Animal Physiology

Question 1

Outline mechanicall digestion

Answer 1

Chewing (Mouth)

Food is initially broken down in the mouth by the grinding action of teeth (chewing or mastication)
The tongue pushes the food towards the back of the throat, where it travels down the esophagus as a bolus
The epiglottis prevents the bolus from entering the trachea, while the uvula prevents the bolus from entering the nasal cavity

Churning (Stomach)

The stomach lining contains muscles which physically squeeze and mix the food with strong digestive juices (‘churning’)
Food is digested within the stomach for several hours and is turned into a creamy paste called chyme

Question 2

Distinguish between Perestalis and Segmentation

Answer 2

Peristalsis

Peristalsis is the principal mechanism of movement in the oesophagus, although it also occurs in both the stomach and gut
Continuous segments of LONGNITUDINAL smooth muscle rhythmically contract and relax
Food is moved unidirectionally along the alimentary canal in a caudal direction (mouth to anus)

Segmentation

Segmentation involves the contraction and relaxation of non-adjacent segments of CIRCULAR smooth muscle in the intestines
Segmentation contractions move chyme in both directions, allowing for a greater mixing of food with digestive juices
While segmentation helps to physically digest food particles, its bidirectional propulsion of chyme can slow overall movement

Question 3

Outline chemical digestion

Answer 3

Stomach Acids

The stomach contains gastric glands which release digestive acids to create a low pH environment (pH ~2)
The acidic environment functions to denature proteins and other macromolecules, aiding in their overall digestion
The stomach epithelium contains a mucous membrane which prevents the acids from damaging the gastric lining
The pancreas releases alkaline compounds (e.g. bicarbonate ions), which neutralise the acids as they enter the intestine

Bile

The liver produces a fluid called bile which is stored and concentrated within the gall bladder prior to release into the intestine
Bile contains bile salts which interact with fat globules and divide them into smaller droplets (emulsification)
The emulsification of fats increases the total surface area available for enzyme activity (lipase)

Enzymes

Enzymes are biological catalysts which speed up the rate of a chemical reaction (i.e. digestion) by lowering activation energy
Enzymes allow digestive processes to therefore occur at body temperatures and at sufficient speeds for survival requirements
Enzymes are specific for a substrate and so can allow digestion of certain molecules to occur independently in distinct locations

Question 4

Where are the following digested?:
Carbohydrates
Proteins
Lipids
Nucelic acids

Answer 4

Carbohydrates

Carbohydrate digestion begins in the mouth with the release of amylase from the salivary glands (amylase = starch digestion)
Amylase is also secreted by the pancreas in order to continue carbohydrate digestion within the small intestine
Enzymes for disaccharide hydrolysis are often immobilised on the epithelial lining of the small intestine, near channel proteins
Humans do not possess an enzyme capable of digesting cellulose (cellulase) and hence it passes through the body undigested

Proteins

Protein digestion begins in the stomach with the release of proteases that function optimally in an acidic pH (e.g. pepsin = pH 2)
Smaller polypeptide chains enter the small intestine where they are broken down by endopeptidases released by the pancreas
These endopeptidases work optimally in neutral environments (pH ~ 7) as the pancreas neutralises the acids in the intestine

Lipids

Lipid breakdown occurs in the intestines, beginning with emulsification of fat globules by bile released from the gall bladder
The smaller fat droplets are then digested by lipases released from the pancreas

Nucleic Acids

The pancreas also releases nucleases which digest nucleic acids (DNA, RNA) into smaller nucleosides

Question 5

Describe the structure of the small intestine

Answer 5

The small intestine is composed of four main tissue layers, which are (from outside to centre):

Serosa – a protective outer covering composed of a layer of cells reinforced by fibrous connective tissue
Muscle layer – outer layer of longitudinal muscle (peristalsis) and inner layer of circular muscle (segmentation)
Submucosa – composed of connective tissue separating the muscle layer from the innermost mucosa
Mucosa – a highly folded inner layer which absorbs material through its surface epithelium from the intestinal lumen

Question 6

What is the function of villi?

Answer 6

• Villi increase the surface area of epithelium over which absorption is carried out
• Villi absorb monomers formed by digestion as well as mineral ions and vitamins

Question 7

How are villi adapted for absorbtion?

Answer 7

Microvilli – Ruffling of epithelial membrane further increases surface area
Rich blood supply – Dense capillary network rapidly transports absorbed products
Single layer epithelium – Minimises diffusion distance between lumen and blood
Lacteals – Absorbs lipids from the intestine into the lymphatic system
Intestinal glands – Exocrine pits (crypts of Lieberkuhn) release digestive juices
Membrane proteins – Facilitates transport of digested materials into epithelial cells

MR SLIM

Question 8

What is the purpose of tight junctions?

