Human bio

Structure of gas exchange

The lungs are adapted for gas exchange due to several structural features:

• The alveoli provide a vast internal surface area, allowing for large amounts of gas exchange quickly. Estimates suggest there are hundreds of millions of alveoli, with a total surface area of 50-80 m², roughly one-third the area of a tennis court.

• Each alveolus is well-supplied with blood vessels, ensuring that blood is close to the air in the alveoli, maintaining concentration differences of oxygen and carbon dioxide.

• The alveolar wall is very thin, only 1 micrometer thick, facilitating easy gas diffusion.

The lungs are located deep within the body to minimize fluid evaporation from the respiratory surfaces. A thin layer of moisture on the alveolar membrane is crucial for gas diffusion, as gases must be dissolved in fluid to move into and out of the blood.

Changes in lung volume, driven by respiratory muscle movements, ensure a constant flow of air in the alveoli, maintaining concentration gradients of oxygen   and carbon dioxide.

The lung structure is optimized for efficient gas exchange between air and blood.

The Process of Gas Exchange

Blood in the capillaries surrounding the alveoli is delivered by the pulmonary arteries. This blood, having circulated through the body, has a low oxygen concentration and a high carbon dioxide concentration.

• Oxygen from the alveolar air dissolves in the moisture lining the alveolus and diffuses through the alveolar membrane into the blood.

• Conversely, carbon dioxide diffuses from the blood into the alveolar air due to its higher concentration in the capillaries compared to the alveolus.

For effective gas diffusion, a concentration gradient must exist between the air in the alveoli and the blood in the capillaries. This gradient is maintained by:

• The continuous flow of blood through the capillaries, which picks up oxygen and releases carbon dioxide, ensuring that new blood with low oxygen and high carbon dioxide replaces it.

• The movement of air in and out of the alveoli with each breath, replacing air that has exchanged gases with fresh air that is low in carbon dioxide and high in oxygen.

Blood

Function

Transport –

·       O2, nutrients, and hormones to cells

·       CO2, urea and other wastes away from cells

Regulation – helps maintain

●        pH of body fluids

●        body temperature

Protection

●        Protects the body against pathogens and toxins

●        Prevents blood loss when blood vessels are damaged by clotting

Process of blood clotting

●        Injury. A cut on the skin or an internal injury creates a small tear in a blood vessel wall, which causes blood flow.

●        Vessel constriction. To control blood loss, the blood vessel immediately narrows (called vasoconstriction), which limits blood flow through the vessel.

●        Platelet plug. In response to the injury, platelets are activated. The platelets stick to one another and to the wound site to form a plug.

●        Fibrin clot. Next, blood clotting factors trigger production of fibrin, which is a strong, strand like substance that surrounds the platelet plug and forms a fibrin clot, a mesh like net that keeps the plug firm and stable. Over the next several days to weeks, the clot strengthens and then dissolves as the wounded blood vessel wall heals.

Circulatory System

• the transport of materials within the internal environment for exchange with cells is facilitated by the structure and function of the circulatory system at the cell, tissue and organ levels

• the components of blood facilitate the transport of different materials around the body (plasma and erythrocytes), play a role in the clotting of blood (platelets) and the protection of the body (leucocytes)

• the lymphatic system functions to return tissue fluid to the circulatory system and to assist in

The Heart and Circulatory System

Overview of the Heart

The heart is a vital organ that functions as a pump to circulate blood throughout the body. It is situated between the lungs in the mediastinum, slightly left of the sternum. The heart has a conical shape, measuring approximately 12 cm in length, 9 cm at its widest point, and 6 cm in thickness, roughly the size of an adult human fist.

The heart is encased in a protective membrane known as the pericardium, which secures it in place while allowing movement during contractions. This membrane also prevents overstretching of the heart.

Structure of the Heart

The heart is divided into two sides, separated by a wall called the septum. Each side consists of two chambers, resulting in a total of four chambers:

·       The right atrium receives blood from the body and transfers it to the right ventricle.

·       The right ventricle pumps blood to the lungs.

