Chapter 11 Flashcards
Animal physiology
How does the immune system recognise that a cell is ‘self?
‘Self’ = cell belongs to our own body and shouldn’t be attacked
- surface of our cells contain large carbs, glycoproteins and other polypeptides that can be recognised by our immune system as self
- bacteria, viruses, parasites, cancer cells and other pathogens have an array of molecules on their surfaces
- immune system can recognise these antigens as ‘non-self’- will trigger an immune response against them
NB/ every organism has unique molecules on the surface of its cells
- helps immune system to recognise cells as self
Antigen
any molecule that can trigger an immune response leading to the generation of antibodies
ABO blood group system
- presence of cell surface antigens is the basis for this system
- there’s also the Rhesus factor
Rh+ = found on surface of RBC in people who are rhesus positive
Rh- = when you don’t have the Rhesus factor on surface of RBC
NB/ all RBC have a standard complex carbohydrate, antigen H, on their surface
Blood group A
a molecule of N-acetylgalactosamine is added to antigen H
- has anti-B antibodies
Blood group B
a molecule of galactose is added to antigen H
- has anti-A antibodies
Agglutination
when antigens on the surface of RBC stimulate antibody production in a person w/ a different blood group
Blood group AB
- has both A and B antigens on RBC surface
- hence, body doesn’t have antibodies to A or B
- universal recipient because they can receive blood from any blood type
Blood group O
- has no A or B antigens
- but it has anti A + B antibodies (don’t affect transfusion)
- universal donor because it contains neither A or B antigens on RBC surface
Haemolysis
breakage of RBC membrane
- leads to release of haemoglobin and other internal components into the surrounding fluid
- agglutination will lead to haemolysis and may result in the death of the patient
Cellular immune response
- Antigen is ingested, via phagocytosis, by macrophages and B cells
- Both process and present antigen on their surface
- only B cells w/ antibodies that can bind the antigen will take in antigen for processing
- only B cells that can produce specific antibodies against antigen are selected for cloning later - Macrophage interacts w/ a helper T cell, activating it
- Activated helper T cell interacts w/ B cell that has antigen on its surface, and activates it
- Activated B cell rapidly divides by mitosis to form clones of plasma cells and memory cells
- Memory B cells stay in body for years after both plasma cells and antibodies have disappeared
- if an infection w/ same antigen recurs, memory B cells quickly divide to form plasma cells and new set of memory cells
- secretes specific antibodies against this specific infection
- memory cells provide long-term immunity to disease
Plasma cells
- produce antibodies of one specific type against the antigen
- have lots of rER and a well-developed Golgi apparatus
Antibodies
Antibodies: proteins that bind to foreign substances
- produced one immune system has reacted to invasion of an antigen
- help to destroy the antigen
- has a constant and variable region
Variable region: part of antibody which is highly specific to a particular antigen- long and short chains are held together by disulphide bonds
Immunity
To be immune against a certain infectious pathogen:
- body needs antibodies that are already in your blood
- or has memory cells that produce a specific antibody against this type of infective agent
Opsonisation
coating of a pathogen w/ antibodies to promote and enhance phagocytosis
Complement proteins
- group of more than 20 proteins that are present in blood and tissue fluid
- normally inactive
- some become activated when they are presented w/ antigens, this is fulfilled by antibodies
Process of opsonisation
- an antigen bound to an antibody is presented to a complement protein
- complement protein is cleaved, to produce activated protein- binds to membrane proteins of pathogens
- this creates pores in the membrane of the pathogenic cell, leading to its lysis or increases its chance of being engulfed by a phagocytic cell
- activated complement proteins may trigger release of histamines from basophils
- can also attract phagocytes to infection sites to enhance elimination of pathogens
Antigens vs. antibodies
Antigens: any entities that trigger an immune response
- eg. virus, bacterium, parasites, fungus or large glycoprotein
Antibodies: proteins produced by plasms cells ( a B cell originally) in response to an antigenic reaction
Primary immune response
Immune response triggered on the first encounter of the body w/ an antigen
What happens after primary immune response?
Following primary response, memory cells produced ensure that, if another infection w/ same pathogen occurs, body responds quickly
Secondary immune response
immune response stimulated on the second exposure to the same antigen
Describe the primary and secondary immune response
- initial conc. of antibodies to antigen A drops quickly
- memory cells produced during first infectious period ensure that when immune system is challenged a second time w/ same antigen A, reaction is faster
- when immune system is challenged a second time, memory cells (formed during primary response) divide by mitosis to form clones of plasma cells and memory cells
- plasma cells produce antibodies to give a fast response to invading pathogen
- the memory cells stay in the body to defend against any future attack
Why is the secondary immune response faster than the primary one?