Answer 8

Tight junctions between epithelial cells occlude any gaps between cells – all monomers must cross the membrane

Question 9

Explain how the following allow for transport with reference to an example in digestion:

Secondary active transport
Facilitated diffusion
Osmosis
Simple diffusion

Answer 9

Secondary Active Transport

A transport protein couples the active translocation of one molecule to the passive movement of another (co-transport)
Glucose and amino acids are co-transported across the epithelial membrane by the active translocation of sodium ions (Na+)

Facilitated Diffusion

Channel proteins help hydrophilic food molecules pass through the hydrophobic portion of the plasma membrane
Channel proteins are often situated near specific membrane-bound enzymes (creates a localised concentration gradient)
Certain monosaccharides (e.g. fructose), vitamins and some minerals are transported by facilitated diffusion

Osmosis

Water molecules will diffuse across the membrane in response to the movement of ions and hydrophilic monomers (solutes)
The absorption of water and dissolved ions occurs in both the small and large intestine

Simple Diffusion

Hydrophobic materials (e.g. lipids) may freely pass through the hydrophobic portion of the plasma membrane
Once absorbed, lipids will often pass first into the lacteals rather than being transported via the blood

Question 10

List and describe the sections of the small intestine

Answer 10

The small intestine is comprised of three distinct regions: duodenum, jejunum and ileum

Duodenum

First segment of the small intestine which is fed by digestive fluids from the pancreas and gall bladder
Bile emulsifies fat globules into smaller droplets and pancreatic juice contains digestive enzymes
Sodium bicarbonate is released from the pancreas to neutralise stomach acids such that intestinal pH is ~ 7

Jejunum

Second segment of the small intestine where the digestive process is largely completed
Pancreatic enzymes and enzymes released from intestinal glands complete the break down of sugars, proteins and lipids

Ileum

Final segment of the small intestine with the principal function of nutrient absorption
The intestinal tract is highly folded (villi and microvilli) to increase surface area and optimise material absorption
Bile is also absorbed here and returned to the liver via blood vessels

Question 11

What did William Harvey propose?

Answer 11

-Arteries and veins were part of a single connected blood network (he did not predict the existence of capillaries however)
-Arteries pumped blood from the heart (to the lungs and body tissues)
-Veins returned blood to the heart (from the lungs and body tissues)

Question 12

How are arteries adapted to their function

Answer 12

-They have a narrow lumen (relative to wall thickness) to maintain a high blood pressure (~ 80 – 120 mmHg)
-They have a thick wall containing an outer layer of collagen to prevent the artery from rupturing under the high pressure
-The arterial wall also contains an inner layer of muscle and elastic fibres to help maintain pulse flow (it can contract and stretch)

Question 13

How do muscle and elastic fibers in arteries allow for blood pressure to be maintained?

Answer 13

The muscle fibres help to form a rigid arterial wall that is capable of withstanding the high blood pressure without rupturing

Muscle fibres can also contract to narrow the lumen, which increases the pressure between pumps and helps to maintain blood pressure throughout the cardiac cycle

The elastic fibres allow the arterial wall to stretch and expand upon the flow of a pulse through the lumen

The pressure exerted on the arterial wall is returned to the blood when the artery returns to its normal size (elastic recoil)
The elastic recoil helps to push the blood forward through the artery as well as maintain arterial pressure between pump cycles

Question 14

Describe the structure of capilleries

Answer 14

-They have a very small diameter (~ 5 µm wide) which allows passage of only a single red blood cell at a time (optimal exchange)
-The capillary wall is made of a single layer of cells to minimise the diffusion distance for permeable materials
-They are surrounded by a basement membrane which is permeable to necessary materials
-They may contain pores to further aid in the transport of materials between tissue fluid and blood (Continous, Fenestrated, or Sinusoid)

Question 15

Describe the blood flow in capillaries

Answer 15

Blood flows through the capillaries very slowly and at a very low pressure in order to allow for maximal material exchange

The higher hydrostatic pressure at the arteriole end of the capillary forces material from the bloodstream into the tissue fluid

The lower hydrostatic pressure at the venule end of the capillary allows materials from the tissues to enter the bloodstream

Question 16

How is the vein adapted to its function?

Answer 16

-They have a very wide lumen (relative to wall thickness) to maximise blood flow for more effective return
-They have a thin wall containing less muscle and elastic fibres as blood is flowing at a very low pressure (~ 5 – 10 mmHg)
-Because the pressure is low, veins possess valves to prevent backflow and stop the blood from pooling at the lowest extremities

Question 17

How is blood in veins transported back to the heart?

Answer 17

The veins contain numerous one-way valves in order to maintain the circulation of blood by preventing backflow

Veins typically pass between skeletal muscle groups, which facilitate venous blood flow via periodic contraction. When the skeletal muscles contract, they squeeze the vein and cause the blood to flow from the site of compression

Question 18

Outline the cardiac cycle

Answer 18

The sinoatrial node sends out an electrical impulse that stimulates contraction of the myocardium (heart muscle tissue)

This impulse directly causes the atria to contract and stimulates another node at the junction between the atrium and ventricle

This second node – the atrioventricular node (AV node) – sends signals down the septum via a nerve bundle (Bundle of His)

The Bundle of His innervates nerve fibres (Purkinje fibres) in the ventricular wall, causing ventricular contraction

Question 19

How is heart rate controlled?

Answer 19

Changes to blood pressure levels or CO2 concentrations (and thereby blood pH) will trigger changes in heart rate

The pacemaker is under autonomic (involuntary) control from the brain, specifically the medulla oblongata (brain stem)

The sympathetic nerve releases the neurotransmitter noradrenaline to increase heart rate
The vagus nerve releases the neurotransmitter acetylcholine to decrease heart rate

Question 20

Discuss Adrenaline briefly

Answer 20

The hormone adrenaline (a.k.a. epinephrine) is released from the adrenal glands (located above the kidneys)
Adrenaline increases heart rate by activating the same chemical pathways as the neurotransmitter noradrenaline

Question 21

Distinguish between systole and Diastole

Answer 21

Systole

Blood returning to the heart will flow into the atria and ventricles as the pressure in them is lower (due to low volume of blood)
When ventricles are ~70% full, atria will contract (atrial systole), increasing pressure in the atria and forcing blood into ventricles
As ventricles contract, ventricular pressure exceeds atrial pressure and AV valves close to prevent back flow (first heart sound)
With both sets of heart valves closed, pressure rapidly builds in the contracting ventricles (isovolumetric contraction)
When ventricular pressure exceeds blood pressure in the aorta, the aortic valve opens and blood is released into the aorta

