Responses to Changes in Environment

Question 1

What is a stimulus?

Answer 1

A change in an organism’s internal or external environment

Question 2

Why is it important that organisms can respond to stimuli?

Answer 2

Organisms increase their chance of survival by responding to stimuli.

Question 3

What is a tropism?

Answer 3

Growth of a plant in response to a directional stimulus. Positive tropism = towards a stimulus, Negative tropism = away from stimulus

Question 4

Describe how indoleacetic acid (IAA) affects cells in roots and shoots

Answer 4

In shoots, high concentrations of IAA stimulates cell elongation. In roots, high concentrations of IAA inhibits cell elongation.

Question 5

Explain gravitropism in flowering plants

Answer 5

1. Cells in tip of shoot / root produce IAA
2. IAA diffuses down shoot / root (evenly initially)
3. IAA moves to lower side of shoot / root (so concentration increases)
4. In shoots this stimulates cell elongation whereas in roots this inhibits cell elongation
5. So shoots bend away from gravity whereas roots bend towards gravity

Question 6

Explain phototropism in flowering plants

Answer 6

1. Cells in tip of shoot / root produce IAA
2. IAA diffuses down shoot / root (evenly initially)
3. IAA moves to shaded side of shoot / root (so concentration increases)
4. In shoots this stimulates cell elongation whereas in roots this inhibits cell elongation
5. So shoots bend towards light
   whereas roots bend away from light

Question 7

Describe the simple responses that can maintain a mobile organism in a
favourable environment

Answer 7

1. Taxes (tactic response)
   * Directional response
   * Movement towards or away from a directional stimulus
2. Kinesis (kinetic responses)
   * Non-directional response
   * Speed of movement or rate of direction change
   * Changes in response to a non-directional stimulus
   * Depending on intensity of stimulus

Question 8

Why are reflexes important?

Answer 8

They are very fast as there are only 3 neurones and therefore few synapses (synaptic transmission is slow). They are also Autonomic (doesn’t involve conscious regions of brain) so doesn’t have to be learnt. They protect us from harmful stimuli, prevents damage to body.

Question 9

Describe the basic structure of a Pacinian corpuscle

Answer 9

They have a single sensory neurone surrounded by gel. Between the neurone and the gel is a stretch mediated sodium ion channel. Within the gel are layers called lamellae.

Question 10

Describe how a generator potential is established in a Pacinian corpuscle

Answer 10

• Mechanical stimulus eg. pressure deforms the lamellae and stretch- mediated sodium ion channels
• So the sodium ion channels in membrane open and sodium ions diffuse into sensory neurone
• This causes depolarisation, leading to a
  generator potential. If generator potential reaches threshold, it triggers an action potential

Question 11

Explain the differences in sensitivity to light for rods & cones in the retina

Answer 11

Rods are more sensitive to light. Several rods are connected to a single neurone. Spatial summation occurs to reach the threshold to generate an action potential.

Cones are less sensitive to light. Each cone is connected to a single neurone and there is therefore no spatial summation.

Question 12

Explain the difference in visual acuity for rods and cones in the retina

Answer 12

Rods give lower visual acuity. 3 rod cells are connected to a single neurone, so 3 rods send a single set of impulses to the brain, and the brain therefore can not distinguish between separate source of light.

Cones give higher visual acuity. Each cone cell is connected to a single neurone, so each cone sends a separate set of impulses to the brain, and the brain therefore can distinguish between separate sources of light

Question 13

Explain the differences in sensitivity to colour for rods & cones in the retina

Answer 13

Rods allow monochromatic vision as there is only 1 type of rod. Cones allow colour vision as there are 3 types of cones (red, green and blue sensitive). These different optical pigments absorb different wavelengths of light, and stimulating different combinations of cones gives the range of colour perception

Question 14

Describe the myogenic stimulation of the heart and transmission of a
subsequent wave of electrical activity

Answer 14

1. Sinoatrial node (SAN) acts as pacemaker, it sends regular waves of electrical activity across the atria. This causes the atria to contract simultaneously.
2. Non-conducting tissue between the atria and ventricles prevents impulse passing directly to ventricles. This prevents immediate contraction of ventricles
3. Waves of electrical activity reach atrioventricular node (AVN) which delays the impulse by 0.1 seconds, allowing atria to fully contract and empty before ventricles contract
4. AVN sends wave of electrical activity down the Bundle Of His, conducting the impulse between ventricles to the apex where it branches into Purkyne tissue, causing ventricles to contract simultaneously from the base up

Question 15

Where are chemoreceptors and pressure receptors located?