The fact that memory cells can directly give rise to plasma cells without the need for antigen presentation or activation of helper T cells and B cells, allows secondary response to be much faster than the primary response
Vaccinations
- inject an attenuated form of pathogen or a toxin that is produced by the pathogen into the body
- vaccines contain antigens that trigger immunity but don’t cause disease
- inactivated toxin triggers a primary immune response resulting in production of antibodies and memory cells
- when actually infected by the toxin, memory cells can produce massive amounts of antibody
- ensures that macrophages and other killer cells can dispose of the infection
Zoonosis
transmission of a disease from animals to humans
Allergen
any substance that can cause an allergic reaction
- body treats the allergen as foreign/dangerous
- thus, produces a strong immune response to a substance that is generally harmless to the body
Histamine
- an organic nitrogenous compound involved in local immune responses
- can also act as a neurotransmitter
- produced by basophils and mast cells (both are types of mast cells) found in the connective tissues
Function:
- dilate and increase permeability of capillaries
- enables WBC, eg. mast cells and some proteins, to invade affected tissues and engage the allergens
Immune response after an allergen has entered the body
- An allergen enters the body
- A B cell comes into contact w/ the allergen
- Plasma cells start producing allergen-specific IgE, an antibody which circulates in the blood and binds to mast cells
- These types of immune cells are involved in acute inflammatory response and are sensitised to allergen
- In a later exposure to the same allergen, allergen will bind to specific IgE and activate mast cells
- This triggers release of histamines (a process called degranulation) and other cellular inflammatory compounds
- Histamine can bind to membrane-bound histamine receptors and cause allergic symptoms
- depending on the tissue, it can cause itchiness, a runny nose, sneezing or, in more serious cases, redness and swelling
- severe allergic reactions can lead to anaphylactic shock, potentially leading to death. - In a normal case of an allergic reaction, use of antihistamine is recommended
Anaphylactic shock
an extreme and often life-threatening allergic reaction to an antigen to which the body has become hypersensitive
Antihistamines
Drugs that inhibit the action of histamine in the body by blocking the receptors of histamine
- reduce leakiness of the capillaries
- in case of a severe allergic reaction, hospitalisation and direct injection with epinephrine may be needed
Monoclonal antibodies
monospecific antibodies produced from one cloned plasma cell
- can recognise and bind to one specific region of the antigen (epitope)
- produced by hybridoma cells
Monospecific antibodies
antibodies that target the same antigen
Epitope
a short AA sequence on the antigen that the antibody is able to recognise
Polyclonal antibodies
- antibodies secreted by plasma cells
- derived from different B cell lineages that have recognised different epitopes of one specific antigen
- hence, are a mixture of antibodies w/ different affinities for same antigen
Hybridoma cell
Fusion of a tumour cell with an antibody-producing plasma cell creates a hybridoma cell
Production of monoclonal antibodies
- A mouse is injected with an antigen X for which a monoclonal antibody is needed
- Once mouse’s spleen starts to produce polyclonal antibodies in its plasma B cells, spleen is removed and fused w/ myeloma cells
- Cells are cultured on a medium that’s selective for hybridoma cells
- The selective culture medium is called HAT medium - it contains hypoxanthine, aminopterin and thymidine 5. Unfused myeloma cells can’t grow because they lack HGPRT, hypoxanthine-guanine-phosphoribosyl transferase (HGPRT) and can’t replicate their DNA
- an enzyme necessary for synthesis of nucleic acids and can’t replicate their DNA - Hybridoma cells can replicate their own DNA because they get their HGPRT from the spleen cells that were used in the fusion
- Each hybridoma cell is then cultured separately and screened using epitope of desired antibody as probe
- Once it’s confirmed that a certain hybridoma is producing right antibody, it’s cultured indefinitely and monoclonal antibodies are harvested from it
How do pregnancy tests work?