Diastole

As blood exits the ventricle and travels down the aorta, ventricular pressure falls
When ventricular pressure drops below aortic pressure, the aortic valve closes to prevent back flow (second heart sound)
When the ventricular pressure drops below the atrial pressure, the AV valve opens and blood can flow from atria to ventricle
Throughout the cycle, aortic pressure remains quite high as muscle and elastic fibres in the artery wall maintain blood pressure

Question 22

Outline Atherosclerosis

Answer 22

Atherosclerosis is the hardening and narrowing of the arteries due to the deposition of cholesterol

Atheromas (fatty deposits) develop in the arteries and significantly reduce the diameter of the lumen (stenosis)
The restricted blood flow increases pressure in the artery, leading to damage to the arterial wall (from shear stress)
The damaged region is repaired with fibrous tissue which significantly reduces the elasticity of the vessel wall
As the smooth lining of the artery is progressively degraded, lesions form called atherosclerotic plaques
If the plaque ruptures, blood clotting is triggered, forming a thrombus that restricts blood flow
If the thrombus is dislodged it becomes an embolus and can cause a blockage in a smaller arteriole

Question 23

List the risk factors for corranry heart disease

Answer 23

Age – Blood vessels become less flexible with advancing age
Genetics – Having hypertension predispose individuals to developing CHD
Obesity – Being overweight places an additional strain on the heart
Diseases – Certain diseases increase the risk of CHD (e.g. diabetes)
Diet – Diets rich in saturated fats, salts and alcohol increases the risk
Exercise – Sedentary lifestyles increase the risk of developing CHD
Sex – Males are at a greater risk due to lower oestrogen levels
Smoking – Nicotine causes vasoconstriction, raising blood pressure

Mnemonic: A Goddess

Question 24

Describe the differnt stages in an electrocardiogram

Answer 24

The P wave represents depolarisation of the atria in response to signalling from the sinoatrial node (i.e. atrial contraction)
The QRS complex represents depolarisation of the ventricles (i.e. ventricular contraction), triggered by signals from the AV node
The T wave represents repolarisation of the ventricles (i.e. ventricular relaxation) and the completion of a standard heart beat
Between these periods of electrical activity are intervals allowing for blood flow (PR interval and ST segment)

Question 25

Describe the first line of Defense against disease

Answer 25

Skin

Protects external structures when intact (outer body areas)
Consists of a dry, thick and tough region composed predominantly of dead surface cells
Contains biochemical defence agents (sebaceous glands secrete chemicals and enzymes which inhibit microbial growth on skin)
The skin also secretes lactic acid and fatty acids to lower the pH (skin pH is roughly ~ 5.6 – 6.4 depending on body region)

Mucous Membranes

Protects internal structures (i.e. externally accessible cavities and tubes – such as the trachea, oesophagus and urethra)
Consists of a thin region of living surface cells that release fluids to wash away pathogens (mucus, saliva, tears, etc.)
Contains biochemical defence agents (secretions contain lysozyme which can destroy cell walls and cause cell lysis)
Mucous membranes may be ciliated to aid in the removal of pathogens (along with physical actions such as coughing / sneezing)

Question 26

Outline the path air takes to the alveoli

Answer 26

Air enters the respiratory system through the nose or mouth and passes through the pharynx to the trachea
The air travels down the trachea until it divides into two bronchi (singular: bronchus) which connect to the lungs
The right lung is composed of three lobes, while the left lung is only comprised of two (smaller due to position of heart)
Inside each lung, the bronchi divide into many smaller airways called bronchioles, greatly increasing surface area
Each bronchiole terminates with a cluster of air sacs called alveoli, where gas exchange with the bloodstream occurs

Question 27

Describe the structure of an alveolus

Answer 27

-They have a very thin epithelial layer (one cell thick) to minimise diffusion distances for respiratory gases

-They are surrounded by a rich capillary network to increase the capacity for gas exchange with the blood

Their internal surface is covered with a layer of fluid, as dissolved gases are better able to diffuse into the bloodstream

Question 28

Distinguish between pneumocytes

Answer 28

Type I pneumocytes

Type I pneumocytes are involved in the process of gas exchange between the alveoli and the capillaries
They are squamous (flattened) in shape and extremely thin (~ 0.15µm) – minimising diffusion distance for respiratory gases
Type I pneumocytes are connected by occluding junctions, which prevents the leakage of tissue fluid into the alveolar air space
Type I pneumocytes are amitotic and unable to replicate, however type II cells can differentiate into type I cells if required

Type II pneumocytes

Type II pneumocytes are responsible for the secretion of pulmonary surfactant, which reduces surface tension in the alveoli
They are cuboidal in shape and possess many granules (for storing surfactant components)
Type II pneumocytes only comprise a fraction of the alveolar surface (~5%) but are relatively numerous (~60% of total cells)

Question 29

Describe the function of Type II pneumocytes

Answer 29

Type II pneumocytes secrete a liquid known as pulmonary surfactant which reduces the surface tension in alveoli

As an alveoli expands with gas intake, the surfactant becomes more spread out across the moist alveolar lining
This increases surface tension and slows the rate of expansion, ensuring all alveoli inflate at roughly the same rate

