3. Organisms Exchange Substances With Their Environment

Question 1

What’s the environment around the cells of multicellular organisms called

Answer 1

Tissue fluid

Question 2

What 2 things affect the amount of each material that’s exchanged

Answer 2

Size
Metabolic rate

Question 3

3 examples of things needed to be interchanged between an organism and its environment

Answer 3

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

Question 4

What are the 2 ways excretory products, nuterients and respitory products are exchanged

Answer 4

-passively (no metabolic energy required), by diffusion and osmosis
-actively (metabolic energy is required) by active transport

Question 5

What’s required for gas exchange to be affective

Answer 5

The exchange surfaces of an organism must be large compared to its volume

(Large surface area to volume ratio)

Question 6

Is a larger surface area to volume ratio better or worse for gas exchange

Answer 6

Better

Question 7

What features have organisms evolved to enable more efficient diffusion?

Answer 7

• a flattened shape so that no cell is ever far from the surface (eg: a leaf)
  -specialised exchange surfaces with large areas to increase the surface area to volume ratio (eg: lungs in mammals, gills of fish)

Question 8

Surface area of a sphere =

Answer 8

4 π r^2

Question 9

Volume of a sphere

Answer 9

4/3 π r^3

Question 10

How to calculate the surface area to volume ratio

Answer 10

Surface area / volume

Make sure volume is 1
Eg:
SA: = 0.6:1

Question 11

Features of specialised exchange surfaces

Answer 11

-Large surface area to volume ratio: increases the rate of change
-very thin = short diffusion distance pathway = materials cross exchange surface rapidly
-selectively permeable = allows selected materials to cross
-movement of the environmental medium (eg: air) to maintain a concentration gradient
-a transport system ensures movement of the internal medium (eg: blood) to maintain a diffusion gradient

Question 12

Diffusion equation

Answer 12

Diffusion ∝ (surface area x difference in concentration ) / length of diffusion pathway

Diffusion is directly proportional to surface area and concentration difference
Diffusion is inversely proportional to length of diffusion pathway

Question 13

Why are specialised exchange surfaces located on the inside of an organism

Answer 13

They’re thin so are easily damaged and dehydrated

Question 14

Describe gas exchange in a single-celled organism

Answer 14

-single called organisms are small -> large surface area to volume ratio
-oxygen is absorbed by diffusion across body surface covered by a cell surface membrane
-CO2 from aerobic respiration diffuses out across their body surface

Cell walls are no additional barrier to the diffusion of gases

Question 15

What types of organisms are insects

Answer 15

Terrestrial

Question 16

What adaptions have insects evolved for efficient gas exchange

Answer 16

tracheae:internal network of tubes, supported by strengthened rings to prevent them from collapsing
They divide into tracheoles: small dead-end tubes that extend throughout body tissue
Allows o2 to be brought directly to respiring tissues due to small diffusion pathway.

Question 17

What are the 3 ways respitory gases move in and out of the tracheal system

Answer 17

1) along a diffusion gradient
2) mass transport
3) ends of the tracheoles are filled with water

Question 18

How do gases enter and leave the tracheae

Answer 18

Through tiny pores called spiracles
-Found on the body surface which open and close by a valve

When open: water vapour evaporates from insect
When closed: prevents water loss

For most of the time, insects keep their spiralled closed

Question 19

What are the limitations of the tracheal system as a method of gas exchange

Answer 19

-relies mostly on diffusion to exchange gases between cells and environment
- for diffusion to be effective, the diffusion pathway needs to be short so insects are small sized. Length of diffusion pathway limits size of that insects attain.

Question 20

Describe the general structure of a fish

Answer 20

Have a waterproof and gas tight outer covering
Small surface area to volume ratio
Therfore:
Their body surface isn’t adequate to supply and remove respiratory gases so…
they’ve evolved specialised internal gas exchange surfaces

Question 21

Describe the structure of the gills

Answer 21

Located within the body, behind the head
Made up of gill filaments stacked up in a pile
Perpendicular to the filaments are gill lamellae, which increase surface area of gills

Question 22

Describe the ventilation of gills

Answer 22

Water is taken through mouth and forced over gills through an opening on each side of the body
Flow of water over the gill lamellae and flow of blood in opposite directions - counter current flow principle
Ensures maximum gas exchange is achieved

Question 23

Describe what the countercurrent exchange principle in fish means will happen

Answer 23

-oxygenated blood meets water with high oxygen concentration.
-deoxygenated blood meets water with low oxygen concentration.
So Diffusion of oxygen from water to blood takes place DOWN a concentration gradient

Diffusion gradient for oxygen is maintained across entire width of gill lamellae

Question 24

Compare the parallel flow and countercurrent flow in the gills of a fish

Answer 24

Countercurrent:
A diffusion gradient is maintained all the way across the gill lamellae. Almost all the oxygen from the water diffuses into the blood

Parallel:
A diffusion gradient is maintained for only half of the distance across the gill lamellae. Only 50% of oxygen in water diffuses into the blood

Question 25

How do respiratory gases move in & out of the tracheal system along a diffusion gradient ?

Answer 25

When cells respire, o2 is used up and so the conc. towards the ends of the tracheoles falls which creates a diffusion gradient. (Allows o2 to diffuse in)
CO2 is produced by respiring cells which creates a diffusion gradient in the opposite direction (allows co2 to diffuse out)

Question 26

How do respiratory gases move in & out of the tracheal system via mass transport ?

Answer 26

The contraction of muscle s in insects can squeeze the trachea enabling mass movement of air in and out

Question 27

How do respiratory gases move in & out of the tracheal system due to the end of the tracheoles being filled with water ?