- home pregnancy tests work by binding to hCG (human chorionic gonadotropin) hormone
- hormone is produced by trophoblast cells of fertilised ova- can be detected after embryo has implanted itself in uterus lining
Process of pregnancy tests
- Dipstick is dipped into a sample of woman’s urine
- If urine contains hCG, anti-hCG will bind to hormone
- This antibody is coupled w/ a blue dye indicator
- If this complex binds to the monoclonal antibody that’s attached to the membrane (in the dipstick), a coloured line will appear
- indicates that the woman is pregnant - As the urine moves up the dipstick, it crosses another line, ‘control line’, that has another antibody anchored to the test membrane
- The control line contains an antibody to the dye
- if this second line is not indicated, the test is invalid
Exoskeleton in insects and crustaceans
- an external structure usually made of chitin (a modified polysaccharide containing nitrogen)
- it protects softer body parts of these animals
Jump sequence of a cockroach
- Flexor muscle contracts, extensor muscle relaxes- tibia flexes
- Extensor muscle contractions, flexor muscle relaxes- tibia extends
- Thus, the cockroach jumps
Biceps
Bicep muscles are located above the humerus and flex (bend) the forearm
Triceps
Tricep muscles are located under the humerus and extend the forearm
Function of bones and exoskeletons
- provide anchorage for muscles
- act as levers
- movement of the body requires muscles to work in antagonistic pairs
Joint
where two bones come together
- allows movements, but only in some directions
- movement is made possible because the cartilage that covers the ends of the bone prevents friction
Composition of bone
- made of many materials, including calcium phosphate, collagen and elastic protein
Cartilage
- composed of specialised cells called chondrocytes
- these cells produce an extracellular matrix, composed of collagen fibres, proteoglycan and elastin fibres
Synovial joints
joints that have a synovial cavity between the two bones
- enclosed in a joint capsule which helps prevent dislocation
- in the joint capsule, synovial fluid reduces friction between the bones by acting as a lubricant
Synovial cavity
cavity is filled w/ synovial fluid that reduces friction at the joint, allowing bones to move freely
- allows a high range of motion
- but synovial joints allow certain movements but not others
Dislocation
an injury to a joint where the bone ends are forced from their normal positions
- painful
- stops you moving your joint
Knee joint
- allows leg to extend and flex
- a synovial joint
- a modified hinge joint
- only extension and flexion
Hip joint
- ball and socket joint
- a synovial joint
- allows leg to flex, extend, rotate and swing from side to side
Sarcolemma
the cell membrane of a striated muscle fibre
Muscle cells
- smooth muscle doesn’t have striations
- muscle cells are elongated, contain many nuclei and mitochondria (to supply high ATP requirements) and are surrounded by a sarcolemma
Structure of musce fibres
- muscle fibres consist of many myofibrils and have a lot of endoplasmic reticulum
- in muscles the ER is called the sarcoplasmic reticulum
- SR stores calcium, which is needed for calcium contraction
- it surrounds each myofibril, allowing it to convey a signal to all parts of the muscle to contract or relax
- each myofibril of the muscle fibre consists of contractile sarcomeres- subunits that contract
Sarcomere
- functional unit of the myofibrils
- contains thick myosin and thin actin filaments
- where actin and myosin filaments overlap (A band), actin and myosin filaments form cross-bridges which bring about muscle contraction
- thin actin filaments are attached to the Z line
I band= region of sarcomere where only actin filaments are present
A band= thin filament partly overlaps thick myosin filaments, appearing as a dark section
H band= in the middle, between the two Z lines, only myosin present
Sliding filament theory
- explains contraction of skeletal muscle
According to this theory:
- actin and myosin filaments slide over each other to make muscle shorter
- actin slides over myosin moving inwards towards the centre of the sarcomere
- makes the length of all the sarcomeres shorten, thus, the entire muscle becomes shorter
Sliding of the filament:
- achieved by an interaction between the myosin heads, actin filaments and ATP hydrolysis
NB/- ATP hydrolysis and cross bridge formation are necessary for the filaments to slide
Sarcomere contraction
- As new ATP attaches to the myosin head, the cross bridge detaches
- As ATP is split into ADP and Pi, cocking of the myosin head occurs
- A cross-bridge forms and the myosin head binds to a new position on actin
- Power stroke- the myosin head pivots and bends as it pulls on the actin filaments, sliding it towards the M line
NB/ - when a muscle contracts, all the sarcomeres contract
- each sarcomere exerts only a tiny amount of force, but all together the result is substantial
Actin filaments
contain actin and two proteins: tropomyosin and troponin
Tropomyosin: forms 2 strands which wind around the actin filament, covering the binding site for the myosin heads
Sarcomere contraction is dependent on:
- Calcium ions
- Tropomyosin
- Troponin
What happens when a muscle receives a neuronal impulse to contract?
