19 June 2016

#127 Summary of Homeostasis and Co-ordination

 1 Animals and plants have internal communication systems that allow information to pass between different parts of their bodies, and so help them to respond to changes in their external and internal environments.

 2 Mammals keep their internal environment relatively constant, so providing steady and appropriate conditions within which cells can carry out their activities. This is known as homeostasis.

 3 Homeostatic equilibrium requires receptors that detect changes in physiological parameters such as the temperature, water potential and pH of the blood. Effectors are the cells, tissues and organs that carry out the functions necessary to restore those parameters to their set points. Homeostatic control systems use negative feedback in which any change in a parameter stimulates actions by effectors to restore the parameter to its set point.

 4 Excretion is the removal of toxic waste products of metabolism, especially carbon dioxide and urea. The deamination of excess amino acids in the liver produces ammonia, which is converted into urea, the main nitrogenous waste product. Urea is excreted in solution in water, as urine. 

 5 The kidneys regulate the concentration of various substances in the body fluids, by excreting appropriate amounts of them. Each kidney is made up of thousands of nephrons and their associated blood vessels. The kidneys produce urine by ultrafiltration and reabsorption, plus some secretion of unwanted substances. Different regions of a nephron have different functions, and this is reflected in the structure of the cells that make up their walls. 

 6 Blood is brought to the glomerulus in an afferent arteriole. High hydrostatic pressure in the glomerulus forces substances through the capillary walls, the basement membrane and inner lining of Bowman’s capsule. The basement membrane acts as a filter, allowing only small molecules through. This filtrate collects in Bowman’s capsule and then enters the proximal convoluted tubule, where most reabsorption occurs by diffusion and active transport; substances are also reabsorbed in the distal convoluted tubule and collecting duct. The loop of Henle acts as a counter-current multiplier, producing high concentrations of sodium and chloride ions in the tissue fluid in the medulla. This tissue has a very low water potential. Water is reabsorbed from fluid in the collecting duct by osmosis if the body is dehydrated. 

 7 The water content of the blood is controlled by changing the amount of water excreted in the urine by the kidneys. This is done by regulating the permeability of the walls of the collecting ducts to water, and hence the volume of water reabsorbed from the collecting ducts into the blood. The permeability is increased by the hormone ADH, which is secreted by the posterior pituitary gland in response to stimulation of osmoreceptors in the hypothalamus. 

 8 Neurones are cells adapted for the rapid transmission of electrical impulses. Sensory neurones transmit impulses from receptors to the central nervous system (brain and spinal cord); motor neurones transmit impulses from the central nervous system to eff ectors; intermediate neurones transmit impulses within the central nervous system. The three neurones are found in series in reflex arcs that control fast, automatic responses to stimuli. In vertebrates, the axons of many neurones are insulated by a myelin sheath, which speeds up transmission. 

 9 Neurones have a resting potential, which is a potential difference across their membranes, with the inside having a negative potential compared with the outside; this potential difference is about −65 mV. An action potential is a rapid reversal of this potential, caused by changes in permeability of the cell surface membrane to potassium and sodium ions. Action potentials are always the same size. Information about the strength of a stimulus is given by the frequency of action potentials produced. 

10 Action potentials are propagated along axons by local circuits that depolarise regions of membrane ahead of the action potential. This depolarisation stimulates sodium ion voltage-gated channels to open, so that the permeability to sodium increases and the action potential occurs further down the axon. Axons are repolarised by the opening of potassium ion voltagegated channels that allow potassium ions to diffuse out of the axon. After a short refractory period when the sodium channels cannot open, the membrane is able to respond again. Refractory periods determine the maximum speed of impulses. 

 11 Action potentials may be initiated within the brain or at a receptor. Receptors respond to information from the environment. Environmental changes result in permeability changes in the membranes of receptor cells, which in turn produce changes in potential diff erence across the membrane. If the potential difference is sufficiently great and above the threshold for the receptor cell, this will trigger an action potential in a sensory neurone. Receptors are transducers converting the energy of stimuli into electrical impulses. 

 12 A synapse is a junction between two neurones or between a motor neurone and a muscle cell. At cholinergic synapses, a transmitter substance, acetylcholine, is released when action potentials arrive. Impulses pass in one direction only, because transmitter substances are released by exocytosis by the presynaptic neurone to bind to receptor proteins that are only found on the postsynaptic neurone. 

