AS Biology Unit 1 Notes

Brief Biology notes for revision on all unit 1 topics. I have missed out "Causes of disease and "Enzymes and the digestive system" this is because i started my notes on paper and didn't want to repeat them. All other topics are here and complete.

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  • Created by: Moulhalel
  • Created on: 18-02-13 14:14

Resolution And Magnification

Magnification is how many times bigger the image is when compared to the object.

Magnification= Size of Image/Size of Object

To calculate the size of an object when knowing the size of the image.

Size of object= Size of Image/Size of Object

Remember to make all units of length the same for both the image and magnification. One may be in NM the other in MM

Resolution- Minimum distance apart that two objects can be in order for them to appear as separate items.

Different Microscopes have different resolutions, a greater resolution means greater clarity. Increasing magnification will increase the size of the image, but not always resolution. Every microscope will have a set limit for its resolution, increasing  the magnification will just blurr the image. 

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The electron microscope

Light microscope have poor resolution as a result of there relatively long wave length. But an electron microscope was developed that used a beam of electrons instead of light. It has 2 main advantages:

  • The electron beam has a very short wavelength, so it has a high resolution - high resolving power
  • As electrons are negatively charged the beam can be focused using electromagnets

Modern electron microscopes can resolve objects that are 0.1 nm apart, 2000 times better than a light microscope. A near vacuum atmosphere has to be created as electrons are abosrbed by molecules in the air.

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Transmission electron microscope limitations:

  • The whole system must be in a vacuum so live specimens cannot be observed
  • Complex staining process is used, even then the image is still in black and white
  • The specimen must be extremenly thin
  • The specimen may contain artifacts, these are things that appear from the way the specimen is prepared

All limitations of the TEM apply to the Scanning electron microscope (SEM) apart from the specimens dont have to be thin.

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Ultracentrifuge is the process by which the fragments in the filtered homogenate are separated in a machine called an ultracentrifuge. This spins tubes of homogenate at very high speeds in order to create a centrifugal force. For animal cells it is as follows:

  • The tube of filtrate is placed in the ultracentrifuge and is spun at a low speed
  • The heaviest organelles, the nuclei, are forced to the bottom of the tube, where they form a thin sediment or pellet
  • The fluid at the top of the tube (supernatant) is removed, leaving just the sediment of nuclei
  • The supernatant is transferred to another tube and spun in the ultracentrifuge at a faster speed than before
  • The next heaviest organelles, the mitochondria, are forced to the bottom of the tube
  • The process is continued in this way so that, at each increase in speed the next heaviest organelle is sedimented and seperated out.
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Investigating the structure of cells

Cell fractionation

Cell fractionation is the process in which cells are broken up and the different organelles they contain are seperated out. Before cell fractionation the tissue is put in a cold isotonic solution because:

  • Cold- to reduce enzyme activity, that might break down organelles
  • Isotonic- means it has the same water potential as the original tissue so the cells down burst or shrink due to osmotic gain or loss of water.
  • Buffered- to maintain pH

There are two stages to cell fractionation:

Homogenation- cells are broken up by a homogeniser(blender) organelles are realesed from the cell, the resultant fluid is known as homogenate, it is filtered to remove any complete cells or large pieces.

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Structure of an epithelial cell - Nucleus

Cells are usually designed for a particular function; each cell has an internal structure that suits its job, known as the ultrastructure of the cell, these are eukaryotic cells, they have a distinct nucleus and membrane bound organelles.

The Nucleus

  • Nuclear envelope: Is a double membrane that surrounds the nucleus. Its outer membrane is continuous with the endoplasmic reticulum of the cell - often has ribosomes on its surface. It controls the entry and exit of material in and out of the nucleus and contains the reactions taking place within it.
  • Nuclear pores: Allow the passage of large molecules, such as messenger RNA, out of the nucleus. There are typically 3000 pores in each nucleus, each 40-100 NM.
  • Nucleoplasm: Is the granular, jelly-like material it makes up the bulk of the nucleus.
  • Chromatin: Is the DNA found in the nucleoplasm, this is the diffuse form it that chromosomes take when the cell is not dividing.
  • Nucleolus: Is a small spherical body within the nucleoplasm. It manufactures ribosomal RNA and assembles the ribosomes.

The functions of the nucleus are toAct as a control centre of the cell throught the production of mRNA and hence protein synthesis. Retain the genetic material of the cell in the form of DNA or chromosomes. Manufacture ribosomal RNA and ribomes

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The Mitochondrion

Mictochondria are rod-shaped and 1-10 um in length. They are made up of the following strucutres:

Double membrance:Surrounds the organelle, the outer membrane controls the entry and exit of material. The inner membrane is folded to form extensions known as cristae.