·       The left atrium receives blood from the lungs and sends it to the left ventricle.

·       The left ventricle pumps blood to the rest of the body.

The left ventricle has a thicker wall than the right ventricle, as it needs to exert more force to pump blood throughout the body.

Key Concept

The heart functions as a four-chambered pump, with atria collecting blood and ventricles pumping it out to the lungs and body.

Heart Valves

Valves in the heart ensure unidirectional blood flow.

The atrioventricular valves are located between the atria and ventricles, consisting of thin tissue flaps held by tendons (chordae tendineae) attached to papillary muscles.

When the ventricles contract, blood pushes against these flaps, sealing the opening between the atria and ventricles.

At the exit of the ventricles are the semilunar valves, which prevent backflow into the ventricles when they relax.

Each semilunar valve has three cusps that flatten against the artery wall when blood flows out and seal off the artery when blood attempts to return.

Key Concept

Valves are crucial for preventing backward blood flow, located between the atria and ventricles, and at the exits of the ventricles.

Blood Vessels

Blood vessels transport blood throughout the body, forming a continuous circulation system. There are three main types of blood vessels:

·       Arteries: Carry blood away from the heart. The aorta is the largest artery, distributing blood from the left ventricle to the body, while the pulmonary artery carries blood from the right ventricle to the lungs.

·       Capillaries: Microscopic vessels that connect arteries and veins, allowing for the exchange of oxygen, nutrients, and waste between blood and cells.

·       Veins: Carry blood back to the heart. They include the superior and inferior vena cava, which bring blood from the body to the right atrium, and pulmonary veins that transport blood from the lungs to the left atrium.

Arteries

Arteries have thick walls composed of smooth muscle and elastic fibers, allowing them to stretch and recoil as blood is pumped. This elasticity maintains blood pressure and flow. Arteries branch into smaller arterioles, which regulate blood flow to capillaries through vasoconstriction and vasodilation.

Capillaries

Capillaries are essential for nutrient and waste exchange due to their thin walls, consisting of a single layer of cells. This structure facilitates the transfer of substances between blood and surrounding tissues.

Veins

Veins have thinner walls than arteries and do not possess muscular walls capable of changing diameter. Blood pressure in veins is lower, leading to the presence of valves that prevent backflow. Blood from capillaries converges into venules, which form larger veins, ultimately returning blood to the heart.

Blood Flow Regulation

Blood flow adapts to the varying needs of cells, especially during physical activity. Changes in blood flow can occur through:

·       Adjusting the output of blood from the heart.

·       Altering the diameter of blood vessels supplying tissues.

Cardiac Cycle

The cardiac cycle refers to the sequence of events during one heartbeat. It consists of:

·       Systole: The pumping phase when the heart muscle contracts.

·       Diastole: The filling phase when the heart muscle relaxes.

During diastole, both atria and ventricles fill with blood. Atrial systole follows, pushing blood into the ventricles, which then contract during ventricular systole to pump blood into the arteries. Both sides of the heart work simultaneously.

Cardiac Output

Cardiac output is the volume of blood ejected from a ventricle per minute, influenced by heart rate (beats per minute) and stroke volume (volume of blood per contraction). The formula for cardiac output is:

Cardiac output (mL/minute) = stroke volume (mL) x heart rate (

Regulation of heart rate

·       Within heart the heart conductive tissue regulates the heartbeat

·       Senatorial node (SA node/pacemaker) initiates heartbeat

·       Atrioventricular node (AV node) regulates beating of ventricle

Cardiac output

·       Heart rate – number of times the heart beats per minute

·       Stroke volume – volume of blood forced from ventricles with each contraction

·       Cardiac output – amount of blood leaving one of the ventricles each minute

·       Cardiac output (mL/min) = heart rate(beats/min) * stroke volume (mL)

Blood pressure

·       The force blood exerts on the walls of arteries

·       Measured with Sphygmomanometer + recorded as 2 numbers eg. 120/80

o   Systolic blood pressure – larger number, indicates pressure in arteries as heart squeezes out blood during each beat

o   Diastolic blood pressure – the lower number, indicates pressure as the heart relaxes before next beat

