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Biology
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Contents
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Respiration
Photosynthesis
Microbiology
Populations
Homeostasis
Nervous system
RESPIRATION
Need for ATP
• Movement
• Homeostasis
• Anabolic processes (synthesis of large
molecules from smaller ones)
• Active transport
• Secretions
ATP
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Temporary energy store
Must be used in cell where it is created
Base = adenine
Pentose sugar = ribose
3 phosphate groups
Adenosine triphosphate
Hydrolysis of ATP is exergonic (energy released) 
catalysed by ATPase
• Phosphorylation of ADP endergonic (energy used)
• 30 kJ mol-1 energy to add/remove phosphate group 
catalysed by ATPsynthetase
Cellular respiration
• Gas exchange = diffusion of gases into and out of
cells that allows respiration to take place
• Respiration = series of oxidation reactions that
take place in living cells resulting in the release of
energy from organic respiratory substrates e.g
glucose
• Aerobic or anaerobic
• Obligate anaerobes = only carry out anaerobic
respiration because they are poisoned by the
presence of oxygen
Glycolysis
• In cytoplasm of cells
• Glucose (6C) phosphorylated to glucose phosphate (6C)
 ATP hydrolysed to ADP
• Glucose phosphate (6C) phosphorylated to fructose
bisphosphate (6C)  ATP hydrolysed to ADP
• Fructose bisphosphate (6C) unstable so breaks down to
form 2x glycerate-3-phosphate (3C)
• 2x glycerate-3-phosphate (3C) converted to pyruvate
(3C)  4 ADP phosphorylated to 4 ATP, 2 NAD reduced
to 2 NADH
• Net gain of 2 reduced NAD and 2 ATP
Link reaction
• In mitochondria
• 2x pyruvate (3C) from glycolysis
decarboxylated and dehydrogenated to form
2x acetate (2C)  carbon dioxide and reduced
NAD formed
• 2x acetate (2C) combines with 2x coenzyme A
to form 2x acetyl coA
• Net gain of 2 carbon dioxide molecules and 2
molecules of reduced NAD
Krebs cycle
• In matrix of mitochondria
• Acetyl coA (2C) from link reaction combines
oxaloacetate (4C) to form citrate (6C)
• Citrate (6C) decarboxylated and dehydrogenated
to regenerate oxaloacetate (4C)  reduced NAD,
reduced FAD, ATP and carbon dioxide formed
• Net gain of 6x reduced NAD, 2x reduced FAD, 2x
ATP and 4x carbon dioxide per molecule of
glucose
Electron transport chain
• Inner membrane of mitochondria
• Hydrogen atoms from NAD and FAD passed down chain of carrier
molecules
• Hydrogen atoms split into hydrogen ions and electrons
• Electrons transferred along electron carriers  each at lower energy level
than previous so energy is released, which is used to make ATP (oxidative
phosphorylation)
• Hydrogen ions stay in solution in inner membrane space of mitochondria
• Oxygen is the final electron acceptor of the electron carrier chain 
electrons, hydrogen ions and oxygen combine to form water, catalysed by
cytochrome oxidase
• 3x ATP made per reduced NAD, 2x ATP made per reduced FAD
• 34x ATP made per glucose molecule (+ 2 from glycolysis and 2 from Krebs
cycle, so overall gain of ATP per glucose molecule for aerobic respiration is
38x ATP)
Chemiosmotic theory
• Mitochondria have a double membrane
• Inner membrane folded to form cristae  large surface
area
• Cristae lines with stalked particles that contain
ATPsynthetase enzymes
• Energy released by electron transport chain pumps
hydrogen ions from matrix to inner membrane space
• Higher concentration of hydrogen ions in inner membrane
space than in matrix sets up an electrochemical gradient
• Hydrogen ions diffuse back into matrix through stalked
particles, down electrochemical gradient
• Electrical potential energy of diffusion of hydrogen ions
used to make ATP, using ATPsynthetase as a catalyst
Anaerobic respiration
• Fermentation = anaerobic respiration of yeast, 2%
efficiency approx  pyruvate converted to ethanal and
carbon dioxide, hydrogen from reduced NAD used to
turn ethanal to ethanol
• Ethanol toxic if accumulated by yeast
• Lactate formed when muscles carry out anaerobic
respiration, 2% efficiency approx  pyruvate reduced
to form lactate
• Lactate transported to liver via bloodstream  1/5
approx converted back to pyruvate and used in aerobic
respiration, 4/5 converted to glycogen
• Oxygen debt = oxygen required to break down lactate
Other respiratory substrates
• Lipids  fats hydrolysed to fatty acids and
glycerol  fatty acids broken down in matrix
to acetyl fragments (2C)  these combine
with coA to form acetyl coA  enters Krebs
cycle  glycerol phosphorylated to
glyceraldehyde-3-phosphate, enters glycolysis
• Protein  hydrolysed to amino acids  these
deaminated in liver  organic acid produced
fed into Krebs cycle
PHOTOSYNTHESIS
Photosynthesis
• Carried out by photoautotrophs
• Takes place in chloroplasts found in the mesophyll cells
and guard cells of green leaves
• Sunlight trapped by the photosynthetic pigments e.g.
