Summary Biology Edexcel SNAB A level Notes ALL TOPICS A*
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Course
EDEXCEL SNAB A LEVEL BIOLOGY
Institution
PEARSON (PEARSON)
Edexcel A LEVEL SNAB Biology
Contains notes on all topics in the specification
Helped me to achieve an A*
Easy to understand notes
Key words
Diagrams to aid understanding
Broken down into topics and subtopics
Topic 1 – Lifestyle, health and risk
Transport of substances
Small organisms have a large surface area: volume ratio (unicellular organisms such as amoeba) as they can
meet their demand through diffusion
however as the size of the organism increases:
- Surface area to volume ratio decreases
- Diffusion distance increases
- Metabolic rate increases (input and output increase)
Therefore diffusion becomes insufficient so larger organisms have mass transport systems which move
substances over long distances by mass flow and allows exchange of substances to take place
Features of a mass transport system:
- A network to move through – vessels
- A medium for movement (fluid) – blood
- Controlled direction – pressure gradient of blood controlled by valves in heart
- Maintenance of speed – contraction of heart and elasticity of arteries maintains pressure gradient
single circulatory system (fish) - mix of oxygenated and deoxygenated blood,
blood flows at a low pressure,
one atrium and one ventricle, heart only transports deoxygenated blood,
bloods flows in a single pathway
double circulatory system (humans) – oxygenated and deoxygenated blood do
not mix, separated (maintains high concentration gradient), blood flows at a
high pressure, two atria and two ventricles, both oxygenated and
deoxygenated blood circulates through the heart and flows in two pathways:
Systemic circulation – blood between the heart and rest of the body, high pressure
Pulmonary circulation – blood between the heart and lungs, low pressure
closed system (all vertebrates) - more efficient as it has different tubes/vessels that have different roles
(specialised), blood vessel generate higher blood pressures - more efficient
open system (invertebrates) – simple heart pumps blood into cavities, substances diffuse between blood and
cells, less efficient
Blood vessels
Tunica
Tunica intima – endothelium that lines the lumen intima
Tunica media – smooth muscle, elastic fibres
Tunica externa – collagen fibres, connective tissue
Tunica
Tunica externa
media
Arteries
- Carry oxygenated blood except pulmonary artery
- Small lumen maintains high blood pressure
- Smooth tunica intima – less resistance for blood flow
- Lots of collagen fibres – strength and support, withstand high pressures
- Arteries nearest the heart have more elastic fibres – stretch and recoil
- Arteries furthest from heart have a greater proportion of muscle tissue – dilate and constrict arteries
,Veins
- Carry deoxygenated blood except pulmonary vein
- Large lumen reduces resistance to flow
- Thinner layer of muscle and elastic fibres – less stretch and recoil
- Reduced collagen fibres – lower pressures
- Semilunar valves close to prevent backflow of blood
Capillaries
- Thin endothelium (one cell thick) – short diffusion distance and fit between cells
- Narrow lumen
- blood flows slower in the capillaries due to their narrow lumens causing more of the blood to be
slowed down by friction against the capillary wall – allows for exchange between the blood and
surrounding cells
Heart and cardiac cycle
The heart is a double pump made of cardiac muscle
Cardiac muscle is supplied with blood through its own coronary
circulation - coronary arteries, veins and a network of capillaries
Contraction of the myocardium generates pressure changes which
results in the constant flow of blood from an area of high pressure to an
area of low pressure, unless flow is blocked by a valve
Cardiac muscle is thicker on the left side of heart as higher pressure is
needed to pump blood further around the whole body
Events on the right and left sides of the heart are the same, but
pressures are lower on the right
Cardiac cycle:
Atrial systole
- Heart is full of blood and ventricles are relaxed
- Atrial walls contract, forces blood into ventricles through atrioventricular (tricuspid and bicuspid)
valves
Ventricular systole
- Atria relax
- Ventricle walls contract, forces blood out - ventricular pressure increases as av valves shut – first
heart sound ‘lub’
- Pressure of blood opens semi-lunar valves, blood flows into aorta and pulmonary arteries
Diastole
- Ventricles relax and pressure falls below that of arteries
- Blood under high pressure in arteries causes semilunar valves to shut – second heart sound ‘dub’
- All the muscle in the heart relaxes
- Blood from vena cava and pulmonary veins enter the atria
* cycle starts again
, Systole – pumping, period of contraction –
systolic pressure
Diastole – filling, period of relaxation – diastolic
pressure
Ventricular contraction is stronger than atrial
contraction – pumps blood into aorta and
pulmonary artery – higher pressure