Question 30

Explain how the muscles contract in ventilation

Answer 30

When the pressure in the chest is less than the atmospheric pressure, air will move into the lungs (inspiration)
When the pressure in the chest is greater than the atmospheric pressure, air will move out of the lungs (expiration)

Inspiration

The muscles responsible for inspiration are the diaphragm and external intercostals (plus some accessory muscles)

Diaphragm muscles contract, causing the diaphragm to flatten and increase the volume of the thoracic cavity
External intercostals contract, pulling ribs upwards and outwards (expanding chest)
Additional muscle groups may help pull the ribs up and out (e.g. sternocleidomastoid and pectoralis minor)

Expiration

The muscles responsible for expiration are the abdominal muscles and internal intercostals (plus some accessory muscles)

Diaphragm muscles relax, causing the diaphragm to curve upwards and reduce the volume of the thoracic cavity
Internal intercostal muscles contract, pulling ribs inwards and downwards (reducing breadth of chest)
Abdominal muscles contract and push the diaphragm upwards during forced exhalation
Additional muscle groups may help pull the ribs downwards (e.g. quadratas lumborum)

Question 31

Causes and Consequences of Emphysema

Answer 31

Emphysema is a lung condition whereby the walls of the alveoli lose their elasticity due to damage to the alveolar walls

The loss of elasticity results in the abnormal enlargement of the alveoli, leading to a lower total surface area for gas exchange
The degradation of the alveolar walls can cause holes to develop and alveoli to merge into huge air spaces (pulmonary bullae)

The major cause of emphysema is smoking, as the chemical irritants in cigarette smoke damage the alveolar walls

The damage to lung tissue leads to the recruitment of phagocytes to the region, which produce an enzyme called elastase
This elastase, released as part of an inflammatory response, breaks down the elastic fibres in the alveolar wall
Elastase activity can be blocked by an enzyme inhibitor (α-1-antitrypsin), but not when elastase concentrations are increased
A small proportion of emphysema cases are due to a hereditary deficiency in this enzyme inhibitor due to a gene mutation

Question 32

State and explain the different types of lung capacity

Answer 32

Total lung capacity – Volume of air in the lungs after a maximal inhalation (~ 6 litres in a normal adult male)

Vital capacity – Volume of air that can be exchanged by the lungs via a maximal inhalation and exhalation

Residual volume – Volume of air that is always present in the lungs (~ 20% of total lung capacity)

Tidal volume – Volume of air that is exchanged via normal breathing (~ 500 ml per breath)

Question 33

Discuss asthma

Answer 33

Asthma is a common, chronic inflammation of the airways to the lungs (i.e. bronchi and bronchioles)

Inflammation leads to swelling and mucus production, resulting in reduced airflow and bronchospasm
During an acute asthma attack, constriction of the bronchi smooth muscle may cause significant airflow obstruction
Common symptoms of an asthma attack include shortness of breath, chest tightness, wheezing and coughing
Severe cases of asthma may be life threatening if left untreated

Asthma may be caused by a number of variable and recurring environmental triggers, including allergens, smoke, cold air, certain medications and arthropods (e.g. dust mites)

Question 34

State the different ways to monitor ventilation rate

Answer 34

Via simple observation (counting number of breaths per minute)
Chest belt and pressure meter (recording the rise and fall of the chest)
Spirometer (recording the volume of gas expelled per breath)

Question 35

Explain Allergic reactions

Answer 35

When a specific B cell first encounters the allergen, it differentiates into plasma cells and makes large quantities of antibody (IgE)
The IgE antibodies attach to mast cells, effectively ‘priming’ them towards the allergen
Upon re-exposure to the allergen, the IgE-primed mast cells release large amounts of histamine which causes inflammation

The release of histamine from IgE-primed mast cells causes an inflammatory response that results in allergic symptoms
Inflammation improves leukocytes mobility to infected regions by triggering vasodilation and increasing capillary permeability

Vasodilation is the widening of blood vessels to improve the circulation of blood to targeted regions
Vasodilation causes redness (as vessel expansion moves blood closer to the skin) and heat (which is transported in blood)

Increased capillary permeability leads to swelling (more fluid leaks from the blood) and pain (swelling causes compression of nerves)
Redness, heat, swelling and localised pain are all typical symptoms of an allergic response

Question 36

How does the body distinguish between self and not self

Answer 36

All nucleated cells of the body possess unique and distinctive surface molecules that identify it as self
These self markers are called major histocompatibility complex molecules (MHC class I)

Any substance that is recognised as foreign and is capable of triggering an immune response is called an antigen (non self)

Question 37

How is disease transmitted?

Answer 37

Direct contact – the transfer of pathogens via physical association or the exchange of body fluids
Contamination – ingestion of pathogens growing on, or in, edible food sources
Airborne – certain pathogens can be transferred in the air via coughing and sneezing
Vectors – intermediary organisms that transfer pathogens without developing disease symptoms themsel

Question 38

Discuss transition of disease between different species

Answer 38

Pathogens are generally species-specific in that their capacity to cause disease (pathogenesis) is limited to a particular species

Certain pathogens may cross the species barrier and be able to infect and cause disease in a range of hosts

Diseases from animals that can be transmitted to humans are called zoonotic diseases (or zoonoses)

Question 39

Explain the role of memory cells in immunity

Answer 39

Memory cells are produced to prevent this delay in subsequent exposures and hence prevent disease symptoms developing

When a B lymphocyte is activated and divides to form plasma cells, a small proportion will differentiate into memory cells
Memory cells are long living and will survive in the body for many years, producing low levels of circulating antibodies
If a second infection with the same pathogen occurs, memory cells will react more vigorously to produce antibodies faster
As antibodies are produced faster, the pathogen cannot reproduce in sufficient amounts to cause disease symptoms
Hence, because pathogen exposure no longer causes the disease to occur, the individual is said to be immune

