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Summary Salters Nuffield A Level Biology - Topic 1 - Lifestyle, Risk & Health
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Edexcel A Biology A level, Salters Nuffield from 2015 Summary notes for Topic 1
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TOPIC 1: LIFESTYLE, HEALTH & RISK 1Circulation
Why do animals have a heart and circulation?
● Very small organisms have short diffusion distances and large SA/V ratios so diffusion is an efficient
way to move substances, such as oxygen and carbon dioxide, around. Diffusion is fast enough to meet
the organism’s needs.
● Animals have a circulatory system to overcome the limits of diffusion (increasing diffusion distance,
increasing requirements)
● Mass flow system = transports substances in bulk down a pressure gradient. It enables substances to
move efficiently over long distances.
● Features of a mass flow system:
○ Circulatory fluid
○ Contracting pump to move fluid
○ Tubes which fluid can circulate
Open circulation
● Blood not fully contained in blood vessels
● Most arthropods + some molluscs
● When heart relaxes, blood is drawn back into
the heart through valves
● Hemolymph = blood mixed with tissues fluid
Closed circulation
● Blood enclosed in blood vessels
● Blood kept at high pressure throughout body
● Oxygen conc. Gradient high because oxyg. &
deoxy. blood do not mix
Single circulation - blood passes through heart once
● Organs in series
● Heart pumps deoxygenated blood to the gills, where gas exchange takes
place and there’s diffusion of CO2 into the water and O2 into the blood
Double circulation - blood passes through heart twice
● 2 circuits: pulmonary and systemic:
○ Blood flows through the heart twice so this gives
blood returning from the lungs an extra ‘boost’,
reducing the time it takes for blood to circulate
around the whole body
○ Birds and mammals can have a high metabolic rate
as oxygen and food required for metabolic
processes can be delivered more rapidly to cells
and meet the organism’s needs
● Organs arranged in parallel:
○ All organs receive the same blood pressure
○ Oxygen concentration is kept high for all organs
● Each circuit will have a different blood pressure:
○ The pulmonary circuit will have a lower blood
pressure because cardiac muscle on the right is
thinner.
○ This prevents any damage to the vessels in the
lung due to high pressure
, TOPIC 1: LIFESTYLE, HEALTH & RISK 2
Circulation in amphibians
● 3 heart chambers
● Double circulation
● Oxygenated blood and deoxygenated mix, but
this doesn't work for humans because:
○ We are warm blooded and therefore have
a higher demand for O2
○ Humans are more active and therefore
have higher requirements
Water as a transport medium
● Good solvent for other polar molecules (salt)
○ V-shaped structure leads to uneven charge distribution so water is a dipole and a
polar molecule
○ Hydrophilic molecules can be transported by water easily. NaCl dissolves in water
because Cl- is attracted to the positive ends and Na+ is attracted to negative ends
○ Vital biochemical reactions can take place in the aqueous cytoplasm
● Bad solvent for hydrophobic, non polar molecules
○ Lipids form a bilayer membrane in water: heads, facing outwards, are hydrophilic while tails are
hydrophobic
○ Lipids can only be transported in blood as lipoproteins
● High specific heat capacity: lots of E required to raise temp because hydrogen bonds are strong
○ Good temperature buffer: organisms maintain steady temperature even when temperature in
surroundings vary
● High latent heat of vaporisation: lots of energy to evaporate water
○ Organisms can cool down effectively as sweat evaporates without losing a lot of water
● High latent heat of fusion: lots of energy has to be lost for it to freeze
○ Organisms can survive in cold places without freezing
● High boiling point: liquid at most temperatures
○ Useful transport medium in mass flow systems
● Higher density than ice: molecules are further apart in liquid water than ice
○ Ice forms at the surface of water, allowing organisms below to survive
● Transmits visible light: transparent and colourless
○ Water plants can receive light for PS
● Low compressibility: resist high pressure
○ Allows buoyancy and organisms to survive in deep oceans
● High cohesion: molecules stick to one another by H bonds
○ High tensile strength: can travel in unbroken columns
○ High surface tension: insects can walk on water
● High adhesion: attracted to surfaces
○ Capillary action: water drawn into small spaces such as xylem vessels, travelling up columns
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