- Calcium ions are released from the sarcoplasmic reticulum
- Calcium ions bind to troponin, which forces tropomyosin to move
- Move exposes actin binding sites
- Myosin heads can now make a cross bridge
- Pivot the actin filaments towards the centre of the sarcomere
Contracted muscle vs. relaxed muscle
- A band (myosin) is the same length in contracted and relaxed muscles
Contracted muscle:
- shorter gap between Z lines
- reduction in H zone
Excretion of nitrogenous waste in animals
Animals excrete nitrogenous waste in 3 forms:
- Urea
- Uric acid
- Ammonia
Type of nitrogenous waste excreted is related to their evolutionary history and habitat
- same time that animals excrete their nitrogenous waste, they control water and electrolyte balance
Ammonia
- toxic at higher conc.
- influences pH and osmolarity of the blood
- hence, it’s immediately converted to less toxic urea, in most mammals, for excretion
- it’s still excreted by fish and aquatic invertebrates
- they have mechanisms that allow fast disposal of ammonia before it builds up to toxic level in their body tissues
Uric acid
- insects, birds and reptiles produce uric acid
- is insoluble in water
- conversion from ammonia to uric acid costs energy, but it helps to conserve water
- important in some arid environments
Osmoregulators
animals that are able to keep/ regulate solute conc. of their body fluids above or below that of their external environment
- organisms have ability to control osmolarity of their tissues within v. narrow limits
- changes in their environment have no effect or cause only small fluctuations in their internal solute conc.
eg. humans and birds
Osmoconformers
marine organisms that actively/passively maintain an internal environment that is isosmotic to their external environment
- means that solute conc. of their body fluid = solute conc. of external medium in which organisms live
- these organisms can’t regulate solutes of their body fluids at a conc. that’s different to external medium
eg. molluscs, jellyfish and other marine invertebrates
Overhydration
excess intake of water
- happens during sporting events
- when normal balance of electrolytes in the body exceeds safe limits by overhydration
- can also be caused by diseases that encourage water accumulation in the body
Consequence of overhydration:
- swelling up of body cells
- swollen cells in brain can lead to intracranial pressure
- as this pressure increases, blood flow to brain can be interrupted
- leads to dysfunction in CNS, seizures, coma or even death
Other consequences:
- vomiting
- changes in mental state
- muscle weakness or cramps
Dehydration
occurs when more fluid is used or lost than amount being taken in
- body doesn’t have enough water and other fluids to carry out its normal functions
Causes:
- vigorous exercise
- intense diarrhoea
- vomiting
- fever
- sweating
- or not taking in enough fluids
It may cause electrolyte imbalance
Consequences:
- urine becomes darker
- skin will become less elastic
- both HR and BR increase
- BP decreases
- may also affect ability to sweat
Osmoregulation
maintenance by an organism of an internal balance between water and dissolved materials, regardless of environmental conditions
- includes control of water balance of the blood, tissue or cytoplasm of a living organism
Kidneys
- play an essential role in osmoregulation and excretion
- filter blood to rid body of nitrogenous waste
- also regulates osmolarity and produces urine
- urine leaves the kidney via the ureter
Composition of blood in the kidney
Blood entering kidney:
- enters through the renal artery
- contains drugs, toxins, nitrogenous waste products (mainly urea), excess water and salt
Blood leaving kidney:
- leaves through the renal vein
- conc. of all those compounds is lower in blood in renal vein
- filtrate contains most compounds found in the plasma apart from large proteins
- glucose conc. is lower than that of renal artery (glucose is needed for some of its metabolic processes)
- oxygen conc. is lower than that of renal artery
- CO2 conc. is higher than that of renal artery
NB/ if proteins are found in the urine, suggests a malfunctioning kidney
Nephron
- basic functional unit of the kidney
- a long tube from the Bowman’s capsule to the collecting duct
- the collecting duct drains into the renal pelvis
Bowman’s capsule
Highly porous wall which collects the filtrate
Glomerulus (is enclosed in the Bowman’s capsule)
Knot-like capillary bed where high-pressure ultrafiltration takes place
- a knot of intertwined capillaries enveloped by podocytes- support capillaries and regulate filtration
Proximal convoluted tubule
Twisted section of the nephron where water, nutrients and salts are reabsorbed back into the blood; contains many mitochondria and microvilli
Loop of Henle
Hairpin shaped tube with a descending and ascending limb; water and salt reabsorption takes place here
Distal convoluted tubule
Another twisted section of the nephron, where water and salts are reabsorbed back into the blood; also contains many mitochondria and microvilli
Collecting duct
A slightly wider tube that carries the filtrate to the renal pelvis
Afferent arteriole
Brings blood from the renal artery
Efferent arteriole
A narrow blood vessel that restricts blood flow, which helps to generate the pressure needed for filtration
Vasa recta
An unbranched capillary shaped like the loop of Henle, w/ descending limb bringing blood deep into medulla
Podocytes
cells of the inner wall of the Bowman’s capsule
- have many extension which fold around the blood capillary forming a network of filtration slits
- filtration slits hold back the blood cells during ultrafiltration w/ help of glomerular basement membrane
Glomerular capillaries and ultrafiltration
- have fenestrations
- covered on the outside by the basement membrane, mainly composed of glycoproteins
- this is where ultrafiltration takes place- process driven by the high pressure in the capillaries
- fenestrations in the capillary wall allows blood to flow out
- basement membrane acts like a sieve during ultrafiltration process and stops blood cells and large proteins
- hence, WBC and RBC can’t pass through, but small proteins, salts and nutrients can
What generates the high capillary pressure in the glomerulus?