Answer 27

During strenuous activity the muscle cells around the tracheoles respire anaerobically, producing lactate which is soluble and lowers the water potential of the muscle cells.
Water can therefore move from the tracheoles and into the cells by osmosis which decreases the volume of water in the tracheoles and allows air to be drawn further in

Question 28

What are the 2 similarities of gas exchange between insects and plant leaf

Answer 28

-no livingcell is far from, the external air, so a source of oxygen and carbon dioxide
-diffusion takes place in air which makes it more rapid than if in water
-short fast diffusion pathway

Question 29

Adaptions of leaf gas exchange for rapid diffusion

Answer 29

• many stomata and so no cell is far from a stoma so short diffusion pathway
• numerous interconnecting air-spaces that occur throughout the mesophyll so gases readily come in contact with mesophyll cells
• large surface area of mesophyll cells for rapid diffusion

Question 30

Structure and function of stomata

Answer 30

-Minute pores mostly on the underside of leaves
-Each stoma is surrounded by a pair of guard cells which open and close the stoma pore to control rate of gas exchange and prevent water loss by evaporation

When guard.cell is turgid (swollen) - stomata is open
When guard cell is flaccid (shrunken) - stomata is closed

Question 31

Do all plant cells photosynthesise

Answer 31

Plant cells respire all the time
But only plant cells with chloroplasts photosynthesise
And when conditions are right

Question 32

How is the diffusion gradient on in and out of leaves maintained

Answer 32

By mitochondria carrying out respiration and chloroplasts carrying out photosynthesis

Question 33

In fish how is a steep diffusion gradient for oxygen maintained

Answer 33

Brining oxygen constantly to the exchange surface by ventilation and carrying it away from the surface by mass transport in the blood

Question 34

What happens in the exchange of oxygen and carbon dioxide in plant cells when photosynthesis is taking place linked to respiration

Answer 34

Some carbon dioxide comes from respiration of cells but most of it.is obtained from external air environment (atmosphere)
Some oxygen from photosynthesis is used in respiration but most of it diffuses out of the plant

Question 35

What happens in the exchange of oxygen and carbon dioxide in plant cells when photosynthesis is NOT taking place, linked to respiration

Answer 35

When it’s dark, oxygen diffuses into the leaf because it’s constantly being used by cells during respiration. In the same way, carbon dioxide produced during respiration diffuses out

Question 36

Compare blood circulation of fish and mammals

Answer 36

Fish:
Single circulation
2 chambers
One artery carrying blood away

Mammal:
Double circulation
4 chambers
Two arteries carrying blood away

Question 37

What features of insects for efficient gas exchange conflicts with conserving water

Answer 37

Thin
Permeable surface
Large area

Question 38

Why is it hard for terrestrial organisms like insects to conserve water

Answer 38

Water easily evaporated from their body surface so they can become dehydrated

Question 39

What are the 3 adaptions insects have evolved to reduce water loss

Answer 39

-small surface area to volume ratio: minimises area over which water is lost
-waterproof covering over body surface: rigid exoskeleton of chitin covered with a waterproof cuticle
-spiracles: openings of tracheae at body surface which close to reduce water loss. But this conflicts with the need for oxygen and so occurs largely when insect is at rest

Question 40

How do terrestrial plants limit water loss

Answer 40

-waterproof covering over parts of the leaves
-ability for guard cells to go flaccid and close stomata
-xerophtypes prevent water loss through transcription

Question 41

What are xerophyte plants

Answer 41

They’re adapted to living in areas where water is in short supply
Evolved adaptions to prevent water loss through transpiration

As regions become drier, the no. of xerophytic plants increase

when explaining adaptions of xerophytic plants to reduce water loss always relate these adaptions to reducing water potential gradient and therefore slower diffusion, less water loss from air spaces and so reduces evaporation of water

Question 42

Leaf adaptions to prevent evaporation to reduce water loss especially when there’s a limited water supply

Answer 42

-thick cuticle
-rolling up of leaves
-Hairy leaves
-stomata in pits or groves
-reduced surface area to volume ratio of leaves

Question 43

Describe how a thick cuticle prevents water loss in plants

Answer 43

Forms a waterproof barrier
The thicker the cuticle, the less water can escape

Question 44

Describe how rolling up of leaves prevents water loss in plants

Answer 44

Protects the lower epidermis that has most of the stomata on it from the outside by trapping air within the rolled leaf
becoming saturated with water vapour so has a high water potential.
There’s no water potential gradient between the inside and outside of the leaf so no water loss.

Question 45

Describe how hairy leaves prevents water loss in plants

Answer 45

thick layer of hair especially on lower epidermis, traps still, moist air next to leaf surface.
Water potential gradient between the inside and the outside of the leaves is reduced so less water is lost by evaporation.

Question 46

Describe how stomata in pits or grooves prevents water loss in plants

Answer 46

Trap still, moist air next to the leaf and reduce the water potential gradient
Eg: pine trees

Question 47

Describe how a reduced surface area to volume ratio of leaves prevents water loss of plants

Answer 47

The smaller the surface area to volume ratio, the slower the rate of diffusion
Leaves that are small and roughly circular in cross-section have a reduced rate of water loss.
Recursion in surface area is balanced against need for sufficient area for photosynthesis

Question 48

How do all aerobic organism release energy

Answer 48

They require a constant supply of oxygen to release energy in form of ATP during respiration

Question 49

Why does the volume of oxygen absorbed and volume of carbon dioxide released have to be large in mammals

Answer 49

-they’re large organisms with a large volume of living cells
-they maintain a high body temp which is related to them having a high metabolic and respiratory rates

So they’ve evolved specialised surfaces called lungs to ensure efficient gas exchange between sir and their blood

Question 50

What’s the site of gas excuse in mammals

Answer 50

Lungs

Question 51

Why are lungs located inside the body

Answer 51

-air isn’t dense enough to support and protect the delicate structures
-the body would lose a lot of water and dry out of not