Answer 15

Chemoreceptors and pressure receptors are located in the aorta and carotid arteries.

Question 16

Describe the roles of chemoreceptors, pressure receptors, the autonomic
nervous system and effectors in controlling heart rate

Answer 16

When Baroreceptors detect a fall in blood pressure or chemoreceptors detect a rise in blood CO2 concentration or fall in pH, they send impulses to the cardiac control centre of the medulla, which sends impulses to the SAN along the sympathetic neurones. Therefore there is more frequent impulses sent from the SAN, so cardiac muscle contracts more frequently and heart rate increases.

When Baroreceptors detect a rise in blood pressure or chemoreceptors detect a fall in blood CO2 concentration or rise in pH, they send impulses to the cardiac control centre of the medulla, which sends impulses to the SAN along the parasympathetic neurones. Therefore there is less frequent impulses sent from the SAN, so cardiac muscle contracts less frequently and heart rate decreases.

Question 17

Describe resting potential

Answer 17

The inside of the axon has a negative charge relative to the outside, as there are more positive ions outside compared to inside).

Question 18

How is a resting potential is established across the axon membrane in
a neurone

Answer 18

Sodium-Potassium ion pump actively transports 3 Sodium ions out of the axon and 2 Potassium ions into the axon. This creates an electrochemical gradient as there is a higher Potassium concentration inside and a higher Sodium ion concentration outside.

Question 19

Explain how changes in membrane permeability lead to depolarisation and the generation of an action potential

Answer 19

1. Stimulus: Sodium ion channels open, so membrane permeability to Sodium ions increases. This means that Sodium ions diffuse into the axon down the electrochemical gradient (causing depolarisation)
2. Depolarisation: If the threshold potential reached, an action potential is generated, because more voltage-gated sodium ion channels open, so more sodium ions diffuse in rapidly
3. Repolarisation: The voltage-gated sodium ion channels close, and voltage-gated potassium ions channels open, so potassium ions diffuse out of the axon
4. Hyperpolarisation: Potassium ion channels are slow to close, so there’s a slight overshoot – too many Potassium ions diffuse
   out
5. Resting potential: Restored by Sodium Potassium ion pump

Question 20

Draw / label a graph showing an action potential

Answer 20

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Question 21

Describe the all-or-nothing principle

Answer 21

For an action potential to be produced, depolarisation must exceed threshold potential. The action potential produced are always same magnitude / size / peak at same potential. Bigger stimuli instead increase frequency of action potentials.

Question 22

Explain how the passage of an action potential along non-myelinated and myelinated axons results in nerve impulses

Answer 22

Non-myelinated axon
* Action potential passes as a wave of depolarisation
* Influx of Sodium ions in one region increases the permeability of adjoining region to Na+ by causing voltage-gated Na+ channels to open
so adjoining region depolarises

Myelinated axon
* Myelination provides electrical insulation
* The axon is depolarised only at the nodes of Ranvier
* This results in saltatory conduction, so there is no need for depolarisation along the whole length of axon

Question 23

Suggest how damage to the myelin sheath can lead to slow responses and/or jerky movement

Answer 23

There will be less / no saltatory conduction, so depolarisation will occur along whole length of axon, so nerve impulses take longer to reach neuromuscular junction and there is a delay in muscle contraction. Also, ions / depolarisation may pass to other neurones, causing wrong muscle fibres to contract

Question 24

What is the refractory period?

Answer 24

It is the time taken to restore the axon to the resting potential when no further action potential can be generated as Sodium ion channels are closed.