Result of the short, large diameter afferent arterioles that convey blood at high arterial pressures directly to the glomerular capillaries
- smaller diameter of efferent arterioles leaving the glomerulus helps to maintain the pressure by restricting the outflow of blood
Function of the nephron
To reabsorb all the important salts and nutrients that have been filtered out of the plasma
Site of reabsorption
- bulk of all reabsorption takes place in PCT immediately after filtrate has left Bowman’s capsule
- most of the water in the filtrate is also recovered
- out of all the dissolved substances present in the filtrate, only useful substances are reabsorbed by active transport
- cells of PCT have large no. of mitochondria to provide energy for active transport
Reabsorption process in the PCT
- Absorbed Na+, Cl-, glucose and AA are quickly removed by the blood plasma
- glucose and AA are passively transported from cells to the blood
- Na+ is moved by active transport via Na+/K+ pumps in tubule membrane
- pumps use active transport to shuttle Na+ (out of the tubule) and K+ (into the tubule_
- Cl- are attracted to the space outside the tubule because of +ve Na+ - Glucose and AA are absorbed from filtrate by specific carrier proteins- powered by conc. gradient of Na+ entering cells of tubule (secondary transport)
- energy to pump Na+ out of the cell, to power the co-porter when it re-enters the cell, comes from Na+K+-ATPase- requires ATP
- conc. gradient of Cl- is kept low within cells of PCT due to their removal by active transport (actively absorbed into blood) - Glucose and AA conc. within PCT increases as they’re absorbed from filtrate
- conc. is higher than of blood plasma, hence, both glucose and AA are reabsorbed into blood by diffusion
- microvilli in tubule wall cells greatly increases SA- enhances diffusion process
NB/ By end of PCT, approximately 80% of all water, glucose and mineral ions have been reabsorbed
Coporters
carrier proteins powered by diffusion of another ion/molecule
- synport- because Na+ and glucose/AA are moving in same direction across membranes
- antiport- if substances moved in opposite directions
Loop of Henle
- uses a countercurrent system to achieve maximum reabsorption of water and sodium ions
Descending loop of Henle
- water moves out into medullary interstitial fluid by osmosis before being absorbed into vasa recta
- impermeable to sodium ions, so, osmolarity of filtrate increases as water is lost
- as Na+ are pumped out of ascending loop of Henle, interstitial fluid in medulla becomes hypertonic- this facilitates absorption of water in the descending limb
NB/ permeable to water, impermeable to Na+
Ascending loop of Henle
- impermeable to water but permeable to Na+
- Na+ moves out of filtrate into interstitial fluid of the medulla
- salt remains near loop of Henle- helps maintain conc. gradient in the medulla
- ultimate absorption of Na+ from interstitial fluid into vasa recta occurs by active transport
- fluid that leaves loop of Henle is less conc. than the tissue fluid around it
NB/ impermeable to water, permeable to Na+
Vasa recta
blood capillaries that run parallel to the Loop of Henle
- water and ions absorbed from the Loop of Henle are taken into the vasa recta
- conc. gradient in the medulla is maintained by vasa recta countercurrent exchange
- there’s no direct exchange between filtrate and blood
- instead, all substances pass through interstitial region of the medulla
Desert animals and loops of Henle
animals living in the desert have longer loops of Henle
- allows for more water reabsorption
- length of loop of Henle is positively correlated w/ need for water conservation in animals
- medulla in the kidneys of these animals is much thicker to accommodate the longer loop
ADH
- antidiuretic hormone
- regulates water reabsorption in the collecting duct and DCT
- secreted by the posterior lobe of the pituitary gland
Secretion of ADH
When dehydrated:
- osmoreceptors in the hypothalamus monitor solute concentration in the blood
- if solute conc. is too high (water conc. is too low), pituitary secretes ADH
- ADH travels through bloodstream to collecting ducts and DCT
- it increases permeability of these structures to water
- hence, more water is reabsorbed
- solute conc. will decrease
- once situation returns to normal, ADH secretion will stop
- example of control by negative feedback
How does ADH work?