Cristae:are shelf like extensions of the inner membrane, some of which extend across the whole width of them mitochondrion. These provide a large surface area for the attachment of enzymes involved in respiration.

Matrix: Makes up the rest of the mitochondrion. It is a semi-rigid material containing protein, lipids and traces of DNA that allows the mitochondria to control the production of their own proteins. The enzymes involved in respiration are found in the matrix.

Mitchondria produce the energy carrier molecule ATP.

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Endoplasmic reticulum (ER)

ER is an elaborate, three dimensional system of sheet-like membranes, spreading through the cytoplasm of cells. The membranes enclose flattened sacs called cisternae. There are 2 types of ER:

Rough endoplasmic reticulum (RER) has ribosomes present on the outer surfaces of the membranes. Its functions are to:

  • Provide a large surface area for the synthesis of proteins and glycoproteins
  • Provide a pathway for the transport of materials, especially proteins, throughout the cell

Smooth endoplasmic reticulum (SER) lacks ribosomes on its surface and is often more tubular in appearance. Its functions are to:

  • Synthesise, store and transport lipids
  • Synthesise, store and transport carbohydrates

Cells which need to store large quantities of carbohydrates, proteins and lipids have a very extensive ER, these cells are liver amd secretory cells.

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Golgi apparatus

Golgi apparatus can be found in most eukaryotic cells, similar to the SER, but is more compact. Stack of membranes- which make flattened sacs or cisternae with small rounded structures called vesicles. Proteins and lipids produced by the ER are sent through the Golgi apparatus in a strict sequence, these will be modified by adding non protein components e.g. carbohydrates. They are also listed so they are sent to the correct place.

Functions of the Golgi apparatus are to:

  • Add carbohydrates to rpoteins to form glycoproteins
  • Produce secretory enzymes, such as those secreted by the pancreas
  • Secrete carbohydrates, such as those used in making cell walls in plants
  • Transport, modify and store lipids
  • Form lysosomes
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Lysosomes are formed when vesicles produced by Golgi apparatus contain enzymes such as protease and lipase. Functions of lysosomes are to:

  • Break down material ingested by phagocytic cells, such as white blood cells
  • Release enzymes to the outside of the cell (exocytosis) in order to destroy material around the cell
  • Digest worn out organelles so that the useful chemicals they are made of can be re-used
  • Completely break down cells after they have died (autolysis)
  • Abundant in secretory cells and in phagocytic cells

Lysosomes are abundant in secretory cells, such as epithelial cells and in phagocytic cells due to there functions.

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Ribosomes And Microvilli

Ribosomes are small cytoplasmic granules found in all cells, They may occur in the cytoplasm or be associated with RER. There are two types, depending on the cells in which they are found:

  • 80S type - found in eukaryotic cells, around 25 NM in diameter.
  • 70S type - found in prokaryotic cells, is slightly smaller.

Ribosomes have 2 sub units, one small and one large, each which contains ribosomal RNA and protein. Ribosomes are important in protein synthesis.


Microvilli are finger-like projections of the epithelial cell that increase its surface area to allow more efficient absorption.

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Lipids are a vary group of substances that share the following characteristics:

  • They contain carbon, hydrogen and oxygen, the proportion of all these is smaller in carbohydrates.
  • They are insoluble in water.
  • They are soluble in organic solvents e.g. alcohol and acetone.
  • Main group of lipids are - triglycerides (fats and oils), phospholipids and waxes.

Role of lipids:

  • Phospholipids help the flexibility of membranes. Transfer lipid-soluble substances
  • Energy source-when oxidised it provides twice the amount of energy as carbs
  • Waterproofing-They are insoluble in water - Helps prevent water loss in animals and plants
  • Insulation-Fats are slow conductors of heat when stored under the skin-retains heat
  • Protection-Fat is often stored around delicate organs e.g. kidney for protection

Phospholipids have two heads:
A hydrophilic head - interacts with water(attracted to it) but not fat
A hydrophobic head - which orients it self away from water but mixes with fats

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The Cell-Surface Membrane

All membranes around and within cells (organelles have the same basic structure, the plasma membrane.


