Cell Exchange

Plasma Membranes

Lipids

• Easily dissolved in organic solvents but not in water
• Triglycerides (fats and oils)
  • Also called triacylglycerides (TAG)
  • Consists of 3 fatty acids linked by ester bonds to glycerol
    • Require 3 condensation reactions (but are not polymers!)
    • Glycerol contains 3 -OH groups
    • One fatty acid contains a -COOH group
  • Excess energy available from food is stored as TAG
  • Can be broken down to yield energy when needed
  • Contain twice as many energy stored per unit of weight as carbohydrates
• Saturated fatty acids
  • -COOH group without double bonds in the carbohydrate chain
  • May cause blockage of arteries which can lead to strokes and heart attacks
  • High melting point / solid at room temperature (fats) / typical animal fats
• Unsaturated fatty acids
  • -COOH group with double bonds in the carbohydrate chain
  • Low melting point / liquid at room temperature (oils)
  • Found in plants
• Phospholipids
  • Found in cell membrane
  • Formed by replacing one fatty acids in a triglyceride with a phosphate group
  • Phosphate is polar / hydrophilic / does mix with H2O
  • Fatty acid tails remain non-polar / hydrophobic / insoluble, does not mix with H2O

Fluid-Mosaic Model

• Membranes consist of a phospholipid bilayer studded with proteins, polysaccharides, lipids
• The lipid bilayer is semipermeable - H2O and some small, uncharged, molecules (O2, CO2) can pass through
• Phospholipids have two parts
  • "Head": hydrophilic → attracts and mixes with H2O
  • Two "fatty acid tails": hydrophobic

Diffusion

• Uses energy from moving particles (kinetic energy)
• Substances move down their conc. gradient until the conc. are in equilibrium
• Fick's law → rate of diffusion across an exchange surfaces (e.g. membrane, epithelium) depends on
  • Surface area across within diffusion occurs (larger)
  • Thickness of surface (thinner)
  • Difference in conc. gradient (larger)
  • (surface area * difference in conc.) / thickness of surface
• Microvilli
  • Extensions of the plasma membrane
  • They increase the surface area of the membrane
  • Accelerate the rate of diffusion
• Temperature increases rate of diffusion due to increasing K.E. (kinetic energy)

Facillitated Diffusion

• Transmembrane proteins form a water-filled ion channel
  • Allows the passage of ions (Ca2+, Na+, Cl-) down their conc. Gradient
  • NB: this is a passive process → no ATP required
  • Some channels use a gate to regulate the flow of ions
  • Selective permeability → not all molecules can pass through selective channels
• Transport mechanism
  • Carrier protein binds to substrate (specific molecule)
  • Molecule changes shape
  • Release of the diffusing molecule (product) at the other side of the membrane
• Example
  • If you want to move a muscle, a nerve impulse is sent to this muscle
  • The nerve impulse triggers the release of a neurotransmitter
  • Neurotransmitter binds to a specific transmembrane protein
  • The protein opens channels that allow the passage of Na+ across the membrane
  • In this specific case, this causes muscle contraction
  • These Na+ channels can also be opened by a change in voltage

Osmosis

• Special term used for the diffusion of water through a differentially permeable cell membrane
• Water is polar and able to pass through the lipid bilayer
• Transmembrane proteins that form hydrophilic channels accelerate osmosis, but water is still able to get through membrane without them
• Osmosis generates pressure called osmotic pressure
  • Water moves down its conc. gradient
  • When pressure is equal on both sites net flow ceases (equilibrium)
  • The pressure is said to be hydrostatic (water-stopping)

Water Potential

• Measurement of ability or tendency of water molecules to move
• Water potential of distilled water is 0, other solutions have a negative water potential
• Hypotonic: solution with a lower conc. of solute / gains water by osmosis
  • Solution is more dilute
  • Cells placed in a solution which is hypotonic will grow as water moves in
  • Red blood cells will swell and burst if it is in a hypotonic solution
  • Plant cells are unable to burst due to their strong cellulose wall
• Hypertonic: solution with a higher conc. of solutes / loses water by osmosis
  • Cells will shrink in hypertonic solutions (eg red blood cells)
• Isotonic: solutions being compared have equal conc. of solutes
  • Cells which are in an isotonic solution will not change their shape
  • The extracellular fluid of the body is an isotonic solution
• Molecules collide with membrane / creates pressure, water potential
• More free water molecules, greater water potential, less negative
• Solute molecules attract water molecules which form a "shell" around them
  • water molecules can no longer move freely
  • less "free water" which lowers water potential, more negative

Active Transport

• Movement of solute against the conc. gradient, from low to high conc.
• Involves materials which will not move directly through the bilayer
• Molecules bind to specific carrier proteins / intrinsic proteins
• Involves ATP by cells (mitochondria) / respiration
  • Direct active transport - transporters use hydrolysis to drive active transport
  • Indirect active transport - transporters use energy already stored in gradient of a directly-pumped ion
• Bilayer protein transports a solute molecule by undergoing a change in shape (induced fit)
• Occurs in ion uptake by a plant root; glucose uptake by gut cells

The Absorption of Glucose from the Small Intestine

The chemical digestion of carbohydrates results in the production of monosaccharides such as glucose. These need to be absorbed by the small intestine and passed into the bloodstream for use by the body. The process of diffusion alone would not result in all of the glucose present in the small intestine being absorbed as an equilibrium would be reached and any remaining glucose would pass out of the body in the faeces. Our digestive systems have evolved to absorb all of the glucose produced.