Question 40

Explain vaccination and herd immunity

Answer 40

ccinations induce long-term immunity to specific pathogenic infections by stimulating the production of memory cells

A vaccine is a weakened or attenuated form of the pathogen that contains antigens but is incapable of triggering disease
The antigenic determinants in a vaccine may be conjugated to an adjuvant, which functions to boost the immune response
The body responds to an injected vaccine by initiating a primary immune response, which results in memory cells being made
When exposed to the actual pathogen, the memory cells trigger a more potent secondary immune response
As a consequence of this more potent immune response, disease symptoms do not develop (individual is immune to pathogen)

Herd immunity is when individuals who are not immune to a pathogen are protected from exposure by the large amounts of immune individuals within the community

Question 41

Distinguis between Active and Passive immunity

Answer 41

Examples of Active Immunity

Natural – Producing antibodies in response to exposure to a pathogenic infection (i.e. challenge and response)
Artificial – Producing antibodies in response to the controlled exposure to an attenuated pathogen (i.e. vaccination)

Examples of Passive Immunity

Natural – Receiving antibodies from another organism (e.g. to the foetus via the colostrum or a newborn via breast milk)
Artificial – Receiving manufactured antibodies via external delivery (e.g blood transfusions of monoclonal antibodi

Question 42

State the function of the skeleton

Answer 42

Skeletons are a rigid framework that function to provide support and protection for body organs. Skeletons provide a surface for muscle attachment and thus facilitate the movement of an organism

Question 43

State the main components of joints

Answer 43

Joint capsule – Seals the joint space and provides stability by restricting the range of possible movements
Cartilage – Lines the bone surface to facilitate smoother movement, as well as absorbing shock and distributing load
Synovial fluid – Provides oxygen and nutrition to the cartilage, as well as lubrication (reduces friction)

Question 44

List the type of joint is order of mobility

Answer 44

There are six main types of synovial joints that allow for different ranges of movement, which are (in order of mobility):

Plane joints (least mobility), hinge joints, pivot joints, condyloid joints, saddle joints, ball and socket joints (most mobility)

Question 45

Explain antagonistic muscle with refence to insect legs

Answer 45

Skeletal muscles exist in antagonistic pairs (when one contracts, the other relaxes) to enable opposing movements

Opposing movements may include: flexion vs extension

When the flexor muscle contracts, the extensor muscle relaxes and the tibia and femur are brought closer together

This retracts the hind quarters in preparation for pushing off the ground

When the extensor muscle contracts, the flexor muscle relaxes and the tibia is pushed away from the femur

This extends the hind quarters and causes the insect to jump

Question 46

State the organisation of muscle fibers

Answer 46

Skeletal muscles consist of tightly packaged muscular bundles (fascicles) surrounded by connective tissue (perimysium

Each bundle contains multiple muscle fibres, which are formed when individual muscle cells fuse together

Muscle fibres contain tubular myofibrils that run the length of the fibre and are responsible for muscular contraction

The myofibrils can be divided into repeating sections called sarcomeres, each of which represent a single contractile unit

Question 47

State the structure of a muscle fiber

Answer 47

They are multinucleate (fibres form from the fusion of individual muscle cells and hence have many nuclei)
They have a large number of mitochondria (muscle contraction requires ATP hydrolysis)
They have a specialised endoplasmic reticulum (it is called the sarcoplasmic reticulum and stores calcium ions)
They contain tubular myofibrils made up of two different myofilaments – thin filament (actin) and thick filament (myosin)
The continuous membrane surrounding the muscle fibre is called the sarcolemma and contains invaginations called T tubules

Question 48

Explain muscle contraction

Answer 48

1. Depolarisation and Calcium Ion Release

An action potential from a motor neuron triggers the release of acetylcholine into the motor end plate
Acetylcholine initiates depolarisation within the sarcolemma, which is spread through the muscle fibre via T tubules
Depolarisation causes the sarcoplasmic reticulum to release stores of calcium ions (Ca2+)

2. Actin and Myosin Cross-Bridge Formation

On actin, the binding sites for the myosin heads are covered by a blocking complex (troponin and tropomyosin)
Calcium ions bind to troponin and reconfigure the complex, exposing the binding sites for the myosin heads
The myosin heads then form a cross-bridge with the actin filaments

3. Sliding Mechanism of Actin and Myosin

ATP binds to the myosin head, breaking the cross-bridge between actin and myosin
ATP hydrolysis causes the myosin heads to change position and swivel, moving them towards the next actin binding site
The myosin heads bind to the new actin sites and return to their original conformation
This reorientation drags the actin along the myosin in a sliding mechanism
The myosin heads move the actin filaments in a similar fashion to the way in which an oar propels a row boat

4. Sarcomere Shortening

The repeated reorientation of the myosin heads drags the actin filaments along the length of the myosin
As actin filaments are anchored to Z lines, the dragging of actin pulls the Z lines closer together, shortening the sarcomere
As the individual sarcomeres become shorter in length, the muscle fibres as a whole contracts

Question 49

Describe the structure of a sacromere

Answer 49

The thick filament (myosin) contains small protruding heads which bind to regions of the thin filament (actin)

Each individual sarcomere is flanked by dense protein discs called Z lines, which hold the myofilaments in place

The actin filaments radiate out from the Z discs and help to anchor the central myosin filaments in place

