AS Biology Edexcel revision notes, unit 1, Topic 1

7/29/2019 AS Biology Edexcel revision notes, unit 1, Topic 1

1/9

Topic 1

Circulatory system

Cardio vascular diseases (CVDs) are diseases of the heart and circulation.

Open circulatory system:

Blood circulates in large open spaces A simple heart pumps blood out into cavities surrounding theanimals organs Substances can diffuse between the blood and the cells When the heart muscle relaxes, blood is drawn from the cavity back into the heart

through small valve openings along its length.

Closed circulatory system:

Blood is enclosed in vessels The blood leaves the heart under high pressure and flowed along the arteries and

then arterioles to capillaries

There are large numbers of capillaries which come into close contact with mostcells in the body

After passing along the capillaries, the blood returns to the heart by means ofvenules and then veins.

Advantages of closed circulatory system:

Blood travels faster due to high pressure which means more efficient at deliveringsubstances around the body quickly.

Oxygen supplied at quicker rate meaning able to meet high metabolic demands.

Heart

Veins Venules Capillaries

ArteriolesArteries


2/9

Single circulatory system (e.g. in fish):

The heart pumps deoxygenated blood to the gills. Here gaseous exchange takes place (diffusion of C02 from blood to water, and

diffusion of 02 into the blood from the water).

After leaving the gills the blood flows around the rest of the body before returningto the heart again.

Double circulatory system:

The right ventricle of the heart pumps deoxygenated blood to the lungs where itreceives oxygen.

The oxygenated blood then returns to the heart to be pumped by the left ventricleout to the rest of the body.

Circulation

The transport medium:

In a circulatory system a liquid and all the particles it contains are transported inone direction in a process known as mass flow.

Blood is made up of plasma (main component), red blood cells, white blood cellsand platelets.

Plasma is mainly water and contains dissolved substances such as food, gases,amino acids, enzymes, proteins, hormones etc.

Properties of water that make it a good transport medium:

Water is an excellent solvent for ions and polar molecules, the chemicals are thenfree to move around and react with other chemicals and most processes taking

place in living organisms, happen like this in solution.

Used in condensation reactions. Hydrogen bonds in water are very strong. This means that a relatively large amount

of energy is required to increase the temperature of water (it has a high specific

heat capacity).

Due to their high water content, the bodies of organisms are also slow to changetemperature and this makes maintaining a stable body temperature easier. No

drastic changes in body temperature.


3/9

The heart and blood vessels

The heart consists of:

Aorta (from heart to body) Pulmonary artery (from heart to lungs) Pulmonary vein (from lungs to heart) Left and right atrium Left and right ventricle Atrio-ventricular valves (separate the atriums and ventricles) Semi-lunar valve (separates the ventricles from the aorta) Inferior vena-cava (takes deoxygenated blood from the lower body to the heart) Superior vena-cava (takes deoxygenated blood from head and arms to the heart)

Blood Vessels:

Arteries Veins

Narrow lumen- increased blood pressure to reach eachcapillary in the body.

Thicker muscle wall- to withstand high pressure of theblood

More collagen, elastic fibre and smooth muscle-

allows for stretching and recoiling as blood passesthrough.

No valves- blood is only travelling in one direction atspeed, unlikely that there would be backflow

Wide lumen- doesnt need to be at highpressure

Thinner muscle walls

Less collagen, elastic fibres and smooth

muscle- blood travelling at a constant not at highpressure

Valves- as blood travels slower in veins itprevents back flow of blood ensuring the blood

reaches the heart.

Capillaries: only one cell thick and join the arterioles to the venules. This is where gas

exchange takes place between the cells of the body and the blood.

How blood moves through the vessels:

Every time the heart contracts (systole) blood is forced into the arteries and theirwalls stretch to accommodate the blood flow.

During relaxation of the heart (diastole) the elasticity of the walls allows them torecoil behind the blood pushing the blood forward.

How valves work:

Skeletal muscles contract once the blood passes through the veins. Contractionpushes blood forward and opens the valves

When the muscles relax, the valves close to prevent backflow.