Question 25

Explain the importance of the refractory period

Answer 25

Ensures that discrete impulses are produced (action potentials don’t overlap). Also limits the frequency of impulse transmission at a certain intensity (prevents over reaction to stimulus). Higher intensity stimulus causes higher frequency of action potentials, but only up to certain intensity. Also ensures action potentials travel in one direction – can’t be propagated in a refractory region

Question 26

Describe the factors that affect speed of conductance

Answer 26

• Myelination: The more myelination, the faster the conduction as depolarisation only occurs at nodes of ranvier (saltatory conduction), and impulse doesn’t travel along whole length of the axon
• Axon diameter: Bigger diameter means less resistance to flow of ions in cytoplasm
• Temperature: Higher temperatures increase the rate of diffusion of Sodium and Potassium ions as there is more kinetic energy. However, enzymes could denature at certain temperatures.

Question 27

What are cholinergic synapses?

Answer 27

Synapses that use the neurotransmitter acetylcholine (ACh).

Question 28

Describe transmission across a cholinergic synapse

Answer 28

At the pre-synaptic neurone, the depolarisation of the pre-synaptic membrane causes opening of voltage-gated Calcium ion channels, so Calcium ions diffuse into the pre-synaptic knob. This causes vesicles containing ACh to move and fuse with the pre-synaptic membrane,
releasing ACh into the synaptic cleft by exocytosis.

At the post-synaptic neurone, ACh diffuses across synaptic cleft to bind to specific receptors on post-synaptic membrane, causing Sodium ion channels to open, so Sodium ions diffuse into the post-synaptic knob causing depolarisation. If the threshold is met, an action potential is initiated

Question 29

What happens to acetylcholine after synaptic transmission?

Answer 29

It is hydrolysed by acetylcholinesterase. Products are reabsorbed by the presynaptic neurone to stop overstimulation - if not removed it would keep binding to receptors, causing depolarisation

Question 30

Explain how synapses result in unidirectional nerve impulses

Answer 30

Neurotransmitters are only released from pre-synaptic neurone, and the receptors are only on the post-synaptic membrane

Question 31

Explain summation by synapses and why is it important?

Answer 31

Addition of a number of impulses converging on a single post-synaptic neurone, causing rapid buildup of neurotransmitter, so threshold more likely to be reached to generate an action potential. Important because low frequency action potentials release insufficient neurotransmitter to exceed threshold

Question 32

What is spatial summation?

Answer 32

Many pre-synaptic neurones
share one post-synaptic neurone, so they collectively release sufficient neurotransmitter to reach threshold to trigger an
action potential

Question 33

What is temporal summation?

Answer 33

One pre-synaptic neurone
releases neurotransmitter many
times over a short period of time, and so over time, there can be sufficient neurotransmitter to reach threshold to trigger an action potential

Question 34

Describe the structure of a synapse

Answer 34

Very similar to a synapse except receptors are on muscle fibre instead of postsynaptic membrane and there are more of them. Also,
muscle fibre forms clefts to store enzyme eg. acetylcholinesterase to break down neurotransmitter

Question 34

Describe inhibition by inhibitory synapses

Answer 34

Inhibitory neurotransmitters hyperpolarise postsynaptic membrane, Cl- channels open, so Cl- diffuse in. K+ channels open, so K+ diffuse out. More Na+ is
required for depolarisation, reduces likelihood of threshold being met and action potential formation
at post-synaptic membranes

Question 35

Compare transmission across cholinergic synapses and neuromuscular
junctions

Answer 35

• In both, transmission is unidirectional
• Cholinergic synapses join neurones to other neurones, whereas Neuromuscular junctions join neurones to muscles
• In Cholinergic synapses, neurotransmitters can be excitatory or inhibitory, whereas in Neuromuscular junctions, they are always excitatory

Question 36

Explain the effect of drugs on a synapse

Answer 36

Some drugs stimulate the nervous system, leading to more action potentials. They have a similar shape to neurotransmitter, so stimulate the release of more neurotransmitter and inhibit enzyme that breaks down neurotransmitter, so Na+ continues to enter

Some drugs inhibit the nervous system, leading to fewer action potentials.. They inhibit the release of neurotransmitter, preventing the opening of calcium ion channels. They block receptors by mimicking shape of neurotransmitter

Question 37

Describe how muscles work

Answer 37

Work in antagonistic pairs and pull in opposite directions e.g. biceps / triceps. One muscle contracts, pulling on bone, producing force
and one muscle relaxes

