OCR Cells, exchange and transport

Revision guide for OCR Biology, Unit 1 (modules 1 and 2).


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  • Created on: 20-01-11 14:25
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Revision notes for biology
Special surfaces for exchange
To keep all cells functioning properly and alive they need...
Oxygen for aerobic respiration
Glucose as a source of energy
Protein for growth and repair
Fats to make membranes and to be a store of energy
Minerals to maintain water potential & to help enzymes action and other
aspects of metabolism
Substances can be absorbed by organisms from the surrounding environment, or
they make them inside their cytoplasm as part of cell metabolism, living cells
MUST be able to take up substances from their environment.
Living cells need to remove waste products from metabolic activities occurring in
their cytoplasm.
Single cells and small organisms can exchange gases, nutrients and wastes across
their outer surfaces; they have a large surface area to volume ratio. When
mulitcellular organisms become larger their surface area to volume ratio becomes
smaller, therefore need a transport system.
Good exchange surfaces
Large surface area provides more space for molecules to pass through
thin barrier; reduces diffusion distance
fresh supply of molecules
removal of required molecules on other side to keep low concentration
The three final features on list are important to help maintain a steep diffusion
Examples of exchange surfaces:
small intestine = where nutrients are absorbed
liver = where level of sugars in blood adjusted
root hairs of plants = where water & minerals absorbed
hyphae of fungi = where nutrients absorbed
The lungs as a organ of exchange
The lungs have a large surface area = provides more space for molecules to pass
through. Individual alveoli are about 100-300 micrometres across. It is said that
the lungs have a total surface area of about 70m2
The plasma membranes that surround the thin cytoplasm of the cells from the
barrier to exchange. These readily allow the diffusion of oxygen and carbon dioxide.
There are a number of adaptations to reduce the distance the gases have to diffuse:
alveolus wall is 1 cell thick
capillary wall 1 cell thick
both walls consist of squamous cells

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A thin layer of moisture lines the alveoli, moisture passes through the cell
membranes from cytoplasm of the alveolus cells. As we breathe out it evaporates
and is lost. The lungs must produce a substance called surfactant to reduce cohesive
forces between the water molecules. With out this the alveolus would collapse due
to cohesive forces between molecules lining the air sac.…read more

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Tissues in the lungs
The trachea, bronchi and bronchioles are airways that always allow passage of
air into the lungs and out again.…read more

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Much narrower than bronchi, the larger bronchioles may have some cartilage but
smaller ones have no cartilage. The wall is made mostly of smooth muscle and
elastic fibres. The smallest have clusters of alveoli at their ends.…read more

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Normally about 5 dm3
- Tidal lung capacity ­ The amount of air your lungs hold during normal
breathing; the amount of air moved in and out of the body in one breath. Normally
about 0.5 dm3
- Expiratory reserve volume ­ the extra air pushed out by the forced
Vital capacity of lungs = TV + IRV +ERV
Transport in animals
The movement of oxygen, nutrients, hormones, waste and heat around the body.…read more

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Respiration is important to produce heat; therefore oxygen and nutrients
need to get to cells quickly; double circulation allows this.
The structure of the mammalian heart
Muscular pump which creates pressure to propel blood through arteries &
around body.
Pressure in arteries produced due to contraction of the left ventricle walls.
Divided into 2 sides.…read more

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Atria- muscle is very thin as these chambers do not need to create much pressure.
They push blood into the ventricles.
Right ventricle- walls thicker than atria, enables atria to pump blood out of heart.
Pumps deoxygenated blood to lungs, pressure must be kept down to prevent the
capillaries in lung bursting.
Left ventricle ­ walls are 2-3 times thicker than right ventricle. Blood is pumped
out through the aorta and needs sufficient pressure to overcome the resistance of
the systemic circulation.…read more

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Note : When pressure in major arteries is higher than pressure in ventricles, semi
lunar valves are closed.
Control of the cardiac cycle
Myogenic ­ can innate its own contraction, e.g. the heart, it will contract and relax
rhythmically even if it is not connected to the body.
Atrial muscles contract and a higher frequency than the ventricular muscle.
Fibrillation occurs when contractions of chambers are not synchronized, so the
heart needs a mechanism that can coordinate in all 4 chambers.…read more

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We of depolarization spreads from SAN over atria
2) Ventricles depolarize, contract and pump blood into ventricles
3) Wave reaches AVN and transmitted down bundle of his
4) Wave transmitted along purkinje fibres into ventricles
5) Ventricles depolarize, contract and pump blood into arteries
6) Atria and ventricles depolarize & relax
7) Atria fill with blood from vena cava and pulmonary vein
monitors electrical activity of heart
ECG used to diagnose problems with heart
P wave= wave of depolarization that spreads from SAN across…read more

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A depressed ST segment can indicate lack of oxygen to heart (myocardial
ischaemia) the drug digoxin can also cause a depressed ST segment. This could also
indicate a heart attack or angina.
Arrhythmias is the condition when the heart beat is too fast or too slow. Some are
considered normal others life threatening.
Ventricular fibrillation is the absence of coordinated contraction of the ventricles
caused by chaotic electrical activity. It is a life threatening condition.…read more


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