These notes got me an A* in OCR A-Level Biology. These notes were made to be bite-sized and straight to the point. Best part? They incorporate MARK SCHEMES answers (in blue) from previous OCR past paper questions so you can get an exact idea of what the exam wants from you. There's also a column fo...
3.1.1 Exchange surfaces
(a) the need for specialised Small organisms have a large SA: V → short diffusion pathway → diffusion is
exchange surfaces To include quick →smaller metabolic demands → diffusion alone is sufficient for exchange
surface area to volume ratio (SA: V),
metabolic activity, single-celled and Larger, multicellular organisms have a small SA: V → diffusion distance is too
multicellular organisms large → diffusion is too slow → higher metabolic demand → so diffusion alone is
insufficient for → needs surface area for exchanges
If told to calculate SA: V → find the SA and V → Then, put it in the form of SA: 1
Cube Length SA V SA : V
2cm 2 × 2 × 6 = 24 2×2×2=8 24 : 8
3:1
3cm 3 × 3 × 6 = 54 3 × 3 × 3 = 27 54 : 27
2:1
(b) the features of an efficient Features of an Efficient Exchange Surface:
exchange surface. To include, • 1) Large surface area
increased surface area – root hair 2) Thin layer → short diffusion pathway
cells • thin layer – alveoli • good 3) Good blood supply → maintains steep conc gradient
blood supply/ventilation to maintain 4) Ventilation → maintains steep conc gradient
gradient – gills/alveolus.
Root Hair Cells:
● Many root hairs → large SA for absorption of water (by osmosis) &
mineral ions (by active transport)
How Root Hair Cells absorb water
● Lower WP inside root hair Alveoli:
cell (1) ● Large no. of alveoli → large SA for gas exchange (1)
● Caused by active transport ● Small size → large SA:V (1)
of ions into the root hair cells ● One cell thick → thin → short diffusion distance (1)
(1) ● Squamous epithelium cells are thin → short diffusion distance (1)
● Good blood supply from capillaries → maintains conc gradient (1)
● Elastic → recoils & helps with ventilation (1)
● Ventilated → maintains steep conc gradient as air is continually replaced
Gills:
● Many lamellae → large SA for gas exchange
● Thin filaments → short diffusion distance between blood and water
● Good blood supply → maintains conc gradient
● Ventilated → maintains steep conc gradient → fresh water constantly
passes over them
(c) the structures and functions of Structure of Gas exchange System: (i.e in the lungs)
the components of the mammalian Air moves through the trachea → bronchi → bronchioles → alveoli (gas ex.)
gaseous exchange system. To Intercostal muscles, diaphragm & ribcage help to move air in/ out
,include the distribution and functions Function: get O2 into the blood and remove CO2 out of the blood
of cartilage, ciliated epithelium,
Goblet cells Secrete mucus (1) → traps pathogens + dust from reaching
goblet cells, smooth muscle and
alveoli
elastic fibres in the trachea, bronchi,
bronchioles and alveoli. PAG1 Ciliated Secrete mucus (1) and beats rhythmically to remove mucus
HSW8 epithelium containing the pathogens & dust up the throat to be swallowed
Elastic Recoils (1) helps expel air (1) prevent bursting (1)
tissue/ When breathing in, elastic fibres stretch and when breathing
Elastic fibres out, elastic fibres relax to push air out.
Smooth Constricts airways (1) by controlling diameter of blood vessels
muscle SM contracts → walls constrict (closes up)
SM relaxes → walls dilate (opens up) + less resistance to
airflow & air moves in/ out easily
Cartilage Holds airways open (1) Reduces friction (1) & provides the
walls support & prevents it from collapsing when breathing out
Bronchioles ● NO cartilage
● NO goblet cells
● Smooth muscle
● Elastic fibres
Alveoli ● Elastic fibres
● Squamous epithelium cells
A Level Biology Revision "Gas Exchange in Mammals"
(d) the mechanism of ventilation in Air moves from high pressure to low pressure, down a pressure gradient
mammals. To include the function of
Inspiration (inhaling) Expiration (exhaling)
the rib cage, intercostal muscles
(internal and external) and Intercostal muscles contract (1) Intercostal muscles relax (1)
diaphragm. Ribcage moves up and out (1) Ribs move down and inwards (1)
Diaphragm moves down Diaphragm moves up (1)
A Level Biology "Breathing in … Volume inside thorax increases (1) Volume of thorax decreases (1)
Pressure inside thorax decreases (1) Pressure inside thorax is more than
Atmospheric pressure is greater than the atmospheric pressure (1)
pressure in thorax so air moves in (1) Air moves out, down a pressure
, Air leaves down pressure gradient (1) gradient (1)
Requires energy Passive (no energy required) unless
forced expiration → intercostal muscle
contracts & rib moves down and in
→active + requires energy
Tidal volume: volume of air inhaled or exhaled (1) in one breath (1)
Vital capacity: max. volume of air (1) inhaled/ exhaled in one forced breath (1)
Inspiratory Reserve Volume: extra air that can be breathed in
Expiratory Reserve Volume: extra air that can be breathed out
Residual volume: volume of air left in the lungs after exhaling as hard as possible
Total capacity: vital capacity + residual volume
Breathing rate: how many breaths taken in a set time (e.g per minute)
Oxygen uptake: the rate at which oxygen is used up
Spirometer: person breathes into a spirometer, through a tube connected to the
oxygen chamber → lid of chamber moves up & down → movements recorded by
Quiz: a spirometer trace or a data logger → soda lime (NaOH) absorbs CO2
https://www.purposegames.com/game/spirometry- ● Total volume of gas in chamber decreases over time as O2 is being used
graph-game
up and CO2 is being absorbed by the soda lime.
(e) the relationship between vital
capacity, tidal volume, breathing rate Describe how the spirometer would be used to measure tidal volume. (3)
and oxygen uptake To include ● Breathe normally (1) Measure the height of wave (1) Measure at least 3
analysis and interpretation of waves and calculate a mean (1)
primary and secondary data e.g. Describe how you could use a spirometer to measure the rate of oxygen
from a data logger or spirometer. uptake. (3)
M0.1, M0.2, M0.4, M1.3 PAG10 ● Measure decrease in volume of O2 in chamber (1) find difference in
HSW2, HSW3, HSW4, HSW5, height between one peak & the next (1) measure time taken (1) divide by
HSW6 time taken (1)
Suggest a chemical that could be used in the chamber to absorb CO2. (1)
● Sodium hydroxide (soda lime)
Explain why a person using the spirometer to measure their vital capacity
should wear a nose clip. (2)
● Otherwise, makes results invalid (1) to ensure all the air inhaled comes
from the chamber (1)
State two other precautions that should be taken when using a spirometer
to measure vital capacity. (2)
● Fresh air (1) clean mouthpiece (1)
Risk assessment (2)
● Oxygen used (1) health of volunteers (1) soda lime working (1)
Suggest why it is not possible to expel all the air from the lungs. (2)
● Lungs/ ribcage cannot be completely compressed (1)
● Trachea is held open by cartilage (1)
● Bronchioles are held open by elastic fibres (1)
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