AQA A2 Biology Chapter 9 -

My AQA Biology Unit 5 Notes.

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  • Created by: Jo Conway
  • Created on: 21-06-12 11:20

Chapter 9: Survival and Response

Organisms increase their chance of survival by responding to their environment.

Tropism - Growth or movement of a plant in response to a directional stimulus
Taxes - Towards or away
Kineses - Speed dependant on strength of stimulus. Just away from any unpleasant stimulus.

Reflex arc: Stimulus - Receptor - Sensory N. - Intermediate N. - Motor N. - Effector - Response.
Involuntary. Brain is not overloaded. Protective. Do not have to be learned. Fast. Few synapses. 

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Chapter 9: Control of Heart Rate and Receptors.

Chemoreceptors: Exercising. CO2 produced. Lots of CO2, low blood pH. In carotid arteries and aorta, this is detected. Freq. of impulses sent to medulla oblongata increases. Heart rate centre sends messages by symp. nervous system to SA node. SA node increases heart rate. CO2 is removed faster.

Pressure receptors: High BP. In carotid arteries, this is detected. Nervous impulse sent to heart rate centre in medulla oblongata. This send messages via parasymp. system to SA node, decreasing rate and blood pressure.

All is by autonomic (automatic) nervous system.

Pacinian Corpuscle: Specific to 1 type of stimulus, mechanical pressure. Membrane becomes stretched, stretch-mediated Na+ channels allows Na+ to diffuse into neurone, causing depolarisation. This makes a generator potential, then an action potential.

Rod cells: Poor visual acuity, sensitive in low light, concentrated around the retina, not at fovea. 3 connections to 1 bipolar neurone. Brain can't distinguish between signals.
Cone cells: Good visual acuity, sensitive in high light, conc. at foveo, fewer around retina. 1 connection to 1 bipolar neurone. Brain distinguishes between singals.

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Chapter 10 - Principles

Nerve cells pass action potentials down their length. They stimulate their target cells by releasing neurotransmitters directly on them for a localised, rapid and short-lived effect.

Hormones pass signals in the blood to produce long-lasting, slow and widespread responses.

Local chemical mediators histamine and prostaglandins are released by mammalian cells for a localised response.

Plant growth factors such as IAA cause response to directional stimuli. The growth factors move from cell to cell where they are required. IAA moves away from light into shaded cells, causing them to enlongate and encourage bending in the stem.

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Chapter 10 - Nerve Impulses

Resting Potential: Na+/K+ pumps out 3 Na and pumps in 2 K by active transport. K+ channels open, so K+ can diffuse out. Na+ channels closed, so it stays outside axon. This is against chemical and electrical gradients.

Action Potential: A stimulus causes depolarisation and there is an influx of Na+ into the cell as the voltage-gates Na+ channels open. This causes more channels to open. At +40mv, the Na+ channels shut and the K+ channels open. K+ diffuses out, causing repolarisation. At -80mv, the K+ ion channels shut and the Na+/K+ pump establishes resting potential again.

Action potentials are all or nothing.

Myelinated axons are faster at conducting than non-myelinated, because they only have action potentials at the node of Ranvier, but non-myelinated action potential all the way down. Higher temp, larger axon, faster diffusion and less leakage lead to a faster impulse.

Refractory period happens because Na+ voltage gated channels are closed. This means action potentials are one way, discrete and the frequency is limited.

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Chapter 10 - Synapses

Action potential causes influx of Ca2+, which causes the synaptic vesicles to fuse with the membrane and release their contents into the cleft. The neurotransmitter fuses to receptor site, generating a new action potential, by opening Na+ channels. Neurotransmitter is broken down, absorbed and reformed using ATP.

Unidirectional - only the presyn. makes the neurotransmitter and only postsyn. has receptors, so action potentials only travel one way.
Summation - Temporal: high frequency impulses. Spacial: more than 1 synapse
Inhibition - Cl- channels on postsyn. open, hyperpolarising membrane. More difficult to establish action potential.

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Chapter 11 - Sliding Filament Mechanism & Muscle C

Muscle>Bundle of Fibres>Fibre>Myofibril>Actin frame, myosin insert.

Actin: thinner, two twisted strands.
Myosin: Thicker, rod shaped, heads.

Slow twitch: Slower, weak contractions, calf muscles, aerobic respiration. Rich blood supply, glycogen, myoglobin, lots of mitchondria.
Fast twitch: Fast, powerful contractions, biceps, anerobic respiration. Thicker myosin, lots of anerobic enzymes, Lots of phosphocreatin.

Phosphocreatin can break down to form Pi to reform ADP in anerobic conditions.

A-band same length
H zone shortens
I band shortens

ATP causes detaching of myosin heads. Ca2+ causes tropomyosin to stop blocking the actin binding sites.

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Chapter 12 - Homeostasis

Homeostasis is the maintenance of a constant internal environment.

Enzymes are sensitive to pH and temperature changes. Changes to these reduce efficiency or denature them. Homeostasis means reactions take place at a constant predictable rate.

Water potential needs to be carefully regulated as the cells can shrink or expand due to osmosis. Cells cannot operate normally. Water potential is controlled by glucose levels. Also, this means a reliable source of glucose for respiration.

Ectotherms - gain and lose their heat from the environment by lying the in sun, taking shelter, from the ground, some from metabolic heat and colour variations.
Endotherms - vasoconstriction, shivering, hair raising, metabolic rate shift, sweating, behaviour.

Blood temp increase: warm receptors - hypothalamus heat loss centre - vasodilation ect. - feedback
Blood temp decrease: cold receptors - hypothalamus heat gain centre - vasoconstriction 

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Chapter 12 - Glucose Regulation & Diabetes

Glucose varies due to ingestion, glycogenolysis (breaking down glycogen) and glyconeogenisis (forming glucose from glycerol and amino acids).
Glycogen is stored in the liver and the liver can do glyconeogenesis.

 Blood glucose level lowers: a cells detect and produce glucagon - glycogenolysis and glyconeogenesis - blood glucose rises.
Blood glucose level rises: b cells detect and produce insulin - glycogen and fat are stored, increased respiration and absorption of glucose - blood glucose lowers.

Adrenaline activates cyclic AMP which activates enzymes that perform gylcogenolysis.

Type I diabetes. Body does not produce insulin. Develops quickly and usually in childhood. Insulin injections needed.
Type II diabetes. Body is resistant to insulin. Develops slowly, usually in obese adults. Diet can control it. 

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Chapter 13 - Feedback & Oestrous Cycle

Negative feedback returns system to normal by turning off corrective response.
Negative feedback allows for greater sensitivity in control.

Positive feedback makes a system deviate even more from the norm than usual.
Positive feedback is associated with breakdown of control systems.

Pituitary gland releases FSH from day 1. The follicle releases oestrogen, which inhibits production of FSH and LH. (-ve feedback). The follice keeps releasing oestrogen until a critical point is reached, where oestrogen now stimulates greater release of FSH and LH. (+ve feedback). There is a surge in FSH and LH. The LH surge causes ovulation and stimulates the empty follicle to develop into a corpus leutum, which releases progesterone. Progesterone inhibits LH and FSH (-ve feedback). The leutium degenerates. It releases less progesterone and so FSH is no longer inhibited.

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