Question 38

Describe the gross and microscopic structure of skeletal muscle

Answer 38

Made of many bundles of muscle fibres packaged together, attached to bones by tendons. Muscle fibres contain:
* Sarcolemma (cell membrane) which folds inwards
(invagination) to form transverse (T) tubules
* Sarcoplasm (cytoplasm)
* Multiple nuclei
* Many myofibrils
* Sarcoplasmic reticulum (endoplasmic reticulum)
* Many mitochondria

Question 39

Describe the ultrastructure of a myofibril

Answer 39

Made of two types of long protein filaments, arranged in parallel
* Myosin - thick filament
* Actin - thin filament
They are arranged in functional units called sarcomeres. The ends form the Z-line, and the middle forms the M-line. The H zone contains only myosin

Question 40

Draw and label a sarcomere

Answer 40

https://images.squarespace-cdn.com/content/v1/5c5aed8434c4e20e953d6011/1603109977095-PM783BB7QO4TM798P4XZ/sarcomere.jpg

Question 41

Explain the banding pattern to be seen in myofibrils

Answer 41

• I-bands - light bands containing only thin actin filaments
• A-bands - dark bands containing thick myosin filaments (and some actin filaments)
• H zone contains only myosin
• Darkest region contains overlapping actin and
  myosin

Question 42

Give an overview of muscle contraction

Answer 42

Myosin heads slide actin along myosin causing the sarcomere to contract. Simultaneous contraction of many sarcomeres causes myofibrils and muscle fibres to contract. When sarcomeres contract (shorten),
* H zones get shorter
* I band get shorter
* A band stays the same
* Z lines get closer

Question 43

Describe the sliding filament theory

Answer 43

FINISH THIS

Question 44

Describe the role of phosphocreatine in muscle contraction

Answer 44

It is a source of inorganic phosphate (Pi), rapidly phosphorylates ADP to regenerate ATP. ADP + phosphocreatine → ATP + creatine. Runs out after a few seconds → used in short bursts of vigorous exercise

Question 45

Compare the structure, location and general properties of slow and fast skeletal muscle fibres

Answer 45

Slow Twitch:
* General Properties: Specialised for slow, sustained contractions (eg. posture, long distance running). Obtains ATP mostly from aerobic
respiration, which releases energy slowly. They fatigue slowly.
* Location: High proportion in muscles used for posture e.g. back, calves. Found in legs of long distance runners.
* Structure: They have a high conc. of myoglobin, which stores oxygen for aerobic respiration. Also have many mitochondria for a high rate of aerobic respiration. Also have many capillaries to supply high conc. of oxygen / glucose for aerobic respiration and to prevent build-up of lactic acid causing muscle fatigue

Fast Twitch:
* General Properties: Specialised for brief, intensive contractions (e.g. sprinting). Obtains ATP mostly from anaerobic respiration, which releases energy quickly. They fatigue quickly due to high lactate conc.
* Location: High proportion in muscles used for fast movement e.g. biceps eyelids. Found in legs of sprinters.
* Structure: They have low levels of myoglobin, and lots of glycogen which is hydrolysed to provide
glucose for glycolysis / anaerobic
respiration which is inefficient so large quantities of glucose is required. There is a high conc. of enzymes involved in anaerobic respiration within the cytoplasm, They also store phosphocreatine

Question 46

Describe homeostasis in mammals

Answer 46

Maintenance of a stable internal environment by physiological control systems (normally involve negative feedback)

Question 47

Explain the importance of maintaining stable body temperature

Answer 47

If temperature is too high: Hydrogen bonds in tertiary structure of enzymes break, so enzymes denature, active sites change shape and substrates can’t bind, so fewer enzyme-substrate complexes form.

If temperature is too low: Not enough kinetic energy so fewer enzyme-substrate complexes will form as there will be less frequent successful collisions

Question 48

Explain the importance of maintaining stable blood pH

Answer 48

If pH is to too far above or below optimal pH, ionic bonds and hydrogen bonds in tertiary structure break. This means the enzymes denature, as active sites change shape and substrates can’t bind. Fewer enzyme substrate complexes will form.

Question 49

Explain the importance of maintaining stable blood glucose concentration

Answer 49

If blood glucose concentration gets too low (hypoglycaemia), there won’t be enough glucose for respiration, so less ATP will be produced, active processes such as active transport can’t happen and this may result in cell death.