chlorophyll
• Light, carbon dioxide, water and a suitable
temperature is needed for photosynthesis to occur
• Carbon dioxide + water (+ light energy)  glucose +
oxygen
• 6CO2 + 6H2O (+ light energy) C6H12O6 + 6O2
• Endergonic reaction which occurs in two stages
catalysed by enzymes; the light-dependent stage and the
light-independent stage
Factors affecting photosynthesis
• Limiting factors are conditions that prevent the rate of
photosynthesis increasing
• For photosynthesis, limiting factors can be light intensity ,
temperature , carbon dioxide concentration , and volume of
water available
• Compensation point is when carbon dioxide produced by
respiration is completely reused during photosynthesis
• Rate of photosynthesis can be found by measuring either
the rate carbon dioxide is used or the rate glucose is
produced or the rate oxygen is produced
• Photosynthometers calculate the rate of photosynthesis by
measuring the volume of oxygen produced in a period of
time
Leaf structure and function
• Large surface area to absorb as much sunlight as possible
• Thin so light can penetrate them, and giving a short
diffusion path for carbon dioxide
• Cuticle and epidermis are transparent so light can pass
through them
• Palisade mesophyll cells contain lots of chloroplasts and
have their long axes parallel to the surface
• Chloroplasts can move intracellularly by cyclosis so they
can arrange themselves for the most efficient absorption of
light
• Chloroplasts hold chlorophyll in an ordered arrangement
• Stomata allow carbon dioxide to enter the leaf
Photosynthetic pigments
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Absorb light energy and convert it to chemical energy
Found in the thylakoid membranes of the chloroplast in groups called antenna
complexes
Photons of light energy are passed along antenna complex until they reach a
chlorophyll a molecule at the reaction centre of a photosystem
Chlorophylls absorb mainly red and blue-violet frequencies of light
Chlorophyll molecules have a hydrophilic head containing a magnesium ion and a
hydrophobic tail
Chlorophyll a and chlorophyll b are the most common types of chlorophyll
Chlorosis is a condition where plants are magnesium deficient so cannot produce
enough chlorophyll and look yellow in colour
Carotenoids are accessory pigments that absorb mainly blue-violet frequencies of
light
Carotenes and xanthophylls are the two main types of carotenoids
Having a range of different photosynthetic pigments allows more energy to be
harnessed for photosynthesis as different pigments have different absorption spectra
Plants look green because very little green light is absorbed by the photosynthetic
pigments
Absorption and action spectra
• Wavelengths of light are either absorbed or
reflected by pigments
• The absorption spectrum indicates which
wavelengths of light are absorbed by pigments
• The action spectrum shows the amount of
carbohydrates synthesized (rate of photosynthesis)
at different wavelengths of light
• The action and absorption spectrums for
chlorophyll are closely correlated, providing
evidence that chlorophyll is a pigment responsible
for absorbing light for photosynthesis
Chromatography
• Separated using chromatography
• Pigments are extracted by grinding a leaf using a pestle and mortar, and a
solvent such as propanone
• Origin line is drawn a couple of centimetres from the bottom of the
chromatography paper, and an extract of the ground leaf is added on the
origin line
• The chromatogram is placed in a glass tank containing a solvent, with the
level of the solvent just below the origin line, and left to allow the solvent to
rise up through the chromatography paper
• Pigments rise up the chromatography paper different distances depending on
the relative solubility in the solvent
• When the solvent front reaches the top of the chromatography paper, the
paper is taken out and dried
• Rf value calculated and used to identify the pigment
• Rf value = distance travelled by pigment/distance travelled by solvent front
Harvesting Light
• Accessory pigments (chlorophyll b and
carotenoids) and primary pigment (chlorophyll a)
found in thylakoid membranes of chloroplasts in
group/clusters called antenna complexes
• Photons of light passed from accessory pigments
to the primary pigment (chlorophyll a) in a
reaction centre
• Two types of reaction centre, photosystems 1 and
2
Light Dependent Stage
• Takes place in thylakoids of chloroplasts
• ADP and Pi synthesised to ATP by
photophosphorylation
• Water is split into 2H+, 2e-’s and ½ O2 by
photolysis
• NADP is reduced by 2H+’s from photolysis of
water
• Two forms of LDS  cyclic photophosphorylation
and non-cyclic photophosphorylation
Non-Cyclic Photophosphorylation
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Also known as Z-scheme
Light absorbed by PSII and passed on to chlorophyll a (P680)
Chlorophyll a emits 2 e-’s, which are raised to a higher energy level and picked up
