Section 3 - Exchange Flashcards
1
Q
How do cells survive?
A
The external environment is different from the internal environment found within an organism and within its cells. Organisms transfer materials between the two environments. This takes place at exchange surfaces.
2
Q
Where does cellular exchanges take place?
A
Takes place at exchange surfaces Always involves crossing cell plasma membranes.
3
Q
What is always involved in cellular exchange?
A
Crossing cell plasma membranes.
4
Q
Why is it essential to have a large surface area to volume ratio?
A
Speeds up the rate of diffusion
5
Q
Why is it essential to have a thin exchange surface?
A
It keeps the diffusion pathway short
6
Q
Why are partially permeable membranes good in exchange surfaces?
A
To allow selected materials to diffuse
7
Q
List the parts of the gas exchange system of terrestrial insects in the order that oxygen passes from outside the body to the respiratory tissues.
A
- Spiricals - Trache - Tracheoles - Air sacs
8
Q
State two ways in which insects ventilate their exchange systems
A
- Abdominal movement - Muscle contracting - Spircle opening and closing
9
Q
Explain the mechanism by which insect respiratory systems respond to oxygen debt during high intensity exercise
A
The tracheole ends are filled with water. Lactate builds up around the muscles. Lactic acid is soluble. Increasing water potential. This means water is drawn into the cell. This gives more space for more oxygen to be available. This extends the reach of air into the tissues as there is less water in the tracheole ends.
10
Q
Explain why there is a conflict in terrestrial insects between gas exchange and water conservation
A
If you want to do gas exchange you need to open the spiricles however this increases the amount of mostiture lost.
11
Q
What is a hypotonic solution?
A
Any solution that has a lower osmotic pressure than another solution.
12
Q
What is a hypertonic solution?
A
Greater concentration of solutes on the outside of a cell when compared with the inside of a cell
13
Q
What is water potential?
A
Water potential is the ‘measure of the ability of water molecules to move freely in solution’. All this means is that is a solution of pure water where there is no solute, all of the water molecules are free to move, so the water potential is high
14
Q
Explain how the tracheal system limits the size of insects
A
if insects were bigger than the diffusion pathway should be longer which would make it inefficient for diffusing oxygen. They would also then lose a lot more moisture.
15
Q
Explain the difference between parallel and counter current
A
In countercurrent all of the oxygen is absorbed where as in parallel only 50% of the oxygen from the water diffuses into the blood.
16
Q
What is always involved in the transfer of materials between the external and internal environment?
A
A cells plasma membrane is always crossed
17
Q
What is the name given for the environment around the cells of multicellular organisms?
A
Tissue fluid
18
Q
What is the mass transport system involved in? What is the role in this process?
A
The movement of tissue fluid to keep its composition constant. It distributes to the tissue fluid and waste products returned to exchange surface for removal. Maintaining the diffusion gradients that bring materials to and from the cell-surface membranes.
19
Q
What are 4 things that need to be interchanged between an organism and its environment?
A
Respiratory gases
Nutrients
Excretory products
Heat
20
Q
What are the respiratory gases?
A
Oxygen Carbon dioxide
21
Q
What must the organism need in order to make the exchange effective?
A
The exchange surface of the organism must be large compared to its volume.
22
Q
Equation - Surface area of cube
A
area of one side x 6
23
Q
Equation - volume
A
length x height x width
24
Q
What adaptions have made larger organisms more efficient?
A
A flattened shape so that no cell is ever far from the surface A specialised exchange surface with large areas to increase the surface area to volume ratio.
25
Equation - Volume of a sphere
4/3 pi r^3
26
What characterises do exchange surfaces show to make them efficient ?
Large surface area relative to the volume of the organism which increases the rate of exchange Very thin so that the diffusion distance is short and therefore materials cross the exchange surface rapidly. Selectively permeable to allow selected materials to cross. Movement of environmental medium eg. air to maintain diffusion gradient Transport system to ensure the movement of the internal medium eg. blood in order to maintain a diffusion gradient.
27
Why does this adaptation make an exchange surface more efficient? Large surface area relative to the volume of the organism
Increases rate of exchange
28
Why does this adaptation make an exchange surface more efficient? Very thin
diffusion distance is short and therefore materials cross the exchange surface rapidly
29
Why does this adaptation make an exchange surface more efficient? Selectively permeable
allow selected materials to cross.
30
Why does this adaptation make an exchange surface more efficient? Movement of environmental medium
maintain diffusion gradient
31
Why does this adaptation make an exchange surface more efficient? Transport system to ensure the movement of the internal medium
maintain diffusion gradient
32
How can you express the relationship between Surface area Length of the diffusion path Diffusion Difference in concentration
Diffusion = SA x Difference in conc. / length of diffusion path
33
How does an increased surface area in terrestrial insects effect water conservation?
Conflicts with it as water will evaporate more from a large SA
34
what is the tracheae in terrestrial insects?
Internal network of tubes Supported by strengthened rings to prevent them from collapsing.
35
What are the tracheoles in terrestrial insects?
Divisions of tracheae They extend throughout all the body tissues of the insect. Oxygen brought directly to respiring tissues as there is a short diffusion pathway from tracheole to any body cell.
36
State the three ways gases move in and out of the tracheal system.
Along a diffusion gradient Mass transport The ends of the tracheoles are filled with water
37
Why does gases move along a diffusion gradient in and out of a tracheal system?