 13 Any one neurone within the central nervous system is likely to have at least several hundred synapses with other neurones, some of which will be stimulatory and some inhibitory. This allows integration within the nervous system, resulting in complex and variable patterns of behaviour, and in learning and memory. 

 14 Hormones are chemicals that are made in endocrine glands and transported in blood plasma to their target cells, where they bind to specific receptors and so affect the behaviour of the cells. 

 15 The concentration of glucose in the blood is controlled by the action of insulin and glucagon, which are secreted by the islets of Langerhans in the pancreas and aff ect liver and muscle cells. The use of negative feedback keeps the blood glucose concentration near the set point. 

 16 Plants produce several chemicals known as plant growth substances that are involved in the control of growth and responses to environmental changes. Auxin is synthesised mainly in growing tips of shoots and roots, and appears to be involved in preventing the growth of lateral buds when an intact and active apical bud is present. Gibberellin is synthesised in young leaves and in seeds. It stimulates growth of stems and germination of seeds such as those of wheat and barley. Abscisic acid is synthesised by any cells in a plant that contain chloroplasts or amyloplasts, especially in stress conditions. The presence of large concentrations of abscisic acid in leaves causes stomata to close.

 1. End-of-chapter questions

1 Which of the  following is an  incorrect statement about the endocrine system?

A   All hormones  bind to receptors on the cell surface of their target  cells.
B   Endocrine glands  are ductless.
C   Endocrine glands secrete hormones into the  blood.
D   Hormones are transported in the blood plasma.

2   Glucose is small enough to be filtered from the  blood  in glomeruli  in the kidney, but is not  normally found in the urine. This is because glucose is:

A   reabsorbed in distal convoluted tubules
B   reabsorbed in proximal  convoluted tubules
C   reabsorbed along the whole length  of the nephrons
D   respired by cells  in the kidney

3 Which  of the  following  is responsible for saltatory conduction in myelinated neurones?

A   axon  membranes
B   nodes   of Ranvier
C   Schwarm   cells
D   voltage-gated channel proteins

4  Which  of the  following correctly identifies the effects of the three plant hormones,abscisic acid  (ABA), auxin and gibberellin?

5 The figure shows the changes in potential  difference across the membrane  of a neurone over a period of time. The membrane was stimulated at time  A and  time B with stimuli of different   intensities.

a    Stimulus B resulted in an action potential.
Describe what is occurring at C,  D and E.    [6]

b    Suggest why stimulus A did  not result in
an action potential being produced  whereas stimulus B did.                                                    [2]
                                                                [Total: 8]

[Cambridge International AS and A Level Biology 9700104, Question 8, October/November 2007]

6 a   Explain  the  meaning   of the  term  excretion. [3]
   b  The figure  is a photomicrograph of part  of the  kidney.
         i   Name  A, B, C and  D.                                  [4]
         ii  Identify the region of the kidneyshown in the figure
 and give a reason for your identification.                 [2]
        iii Calculate   the  actual  maximum    width   of the
structure   labelled  A. Show  your  working.            [2]
[Total: 11]

7 The control of the water content of the blood is an example of homeostasis.
    a Name the part of the body that monitors the water potential of the blood.    [1]

 In an investigation of the factors that influence urine production, a person drank one litre of water.  The person's urine was collected  at half-hourly intervals for four hours  after drinking. The results  are shown as line A on the figure. On the following day, the same person drank one litre of a dilute    salt solution and the urine was collected in the same way (line  B). Dilute salt solution has about the  same water potential as blood plasma.

   b  Calculate how much urine was produced in the two hours after drinking the litre of water. [1]
   c  Explain why the person produced so much urine after drinking  the litre of water.  [4]
  d   Suggest  why the results during the second day were so different from those on the first day. [2]
  e Explain why negative feedback, and not positive feedback is involved in homeostatic mechanisms.