Depends on :

o   Cardiac output (as increase so does pressure)

o   Diameter of vessels (constriction increase pressure)

o   Any increase in heart rate will also increase BP and vice versa

Blood Vessels

·       Arteries carry blood from heart toward capillaries and veins return blood to  heart from capillaries

Arteries

·       Carry blood away form heart

·       Walls contain smooth muscle and elastic fibres:

o   Elastic walls stretch to accommodate extra blood at high pressure

o   Ventricles relax, elastic walls recoil

o   Muscles in walls does not contract + relax to pump blood along, it is elastic recoil that keeps blood moving

·       Pressure is variable (high compared to veins)

o   Pressure increase as blood is pumped through artery and then decreases as ventricles relax again

Smooth muscles

·       In artery walls, can contract to reduce diameter of artery

o   reduce blood flow (vasoconstriction)

o   relax to dilate artery and increase blood flow (vasodilation)

Veins

·       Carry blood back towards heart

·       Walls do not have elastic fibres

·       Blood pressure is constant and relatively low

·       Blood loses most of its pressure as it flows through capillary bed

·       Many veins (particularly in legs and arms) have valves to prevent backflow of blood

·       Muscles of limbs also contract squeezing the veins and pushing blood along

Capillaries

·       Tiny arteries (arterioles) joined to tiny veins (venules) by network of microscopic blood vessels – capillaries

·       Carry blood close to cells so they can get their oxygen and nutrients from blood and get rid of waste

·       Walls are only one cell thick to make diffusion more effective

Tunicae

Arteries and  veins have three layers (tunicae)- tunica externa, tunica media, tunica interna

Antigen and antibodies

·       The surface of RBC are coated with protein and sugar molecules:

·       antigens - a substance that is capable of stimulating th v e formation of antibodies.

·       Antibody – protein formed by immune system in response to antigen

·       They can combine to form a complex

 Blood groups ABO

·       There are  more than 20 genetically determined blood group system known today – ABO and Rh blooding grouping

·       2 different antigens involved in determining which blood group you are in the ABO grouping. Which one/s yoy have determined by your DNA

·       According to the ABO blood typing system there four different kinds of blood types: A, B, AB or O (null)

·       The immune system produces antibodies to antigens

·       Immune system recognises out own antigens and wont produce antibodies against them but will produce antibodies against non-self antigens

·       Therefore, if u have RBC’s w/ antigen A you won’t make antibodies against it but will make antibodies against antigen B

Blood transfusion

·       Transfers blood or one of the components of blood from one person to another

·       Eg after blood loss / treat conditions such as anaemia, leukaemia/haemophilia

·       Transfusion require blood types of two individuals to be matched

·       If transfusion is different to receiver’s blood, the receiver will produce antibodies against the antigen of the donor’s blood

·       Mixing blood types that are incompatible will result in agglutination where RBCs clump together

·       Only certain blood types can donated and received by each other to avoid this

Rh blood groups

Based on antigens that are found on surface of RBC:

·       RH antigen present = Rh+

o   Does not produce any antibodies

·       Rh antigen not present = Rh-

o   Produces anti-Rh antibody

Rh blood groups alaso matched for transfusion

Anti-Th antibody is not normally present individuals who are Rh- however it will be produced on exposure to a Rh antigen

First transfusion of Rh+ blood to Rh- person will not cause problems initially, as antibodies are producers slowly

However subsequent exposure result in much faster production of the antibodies (due to memory cells) and aggulation

Digestive system

• the supply of nutrients in a form that can be used in cells is facilitated by the structure and function of the digestive system at the cell, tissue and organ levels

 • digestion involves the breakdown of large molecules to smaller ones by mechanical digestion (teeth, peristalsis, churning and bile) and chemical digestion (by enzymes with distinctive operating conditions and functions that are located in different sections of the digestive system)

 • the salivary glands, pancreas, liver and gall bladder produce or store secretions which aid the processes of digestion

 • absorption requires nutrients to be in a form that can cross cell membranes into the blood or lymph and occurs at different locations, including the small intestine and large intestine

• elimination removes undigested materials and some metabolic wastes from the body

Digestive System Overview

Importance of Nutrients

All cells require nutrients for energy, growth, reproduction, secretion, and metabolic processes. The digestive system extracts nutrients from food and absorbs them into the body for cellular use.