by an electron acceptor
Electron’s passed along a chain of carrier molecules until it is eventually accepted
by PSI
Energy released when electrons are passed down chain of carrier molecules is
used for photophosphorylation of ADP and Pi to ATP
Light absorbed by PSI and passed onto chlorophyll a (P700), which emits 2 e-’s
Electrons raised to higher energy level and picked up by an electron acceptor
Electrons passed down (shorter) carrier molecule chain until accepted by NADP
Photolysis of water produces 2H+, which combines with NADP to give reduced
NADP, 2e-’s which replace electrons lost from PSII and ½ O2 which is emitted as a
waste product
Reduced NADP and ATP passed onto light independent stage
Cyclic Photophosphorylation
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Only involves PSI
Light absorbed by PSI and passed onto chlorophyll a (P700)
Chlorophyll a molecule emits an electron
Electron is raised to a higher energy level and is picked up
by an electron acceptor
Electron passed along a carrier molecule chain until it
recombines with PSI
Energy emitted when electrons are passed along carrier
molecule chain used for photophosphorylation of ADP and
Pi to ATP
No reduced NADP is made
ATP is passed onto the light independent stage
Chemiosmosis
• ATP is synthesised form ADP and Pi by enzyme ATP
synthetase found in the thylakoid membranes
• Energy emitted as electrons are passed down the
carrier molecule chain in light dependent stage used to
pump hydrogen ions from stroma to the thylakoid
membrane space, creating an electrochemical gradient
across the thylakoid membrane
• Hydrogen ions diffuse down the electrochemical
gradient through the thylakoid membrane via protein
channels
• Shape of ATP synthetase changed so that ATP can be
synthesised from ADP and Pi
Light Independent Stage
• Also known as Calvin Cycle
• Carbon dioxide combines with ribulose bisphosphate (RuBP) using
enzyme RuBP carboxylase as a catalyst
• Product of unstable 6C compound formed, which decomposes into
2x 3C molecules of glycerate 3-phosphate (GP)
• ATP used to phosphorylate 2x 3C molecules of GP to 2x 3C
molecules of glycerate bisphosphate
• Reduced NADP acts as reducing agent to reduce glycerate
bisphosphate to glyceraldehyde 3-phosphate (GALP)
• 1/6 of GALP produced is converted to glucose and other respiratory
substrates
• 5/6 of GALP produced is used in a series of enzyme catalysed
reactions to regenerate RuBP
MICROBIOLOGY
White blood cells
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Also known as leucocytes
Defend body against pathogens
Pathogens are disease causing organisms
Made in bone marrow by division of stem cells
Neutrophils are lobed and largest of leucocytes  role is
phagocytosis
Lymphocytes are small with a large, round nucleus
B-lymphocytes produce antibodies (humoral response)
T-lymphocytes are involved with the cell-mediated response
Monocytes are large with a kidney shaped nucleus  develop into
macrophages
Eosinophils are associated with allergies
Basophils release chemicals such as histamines that cause
inflamation
Types of bacteria
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Coccus  spherical
Spirillum  spiral shape
Bacillus  rod shaped
Gram positive  retains crystal violet dye because
crystal violet is trapped in the peptidoglycan wall
• Gram negative  retains saffronin because
lipopolysaccharide layer that prevents crystal violet
being trapped in the peptidoglycan wall is made more
permeable by the crystal violet dye, so that the counter
stain saffronin can be taken up by the peptidoglycan
wall
Bacterial growth
• Lag phase is when the pathogen is active but there is
little growth as they are taking up water and producing
enzymes
• Exponential/log phase is where the population size
increases rapidly
• Carrying capacity is when the maximum population the
environment can support is reached
• Stationary phase is when the pathogens are dying at
the same rate as they are produced
• Death phase is when pathogens are dying faster than
they are being produced due to lack of nutrients, lack
of oxygen or accumulation of toxic waste products
Factors affecting growth
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Temperature
– Thermophiles have optimum temperature of above 40 degrees  grow in hot springs,
compost heaps and water heaters
– Mesophiles have optimum temperature between 20 and 40 degrees  most bacteria
including human pathogens
– Cryophiles have optimum temperature below 20 degrees  live in Arctic and Antarctic
Oceans, fridges and freezers
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pH
– Most have optimum of pH 7 and cannot function below pH 4
– Bacteria produce waste products with low pH and can lead to death of bacteria population
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Oxygen
– Needed for aerobic bacteria to produce ATP
– Obligate anaerobes are killed if oxygen is present