4/9

How the heart works

There are four chambers of the heart which alternately contract and relax in a

sequence known as the cardiac cycle. Contraction of a chamber is known as

systole and relaxation is diastole.

Phase 1: Atrial systole

Blood under low pressure flows into the left and right atrium from thepulmonary veins and vena cava

As the atria fill up, pressure against the atrio-ventricular valves startspushing them open and blood starts leaking into the ventricles

The atria then contract forcing the remaining blood into the ventricles.Phase 2: Ventricular systole

The ventricles contract from base upwards increasing the pressure in theventricles

This pushes blood up and out through the arteries opening the semi-lunar valves.

The pressure of the blood against the atrio-ventricular valves closesthem and prevents back flow into the atria.

Phase 3: Diastole

Atria and ventricles then relax during diastole Elastic recoil lowers pressure in the atria and ventricles Blood under high pressure is drawn back to the ventricles closing the

semi-lunar valves

The coronary arteries fill during diastole Low pressure in the atria helps draw blood into the heart from the veins.

The closing of the atrio-ventricular and semi-lunar valves is what creates the

characteristic sound of the heart beat.


5/9

What is atherosclerosis?

Atherosclerosis is a disease process that leads to coronary heart disease and

strokes.

In atherosclerosis fatty deposits can either block an artery directly, or increase

its chance of being blocked by a blood clot (thrombosis).

Stages of atherosclerosis:

Endothelium lining in an artery becomes damaged (e.g. due to high bloodpressure or toxins from smoking cigarettes)

Damage causes the inflammatory response where white blood cells leave theblood vessel and move to the artery wall. The white blood cells accumulate

chemicals from the blood, particularly cholesterol. A deposit, called atheroma, builds up Calcium salts and fibrous tissue also build up at the site, resulting in a hard

swelling (plaque) - this means the artery wall loses some of its elasticity.

Plaques also cause the artery to become narrower- makes it difficult for theheart to pump blood around the body and as a result can lead to high blood

pressure.

Positive feedback builds up- plaques lead to raised blood pressure and raisedblood pressure makes it more likely that more plaques will form.

Blood clotting process:

When blood vessel walls are damaged, a blood clot is more likely to form. When platelets come into contact with a damaged vessel wall they change from

flattened discs to spheres with long thin projections.

This change causes them to stick to the exposed collagen in the wall and each otherand form a temporary platelet plug

They also release substances that activate more platelets.The direct contact of blood with collagen triggers a series of chemical changes in the

blood:

Soluble plasma protein (prothrombin) is converted into thrombinthrombin catalyses the conversion of another soluble plasma protein call fibrinogen

into long soluble strands of fibrinthe fibrin strands form a mesh that traps blood cells to form the clot


6/9

Cardiovascular disease

Identifying risk factors

Risk factors that increase the chance of getting CVD:

High blood pressure Obesity Blood cholesterol and other dietary factors Smoking Genetic inheritance

Some of these can be controlled.

The risk of CVD is higher for men than woman in the UK. The risk of CVD also

increases with age.

High blood pressure

Elevated blood pressure (hyper tension) is one of the most common factors in

the development of cardio vascular disease.

Blood pressure is the measure of hydrostatic force of the blood againstthe walls of the blood vessel.

Blood pressure is measured using a sphygmomanometer; it measures thesystolic pressure and diastolic pressure within the arteries.

SI unit is mmHg Shown by placing the systolic pressure over the diastolic pressure In a healthy person a systolic pressure between 100 and 140 is expected

and a diastolic pressure between 60 and 90 is expected.

Any factor which causes arteries or arterioles to constrict will lead to high blood pressure

or hypertension.

These include:

Loss of elasticity with age Atherosclerosis Adrenaline High sodium diet


7/9

Carbohydrates

Sugars

General formula= Cx(H20) n Sugar and starch most familiar carbohydrate Monosaccharides are single sugar units, disaccharides are single sugars which have

combined in a condensation reaction and polysaccharides are long straight orbranched chains of sugar units.

When sugars join together via carbon 1 and carbon 4 on another, a 1, 4 glycosidic

bond is formed and produces water.

Monosaccharides provide a rapid source of energy. They are readily absorbed and

require little or no change before being used in cellular respiration.