Basic Activities of the Digestive System

The digestive system performs six essential activities:

• Ingestion of food and water

• Mechanical digestion of food

• Chemical digestion of food

• Movement of food along the alimentary canal

• Absorption of digested food and water into the blood and lymph

• Elimination of unabsorbed material

Types of Digestion

Body cells need simple sugars, amino acids, fatty acids, vitamins, minerals, and water. These nutrients are derived from complex carbohydrates, proteins, and fats, which must be broken down into smaller units through digestion.

Key Concept

Digestion is the process of breaking down food into particles small enough to be absorbed into the bloodstream.

Mechanical Digestion

Mechanical digestion involves the physical breakdown of food particles through:

• Teeth cutting, tearing, and grinding food

• Churning action in the stomach

• Bile release from the gall bladder to emulsify fats

The goal is to increase the surface area for more effective chemical digestion.

Key Concept

Mechanical digestion increases the surface area of food for chemical digestion.

Chemical Digestion

Chemical digestion breaks down large molecules into smaller ones using enzymes:

• Carbohydrates into monosaccharides

• Proteins into peptides and amino acids

• Lipids into fatty acids and glycerol

• Nucleic acids into nucleotides

Enzymes act as biological catalysts to speed up reactions without being consumed.

Key Concept

Chemical digestion uses enzymes to convert large molecules into smaller, absorbable ones.

Alimentary Canal Structure

The alimentary canal is a continuous tube from the mouth to the anus, including associated organs like the pancreas and gall bladder. Its lining is where nutrient absorption occurs.

The Mouth

Ingestion occurs in the mouth, where food is chewed (mastication) and mixed with saliva containing mucus (for lubricating bolus) and salivary amylase for starch digestion.

Teeth and Mechanical Digestion

There are four types of teeth:

• Incisors for biting

• Canines for tearing

• Premolars for grinding

• Molars for crushing

The tongue shapes food into a bolus towards pharynx for swallowing.

The Oesophagus

The oesophagus is 23-25 cm long, connects the pharynx to the stomach, passes through diaphragm.

Movement of food lubricated by mucous

Longitudinal – runs along length

Circular – arranged in circles around canal

When food enters pharynx and oesophagus, circular muscles contract behind the food known as peristalsis (wave-like contractions) to move food along. Food is moved unidirectionally along alimentary canal from mouth to anus.

Key Concept

Peristalsis is the muscle contraction that moves food through the oesophagus.

The Stomach

·       The stomach performs mechanical digestion through muscular contractions and chemical digestion with gastric juice, which contains hydrochloric acid and enzymes.

·       Contains hydrochloric acid, mucus and digestive enzymes

·       Mechanical digestion occurs by peristalsis along stomach wall

·       Has oblique muscle layer, circular and longitudinal layer which allows variety of ways to churn food and mix with juices

·       Food is mixed with gastric juice to becomes a thick soupy liquid called chyme

·       Mucus - prevents stomach digesting itself

·       Pepsinogen converts to pepsin in the acidic environment, initiating protein digestion.

·       Hydrochloric acid allows pepsin to act and is able to kill bacteria that enters with the food.

·       Stomach mucosa

o    The lining of the stomach - specialised for the secretion of gastric juice.

o   The gastric glands which secrete gastric juice are located in narrow tube-like structures called gastric pits.

o   The HCl, mucus and pepsin are all secreted by different cells in the gastric pits.

o   Hydrochloric acid converts pepsinogen to the active pepsin only when both reach the opening of the pits. 

o   This along with a mucous lining helps to stop the pepsin and HCl acid destroying the stomach itself.

