[Y1] Organisms Exchange Substances With Their Environment.

Question 1

What is tissue fluid?

Answer 1

The environment around the cell of multicellular organisms.

Question 2

What are examples of things that need to be interchanged between an organism and its environment?

Answer 2

• Respiratory gases (oxygen and carbon dioxide)
• Nutrients (glucose, fatty acids, vitamins, minerals)
• Excretory products (urea, carbon dioxide)
• Heat

Question 3

What are two ways in which physical exchange can take place?

Answer 3

• Passively

- Actively

Question 4

Why does simple diffusion of substances across the outer surface only meet the needs of smaller organisms?

Answer 4

Because as organisms become larger, their volume increases at a faster rate than their surface area.

Question 5

What have organisms evolved to have due to the fact that diffusion alone isn’t suitable?

Answer 5

• A flattened shape so that no cell is ever far from a surface.
• Specialised exchange surfaces with large areas to increase the surface area to volume ratio.

Question 6

How do you calculate the surface area to volume ratio?

Answer 6

Surface area / Volume

Question 7

What are features of specialised exchange surfaces?

Answer 7

• A large SA:V of the organism which increases the rate of exchange.
• Very thin so that diffusion distance is short and therefore material cross the exchange surface rapidly.
• Selectively permeable to allow selected material to cross.
• Movement of the environmental medium (e.g. air to maintain a diffusion gradient)
• A transport system to ensure the movement of the internal medium (e.g. blood in order to maintain a diffusion gradient)

Question 8

Diffusion is directly proportional to…

Answer 8

(Surface area x difference in concentration) / length of diffusion path

Question 9

Describe gas-exchange in single celled organisms.

Answer 9

• They are small and so have a large SA:V.
• Oxygen is absorbed by diffusion across their body surface, which is only covered by a cell-surface membrane.
• In the same way, carbon dioxide from respiration diffuses out.
• The cell wall doesn’t act as an additional barrier to the diffusion of gases.

Question 10

How does respiratory gases move in and out of the tracheal system in insects?

Answer 10

• Diffusion gradient:
  More oxygen in the air than the end of the tracheoles. This means there is an oxygen gradient outside into the tracheoles. The same is with carbon dioxide made in the cells.
• Mass transport:
  The contraction of muscles in insects can squeeze the trachea enabling mass movement of air in and out. This further speeds gas exchange.
• Ends of tracheoles filled with water:
  After the major activity, cells around the tracheoles respired via anaerobic respiration. This produces lactate in the cells. The soluble lactate creates a low water potential in the cells. water moves out of the tracheoles by osmosis. The volume of water in the tracheoles decreases in volume. This draws air further down them. Thus the final diffusion pathway is gas rather than a liquid phase. Thus diffusion is more rapid.

Question 11

List the parts of an insects respiratory system.

Answer 11

• Spiricles
• Tracheae
• Tracheoles
• Air Sacs

Question 12

Why do insect often keep their spiracles closed?

Answer 12

To prevent water loss.

Question 13

What are the limitations of a terrestrial insect’s tracheal system?

Answer 13

• Realise on diffusion to exchange gases between the environment and cells.

This means diffusion pathways must be short (to be effective).

Hence limiting their size.

(Though being small hasn’t hindered them. They are one of the most successful groups of organisms on earth.)

Question 14

Why have fish evolved a specialised internal gas exchange surface and what is it?

Answer 14

• Fish have waterproof, thus gas-tight, outer covering.
• Being large means they have a small SA:V.
• Thus their body is not adequate to supply and remove respiratory gases.
• So they have gills.

Question 15

Describe the structure of the gill.

Answer 15

• The gills are located behind the head of the fish.
• They are made of gill filaments which stack up in a pile (like pages in a book) along a gill bar (like a book spine).
• At right angles to the filaments are gill lamellae which increase the SA of the gills.

Question 16

How do the gills work?

Answer 16

• Water is taken in through the mouth and force over the gills and out through an opening on each side of the body.
• This flows water over the gill lamellae and flows in the opposite direction to the flow of blood in the capillary.
• This is known as a countercurrent flow.

Question 17

Why is countercurrent flow effective?

Answer 17

It maintains a diffusion gradient.

Question 18

How is countercurrent flow effective?

Answer 18

• Water flows in the opposite direction to the flow of blood.
• This means blood that is loaded with oxygen meets water which has its maximum concentration of oxygen.
• This also means blood with little oxygen meets water which has most, but not all, of its oxygen, removed.
• Therefore a diffusion gradient is maintained.

Question 19

How effective is countercurrent flow?

Answer 19

• About 80% of the available oxygen is absorbed.

If oxygen and blood flowed in the same direction, only 50% of available oxygen would be absorbed.

Question 20

What is the main difference between gas exchange in plants compared to animals?

Answer 20

Some plant cells carry out photosynthesis.

Question 21

How is gas exchanged between plant cells and the environment reduced?

Answer 21

At times, the gas produced by the cells in one process (e.g. respiration) is used in the other (e.g. photosynthesis).

Question 22

What effects the volume and types of gases being changed by a plant leaf?

Answer 22

The rate of photosynthesis and the rate of respiration.

Question 23

Where do the gases come from and go when photosynthesis is taking place?

Answer 23

• Most gases are obtained from the external air.
• Some oxygen is used in respiration.
• Most oxygen diffuses out of the plant.

Question 24

Where do the gases come from and go when photosynthesis is not taking place?

When might this happen?

Answer 24

• Oxygen diffuses into the leaf from the external air.
• Carbon dioxide diffuses out of the plant.

This may happen when it is dark, resulting in photosynthesis not occurring.

Question 25

How is gas exchange similar in plants and in terrestrial insects?

Answer 25

• No living cell is far from the external air, and thus a source of oxygen and carbon dioxide.
• Diffusion takes place in the gas phase (air), which makes it more rapid than if it were in the water.

Question 26

List the components in the structure of the leaf. (From top to bottom. Include components multiple times if they occur at different levels)

Answer 26

• Waxy cuticle
• Upper epidermis
• Palisade mesophyll
• Spongy mesophyll (with air gaps)
• Veins (for liquid/solid transport)
• Lower epidermis
• Guard Cells
• Stomata
• Waxy cuticle

Question 27

How are leaves adapted for effective gas exchange?

Answer 27

• Many stomata, so no cell is far from a stoma and thus the diffusion pathway is kept short.
• Numerous interconnecting air-spaces throughout the mesophyll so gases readily come into contact with mesophyll cells.
• Large SA of mesophyll cells for rapid diffusion.

Question 28

Why do leaves have to be adapted for effective gas exchange?