[Total: 13]

8 Mammals have internal communication systems for signalling between cells.
   a Explain why animals such as mammals have internal communication systems.    [4]

Organs that secrete substances through ducts are known as exocrine glands. Most of the pancreas is made up of exocrine  tissue. The rest, about 2%, is endocrine tissue which secretes insulin and glucagon.
   b Describe the structure of the endocrine tissue in the pancreas.   5]

An investigation  was carried out to determine the response of pancreatic cells to an increase in the  glucose concentration of the blood. A person who had been told not to eat or drink  anything  other  than water for 12 hours took a drink of a glucose solution. Blood samples were taken from the person at one hour  intervals for five hours, and the concentrations of glucose, insulin and glucagon   in the blood were determined. The results are shown in the figure.

c   i  Explain why the person was told not to eat or drink anything other than water for 12 hours                   before having the glucose drink.  [3]
    ii  Use the information in the figure to describe the response  of the pancreatic cells to an increase            in the glucose concentration.   [4]
    iii  Outline the role of insulin when the glucose concentration in the blood increases. [5]
 [Total:  21]

9 Gibberellin is a plant growth regulator.
   a  Outline the role of gibberellin in the germination of seeds such as those of wheat and barley.   [5]

In an investigation of the effects of gibberellin, plants  of short-stemmed and long-stemmed varieties of five cultivated species were grown from seed. The young plants of each species were divided into two groups. One group of plants was sprayed with a solution of gibberellin each day. A control group was sprayed with the same volume  of water. After eight weeks, the stem length of each plant  was measured and means calculated for each group of plants. A statistical test was carried out to determine whether the difference between the treatments for each species was significant.

The results are shown in the figure. The p value for each species is given.

b  Using the information in the figure, describe the effect of adding gibberellin  solution to the two varieties of the five species.              [5]
c Explain why the short-stemmed variety of pea showed a more significant growth  in height when  treated with gibberellin than the long-stemmed variety.   [3]
d Suggest  the advantages of cultivating crops of short-stemmed varieties of peas and beans rather  than long-stemmed varieties.    [3]
 [Total: 16]

10 Abscisic acid (ABA) is a weak acid. Its structure can be represented as ABA-H. It dissociates   into positively charged H+ ions (protons) and negatively charged  ABA-  ions as shown:
The following observations have been  made by scientists:

   light stimulates proton (H+ ion) uptake into the grana of chloroplasts; 
  ABA-H can diffuse into and out of chloroplasts, but  ABA-  cannot.

This information is summarised in the figure.

a   Using all the information provided, predict what  happens to the  pH in the stroma in the light.[1]
b   i When light shines on the chloroplast, dissociation of ABA-H is stimulated. Explain why this             happens.                                      [2] 
    ii Explain the effect that this will have on diffusion of ABA-H into or out of the chloroplast.  [2]

    When the mesophyll cells of leaves become dehydrated, some of the ABA stored in the                       chloroplasts is released into the transpiration stream in the apoplast,
c  ABA travels in the apoplast pathway to the  guard cells. Explain why this is an advantage when            the leaf is dehydrated.                    [2]
[Total:  7]

11 a  Explain how a nerve impulse is transmitted along a motor neurone. [9] 
     b  Describe how an impulse crosses a synapse.  [6]
 [Total:  15]

12  a   Describe a reflex arc and explain why such reflex arcs are important.   [7]
      b   Describe the structure of a myelin  sheath and explain its role in the speed of transmission of a            nerve impulse.     [8]
[Total:   15]

[Cambridge International AS and A Level Biology Paper 41, Question 10, October/November2009]

13  a   Compare  the roles of the endocrine  and nervous systems in control and coordination in                    animals.         [8]
      b Describe the part played by auxins in apical dominance in a plant shoot.                         [7]

[Cambridge International AS and A Level Biology 9700/04, Question 10, May/June 2008]

2. End-of-chapter answers

Cambridge International Examinations bears no responsibility for the example answers to questions taken from its past question papers which are contained in this publication.

 1 A
 2 B
 3 B
 4 C

Exam-style questions

5 a C: depolarisation/the inside of the membrane becomes more positive/less negative; 
           sodium ions/Na+ , flow in; 
       D: repolarisation/inside of the membrane becomes more negative/less positive; 
            potassium ions/K+ , flow out; 
       E: hyperpolarisation/refractory period; 
            more negative than resting potential;                                                                              [6]

   6 a excretion: removal from the body; of waste products of metabolism; 
       carbon dioxide/nitrogenous waste/urea/uric acid/ any other example; 
       substances in excess of requirements; 
        water/salts/sodium ions/potassium ions/any other example;                                [max. 3] 

      b i A: distal convoluted tubule; 
            B: Bowman’s capsule; 
            C: glomerulus/capillary; 
            D: proximal convoluted tubule;                                                                                  [4]

       ii cortex; glomeruli/convoluted tubules, are only found in the cortex;                         [2]