Phospholipids form a bilayer sheet. They are important components of the cell surface membrane for the following reasons:

  • One layer of phospholipids has its hydrophilic heads pointing inwards (interacting with the water in the cell cytoplasm).
  • The other layer of phospholipids has its hydrophilic heads pointing outwards (interacting with the water which surrounds all cells).
  • The hydrophilic tails of both phospholipids layers point into the centre of the membrane-protected, from the water on both sides.
  • Lipid soluble material moves through the membrane through phospholipids, the functions of the phospholipids are to:
  • Allow lipid soluble substances to enter and leave the cell
  • Prevent water soluble substances from entering and leaving the cell
  • Make the membrane flexible
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The Cell-Surface Membrane - Proteins

Proteins are randomly embedded in the phospholipid bilayer in two main ways:

Extrinsic proteins occur either on the surface of the bilayer or partly embedded in it, they give mechanical support to the membrane or act as cell receptors for molecules and hormones.

Intrinsic stretch across the whole phospholipid bilayer. Some act as carrier pathways to transport water soluble material across the membrane while other are enzymes.

Functions of proteins in the membrane are to:

  • Provide structural support
  • Act as carriers transporting water-soluble substances across the membrane.
  • Allow active transport across the membrane by forming ion channels for sodium, potassium.
  • Form recognitions sites by identifying cells and act as receptors e.g. for hormones. 

Fluid mosaic model for the cell surface membrane:

  • Fluid because the molecules can move relative to one another-more flexibility
  • Mosaic because the proteins embedded vary in shape,size,pattern-like a mosaic
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Diffusion is the net movement of molecules or ions from a region where they are more highly concentrated to one where their concentration is lower.

Diffusion is only possible because:

  • All particles are constantly in motion due to kinetic energy that they posses,
  • This motion is random, with no set pattern to the way the particles move around,
  • particles are constantly bouncing off one another as well as off other objects e.g. the sides of the vessel in which they are contained

There are several factors affecting the rate at which diffusion occurs:

  • Concentration gradient: The greater the difference in concentration of molecules or ions the faster diffusion Will occur, if there is little difference diffusion will be slower and vice versa
  • Area over which diffusion takes place: the larger the area of an exchange surface, the faster the rate of diffusion
  • Thickness of exchange surface: the thinner the exchange surface, the faster the rate of diffusion and vice versa
  • Temperature: molecules will have more kinetic energy. so diffusion will occur faster
  • Surface area x Difference in concentration/ length of diffusion pathway
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Facilitated Diffusion

Facilitated diffusion is a passive process, It relies only on the inbuilt motion (kinetic energy) of the diffusing molecules. There is no external input of energy. Like diffusion, it occurs down a concentration gradient, but it differs in the it occurs at specific points on the plasma membrane where there are special protein molecules. These proteins form water-filled channels (protein channels) across the membrane. These allow water-soluble ions and molecules, such as glucose and amino acids, to pass through. Such molecules would usually diffuse only very slowly through the phospholipid bilayer of the plasma membrane. The channels are selective, each opening only in the molecule is not present, the channel remains closed. In this way, there is some control over the entry and exit of substances.

An alternative form of facilitated diffusion involves carrier proteins that span the plasma membrane. When a particular molecule that is specific to the protein that is present, it binds with the protein. This causes it to change shape in such a way that the molecule is released inside the membrane. No external energy is needed for this. The molecules move sfrom a region where they are highly concentrated to one where they are less concentrated, using only the kinetic energy of the molecules themselves.

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Osmosis is the passage of water from a region where it has a higher water potential to a region where it has a lower water potential through a partially permeable membrane.

Solute molecules cannot pass through a partially permeable membrane.

If animals cells are placed in pure water they will burst as they have a negative water potential so water will move in from osmosis, animals cells do not have a cell wall to prevent this. Animals cells in a more negative water potential will cause them to shrink and shirvell.

Plant cells have a cell wall which gives extra support and protection so the cell will not burst, but the content inside of the plant cell will be pushed against the cell wall in a higher water potential. In a lower water potential the contents will pull away from the cell wall.

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Active Transport

 Active transport is the movement or ions into or out of a cell from a region of lower concentration to a region of higher concentration using ATP and carrier molecules.

Active transport differs from passive forms of transport in the following ways:

Active transport uses ATP, materials are moved against a concentration gradient- lower to higher, carrier molecules act as pumps and the process is very selective.

Carrier proteins span the cell surface membrane and accept the molecules/ions to be transported on one side of it. The molecules/ions bind to receptors on the channels of the carrier protein. On the inside of the cell, ATP binds to the protein, causing it to split into ADP and a phosphate molecule. As a result, the protein molecule changes shape and opens to the opposite side of the membrane. The molecules/ions are then released from the protein and recombines with ADP to form ATP. This causes the protein to revert to its original shape, ready for the proccess to be repeated.