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Nutrients
– Essential for growth
– Nitrogen needed for protein synthesis
Culturing bacteria  aseptic
technique
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Cuts covered with clean, waterproof dressing
No food or drink in lab
Windows and doors closed to avoid airborne contamination
Wash hands with anti-bacterial soap before and after
Wipe down bench with disinfectant before and after
Tape petri dish securely after inoculation and label them
Keep temperature below 30 degrees
Sterilise all containers using autoclave (121 degrees for 15 mins) before
and after to destroy spores
Sterilise equipment throughout innoculation by placing in alcohol then
burning off alcohol with bunsen flame
Work near lit bunsen burner to produce convection currents to kill
airborne infections
Lift petri dish lid at 45 degree angle
Do not open petri dishes after inoculation
Culturing bacteria
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Wash hands and disinfect bench
Label petri dish
Dip inoculating loop in alcohol and burn off using bunsen flame
Unscrew bottle of microbe sample and hold opening in bunsen
flame for 2 seconds
Dip sterile inoculation loop into microbe sample and replace lid of
bottle
Lift lid of petri dish slightly and streak inoculating loop over surface
of the agar
Replace lid and seal with tape
Put dish upside down in incubator at 25 degrees for 2-3 days
Wash hands and disinfect bench
Monitoring growth
• Haemocytometers are modified microscope
slides divided into squares
• A type squares have side length 1mm
• B type squares have area of 0.04 square mm 
25 B squares per A square
• C squares have area 0.0025 square mm  16 C
squares per B square
• Number of cells in a particular type of square
counted using a microscope
• Used to calculate number of cells per cubic mm
Disadvantages of haemocytometer
method of monitoring growth
• Unreliable due to small volume of sample
used
• Can’t differentiate between viable (living and
able to reproduce) and dead cells so can get
inaccurate totals
• Debris in sample may obscure cells to be
counted
Dilution plating
• Culture medium diluted
• Small sample of each dilution placed on agar
plate
• Plates incubated between 25 and 30 degrees for
2-5 days
• Plates examined and colonies counted
• Assumption that each colony comes from a single
cell
• Total viable cell count = number of colonies x
dilution factor
Turbidimetry
• Colorimeter used to measure turbidity
(cloudiness)
• Amount of light absorbed measured
• More cells = more light blocked
• Results compared to calibration curve  graph of
absorbance of known concentrations of cells
• Assumes turbidity caused solely by
microorganisms
• Mixture must be continually stirred to prevent
settling
POPULATIONS
Carbon cycle
• Carbon used for photosynthesis, dissolves
from atmosphere into seawater
• Released back into environment by
respiration, respiration of decomposers,
combustion of fossil fuels
Nitrogen cycle
• Nitrogen is an unreactive gas  converted to nitrates so it can be used by
plants and transferred along food chain
• Ammonification is the breakdown of proteins, amino acids and urea by
decomposing bacteria to form nitrogen
• Nitrification is the conversion of ammonium ions to nitrates under aerobic
conditions by nitrofying bacteria  nitrosomonas oxidises ammonium
ions to nitrites, nitrobacter oxidises nitrites to nitrates, which can enter
the food chain
• Nitrogen fixation is the conversion of nitrogen in the atmosphere to
nitrates by nitroge-fixing bacteria  azotobacter in the soil, nostoc in
freshwater, rhizobium found in root nodules of legume plants
• Denitrification is the conversion of nitrates and ammonium ions back to
nitrogen gas by denitrifying bacteria in the absence of oxygen 
pseudomonas and thiobacillus in water-logged soils carry out
denitrification to gain their energy
Populations
• Group of individuals of same species living in same
place at same time and interbreeding
• Population growth causes competition for resources
and space
• Better adapted individuals of the population are more
likely to survive and reproduce, passing on their genes
to their offspring
• Adaptations may make the individual more successful
in breeding and rearing their young, better at
protecting themselves and offspring from predators,
better at locating food sources etc
Determining population growth
• Birth rate = reproductive capacity of
population
• Mortality = death rate of organisms in the
population
• Immigration = movement of individuals into
the population
• Emigration = movement of individuals out of a
population
Population growth
• Exponential growth when conditions are favourable
• Boom and bust curve caused by exponential growth followed by
rapid decrease in population caused by a limiting factor
• Biotic potential = maximum rate of reproduction when their are no
other limiting factors