Answer 28

There is no specific transport system for gases, and so must move in and through the plant by diffusion.

Question 29

What are stomata?

Answer 29

Minute pores that occur mainly, but not exclusives, on the leaves, especially the underside.

Question 30

How have plants evolved to balance the conflicting need for gas exchange and control water loss?

Answer 30

By closing stomata at times when water loss would be excessive.

Question 31

Describe the structure of guard cells around a stoma.

Answer 31

• Two guard cells.
• The outer walls of both cells are thin.
• The inner walls of both cells are thick.
• (when the stoma is open, the gap is the stomatal aperture.)

Question 32

Why are gas exchange surfaces more or less 100% saturated with water vapour?

Answer 32

So there is less evaporation of water from the exchange surface as an osmotic gradient cannot be set up.

Question 33

What is a problem for all terrestrial organisms?

Answer 33

Water can easily evaporate from the surface of their bodies and they can become dehydrated.

Question 34

How have terrestrial insects evolved to reduce water loss?

Answer 34

• Small SA:V ratio to minimise the area over which water is lost.
• Waterproof covering forming a rigid outer skeleton (exoskeleton) made od chitin that are covered with a waterproof cuticle.
• Spiricles at the opening to their tracheal system which can be closed to reduce water loss. This often occurs when the insect is at rest as it conflicts with the need for oxygen.

Question 35

Why cant plants have a small SA:V ratio like terrestrial insects?

Answer 35

Photosynthesis requires a large leaf SA:V for the capture of light and exchange of gases.

Question 36

How have terrestrial plants (in general) evolved to reduce water loss?

Answer 36

• Waterproof cuticle over parts of the leaf.

- The ability yo close stomata when necessary.

Question 37

What is a xerophyte?

Answer 37

A plant with a restricted supply of water, that has evolved a range of adaptions to limit water loss through transportation.

Question 38

Waht does desiccated mean?

Answer 38

Having had all moisture removed; dried out.

Question 39

Give 5 examples of xerophytes.

Answer 39

• Cacti
• Confers
• Holly
• Acacia
• Pine

Question 40

How might xerophytes be adapted to reduce water loss?

Answer 40

• Thick cuticle:
  Although waterproof, a normal cuticle can loose up to 10% of water. Being thicker lessens this. (e.g. holly).
• Rolling up of leaves:
  This protects the lower epidermis from the surrounding and creates a region of still air within the leaf. This region becomes saturated and increases the water potential around the leaf. (E.g. marram grass).
• Hairy leaves:
  This traps still, moist air next to the leafs surface (especially on the lower epidermis) thus increasing the water potential outside the leaf. (E.g. one type of heather plant).
• Stomata in pits or grooves:
  This traps still, moist air next to the leaves and reduces the water potential gradient. (E.g. cacti).
• A reduced SA:V of leaves:
  By having smaller and roughly circular leaves (in cross-section) rather than a broad flat leaf, the rate of water loss will reduce considerably. This must be balanced with the need for sufficient area for photosynthesis. (E.g. pine trees).

Question 41

How do the guard cells open and close the stomata?

Answer 41

OPEN:
- Blue stimulates the production of ATP in the guard cell

• This allows for the H+ K+ ion pump to transport H+ ions out and K+ ions in via co-transport.
• Increasing the solute potential inside the guard cell.
• Decreasing the water potential inside the guard cell.
• Causing water to move into the guard cell via osmosis.
• Making the guard cell more turgid, bending the cells away from each other (as the outer membrane is thinner and thus more flexible it pulls the thicker inner membrane).
• Opening the stoma.

CLOSE:
- In the absence of blue light or in conditions of water stress, ABA is produced.

• This allows for K+ ions to exit the guard cell via facilitated diffusion.
• Decreasing the solute potential inside the guard cell.
• Increasing the water potential inside the guard cell.
• Causing water to move out of the guard cell via osmosis.
• Making the guard cell less turgid, relaxing the cells towards each other.
• Closing the stoma.

Question 42

What is required to release energy in the form of ATP during respiration?

Answer 42

• A constant supply of oxygen.

Question 43

What must be removed when the energy in the form of ATP is released during respiration and why?

Answer 43

• Carbon Dioxide must be removed.

- As its build-up could be harmful to the body.

Question 44

Why are the volumes of oxygen absorbed and carbon dioxide removed large in mammals?

Answer 44

• They are relatively large organisms which a large volume of living cells.
• They maintain high body temperature as a result of them having a high metabolic and respiratory rates.

Question 45

List the structures involved in the system that ensure efficient gas exchange in mammals

Answer 45

• Nasal Cavity and mouth.
• Pharynx.
• Larynx.
• Trachea.
• Bronchi.
• Bronchioles.
• Alveoli.
• Lungs.
• Diaphram.
• Ribs.
• Intercostal muscles (internal and external).

Question 46

Why are lungs located inside mammalian bodies?

Answer 46

• Air is not dense enough to support and protect its delicate structures.
• The body as a whole would otherwise lose a great deal of water and dry out.

Question 47

How ar the lungs ventilated?

Answer 47

By a tidal stream of air such that the air within them in constantly replenished.

Question 48

What are lungs?

Answer 48

• A pair of lobed structures.
• With highly branched tubules called bronchioles.
• That end in tiny air sacs called alveoli.

Question 49

What is the trachea?

Answer 49

• A flexible airway.
• Supported by rings of cartilage (that prevent it from collapsing under the air pressure as it falls during inspiration.
• Made of muscle lined with ciliated epithelial cells and goblet cells.

Question 50

What are bronchi?

Answer 50

• Two Divisions of the trachea, leading to each lung.
• Similar to the structure of trachea.
• Cartilage is reduced as the bronchi get smaller.

Question 51

What are bronchioles?

Answer 51

• Branching subdivisions of the bronchi.

- Walls made of muscle line wit epithelial cells, allowing them to constrict to control airflow in and out.

Question 52

What are alveoli?

Answer 52

• Minute air sacs of diameter 100um - 300um.
• Between alveoli are collagen and elastic fibres.
• Are lined with epithelial cells.
• Elastic fibres allow them to stretch as they fill with air on inspiration. They then spring back on expiration.
• The alveolar membrane is the gas-exchange surface.

Question 53

What is inspiration (inhalation)?

Answer 53

• When air is forced into the lungs.

- Due to the air pressure of the atmosphere being greater than the air pressure inside the lungs.

Question 54

What is expiration (exhalation)?

Answer 54

• When air is forced out of the lungs.

- Due to the air pressure in the lungs being greater than the air pressure of the atmosphere.

Question 55

What muscles in the lungs help create pressure change?

Answer 55

• Diaphragm.
• Internal intercostal muscle.
• External intercostal muscle.