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Absorption in the small intestine

Glucose is absorbed through the walls of the small intestine via the villi, they increase surface area and increase the rate of absorption. Villi are part of a specialised exchange furface apdatped for the absorption of the products of digestion, there properties increase the efficiency of absorption in the following ways:

  • They increase the surface for diffusion
  • They are very thin walled, thus reducing the dsitance over which diffusion takes place.
  • They are able to move and so help maintain a diffusion gradient
  • They are well supplied with blood vessels so that blood can carry away absrobed molecules and hence maintain a diffusion gradient.

Epethelial cells lining the villi posses micro villi.

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Role of active transport in absorption

Sodium ions are actively transported out of epithelial cells, by the sodium-potassium pump, into the blood. This takes place in one type of protein-carrier molecule found in the cell-surface membrane of the epithelial cells.

There is now a much higher concentration of sodium ions in the lumen of the intestine than inside the epithelial cells.

The sodium ions diffuse into the epithelial cells down this  concentration gradient through a different type of protein carrier ( co -transport protein) in the cell-surface membrane. As the sodium ions flood back in through this second carrier, they couple into the cell with them

The glucose passes into the blood plasma by facilitated diffusion using another type of carrier protein.

Both sodium ions and glucose molecules move into the cell, but while the sodium ions move down their concentration gradient. the glucose molecules move up their concentration gradient. It is the sodium ion concentration gradient, rather than ATP directly, that powers the movement of glucose into the cells. This make it an indirect rather than a direct form of active transport.

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Cell structure of  bacteria:

  • Cell wall: physical barrer that protects against mechanical damage and excludes certain substances
  • Capsule: Protects bacterium from other cells and helps groups of bacteria stick together for further preotection, also has a mucus layer to protect it from low PH- stomach.
  • Cell-surface membrane: acts as a differentially permeable laer which cotnrols the entry and exit of chemicals.
  • Flagellum: Aids the mvoemnt of bacterium becuase of its rigid, corcksrew shape and rotating base help the cell spin through fluids
  • Circular DNA: possess the genetic information for the replication of bacterial cells
  • Plamsid: possesss genes that aid the survival of the bacteria in adverse condiotions e.g. produces enzymes which break down antibodies

Main symptoms of cholera are diarrhoea and consequently dehydration. Vibrio cholerae is transmitted by the ingestion of water/food.

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Cholera continued

  • Almost all Vibrio cholerae bacteria ingested by humans are killed by the acidic conditions in the stomach. A few may survive due there mucus capsule, especially if the PH is above 4.5.
  • When the bacteria reach the small intestine they use their flagella to propel themselves, in a corkscrew-like fashion, through the mucus lining of the intestinal wall.
  • They then produce a toxic protein . This protein has two parts . One part binds to specific carbohydrate receptors on the cell surface membrane. As only  the epithelial cells of the small intestine have these specific receptors, this is why the cholera toxin only affects this region of the body. The other, toxic part, enters the epithelial cells. This causes the ion channels of the cell-surface membrane to open, so that the chloride ions that normally contained within the epithelial cells flood into the lumen of the intestine.
  • The loss of chloride ions from the epithelial cells raises their water potential, while the increase of chloride ions in the lumen of the intestine lowers its water potential. Water therefore flows from the cells into the lumen
  • The loss of ions from the epithelial cells establishes a concentration gradient. Ions therefore move by diffusion into the epithelial cells from the surrounding tissues, including the blood. This, in turn establishes a water potential gradient that causes water to move by osmosis from the blood and other tissues into the intestine.
  • It is this loss of water from the blood and other tissues into the intestine which causes a the symptoms.
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Oral rehydration therapy

When treating Diarrhoeal diseases drinking water is ineffective for two reasons:

  • Water is not being absorbed from the intestine. Indeed, as in the case of cholera, water is actually being lost from cells.
  • Drinking of water does not replace the electrolytes (ions) that are being lost from the epithelial cells of the intestine.

Rehydration solution needs to contain: 

Water - to rehydrate the tissues

Sodium - to replace the sodium ions lost from the epithelium of the intestine and to make optimum use of the alternative sodium-glucose carrier proteins

Glucose - stimulate the uptake of sodium ions from the intestine and to provide energy, glucose is given in the form of starch.

Potassium - such as chloride and citrate ions, to help prevent electrolyte imbalance.