Glucose is therefore absorbed by the small intestine using an active process. It is considered an active process because ATP is required for it to happen. However it uses the ATP indirectly as it is the movement of sodium ions which actually powers the movement of glucose into the cells. It is also an example of CO-TRANSPORT because two molecules (glucose AND sodium) are involved.

1. Sodium ions are actively transported out of the epithelial cell into the blood by the sodium potassium ATPase. This protein pump is present in the membrane of all eukaryotic cells.
2. Sodium ions are now at a lower concentration in the epithelial cell than in the lumen of the small intestine.
3. Sodium ions now diffuse down their concentration gradient through a co-transport protein present in the plasma membrane of the epithelial cell. The energy released as the sodium ions move down their concentration gradient allows glucose molecules to pass through the co-transporter too despite the epithelial cell having a higher concentration of glucose than the lumen of the small intestine.
4. The glucose now passes into the blood via facilitated diffusion.

Cholera

Prokaryotic Organisms - Bacteria

• Bacteria are prokaryotes
  • Nucleus (5µm)
    • Contains chromosomes (genes made of DNA which control cell activities)
    • Separated from the cytoplasm by a nuclear envelope
    • The envelope is made of a double membrane containing small holes
    • These small holes are called nuclear pores (100nm)
    • Nuclear pores allow the transport of proteins into the nucleus
  • Undergo asexually reproduction by binary fission / 2 identical daughter cells
• Classification
  • Most bacteria require oxygen to survive: aerobic bacteria
  • Bacteria that are growing in the absence of oxygen: anaerobic bacteria
• Grow best at optimum conditions (human body)
  • Constant temperature
  • Neutral pH
  • Constant supply of food, H2O, O2
  • Mechanism removing waste
• Only a small number are pathogens. Pathogens cause disease by:
  • Damaging our cells; or
  • Producing toxins; or
  • Directing our immune system against our own cells

Vibrio Cholera

• Produces enterotoxins released from bacteria
  • Enters enterocytes (cells lining the surface of the intestine) by endocytosis
  • Activates the CFTR protein (cystic fibrosis transmembrane regulator)
  • Causes secretion of sodium, chloride and bicarbonate ions from enterocytes
  • Water follows sodium into the intestinal lumen
• Osmotic loss of up to 10L of water per day!
  • Results in severe watery diarrhoea of sudden onset
  • Dehydration leads to death within hours if untreated
• Giving oral sodium would cause more water to be secreted into the intestine, worse!
• Giving oral glucose and sodium (oral rehydration therapy)
  • Glucose is still absorbed through the intestinal wall
  • This is done by a glucose-sodium co-transporter
  • Carries one glucose molecule and one sodium ion across the intestine into the blood
  • Water always follows sodium
  • Diarrhoea is less severe and body becomes rehydrated
• Oral rehydration therapy (ORT) also contains potassium and bicarbonate ions
  • Prevents electrolyte imbalance
  • Prevents metabolic acidosis

Cell Structure

Cells

You need to be able to use your knowledge of the structure of cells and the function of the organelles to interpret electronmicrographs of cells. In particular the epithelial cells lining the small intestine. These are cells which are specialised for absorbing the products of digestion. They will therefore have a large surface area provided by the microvilli and due to the need for active transport across their cell membranes they will contain a large number of mitochondria providing them with ATP. Epithelial cells also secrete enzymes and other proteins. This means that they will have a large and visible endoplasmic reticulum, Golgi apparatus to allow protein production and secretion.

Below is a list of the most important organelles that you are required to know about. It is worth you using textbooks and perhaps even other websites such as www.cellsalive.com to look at actual images of different types of cells and their organelles.