• Environmental resistance = factors that limit growth of population
e.g accumulation of waste products, lack of resources, climatic
conditions, predators, parasites, competitors
• Carrying capacity = maximum population size that can be supported
by a particular environment
• S-shaped curve = lag phase, log phase, stationary phasem decline
phase (see microbiology)  occurs when species colonise new
habitats
Environmental resistance
• Abiotic factors = climate, oxygen levels, water
quality, pollution
• Biotic factors = competition for resources/space,
parasites, predators
• Density-independent factors affect all plants and
animals of the population, regardless of
population size  climate, pollution, disease
• Density-dependent factors vary in effect on
population depending on population size 
competition for resources, predation, parasites
(always biotic, never abiotic)
Competition
• Intraspecific competition = competition
between organisms of the same species
caused by over-reproduction  densitydependent
• Interspecific competition = competition
between organisms of different species
• Competitive exclusion principle says
interspecific competition is most intense when
two different species occupy the same niche
Predation
• Good predators have means to kill, speed to pursue prey
and camouflage for stalking prey
• Group hunting allows prey to be surrounded
• Young, old or sick prey targeted as easier to kill
• Large prey gives more food per kill
• Variety of prey species reduces chance of starvation
• Migration to areas with plenty of prey species reduces
chance of starvation
• Prey adapt to be faster than predator, stay in large groups,
have stings, taste bad and have warning coloration, are
camouflaged in the environment, have startle mechanisms
to confuse predator
Predator-prey cycles
• Fluctuations in predator numbers smaller than
fluctuations in prey numbers
• Fluctuations in predator numbers lag behind
fluctuations in prey numbers
• Fewer predators than prey
Biological control
• Use of predators, parasites and pathogens to keep
pests levels below the economic damage threshold
• Economic damage threshold = pests causing enough
damage that it is worth spending money to control the
pest
• Biological control agent = predator, parasite or
pathogen used  must be specific to the pest, high
initial expense, inexpensive once established, slow to
react, crops may suffer from several pests so more than
one biological control may be required, biological
control agent may need to be reintroduced if crop is
only sometimes affected by the pest
HOMEOSTASIS
Homeostasis
• Maintenance of a constant internal environment
• Receptors detect a stimulus
• Stimulus is a change in the level of the factor
being regulated
• Input is a detectable change
• Coordinator receives info from receptor and
triggers action to correct the change
• Effector brings about a change  corrective
mechanism
Thermoregulation
• Regulation of body temp
• Heat is transferred to and from organisms by
radiation, conduction and convection
• Heat is gained from respiration, conduction
from surroundings, convection from
surroundings and radiation from surroundings
• Heat is lost by evaporation of water,
conduction to surroundings, convection to
surroundings and radiation to surroundings
Ectotherms
• Animals that don’t generate much body heat
• All animals except birds and mammals
• Body temperature fluctuates with
environment (fish, amphibians) or is
controlled by increasing activity levels (lizards)
Endotherms
• Generate their own body heat
• Mammals and birds
• Vasoconstriction involves the contractions of muscles
in the arteriole wall, reducing blood flow to the
capillaries so less heat is lost
• Vasodilation is the opposite of vasoconstriction
• Sweat glands release sweat, which evaporates from the
skin, giving a cooling effect
• Erector muscles connected to the hair follicles contract
causing the hairs to stand up on end and trap a layer of
air between the hairs and the skin, giving a warming
effect
Controlling body temp
• Hypothalumus in brain controls body temp by
monitoring the temp of blood passing through
it
• Core body temp around 37 degrees in humans
Overcooling
• Vasoconstriction of arterioles; divert blood away from
skin, less heat lost via radiation
• Sweating reduced; prevent heat being lost from
evaporation of sweat from skin
• Erector muscles contract; hairs raised and layer of air
trapped between hairs and skin, acting as insulation
• Shivering; contraction and relaxation of muscles
produces heat energy
• Behavioural adaptations; putting on more clothes,
staying in heated rooms, being more active during the
day when it is warmer, huddling
Overheating
• Vasodilation of arterioles; more blood reaches
capillaries near skins surface, more heat lost by
radiation
• Sweating increases; more sweat on skins surface
to evaporate, cooling the skin
• Erector muscles relax; hairs flatten to reduce