- ADH increases permeability to water by acting on receptors in basolateral membrane of cells in collecting tubules
- binding of ADH to these receptors initiates a series of events- causes specific vesicles w/ aquaporin on their membranes to move and to fuse w/ apical membrane
- insertion of aquaporin water channels makes apical membrane permeable to water
- water moves into the cell through these channels in response to osmotic gradient
- it then passes into circulation across basolateral membrane
- basolateral membrane is always freely permeable to water but apical membrane is permeable only when water channels are inserted
- when osmolarity is back to normal, ADH secretion stops, and water channels are removed from membrane and reform as vesicles
- collecting duct is then impermeable to water
- filtrate flowing through is lost as water without further water absorption
Aquaporins
membrane proteins that form water channels allowing water to pass through the cell membrane
Causes of kidney failure
- diabetes
- hypertension
- untreated urinary tract infections
Treatment for kidney failure
- kidney dialysis (haemodialysis)
- kidney transplant
Haemodialysis
- uses an artificial membrane, dialysis tubing, to remove wastes from the blood, restore proper balance of electrolytes in the blood and eliminate extra fluid from the body
- dialyser is made of two parts: one for the blood, one for dialysate- dialysis tube is a thin membrane that separates these 2 parts
- during haemodialysis, blood cells, proteins and other important molecules remain in the blood- too big to pass through membrane
- smaller waste products eg. urea, creatinine, potassium and extra fluid, pass through membrane and are washed away in dialysate
Urine analysis
- used to detect drug abuse, kidney failure or diabetes
- used for testing kidney function
- tests for presence of cells and a range of compounds
Too much glucose = diabetes
Presence of blood or leucocytes = infection/kidney tumour
Too much protein = ultrafiltration process may be failing, could be an indication of advance and prolonged hypertension
Erythrocytes in urine = indicates a sever infection or tumour
Haemolymph
- all arthropods have haemolymph
- fluid that combines characteristics of blood and interstitial fluid
- their internal organs are bathed in haemolymph
Malpighian tubule system
- insects don’t have a closed circulatory system and a kidney that could cleanse the circulating haemolymph
- instead, they rely on tubules that branch off their hind gut- Malpighian tubule system
- consists of branching tubules extending from alimentary canal
- absorb solutes, water and waste from the surrounding haemolymph
Malpighian tubule system in insects
carries out osmoregulation and removal of nitrogenous wastes
How do insects remove nitrogenous wastes?