Nucleus

• Contains DNA
• DNA arranged into long thin threads known as chromosomes
• In most cells the chromosomes are arranged in homologous pairs
• Surrounded by nuclear envelope
• This has pores to allow communication between the nucleus and cytoplasm

Plasma Membrane

• Sea of phospholipids - arranged as a bilayer
• Intrinsic and extrinsic proteins float within the phospholipids
• Selectively permeable barrier - controls movement of substances between the internal and external environments

Microvilli

• Adaptation of cells to increase surface area for absorption or secretion
• Found on epithelium of the small intestine

Lysosomes

• Formed by the golgi apparatus
• Contain digestive enzymes - proteases and lipases
• Important to protect the cell from the effect of these enzymes before they are released at the cell surface membrane or into a phagocytic vesicle

Mitochondria

• 1µm in diameter and 7µm in length
• Mostly protein, but also contains some lipid, DNA and RNA
• Power house of the cell
• Energy is stored in high energy phosphate bonds of ATP
• Mitochondria convert energy from the breakdown of glucose into adenosine triphosphate (ATP)
• Responsible for aerobic respiration
• Metabolic activity of a cell is related to the number of cristae (larger surface area) and mitochondria
• Cells with a high metabolic activity (e.g. heart muscle) have many well developed mitochondria

Ribosomes

• 20-30nm in size
• Small organelles often attached to the ER but also found in the cytoplasm
• Large (protein) and small (rRNA) subunits form the functional ribosome
  • Subunits bind with mRNA in the cytoplasm
  • This starts translation of mRNA for protein synthesis (assembly of amino acids into proteins)
• Free ribosomes make proteins used in the cytoplasm. Responsible for proteins that
  • go into solution in cytoplasm or
  • form important cytoplasmic, structural elements
• Ribosomal ribonucleic acid (rRNA) are made in nucleus of cell

Endoplasmic Reticulum (ER)

• Rough ER
  • Have ribosomes attached to the cytosolic side of their membrane
  • Found in cells that are making proteins for export (enzymes, hormones, structural proteins, antibodies)
  • Thus, involved in protein synthesis
  • Modifies proteins by the addition of carbohydrates, removal of signal sequences
  • Phospholipid synthesis and assembly of polypeptides
• Smooth ER
  • Have no ribosomes attached and often appear more tubular than the rough ER
  • Necessary for steroid synthesis, metabolism and detoxification, lipid synthesis
  • Numerous in the liver

Golgi Apparatus

• Stack of flattened sacs surrounded by membrane
• Receives protein-filled vesicles from the rough ER (fuse with Golgi membrane)
• Uses enzymes to modify these proteins (e.g. add a sugar chain, making glycoprotein)
• Adds directions for destination of protein package - vesicles that leave Golgi apparatus move to different locations in cell or proceed to plasma membrane for secretion
• Involved in processing, packaging, and secretion
• Other vesicles that leave Golgi apparatus are lysosomes

Techniques used in Cell Biology

• Microscopy
  • Magnification → increases the size of an object
  • Resolution/resolving power → ability to distinguish between adjacent points
• Calculating magnification
  • X = size of picture (measure the size of the diagram in the question)
  • Y = size of object in real life (often given in exam question)
  • Make sure Y has the same unit as X!
    • If X = mm and Y = μm
    • Convert mm to μm = X * 1000
  • Magnification = Xμm / Yμm

Feature	Optical microscope	Electron microscope
Radiation	Light	Electrons
Magnification	400x (max1500)	≈500 000x
Resolution	2µm	1nm / 0,001µm Electrons have a small wavelength Thus, higher resolution
Vacuum in microscope	Absent	Present
Specimen is	- Alive or dead - Stained	- Dead (vacuum!) Transmission microscope: Electrons pass through internal structure of specimen Scanning microscope: Beams of electrons are reflected off specimens surface. Allows a three dimensional view

Cell Fractionation

To study the function of individual organelles large numbers of isolated organelles need to be obtained. Cell fractionation is used to gather these organelles.

It is worth looking on the internet or in your text books for a step by step diagram of the process to use alongside this explanation.

• The tissue from which the organelles are to be harvested from is firstly placed in a COLD, ISOTONIC, BUFFERED solution
  • The solution is cold to minimise enzyme activity
  • The solution is isotonic to prevent organelle damage due to osmotic water gain or loss
  • The solution is buffered to maintain a constant pH
• The solution containing the cells is then HOMOGENISED in a blender to release organelles from the cells. After homogenising, the fluid is known as the HOMOGENATE, it is now FILTERED to remove any large pieces of cell debris
• The filtered homogenate is then centrifuged in an ultracentrifuge at progressively greater speeds in order to separate the different components. When spun in the centrifuge at a low speed, the largest organelle - the nucleus will be forced to the bottom of the tube and form a pellet.
• The SUPERNATANT (fluid above the pellet) now contains cell components too small to sediment at this speed. This fluid is centrifuged at a higher speed to form another pellet which will contain organelles such as mitochondria
• The supernatant can then again be centrifuged at an even higher speed to separate out even smaller organelles
• Once the organelle that is to be studied has been extracted from the homogenate it can be resuspended in distilled water to make it easier to use in experiments

The Lungs

The Lungs

The structure of the respiratory system

You need to recall the structure and function of each component of the respiratory system:

• Nose
• Pharynx
• Larynx
• Epiglottis
• Trachea
• Bronchus
• Bronchiole
• Alveolus

Air comes into the respiratory system through the nose. The air is filtered in the nostrils due to the presence of small hairs. It is also moistened and warmed by the nasal cavities and the mucus present traps foreign particles which are then propelled towards the throat by the cilia on the epithelial cells.