Question 56

What happens when the internal intercostal muscles contract?

Answer 56

Expiration.

Question 57

What happens when the external intercostal muscles contract?

Answer 57

Inspiration.

Question 58

What happens when we breathe in?

Answer 58

• The external intercostal muscles contract, while the internal intercostal muscles relax
• The ribs are pulled upwards and outwards, increasing the volume of the thorax.
• The diaphragm muscles contract, causing it to flatten, which increases the volume of the thorax.
• The increased volume of the thorax results in reduction of pressure in the lungs.
• Atmospheric pressure is now great then pulmonary pressure, so air is forced into the lungs.

Question 59

What happens when we breathe out?

Answer 59

• The internal intercostal muscles contract, while the external intercostal muscles relax.
• The ribs move downwards and inwards, decreasing the volume of the thorax.
• The diaphragm muscles relax and so it is pushed up again by the contents of the abdomen that were compressed during inspiration. The volume of the thorax is therefore further decreased.
• The decreased volume of the thorax increases the pressure in the lungs.
• The pulmonary pressure is now greater than that of the atmosphere, and so air is forced out of the lungs.

(Though during normal quiet breathing, the recoil of the elastic tissue in the lungs is the main cause of air being forced out.)
(muscles play a major part in more strenuous conditions like exercise)

Question 60

Why must a diffusion gradient be maintained that eh alveolar surface?

Answer 60

To ensure a constant supply of oxygen.

Question 61

Other than the usual features of an efficient exchange surface, what also must there be?

Answer 61

Movement of the environmental medium (e.g. air).

Movement of the internal medium (e.g. blood).

(This is done via respiration and circulation)
(as diffusion isn’t efficient enough by itself to get air into and out of the alveoli to/form the environment)

Question 62

Why is the diffusion of gases between the alveoli and the blood so rapid?

Answer 62

• Red blood cells are slowed as they pass through pulmonary capillaries, allowing more time for diffusion.
• The distance between the alveolar air and red blood cells is reduced as the red blood cells are flattened against the capillary walls.
• The walls of both alveoli and capillaries are very thin and therefore the distance over which diffusion takes place is very short.
• Alveoli and pulmonary capillaries have a very large total SA.
• Breathing movements constantly ventilate the lungs, and the action of the heart constantly circulates blood around the alveoli. Together, these ensure that a steep concentration gradient of the gases to exchange is maintained.
• Blood flow through the pulmonary capillaries maintains a concentration gradient.

Question 63

What cells line the alveoli and pulmonary capillaries?

Answer 63

Alveoli: Epithelial.

Pulmonary Capillary: Endothelial.

Question 64

What are the risk factors of lung disease (COPD)?

Answer 64

• Smoking.
• Air Pollution.
• Genetic make-up.
• Infections.
• Occupation.

Question 65

What do glands do in the digestive system?

Answer 65

They produce enzymes that hydrolyse large molecules into smaller ones ready for absorption.

Question 66

What are the major parts of the digestive system and their roles?

Answer 66

• Salivary glands:
  Pass their secretion via ducts into the mouth, there contain amylase which hydrolyses starch into maltose.
• Oesophagus:
  Carries food from mouth to stomach.
• Stomach:
  A muscular sac with an inner layer that produces enzymes and secretes them through glands. It stores and digests food (especially proteins (pepsin)).
• Pancreas:
  A large gland below the stomach the produces pancreatic juice. This contains protease that hydrolyses proteins, lipase that hydrolysis lipids, and amylase that hydrolysis starch.
• Ileum:
  A long muscular tube where food is further digested by enzymes produces in its walls and by glands. Its inner walls are folded into villi and have further microvilli on its epithelial cells to increase the SA:V to absorb products into the bloodstream better.

- Large Intestine:
Absorbs water (mostly from the secretion of previous glands).

• Rectum:
  Where faeces is stored before being periodically removed via the anus during egestion.

Question 67

What are the stages of digestion (common in most organisms)?

Answer 67

• Physical breakdown.

- Chemical breakdown.

Question 68

Why is the first stage of digestion important?

Answer 68

Physical Breakdown:
this is important because it makes it possible to ingest the food but also provides a large surface area for chemical digestion.

Question 69

What are examples of the first stage of digestion?

Answer 69

• Chewing/biting with teeth.

- Stomach muscles churning up food.

Question 70

Why is the second stage of digestion important?

Answer 70

Chemical Digestion:

this is important because it hydrolyses large, insoluble molecules into smaller soluble ones.

Question 71

What are examples of the second stage of digestion?

Answer 71

• Digestive enzymes (most importantly: Carbohydrases, Lipases, Proteases)

Question 72

How is do enzymes digest starch?

Answer 72

• Firstly, amylase is produced in the mouth and the pancreas, this hydrolyses the alternate glycosidic bonds of starch molecules, producing the disaccharide maltose.
• Maltose is then hydrolysed into the monosaccharide α-glucose by the disaccharidase called maltase (which is produces by the lining of the ileum).

Question 73

What is the full process that takes place to digest starch in humans?

Answer 73

• Saliva enters the mouth from salivary glands and is thoroughly mixed with food during chewing.
• This saliva contains salivary amylase which starts hydrolysing any starch into maltose. The saliva also contains mineral salts that help maintain the pH at around 7 (optimum for salivary amylase).
• Food is then swallowed and enters the stomach which, with its acid conditions, denature the amylase and prevents further hydrolysis of starch.
• After a time, food is mixed with pancreatic juice (from the pancreas) as it passes into the small intestine.
• This pancreatic juice contains pancreatic amylase, which continues to hydrolise any remaining starch into maltose. Alkaline slats are used (produced by both the pancreas and the internal walls) to maintain the pH at around 7 (so the amylase can function).
• Muscles in the intestine push food along the ileum. And its epithelial lining produce maltase. This maltase is part of the cell-surface membrane of the epithelial cells that line the ileum, therefore the maltase is a membrane-bound disaccharidase. Maltose is hydrolysed into α-glucose.

Question 74

Other than Maltose, what disaccharides are common in the human diet, and how are they hydrolysed?

Answer 74

Sucrose (common in fruit):
- hydrolysed the single glycosidic bond by the membrane-bound disaccharidase, sucrase into (the monosaccharides) glucose and fructose.

Lactose (common in milk products):
- hydrolysed the single glycosidic bond by the membrane-bound disaccharidase, Lactase into (the monosaccharides) glucose and galactose.

Question 75

How are lipids digested?