·       Pyloric sphincter – a thickening of circular muscle, regulates flow of material from stomach to duodenum. After 2-8 hours contents in stomach pushed to duodenum

Key Concept

Food undergoes both mechanical and chemical digestion in the stomach.

The Small Intestine

The small intestine, about 6-7 m long, consists of three regions:

• Duodenum: primary site for chemical digestion. Receives material from stomach and continues digestion

• Jejunum: effective absorption of carbohydrates and proteins

• Illeum: absorption of vitamin B12 and bile salts

Digestion in the Small Intestine

Continues with:

• Pancreatic juice for neutralizing stomach acid and digesting food

o   Pancreatic amylase – breaks down starch

o   Pancreatic protease/trypsin – breaks down proteins into small chains of amino acids

o   Ribonuclease and deoxyribonuclease – digest RNA and DNA

o   Pancreatic lipases – breaks down fats into fatty acids and glycerol

• Bile for emulsifying fats

o   No enzymes

o   Has bile salts important for digestion of fats into droplets. This is mechanical digestion that increases surface area which lipases can act to chemically break down fat

• Intestinal juice for completing digestion

o   Lining of small intestine secretes intestinal juice containing enzymes:

Peptidase, sucrase, lactase, maltase, and lipases

Segmentation also aids in mechanical digestion.

·       Contraction of circular muscles narrow intestine, helps to break up bolus and mix with juices and bile

·       Brings into contact with lining for absorption

·       Move chyme in both directions, allowing mix with intestinal juice and bile

Key Concept

Enzymes in pancreatic and intestinal juices facilitate chemical digestion, while bile emulsifies fats.

Absorption of Nutrients

After digestion, nutrients are absorbed through the small intestine's wall, which has a large surface area due to folds, villi, and microvilli.

·       Villi absorb digested food. Each villus is ~1 mm long. Inside villus is lymph capillary – lacteal, which is surrounded by network of blood capillaries. Absorption is helped by muscular movements of the intestinal wall that keep villi moving

Absorption occurs via simple diffusion and active transport, with nutrients entering blood capillaries or lacteals.

o   Amino acids absorbed by active transport in to blood capillaries

o   Simple sugars absorbed by active transport. Pass through cells outside of villi and in to blood capillaries

o   Fatty acids and glycerol absorbed by simple diffusion. In cells og villi fatty acids and glycerol recombine to form fat, fat droplets enter lacteals.

o   Water and water soluble vitamins absorbed into blood capillaries by diffusion.

Substances absorbed into blood capillaries are carried by hepatic portal vein to liver or remain in blood to other body cells. Substances absorbed into lacteals are transported in lymph system, and then emptied into blood through veins in upper part of chest.

Key Concept

The small intestine's structure maximizes nutrient absorption.

The Large Intestine

The large intestine, about 1.5 m long, includes the caecum, colon, rectum, and anus. It absorbs remaining water and contains bacteria that break down organic compounds.

o   Caecum – a pouch,, where small intestine joins large

o   Colon – longest part, inverted u shape

o   Rectum – last part, semi solid material left in colon after water absorption is pushed into rectum by peristalsis. As walls stretch, this triggers defecation, muscle around anus relaxes and faeces can be passed out

o   Anus – external opening at end of rectum, circular muscles known as anal sphincter. Faeces consist of water, undigested material, bacteria, and bile pigments

.

Key Concept

Slow movement through the large intestine allows for water absorption, producing faeces.

Effects of Diet on the Alimentary Canal

Meal size and content affect the speed of material movement through the digestive system. High protein or fat meals slow down digestion, while alcohol and caffeine stimulate it.

Constipation

Caused by reduced large intestine movement, leading to hard, dry faeces. Often due to lack of roughage, exercise, or emotional issues.

Diarrhoea

Frequent watery faeces due to irritation of the intestines, which can lead to dehydration. Causes include bacteria, viruses, parasites, and certain diseases.

Importance of Soluble Fibre

Soluble fibre helps lower cholesterol and improve blood glucose levels. Sources include fruits, vegetables, and oats.