From the nose, the air passes into the pharynx and is drawn into the larynx and then the trachea. The epiglottis is found within the larynx. This structure prevents food and drink passing into the respiratory system. When swallowing, the larynx is pulled up and the epiglottis flaps back to block the entrance of the larynx.

The trachea contains C-shaped cartilage rings which prevent the tube collapsing due to the change of pressure. It divides into 2 tubes with smaller diameter called bronchi. Each bronchus is lined with ciliated epithelia to waft mucus upwards towards the throat. There is asymmetry in the respiratory system - the right bronchus is bigger than the left one and at a more vertical angle. This makes it a common site for inhaled foreign bodies.

The bronchi further divide into bronchioles. These are important because their diameter can be controlled by smooth muscles contraction or relaxation. The bronchioles terminate with alveoli (100µm in diameter) which are the site of gas exchange.

Mechanism of breathing

The process of breathing in and out is known as ventilation. Breathing in (inhalation) relies on air being drawn into the lungs because the air pressure in the lungs is lower than that in the atmosphere. This lower pressure is created by the contraction of the diaphragm and the external intercostal muscles. This flattens the diaphragm and moves the ribs up and out. The volume of the thorax therefore increases, decreasing the pressure and allowing air to enter into the lungs.

Exhalation (breathing out) requires the air pressure in the lungs to be increased. As with inhalation this occurs because of changes in the diaphragm and intercostal muscles. The diaphragm and external intercostal muscles relax, raising the diaphragm back up into a dome shape and pulling the ribs down and in. These changes reduce the volume of the thoracic cavity. This rise in air pressure in the lungs means that air is moved out of the lungs. If exhalation is forced then the internal intercostal muscles contract further raising the pressure in the thoracic cavity.

Fick’s law

Fick was a busy man, he has both a law and a principle named after him AND is credited with the invention of the contact lens! Fick’s law states that:

The rate of diffusion across a fluid membrane is proportional to (surface area x conc. difference) /distance

Efficient gas exchange therefore requires:

• Large surface area
• Large concentration
• Short diffusion pathway (thickness of the membrane molecules must travel to diffuse across)

Large organisms have a small surface area : volume ratio

• Decreases the rate of diffusion
• Large animals loose less heat than small animals
• Don't require a high metabolism to maintain body temperature
• Feed only once

Small organisms have a large surface area: volume ratio

• Lose heat very readily
• Need a high metabolism to maintain body temp
• Must feed continuously

Alveolar Gas Exchange

The greater the partial pressure of O2 in alveolar air the more O2 will dissolves in blood (Henry's Law)

It seems that Henry was a master of stating the obvious! Let’s try to further understand the process of how oxygen moves across the alveolus and into the blood.

The alveoli - the site of gas exchange in the lungs is composed of epithelial cells. This has evolved to allow efficient diffusion of gases (large surface area short diffusion pathway) down their concentration gradients. O2 therefore diffuses from air to blood where it then associates with haemoglobin and CO2 diffuses from blood to the air in the alveolus.

A protein called surfactant is produced by the alveoli, which prevents the alveolar surfaces from sticking together when they deflate. The alveoli also contain phagocytes to kill bacteria that have not been trapped by mucus which may later cause disease.

Lung Disease

There are many different conditions which can impair the function of the lungs. Obviously with such an important role, any harm done to the lung tissue can have major effects on a person’s health and fitness. With such a clinical focus, this is a key area of ‘How Science Works’ and you should be able to analyse data for correlations and causal relationships between human activity/behaviour and the onset of disease.

Asthma

At least one in ten people suffer from asthma at some point during their lives with the majority of cases presenting in childhood. The condition is caused by inflammation of the bronchioles. In an asthma attack:

1. The smooth muscle in the bronchiole wall contracts which narrows the lumen
2. The epithelial cells lining the bronchiole secrete more mucus than normal which obstructs the movement of air through the respiratory system
3. Breathing rate increases but the tidal volume is reduced
4. Gas exchange in the alveoli is reduced

Asthma attacks can be triggered by many different stimuli. Common triggers include:

• Some diseases e.g. the common cold and flu
• Exposure to air pollution or dust
• Exposure to known allergens such as pollen, animal fur or certain foods
• Exercise - especially in cold air
• Psychological factors such as stress

There is no ‘cure’ for asthma but the condition can be treated and managed through the use of inhalers which administer drugs to the respiratory system. Normally an asthma sufferer would have two inhalers to be used in different ways. They will have one ‘Preventer’ which when used release anti-inflammatory drugs such as steroids to reduce the underlying inflammation and hopefully reduce the likelihood that a person will have an asthma attack. They will also be given an inhaler which should be used to relieve symptoms during an attack. This contains substances which dilate the bronchioles which should make breathing easier.