Answer 75

• First lipids are emulsified into tiny droplets called micelles by bile salts, produced by the liver.
• This increases its SA:V so enzyme action is quicker.
• The ester bonds in the triglycerides are hydrolysed by lipase, produces in the pancreas, into fatty acids and monoglycerides.
• A monoglyceride is a glycerol molecule with a single fatty acid molecule attached to it.

Question 76

How are proteins digested?

Answer 76

Because proteins are large and complex they are hydrolysed by different peptidases:

• Endopeptidases hydrolyse the peptide bonds between amino acids in the central region of the protein. This makes a series of smaller polypeptide molecules.
• Exopeptidases then hydrolyses the peptide bonds on the terminal amino acids of the polypeptide. This progressively releases dipeptides and single amino acids.
• Dipeptidases can then hydrolyse the bonds between the two amino acids (of a dipeptide). The dipeptidase is a membrane-bound peptidase, found on the cell-surface membrane of the epithelial cells lining the ileum.

Question 77

How is the ileum adapted for its function?

Answer 77

• Its walls are folded and have finger-like projections, about 1mm long, called villi.
  (increase SA:V of the ileum and thus the rate of absorption)
• They have thin walls, lined with epithelial cells on the other sides of which a rich network of capillaries. (so molecules can be easily absorbed into the bloodstream)

Question 78

Where are Villi situated in the ileum?

Answer 78

At the interface between the lumen of the intestine and the blood/other tissue inside the body.

Question 79

How are villi (in the ileum) adapted for absorbtion of the products of digestion?

Answer 79

• Increase SA for diffusion.
• Very thin-walled, thus reducing the distance over which diffusion takes place.
• They contain muscle and are able to move (helping to maintain a diffusion gradient as when products of digestion are absorbed new material rich products replace it).
• Well supplied with blood vessels which carry away absorbed molecules (maintaining a diffusion gradient).
• Epithelial cells lining the villi contain microvilli on its cell surface, further increasing the SA for absorption.

Question 80

How are amino acids and monosaccharides absorbed in the ileum?

Answer 80

• Dffusion

- co-transport.

Question 81

How are triglycerides absoebes in the ileum?

Answer 81

• After digestion, micelles break down into monoglycerides and fatty acids as a result of contact with epithelial cells through movement.
• In the epithelial cells, they are transported to the ER which recombine them into triglycerides.
• Starting in the ER then into the Golgi apparatus, triglycerides associate with cholesterol and lipoproteins to form chylomicrons.
• Chylomicrons are adapted for the transport of lipids. They move out of the epithelial cell through exocytosis and enter lymphatic capillaries called lacteals (found at the centre of each villus).
• The lymphatic vessels then take it into the bloodstream. The triglycerides in the chylomicrons are hydrolysed by an enzyme in the endothelial cells of the blood capillaries (where they diffuse into the cells)

Question 82

What are Haemoglobins?

Answer 82

Protein molecules with a quaternary structure that has evolved to make it efficient at loading oxygen under one set of conditions but unloading it under a different set of conditions.

Question 83

Describe the structure of Haemoglobins.

Answer 83

Primary: a sequence of amino acids in four polypeptide chains.

Secondary: Each of these polypeptide chains is coiled into an α-helix.

Tertiary: Each helix folds into a precise shape - important in its ability to carry oxygen.

Quaternary: The four chains are linked to form an almost spherical molecule, each associated with a haem group - containing a ferrous (Fe²⁺) ion.

Question 84

How many oxygens molecules can be carried by a single haemoglobin molecule in humans?

Answer 84

4 - as their are four ferrous ions.

Question 85

What is haemoglobin loading, or associating?

Where does this take place?

Answer 85

The process by which haemoglobin binds with oxygen.

Takes place in lungs.

Question 86

What is haemoglobin unloading, or dissociating?

Where does this take place?

Answer 86

The process by which haemoglobin releases oxygen.

Takes place in the tissues.

Question 87

What does affinity mean?

in the context of haemoglobin

Answer 87

The level at which a molecule is chemically attracted to haemoglobin.

Hight affinity = take up oxygen more easily BUT releases it less easily.

Low affinity = take up oxygen less easily BUT releases it more easily.

Question 88

What must haemoglobin do to be efficient at transporting oxygen?

Answer 88

Haemoglobin must:

• readily associate with oxygen at the surface where gas exchange takes place.
• readily dissociate from oxygen at those tissues requiring it.

Question 89

How does haemoglobin change its affinity for oxygen?

Answer 89

Due to its shape-changing in the presence of certain substances.

• In the presence of high carbon dioxide concentration,
• when oxygen concentration are low,
• the new shape of haemoglobin binds more loosely to oxygen.
• So it’s affinity to oxygen is low
• and oxygen is dissociated.
• In the presence of low carbon dioxide concentration,
• when oxygen concentration are high,
• the new shape of haemoglobin binds more strongly to oxygen.
• So it’s affinity to oxygen is high
• and oxygen is associated.

Question 90

Why do different haemoglobins have different affinities to oxygen?

Answer 90

• Different shape of molecules*
• Different species produce haemoglobin with a slightly different amino acid sequence (primary).
• Therefore different species’s haemoglobin have slightly different tertiary and quaternary structures.
• and so different oxygen binding properties.

Question 91

Explain the oxygen dissociation curve?

Answer 91

S- shaped.

• Due to shape making it difficult for first oxygen to bind (as the four polypeptide subunits are closely united), little oxygen binds to haemoglobin. Shallow gradient.
• The binding of the first oxygen changes the molecules quaternary structure (an thus shape), making it easier for the other subunits to bind to oxygen.
• Thus takes a smaller increase in the partial pressure of oxygen to begin the second oxygen than it did the first one. This is known as positive cooperativity. The gradient steepens.
• After the third molecule has bound, it’s harder to bind the fouth oxygen. This is due to the probability; when more sites are occupied, it’s less likely that a single oxygen molecule will find an empty site to bind to. Gradient reduces and flattens off.

Question 92

What are two interpretations of an oxygen dissociation curve?

Answer 92

• Further left the curve = greater affinity of haemoglobin to oxygen.
  (loads readily, unloads less easily)
• Further right the curve = lower affinity of haemoglobin to oxygen.
  (loads less easily, unloads readily)

Question 93

What is partial pressure?

Answer 93

The amount of a gas that is present in a mixture of gasses that contributes to the total pressure.

written as: p(symbol); e.g pO₂

Unit: kPa

Question 94

What is normal atmospheric pressure?

Answer 94

100kPa
so pO₂ = 21kPa
(in the atmosphere^)

Question 95

What is the Bohr effect?

Answer 95

The greater the concentration of carbon dioxide, the more readily the haemoglobin releases its oxygen.