- Uric acid, Na+ and K+ are transported into the Malpighian tubules, water follows
- Contents of Malpighian tubules are discharged into the gut
- Some Na+ and K+ are actively transported from the hindgut and rectum back to the haemolymph, water follows
- Uric acid precipitates in rectum and is excreted along w/ wastes
NB/ hind gut absorbs most of the water and mineral salts
- inserts convert ammonia into uric acid, which is insoluble in water, is excreted as a semi-solid in faeces
- formation of uric acid instead of urea is a successful adaptation to water conservation in a terrestrial environment
- less water is lost during its excretion
Sexual reproduction
a form of reproduction that involves fusion of 2 haploid gametes
Gametogenesis
process by which cells of germinal epithelium undergo cell division and differentiation to form haploid gametes
Spermatogenesis
- production of sperm
- takes place in testes
- testes are composed of seminiferous tubules w/ interstitial cells filling up the gaps in between
Process of spermatogenesis
- Spermatogonia (diploid cells, 2n) divide by mitosis to produce more spermatogonia
- Out of the two daughter cells produced, one renews the stock of spermatogonia in the germinal epithelium
- other is involved in sperm production - Spermatogonia grow into larger cells called primary spermatocytes (2n)
- Each primary spermatocyte carries out first division of meiosis to produce 2 secondary spermatocytes (n)
- Each secondary spermatocyte, in turn, carries out second division of meiosis to produce 2 spermatids (n)
- Spermatids then become associated w/ Sertoli cells, that help them to develop into spermatozoa (n)
- Sperm detaches from Sertoli cells and is carried out of testis by fluid in the centre of the seminiferous tubule
Sertoli cells
- nurse cells
- provide nurture and support
- function is to nourish the developing sperm cells through stages of spermatogenesis
Germinal epithelium
the layer of cells on the outer layer of the seminiferous tubule
Spermatogonia
stem cells of the germinal epithelium
Leydig cells
in interstitial space of the testis produce testosterone
- allows spermatocytes to complete meiotic division and mature into spermatozoa
Oogenesis
- involves production of ova
- takes place in the ovaries
- female is born w/ 400 000 primary follicles
- primary follicle consists of primary oocyte, surrounded by a single layer of follicle cells
- but, their development has been halted at the first meiotic division Nothing happens to primary follicle until female reaches puberty
Process of oogenesis
- At the start of each menstrual cycle, FSH stimulates development of primary follicles
- usually, only one primary follicle becomes a mature follicle
- mature follicle contains multiple layers of follicle cells, and a fluid-filled cavity - In mature follicle, oocyte begins 2nd meiotic division
- Oogonia of germinal epithelium divide by mitosis to form more oogonia
- Some of these cells grow and then complete prophase I of meiosis to form primary oocytes
- These become surrounded by a layer of follicle cells- called primary follicles
- After puberty, under influence of FSH, normally one follicle matures every month to form a mature follicle- at the same time, primary oocyte completes meiosis I stopping at metaphase II- forms secondary oocyte
- During ovulation, egg released is a secondary oocyte
- IF fertilisation occurs, meiosis II is completed, otherwise egg remains as secondary oocyte
- Each time a meiotic division is completed, cytoplasm is unequally shared between daughter cells to form a smaller polar body and oocyte (egg that can be fertilised to form a zygote)
NB/ happens every month after puberty when primary follicle starts to mature
Compare and contrast spermatogenesis and oogenesis
Similarities:
- both involve mitosis, cell growth, two divisions of meiosis and differentiation
Differences:
- at the end of their development, two different types of cell result
- sperm cells have almost no cytoplasm, egg cells have increased cytoplasm
S: each complete meiotic division results in 4 spermatids
O: 1st division produces 1 large and 1 small cell
- large cell goes onto 2nd division of meiosis, completing it after fertilisation
- only large cell survives
S: sperm differentiation eliminates most of the cytoplasm
O: egg must increase its cytoplasm
S: from puberty on, teste produce sperm continuously
O: egg formation happens once per menstrual cycle
S: millions of sperm are produced daily
O: only one egg cell matures, and is released during every menstrual cycle
Graafian follicle
an advanced tertiary follicle where all the smaller fluid-filled cavities have fused together (forming one main cavity)
Fertilisation
fusing of two gametes: an egg cell and a sperm cell
- both are haploid, so resulting zygote is diploid
Polyspermy
if more than one sperm could fertilise an egg, it would result in polyploid offspring- this isn’t desirable
Steps of fertilisation
- A sperm cell penetrates follicle cells and binds to receptors of zona pellucida
- The acrosomal reaction occurs: hydrolytic enzymes are released from acrosome by exocytosis
- These enzymes make a hole in the zona pellucida allowing sperm to penetrate plasma membrane of egg
- There is contact between sperm and egg, w/ fusion of the sperm and egg plasma membranes
- The sperm nucleus enters egg cytoplasm, activating the egg
- Results in completion of 2nd meiotic division and a rise in intracellular conc. of calcium, in preparation for the cortical reaction
- The cortical reaction hardens zona pellucida, preventing polyspermy
How is polyspermy prevented?