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Structure of the human gas-exchange system

Main parts of the human gas-exchange system and there funtions:

  • Lungs: are a pair of lobed structures made up of a series of branched tubules, called bronchioles, which end in tiny air sacs called alveoli.
  • Trachea: is a flexible airway protected by rings of cartilage which prevent the trachea from collapsing, when air pressure falls when breathing in. Tracheal walls are made up of muscle, lined with epithelial and goblet cells. Goblet cells produce mucus, which traps dirt/bacteria, the cilia move the mucus containing dirt/bacteria up the throat where it passes down the oesaphagus into the stomach.
  • Bronchi: are two division, each leading to one lung. They also produce mucus and trap dirt and have cilia which move dirt/bacteria towards the throat. Larger bronchi are supported by cartilage, as the size of the bronchi reduces so does the cartilage.
  • Bronchioles: are a series of branching subdivisions of the bronchi. Their walls are lined with muscle, this allows them to constrict so they can control air flow in and out of the alveoli.
  • Alveoli: are air minute sacs. They contain collagen and elastic fibres, the elastic fibres allow the aveoli to expand when breathing in, they then spring in order to expel Co2. The alveolar membrane is the gas exhange surface.
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The mechanism of breathing

Inspiration takes place when the atmospheric pressure is greater than the pressure in the lungs (Inhalation). Expiration is when the lungs pressure is greater than the atmospheric pressure, air moves out (Exhalation).

  • Internal intercostal muscles, whole relaxation leads to inspiration
  • External intercostal muscles, whose contraction leads to expiration

Inspiration: External intercostal muscles contract, while internal intercostal muscles relax. Ribs are pulled upwards and outwards, increasing the volume of the thorax. Diaphragm muscles contract, causing it to flatten, which also increases the volume of the thorax. The increased volume of the thorax results in reduction of pressure in the lungs. Atmospheric pressure is now greater than pulmonary pressure, and so air is forced into the lungs.

Expiration: Internal costal muscles contract, while external intercostal muscles relax. Ribs move donwards and inwards, decreasing the volume of the thorax. The diaphragm muscles relax, making it return to its upwardly domed position,again decreasing the volume of the thorax. The decreased volume of the thorax increases the pressure in the lungs. The pulmonary pressure is now greater thena that of the atmospere, and so air is forced out of the lungs.

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Exchange of gases in the lungs

The site of gas-exchange in mammals takes place in the alveoli. A constant diffusion gradient must be maintained.

Essential features of exchange surfaces:

  • Large surface area to volume ratio-speeds up the rate of diffusion
  • Very thin exchange surface-to keep the diffusion pathway short so materials can cross rapidly.
  • Movement of environmental medium- e.g. air to maintain concentration gradient.
  • Movement of the internal medium- e.g. blood to maintain a diffusion gradient.

Diffusion between the alveoli and the blood will be very rapid because:
Red blood cells are slowed as they pass through pulmonary capilliaries as they are very narrow allowing more time for diffusion. Distance between the alveolar, air and blood cells is reduced as blood cells are flattened against the capillary walls. Alveoli and capillaries walls are very thin and therefore distance over which diffusion takes place is very short. Alveoli and pulmonary capillaries have a very large total surface area, breathing movements constantly ventilate the lungs and the action of the heart constantly circulates blood around the alveoli. Together, these ensure that a steep concentration gradient of the gases to be exchanged is maintained, blood flow through pulmonary capillaries maintains a concentration gradient.

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Lungs disease - pulmonary tuberculosis

TB is spread through air droplets in the air, when the infected cough, sneeze, laugh or talk. M.turberculosis is a resistant strand which can survive serveral weeks after the droplets have dried.

Some groups are at greater risk of contracting TB, these groups include people who:

  • Are in close contact with infected individuals for long periods of time-crowded conditions etc.
  • Work or reside in long term care facilities- old people homes, hospitals etc.
  • Are from countries where TB is common
  • Have reduced immunity - babies, the old or the sick.
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Lungs disease - pulmonary tuberculosis continued

Course of infection:

  • The bacteria grow and divide in the upper regions of the lungs where there is a plentiful supply of oxygen.
  • The body's immune system responds and white blood cells accumulate at the site of infection to ingest the bacteria.
  • This leads to inflammation and the enlargement of the lymph nodes that drain that area of the lungs. This is called the primary infection and usually occurs in children.
  • In a healthy person there are few symptoms, i any and the infection is controlled within a few weeks.However, some bacteria usually remain.
  • Many years later these bacteria may re-emerge ti cause a second infection of TB. This is called post primary TB and typically occurs in adults.
  • This infection also arises in the upper regions of the lungs, but is not so easily controlled. The bacteria destroy the tissue of the lungs. This results in cavities and, where the lung repairs itself, scar tissue.
  • The sufferer coughs up damaged tissue containing bacteria, along with blood. Without treatment TB spreads to the rest of the body and can be fatal.
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Lung disease - fibrosis