Pulmonary Fibrosis

Fibrosis is the scarring of body tissue in this case - the lungs. The scarring causes a loss in elasticity of the tissue between the alveoli and contorts the bronchioles and alveoli. These pathological effects reduce lung capacity. The condition is highly linked to occupational hazards such as working with substances such as asbestos, coal dust and metal dust. Widespread fibrosis caused by inhalation of harmful substances is called emphysema. Infectious diseases such as tubercolosis can also leave small regions of the lungs with scarring. Fibrosis therefore refers to the consequence of diseases which produce lung damage.

A patient suffering from pulmonary fibrosis would have shortness of breath and/or a cough. At present there is no treatment for this condition and a lung transplant is the only treatment option which will improve long - term survival.

Pulmonary Tuberculosis

This condition is caused by rod-shaped bacteria: Mycobacterium tuberculosis OR Mycobacterium bovis.

Symptoms:

• persistent cough
• tiredness
• loss of appetite and weight loss
• fever
• coughing up blood

Transmission:

This disease is spread through the air in droplets released when an infected person coughs or sneezes. Coughs and sneezes really do spread diseases! Unusually the bacterium causing TB can survive for a long period of time even in dried droplets. This means that close contact with an infected person over a period of time can lead to transmission of the disease.

It is especially common in communities where living space is relatively small or crowded working environments. There are some countries in which TB is particularly prevalent and anyone moving to Britain from these countries will need to be tested for the bacteria before being allowed to enter the country without first being treated for TB.

People most at risk of contracting TB are those who:

• are in close contact with infective individuals
• live or work in care facilities
• are from countries in which TB is prevalent
• have reduced immunity (the very young or very old, those with AIDS, people taking immunosuppresants, malnourished individuals, alcoholics, homeless people)

Once inside a person’s respiratory system there is a plentiful supply of oxygen allowing rapid growth and division of the bacteria. At this early stage the person often develops pneumonia. The white blood cells of the immune system respond rapidly to the infection to try and prevent the bacteria from spreading. This process results in the formation of scar tissue which contains the infection in an inactive state.

If the body’s immune system becomes weakened the TB bacteria can break through the scar tissue resulting in the return of pneumonia and the spreading of the bacteria to other parts of the body (the kidneys, bone and linings of the brain and spinal cord are the most common sites affected).

Symptoms and Treatment:

There is a relatively long time between the time of infection and the onset of symptoms. A patient will present with tiredness, weight loss, fever, coughing, chest pain and shortness of breath (this is due to the presence of scar tissue in the lungs).

Inactive TB may be treated with an antibiotic and active TB will usually require several antibiotics to combat the infection.

Emphysema

Emphysema can be an inherited condition but most cases arise as a result of smoking. The toxins passed into the lungs when smoking trigger an immune response which ultimately leads to the destruction of the lung tissue.

Your lungs contain white blood cells which ‘patrol’ the lungs phagocytosing any harmful pathogens or particles which are inhaled. These phagocytes release enzymes which catalyse the breakdown of proteins found in the connective tissue between the alveoli and bronchioles. This makes it easier for them to move around the lung tissue to engulf and kill the invading pathogen or particle. Elastin is one of these proteins which phagocytes destroy. The elastin fibres act as an elastic band would, snapping back into shape after they have been stretched. This allows the stretch and recoil of the alveoli (the site of gas exchange).

Excessive destruction of elastin is normally prevented by the production of a substance called α1-antitrypsin which acts to prevent the action of elastase (the enzyme which catalyses the breakdown of elastin). The smoke from cigarettes contains several chemicals which stop lung cells from producing α1-antitrypsin. This means, that the destruction of elastin will increase, damaging the elastic tissue of the lungs making it harder for a person to exhale. Other proteins are also destroyed by the enzymes secreted by the pathogens meaning that the alveoli walls can be damaged and the surface area available for gas exchange is reduced.

A reduced area for gas exchange means that a person with emphysema’s blood will contain a reduced concentration of oxygen. This will limit the amount and rate of aerobic respiration achievable by their cells making any activity a great effort.

The Heart

The Heart

When learning about the structure and function of the heart, it is useful to have a labelled diagram close to hand. Even better still would be to get to a butcher or supermarket and buy a lamb heart and investigate for yourself!

It is also worth looking for animations of the cardiac cycle on the internet to try and give you an image of how this organ functions.

Anatomy

Until you are comfortable with the structure of the heart, it will be very difficult to understand HOW the heart works. You should be able to identify the following structures:

• Right atrium (RA)
• Right ventricle (RV)
• Left atrium (LA)
• Left ventricle (LV)
• Sinoatrial node (SAN)
• Atrioventricular node (AVN)
• Tricuspid valve
• Mitral valve
• Semilunar valves

The Cardiac Cycle

The four chambers of the heart are continually contracting and relaxing in a sequence known as the cardiac cycle. Contraction of a chamber is SYSTOLE (pronounced sistolee) and relaxation DIASTOLE (pronounced diastole). The left and right sides of the heart actually contract simultaneously but in order to understand how blood moves through the circulatory system we will consider each half separately.