Question 96

How does the Bohr effect explain why the behaviour of haemoglobin changes at the gas-exchange surface (e.g the lungs)?

What is its overall effect on the oxygen dissociation curve?

Answer 96

• The concentration of carbon dioxide is low,
  (because it diffuses across the exchange surface and is excreted from the organism.)
• The affinity of haemoglobin for oxygen is increased, which coupled with the high concentration of oxygen at the gas-exchange surface,
• oxygen is readily loaded by haemoglobin.

The reduced carbon dioxide concentration has shifted the oxygen dissociation curve to the left.

Question 97

How does the Bohr effect explain why the behaviour of haemoglobin changes at a rapidly respiring tissue (e.g muscles)?

What is its overall effect on the oxygen dissociation curve?

Answer 97

• The concentration of carbon dioxide is high,
  (because it is a product of respiration.)
• The affinity of haemoglobin for oxygen is reduced, which coupled with the low concentration of oxygen at the rapidly respiring tissue,
• oxygen is readily unloaded by haemoglobin into rapidly respiring tissue cells.

The increased carbon dioxide concentration has shifted the oxygen dissociation curve to the right.

Question 98

Why does carbon dioxide change the shape of haemoglobin?

at specific sights in an organism

Answer 98

At gas-exchange surface:

• carbon dioxide is constantly being removed.
• The pH is slightly raised due to low concentrations of carbon dioxide.
• Higher pH changes shape of haemoglobin into one that enables it to load oxygen readily.
• This shape increases the affinity of haemoglobin for oxygen, so is not released as it passes in the blood.

In tissues:

• carbon dioxide is produced by respiring cells.
• Carbon dioxide is acidic in solution, so pH of blood within the tissue is lowered.
• Lower pH changes shape of haemoglobin into one with a lower affinity for oxygen.
• Haemoglobin releases its oxygen into the respiring tissue.

Question 99

Why is it that the more active a tissue, the more oxygen is unloaded from haemoglobin?

Answer 99

• Higher rate of respiration.
• More carbon dioxide produced by tissue.
• Lower pH of the blood.
• Greater haemoglobin shape change.
• More readily oxygen is unloaded (lower affinity of Hame to oxy).
• More oxygen is available for respiration

Question 100

Explain what can be seen from saturation of haemoglobin with oxygen(%) by partial pressure of oxygen (kPa)?

Answer 100

When the partial pressure of oxygen is low, a saturation of haemoglobin with oxygen is low:
Here Haemoglobin molecules in an active tissue unloads 75% of its oxygen.

When the partial pressure of oxygen is high, a saturation of haemoglobin with oxygen is high:
Here Haemoglobin molecules in a resting tissue unloads 25% of its oxygen.

The graph flattens off at 97% as not all haemoglobin molecules are loaded with their maximum four oxygens.

Question 101

Why is a mass transport system needed?

Answer 101

To have an efficient supply on materials over a larger distance.

Question 102

What two factors determin wether or not there is a specialised transport medium?

Answer 102

• The SA:V ratio.

- How active the organism is.

Question 103

What are common fratures of transport systems?

Answer 103

• A suuitbale medium in which to carry materials (e.g blood)
• A form of mass transport where the medium can be moved rapidly in bulk better than diffusion.
• A closed system of tubular vessels that contain the transport medium, that forms a branding out network to distribute to all parts of the organism.
• A mechanisms for moving the transport medium within vessels, requiring a pressure difference between part A and B of the system.
• A mechanism to maintain a direction (e.g valves)
• A means of controlling the flow of the transport medium to suit the changing needs of different parts of an organism.
• A mechanism for the mass flow of water or gasses.

Question 104

Why is blood a good medium in which to carry material?

Answer 104

It is liquid based on water so can readily disolve substances and can be moved around easily.

Question 105

How do animal, in general, achieve the features of a mass transport system?

Answer 105

Using muscular contractions either of the body muscles or of a specialised pumping organ.

(like a heart)

Question 106

How do plants, in general, achive the features of a mass transport system?

Answer 106

Uingin natural, passive processes.

like evaporation of water

Question 107

What type of circulatory system do mamals have?

Answer 107

A closed, double criculatory system.

blood is confined to vessels and passes twice through the heart in for each complete cycle.

Question 108

Why does blood need to pass through the heart twice?

Answer 108

• It needs to go to the lungs at a lower pressure than the rest of the body.
• If it were all at ‘lung’ pressure it would pass too slow around the body (may even pool)
• If it were all at ‘body’ pressure not enough gas exchange will take place or the capilaries will burst.

Question 109

Why do mammals need blood pumped arouynd the body at a high rate?

Answer 109

Necessary for a high body temeprature, and hence a high metabolic rate.

Question 110

Where is the heart located?

Answer 110

In the thoracic cavity behind the sternum.

Question 111

List the parts of the heart in order of the flow of blood, starting with it’s entry after coming back from the rest of the body.

Answer 111

• Superior (anterior) vena cava.
• Inferior (posterior) vena cava.
• Cavity of right atrium.
• Right atrioventricular valve.
• Cavity of right venticle.
  (passed septum)
• Semi-lunar vavles.
• (right and left) Pulmonry artery.
• Pulmonary veins.
• Cavity of left atrium.
• Left atrioventricular valve.
• Cavity of left ventricle.
  (passed sptum (and thick muscular walls)).
• Semi-lunar valves.
• Aorta.

Question 112

Describe the two chambers of the heart.

Answer 112

• Atruim: thin walled and elastic, and stretches as it collects blood
• Ventricle: much thicker muscular wall as it has to contact strongly to pump blood some distances (either to lungs or the rest of the body).

Question 113

Describe the change in pressure as blood passes through the body?

Answer 113

Starting from the aorta it is at its highest.

As it passes through larger arteries and small arteries it changes often but trending to decrease.

As is passes through capilleries, it drastically decreases.

When in small veins, then large veins it is low and decrease at a lessening rate.

At the vena cava it is at its lowest.

Question 114

Why does the left venricule pump have a much thicker wall than the right?

Answer 114

The right only need to pump blood to the lungs.

Thick walls allow the left ventricle to contract more to createenough pressure to pump blood to the rest of the body.

Question 115

What is the differnce between the left and right atrioventricular valves?

Answer 115

Left is bicuspid.

Right is tricuspid.

Question 116

Describe the function of the four vessels connected to the heart.

Answer 116

• Aorta: connected to the left ventricle and carries oxygenated blood to all parts of the body (except lungs).
• Vena cave: connected to the right atrium and brings deoxygenated blood back from the tissue of the body (except the lungs)
• Pulmonary artery: connected to the right ventricle and carries deoxygenated blood to the lungs, where oxygen is replenished and carbon dioxide is removed. (unusual for an artery to carry deoxygenated blood.)
• Pulmonary vein: connected to the left atreum and brings oxygenated blood back from the lungs. (unusual for a vein to carry oxygenated blood.)