If blood glucose concentration gets too high (hyperglycaemia), the water potential of blood decreases, so the water from tissues will move to blood via osmosis. Also the kidneys can’t absorb all glucose, so more water will be lost in urine causing dehydration.

Question 50

Describe the role of negative feedback in homeostasis

Answer 50

Receptors detect change from optimum, and Effectors respond to counteract change, returning levels to optimum / within normal range

Question 51

Describe positive feedback

Answer 51

Receptors detect change from optimum, and Effectors respond to amplify change, producing a greater deviation from normal. This is not involved in homeostasis.

Question 52

Describe the factors that influence blood glucose concentration

Answer 52

• Consumption of carbohydrates → glucose absorbed into blood
• Rate of respiration - e.g. increases during exercise due to muscle contraction

Question 53

Describe glycogenesis, glycogenolysis and gluconeogenesis

Answer 53

Glycogenesis: Converts glucose → glycogen
Glycogenolysis: Converts glycogen → glucose
Gluconeogenesis: Converts amino acids and glycerol → glucose

Question 54

Explain the action of insulin in decreasing blood glucose concentration

Answer 54

Beta cells in islets of Langerhans in the pancreas detect the increase in blood glucose concentration and they secrete insulin. Insulin attaches to specific receptors on the cell surface membrane of liver and muscle cells. This causes more glucose channel proteins to join cell surface membrane, increasing permeability to glucose, so more glucose can enter the cell by facilitated diffusion. This also activates enzymes involved in conversion of glucose to glycogen (glycogenesis), lowering glucose concentration in cells, creating a concentration gradient, so glucose enters cell from blood by facilitated diffusion

Question 55

Explain the action of glucagon in increasing blood glucose concentration

Answer 55

Alpha cells in the islets of Langerhans in the pancreas detect the decrease in blood glucose concentration and secrete
glucagon. Glucagon attaches to specific receptors on the cell surface membranes of liver cells. This activates enzymes involved in hydrolysis of glycogen to glucose (glycogenolysis), and activates enzymes involved in conversion of glycerol and amino acids to glucose (gluconeogenesis). This establishes a concentration gradient, so glucose enters blood from cells by facilitated diffusion

Question 56

Explain the role of adrenaline in increasing blood glucose concentration

Answer 56

At times of fear or stress, the adrenal glands secrete adrenaline. Adrenaline attaches to specific receptors on cell surface membranes of liver cells, activating enzymes involved in hydrolysis of glycogen to glucose (glycogenolysis). This establishes a concentration gradient and glucose enters blood from cells by facilitated diffusion

Question 57

Describe the second messenger model of adrenaline and glucagon action

Answer 57

Adrenaline or glucagon attach to specific receptors on cell membrane which activates enzyme adenylate cyclase, which converts ATP to cyclic AMP (cAMP). cAMP activates protein kinase enzymes, which activate enzymes to break down glycogen to glucose (Glycogenolysis)

Question 58

What is the cause of Type I Diabetes?

Answer 58

Beta cells in islets of Langerhans
in the pancreas produce insufficient insulin. Normally develops in childhood due to an autoimmune response destroying beta
cells of Islets of Langerhans

Question 59

What is the cause of Type II Diabetes?

Answer 59

Receptors lose responsiveness /
sensitivity to insulin, so there will be fewer glucose transport proteins and therefore less uptake of
glucose and less conversion of glucose to glycogen. Caused by lifestyle, risk factors include obesity

Question 60

Describe how Type I Diabetes can be controlled

Answer 60

Can be controlled through injections of insulin. Blood glucose concentration can be monitored with biosensors, and dose of insulin matched to glucose intake.
Should also control carbohydrate intake to avoid sudden rise in glucose

Question 61

Suggest why insulin can’t be taken as a tablet by mouth

Answer 61

Insulin is a protein so it would be hydrolysed by peptidases

Question 62

Describe how Type II Diabetes can be controlled

Answer 62

Not normally treated with insulin injections but may use drugs which target
insulin receptors to increase their sensitivity, in order to increase glucose uptake by cells and tissues. Should also reduce sugar intake (carbohydrates) and reduce fat intake, so less glycerol converted to glucose. Also, should do more regular exercise to use up more glucose by increasing respiration. Losing weight can also increase sensitivity of receptors to insulin