Question 52

What bony box protects the lungs

Answer 52

Rib cage

Question 53

Describe relationship between rib cage and lungs

Answer 53

Ribs can be moved by the muscles between them
Lungs are ventilated by a tidal stream of air, ensuring the air within them is constantly replenished

Question 54

What are the main parts of the human gas-exchange system

Answer 54

-lungs
-trachea
-bronchi
-bronchioles
-alveoli

Question 55

Describe the structure and function of the lungs in the human gas-exchange system

Answer 55

Pair of lobed structures made of a series of highly branched tubules called bronchioles which end in tiny air sacs called alveoli

Question 56

Describe the structure and function of the trachea in the human gas-exchange system

Answer 56

Flexible airway that’s supported by rings of cartilage preventing trachea collapsing as the air pressure inside falls when breathing in. Walls are made up of muscle, lined with ciliates epithelium and goblet cells

Question 57

Describe the structure and function of the bronchi in the human gas-exchange system

Answer 57

2 divisions of trachea, each leading to one lung.
Like the trachea, they produce mucus to trap dirt particles and have cilia that move dirty mucus towards the throat.
The larger bronchi are supported by cartilage.
As bronchi decreases in size, the amount of cartilage also decreases

Question 58

Describe the structure and function of the bronchioles in the human gas-exchange system

Answer 58

Series of branching subdivisions of the bronchi.
Walls are made of muscle lined with epithelial cells allowing them to constrict so they can control flow of air in and out of alveoli

Question 59

Describe the structure and function of the alveoli in the human gas-exchange system

Answer 59

Minute air-sacs at the end of bronchioles.
Colleges and elastic fibres between them.
Lined with epithelium
Elastic fibres allow alveoli to stretch as they full with air in inspiration
Spring back during expiration to remove carbon dioxide air.
Alveolar membrane is the gas-exchange surface
Each alveoilus has a network of pulmonary capillaries around it, so borrow that red blood cells flatten against the thin capillary walls to squeeze through

Question 60

Define process of ventilation / breathing

Answer 60

Air is constantly moved in and out of the lungs to maintain diffusion of gases across the alveolar epithelium

Question 61

State the 2 processes in ventilation / breathing

Answer 61

inspiration - inhalation / breathing in
expiration - exhalation / breathing out

Question 62

What 3 muscles control pressure changes in the lungs

Answer 62

Diaphragm- a sheet of muscle that separates the thorax from the abdomen

Intercostal muscles - lie between the ribs
- 2 sets:
-internal intercostal muscles: contraction leads to expiration
-external intercostal muscles: contraction leads to inspiration

Question 63

Is inspiration or expiration an active process

Answer 63

Inspiration - requires energy

Question 64

Describe process of inspiration

Answer 64

1) external intercostal muscles contract as internal intercostal muscles relax
2) ribs are pulled upwards and outwards, increasing volume of thorax
3) diaphragm muscles contract, so flattens, increasing volume of thorax
4) results in reduction of pressure in the lungs
5) atmospheric pressure is greater than pulmonary pressure in lungs so air is forced into lungs

Question 65

Describe process of expiration

Answer 65

1) internal intercostal muscles contract as external intercostal muscles relax
2) ribs are pulled downwards and inwards, decreasing volume of thorax
3) diaphragm muscles relax, so is pushed up by contents of abdomen that were compressed during inspiration, decreasing volume of thorax
4) results in increase of pressure in the lungs
5) atmospheric pressure is lower than pulmonary pressure in lungs so air is forced out

Question 66

What’s the main cause of air being forced out during normal quiet breathing

Answer 66

Recoil of the elastic tissue in the lungs
Only under strenuous activity like exercise do muscles play a major part

Question 67

What’s the pulmonary ventilation rate and how is it calculated

Answer 67

Total volume of air moved into the lungs during one minute

Pulmonary ventilation rate (dm^3 min^-1) -
= tidal volume (dm^3) x breathing rate (min^-1)

Tidal: volume of air taken in at each breath when body is at rest. Usually 0.5dm^3
Breathing (ventilation) rate: number of breaths taken in 1 min. Usually 12-20

Question 68

Where’s the site of gas exchange in mammals

Answer 68

Epithelium of the alveoli

Question 69

How is a constant supply of oxygen to the body ensured

Answer 69

A diffusion gradient is maintained at the alveolar surface

Question 70

4 factors that enable efficient transfer of materials over an exchange surface…

Answer 70

Thin
Semi-Permeable
Large surface area

Question 71

What’s the total surface area of the alveoli in the lungs

Answer 71

70 m^2

Question 72

Why is the diffusion of gases between the alveoli and the blood very rapid

Answer 72

• red blood cells slowed as they pass through pulmonary capillaries = more time for diffusion
  •the distance between the alveolar air & red blood cells is reduced as red blood cells are flattened against capillary walls
  • the walls of both alveoli & capillaries are very thin = very short diffusion distance
  • alveoli & pulmonary capillaries have large total surface area
  • breathing movements constantly ventilates lungs, action of the heart constantly circulates blood around the alveoli.
  Together, ensure steep concentration gradient is maintained
  • blood flow through pulmonary capillaries maintains a concentration gradient.

Question 73

Define lung disease

Answer 73

Chronic obstructive pulmonary disease (COPD)

Question 74

What are the risk factors the increase the probability of getting lung disease

Answer 74

1) Smoking
2) air pollution
3) genetic make-up
4) infections
5) occupation eg: working with harmful chemicals, gases and dusts

Question 75

Describe the human digestive system.
What’s it made up of?
How is it an exchange system through which food substances can be absorbed ?