Question 117

how does the heart get its blood?

Answer 117

Cornary artery, which branched from the arota.

Question 118

What can happen if the blood vessel that supplies blood to the heart is blocked?

Answer 118

If the cornary artery is blocked (e.g by bloot clotting):

• Can lead to myocardial infarction, it heart attack
• as an area of the heart muscle is deprived of blood
• and thus deprived of oxygen.
• The muscles are unable to respured (aerobivally),
• leading to cell death.

Question 119

What are the two phases of the cardiac cycle?

Answer 119

• Systol (contraction).

- Diastole (relaxation).

Question 120

What happens during diastole?

Answer 120

• Blood returns to the atria through the pulmonnry vien (from lungs) and vena cava (from body).
• The pressure in atria rise is it fills.
• When pressure > ventricle pressure…
• Atrioventricular valve open.
• Blood passes into ventricles. (aided by gravity)
• The muscles of the atria and ventricles are relaxed.
• Ventrical walls recoil, lowering the pressure.
• When pressure < Aorta and pulmonary artery…
• Semi-lunar vlaves close. (“dub” sound)

Question 121

What happens during atrial systole?

Answer 121

• Artial walls contract.
• Ventrical walls remain relax and recoiled (from diatole).
• Forcing remaining blood from atria to ventricals.

Question 122

What happens during ventricular systole?

Answer 122

• Short delay allows ventricals to fill with blood.
• Ventrical walls contract.
• Blood pressure in ventricals increase.
• Forcing shut the atrioventricular valves. (preventing backflow of blood into artia) (“lub” sound)
• Blood pressure rises further (due to valve being shut).
• When pressure > aorta and pulmonary artery…
• Blood is forced from ventricles into the vessels.

Question 123

Explain the function of three types of valves in the cardiac cycle.

Answer 123

• Atrioventricular between artia and ventricals on each side. Prvent backflow when contraction of the ventricales means that the ventricular pressure > atrial so blood flows into (arterial) vessels.
• Semi-lunar are in aorta and pulmonary arterys. Prevent backflow into ventricals when pressure in vessels > ventricles, when elastic walls of the vessels recoil increasing pressure within them and when venticle walls relax reducing reducing the pressure in the ventricles.
• Pocket are in veins that occur throughout the venous system. When veins are squeezed (e.g. when skeletal muscles contract) blood flows to heart.

Question 124

Describe the shape and strucute of the valves in the mammalian circularoy system.

Answer 124

• Made up of a number of flaps of tough, but flexible, fibourous tissue.
• Cusp-shaped, like deep bowls.

Question 125

What is the cardiac output?

Answer 125

The volume of blood pumped by one ventricle of the heart in one minute.

It is usually measured in dm³min⁻¹.

Question 126

What are the factors that effect the cardiac output?

Answer 126

• Heart Rate: rate at which the heart beats

- Stroke Volume: volume of blood pumped out at each beat

Question 127

What is the formula for cardiac output?

Answer 127

Cardiac output = heart rate x stroke volume

Question 128

Desribe the ventricular pressure in the left side of the heart during the cardiac cycle.

Answer 128

• Low at first.
• Gradually increases as the ventricle fills with blood (when the artia contract).

(left atrioventricular vlave closes)

• Pressure rises above that of the aorta.

(blood is fired into aorta past the opened semi lunar valve)

• Pressure falls as the ventricles empty and the walls relax.

Question 129

Desribe the atrial pressure in the left side of the heart during the cardiac cycle.

Answer 129

• Always relativly low (due to thin walls of the atrium not being able to exert much force)
• Highest when they are contracted.
• Drops when the left atrioventricular valve closes and its walls relax.

(Atria then fill with blood).

• Leads to a gradual build-up of pressure.
• Until a slight drop.

(when the left atrioventicular valve opens and some blood moves into the ventricle.)

Question 130

Desribe the arotic pressure in the left side of the heart during the cardiac cycle.

Answer 130

• Rises when ventricle contracts (as blood is forces into aorta)
• It gradually falls ( never below 12kPa due to the elasticity of its walls creates recoil).
• Recoil creates a temporary rise in pressure at the strat of the relaxation phase.

Question 131

Desribe the ventricular volume in the left side of the heart during the cardiac cycle.

Answer 131

• Rises as the atria contract and ventricles fill with blood.
• Drops suddenly as blood is forced out into the aorta (when the semi lunar vlave opens).
• Increases again as the ventricles fill with blood.

Question 132

Describe the function of differnt blood vessels.

Answer 132

• Arteries: carries blood away from the heart and into arterioles.
• Aterioles: are smaller arteries that control blood flow from arteries to capillaries.
• Capillaries: are tiny vessels that link arterioles to veins.
• Veins: carries blood from capillaries back to the heart.

Question 133

Describe the function of the basic layered structures that make up blood vessels.

(Outside inwards)

Answer 133

• Tought fibourous outer layer: resists pressure changes from both within and outside.
• Muscle layer: can contract and so control the flow of blood.
• Elastic layer: helps to maintain blood pressure by strechingand springing back (recoiling).
• Thin endothelium: is smooth to reduce friction and thin the allow diffusion.
• Lumen: (not actually a layer) the central cavity of the blood vessel through which blood flows.

Question 134

How do the arteries’ structure relate to its function?

Answer 134

• Thick muscle layer (compared to veins): means ateries can be constructed and dilated in order to control the volume of blood passing through them.
• Thick elastic layer (compared to veins): means it can stech during systole and relax during diastole. This helps maintain a high pressure and smooth pressure surges created by heart.
• (overall) Thick walls: prevents vessel from bursting under pressure.
• No valves (except from aterties leaving the heart): as blood is already under pressure due to heart pumping there tends to be no back flow.

Question 135

How do the aterioles’ structure relate to its function?

Answer 135

• Thicker muscle layer (than ateries): means the lumen can contract and restict the flow of blood to control its movement into capileries.
• Thinner elastic layer (than arteries): as blood pressure is lower.

Question 136

How do the veins’ structure relate to its function?

Answer 136

• Thinner muscular layer (than arteries): no need to control the flow of blood as it does not flow to the tissue.
• Thinner elastic layer (than arteries): as pressure is too low to cause veins to burst or to cause recoil action.
• (overall) thin walls: no need for thick walls as pressure is too low to cause bursts. Also allows them to be flattened to aid the flow of blood.
• Valves at intervals throughout: prevent backflow as pressure is low

Question 137

How do the capileries’ structure relate to its function?