Bowel Cancer

Linked to diet, alcohol, and smoking. High red meat and low fibre diets increase risk.

Coeliac Disease

Inability to tolerate gluten, leading to villi damage and malnutrition. Symptoms vary, and the only treatment is a gluten-free diet.

Key Concept

A healthy diet is crucial for maintaining a healthy digestive system

Excretory

Urine formation

By nephrons through:

·       Glomerular filtration

·       Selective reabsorption

·       Tubular secretion

Glomerular filtration

·       Takes place in renal corpuscle

·       Materials filtered out of blood in the glomerulus and collected in glomerular capsule

·       Filtrate – fluid collected

·       Non-selective passive process

Kidney structure suited for function

·       Efferent arteriole leaving glomerulus has smaller diameter / is narrower compared to afferent arteriole entering it

o   Increase resistance to blood flow in efferent arteriole

o   Creates high blood pressure in capillaries

o   Forces water and solutes smaller than proteins through capillary membranes into the capsule (filtration)

o   Easy for blood to enter the glomerulus but difficult for it to exit- Increase pressure within the glomerulus

o   Glomerulus forms extensive narrow branches which increases surface area available for filtration

o   The net pressure gradient within glomerulus forces water and dissolved substances to move into capsule space (forming filtrate)

·       Glomerular capsule directly surrounds glomerulus- allows filtrate to pass efficiently into capsule from blood in capillaries

·       Walls of both capillaries and capsule are one cell thick- less distance for filtrate to travel so it passes efficiently into capsule.

·       Blood continually flowing through kidney which maintains concentration gradient

Filtrate composition

Filtrate consist of components of blood, except red blood cells, WBC, platelets and plasma proteins as they are too large

Filtrate:

Water, various ions and salts

Amino acids, fatty acids glucose

Urea, uric acid and creatinine

Vitamins, hormones and toxins

Selective reabsorption

Returned to bloodstream from filtrate in peritubular capillaries by selective reabsorption

Occurs along tubules of nephron and surrounding peritubular capillaries

For efficient reabsorption

·       Large surface are :

·       Long length of renal rubule created by convolutions of tubule and long loop of Henle

·       Large number of nephrons in each kidney

·       Proximal convoluted tubules are lined with cuboidal epithelium with brush border or microvilli which increases surface area

·       Some is passive (water osmosis, small ions by diffusion

·       Most active by protein carriers (active transport)

§  Water – osmosis

§  Glucose – active transport

§  Amino acids – active transport

§  Ions

o   Sodium active transport

o   Potassium, chloride and bicarbonate – passive transport

o   Calcium – active and passive transport

·       Urea – partially absorbed

Proximal convoluted tubule – water, glucose, amino acids, vitamins and some salts are reabsorbed

      When solutes removed, glomerular filtrate becomes dilute so water reabsorbed into blood capillaries

Loop of Henle – ions are reabsorbed to adjust and maintain pH of 7.3 – 7.4

Distal convoluted tubule – further reabsorption of water and salts takes place depending on body needs.

Permeability of membrane of cells of distal convolted tubule and collecting duct can change so more or less waater can be reabsorbed

   Process is under hormonal control of Antidiuretic Hormone which increases permeability of walls DCT and collecting duct so more water can be absorbed

Tubular secretion

·       Adds substances to filtrate form blood

o   Potassium and hydrogen ions

o   Creatinine

o   Drugs

·       Movement of these materials is by active transport

·       Efficient secretion

Large surface area

Long length

Large number of nephrons

Purpose 

Allow body to remove unwanted materials

Regulate pH levels of blood

Urine is slightly acidic

Urine formation

·       Collecting duct drains filtrate form several nephrons and passes to renal pelvis where it pass to ureter to bladder to be stored

·       Main components of urine – water, urea, uric acid, ions and creatinine

o   Urine doesn’t normally contains proteins or glucose

o   99% of water enters nephron is reabsorbed

o   Uric acid – produced by metabolism of purines which come form breakdown of nucleic acid

Component

Water – 96%

Urea-2%

Uric acid

Movements by synovial joints

flexion – bending movement that decreases the angle

extension – bending movement that increases the angle

abduction – moving away from longitudinal axis

adduction – movement toward the longitudinal axis

rotation – turning the bone or limb around its long axis

  Metabolism

All chemical reactions that occur in cells of organisms in which attempts to balance release and utilisation of energy.