stationary layer of insulating air
• Behavioural adaptations; aestivation (hibernating
in hottest months), nocturnal to avoid hot
daytime temperatures, removing clothing
Pancreas
• Exocrine gland  release secretions along ducts/tubes;
secretes pancreatic juice down pancreatic duct to
duodenum
• Endocrine gland  secrete hormones into
bloodstream; insulin and glucagon secreted into blood
to control blood glucose levels
• Islet’s of Langerhans are made up of alpha cells which
secrete glucagon, and beta cells which secrete insulin
• Glucagon converts glycogen to glucose, so it can be
used
• Insulin converts glucose to glycogen, so it can be stored
Control of blood glucose levels
• Normal blood glucose level is 80-90 mg per 100 cubic cm
• Absorption of carbohydrates from alimentary canal,
glycogenolysis (conversion of glycogen to glucose) and
gluconeogenesis (conversion of amino acids and glycerol to
glucose) cause an increase in blood glucose levels
• Excess amino acids broken down in liver by deamination 
amino part excreted, rest converted to glucose
• Blood glucose levels maintained during fasting by
conversion of lipid stores and use of existing proteins
leading to muscle wastage
Glucagon
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Blood glucose levels decrease
Detected by alpha cells in islets of Langerhan
Glucagon secreted
Glycogen converted to glucose and increased
rate of gluconeogenesis
Insulin
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Blood glucose levels increase
Detected by beta cells in islets of Langerhan
Insulin secreted
Insulin attaches to receptor sites on cell membrane of liver,
muscle and adipose )fat store) cells
Permeability of cells to glucose changes; increased activity
of carrier molecule that transports glucose across cell
membrane
Rate of respiration increases as more glucose is available
Glycogen stored in liver and muscles (glycogenesis)
Rate of conversion of glucose to fats to be stored in adipose
tissue increases
Diabetes mellitis
• Inability to control blood glucose levels due to lack of insulin
• Permeability of cells to glucose not increased so fats and proteins
used for respiration, causing weight loss
• Increased water potential of blood causes thirst as more water is
needed to dilute the blood
• Glucose found in the urine because kidneys are unable to reabsorb
high levels of glucose filtered into the tubules
• Causes of diabetes are insulin receptors failing to recognise insulin
despite insulin being produced or beta cells of islets of Langerhans
being destroyed by the body’s immune system so insulin is no
longer secreted
• Controlled by regulating carbohydrate intake and injecting insulin
Excretion
• Removal of waste products made in the cells during metabolism
• Carbon dioxide, nitrogenous waste from breakdown of amino acids, bile
pigments from breakdown of red blood cells
• Nitrogenous waste released by fish as ammonia because it diffuses out
across gills and is diluted down by water as it is extremely soluble
• Nitrogenous waste excreted by birds and insects as uric acid, a white paste
made from ammonia and a small volume of water
• Nitrogenous waste excreted by mammals as urea
• Excess amino acids broken down in liver by deamination, producing
ammonia
• Urea is made in the liver by combining 2x ammonia with 1x carbon dioxide
• Urea less toxic than ammonia so tissues can tolerate higher
concentrations of it
• Urea is filtered out of the bloodstream by the kidneys and is excreted as
urine
Kidneys
• Two at back of abdomen
• Main organs of urinary system
• Filter waste products out of blood; about 180l of fluid filtered but
only 1l of urine produced per day
• Receives blood supply from renal artery
• Consists of units called nephrons
• Blood enters kidney at high pressure to help with filtration
efficiency
• Filtered blood leaves kidneys via renal veins
• Filtered waste products excreted as urine
• Urine passes down ureter to the bladder where it is stored
• Urination occurs when sphincter muscles relax and urine passes
from the bladder out of the body via the urethra
Kidney structure
• Surrounded by adipose (fat) tissue and fibrous connective
tissue to keep kidneys in the correct position and protect
them from damage
• Outer region is the cortex, where filtration is carried out by
the nephrons  dense capillary network receiving blood
from the renal artery
• Inner region is the medulla  nephrons extend across
medulla to form renal pyramids
• Renal pyramids project into pelvis in centre of the kidney
 urine passes out of the pelvis before it passes down the
ureter to the bladder
• Function of kidneys to remove nitrogenous waste, control
water content and pH of blood
Structure of nephron
• Bowman’s capsule in cortex of kidney
• Proximal convoluted tubule below Bowman’s capsule
• Tubule leads into loop of Henle in the medulla that goes out
through cortex