Blood moves from high to low pressure at each
stage of the cycle
Atherosclerosis
The hardening of arteries caused by the buildup of fibrous plaque – atheroma
Cause of many cardiovascular diseases – myocardial infarction, stroke
Blocks blood supply resulting in permanent damage to tissue
1. Endothelial dysfunction – artery wall becomes damaged – high cholesterol, smoking, high BP
2. Inflammatory response – monocytes turn into macrophages which form foam cells – accumulate LDL
3. Foam cells die and release LDL which causes fibrous tissue to accumulate – plaque formation
4. Outer portions of plaque become calcified and harden
5. Artery narrows and becomes less flexible, elastic – atheroma forms
6. Increases blood pressure further as there is less space for blood to flow –
positive feedback plaque leads to raised BP and this makes it more likely that
further plaques will form as endothelial dysfunction is more likely
Blood clotting (thrombosis)
7. Damaged tissue due to atheroma rupture
8. Platelets attach to exposed collagen fibres on artery wall – platelet plug forms
9. Platelets release thromboplastin which triggers the conversion of prothrombin (protein - inactive)
into thrombin (enzyme - active) – clotting factors are released for the conversion – Ca2+ and vitamin K
10. Thrombin catalyses the conversion of soluble fibrinogen into insoluble fibrin
11. Fibrin forms a network of fibres which traps platelets and red blood cells– insoluble blood clot
the blood clot can cause a complete blockage within the artery preventing blood flow meaning that oxygen is
not delivered to the tissues downstream of the artery. This means that cells within the tissue cannot carry out
aerobic respiration – start to die
angina – chest pain caused when the cardiac muscle is working harder and needs to respire more and as the
heart muscle lacks oxygen, it is forced to respire anaerobically
if the heart muscle is not supplied with blood it is ischaemic – permanent damage if muscle cells are starved
, arrythmia is when the heart beat irregularly, can lead to heart failure
If the blockage happens in a coronary artery – heart attack
If it occurs in a blood vessel leading to the brain – stroke
If it occurs in an artery in the legs – deep vein thrombosis
Risk factors of CVD
High blood pressure (hypertension)
- Contact between blood and the walls of the vessels causes friction and this impedes blood flow –
peripheral resistance
- More strain on the artery walls – more likely to suffer damage which can lead to atherosclerosis
- A diet high in salt, high alcohol consumption, inactivity and stress can increase blood pressure
Smoking
- Carbon monoxide in cigarette smoke binds to haemoglobin in red blood cells which reduces the
amount of oxygen that can be transported in the blood - heart has to pump faster to get same
amount of oxygen – increases blood pressure
- Reduces the levels of antioxidants in the blood which can protect arteries from endothelial damage
- Nicotine in cigarette smoke makes platelets more sticky, higher chance of blood clot and blockage in
arteries – narrow arteries, less blood flow
Weight
- being overweight puts greater strain on the heart and increases blood pressure
- more fat in arteries and around organs
- blood lipid levels increase, greater chance of type 2 diabetes, greater risk of CVD
- positive correlation between obesity (excess fat, BMI >30.0), increased blood pressure and CVD
Exercise
- a sedentary lifestyle can increased the risk of CVD by increasing blood pressure
- regular aerobic exercise lowers resting HR, lowers BP, lowers cholesterol and lipoproteins in the blood
Stress
- prolonged stress can lead to fight or flight response – cortisol and adrenaline are released
- cause elevated blood pressure and heart rate which can put strain on the arteries – damage could
lead to atherosclerosis
Diet
- High in saturated fats and low in unsaturated fats can increase the risk of CVD
- Consumption of saturated fats increases LDL cholesterol levels – builds up to form plaque
Gender
- Oestrogen provides women with some protection from CVD before menopause (decrease in
oestrogen levels)
- It increases HDL cholesterol and decreases LDL cholesterol, promotes blood clotting, smooths and
dilates blood vessels so blood flow increases
Age
- As you get older, the elasticity and width of arteries decreases
- Arteries become more easily damaged – atherosclerosis
Genetics
- Inherit tendency to: high blood pressure, poor cholesterol metabolism, arteries that are more easily
damaged, relative HDL: LDL levels in the blood
- In familial hypercholesterolaemia (FH), mutations in the LDLR gene cause the LDL receptors not to
form which means that the removal of LDL from the blood is less efficient – atheroma can form
Identifying risk factors for CVD
Cohort studies follow a large group of people overtime to see who develops the disease and who does not
The exposure to risk factors is compared to look for correlations and draw conclusions
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