Answer 137

• Walls mostly lining layer: so they are thin, decreasing the diffusion pathway, allowing for rapid diffusion of materials between the blood and the cell.
• Numerous and highly branched: providing a larger SA for exchange.
• Noarrow diameter: so permeate tissues, so no cells is far from a capillary and there is a short diffusion pathway.
• Narrow lumen: so RB cells are squeezed flat against the side of them, reducing the distances for diffusion.
• Spaces between the endothelia cells: allowing WB cells to escape in order to deal with infections within tissues..

Question 138

What is the function of tissue fluid?

Answer 138

To supply glucose, amino acids, fatty acids , and ions in solution (and oxygen) to the tissues. In exchange for carbon dioxide.

Question 139

How deos tissue fluid maintain a mostly constant environment feo cells it surrounds?

Answer 139

As it is controlled by variou homeostatic systems.

Question 140

What si hydrostatic pressure?

Answer 140

Pressure caused by the pumping of th heart.

Question 141

What opposes the outwards pressure of tissue fluid from blood plasma?

Answer 141

• Hydrostatic pressure of the tissue fluid outside the capileleries, resisting the outward movement of liquid.
• The lower water potential pf the blood, due to the plasma proteins that causes water to move backinto the blood withing the capillaries.

Question 142

What is ultrafiltration (in terms of tissue fluid)?

Answer 142

Filtration caused by the combined forces creating an overall pressure that pushes tissue fluid out of capillaries at the arterial end…

…with the pressure only being big enough to force out small molecules, leaving all the cells and proteins in the blood, as they are too large to cross the membrane.

Question 143

How does tissue fluid return to the circulatory system?

Answer 143

• The loss of tissue fluid from capillaries reduces the hydrostatic pressure inside them.
• By the time blood has reached the venous end of the capillaries its hydrostatic pressure is lower than that of tissue fluid outside it.
• Tissue fluid is forced back into capillaries by the higher outside hydrostatic pressure
• The plasma has also lost water and still contains proteins. It therefore has a lower water potential than the tissue fluid.
• Water leave the tissue by osmosis down a water potential gradient.

Question 144

What happends to tissue fluid that is unable to reenter the circulatory system?

Answer 144

• Carried out by the lymphatic system.

- Drain its contents into the bloodstream via two ducts that rejoin veins close to the heart.

Question 145

Describe the structure of the lymphatic system.

Answer 145

• Starts in the tissues.
• Resembling capilaries, but have dead ends.
• They gradually merge into larger vessels that form a network throughout the body.
• Larger vessels drain their contents into the bloodstream via two ducts that join veins close to the heart.

Question 146

How are the contents of the lymphatic system moved?

Answer 146

• Hydrostatic pressure of the tissue fluid that has left the capillaries.
• Contration of body muscles that squeeze lympth vessels. Valves in the lymph vessels ensure that fluid inside moves away from the tissue n the direction of the heart.

Question 147

What are the overall ways that tissue fluid is moved around the body?

Answer 147

Ulrafiltration: from blood plasma to tissue fluid.

Reabsorbsion: from tissue fluid to blood plasma.

Drainage: from tissue fluid to lymph.

The return of lymph via lymph vessels.

Question 148

What is transpiration?

Answer 148

The process by which water is pulled through xylesm vessels in the stem and is evaporated from leaves.

Question 149

Why is it beneficial for the humidity of the atmosphere to be less than that of the air spaces through the stomata?

Answer 149

• So there is a wster potential gradient from the air spaces through the stomata to the air.
• Therefore when the stomata is open, water vapour molecules can diffuse from the air soaes into the surrounding air.

Question 150

How is the water lost by diffusion from air spaces in leaves replaced?

Answer 150

Replaced by water evaporating from the cell walls of the surrounding mesophyll cells.

Question 151

How can a plant control the rate of transpiration?

Answer 151

By changing the size of the stomatal pores.

Question 152

How ia the water lost to air spaces from the mesophyll cells replaced?

Answer 152

Replaced by water from xylem either via cell walls or cytoplasms.

Question 153

Why does water moving from the xylem into mesophyll cells via the cytoplasmic route occur?

Answer 153

• mesophyl cells lose water to the air spaces by evaporation due to heat supplied by the sun.
• these cells now have a lower water potential and so water enters by osmosis from neighbouring cells.
• the loss of water from these neighbouring cells lowers their water potential.
• they, in turn, take in water from these neighbours by osmosis.

Question 154

What is the main factor that is responsible fro the movement of water up the xylem?

Answer 154

cohesion-tension.

Question 155

How does water moeve up the stem?

Answer 155

• Water evaoprates from mesophyll cells due to heat fro the sun leading to transiration.
• Water molecules from hydrogen bonds between on another and hence stick together (cohesion).
• Water forms a continuous, unbroken column across the mesoplyll cells and down the xylem.
• As water evaoprates from the mesophyll cells in the leaf into the air spaces beneath the stomata, more molecules of water are drawn up behind it as a result of cohesion.
• A column of water is therefore pulled up the xylem as a result of transiration (transpiration pull)
• Transpiration pull puts the xylem unter tension, that is, there is a negative pressure within the xylem, hence the name cohesion tension theory.

Question 156

What are some peices of evidence to support cohesion-tension theory?

Answer 156

Change in the diameter of the tree trunks according to the rate of transpiration:

• During the day, when transpiration is at it’s greatest, there is more tension (negative pressure) in the xylum.
• This pulls the walls of the xylem vessels inwards and causes the trunk to shrink in diameter.
• At night, when transpiration is at its lowest, these is less tension in the xylem and so the diameter of the trunk inreaces.

If the xylem vessel is broken and air enters it, the tree can no longer draw up water.
- This is beacuse the continuous column of water is broken and so the water moilecules can no longer stick together.

When a xylem vessel is broken, water does no leak out, as would be the case if it were under pressure.
- Intead air is drawn in, which is consistent with being under tension.

Question 157

How much energy does transporatonal pull require?

Answer 157

NONE - it’s a passive process.

infact the xylem vessesl are dead cells and cannot actively pull water

Question 158

Ho]w many end does the xylem vessel have? What does this mean for the plant?

Answer 158

The xylem vessel has has no end walls.

This means it forms a series of continuous, unbroken tubes from root to leaves, which is essential to the cohesion-tension theory of water flow, up the stem.

Question 159

Does the movement of water as a whole require energy?

Answer 159

Yes, to dirve the proess of transpiration.

In the form of heat (from the sun) that evaporates water from the leaves.

Question 160

Whats is translocation?

Answer 160

The preocess by which organic molecules and some mineral ions are transported from one part of a plant to another.

