Topic 6 - Organisms respond to changes in their internal and external environments

Question 1

How do organisms increase their chances of survival in changing environments?

Answer 1

By responding to internal and external stimuli through receptors and effectors, enabling appropriate behavioural and physiological changes.

Question 2

What are taxes and kineses?

Answer 2

Taxes are directional movements toward or away from a stimulus (e.g., phototaxis)

kinases are non-directional movements where the rate of movement is affected by stimulus intensity (e.g., woodlice moving more in dry areas).

Question 3

How does indoleacetic acid (IAA) affect phototropism in shoots?

Answer 3

IAA accumulates on the shaded side of the shoot, promoting cell elongation and causing the shoot to bend toward light.

Question 4

How does IAA influence gravitropism in roots?

Answer 4

IAA accumulates on the lower side of the root, inhibiting cell elongation, causing the root to bend downward in response to gravity

Question 5

Describe the structure and function of a simple reflex arc

Answer 5

Simple reflex arc involves a sensory neuron, a relay neuron, and a motor neuron, allowing rapid, involuntary responses to stimuli for protection.

Question 6

What is the role of receptors in the nervous system?

Answer 6

Receptors detect specific stimuli and convert them into electrical impulses (generator potentials) for processing.

Question 7

How does a Pacinian corpuscle function as a mechanoreceptor?

Answer 7

Pressure deforms its lamellae, opening stretch-mediated sodium channels, leading to depolarization and a generator potential

Question 8

Compare rod and cone cells in the retina

Answer 8

Rods are sensitive to low light, provide black and white vision, and have low visual acuity - due to retinol convergence, there is spatial summation of rod cells impulses to a single ganglion cell.

Cones detect colour, require bright light, and offer high visual acuity - due to no retinol convergence, each cell connected to its own ganglion cell, sending many individual signals through the optic nerve

Question 9

How is the heart’s rhythm initiated and controlled?

Answer 9

The sinoatrial node (SAN) generates impulses causing atrial contraction, moving from right to left

Impulses reach the atrioventricular node (AVN), which has a layer of non conductive tissue which delays them

Before passing to the bundle of His and Purkyne fibers, leading to ventricular contraction from the apex upwards.

Question 10

What is the role of chemoreceptors in heart rate regulation?

Answer 10

Chemoreceptors detect changes in blood pH due to CO₂ levels and send signals to the medulla, which adjusts heart rate via the autonomic nervous system.

Question 11

Describe the resting potential of a neuron

Answer 11

The resting potential is approximately -70 mV, maintained by the sodium-potassium pump and differential membrane permeability to ions.

Question 12

What occurs during an action potential?

Answer 12

Upon reaching the threshold, voltage-gated sodium channels open, causing depolarization; subsequently, potassium channels open for repolarization, followed by hyperpolarization before returning to resting potential

Question 13

How does myelination affect nerve impulse conduction?

Answer 13

Myelination allows saltatory conduction, where impulses jump between nodes of Ranvier, increasing conduction speed.

Question 14

Outline the process of synaptic transmission at a cholinergic synapse

Answer 14

An action potential triggers voltage gated calcium channels to open, and an influx of calcium ions via facilitated diffusion, causing vesicles to move and fuse with the pre synaptic membrane to release acetylcholine into the synaptic cleft; acetylcholine binds to receptors on the postsynaptic membrane, allowing for an influx of Na+ leading to depolarization.

Question 15

What is the difference between spatial and temporal summation?

Answer 15

Spatial summation involves multiple presynaptic neurons releasing neurotransmitters simultaneously

Temporal summation involves one neuron releasing neurotransmitters rapidly in succession.

Question 16

How do neuromuscular junctions differ from typical synapses?

Answer 16

Neuromuscular junctions always result in muscle contraction, have more receptors, and the neurotransmitter is always acetylcholine.

Question 17

Describe the structure of a myofibril

Answer 17

Myofibrils consist of repeating units called sarcomeres, containing actin (thin) and myosin (thick) filaments arranged in a specific pattern.

Question 18

Explain the sliding filament theory of muscle contraction

Answer 18

Calcium ions bind to tropomyosin, exposing binding sites on actin; myosin heads attach, perform a power stroke using ATP, pulling actin filaments inward, shortening the sarcomere.

Question 19

What are the roles of ATP and phosphocreatine in muscle contraction?

Answer 19

ATP provides energy for cross-bridge cycling; phosphocreatine regenerates ATP from ADP during short bursts of intense activity.

Question 20

Compare slow and fast skeletal muscle fibres

Answer 20

Slow fibres are adapted for endurance, rich in mitochondria and myoglobin

Fast fibres are adapted for rapid, powerful contractions, with more glycogen stores and anaerobic capacity.

Question 21

Why is homeostasis important?

Answer 21

It maintains a stable internal environment, essential for optimal enzyme function and overall cellular processes.

Question 22

How does negative feedback regulate physiological processes?