The cortical reaction hardens zona pellucida, preventing polyspermy
- reaction involves release of a mixture of enzymes, including several proteases, from cortical granules by exocytosis
- These enzymes diffuse into zona pellucida and alter its structure by inducing hardening of layer and destroying sperm receptors
- hence, additional sperms can’t bind to zona pellucida or digest it to reach oocyte
External fertilisation
a method of fertilisation in which 2 haploid gametes, a sperm and an egg cell, fuse together outside of the parent’s body
- works best when vast quantities of eggs and sperm are produced to increase chances of fertilisation
- needed because eggs and sperm can be predated upon by other animals and environmental factors, could influence success rates of fertilisation
- courtship rituals ensure that male and female sex cells are released close to each other
- nevertheless, chances of fertilisation are low
Internal fertilisation
involves transfer of sperms into the female’s body for fertilisation to occur
- chance of fertilisation are much greater as sex cells are closer together when released
Miscarriages
- loss of corpus luteum before placenta is fully established
Blastocyst
- made of a thin-walled hollow structure that contains an outer of cells and the inner cell mass
- inner cell mass gives rise to embryo
- outer cell layer develops into placenta and other supporting tissues needed for foetal development within the uterus
Stages of pregnancy
- after fertilisation, ovum divides by mitosis
- after 48 hours, there’s a 4-cell embryo
- at this stage, embryo is still in the oviduct, where fertilisation took place
- embryo slowly divides further and migrates down the oviduct to the uterus
- after 7 days, embryo reaches the uterus- has around 125 cells and is called a blastocyst
- blastocyst implants itself in the endometrium of uterus
- it starts to develop finger-like projections that become intertwined w/ maternal blood vessels- this eventually develops into placenta
- the endometrium is maintained throughout pregnancy, by continued production of progesterone and oestrogen
- early in pregnancy, embryo starts to produce hCG
- this stimulates the corpus luteum to continue the production of progesterone and oestrogen (essential for maintaining the endometrium)
- hCG ensures that corpus luteum remains until placenta is fully established and can take over role of progesterone and oestrogen secretion
Placenta
- an organ which facilitates exchange of material between the foetus and its mother
- placenta is foetal in origin
- chorionic villi provide a maximum SA for contact w/ maternal blood
- villi project into intervillous space
- maternal blood collects in these spaces
- foetal blood circulates in capillaries which lie very close to surface of the villus w/ only a few micrometres separating them from the maternal blood pools
- short distance facilitates diffusion between maternal blood and foetal blood
Monotremes
mammals without a placenta
- echidna
- platypus
they lay eggs but still suckle their young
Role of placenta during pregnancy
- Exchange of material to keep foetus alive during pregnancy
- Production of progesterone and estrogen
- at start of pregnancy, this is performed by corpus luteum
- but, activity of corpus luteum progressively decreases from the start of the 8th week
- Its role is entirely replaced by placenta at the end of the first trimester (about 12 weeks)
What passes from the mother to the foetus?
- oxygen by diffusion
- glucose, AA, vitamins and minerals by facilitated diffusion
- water by osmosis
- hormones by endocytosis
- drugs by diffusion
- some viruses via receptors
What passes from foetus to mother?
- carbon dioxide by diffusion
- urea by facilitated diffusion
- water by osmosis
- hormones by exocytosis
Foetal arteries
bring deoxygenated blood from foetus to placenta
Foetal vein
- there is only one vein
- brings oxygenated blood back from placenta to the foetus
Gestation period
refers to the time that the embryo develops within the female’s body
- starts w/ fertilisation and ends at birth
- higher the average mass of an animal, longer the gestation period
Birth
- at the end of the gestation period, birth is induced by a combination of certain hormones
- foetus signals to placenta to stop producing progesterone
- triggers secretion of oxytocin
- while progesterone level drops prior to birth, oestrogen level continues to rise and induces development of oxytocin receptors on muscles of uterine wall
- increases responsiveness of uterus to oxytocin
- endometrium secretes prostaglandins that start the uterine contractions
- oxytocin stimulates myometrium to contract- it’s the strongest simulator of uterine contractions
- oxytocin is controlled by positive feedback- contractions are mild initially, but become stronger as more oxytocin is released (labour)
- strength of contractions increases until baby is expelled from uterus (birth)
- as uterus contracts, cervix dilates
- amniotic sac breaks
- contractions may continue for hours until baby is pushed through cervix and vagina
- after baby is born, prolactin, stimulates milk production
- after birth, prolactin production is stimulated when the baby suckles
Positive feedback
a process involving a feedback loop where a change in a process further increases the change
- birth is mediated by +ve feedback involving oestrogen and oxytocin
Oxytocin
- produced by the posterior lobe of the pituitary gland and by the foetus
Oestrogen pollution
widespread use of contraceptives has increased levels of synthetic oestrogen in the environment
Consequences of higher than normal levels of oestrogen in the environment?
- Lowers average sperm count in males
2. Cause certain fish to show female characteristics
