#aqa science

Homeostasis and the reflex arc

Homeostasis: maintaining/controlling your internal environment (e.g. body temperature, blood glucose levels, glucose content)

Stimulus is detected by receptor cells, information is sent as an electrical impulse, impulse travels to brain and spinal cord, brain coordinates a response, impulse goes to motor neurones, which sends information to the effector (gland or muscle)

In reflexes, the impulse bypasses the conscious part of the brain (involves chemical diffusion between the synapses of two neurones) so they are rapid and automatic

The brain

Cerebral cortex: consciousness, intelligence, memory and language

Hypothalamus: involved in controlling body temperature

Pituitary gland: produces hormones and chemicals

Medulla: concerned with unconscious activities (heartbeat, breathing etc)

Cerebellum: controls muscles activity and balance

Brain is protected by skull and encased in meninges

Can be studied by matching changes in memory/behaviour to areas of trauma, electrical stimulation, or MRI scans

The eye

Made up of cornea (lets light in), sclera (protects eye, outer layer), retina (light sensitive cells send impulses via optic nerve), suspensory ligaments/ciliary muscles (holds lens), lens (fine-tunes light), iris (controls pupil size), and pupil (changes light entry)

Bright light causes pupils to constrict – circular muscles contract, radial muscles relax

Dim light causes pupils to dilate – circular muscles relax, radial muscles contract

Near objects are refracted strong as the lens thickens – ciliary muscles contract, suspensory ligaments loosen

Far objects are refracted only slightly as lens thins – ciliary muscles loosen, suspensory ligaments contract

Hormones

Hormones are chemical messengers released by a gland in response to an internal change and carried in the blood

Negative feedback is doing the opposite of something to return it to normal

Example: thyroxine (thyroid gland in the throat) – uses iodine in your diet to control metabolic changes (remains stable in adulthood)

Adrenaline is not caused by negative feedback – triggers ‘fight or flight’ response (heart rate increase, glycogen converted to glucose, pupils dilate, mental awareness increase, blood diverted from digestive system to big muscles)

Insulin is released when blood glucose is high, glucose taken in by cells or converted to glycogen in the liver

Glucagon is released if too low, and glycogen is broken down into fats/amino acids, fat broken down for energy

Diabetes symptoms include thirst, glucose in urine, fatigue, weight loss, and overactive bladder

Treated by injecting insulin, pancreas/pancreatic cells implant, embryonic stem cells, diet, exercise and weight loss

Male reproductive hormone is testosterone – causes puberty (growth spurt, pubic and facial hair, larynx grows, testes grow/become active, shoulders and chest broaden)

Female puberty is caused by oestrogen (growth spurt, pubic hair, breast enlargement, external genitals grow, fat deposits on hips/buttocks/thighs, brain changes and matures, menstruation begins)

The menstrual cycle: egg matures, uterus lining thickens to prepare for pregnancy, at 14 ays a mature egg is released (ovulation), if egg is not fertilise by day 28, the uterus lining sheds along with egg

Controlled by 4 hormones – FSH (matures eggs in follicle, stimulates ovaries to create oestrogen), oestrogen (secreted with rising FSH levels, stimulates build of uterus lining, high levels inhibit FSH and stimulate LH), LH (stimulates release of egg, drops after ovulation), and progesterone (inhibits FSH and LH, secreted by empty egg follicle, maintains uterus lining/pregnancy)

Fertility

In IVF, mother is given LH and FSH to stimulate oestrogen, eggs are collected and fertilised with father’s sperm in the lab, fertilised eggs are kept in a special solution until they become embryos, and are then implanted back into the uterus

Contraception is a method of preventing the sperm cells and egg cell meeting fertilised

Includes hormonal methods (pill, implants, injections, patches), chemical methods (spermicides), barrier methods (condoms, diaphragms, IUD), and surgical methods (vasectomy, cutting oviducts)

Thermoregulation

Too cold = vasoconstriction (shunt vessel opens), blood vessels keep blood way from skin surface, less heat loss, muscles contract to raise hair to trap heat, shivering

Too hot = vasodilation (shunt vessel narrows), blood travels near surface of skin, sweat glands produce sweat, takes heat away from skin with radiation as it evaporates

A change in a few degrees can affect enzyme activity and how energy is transferred (i.e. CO2 diffusing out of cells down conc. gradient, body must maintain this steep conc. gradient)

Kidneys

Urea made when amino acids are broken down by deamination (removes amide group), makes ammonia which is converted to urea for safe excretion

Nephrons filter blood

Glucose, mineral ions, water and urea move out of blood

RBC/WBC/proteins are too large to be filtered so bypass kidneys

All glucose is reabsorbed

Water and mineral ions reabsorbed by selective reabsorption

Urine trickles to the bladder

Receptor cells in the brain detect conc. of solutes in plasma

If the blood is too concentrated, lots of ADH is produces, and a small volume of concentrated urine is produced

If blood is too dilute, less ADH is produced, and a large volume of dilute urine is produced

Doing lots of exercise/sweating/drinking little means less urine as more water/mineral ions are absorbed to make sweat

Doing little activity/drinking a lot produces lots of urine

Dialysis involves blood passing between partially permeable membrane surrounded by dialysis fluid with the same conc. of dissolved ions and glucose as a healthy person