• Right atrium receives blood from
  • Superior vena cava (SVC) - carries blood from upper body (head, arms)
  • Inferior vena cava (IVC) - carries blood from lower body (chest, abdomen, legs)
• Blood flows from right atrium, across tricuspid valve, into right ventricle
• Blood leaves right ventricle and enters pulmonary artery
  • Backflow into RV prevented by semilunar pulmonic valve
  • Deoxygenated blood arrives at lungs via pulmonary artery
  • Oxygenated blood leaves lungs via pulmonary vein
• Blood from pulmonary vein enters left atrium
• Blood flows from left atrium, across mitral valve, into left ventricle
• Left ventricle has a thick muscular wall / generates high pressures during contraction
• Blood from LV is ejected, across aortic valve, into aorta

TASK: using a simple diagram (boxes will do), draw arrows showing how the blood moves through the chambers and blood vessels.

A common exam question at both GCSE and A Level is why is the muscle of left ventricle is thicker than right ventricle? If you’ve done a heart dissection at school, this will certainly be something which the teachers pointed out and there is in fact a considerable difference between the two chambers. The reasons for this are outlined below:

• The pressure of the blood in the aorta is higher than pulmonary artery
• The left ventricle must therefore generate more pressure to overcome pressure of aorta
• Therefore, thicker muscle required in left ventricle

The problem of backflow:

• Between each chamber of the heart are valves which prevent the blood being forced back into the chamber from which it was just pushed out. Between the atria and the ventricles are the tricuspid and mitral valves (mitral is on the left and tricuspid on the right). These are known as the atrioventricular valves. If you’ve dissected a heart you will have seen fibrous strands leading from ‘flaps’ at the top of the ventricles. These strands (cordae tendinae) are attached to papillary muscles which contract during ventricular systole which generates tension pulling the AV valves shut.
• The pulmonary artery and the aorta also contain valves to prevent the blood from these vessels falling back into the ventricles. These are known as the Semilunar valves (pulmonic and aortic). They do not work in the same way as the AV valves. Instead, the pressure of blood within the vessel actually causes the closure of the semilunar valves.

Pressure Changes

At several points so far, pressure has been mentioned. It is an important aspect of the cardiac cycle and a factor which can be used to identify which stage of the cardiac cycle a heart is in. In fact, examiners love to provide you with pressure graphs and ask you to analyse the cardiac cycle. It is therefore worth us spending a little time going over the principles of ‘Isovolumetric contraction’ - it sounds worse than it is!

As a chamber fills with blood, the pressure is going to rise. When a chamber contracts, the pressure is going to rise. Changes in pressure affect whether a valve is open or closed. Fluids always move from areas of high pressure to areas of low pressure. Let us think through the cardiac cycle in terms of pressure:

• As the blood passes into the atria, the valves are open so most will fall immediately into the ventricle. There is a gradual rise in pressure in the atria until the end of atrial systole when the blood has moved into the ventricles.
• The intraventricular pressure rises as the ventricles fill with blood. This closes the AV valves.
• Contraction of the ventricles means that the intraventricular pressure is higher than the pressure in the artery which forces the blood out of the ventricle and into the aorta or pulmonary artery (depending on which side of the heart you’re looking at).
• The increase in pressure of the artery causes the closing of the semilunar valves preventing the back flow of blood into the ventricle.

All good text books should have a pressure graph for you to look at and try to understand how the pressure changes relate to the cardiac cycle.

Electrical Activity of the Heart - Controlling the Cardiac Cycle

The heart has a unique ability to beat (contract) on its own. The cardiac muscle cells are therefore myogenic. Regulation of this contraction though is required to ensure that the muscle cells contract in a specific way and that your heart can respond to meet the energy demands of your body. Nervous and hormonal stimulation both have an effect on the way that the heart contracts.

On the right atrium is a structure called the Sinoatrial Node, or the SAN. This bundle of cells acts as a pacemaker controlling the rate of contraction - the heart rate. Stimulation of this node initiates a wave of electrical impulses which spread aross the atria causing atrial systole. If cardiac cells are stained with a voltage sensitive dye then a wave of contraction can be seen rippling across the atria (all muscular contraction relied on electrical changes).

The electrical signal in the atria is picked up by a second node, the AtrioVentricular Node (or the AVN) which passes the signal down to the apex of the heart (bottom of the ventricles). This is passed through specialised conducting cardiac muscle fibres called the Bundle of His. From the apex, the electrical activity is spread throughout the ventricles along Purkinje fibres. This means that the ventricles contract from the bottom up once they have filled with blood.