Answer 75

Made up it a long muscular tube and it’s associated glands
These glands produce enzymes that hydrolyse large molecules into small ones ready for absorption

Question 76

Describe the role of the œsophagus

Answer 76

Carries food from the mouth to the stomach

Question 77

Describe the role of the stomach

Answer 77

The stomach is a muscular sac with an inner layer that produces enzymes.
Its role is to store and digest food, especially proteins
It has glands that produce enzymes which digest protein

Question 78

What is the ileum (small intestine) and what’s its role

Answer 78

Long muscular tube.
Food is digested by enzymes that are produced by its walls & by glands that pour their secretions into it
Inner walls are folded into villi = large sa
sa of villi increased further by millions of tiny projections (microvilli) on the epithelial cells of each villus
this adapts the ileum for its purpose of absorbing products of digestion into the bloodstream

Question 79

Describe the role of the large intestine

Answer 79

Absorbs water
Most of which is from the secretions of many digestive glands

Question 80

Describe the role of the rectum

Answer 80

Final section of the intestines
Faeces are stored here before periodically being removed via the anus in a process called egestion

Question 81

Describe the role of the salivary glands

Answer 81

Situated near the mouth
Pass their secretions via a duct into the mouth
Secretions contain amylase which hydrolyses starch into maltose

Question 82

Describe the role of the pancreas

Answer 82

Large gland situated below stomach
Produces a secretion called pancreatic juice
Contains protease, lipase and amylase to hydrolyse proteins, lipids and starch

Question 83

What are the 2 stages of digestion in humans

Answer 83

Physical breakdown
Chemical breakdown

Question 84

Describe and explain process of physical breakdown

Answer 84

If the food is large it is broken down into smaller pieces by the teeth
Makes it possible to ingest & provides a larger sa for chemical digestion
Food is also physically broken down by being charged up by muscles in stomach wall

Question 85

Describe and explain chemical digestion

Answer 85

Hydrolysis large insoluble molecules into smaller soluble ones
Carried out by enzymes which function by hydrolysis
Enzymes are specific so more than one is needed to hydrolyse a large molecule

Question 86

Describe and explain the role of carbohydrases

Answer 86

Hydrolyse carbohydrates, ultimately to monosaccharides

Question 87

Describe the role of lipases

Answer 87

Hydrolyse lipids (fats and oils) into glycerol and fatty acids

Question 88

Describe the role of proteases

Answer 88

Hydrolyse proteins, ultimately to amino acids

Question 89

Give the step by step process of starch digestion in humans

Answer 89

• Salivaenters the mouth from salivary glands & mixes thoroughly with food during chewing
• Amylase hydrolyses the alternate glycosidic bonds of the starch molecule producing maltose
• Food is swallowed, enters the stomach where conditions are acidic- denatures amylase & prevents further hydrolysis of the starch
• After a time, food is passed into small intestine where it mixes with pancreatic juice (containing pancreatic amylase)
• Allows hydrolysis of remaining starch to maltose to continue
• Muscles in the intestine wall push the food along the ileum. Epithelial lining produces disaccharide maltase
• Maltase hydrolyses the maltose into a-glucose

Question 90

How is the pH of salivary amylase maintained at around neutral and why is this important ?

Answer 90

Contains mineral salts - help to maintain neutral pH which is the optimum pH for salivary amylase to work

Question 91

How is the pH of pancreatic amylase maintained at around neutral and why is this important ?

Answer 91

Alkaline salts are produced by both the pancreas and the intestinal wall to maintain the pH at round neutral so the amylase can function

Question 92

Why is maltase referred to as a membrane-bound disaccharide?

Answer 92

Not released into the lumen of the ileum but is part of the cell-surface membranes of the epithelial cells that line the ileum

Question 93

What does the membrane-bound disaccharide sucrase hydrolyse ?

Answer 93

Hydrolyses the single glycosidic bond in the sucrose molecule.
Produces glucose and fructose

Question 94

What does the membrane-bound disaccharide lactase hydrolyse ?

Answer 94

Hydrolyses the single glycosidic bond in the lactose molecule
Produces glucose and galactose

Question 95

Describe the process of lipid digestion

Answer 95

Hydrolysed by lipases - produced in the pancreas (hydrolyse the ester bond in triglycerides to form fatty acids & monoglycerides)
- first split up into tiny droplets called micelles by bile salts (produced by the liver)
- process is called emulsification (increases sa of lipids so that Lipase action is sped up)

Question 96

Describe the process of protein digestion

Answer 96

Proteins are large complex molecules hydrolysed by a group of enzymes called peptidases (proteases)
3 types:
- endopeptidases
- exopeptidases
- dipeptidases

Question 97

What is the role of endopeptidases in protein digestion

Answer 97

Hydrolyses the peptide bonds between amino acids in the central region of a protein forming a series of peptide molecules

Question 98

What is the role of exopeptidases in protein digestion

Answer 98

Hydrolyses the peptide bonds on the terminal amino acids of the peptide molecules formed by endopeptidases
Release dipeptidases and single amino acids

Question 99

What is the role of dipeptidases in protein digestion

Answer 99

Hydrolyses the bond between 2 amino acids of a dipeptidases.
Dipeptidases are membrane-bound, being part of the epithelial cells lining the ileum

Question 100

Which properties of the villi increase the efficiency of absorption?

Answer 100

• increase SA for diffusion
• thin walled, reducing diffusion distance
• contain muscle so are able to move - maintains diffusion gradients
• well supplied with blood vessels, blood can carry away absorbed materials (maintains diffusion gradient)
• villi possess microvilli - further increase SA for absorption

Question 101

Which methods are used to absorb amino acids and monosaccharides?