7 a hypothalamus;                                                                                                                 [1]

   b 1555 cm3 (or any answer within the range 1150 to 1160 cm3 or equivalent in dm3 ); [1] 

   c any four from: 
     water was absorbed into the blood; 
     water increases the water potential of the plasma; 
     any effect of an increase in water potential of the plasma on, cells/tissues, e.g. water enters cells          by osmosis/cells will swell/decreases efficiency of reactions inside cells/cells may burst;                  osmoreceptors detect increase in water potential; do not, secrete/release, ADH; collecting ducts     remain impermeable to water; excess water lost in urine; until water potential returns to normal/ set     point;                                                                                                                               [max. 4]

d (after absorption of dilute salt solution) no change in water potential of blood plasma; water and salt is not lost in the urine, so must remain in the body; giving an increase in volume, of blood or body fl uids; body tolerates changes in blood volume, but not its water potential;         [max. 2] 

 e homeostasis is maintenance of constant internal conditions; 
    negative feedback: a deviation from the set point; 
    is detected by a receptor; a control centre instructs eff ector to carry out an action; 
    to reverse the change/return factor to set point; 
    positive feedback: any (small) deviation in a factor leads to an increase in the change (not a reversal); e.g. opening of voltage-gated sodium ion channels in rising phase of action potential; [max. 5] 
 [Total: 13]

8 a animals are multicellular/complex organisms; 
  cells are long distances apart;
  of cells/tissues/organs, so that they work together; 
  regulation of internal environment/ refer to homeostasis;
  response to, changes in the environment/external stimuli; 
  any example;                                                                                                                   [max. 4]

 b islets of Langerhans; 
    small groups of cells; 
    scattered among the exocrine tissue;
     blood spaces/ capillaries, in between the cells;
     α cells;
     β cells; 
     cells full of vesicles containing (molecules of), hormones/protein; 
      cells with rough endoplasmic reticulum;                                                                [max. 5] 

 c i glucose concentration may already be high; 
      if person had eaten within 12 hours;
      effect of sudden increase would not be seen/so there was a sudden increase; 
     may already be a high concentration of insulin;                                                        [max. 3] 
ii β cells secrete insulin; concentration of insulin increases over first hour after taking the glucose solution; 
   insulin concentration increases from 60 to 300 pmol dm−3 ; 
   α cells do not secrete glucagon; 
   glucagon concentration, remains constant/decreases;
    from 42 to 36 pmol dm−3 ;                                                                                              [max. 4] 

iii insulin: stimulates, liver/muscle, cells; 
       increase in uptake of glucose from the blood; 
      stimulates enzymes; to increase conversion of glucose to glycogen; 
     brings about a decrease in the blood glucose concentration;                                                    [5] 
  [Total: 21]

9 a gibberellin secreted by embryo; 
    stimulates cells in the aleurone layer; 
    stimulates protein synthesis;
     to make amylase; 
     to break down starch in the endosperm; 
     mobilises glucose; 
    for respiration; 
    to provide energy for germination;                                                                                    [max. 5] 

   b gibberellin increases the mean length of the stem in the short-stemmed varieties of all species; 
     figures for any one species; 
    increase is significant in all but one species;
     gibberellin increases the mean length of the stem in the long-stemmed varieties of four of the species; not tomato; 
   increases are not signifi cant;                                                                                               [max. 5]

c short-stemmed plants do not make (much) gibberellin; 
(because) they do not have the allele for the enzyme that makes gibberellin; 
long-stemmed variety has dominant allele for gibberellin synthesis; 
gibberellin supplied each day promotes growth of stems;                                              [max. 3] 

d less energy used for growth of stem; (therefore) more energy in, peas/beans/seeds; plants do not need (as much) support; less likely to be damaged by wind; less plant material to harvest/less wastage at harvest;                                                                                                                        [max. 3] 

 [Total: 16]