• Loop of Henle leads into distal convoluted tubule, which joins to
collecting duct that carries urine through medulla to the pelvis of
the kidney
• Nephron’s have rich blood supply brought to kidney by renal artery
• Afferent arteriole supplies Bowman’s capsule with blood
• Afferent arteriole branches into capillaries called glomerulus, which
rejoin to form efferent arteriole
• Afferent arteriole has wider diameter than efferent arteriole, so
high pressure maintained in the glomerulus
Ultrafiltration
• Filtering small molecules out of blood into
Bowman’s capsule under pressure
• Bowman’s capsule has 2 layers
• Endothelium of capillaries of first layer have tiny
gaps; allow molecules to pass through
• Basement membrane between two layers made
of glycoprotein and collagen fibres; prevents large
molecules passing through (acts as a filter)
• Epithelial cells in second layer are podocytes;
foot-like projections, gaps between the cells
Reabsorption
• Selective reabsorption
• Glucose, amino acids, vitamins and many sodium
and chloride ions actively transported from
proximal convoluted tubule back into blood
• Microvilli provide large surface area
• Mitochondria provide ATP for active transport
• Capillaries surrounding nephron have high solute
concentration so water passes out of filtrate in
proximal convoluted tubule into blood by osmosis
Loop of Henle
• Runs into medulla and back to cortex of kidneys
• First part is the descending limb  water passes out via osmosis
due to higher solute concentration outside in surrounding tissue
• Second part is ascending limb; more permeable to salts, less
permeable to water  sodium and chloride ions moved first
passively, then actively out into surrounding tissue
• Creates high solute concentration in medulla, where collecting
ducts of nephrons pass through, so water can be reabsorbed from
collecting ducts by osmosis, producing a concentrated urine in the
collecting ducts
• Countercurrent multiplier mechanism  solute conc lower in
ascending limb than descending limb so water drawn out of
collecting ducts by osmosis
Distal convoluted tubule and collecting
duct
• Permeability of both affected by hormones  regulate
how much water passes into the medulla of the kidney,
and how concentrated the urine will be
• Distal convoluted tubule made of cells like those in
proximal convoluted tubule  microvilli on surface,
many mitochondria
• Function to pump sodium ions out of nephron into
blood by active transport
• Hydrogen carbonate ions dissociate from carbonic acid
and pass out of distal convoluted tubule into the blood
 raises pH of blood
Water balance in desert animals
• Longer loop of Henle  greater solute
concentration in medulla  more water
reabsorbed  urine more concentrated
• Thicker medulla  longer loop of Henle
• Water comes from food  metabolic water from
respiration
• Remaining underground during day prevents
water loss by evaporation
• Nasal passages cool air so moisture condenses
before it’s exhaled
Osmoregulation
• Homeostatic control of body water
• Most water gained from eating and drinking, some from metabolic
reactions e.g respiration
• Most water lost is from urine, some from sweat, breathing and faeces
• Negative feedback
• Osmoreceptors in hypothalumus of brain detect a change in solute
concentration
• Low water potential stimulates pituitary gland to release antidiuretic
hormone (ADH) into blood, which makes distal convoluted tubule and
collecting duct more permeable to water so more water is reabsorbed,
producing a smaller volume of more concentrated urine
• Low water potential also activates thirst centre in the brain, causing thirst
so more water is consumed and blood is diluted
• High water potential stimulates the pituitary gland to release less ADH
NERVOUS SYSTEM
Nervous system
• Stimulus  receptor  CNS  effector 
response
• Stimulus = detectable change
• Receptor = sensory cells
• CNS (central nervous system) = brain and spinal
chord  info brought to and from CNS by the
PNS (peripheral nervous system)
• Effector = muscle or gland
• Response = action taken
Neurones
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Generate and transmit nerve impulses
Motor neurone carries nerve impulses from the CNS to an effector
Cell body = part containing nucleus, other organelles
Dendrites = short, thin, cytoplasmic extensions of cell body, carrying
impulses towards cell body
Axon = long extension of cell body, carries impulses away from cell body
Motor end plates = connection between axon of motor neurone and
effector
Myelinated axon = axon surrounded by fatty sheath of myelin formed by
Schwann cells wrapping themselves around the axon
Nodes of Ranvier = gaps in myelinated sheath where axon is exposed
Sensory neurone carries impulses from receptor cells to CNS, has one long
dendrite bringing info to cell body
Resting potential
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-65/-70 mV
Axon polarised
More positive outside axon than inside