Answer 101

Diffusion and co-transport

Question 102

How are triglycerides formed (from digestion products)

Answer 102

• Micelles is the name given to monoglycerides and fatty acids when in combination with bile salts
• They (micelles) come into contact with the epithelial cells lining the villi of the ileum and break down, releasing the monoglycerides and fatty acids.
• As these are non-polar they can diffuse across the cell-surface membrane into the epithelial cells
• they are then transported to the Endoplasmic Reticulum where they’re recombined to form triglycerides

Question 103

How are the triglycerides absorbed once formed ? (from products of digestion)

Answer 103

• Starting in the Endoplasmic Reticulum and continuing in the Golgi apparatus the triglycerides associate with cholesterol and lipoproteins to form structures called chylomicrons(specially adapted for transport of lipids)
• Chylomicrons move out of the epithelial cells by exocytosis and enter lymphatic capillaries called lacteals and then into the bloodstream via lymphatic vessels
• The triglycerides in the chylomicrons are hydrolysed by an enzyme in the endothelial cells of blood capillaries where they diffuse into cells

Question 104

Describe the structure of haemoglobin

Answer 104

• primary structure- sequence of amino acids in 4 polypeptide chains
• secondary structure- each of the polypeptide chains are coiled into helix
  -tertiary structure- each polypeptide folded into precise shape - important factor in ability to carry o2
  -quaternary structure- all 4 polypeptides are linked to form globular molecule

Question 105

Describe in detail the quaternary structure of haemoglobin

Answer 105

All 4 polypeptides are linked to form globular molecule
Each polypeptide is associated with harm group containing a ferrous ion (Fe2+)
Each Fe2+ can combine with a single oxygen molecule(O2) making x4 O2 molecules that can be carried by a single haemoglobin molecule in humans

Question 106

What is the name of the process by which haemoglobin binds with oxygen and where does it take place in humans ?

Answer 106

Loading/ associating
Takes place in lungs

Question 107

What is the name of the process by which haemoglobin releases its oxygen and where does it take place in humans ?

Answer 107

Unloading/dissociation
Takes place in the tissues

Question 108

Haemoglobins with a high affinity for oxygen…

Answer 108

take up oxygen more easily but release it less easily

(Opposite for haemoglobins with low affinity for oxygen)

Question 109

To be efficient at transporting oxygen, haemoglobin must:

Answer 109

• readily associate with oxygen at the surface where gas exchange takes place
• readily dissociate form oxygen at those tissues requiring it
  achieved by ability to change affinity for oxygen under different conditions as a result of shape changes

Question 110

At the gas exchange surface, under which conditions is the affinity of oxygen optimal ?

Answer 110

Affinity of haemoglobin for oxygen should be high as oxygen needs to be associated
High O2 concentration
Low CO2 concentration

Question 111

At the respiring tissues, under which conditions is the affinity of oxygen optimal ?

Answer 111

*Affinity of haemoglobin for oxygen** should be low as oxygen needs to be dissociated
Low O2 concentration
High CO2 concentration

Question 112

What is oxygen dissociation curve measuring ?

Answer 112

The relationship between the saturation of haemoglobin with oxygen (%) and the partial pressure of oxygen (KPa)

Question 113

Explain the shape of the oxygen dissociation curve (why is the gradient of the curve initially shallow?)

Answer 113

Shape of the haemoglobin molecule makes it difficult for the 1st oxygen molecule to bind to 1 of the sites on its 4 polypeptide subunits because they are closely united.
Therefore at low concentrations, little oxygen binds to the haemoglobin

Question 114

Explain the shape of the oxygen dissociation curve (why does the gradient of the curve steepen?)

Answer 114

The binding of the 1st oxygen molecule changes the quaternary structure of the haemoglobin molecule, causing it to change shape
This change makes it easier for the other subunits to bind to an oxygen molecule

Question 115

What does the term positive cooperativity refer to ?

Answer 115

There is a smaller increase in the partial pressure of oxygen to bind the second oxygen molecule than it did to bind the first one. (Binding becomes easier)

Question 116

Explain the shape of the oxygen dissociation curve (why does the gradient of the curve reduce and flatten off?)

Answer 116

After the binding of the 3rd oxygen molecule probability makes it harder for 4th to bind.
With majority of binding sites occupied, it is less likely that single oxygen molecule will find an empty site to bind to

Question 117

The further to the right the oxygen dissociation curve is, the…

Answer 117

lower the affinity of haemoglobin for oxygen (loads less readily but unloads readily)

Opposite for a curve further to the left

Question 118

What is the Bohr effect

Answer 118

Haemoglobin has a reduced affinity for oxygen in the presence of co2
The greater the co2 conc the more readily pay the haemoglobin releases its oxygen

Question 119

What is the advantage of having a low co2 concentration at the gas-exchange surface (lungs)

Answer 119

At the lungs co2 diffuses across the exchange surface and is excreted from the organism.
The affinity for oxygen is increased, which, coupled with the high conc of oxygen in the lungs means that oxygen is readily associated with haemoglobin.
Oxygen dissociation curve is shifted to the left

Question 120

What is the advantage of having a high co2 concentration at the respiring tissues (lungs)

Answer 120

The affinity of haemoglobin for oxygen is reduced, which, coupled with the low conc of o2 in muscles, means that oxygen is readily dissociated from the haemoglobin to the muscle cells.
Oxygen dissociation curve is shifted to the right

Question 121

How does the body ensure that there is always sufficient oxygen for respiring cells (step by step process of loading, transport and unloading of o2)

Answer 121

• At the gas-exchange surface co2 us constantly being removed
• The pH is slightly raised due to the low conc of co2
• The higher pH changes the shape of haemoglobin into one that enables o2 to be readily loaded
• Shape increases the affinity of haemoglobin for o2 so it is not released while being transported in blood to tissues
• In the tissues co2 is produced by respiring cells
• CO2 is acidic in solution so the pH of the blood within the tissues is lowered
• Lower pH changes shape of haemoglobin into one with a lower affinity for o2
• Haemoglobin releases its o2 into respiring tissues

Question 122

Why does the tissue being more active have an effect in the volume of o2 unloaded

Answer 122

The higher the rate of respiration -> more co2 produced -> lower the pH -> greater haemoglobin shape change -> more readily o2 is unloaded -> more o2 available for respiration

Question 123

Whether or not there is a specialised transport medium and whether or not it is circulated by a pump depends on which 2 factors?