The effects of fibrosis on lung function, along with an explanation of their causes, are listed below:

Shortness of breath, especially when exercising: Large amount of air space is occupied by fibrous tissue, so less oxygen is being taken in at each breath. Also the thickened epithelium of the alveoli means that the diffusion pathway is increased and so the diffusion of oxygen is extremely slow. The loss of elasticity of the lungs makes it very difficult to ventilate the lungs, this also makes it hard to maintain a diffusion.

Chronic dry cough: The fibrous tissue creates an obstruction in the airways of the lungs. The body's reflex reaction is to try to remove the obstruction by coughing. Since the tissue is more or less immovable, nothing is expelled and the cough is described as "dry". 

Pain and discomfort in the chest: are the consequences of pressure, and hence damage, from the mass of fibrous tissue in the lungs and further damage and scarring due to coughing.

Weakness and fatigue: results from the reduced intake of ocygen into the blood. This means that the release of energy cellular respiration is reduced, leading to tiredness.

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Asthma can be triggered and made worse by several factors such as, air pollutants, exercise, anxiety/stress and allergens such as fur, faeces and pollen. This causes white blood cells in the lining of the bronchi/bronchioles to release a chemical called histamine, this has the following results:

  • The lining of these airways becomes inflamed 
  • The cells of the epithelial lining secrete larger quantities of mucus than normal
  • Fluid leaves the capillaries and enters the airways. 
  • The muscle surrounding the bronchioles contracts and so constricts the airways.

The symptoms of asthma:

  • Difficulty in breathing is due to the constriction of the bronchi and bronchioles, their inflamed linings and the additional mucus and fluid within them.
  • A wheezing sound when breathing is caused by the air passing through the very constricted bronchi and bronchioles
  • A tight feeling in the chest is the consequences of not being able to ventilate the lungs adequately because of the constricted bronchi and bronchioles.
  • Coughing is the reflex response to the obstructed bronchi and bronchioles in an effort to clear them.
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Symptoms of emphysema:

  • Shortness of breath results from difficulty of exhaling air as elasticity is lost in the lungs. Also the lungs cannot be emptied of much of their air so it difficult to breath in fresh air containing oxygen so the patient feels breathless. The smaller alveolar surface area leads to reduced levels of oxygen in the blood so the patient breaths more rapidly trying to increase oxygen supply.
  • Chronic cough is the consequence of lung damage and the body's effort to remove damaged tissue and mucus that cannot be removed naturally because the cilia on the bronchi and bronchioles have been destroyed.
  • Bluish skin coloration is due to low levels of oxygen on the blood as a result of poor gas diffusion in the lungs.

Lung functions                                                        Disease that interferes with function

  • to ventilate by inhalation                                      asthma
  • to ventilate by exhalation                                     fibrosis and emphysema
  • to provide a large surface area                             emphysema
  • to provide a short diffusion pathway                      fibrosis
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The structure of the heart

The human heart is two separate pumps, the pump on the left deals with oxygenated blood from the lungs while the pump on the right deals with deoxygenated blood from the body. Each pump has two chambers:

Atrium: Thin walled-elastic  and stretches as it collects blood- pumps blood a short distance to the ventricle so it has a thin muscular wall.

Ventricle: Thicker muscular wall as it has to pump blood a longer distance to either the lungs or the rest of the body.

Between each atrium and ventricle are valves that prevent the back flow of blood into the atria when the ventricles contract. There are two sets of valves:

  • The left atrioventricular (bicuspid) valves, formed of two cup shaped flasp on each left side of the heart.
  • The right atrioventricular (tricuspid) valves, formed of three cup shaped flaps on the right side of the heart.
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Structure of the heart continued

Each of the four chambers of the heart is served by large blood vessels that carry blood towards or away from the heart. The ventricles pump blood away from teh ehart and into the arteries. The atria receive blood from the veins.

Vessels connecting the heart to the lungs are called pulmonary vessels. The vessels connected to the four chambers:

Aorta is connected to the left ventricle and carries oxygenated blood to all parts of the body except the lungs.

Vena cava is connected  to the right atrium and brings deoxygenated blood back from the tissues of the body.

