OCR AS BIOLOGY: Exchange and Transport Spec Notes

Specification notes I made for AS Biology OCR. 

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  • Created on: 06-04-12 15:21
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Module 1.2: Exchange and Transport
Explain, in terms of surface-area-to-volume ratio, why multicellular organisms need specialised
exchange surfaces and single celled organisms do not.
Multi cellular organisms need more nutrients and gases than a single celled organism and also the
substances need to travel a greater distance from the surface of the organism to the centre.
Because the multicellular organisms have a small surface area to volume ratio it cannot rely on
diffusion because it is not fast enough or efficient enough. Therefore the multicellular organism
needs transport systems.
Describe the features of an efficient exchange surface with reference to diffusion of oxygen and
carbon dioxide.
An efficient exchange surface (like the alveolus) needs to have a large surface area so that large
amounts of molecules can diffuse in and out of the alveoli. They need a thin barrier (1 cell thick)
to reduce the diffusion distance. There needs to be a constant supply of new molecules in the
capillaries surrounding the alveolus and there needs to be a removal of required molecules
(oxygen) in the body to keep the concentration low. All features except for the first help
maintain a steep diffusion gradient.
Describe the features of the mammalian lung that adapt it to efficient gas exchange.
The lungs have a large surface area which allows more molecules of oxygen and carbon dioxide
to pass through. The plasma membranes around cells allow diffusion of oxygen and carbon
dioxide. Thin barriers in vessels and alveoli and close contact between the two allow barrier of
diffusion to be as small as possible making diffusion faster. Lungs produce surfactant to reduce
cohesive forces in water molecules and it stops the alveoli collapsing.
Outline the mechanism of breathing (inspiration and expiration) in mammals, with reference to
the function of the rib cage, intercostals muscles, and diaphragm.
In inspiration the diaphragm contracts and becomes flatter which pushes digestive organs down.
The external intercostals muscles contract and raise the ribs. The volume of the chest cavity
increase and the pressure drops below 1atmp so air moves into the lungs.
In expiration the diaphragm relaxes and is pushed up by the organs underneath. The external
intercostals muscles relax and the ribs fall, the volume of the chest cavity decreases. The
pressure rises above atmospheric pressure and air moves out of the lungs.
Describe the distribution of cartilage, ciliated epithelium, goblet cells and smooth muscle and
elastic fibres in the trachea, bronchi and bronchioles and alveoli of the mammalian gaseous
exchange system.

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Ciliated Goblet cells Smooth Elastic fibres
epithelium muscle
Trachea Lots of it yes yes yes yes
Bronchi Less than yes yes yes yes
trachea but
still there
bronchioles Large ones no no yes yes
yes, small
ones no.
alveoli No No No no no
Describe the functions of cartilage, cilia, goblet cells, smooth muscle, and elastic fibres in the
mammalian gaseous exchange system.
Cartilage Holds trachea and bronchi open. Prevents collapse during inspiration with low
pressure. Some flexibility because it is in C shape.…read more

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Vital capacity is the largest volume of air that can be moved into and out of the lungs in any one
breath. Approx 5 dm3 .
Describe how a spirometer can be used to measure vital capacity, tidal volume, breathing rate
and oxygen uptake.
A spirometer has a chamber filled with oxygen which floats on a tank of water. When the person
breathes in from a mouthpiece the inspiration makes the container move down and expiration
makes in the container move up.…read more

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Mammals need to keep themselves warm so
need even more energy.
Explain the meaning of the terms single and double circulatory systems with reference to the
circulatory systems of fish and mammals.
A single circulatory system is like 1 loop. In a fish the blood flows from heart -> gills -> body ->
heart. The double circulatory system is in two loops.…read more

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Diastole: The heart fills with blood, the muscles in the heart are relaxed and the atrioventricular
valves are open with the semi lunar valves shut.
Atrial Systole: The atria contract, blood moves from atria to ventricles, the AV valves open and
the SL valves shut.
Ventricular systole: Ventricles contracts, blood moves from ventricles into aorta and pulmonary
artery, the AV valves are shut and SL valves are open.…read more

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Open Circulatory system Close Circulatory system
Insects have open systems so the blood never Fish have a closed system. Blood stays inside
gets out of body cavity. Blood is not always vessels always. Tissue fluid bathes the blood in
held in vessels. The blood fluid bathes the nutrients that the cells need. The enables the
tissues and cells to give them the nutrients. heart to pump at higher pressure and so the
Blood enters heart through ostia pores and
blood travels faster throughout the body.…read more

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Tissue fluid doesn't contain most of the cells in blood or plasma proteins. It has to carry oxygen
and nutrients in the blood to cells and needs to carry carbon dioxide and wastes back to the
Lymph is what is formed when tissue fluid bathes the cells. Not all of the fluid returns to the
capillaries and some are drained into the lymphatic system. These drain the excess fluid into
large vessels which eventually rejoin the blood system in the chest cavity.…read more

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Explain the significance of the different affinities for oxygen of fetal haemoglobin and adult
Fetal haemoglobin has a high affinity for oxygen than that of adult haemoglobin. In the placenta
the fetal haemoglobin absorbs the oxygen from the fluid in the mother's blood. This reduces the
oxygen tension within the blood fluid which makes the adult haemoglobin release oxygen. So
the oxyhaemoglobin dissociation curve is to the left of the curve for the adults.
Describe the role of haemoglobin in carrying carbon dioxide.…read more

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In a multicellular plant organism the cells on the outside of the plant close to the environment
would gain all they need by simple diffusion but because there are many cells the cells inside the
plant would not receive enough water or nutrients to survive. In plants roots can obtain ware
easily but cannot absorb sugars from the soil, and leaves can produce sugars but cannot obtain
water from the air.…read more

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Describe the structure and function of xylem vessels, sieve tube elements and companion cells.
Xylem vessels are long dead cells from end to end. They are hollow and are impregnated by
lignin which waterproofs the xylem as well as making it strong and preventing collapse. The lignin
forms patterns which allow flexibility and in some places there are pits or bordered pits which
allow the water to leave one vessel and enter into another or pass into living parts of the plant.…read more


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