Answer 22

Negative feedback mechanisms detect deviations from a set point and initiate responses to restore equilibrium

Question 23

How does insulin regulate blood glucose levels?

Answer 23

Insulin binds to complementary receptors on liver cells

Promoting glucose uptake by cells by sending a chemical signal causing GLUT4 channels to move and fuse with the membrane

Glucose can move into the cell via facilitated diffusion,

Insulin then stimulates glycogenesis in the liver, lowering blood glucose concentration.

Question 24

What is the role of glucagon in blood glucose regulation?

Answer 24

Glucagon stimulates glycogenolysis (conversion of glycogen to glucose)

AND gluconeogenesis (formation of glucose from non-carbohydrates like glycerol and amino acids), increasing blood glucose levels.

Question 25

How does adrenaline affect blood glucose levels?

Answer 25

Adrenaline binds to receptors on liver cells, activating enzymes that cause glycogenolysis and inhibit glycogenesis, increasing blood glucose concentration during stress or exercise

Question 26

What is the second messenger model used by adrenaline and glucagon?

Answer 26

Hormone binds to cell surface receptor → activates adenylate cyclase → converts ATP to cAMP (cyclic AMP) → activates protein kinase → triggers enzyme cascades (e.g., glycogen breakdown).

Question 27

Define glycogenesis, glycogenolysis, and gluconeogenesis

Answer 27

Glycogenesis: Formation of glycogen from glucose. Glycogenolysis: Breakdown of glycogen into glucose. Gluconeogenesis: Formation of glucose from non-carbohydrate sources

Question 28

What causes Type I diabetes, and how is it managed?

Answer 28

Type I diabetes is caused by an autoimmune response destroying β cells in the pancreas. Managed by insulin injections and careful blood glucose monitoring.

Question 29

What causes Type II diabetes, and how is it managed?

Answer 29

Type II diabetes results from insulin resistance (target cells don’t respond to insulin). Managed through diet, exercise, weight loss, and sometimes medication.

Question 30

How does the food industry contribute to Type II diabetes, and what is the role of health advisers?

Answer 30

Food industry: Promotes high-sugar processed foods contributing to poor diets. Health advisers: Encourage public education, clearer food labelling, and policies to reduce sugar consumption.

Question 31

What is osmoregulation?

Answer 31

Osmoregulation is the homeostatic control of the water potential of the blood, preventing excessive water loss or gain to maintain stable internal conditions.

Question 32

Which organ monitors blood water potential and how?

Answer 32

The hypothalamus contains osmoreceptors that detect changes in blood water potential via osmosis, triggering a hormonal response via the pituitary gland.

Question 33

What hormone is involved in osmoregulation and where is it released from?

Answer 33

Antidiuretic hormone (ADH), produced by the hypothalamus and released from the posterior pituitary into the bloodstream.

Question 34

How does ADH act on the kidney?

Answer 34

ADH binds to receptors on the distal convoluted tubule (DCT) and collecting duct, causing aquaporins to be inserted into membranes, increasing water reabsorption into the blood.

Question 35

What happens when blood water potential is too low (e.g., after sweating)?

Answer 35

More ADH is released → more aquaporins → more water reabsorbed → concentrated urine, conserving water.

Question 36

What happens when blood water potential is too high (e.g., after drinking lots)?

Answer 36

Less ADH is released → fewer aquaporins → less water reabsorbed → dilute urine, allowing excess water to be excreted.

Question 37

What is the structure of the nephron and its key regions?

Answer 37

Bowman's capsule → glomerulus Proximal convoluted tubule (PCT) Loop of Henle Distal convoluted tubule (DCT) Collecting duct

Question 38

What is ultrafiltration and where does it occur?

Answer 38

Ultrafiltration occurs in the glomerulus and Bowman’s capsule due to high hydrostatic pressure, filtering small molecules (e.g. water, glucose, urea) into the nephron.

Question 39

How is glucose reabsorbed in the nephron?

Answer 39

In the proximal convoluted tubule, glucose is reabsorbed by active transport and co-transport with sodium ions into the blood.

Question 40

What is the role of the loop of Henle in osmoregulation?

Answer 40

The loop creates a sodium ion gradient in the medulla via counter current multiplication. This lowers water potential, enabling water reabsorption from the collecting duct

Question 41

How does the descending limb of the loop of Henle function?

Answer 41

It is permeable to water but not salts, so water leaves by osmosis into the salty medulla, concentrating the filtrate.

Question 42

How does the ascending limb of the loop of Henle function?

Answer 42

It is impermeable to water but actively transports Na⁺ and Cl⁻ out into the medulla, reducing filtrate concentration and maintaining medullary gradient.

Question 43

What is the role of the collecting duct in water reabsorption?

Answer 43

It passes through the medullary sodium ion gradient. With ADH, water is reabsorbed by osmosis, concentrating urine and conserving water.

Question 44

What is the significance of countercurrent flow in the nephron?

Answer 44

It maintains a steep concentration gradient throughout the length of the collecting duct, maximizing water reabsorption.