Active transport (requires ATP) of sodium (out
of axon) and potassium ions (into axon)
against conc gradient via sodium-potassium
pump (carrier protein in axon membrane)
• Sodium ions diffuse back out of axon faster
than potassium ions diffuse back in
Action potential
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Nerve impulse initiated by stimulation of neurone
+40 mV
More positive inside axon than outside
Lasts about 3 miliseconds
Axon depolarised
Change in permeability of axon membrane to sodium and
potassium ions
• Sodium channels open when neurone is stimulated, so influx of
sodium ions causes change in potential of axon membrane
• Axon repolarised when resting potential is restored
• Potassium channels open causing outflux of potassium ions,
meanwhile sodium channels close, so axon membrane is
repolarized
Progress of impulse
• All or nothing law = stimulus is either strong
enough to generate impulse or not; strength of
stimulus does not effect strength of impulse
• Action potential generated when neurone
stimulated beyond threshold intensity
• Size of action potential same regardless of
strength of stimulus  stronger stimulus results
in greater frequency of action potentials NOT
greater size
• Local circuits occur in axon, passing action
potential along axon
Refractory period
• Time delay between action potentials  few
milliseconds
• Absolute refractory period = sodium channels in axon
membrane closed, no inward movement of sodium
ions, another impulse cannot be generated
• Relative refractory period = after potassium channels
open, action potential can only occur if stimulus is
more intense than usual threshold level
• Ensures impulses flow in one direction only because
region of axon membrane behind impulse cannot be
depolarised
• Limits frequency of impulses
Factors affecting speed of transmission
• Axon diameter  thicker = faster because greater
surface area means greater exchange of ions
across surface
• Myelin sheath  insulates axon, no ion exchange
across myelinated part of axon, action potential
only occurs at nodes of Ranvier and jump from
one node to the next
• Saltatory conduction  occurs when axon is
myelinated, increases speed of transmission,
conserves energy as sodium-potassium pumps
only operate at nodes
Synapse
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Where 2 neurones functionally meet (do not touch)
Synaptic cleft = gap between 2 neurones
Presynaptic = before synapse
Postsynaptic = after synapse
Neurotransmitters = chemicals released by presynaptic
neurone that diffuse across synaptic cleft and trigger
action potential in the postsynaptic neurone
• Neuromuscular junctions = synapse between motor
neurones and muscles
Structure of synapse
• Axon terminals/synaptic bulbs = end of axon of
presynaptic neurone
• Presynaptic membrane
• Postsynaptic membrane contains ion-specific
channels, protein molecules on surface that act
as receptors for neurotransmitters
• Large numbers of mitochondria in synaptic bulb
provide ATP for active transport
• Synaptic vesicles in synaptic bulb contain
neurotransmitter molecules
Synaptic transmission  acetylcholine
as neurotransmitter
• Action potential arrives at synaptic bulb
• Calcium channels open in presynaptic membrane open
• Influx of calcium ions because higher conc outside synaptic cleft
than outside
• Vesicles containing acetylcholine move to and fuse with presynaptic
membrane
• Acetylcholine released into synaptic cleft, diffuses across and
attaches to receptor proteins on postsynaptic membrane
• Sodium channels in postasynaptic membrane open, creating action
potential
• Acetylcholinesterase (enzyme) hydrolyses acetylcholine into acetate
and choline
• Choline taken up by presynaptic membraneby active transport and
combined with coenzyme A to reform acetylcholine
Function of synapse
• Temporal summation = number of action potentials required from
presynaptic neurone to release enough neurotransmitter (beyond
threshold level) to stimulate action potential
• Spatial summation = many presynaptic neurones synapse with one
postsynaptic neurone, so action potentials arriving from many
presynaptic neurones allows neurotransmitter level at protein
receptors to go beyond threshold level
• Synaptic vesicles only in synaptic bulb, not dendrite, so impulses
only travel in one direction
• Excitatory synapse = open sodium channels to generate action
potential
• Inhibitory synapse = open potassium channels, close sodium
channels to prevent action potential
Spinal chord
• Hollow tube running from base of brain to end of
spinal chord
• Grey matter in centre contains cell bodies of relay and
motor neurones
• White matter surrounding grey matter contains
myelinated axons
• Central canal is in centre of grey matter and contains
cerebrospinal fluid
• Sensory neurones enter via dorsal root  cell bodies
form dorsal root ganglions
• Motor neurones leave via ventral root