The centre of the sarcomere appears darker due to the overlap of both actin and myosin filaments (A band)

The peripheries of the sarcomere appear lighter as only actin is present in this region (I band)

The dark A band may also contain a slightly lighter central region where only the myosin is present (H zone)

Question 50

Discuss osmoconformers and osmoregulators

Answer 50

Water levels within an organism are constantly changing as a result of metabolic activity

The concentration of water within cells (osmolarity) will impact tissue viability

Osmoconformers maintain internal conditions that are equal to the osmolarity of their environment

By matching internal osmotic conditions to the environment, osmoconformers minimise water movement in and out of cells
Less energy is used to maintain internal osmotic conditions within an osmoconformer

Osmoregulators keep their body’s osmolarity constant, regardless of environmental conditions

While osmoregulation is a more energy-intensive process, it ensures internal osmotic conditions are always tightly controlled
Osmoregulators can maintain optimal internal conditions whereas osmoconformers are affected by environmental conditions

Question 51

Define excretion

Answer 51

Excretion is the removal from the body of the waste products of metabolic activity

Question 52

Discuss why and how different animals get rid of nitrogenous waste

Answer 52

Nitrogenous wastes are toxic to the organism and hence excess levels must be eliminated from the body
The type of nitrogenous waste in animals is correlated with the evolutionary history of the animal and the habitat

Most aquatic animals eliminate their nitrogenous wastes as ammonia (NH3)

Ammonia is highly toxic but also very water soluble and hence can be effectively flushed by animals in aquatic habitats

Terrestrial animals have less access to water and hence must package nitrogenous waste in less toxic forms

Mammals eliminate their nitrogenous wastes as urea, which is less toxic and hence can be stored at higher concentrations
Reptiles and birds eliminate wastes as uric acid, which requires more energy to make but is relatively non-toxic and requires even less water to flush (it is eliminated as a semi-solid paste)

Question 53

Outline excretion in insects

Answer 53

Insects have a circulating fluid system called hemolymph that is analogous to the blood system in mammals
Malpighian tubules branch off from the intestinal tract and actively uptake nitrogenous wastes and water from the hemolymph
The tubules pass these materials into the gut to combine with the digested food products
Solutes, water and salts are reabsorbed into the hemolymph at the hindgut, whereas nitrogenous wastes (as uric acid) and undigested food materials are excreted via the anus

Question 54

How does blood in renal artery and renal vein differ

Answer 54

Blood in the renal vein (i.e. after the kidney) will have:

Less urea (large amounts of urea is removed via the nephrons to form urine)
Less water and solutes / ions (amount removed will depend on the hydration status of the individual)
Less glucose and oxygen (not eliminated, but used by the kidney to generate energy and fuel metabolic reactions)
More carbon dioxide (produced by the kidneys as a by-product of metabolic reactions)

Question 55

State the features of a nephron

Answer 55

Bowman’s capsule – first part of the nephron where blood is initially filtered (to form filtrate)
Proximal convoluted tubule – folded structure connected to the Bowman’s capsule where selective reabsorption occurs
Loop of Henle – a selectively permeable loop that descends into the medulla and establishes a salt gradient
Distal convoluted tubule – a folded structure connected to the loop of Henle where further selective reabsorption occurs

Question 56

Explain ultrafiltration

Answer 56

Structure of the Bowman’s Capsule

As the blood moves into the kidney via afferent arterioles it enters the glomerulus
This glomerulus is encapsulated by the Bowman’s capsule, which is comprised of an inner surface of cells called podocytes
Between the podocytes and the glomerulus is a glycoprotein matrix called the basement membrane that filters the blood

Basement Membrane

Glomerular blood vessels are fenestrated (have pores) which means blood can freely exit the glomerulus
The podocytes of the Bowman’s capsule have gaps allowing for fluid to move freely into the nephron
The basement membrane is size-selective and restricts the passage of blood cells and large proteins

Hydrostatic Pressure

Ultrafiltration involves blood being forced at high pressure against the basement membrane, optimising filtration
This high hydrostatic pressure is created in the glomerulus by having a wide afferent arteriole and a narrow efferent arteriole
The glomerulus forms extensive narrow branches, which increases the surface area available for filtration

Question 57

Outline selective reabsorbtion in the nephrons

Answer 57

It involves the reuptake of useful substances from the filtrate and occurs in the convoluted tubules (proximal and distal)
The majority of selective reabsorption occurs in the proximal convoluted tubule, which extends from the Bowman’s capsule

The proximal convoluted tubule has a microvilli cell lining to increase the surface area for material absorption from the filtrate
The tubule is a single cell thick and connected by tight junctions, which function to create a thin tubular surface with no gaps
There are also a large number of mitochondria within these tubule cells, as reabsorption involves active transport

The tubules reabsorb all glucose, amino acids, vitamins and hormones, along with most of the mineral ions (~80%) and water
Mineral ions and vitamins are actively transported by protein pumps and carrier proteins respectively
Glucose and amino acids are co-transported across the apical membrane with sodium (symport)
Water follows the movement of the mineral ions passively via osmosis

Question 58

Explain the formation of a salt gradient by loop of henle

Answer 58

The function of the loop of Henle is to create a high solute (hypertonic) concentration in the tissue fluid of the medulla
The descending limb of the loop of Henle is permeable to water but not salts
The ascending limb of the loop of Henle is permeable to salts but not water
This means that as the loop descends into the medulla, the interstitial fluid becomes more salty and hypertonic