Pulmonary artery is connected to the right ventricle and carries deoxygenated blood to the lungs, where its oxygen is replenished and its carbon dioxide is removed. Unusually for an artery, it carries deoxygenated blood.

Pulmonary vein is connected to the left atrium and brings oxygenated blood back from the lungs. Unusually for a vein, it carries oxygenated blood.

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The cardiac cycle


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Heart disease - Atheroma

Atheroma is a fatty deposit that forms within the wall of an artery.

It begins as fatty streaks that are accumulations of white blood cells that have taken up low density lipoproteins (LDL).

These streaks can enlarge to form atheromatous plaque, these usually occur in large arteries, these are made up of cholesterol, fibres and dead muscle cells.

They build up in the lumen of an artery, causing it to narrow so that blood flow through it is reduced.

Atheromas increase the risk of two dangerous conditions: Thrombosis and aneurysm.

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If an atheroma breaks through the lining (endothelium) of the blood vessel, it forms a rough surface that interrupts the otherwise smooth flow of blood. This may result in the formation of a blood clot, or thrombus, known as thrombosis. This may block or reduce blood flow to tissues beyond it. The region of tissue deprived of blood often dies due to the lack of oxygen, glucose and other nutrients.

The thrombus can break off and can be carried to different places where it lodges in and blocks other arteries.

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Atheromas can lead to the formation of a thrombus and weaken the artery walls. These weakened points swell to form a balloon like blood filled structure called an aneurysm. Aneurysms frequently burst, leading to a hemorrhage and there fore loss of blood to the region of the body served by that artery. A brain aneurysm is known as a stroke.

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Myocardial infarction (Heart attack)

This is known as a heart attack, myocardial infarction means reduced supply of oxygen to the muscle of the heart. It results from a blockage in the coronary arteries. If this occurs close to the junction of the coronary artery, the symptoms will be milder because a small area of muscle will suffer oxygen deprivation. If this occurs to a main blood vessel than the results can be deadly as the heart will be deprived of ocygen, glucose and nutrients and will die.

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Risk factors associated with coronary heart diseas

Smoking: Carbon monoxide combines easily with haemoglobin in red blood cells to form carboxyhemoglobin. This means less oxygen will be supplied to the body and muscles, so the heart must work harder resulting in raised blood pressure increasing the risk of CHD and in exercise this can cause chest pain.

Nicotine stimulates the production of the hormone adrenaline, which increases heart rate and raises blood pressure, results in an increased chance of CHD or a stroke. Also nicotine makes red blood cells more sticky leading to a higher chance of strokes, thrombosis and heart attack.

Factors effecting high blood pressure: Genes, stress, unhealthy diet, lack of exercise. High blood pressure increases the risk of heart disease because: 

  • There is already a higher pressure in arteries, so the heart must work harder to pump blood.
  • Higher blood pressure within the arteries means that they are more likely to develop an aneurysm and burst, causing haemorrhage.
  • To resist the higher pressure within them, the walls of the arties  tend to become thickened and may harden, restricitng the flow of blood.
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Defence mechanisms

The human body has a range of defences to protect itself from pathogens. They are of two main types:

Non-specific mechanisms that do not distinguish between one type of pathogen and another, but respond to all of them in the same way. The mechanism take two forms: 

A) a barrier to the entry of pathogens  B) phagocytosis

Specific mechanism that do distinguish between different pathogens. The responses are less rapid but provide long-lasting immunity. The responses are less rapid but provide long-lasting immunity. The responses involve a type of white blood cell called a lymphocyte and again take two forms:

A) cell-mediated responses involving T lymphocytes  B) humoral responses involving B lymphocytes

Lymphocytes already exist about 10 millions of them, so there is a high probability that when a pathogen enters the body that there is a lymphocyte will have a complementary protein for the protein on the pathogen, then that specific lymphocyte can build up its numbers to effectively destroy the pathogen.

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Phagocytosis (Non-specific response)

Barriers preventing entry of pathogens take a number of different forms in humans, including:

a protective covering. The skin covers the body surface, providing a physical barrier that most pathogens find hard to penetrate.

epithelia covered in mucus. Many epithelial layers produce mucus, which acts as a further defence against invasion. In the lungs, pathogens stick to this mucus, which is then transported away by cilia, up the trachea, to be swallowed into the stomach.

Hydrochloric acid in the stomach. This provides such a low pH that the enzymes of most pathogens are denatured and there the organisms are killed.

