of the capillary tube from the water beaker until one hair bubble has
formed, then put the tube back into the water
- Record the position of the air bubble
- Use a stopwatch to record time e.g. one minute
- Record distance moved per unit time
- Rate of air movement = estimate of transpiration rate
- Change one variable at a time and keep all other variables constant (wind, humidity, light
and temperature)
How different environmental variables affect the transpiration rate
- Light
- The higher the light intensity, the faster the transpiration rate (positive
correlation)
- Because stomata open in light to let in CO2 for photosynthesis
- Allowing more water to evaporate faster
- Stomata close when it’s dark so there is a low transpiration rate
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Temperature
- The higher the temperature, the faster the transpiration rate (positive
correlation)
- Water molecules gain kinetic energy as temperature increases
- Move faster
- Water evaporates faster
Humidity
- The lower the humidity, the faster the transpiration rate (negative correlation)
- Because as humidity increases, more water is in the air so it has a higher water
potential
- Decreasing the water potential gradient from leaf to air
- Water evaporates slower
Wind
- The windier, the faster the transpiration rate (positive correlation)
- Wind blows away water molecules from around the stomata
- Decreasing the water potential of the air around the stomata
- Increasing the water potential gradient
- Water evaporates faster