Question 59

Explain how water reabsorbtion is hormone controlled

Answer 59

Water Reabsorption

As the collecting duct passes through the medulla, the hypertonic conditions of the medulla will draw water out by osmosis
The amount of water released from the collecting ducts to be retained by the body is controlled by anti-diuretic hormone (ADH)
ADH is released from the posterior pituitary in response to dehydration (detected by osmoreceptors in the hypothalamus)
ADH increases the permeability of the collecting duct to water, by upregulating production of aquaporins (water channels)
This means less water remains in the filtrate, urine becomes concentrated and the individual urinates less (i.e. anti-diuresis)
When an individual is suitably hydrated, ADH levels decrease and less water is reabsorbed (resulting in more dilute urine)
Remember: ADH is produced when you Are DeHydrated

Question 60

State consequences of over and Under dehydration

Answer 60

Dehydration

Dehydration is a loss of water from the body such that body fluids become hypertonic
Individuals will experience thirst and excrete small quantities of heavily concentrated urine (to minimise water loss)
Blood pressure will drop (less water in plasma) and the heart rate will increase to compensate for this
The individual will become lethargic and experience an inability to lower body temperature (due to lack of sweat)
Severe cases of dehydration may cause seizures, brain damage and eventual death

Overhydration

Overhydration is a less common occurrence that results when an over-consumption of water makes body fluids hypotonic
Individuals will produce excessive quantities of clear urine in an effort to remove water from the body
The hypotonic body fluids will cause cells to swell (due to osmotic movement), which can lead to cell lysis and tissue damage
Overhydration can lead to headaches and disrupted nerve functions in mild cases (due to swelling of cells)
In severe cases, overhydration may lead to blurred vision, delirium, seizures, coma and eventual death

Question 61

How does loop of henle length correlate to environment

Answer 61

With more drought conditions the henle increase in length to preserve more water

Question 62

State what can be found in a urinary anlaysis

Answer 62

Glucose: The presence of glucose in urine is a common indicator of diabetes (high blood glucose = incomplete reabsorption)

Proteins: High quantities of protein in urine may indicate disease (e.g. PKU) or hormonal conditions (e.g. hCG = pregnancy)

Blood cells: The presence of blood in urine can indicate cancer

Drugs / toxins: Many drugs pass through the body into urine and can be detected (e.g. performance enhancing drugs)

Question 63

Discuss treatment of kidney failure

Answer 63

Kidney dialysis involves the external filtering of blood in order to remove metabolic wastes in patients with kidney failure

Blood is removed and pumped through a dialyzer, which has two key functions that are common to biological membranes:

It contains a porous membrane that is semi-permeable (restricts passage of certain materials)
It introduces fresh dialysis fluid and removes wastes to maintain an appropriate concentration gradient

Kidney dialysis treatments typically last about 4 hours and occur 3 times a week – these treatments can be effective for years

The best long-term treatment for kidney failure is a kidney transplant:

The transplanted kidney is grafted into the abdomen, with arteries, veins and ureter connected to the recipient’s vessels
Donors must typically be a close genetic match in order to minimise the potential for graft rejection
Donors can survive with one kidney and so may commonly donate the second to relative suffering kidney failure

Question 64

Outline Spermatogenesis

Answer 64

Spermatogenesis describes the producton of spermatozoa (sperm) in the seminiferous tubules of the testes

The process begins at puberty when the germline epithelium of the seminiferous tubules divides by mitosis

These cells (spermatogonia) then undergo a period of cell growth, becoming spermatocytes

The spermatocytes undergo two meiotic divisions to form four haploid daughter cells (spermatids)

The spermatids then undertake a process of differentiation in order to become functional sperm cells (spermatozoa)

Question 65

Outline oogenesis

Answer 65

Oogenesis describes the production of female gametes (ova) within the ovaries (and, to a lesser extent, the oviduct)

The process begins during foetal development, when a large number of primordial cells are formed by mitosis (~40,000)

These cells (oogonia) undergo cell growth until they are large enough to undergo meiosis (becoming primary oocytes)

The primary oocytes begin meiosis but are arrested in prophase I when granulosa cells surround them to form follicles

The primary oocytes remain arrested in prophase I until puberty, when a girl begins her menstrual cycle

Each month, hormones (FSH) will trigger the continued division of some of the primary oocytes

These cells will complete the first meiotic division to form two cells of unequal size

One cell retains the entirety of the cytoplasm to form a secondary oocyte, while the other cell forms a polar body

The polar body remains trapped within the follicle until it eventually degenerates

The secondary oocyte begins the second meiotic division but is arrested in metaphase II

The secondary oocyte is released from the ovary (ovulation) and enters into the oviduct (or fallopian tube)

The follicular cells surrounding the oocyte form a corona radiata and function to nourish the secondary oocyte

If the oocyte is fertilised by a sperm, chemical changes will trigger the completion of meiosis II and the formation of another polar body (the first polar body may also undergo a second division to form a third polar body)

Once meiosis II is complete the mature egg forms a ovum, before fusing its nucleus with the sperm nucleus to form a zygote

Question 66

Contrast spermatogenisis and oogenesis

Answer 66

1. Number of cells produced

In spermatogenesis, the cells divide equally during meiosis to produce four functional gametes
In oogenesis, the cells do not divide equally and as a result only one functional gamete is formed (plus 2 – 3 polar bodies)

2. Size of cells produced

In spermatogenesis, the cells that are formed following differentiation are all of equal size with equal amounts of cytoplasm
In oogenesis, one daughter cell (the ovum) retains all of the cytoplasm, while the other daughter cells form polar bodies
The polar bodies remain trapped within the surrounding layer of follicle cells until they eventually degenerate