You may have heard the terms fibrillation or VF (ventricular fibrillation) on TV shows or films. This refers to changes in the electrical activity of the heart muscle cells. Fibrillation occurs when the cells are not contracting in a regular fashion which, if it’s happening in the ventricles will mean that blood is not being forced into the blood vessels.

QUESTION: How and why would using a defibrillator help in this situation?

Pulling it all together

You should now understand the following things:

• The structure of the heart - atria, ventricles, valves, blood vessels leading to and from the heart.
• The route blood takes through the heart - the cardiac cycle
• The terms systole and diastole
• The function and role of valves within the heart
• The changes in pressure within the chambers and blood vessels
• The electrical control of the cardiac cycle.

TASK: Test your knowledge by rearranging the following statements into a logical order!

1. Atria receive blood from veins and store it prior to each heart beat
2. Ventricular diastole
   End of cardiac cycle, all chambers relax. Aortic and pulmonary valves close → 2nd heart sound. This prevents backflow into ventricles.
3. Pressure of RA > RV - forces tricuspid valve to open
   Pressure of LA > LV - forces mitral valve to open
4. Atrial systole
   SAN stimulated and wave of electrical activity spreads across atria.
   Both atria contract and move blood across AV valves into ventricles. This reduces volume of atria but increases pressure
5. Atria fill up again to start next cycle. The volume increases while pressure decreases.
6. Ventricular systole
   Contraction of ventricles increases pressure. The AV valves close as blood is forced against them → 1st heart sound. This prevents backflow into atria. Instead, blood is ejected into arteries through aortic and pulmonary valves.
7. Electrical signal picked up by AVN. Bundle of His transfers electrical activity down to apex of heart and along purkinje fibres to intiate contraction of the ventricles.

Digestive System

The Digestive System

The digestive system is a tube through which food passes from the mouth where food is ingested to the anus where it is egested. It consists of a series of organs, each with a distinct structure and function. During the digestive transit food is broken down into substances suitable for absorption into the bloodstream.
The gut wall has the same basic structure along its length. There are three main layers:

• An outer, muscular layer. Circular and longitudinal layers of smooth muscles are present. Alternate contraction of these muscles moves food along the digestive tract (peristalsis)
• A middle layer of connective tissue - submucosa
• An inner layer - mucosa

These three layers have different adaptations in different parts of the alimentary canal. The adaptations are underlined below.

Oesophagus

• Muscular tube carrying food from the mouth to the stomach

Stomach

• Elastic and muscular organ which can expand
• Highly folded mucosa
• Gastric pits secreting gastric juices containing digestive enzymes (proteases)
• Contraction and relaxation of the muscular wall mix the food thoroughly

Small Intestine

• The site of chemical digestion and absoption of the products of lipids, polysaccharides and proteins
• Highly folded mucosa - arranged in villi (finger like projections to increase surface area for absorption)
• Epithelial cells lining the small intestine have a folded cell membrane - microvilli to further increase the surface area for absorption

Large Intestine

• The site of absorption of water
• Undigested food matter forms faeces

Rectum

• Faecal matter is stored here before egestion

Digestion

• Large molecules (starch, proteins, TAG) are too big and insoluble to be absorbed
  • Polymers have to be broken down into monomers
  • With help of hydrolytic enzymes - reaction requires H2O
  • Note: TAGs are not polymers but also need to be broken down
• Different enzymes break down different food
  • Work best at body temperature (37°)
  • Work in different conditions at different pH (stomach is acidic, intestine is alkaline)
• Hydrolysis
  • Proteins → amino acids
    • Essential amino acids: cannot be synthesised and must be present in diet
    • Non-essential amino acids: synthesised from essential amino acids by transamination in the liver
  • TAG → glycerol and fatty acids
  • Polysaccharides → monosaccharides

Proteins

• Proteins are made up by different combinations of 20 amino acids
  • Common structure
    • -COOH group
    • -NH2 group
  • Amino acids differ in their R-group
• Tertiary structure
  • Complex globular 3D shape
  • Folding and twisting of polypeptides (H-bond, ionic bonds, disulphide bridges)
  • Polypeptides contain many peptide bonds
• Same amino acid sequence → ALWAYS same shape
• Bonds found in proteins
  • Hydrogen bonds
    • Between R-groups
    • Easily broken, but present in larger numbers
    • The more bonds, the stronger the structure
  • Ionic bonds
    • Between -COOH and -NH2 groups
  • Disulphide bridges
    • Between two sulphur-containing cysteine side chains
    • Strong bonds found in skin and hair
• Denaturation
  • Destruction of tertiary structure, can be done by heat
  • Protein structure is lost and cannot reform → dysfunctional
• Background Reading: Structure of Proteins

What are enzymes?