Answer 123

The surface area to volume ratio
How active the organism is

(The lower the sa:v & the more active the organism is, the greater the need is for a specialised transport system with a pump)

Question 124

What are the common features of transport systems

Answer 124

A suitable medium in which to carry materials eg. Blood
A form of mass transport in which the transport medium is moved around in bulk over large distances
A closed system of tubular vessels that contains the transport medium and forms a branching network for distribution
A mechanism for moving the transport medium within vessels (requires pressure differences)

Question 125

Describe the circulatory system in mammals

Answer 125

Closed, double circulatory system
Blood is confined to vessels and passes twice through the heart for each complete circuit of the body

Question 126

Why does the blood pass twice through the heart for each complete circuit of the body ?

Answer 126

Because when blood is passed through the lungs, it’s pressure is reduced.
If it were to pass immediately to the rest of the body its low blood pressure would make circulation very slow.
Blood is therefore returned to the heart to boost its pressure before being circulated to the rest of the tissues - substances are delivered to the rest of the body quickly, necessary as mammals have high body temp and high rate of metabolism

Question 127

What is the atrium

Answer 127

Thin walled and elastic chamber - stretches as it collects the blood

Question 128

What is the ventricle

Answer 128

Much thicker muscular walled chamber as it has to contract strongly to pump blood some distance, either to lungs or rest of body

Question 129

What do the left and right pumps of the heart individually deal with

Answer 129

Left deals with oxygenated blood from lungs
Right deals with deoxygenated blood from body

Question 130

What are the two valves in the chambers of the heart

Answer 130

The left atrioventricular (bicuspid) valve
The right atrioventricular (tricuspid) valve

Question 131

What is the aorta

Answer 131

Connected to the left ventricle and carries oxygenated blood to all parts of the body except the lungs

Question 132

What is the vena cava

Answer 132

Connected to the right atrium and brings deoxygenated blood back from tissues of the body (except the lungs)

Question 133

What is the pulmonary artery

Answer 133

Connected to the right ventricle and carries deoxygnated blood to the lungs where it’s oxygen is replenished and co2 is removed.

Question 134

What is the pulmonary vein

Answer 134

Connected to the left atrium and brings oxygenated blood back from the lungs

Question 135

What happens if the coronary arteries (which branch off from the aorta) is blocked ?

Answer 135

Leads to myocardial infarction or a heart attack because an area of the heart muscle is deprived from blood and therefore oxygen also.
Muscle cells are therefore unable to respire aerobically and so die

Question 136

Which factors can increase the risk of an individual suffering from cardiovascular disease ?

Answer 136

Smoking
High blood pressure
Blood cholesterol
Diet

Question 137

How does smoking increase likelihood of suffering from heart disease

Answer 137

Carbon monoxide - combines easily & irreversibly with haemoglobin to form carboxyhemoglobin reducing the oxygen carrying capacity of the blood. Heart has to work harder and blood may be insufficient to supply the heart muscle during exercise = raised blood pressure or chest pain (angina) or in severe cases, heart attacks
Nicotine - stimulates production of adrenalin = increases heart rate and blood pressure - greater risk of heart disease/stroke. Also makes platelets ‘sticky’ - higher risk of thrombosis

Question 138

How does high blood pressure increase risk of heart disease ?

Answer 138

As there is already a higher pressure in the arteries, heart has to work harder to pump blood into them- more prone to failure
Also mens more likely to develop an aneurysm and burst causing a haemorrhage.
To resist higher pressure the walls of the arteries tend to become thicker and harden restricting blood flow

Question 139

Describe and explain the relaxation of the heart (diastole)

Answer 139

Blood returns to the atria of the heart through the pulmonary vein and vena cava.
As atria fill pressure inside them rises & when the this pressure exceeds that in the ventricles the atrioventricular valves open allowing the blood to pass through to ventricles.
At this stage the muscular walls of both the atria and ventricle are relaxed which causes them to recoil and reduces pressure within ventricle.
The ventricle pressure is lower and so the semi-lunar valves in aorta and pulmonary artery close producing the ‘dub’ sound of the heart beat.

Question 140

Describe and explain the contraction of the atria (atrial systole)

Answer 140

The contraction of the atrial walls, along with the recoil of the relaxed ventricle walls, forces the remaining blood into the ventricles from the atria.
Throughout the muscles of the ventricle walls remain relaxed

Question 141

Describe and explain the contraction of the ventricles (ventricular systole)

Answer 141

After a short delay to allow the ventricles to fill with blood their walls contract simultaneously.
Increases the blood pressure within them, forcing the atrioventricular valves to shut, preventing the backflow of blood into the atria, producing a ‘lub’ sound.
The closing of the valves increases the pressure in the ventricles, forcing blood into the aorta and pulmonary artery

Question 142

What are valves and what are their roles

Answer 142

Valves in the cardiovascular system are designed so that they open whenever the difference in blood pressure either side of them favours the movement of blood in the required direction.