Through the phloem in flowering plants

Question 161

Describe the strusture of the phloem.

Answer 161

Made up of sieve tube elements, which are long and thin, and are arranged end to end. The sieve tubes have:

• cellulose walls.
• a central lumen vertically throughout the tube.
• cytoplasm around the lumen.

Their end walls form sieve plates, with pores in them.

Associated with the sieve tubes are companion cells, these have:

• cellulose walls.
• a central nucleus.
• cytoplasm.

Question 162

Whats are the sites of production know as in a plant?

Answer 162

Sources.

Question 163

What are the sites of usage or storage known as in a plant?

Answer 163

Sinks.

Question 164

Why can material in the phloem flow in both directions?

Answer 164

Beacuse plants have sinks above and below sources.

Question 165

List the substances a phloem may carry.

Answer 165

• Sucrose.
• Amino acids
• Potassium ions
• Chlorinde ions
• phosphate ions.
• magnesium ions.

Question 166

Why cant the mechanism the phloem uses to translocate material be diffusion? What is the method?

Answer 166

Too fast.

Mass flow theroy is the current best method.

Question 167

What are the three stages of the mass flow theorem?

Answer 167

1. Transfer of sucrose into sieve elements from photosynthesising tissue.
2. Mass flow of sucrose through sieve tube elements.
3. Transfer of sucrose from the sieve tube elements into storage or other sink cells.

Question 168

Describe what happens in the first stage of the mass flow theorem?

Answer 168

• Surose is made from the products of photosynthesis in cells with chloroplasts.
• Sucrose diffuses down a concentration gradient by facilitated diffusion from the photosynthesising cells into companion cells.
• Hydrogen ions then diffuse down a concentration gradient through carrier proteins into the sive tube elements.
• Sucrose molecules are transported along with the hydrogen ions via co-transport. The protein carriers are therefore also known as co-transport proteins.

Question 169

Define mass flow.

Answer 169

The bulk movement of a substance through a given channel or area in a specified time

Question 170

Describe what happens in the second stage of the mass flow theorem?

Answer 170

• (because of 1st stage) The sieve tubes have a lower (more negative) water potential.
• Water moves from the xylem into the sieve tubes by osmosis (as it has a much higher (less negative) water potential).This creates hydrostatic pressure within the sieve tubes.
• At the respiring cells, sucrose is being used up or converted into starch (for stogage).
• Once they have a lower sucrose concentration, sucrose is actively transported into them from the sieve tubes, lowering their water potential.
• Because of this water moves into the repairing cells , from the sieve tubes, by osmosis.
• This lowers the hydrostatic pressure in that region of the sieve tube.
• As a result of water enitering the sieve tube elements at the source and leaving at the sink, there is a high hydrstaic pressure at the source and a low one at the sink.
• Therefore the mass flow of sucrose solution is down the hydrostatic gradient in sieve tubes.

Question 171

Why is the mass flow of sugars (in the phloem) as a whole affected by temperature and metabolic posions?

Answer 171

Although the mass flow process is pasive, it occurs as a result of the active transport of sugars.

Therfore the processes active and so is affected by temperature and metabolic poisions.

Question 172

Describe what happens in the third stage of the mass flow theorem?

Answer 172

Sucrose is actively transported by companion cells out of the sieve tubesa and into the sink cells.

Question 173

What are some peices of evidence supporting the mass flow hypothesis?

Answer 173

• When cut sap is released, therefore there must be pressure within sieve tubes.
• The concentration of sucrose is higher in leaves( source) than in roots (sink).
• Downwards flow in the phloem occurs in daylight, but stops when leaves are shaded, or at night.
• Increses in sucrose levels in the leaf are followed by increases in sucrose levels in the phloem a little later.
• Metabolic poisons and/or lack of oxygen inhibit translocation of sucrose in the phloem.
• Companion cells posses many mitochondria and readily procude atm.

Question 174

What are some peices of evidence questioning the mass flow hypothesis?

Answer 174

• Sieve tube function is unclear as it seems to hinder mass flow.
  (suggested it may help tube from bursting though)
• Not all solutes move at the same speed, which they should if movement is by mass flow.
• Sucrose is delivered at more or less the same rate to all regions, rather than going more quickly to the ones with lowest sucrose concentration (which is wat mass flow would suggest)

Question 175

Describe the ringing experiment.

Answer 175

• A section of the outer layers (protective layers and phloem) are removed around the circumference of a woody stem (but still attached to other parts).
• After a period of time the region immediately above the missing ring of tissue has swollen.
• Samples of the accumulated liquid are taken and are found to be rich in sugars and other dissolved organic substances.
• Some of the non-photosytehic regions below the ring (near the roots)are found to wither and die.
• The above continue to grow.

Question 176

What do observarvations from the ringing experiment suggest?

Answer 176

• The sugars of the phloem accumulate above the ring, leading to swelling in this region
• The inturupteion of flow of sugars to the region below the ring leads to the death of tissue in this region.

Question 177

What conclusion can be drawn from the ringing experiment?

Answer 177

The phloem, rather than the xylem is responsible for translocating sugars in plants.

If this was not the case, we wouldn’t see any swelling, and tissue below would not die.

Question 178

Describe the tracer experiment.

Answer 178

• Isotope ¹⁴C can be used to make radiocavly labelled ¹⁴CO₂.
• A plant can then be grown in an atmosphere containing ¹⁴CO₂.
• The ¹⁴C will then be incorated into sugars produced by photosythesis.
• These radioactive sugars can then be traced as they move within the plant using autoradiography.
• This involves taking thin cross-sections of the plant stem and placing them on a piece of x-ray film.
• The film becomes blackend where it has been exposed to the radiation produced by ¹⁴C in the sugars.
• The backend regions are found to correspond to where phloem tissue is in the stem.

Question 179

What conclusion can be drawn from the tracer experiment?

Answer 179

As other tissue (fromt he phloem) do not blackend the film, they do not carry sugars and the phloem alone is responsible for their translocation.

Question 180

What are some peices of evidence that translocation of organic molecules occurs in the phloem?

Answer 180

• When a phloem is cut, a solution of organic molecules flows out.
• Plants provided with radioactive CO₂ can be show to have radioactivly labelled carbon in phloem after a short time.
• Aphids feed on sucrose using a needle-like mouthpart to pentrate the phloem. They can be used to extract the contents of the sieve tubes. The extracted substance show daily variation in the sucrose contents of the leaves that are mirroed (a bit) by an identical change in the sucrose content of the phloem.
• The removal of a ring of phloem around a whole circumference leads to the accumulation of sugars above the ring and their disappearance from below it.