Anabolic – small molecules join together to produce larger molecules, requires energy eg protein synthesis

Catabolic – breakdown of large molecules into smaller molecules, does not require energy eg digestion

Enzymes – a biological catalyst that speeds up chemical reaction by decreasing the activation energy. Can control chemical reactions that occurs in body, without being used ore altered. Without enzymes reactions will be slow.

They are specific due to structure and active sites that is suited with a particular substrate due to complementary shape between enzymes active site and substrate molecule. This ensures only intended reaction occurs within cell.

Cellular respiration

The process by which organic molecules, taken in as food is broken down in cells to release energy for cells activities such as movement of cell, uptake of material and secretion of new chemical compounds. This occurs in all cells to supply each cell with energy in the form of ATP.

Factors affecting enzyme activity

A number of factors influence the activity of enzymes and the

rates of chemical reactions in which they are involved.

. The higher the concentration of enzyme, the faster the rate

of a chemical reaction because there are more enzyme

molecules to influence reactants. By regulating the type and

number of enzymes present, the body is able to control which

reactions occur and the rate at which they proceed.

. Increasing substrate concentration also increases the rate of

the reaction. This occurs because there will be more substrate

molecules coming into contact with the enzyme molecules.

However, increasing the substrate beyond a certain

concentration will cease to have an effect because the active

sites on all the enzyme molecules will be fully occupied.

. The products of the reaction must be continually removed,

otherwise the rate of the reaction will slow because it

becomes more difficult for the substrate molecules to make

contact with the enzyme molecules.

. Temperature influences enzyme activity. The rate of most

chemical reactions increases as temperature increases. This

is true of most enzyme reactions but only within a limited

temperature range. Because enzymes are proteins, beyond

about 45-50℃ their structure changes; they are denatured.

As the shape of the enzyme is crucial for its functioning,

denatured enzymes are inactive. The temperature at which an

enzyme works best is called the optimum temperature. For

most enzymes in the human body, this is 30C to 40C.

. Enzymes are very sensitive to the pH of the medium in which

a reaction is taking place. Each enzyme has an optimum pH at

which it will work most effectively.

. Many enzymes require the presence

of certain ions or non-protein

molecules before they will catalyse

a reaction. Such substances are

called cofactors. Cofactors change

the shape of the active site so that

the enzyme can combine with the

substrate. Without a cofactor the

enzyme molecule is intact, but

cannot function. Some cofactors are non-protein organic

molecules. They are then called coenzymes. Many vitamins

function as coenzymes.

. Enzyme inhibitors are substances that slow or even stop the

enzyme's activity. Inhibitors may be used by cells to control

reactions so that products are produced in specific amounts.

Many drugs are enzyme inhibitors; for example, penicillin

inhibits an enzyme in bacteria that is involved in construction

of the cell wall.

Glycolysis

The first phase in the breakdown of glucose does not require oxygen. It is called glycolysis, which means 'splitting glucose'. A glucose molecule is broken down, in a series of 10 steps, to two molecules of pyruvate. Sometimes this molecule is called pyruvic acid; however, the two substances differ slightly in their structure.

Anaerobic respiration

If no oxygen is available, the pyruvate produced in glycolysis is then converted to lactic acid by fermentation. The production of lactic acid from glucose is called anaerobic respiration, which means respiration without oxygen. The fermentation stage of anaerobic respiration does not produce any additional ATP; however, the glycolysis of one molecule of glucose releases enough energy to convert two molecules of ADP to ATP. Therefore, anaerobic respiration allows cells to produce some energy in the absence of oxygen. The enzymes required for anaerobic respiration are available in the cytosol of the cell; therefore, glycolysis and the conversion of pyruvate to lactic acid occur in the cytosol. Anaerobic respiration is very important during vigorous physical activity, when the respiratory and circulatory systems are unable to supply muscle cells with enough oxygen to meet all the energy demands of the contracting muscles. In such circumstances, anaerobic respiration supplies the extra energy. This results in the accumulation of lactic acid in the muscles, and lactic acid may cause muscle pain.