3. Timing of the process

In spermatogenesis, the production of gametes is a continuous process that begins at puberty and continues until death
In oogenesis, the production of gametes is a staggered and finite process:
It begins before birth (prenatally) with the formation of a fixed number of primary oocytes (~40,000)
It continues with the onset of puberty according to a monthly menstrual cycle
It ends when hormonal changes prevent the further continuance of the menstrual cycle (menopause)

Question 67

Outline fertilization

Answer 67

1. Capacitation

Capacitation occurs after ejaculation, when chemicals released by the uterus dissolve the sperm’s cholesterol coat

This improves sperm motility (hyperactivity), meaning sperm is more likely to reach the egg (in the oviduct)
It also destabilises the acrosome cap, which is necessary for the acrosome reaction to occur upon egg and sperm contact

2. Acrosome Reaction

When the sperm reaches an egg, the acrosome reaction allows the sperm to break through the surrounding jelly coat

The sperm pushes through the follicular cells of the corona radiata and binds to the zona pellucida (jelly coat)
The acrosome vesicle fuses with the jelly coat and releases digestive enzymes which soften the glycoprotein matrix
The sperm then pushes its way through the softened jelly coat and binds to exposed docking proteins on the egg membrane
The membrane of the egg and sperm then fuse and the sperm nucleus (and centriole) enters the egg

3. Cortical Reaction

The cortical reaction occurs once a sperm has successfully penetrated an egg in order to prevent polyspermy

Cortical granules within the egg’s cytoplasm release enzymes (via exocytosis) into the zona pellucida (jelly coat)
These enzymes destroy sperm binding sites and also thicken and harden the glycoprotein matrix of the jelly coat
This prevents other sperm from being able to penetrate the egg (polyspermy), ensuring the zygote formed is diploid

Question 68

Outline the development of a zygote after fertilization

Answer 68

Blastocyst Formation

Following the fusion of an egg and sperm (fertilization), an influx of Ca2+ into the ova prompts the completion of meiosis II

The egg and sperm nuclei combine to form a diploid nuclei and the fertilized cell is now called a zygote

The zygote will undergo several mitotic divisions to form a solid ball of cells called a morula. As the morula continues to divide, it undergoes differentiation and cavitation (cavity formation) to form a blastocyst

Implantation of Blastocyst

The final stage of early embryo development is the implantation of the blastocyst into the endometrial lining of the uterus

The blastocyst breaches the jelly zona pellucida that was surrounding it and preventing its attachment to the endometrium

Digestive enzymes are released which degrade the endometrial lining, while autocrine hormones released from the blastocyst trigger its implantation into the uterine wall from the trophoblastic end

Only once the blastocyst is embedded within the uterine wall can the next stage of embryogenesis occur

The growing embryo will gain oxygen and nutrients from the endometrial tissue fluid, ensuring its continued development
The entire process (from fertilization to implantation) takes roughly 6 – 8 days

Question 69

Explain the role of hCG in maintaining pregnancy

Answer 69

hCG promotes the maintenance of the corpus luteum within the ovary and prevents its degeneration

As a consequence of this, the corpus luteum survives and continues to produce both oestrogen and progesterone

The levels of hCG are maintained for roughly 8 – 10 weeks while the placenta is being developed

After this time, the placenta becomes responsible for progesterone secretion and nourishing the embryo
At this point the corpus luteum is no longer required and begins to degenerate as hCG levels drop

Question 70

How does material exchange between fetus and mother occur via the placenta

Answer 70

The chorionic villi extend into the intervillous space (lacuna) and exchange materials between the mother and foetus
Chorionic villi are lined by microvilli to increase the available surface area for material exchange
Foetal capillaries within the chorionic villi lie close to the surface to minimise diffusion distance from blood in the lacunae
Materials such as oxygen, nutrients, vitamins, antibodies and water will diffuse from the lacunae into foetal capillaries
Foetal waste (such as carbon dioxide, urea and hormones) will diffuse from the lacunae into the maternal blood vessels

Question 71

Outline the hormonal control of birth

Answer 71

After 9 months, the baby is fully grown and stretches the walls of the uterus – placing a strain on both mother and infant

This stress induces the release of chemicals which trigger a rise in the levels of estrogen which prepares the smooth muscle of the uterus for hormonal stimulation by increasing its sensitivity to oxytocin

Estrogen also inhibits progesterone, which was preventing uterine contractions from occurring while the foetus developed

Now that the uterus is primed for childbirth, the brain triggers the release of oxytocin from the posterior pituitary gland. Oxytocin stimulates the uterine muscles to contract, initiating the birthing process (it also inhibits progesterone secretion)

The foetus responds to this uterine contraction by releasing prostaglandins, which triggers further uterine contractions

As the uterine contractions trigger the release of chemicals that cause further contractions, a positive feedback loop ensues

Contractions will stop when labour is complete and the baby is birthed (no more stretching of the uterine wall)

Question 72

What two factors affect gestation period?

Answer 72

Animal size / mass – larger animals tend to have longer gestation periods (as they tend to produce larger offspring)

The level of development at birth – more developed infants will typically require a longer gestation period

Question 73

State the role of the amniotic sac

Answer 73

The fluid is largely incompressible and good at absorbing pressure, and so protects the foetus from impacts to the uterus

The fluid also creates buoyancy so that the foetus does not have to support its own weight while a skeletal system develops

Amniotic fluid prevents the dehydration of foetal tissues

Question 74

What hormones does the placenta release?

Answer 74

Estrogen and Progesterone