10 a increases;                                                                                                                   [1]
     b i concentration of protons in the stroma decreases (as enter grana); shifts equilibrium to the right;                                                                                                                                 [2]  
        ii increases diff usion into the chloroplast; as concentration of ABA–H decreases; so maintaining a concentration gradient into the chloroplast;                                                                 [max. 2]
      c ABA stimulates closure of stomata; less water vapour is lost;                                 [2] 
                                                                                                                                      [Total: 7]

11 a resting potential; 
      anything in the range −60 to −70 mV; 
       sodium–potassium pump uses ATP to pump Na+ out and K+ in; 
       many anions inside the neurone; resting potential is due to leakage of K+ out; 
       action potential is depolarisation of membrane;
       up to +40 mV; 
      opening of voltage-gated sodium ion channels/ sodium ions flow in; 
     closing of voltage-gated sodium ion channels; 
      voltage-gated potassium ion channels open; 
      K+ flow out; resting potential restored; 
     local circuits depolarise next part of, membrane/ axon; 
     refractory period ensures impulse does not travel backwards; 
     saltatory conduction in myelinated neurones; 
      action potential only at nodes of Ranvier; 
[max. 9]

b action potential arrives at presynaptic membrane; voltage-gated calcium ion channels open; calcium ions enter to stimulate vesicles to move to membrane; 
vesicles fuse with membrane/ exocytosis, to release (named) neurotransmitter; 
 (named) neurotransmitter diff uses across (synaptic) cleft; 
 binds with receptor on postsynaptic membrane; 
stimulates opening of sodium ion channel proteins;
 sodium ions flow in through postsynaptic membrane/depolarisation of postsynaptic membrane;                                                                                                                                         [max. 6] 
 [Total: 15]

12 a reflex arc: to max 5 – these points may be on a diagram

 strong stimulus in receptor/AW; 
 action potential/impulses, along sensory neurone; 
 dorsal root of spinal nerve; into spinal cord; synapse with intermediate neurone;
 (then) motor neurone; 
 action potential/impulses, to eff ector;
 action potential/impulses, to brain; 
 response; e.g. knee jerk; 
 other points which may also be given on a diagram: fast/immediate;
 stops/limits, damage/danger;
 automatic/no conscious thought; 
 innate/stereotyped/instinctive;                                                                                [max. 7] 

b myelin sheath: Schwann cells; 
 wrap around axon; 
 sheath mainly lipid; 
 (sheath) insulates axon (membrane); 
 Na+ /K+ , cannot pass through sheath/can only pass through membrane at nodes;
depolarisation (of axon membrane) cannot occur where there is sheath/only at nodes of Ranvier; 
 local circuits between nodes; 
 action potentials ‘jump’ between nodes; 
saltatory conduction; 
 increases speed/reduces time, of impulse transmission; 
up to 100 m s−1 ; 
 speed in non-myelinated neurones about 0.5 m s−1 ;                                                          [max. 8] 
 [Total: 15]

13 a endocrine system: uses hormones; 
 which are chemical messengers/chemicals that transfer messages; 
 secreted/released, into blood by ductless glands;
 influence target/organs/cells; 
 which have receptors on cell membranes; 
 an example of named hormone and eff ect; 
 e.g. insulin, stimulates decrease in blood glucose concentration nervous system: use impulses/action potentials; 
 not electrical, signals/current along neurones/nerve fi bres; 
 not nerves synapse (at target cell)/neuromuscular junction; 
 named neurone; 
e.g. receptor/sensory/motor/ eff ector/intermediate/relay
differences between the two systems: endocrine has slow effect/nervous is fast;
 endocrine has long-lasting eff ect/nervous has short-term eff ect;
 endocrine has widespread eff ect/nervous has very localised effect; 
 any other detail; 
e.g. extra detail of synapse, such as neurotransmitters                                                   [max. 8] 

b auxins: indole 3-acetic acid (IAA/plant growth regulator); 
synthesised in, growing tips/apical buds/meristems; 
 moves by diff usion; 
 from cell to cell; 
 also, by mass flow/in phloem; 
stimulates cell elongation; 
 not cell enlargement inhibits, side/lateral, buds/ growth accept inhibits branching plant grows, upwards/ taller;
 accept stem elongates interaction between IAA and other plant growth regulators; 
 AVP; e.g. role of abscisic acid (ABA) and lateral bud inhibition 
 AVP; e.g. cytokinins antagonistic to IAA/ gibberellins enhance IAA [max. 7] 
 [Total: 15]

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