Fluid contains urea, meaning a steep concentration gradient means only urea and excess mineral ions diffuse across

No active transport unlike in the kidneys because, contrary to what AQA believe, inanimate objects cannot respire

A transplant can be given but the risk of cell rejection means a patient must be on immune-suppressant drugs for life

Plant hormones

Roots grow down due to gravity (positive gravitropism)

Shoots grow up towards the light (phototropism)

Auxin distribute to bend due to gravity/the shaded side respectively to elongate plant cells and curve growth

Gibberellins stimulate breakdown of food stores in seeds at germination – used to speed up germination

Ethene controls cell division and ripens fruit

To make clones of the same plant (i.e. it has good properties/can be grown quickly), cuttings are taken and mixed with rooting powders

Auxin can be used as a weed killer

Reproduction

Asexual reproduction: one parent, genetically identical offspring, mitosis

Sexual reproduction: two parents, variation in offspring, gametes created by meiosis

In meiosis, genetic information is copied so there are 4 sets of each chromosome, each chromosome forms a pair of chromatids, and the cell divides twice in quick succession to form 4 gametes, each with a single set of chromosomes (23)

Fungi are made of hyphae, in which spores reproduce asexually but undergo meiosis to make the spores

Genetics

DNA: long polymer chain of repeating units, twists and folds into a double helix structure, paired up homologous chromosomes

Genes code for specific sequence of amino acids to make specific proteins by protein synthesis (transcription – DNA unzips, mRNA complements strand, carried out by ribosomes, and translation – tRNA complements mRNA codon and brings amino acid to ribosome, mRNA is entirely decoded until a chain of amino acids forms polypeptides)

The genome is the entire genetic material of an organism

A nucleotide is the combination of a sugar, phosphate backbone, and base

Nucleotides are grouped into 3 (codons)

The complementing base pairs are adenine & thymine, and cytosine & guanine

Non-coding parts of DNA are used for switching certain genes on/off (controls gene expression)

Mutation: change in sequence of bases in DNA, forming a different protein

Mutations in non-coding DNA won’t affect the phenotype, but how certain genes are expressed

Alleles are different versions of the same gene – each code for different protein

Inheritance was first developed as an idea by Gregor Mendel

If only one gene is involved, it is monohybrid inheritance

Polydactyl – dominant, cystic fibrosis (in the paper it’s meant to be in) – recessive, sickle cell anaemia – recessive, Huntington’s disease – dominant

Genetic screening can be done by amino centesis (fluid taken from foetal fluid via needle), chorionic villus sampling (sample of tissue from placenta), or by screening embryos (DNA isolated from embryo cell and tested with fluorescent dye which binds to specific alleles)

Variation and evolution

Discontinuous variation: characteristics can only result in certain values

Continuous variation: characteristics can take any value within a certain range (shown by normal distribution curve)

Speciation is natural selection due to a change in location

Darwin proposed the theory of natural selection (good mutation leads to higher survival rate and therefore is inherited more and more over time)

Lamarck said that animals change to suit their environment, but this was disproven as animals cannot change their genotype

Classification

Animalia, plantae, fungi, protoctista, prokaryote – kingdoms

Archaea, bacteria, eukaryotes – domains (Carl Woese)

Mammals, reptiles, fish, amphibians, birds – class

Kingdom, phylum, class, order, family, genus, species

Named by binomial system (two components) in Latin from the genus and species

Ecology

Ecosystem: all the organism living in a habitat with all the non-living parts

Individual organisms -> populations -> communities

Interdependence is when species rely on each other

When all the species and environmental factors are in balance, it is a stable community

Abiotic factors – light, temperature, moisture, soil pH, minerals, wind, CO2, and Oxygen levels

Biotic factors – food, predators, pathogens, competition between species

Adaptations may be structural, behavioural or functional

Extremophiles live in extreme environments

Population: group of one species living in a habitat

Biodiversity: the variety of different species on earth – allows interdependence

Threats include pollution, over-exploitation, decay/burning of peat, deforestation and global warming (rising sea levels, changes migration, disrupt species distribution etc)

Solutions include breeding programs, protecting rare species, encouraging farmers to leave hedgerows (stop monoculture), reducing deforestation/CO2 emissions, fishing quotas, and adapting net sizes

Decomposition occurs by detrivores beginning the breakdown and then decomposers secreting enzymes to partially digest waste and leave small, soluble food molecules which are absorbed by soil

Producers -> primary consumers -> secondary consumers -> tertiary consumers (apex predators have no predators above them)

In a stable community, predators and prey rise/fall in cycles

Carbon cycle:

Water cycle:

Pyramids of biomass show trophic levels

Energy is lost due to faeces, excretion, urine, respiration etc so there are less organisms in higher trophic levels

Feeding animals high-protein food increases energy transfer, along with lowering temperatures and movement (although this is unethical)

Biotechnology uses Fusarium in large vats called fermenters with glucose syrup to make mycoprotein, which is then harvested and purified

Mycoprotein is low in fat, high in fibre/protein

Genetically modified crops can provide nutritional value (i.e. golden rice in Asia)