Question 143

What are atrioventricular valves

Answer 143

Between left atrium and right atrium and ventricle.
These prevent back flow of blood when contraction of ventricles means that ventricular pressure exceeds atrial pressure. Closure of these valves ensures that when the ventricles contract blood within them moves to the aorta and pulmonary artery rather than back to the atria

Question 144

What are semi-lunar valves

Answer 144

Found but the aorta and pulmonary artery.
Prevent backflow of blood into the ventricles when the pressure in these vessels exceeds that of the ventricles. This arises when the elastic walls of the vessels recoil increasing the pressure within them

Question 145

What are pocket valves

Answer 145

In veins.
Ensures that when veins are squeezed blood flows back towards the heart rather than away from it

Question 146

Describe structure of valves

Answer 146

Made up of a number of flaps of tough, but flexible, fibrous tissue which are cusp shaped.

Question 147

Cardiac output (dm3min-1) =

Answer 147

Heart rate x stroke volume
(Stroke volume is the volume of blood pumped out at each beat)

Question 148

What are the 4 different types of blood vessels and what are their functions?

Answer 148

Arteries : carry blood away from heart and into arterioles
Arterioles: smaller arteries that control blood flow frown arteries to capillaries
Capillaries: tiny vessels that link arterioles to veins
Veins : carry blood from capillaries back to heart

Question 149

Describe the basic layered structure of arteries, arterioles and veins from outside in (all have the same)

Answer 149

tough fibrous outer layer - resists pressure changes
muscle layer - can contract and so control the flow of blood
elastic layer - helps to maintain blood pressure by stretching and springing back (recoiling)
thin inner lining (endothelium) - smooth to reduce friction and thin to allow diffusion
lumen- central cavity through which the blood flows

Question 150

Describe the structure of the artery relative to its function

Answer 150

thicker muscle layer than veins - smaller arteries can be constructed and dilated in order to control the vol of blood passing through
thicker elastic layer than veins - important for blood pressure to remain high if it is to reach extremities of the body. stretching and recoiling action helps smooth pressure surges caused by the beating of the heart
overall thickness of wall is great - resists the vessel bursting under pressure
there are no valves - due to high pressure blood tends not to flow backwards

Question 151

Describe the structure of the arteriole related to its function

Answer 151

thicker muscle layer than in arteries - contraction of this layer allows constriction of the lumen which restricts blood flow and so controls movement into capillaries
thinner elastic layer than arteries - blood pressure is lower

Question 152

Describe the structure of the vein relative to its function

Answer 152

thin muscle layer- constriction & dilation can’t control blood flow to tissues
thin elastic layer - blood pressure is too low to create recoil action
overall wall thickness is small - pressure within is too low for risk of bursting. Also can be easily flattened aiding blood flow
valves throughout - ensure no backflow of blood which might otherwise occur due to low pressure

Question 153

Describe the structure of the capillary related to its function

Answer 153

walls consist mostly of lining layer - short diffusion distance as thin = rapid diffusion
numerous and highly branched - large SA for exchange
narrow diameter - permeate tissues which means that no cell is far from a capillary
narrow lumen - red blood cells are squeezed flat, bringing them closer to cells supplying o2, reducing diffusion distance
spaces between lining - allow white blood cells to escape in order to deal with infections within tissues

Question 154

What is tissue fluid

Answer 154

supplies glucose, amino acids, fatty acids, ions in solution and o2 to the tissues.
Refugees co2 and other waste materials
Provides a mostly constant environment for the cells it surrounds

Question 155

In the case of the cytoplasmic route, water movement occurs because:

Answer 155

• Mesophyll 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 their neighbours by osmosis

Question 156

What is ultrafiltration

Answer 156

A type of filtration under pressure that removes tissue fluid from blood plasma
- pressure is only enough to force small molecules out of the capillaries leaving all cells and proteins in the blood because they are too large to cross cell-surface membranes

Question 157

Give the steps for the return of tissue fluid to the circulatory system

Answer 157

• The loss of tissue fluid from capillaries reduces the hydrostatic pressure inside them
• So, by the time the blood has reached the venous end of the capillary network, hydrostatic pressure is usually lower than that of the tissue fluid outside of it
• Therefore tissue fluid is forced back into the capillaries
• Also the plasma has lost water and still contains proteins- has a lower water potential than the tissue fluid
• Water leaves tissue by osmosis down a water potential gradient

Question 158

What happens to the tissue fluid that isn’t returned to the capillaries

Answer 158

Carried back by the lymphatic system (system of vessels that begin in the tissue)
These vessels drain their contents back into the bloodstream via 2 ducts that join veins close to the heart

Question 159

How are the contents of the lymphatic system moved if not by the pumping of the heart?

Answer 159

Hydrostatic pressure of the tissue fluid that has left the capillaries
Contraction of body muscles that squeezes the lymph vessels

Question 160

The movement of water up the stem occurs as follows:

Answer 160

• Water evaporates from mesophyll cells due to heat from sun, leading to transpiration
• Water molecules form hydrogen bonds between one another - cohesion
• Water forms a continuous unbroken column across the mesophyll cells & down the xylem
• As water evaporates from mesophyll cells in the leaf into the air spaces beneath the stomata, more molecules are drawn up behind it
• The column of water is therefore pulled up the xylem as a result of transpiration (transpiration pull)
• This puts the xylem under tension, creating a negative pressure in the xylem (cohesion-tension theory)

Question 161

What evidence is there for the cohesion tension theory

Answer 161

Change in diameter of tree trunks according to the rate of transpiration- during day, when transpiration is at its greatest, there is more tension in xylem- pulls the walls of xylem vessels inwards and causes trunk to shrink in diameter
If xylem vessel is broken and air enters the tree can no longer draw up water as continuous column is broken and so no more cohesion
When a xylem vessel is broken water does not leak out as it would if under pressure. Instead air is drawn in (consistent with it being under tension)