The glycosidic link between two sugar units in a disaccharide can be split by

hydrolysis. Hydrolysis of carbohydrates takes place when carbohydrates aredigested in the gut, and when carbohydrate stores in a cell are broken down to

release sugar

Polysaccharides:

There are three main polysaccharides that are found in food: starch and cellulose in

plants and glycogen in animals.

Starch and glycogen is an energy storage molecule within cells. They are suitable forstorage as they are compact with low solubility in water.

Starch is made up of amylose and amylopectin.

Amylose:

a polymer of glucose forming a straight chain 1, 4 glycosidic links between adjacent glucose molecules Chain is coiled into a helix shape

Amylopectin:

A polymer of glucose with side branches 1, 6 glycosidic links branched not coiled

Good for storage as it is insoluble and therefore does not diffuse across cell membranes

and has little osmotic effects within a cell, amylose helix form is compact and the

branches of the amylopectin allows compound to be easily hydrolysed to release glucose

monomers quickly.

Cellulose is known as dietary fibre. It has an important function as it helps the movement

of material through the digestive tract.


8/9

Glycogen is used instead of starch for storage in bacteria, fungi and animals. It is similar

to amylopectin but has more side branches. Numerous side branches means that it can be

rapidly hydrolysed giving easy access to stored energy. (In humans it is stored in the liver

and muscles).

Lipids

Lipids enhance the flavour and palatability of food.

They are insoluble in water but soluble in organic solvents such as ethanol.

The most common lipids we eat are triglycerides:

Used as energy stores in plants and animals Made up of one glycerol molecule and three fatty acids Glycerol molecule and fatty acids linked by condensation reaction The bond between the glycerol and fatty acids is called an ester bond Three ester bonds in one triglyceride

Saturated fats: if the fatty acid chains in a lipid contain the maximum number

of hydrogen atoms they are saturated. There are no double bonds.

Unsaturated fats: monounsaturated fats have one double bond between two

of the carbon atoms in the chain. Polyunsaturated fats have a larger amount

of double bonds.

Cholesterol is a short lipid molecule. Important for cell membranes, hormones

and bile salts. Made in the liver from saturated fats and also obtained in out

diet. Too much can lead to a high blood cholesterol level.

Energy Balance

A constant supply of energy is needed to maintain your essential body processes. The

amount of energy needed for this is called the basal metabolic rate (BMR)

BMR is higher in:

Males Heavier people Younger people More active people

Body mass index (BMI) is a way of classifying body weight relative to a persons height.

If one of the fatty acids in a triglyceride is replaced with a phosphate group, a

phospholipid is formed. These molecules make up part of the cell membrane.


9/9

Cholesterol

Cholesterol is not soluble in water. To be transported in the bloodstream it is

combined with proteins to form soluble lipoproteins.

Two types of lipoproteins:

Low density lipoproteins (LDLs)

Main cholesterol carrier in the blood Triglycerides from saturated fats in our diet combine with the cholesterol

and protein to form LDLs

they circulate and are stored in the blood stream and bind to receptorsites on cell membranes

they consist of more cholesterol than proteinHigh density lipoproteins (HDLs)

Made up of more protein than cholesterol hence the high density Are made when triglycerides from unsaturated fats combine with

cholesterol and protein

They transport cholesterol from the body tissues to the liver where it isbroken down.

It helps lower blood cholesterol and helps remove plaques in arteriesReducing risk of CVD

The risk of getting CVD can be reduced by:

Stopping smoking Maintaining a normal blood pressure (below 140/85 mmHG) Maintaining a low blood cholesterol level Maintaining a normal BMI/ low waist-to-hip ratio Doing more physical exercise Reduced or no consumption of alcohol

Ways of controlling blood pressure:

ACE inhibitors- antihypertensive drug Calcium channel blockers Diuretics

Reducing cholesterol levels:

Statins are the main drug used. They inhibit an enzyme involved in the productionof LDL cholesterol.

AS Biology Edexcel revision notes, unit 1, Topic 1

Documents

Transcript of AS Biology Edexcel revision notes, unit 1, Topic 1