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AQA A level Biology Topic 3 - Organisms exchange substances with their environment notes (A* achieved)

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Grade achieved: A* I have put so much hard work into making this set of notes. Achieving grade A* and constantly getting top grades at school proved that my hard work paid off. My notes are mostly based on 10+ years of mark schemes from past papers, and some from revision guides + online res...

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  • April 14, 2022
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AQA AS Biology notes Topic 3 – Organisms exchange substances with their environment
Topic 3 – Organisms exchange substances with their environment
Table of Content

3.1 Surface area to volume ratio..............................................3
3.2 Gas exchange.....................................................................3
In tracheal system of an insect...................................................................................................................3
....................................................................................................................................................................5
Across gills of fish (gill lamellae & filaments including counter-current principle)...................................5
By leaves of dicotyledonous plants............................................................................................................7
Structural and functional compromises between the opposing needs for efficient gas exchange and
the limitation of water loss shown by….....................................................................................................7
Gross structure of human gas exchange system limited to alveoli, bronchioles, bronchi, trachea and
lungs...........................................................................................................................................................7

3.3 Digestion and absorption.................................................10
Starch / polysaccharide digestion............................................................................................................13
Disaccharides digestion............................................................................................................................13
Lipid digestion..........................................................................................................................................14
Protein digestion......................................................................................................................................14
Absorption of amino acids & monosaccharides (Co-transport)..............................................................15

3.4 Mass transport.................................................................16
3.4.1 Mass transport in animals..................................................................16
Haemoglobin............................................................................................................................................16
Oxyhaemoglobin dissociation curve........................................................................................................17
Heart.........................................................................................................................................................18
Cardiac cycle.............................................................................................................................................20
Artifical heart............................................................................................................................................22
Data interpretation questions..................................................................................................................23
Circulatory system....................................................................................................................................23
Tissue fluid................................................................................................................................................25

3.4.2 Mass transport in plants.....................................................................26
Investigate effect of named environmental variable on rate of transpiration........................................27
Investigate transport of plants.................................................................................................................30



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, AQA AS Biology notes Topic 3 – Organisms exchange substances with their environment




2

, AQA AS Biology notes Topic 3 – Organisms exchange substances with their environment

3.1 Surface area to volume ratio
Relationship between size of organism & its SA:V
 Smaller organisms tend to have a higher SA:V than larger organisms


Relationship between SA:V (& thus size of organism) & metabolic rate
 ↑ SA:V, ↑ rate of heat loss per gram of tissue
 So they need higher metabolic rate / faster respiration to generate
enough heat to maintain a constant body temp to replace heat lost

Organisms with no transport system / specialised Why do large / active organisms need a transport
exchange surfaces system?
 Unicellular & multicellular  Large
 Few layers of cells o Small SA:V ratio
o Large SA:V ratio o Long diffusion distance
o Short diffusion distance o Slow rate of diffusion
o High rate of diffusion  Active
 Not very active o High metabolic rate (so high demand for
o Low metabolic rate O2 to remove CO2)
o Don’t need lots of nutrients to carry out o Need lots of nutrients to carry out
function function
o Little waste produced o More waste produced
 Need transport system to speed up diffusion




3.2 Gas exchange
Adaptations of gas exchange surfaces shown by gas exchange…

Across body surface of a single-celled organism / unicellular
 Thin, fat shape
 Large SA:V, short diffusion pathway / distance (all parts of cell are small distance away from
exchange surfaces)
o Rapid diffusion eg O2 / CO2


In tracheal system of an insect

1. Air moves via spiracles
(pores) on surface of insect
2. Air moves via tracheae
3. Gas exchange at tracheoles
directly to / from cells
a. O2 diffuses down
concentration gradient to
respiring cells
b. CO2 diffuses down
concentration gradient from
respiring cells
3

, AQA AS Biology notes Topic 3 – Organisms exchange substances with their environment


Resting tissue Active tissue
 Low respiration rate so no lactic acid in  Lactic acid ↓ osmotic potential in cells
tissue  Tracheal fluid withdrawn out of tracheoles into
 Tracheal fluid seeps into tracheoles from respiring cell (muscle tissue) by osmosis
cells  ↓ air pressure in tracheoles
 O2 from air in tracheoles dissolves in fluid &  Air moves in to replace fluid & gases diffuse more
moves slowly via fluid into cell rapidly across gas filled tracheoles
 CO2 diffuses from cells into fluid then into air
in tracheoles

Explain movement of O2 into gas exchange system of insect when it's at rest (3)
 O2 used in aerobic respiration so O2 diffuses down concentration gradient from fluid into cell


Explain how insect's tracheal system is adapted for efficient gas exchange (3)
 Tracheoles have thin → short diffusion distance to cells
 Highly branched tracheoles → large SA (for gas exchange) & short diffusion distance to cells
 Tracheae provide tubes full of air → fast diffusion (into insect tissue)
 Fluid in the end of tracheoles moves out into tissues during exercise → large SA / faster diffusion via
air to gas exchange surface
 Body can be moved by muscles to move air → maintains concentration gradient for O 2 / CO2
diffusion
 Rhythmic abdominal movement → more efficient gas exchange between tracheoles & muscle
tissue coz O2 enters quickly so maintains diffusion gradient

Explain why water is always lost from gas exchange surfaces of terrestrial organisms (2)
(Terrestrial animals that live entirely on land)
 Gas exchange surfaces are permeable (to small molecules)
 Water diffuses from inside animal to out down concentration gradient


Why rate of water loss during gas exchange is very low in most insects? (4)
 Spiracles on surface of insect limits exposure of respiratory surface & can be closed
 Trachea / tracheoles inside insect limits exposure of respiratory surface
 Sunken spiracles / hair around spiracles trap moist air / reduce water potential gradient
 Trachea cuticle lined so only lose water via tracheoles


Explain why increase CO2 conc increases water loss
 CO2 in tracheae so spiracles open, allowing water vapour to escape




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