Lactic acid from anaerobic respiration is taken by the blood to the liver, where it can be recombined with oxygen to form glucose and eventually glycogen. As this process requires oxygen, physiologists say that, when cells are respiring anaerobically, the body is incurring an oxygen debt. After vigorous exercise, one continues to breathe heavily for some time because the oxygen debt must be 'repaid' by converting lactic acid to glucose. The extra oxygen required after exercise may also be called recovery oxygen.

Key concept

Anaerobic respiration uses glucose to produce lactic acid and two ATP molecules in the absence of

oxygen.

Aerobic respiration

The complete breakdown of glucose to carbon dioxide and water requires oxygen. The pyruvate from glycolysis is completely broken down to carbon dioxide and water. This is known as aerobic respiration - respiration requiring oxygen. Aerobic respiration occurs in the mitochondria of the cell. Mitochondria are organelles constructed with a double membrane - an outer membrane that forms the shape of the organelle, and an inner membrane, called cristae, that is folded inwards. The enzymes for the reactions of aerobic respiration are attached to the internal membrane, so folding produces a large surface area on which the reactions of aerobic respiration can take place.

1 For the pyruvate to enter the next pathway it is first converted to acetyl coenzyme A (acetyl CoA).

To do this, a carbon dioxide molecule is removed from the pyruvate and the remaining

two-carbon structure joins to coenzyme A. No ATP is produced during this process.

2 The acetyl CoA then enters the citric acid cycle, also known as the Krebs cycle. Here the carbon atoms in the acetyl CoA are released in carbon dioxide. For every acetyl CoA that enters the citric acid cycle, one molecule of ATP is also produced. This means that two ATP molecules are produced per glucose molecule.

3 The final stage of cellular respiration is the electron transport system; the only stage that uses oxygen. This stage is also called oxidative phosphorylation. Here electrons are passed between molecules, finally resulting in oxygen molecules forming water. There is some debate regarding the exact number of ATP molecules that are produced during this process. Estimates range between 26 and 34 molecules.

Thus, aerobic respiration of one molecule of glucose has the potential to generate up to 38 molecules of ATP - two from glycolysis, two from the citric acid cycle and up to 34 from the electron transport mechanism. This can be represented as:

The processes of anaerobic and aerobic respiration are summarised in Figure 3.21.

Key concept

Aerobic respiration uses oxygen to convert glucose into carbon dioxide and water, producing up to 38 molecules of ATP per glucose molecule. A yield of 38 ATP molecules from the energy contained in one molecule of glucose is the theoretical maximum. The actual ATP yield is often lower than this. Because the reactions of aerobic respiration take place in the mitochondria, and because aerobic respiration releases about 95% of the energy needed to keep a cell alive, the mitochondria are often known as the powerhouses of the cell.

energy used by cell

Cells need the energy that is temporarily stored in the ATP molecule for a variety of processes. These are summarised in Figure 3.22. Each of the chemical reactions involved in cellular processes produces a certain amount of heat. In cellular respiration, only about 40% of the energy released is incorporated into ATP; the other 60% is lost as heat. Therefore, energy must be continually consumed in the form of food to replace what is lost as heat and utilised for other purposes. The reactions of cellular respiration are catabolic; that is, they release energy as larger molecules are broken down into smaller ones. ATP may be used to transfer energy produced in catabolic reactions to anabolic reactions that require energy. For example, when lactic acid is recombined with oxygen in the liver to form glucose, or when glucose molecules are joined to form glycogen, the energy required comes from the breakdown of ATP to ADP. Similarly, energy for the build-up of proteins, lipids and other molecules is transferred from cellular respiration by ATP.