• All enzymes are globular proteins → spherical in shape
• Control biochemical reactions in cells
• They have the suffix "-ase"
• Intracellular enzymes are found inside the cell
• Extracellular enzymes act outside the cell (e.g. digestive enzymes)
• Enzymes are catalysts → speed up chemical reactions
  • Reduce activation energy required to start a reaction between molecules
  • Substrates (reactants) are converted into products
  • Reaction may not take place in absence of enzymes (each enzyme has a specific catalytic action)
  • Enzymes catalyse a reaction at max. rate at an optimum state
• Lock and key theory
  • Only one substrate (key) can fit into the enzyme's active site (lock)
  • Both structures have a unique shape
• Induced fit theory
  • Substrate binds to the enzyme's active site
    • The shape of the active site changes and moves the substrate closer to the enzyme
    • Amino acids are moulded into a precise form
    • Enzyme wraps around substrate to distort it
  • This lowers the activation energy
  • An enzyme-substrate complex forms → fast reaction
  • E + S → ES → P + E
• Enzyme is not used up in the reaction (unlike substrates)

Enzyme Activity

• Changes in pH
  • Affect attraction between substrate and enzyme
  • Ionic bonds can break and change shape → enzyme is denatured
  • Charges on amino acids can change → ES complex cannot form
  • Optimum pH (enzymes work best)
    • pH 7 for intracellular enzymes
    • Acidic range (pH 1-6) in the stomach for digestive enzymes (pepsin)
    • Alkaline range (pH 8-14) in oral cavities (amylase)
  • pH measures the conc. of hydrogen ions → higher conc. will give a lower pH
• Enzyme conc
  • Proportional to rate of reaction, provided other conditions are constant
  • Straight line
• Substrate conc.
  • Proportional to rate of reaction until there are more substrates than enzymes present
  • Rate of reaction increases
    • Substrate binds to active site, but more enzymes are available
    • Rate increases if more substrate is added
  • Eventually, curve becomes constant (no increased rate)
    • Substrates occupy all active sites (all enzymes)
    • Adding more substrate won't yield more product, as no more active sites are available
• Increased Temperature
  • Increases speed of molecular movement → chances of molecular collisions → more ES complexes
  • At 0-42°C rate of reaction is proportional to temp
  • Enzymes have optimum temp. for their action (usually 37°C in humans)
  • Above ≈42°C, enzyme is denatured due to heavy vibration that breaks -H bonds
  • Shape is changed → active site can't be used anymore
• Decreased Temperature
  • Enzymes become less and less active, due to reductions in speed of molecular movement
  • Below freezing point
    • Inactivated, not denatured
    • Regain their function when returning to normal temperature
  • Thermophilic: heat-loving
  • Hyperthermophilic: organisms are not able to grow below +70°C
  • Psychrophiles: cold-loving
• Monomer (-OH) + monomer (-H) ↔ polymer + H2O(l)
• Condensation: monomers join to form polymers
  • Amino acids join to form a dipeptide (protein)
    • Two amino acids release -H and -OH groups (H2O)
    • Peptide bond forms between the alpha-carbon and nitrogen
  • Monosaccharides join to form disaccharides
    • Glycosidic bond forms between both monomers
• Hydrolysis: break down of a polymer
  • Reverse of the condensation reaction
  • This is the process of digestion

Carbohydrates

• Organic molecules which contain C, H and O
• Bind together in the ratio Cx(H2O)y
• Monosaccharides → single sugar (monomer)
  • Ribose found in RNA and DNA
  • Deoxyribose part of nucleic acids
  • Glucose is the main energy source in brain
  • Fructose is found in sweet-tasting fruits
• Disaccharides → two sugar residues (2 monomers)
  • Sucrose (glucose + fructose) → transport carbohydrates in plants
  • Maltose (glucose + glucose) → formed from digestion of starch
  • Lactose (glucose + galactose) → found in milk
  • Lactose intolerance
• Polysaccharides → many sugar residues (polymer)
  • Starch (alpha-glucose) → main storage of carbohydrates in plants
  • Glycogen (alpha-glucose) → main storage of carbohydrates in humans
  • Cellulose (beta-glucose) → component of plant cell wall, important for digestion

Starch

• Consists of amylopectin and amylose (both are made of α-glucose)
  • Amylopectin is branched via 1,6-glycosidic bonds
  • Amylose forms a stiff helical structure via 1,4-glycosidic bonds
  • Both are compact molecules → starch can be stored in small space
• The ends are easily broken down to glucose for respiration
• Does not affect water potential as it is insoluble
• Readily hydrolysed by the enzyme amylase produced by the pancreas and present in saliva
• Found in corn (maize), wheat, potato, rice

Biochemical Tests

• Reducing sugars (all monosaccharides and some disaccharides) can be tested for using Benedict’s reagent. After placing the sample and the reagent in a hot water bath a brick red precipitate will be produced if reducing sugars are present.
• Non reducing sugars require a negative result using Benedict’s reagent. Add hydrochloric acid to the sample and heat. Neutralize the solution using sodium hydrogencarbonate and then test again with Benedict’s solution. A positive result will be found.
• Starch can be tested for using iodine. In the presence of starch iodine will turn blue - black.