Question 63

Summarise the role of different parts of the nephron

Answer 63

1. Bowman’s Capsule: Formation of glomerular filtrate (ultrafiltration)
2. Proximal convoluted tubule: Reabsorption of water and glucose (selective reabsorption)
3. Loop of Henle: Maintenance of a gradient of sodium ions in the medulla
4. Distal convoluted tubule and 5. Collecting Duct: Reabsorption of water (permeability controlled by ADH)

Question 64

Describe the formation of glomerular filtrate

Answer 64

1. There is a high hydrostatic pressure in glomerulus as the diameter of afferent arteriole is wider than efferent arteriole (out)
2. Small substances eg. water, glucose, ions, urea are forced into glomerular filtrate
3. Large proteins / blood cells remain in blood

Question 65

Describe the reabsorption of glucose
by the proximal convoluted tubule

Answer 65

1. Na+ is actively transported out of epithelial cells to the capillary
2. Na+ moves by facilitated diffusion into epithelial cells down a concentration gradient, bringing glucose against its concentration gradient
3. Glucose moves into the capillary by facilitated diffusion down its concentration gradient

Question 66

Describe the reabsorption of water
by the proximal convoluted tubule

Answer 66

Glucose in the capillaries lower water potential, so water moves by osmosis down the water potential gradient

Question 67

Describe and explain how features of the cells in the PCT allow the rapid
reabsorption of glucose into the blood

Answer 67

• Microvilli provides a large surface area
• Many channel / carrier proteins for facilitated diffusion / co-transport
• Many carrier proteins for active transport
• Many mitochondria, which produce ATP for active transport
• Many ribosomes to produce carrier / channel proteins

Question 68

Suggest why glucose is found in the urine of an untreated diabetic person

Answer 68

Blood glucose concentration is too high so not all glucose is reabsorbed at the PCT. Glucose carrier / cotransporter proteins are saturated / working at maximum rate

Question 69

Explain the importance of maintaining a gradient of sodium ions in the medulla (concentration increases further down)

Answer 69

So that water potential decreases down the medulla (compared to filtrate in collecting duct), so a water potential gradient is maintained between the collecting duct and medulla, to maximise reabsorption of water by osmosis from filtrate

Question 70

Describe the role of the loop of Henle in maintaining a gradient of sodium
ions in the medulla

Answer 70

In the ascending limb: Na+ is actively transported out (so filtrate concentration decreases). Water remains inside as the ascending limb is impermeable to water. This increases concentration of Na+ in the medulla, lowering water potential.

In the descending limb: Water moves out by osmosis then is reabsorbed by capillaries (so filtrate concentration increases). Na+ diffuses back in.

Question 71

Suggest why animals needing to conserve water have long loops of Henle
(thick medulla)

Answer 71

More Na+ moves out, so Na+ gradient is maintained for longer in medulla, so water potential gradient is maintained for longer, and more water can be reabsorbed from collecting duct by osmosis

Question 72

Describe the reabsorption of water by the distal convoluted tubule and
collecting ducts

Answer 72

Water moves out of distal convoluted tubule and collecting duct by osmosis, down a water potential gradient, controlled by ADH which increases their permeability

Question 73

What is osmoregulation?

Answer 73

Control of water potential of the blood (by negative feedback)

Question 74

Describe the role of the hypothalamus in osmoregulation

Answer 74

Contains osmoreceptors which detect increase or decrease in blood water potential. It then produces more ADH when water potential is low or less ADH when water potential is high

Question 75

Describe the role of the posterior pituitary gland in osmoregulation

Answer 75

Secretes (more / less) ADH into blood due to signals from the hypothalamus

Question 76

Describe the role of antidiuretic
hormone (ADH) in osmoregulation

Answer 76

1. Attaches to receptors on collecting duct and distal convoluted tubule
2. Stimulating addition of channel proteins into cell-surface membranes
3. So increases permeability of cells of collecting duct and DCT to water
4. So increases water reabsorption from collecting duct / DCT (back into blood) by osmosis
5. So decreases volume and increases
   concentration of urine produced