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Phagocytosis (Non-specific response) continued


  • Chemical products of the pathogen act as attractants causing phagocytes to move towards the pathogen
  • Phagocytes attach themselves to the surface of the pathogen
  • They engulf the pathogen to form a vesicle, known as a phagosome
  • Lysosomes move towards the vesicle and fuse with it
  • Enzymes within the lysosomes breakdown the pathogen. The process is the same as that for the digestion of food in the intestines, namely hydrolysis of larger, insoluble molecules into smaller, soluble ones.
  • The soluble products from the breakdown of the pathogen are absorbed into the cytoplasm of the phagocyte.

The phagocytosis causes inflammation at the site of infection. This swollen area contains dead pathogens and phagocytes, which are known as pus. Inflammation is the result of the release of histamine, which causes dilation of the blood vessels. This in turn, speeds up the delivery of phagocytes to the site of infection.




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T cells and cell-mediated immunity ( Specific resp

An antigen is an organism or substance that is recognised as being foreign by the immune system and stimulates an immune response. Antigens are usually proteins that are part of the cell surface membrane, cell walls of invading cells e.g. micro-organsims, diseased cells and cancer cells.

Lymphocytes - There are two types of lymphocytes, each with its own immune response:

B lymphocytes (B cells) are associated with humoral immunity, i.e immunity involving antibodies that are present in body fluids, or "humour". THese mature in bone marrow

T lymphocytes (T cells) are associated with cell-mediated immunity, i.e. immunity involving body cells. These mature in the thymus gland.

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Cell- mediated immunity

T lymphocytes respond to an organisms own cells that have been invaded by non-self material e.g. a virus or cancer cell. They also respond to transplanted material which is genetically different. T lymphocytes can distinguish between normal and invader cells:

  • phagocytes that have engulfed and broken down a pathogen present some of the pathogens antigens on their own cells-surface membrane
  • body cells invaded by a virus also manage to present some of the viral antigens on their own cell-surface membrane, as sign of distress.
  • caner cells likewise present antigens on their cells-surface membranes.

T lymphocytes response to infection by a pathogen:

  • pathogens invade body cells or are taken in by phagocytes
  • The phagocytes places antigens from the pathogen on its cell surface-membrane
  • Receptors on certain T helper cells fit exactly onto these antigens
  • This activates other T cells to divide rapidly by mitosis and form a clone. The cloned T cells: Develop into memory cells that enable a rapid response to future infections by the same pathogen. Stimulate phagocytes to engulf pathogen by phagocytosis. Stimulate B cells to divide and kill infected cells.
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B cells and humoral immunity

B cells can become one of two clones:

Plasma cells secrete antibodies. They only survive for a few day, but each can make around 2000 antibodies a second. These antibodies destroy any pathogens and toxins. Plasma cells are responsible for the immediate defence of the body against infection, known as primary immune response.

Memory cells live longer than plasma cells, they circulate in blood fluid until they encounter the same antigen at a later date, they divide rapidly and form into plasma cells and more memory cells. The plasma cells produce antibodies to kill the pathogen. Memory cells provide long term immunity. This is known as a secondary immune response. It is more rapid and and of greater intensity than the primary immune response.

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B cells and humoral immunity continued

The role of B cells in humoral immunity:

  • The surface antigens of the invading pathogen are taken up by B cells.
  • The B cells process the antigen and present them on their surfaces
  • T helper cells attach to the processed antigens and on the B cells thereby activating them
  • The B cells are now activated and divide by mitosis to give a clone of the plasma cells
  • The cloned plasma cells produce antibodies they exactly fit the antigens on the pathogens surface
  • The antibodies attach to antigens on the pathogen and destroy them. Primary response
  • Some B cells develop into memory cells. These can respond to future infections by the same pathogen by dividing rapidly and developing into plasma cells that produce antibodies. This is the secondary immune response.
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Passive immunity is produced by the introduction of antibodies into individuals from an outside source. As the antibodies are not being produced by the individuals themselves, they are not replaced when they are broken down in the body and so the immunity is generally short lived.

Active immunity is produced by stimulating the production of antibodies by the individuals' own immune system. It is generally long-lasting.

The success of a vaccination programme depends on a number of factors:

  • Economically available in sufficient quantities.
  • A few side effects, if any.
  • Easy to produce, store and transport.
  • Easy to administer with train staffed across the population and the majority of the population should be vaccinated if not all. If not possible and then at least the vulnerable

Why a vaccine does not eliminate a disease: Vaccination fails to induce immunity in certain individuals, individuals may develop the disease immediately after vaccination before their immunity levels are high enough to prevent it. The pathogen may mutate or there may be many varieties, pathogens may hide within cells, religious objections. 

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