Question 162

Describe the structure of xylem vessels

Answer 162

Hollow, dead, thick walled tubes
No end walls

Question 163

Is transpiration pull passive or active

Answer 163

Passive

Question 164

Explain the steps to find the rate of water loss using a potometer

Answer 164

• A leafy shoot is cut under water. Don’t get water on leaves
• Potometer is filled completely with water, no air bubbles
• Using a rubber tube, the leafy shoot is fitted to the Potometer underwater
• The Potometer is removed form under water and joints are sealed with waterproof jelly
• An air bubble is introduced to capillary tube
• Distance moved by air bubble in given time is measured a number of times & mean is calculated
• Volume of water loss is calculated & plotted against time on graph

Question 165

Describe the structure of the phloem and how is it adapted for mass transport

Answer 165

Made up of sieve tube elements
Walls are perforated to form sieve plates
Companion cells are associated with sieve tube elements (contain many mitochondria for production of ATP)
Few organelles to allow more room for mass flow

Question 166

What are ‘sources’ and ‘sinks’

Answer 166

Source - site of production of sugars during photosynthesis
Sink - places where sugars produced during photosynthesis can be used directly or stored for future use

(Sinks can be found anywhere in plant, sometimes above a source. Therefore translocation in phloem can be in either direction)

Question 167

What does mass-flow theory refer to

Answer 167

The mechanism of translocation

Question 168

Give the steps in the 1st step of translocation: transfer of sucrose into sieve elements from photosynthesising tissue

Answer 168

• Sucrose is manufactured from the products if photosynthesis in cells with chloroplasts
• Sucrose diffuses down a conc gradient by facilitated diffusion from photosynthesising cells into companion cells
• Hydrogen ions are actively transported from companion cells into spaces within cell walls using ATP
• They then diffuse down a conc gradient through carrier proteins into sieve tube elements
• Sucrose molecules are transported along with the hydrogen ion - co-transport. Protein carriers are therefore known as co-transport proteins

Question 169

Give the steps in the 2nd step of translocation: Mass flow of sucrose through sieve tube elements

Answer 169

sucrose has been actively transported into sieve tubes (as described in previous step)…

• This causes sieve tubes to have a lower water potential
• As the xylem has a much higher water potential, water moves from xylem into the sieve tubes by osmosis creating a high hydrostatic pressure within them
• At the sinks, sucrose is being used up for respiration or converted to starch for storage
• Sinks therefore have a low sucrose content and so sucrose is actively transported into them from sieve tubes, lowering water potential
• Due to lowered water potential, water also moves in from sieve tubes by osmosis
• Hydrostatic pressure if sieve tubes in this region is lowered
• Therefore there is a mass flow of sucrose solution down this hydrostatic gradient in sieve tubes

Question 170

Is mass flow passive or active ?

Answer 170

While mass flow is passive it occurs as a result of the active transport of sugars.
Therefore the process as a whole is active which is why it’s affected by temperature and metabolic poisons

Question 171

Give evidence supporting the mass flow theory

Answer 171

• there is pressure within sieve tubes, as shown by sap being released when cut
• conc of sucrose is higher in leaves (source) than in roots (sink)
• downward flow in the phloem occurs in daylight but ceases when leaves are shaded or at night
• increase in sucrose levels in the leaf followed by similar increases in sucrose levels in phloem a little later
• metabolic poisons and/or lack of oxygen inhibit translocation of sucrose in phloem
• companion cells contain many mitochondria and readily produce ATP

Question 172

Give evidence questioning the mass flow theory

Answer 172

• function of sieve plates is unclear, as they would seem to hinder mass flow (suggested structural function)
• not all solutes move at same speed - they should do 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 ones with lowest sucrose conc

Question 173

Give the steps in the 3rd step of translocation: transfer of sucrose from sieve tube elements into storage or other sink cells

Answer 173

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

Question 174

The observations made in ringing experiments suggest that removing the phloem around the stem has led to…

Answer 174

The sugars of the phloem accumulating above the ring, leading to swelling in this region
The interruption of flow of sugars to the region below the ring and the death of tissues in this region

shows us that phloem rather than xylem is responsible for trans locating sugars in plants

Question 175

What is a ringing experiment and how is it used ?

Answer 175

Woody stem cross section : layer of bark > layer of phloem > layer of xylem

Section of outer layers (bark and phloem) are removed around the complete circumference of the stem while it is still attached to rest of the plant.
After a period of time the region above the missing ring of tissue will swell.
Samples of the liquid accumulated in the swollen region are found to be rich in sugars and other dissolved substances.
Below the ring tissue withers and dies

Question 176

What are tracer experiments and how are they used?

Answer 176

Radioactive isotopes are useful for tracing movement of substances in plants
Isotopes can radioactively label co2 for example.
If a plant is then grown in an atmosphere containing that substance the isotope will be incorporated into the sugars during photosynthesis.
These sugars can then be traced as they move within the plant using autoradiography

Question 177

Explain how autoradiography is used in tracer experiments

Answer 177

Involves taking thin cross sections of the plant stem and placing them on a piece of x-ray film. The film becomes black where it has been exposed to the radiation produced by the radioactive isotope in the sugars.
Blackened regions are found to correspond to where the phloem tissue is in the stem.
The non-blackened regions of the film do not carry sugars which shows that the phloem alone is responsible for their translocation

Question 178

State what assumption must be made if a Potometer is used to measure the rate of transportation

Answer 178

That all water taken up is transpired
(Some can be used in respiration)

Question 179

Explain why phloem pressure is reduced in the hottest part of the day (use understanding of transpiration and mass flow)

Answer 179

At the hottest part of the day, the rate of water lost by transpiration is high.
This creates high tension in the xylem.
Means that less water is diffused from xylem by osmosis because of insufficient water potential gradient