When respiring, oxygen is used up so concentration falls. This creates a diffusion gradient causing gaseous oxygen to diffuse
38
Explain the movement of oxygen through a tracheal system
A diffusion gradient caused gaseous oxygen to diffuse from the atmosphere along the tracheae and tracheoles to the cells. Carbon dioxide is produced during respiration. This creates diffusion gradient in the opposite direction. This causes gaseous carbon dioxide to diffuse along the tracheoles and tracheae from the cells to the atmosphere.
39
Why are respiratory gases exchanged quickly by the process of diffusion?
As diffusion in the air is more rapid than diffusion in water.
40
Explain how respiratory gases move in and out of the tracheal system by mass transport
The concentration of muscles in insects can squeeze the trachea enabling mass movements of air in and out. This further speeds up the exchange of respiratory gases.
41
What happens to the tracheoles during periods of major activity in terrestrial animals?
During periods of major activity, the muscle cells around the tracheoles respire carry out out come anaerobic respiration. This produces lactate
42
What is lactate?
Soluble and lowers the water potential of muscle cells around the tracheoles.
43
What process means that water can move into muscle cells around the tracheoles?
Osmosis
44
What happens when water goes through osmosis and draws water into muscle cells around the tracheoles?
Water in the ends of the tracheoles decrease in volume and in doing so they draw air further into them. This means the the final diffusion pathway is in a gas rather than a liquid phase, and therefore diffusion is more rapid. This increases the rate at which air is moved in the tracheoles but leads to greater water evaporation.
45
What are spiracles?
Tiny pores that mean gases can enter and leave tracheae.
46
How do spiracles open and close?
A valve
47
What happens when spiracles are open?
Water vapour can evaporate from the insect.
48
Why do insects normally keep their spiracles closed?
Prevent water loss
49
Why do spiracles open?
To allow gas exchange.
50
What are the limitations of the tracheal system?
- Relies mostly on diffusion to exchange gases between the environment and the cells. - Diffusion pathway limits the size that insects can attain.
51
Why are insects such a small size?
To be effective, the diffusion pathway needs to be short so they need to be small.
52
Explain the structure of a fishes outer covering
Waterproof and therefore gas-tight.
53
As fish are relatively large what does this mean for their SA:V?
Small surface area to volume ratio. So body surface not adequate to supply and remove their respiratory gases and so they have evolved a specialised internal gas exchange.
54
What specialised internal gas exchange system do fish use?
The gills
55
Where are the gills located?
Within the body of the fish behind the head.
56
What are the gills made up of?
Gill filaments
57
What is the structure of gill filaments?
Gill filaments are stacked up in a pile like pages of a book. At right angle to the filaments are fill lamellae.
58
What is the role of gill lamellae?
Increase the surface area of the gills
59
Where is the gill lamellae found?
At right angle to the gill filament.
60
Explain the water movement in fish?
Water is taken in through the mouth and forced over the gills and out through an opening on each side of the body.
61
What is countercurrent flow?
The flow of water over the gill lamellae and flow of blood within them are in opposite directions.
62
Why is the countercurrent flow in fish gills important?
To ensure the maximum possible gas exchange is achieved within the fishes gills
63
What is the essential feature of the countercurrent exchange system?
The blood and the water that flow over the gill lamellae do so in opposite directions.
64
What does the arrangement of the countercurrent flow mean?
- Blood that is already well loaded with oxygen meets water which has its maximum concentration of oxygen. Therefore diffusion of oxygen from the water to the blood takes place. - Blood with little oxygen in it meets water which has had most, but not all, of its oxygen removed. Again, diffusion of oxygen from the water to blood takes place.
65
What proportion of all the available oxygen in the water is absorbed by countercurrent flow in fish gills?
80%
66
Define - parallel flow
The flow of water and blood had been in the same direction.
67
In parallel flow what proportion of available oxygen would be absorbed by the blood?
50%
68
Why is the oxygen absorption in parallel flow so low?
The diffusion gradient would only be maintained across part of the length of the gill lamellae. Only 50%
69
Explain the diffusion of oxygen due to the countercurrent flow
A diffusion gradient is maintains all the way across the gill lamellae. Almost all the oxygen from the water diffuses into the blood
70
What is the diffusion of oxygen like in a parallel flow system?
A diffusion gradient is maintained for only half of the distance across the gill lamellae. Only 50% of the oxygen from the water diffuses into the blood.
71
What processes do plants use in gaseous exchange?
Respiration and photosynthesis
72
What can reduce gas exchange with the external air in leaves?
Gases produced in either respiration or photosynthesis can be used for the other.
73
Where is carbon dioxide found for photosynthesis to occur in plants?
Some from respiration off cells but most from the external air.
74
What happens when plants need energy but it is night time or dark?
Photosynthesis can not occur so oxygen diffused into the leaf so it can be constantly used by the cells during respiration. The carbon dioxide produced during respiration diffuses out.
75
How is gas exchange in plants similar to that in insects?
- No living cell is far from the external air and so 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 water. Overall, there's a short diffusion pathway.
76
What adaptations have leaves made to make diffusion for rapid?
Small pores (stomata) so no cell is far from a stoma and therefore the diffusion pathway is short. Numerous interconnecting air-spaces that occur throughout the mesophyll so that gases can readily come in contact with mesophyll cells. Large SA of mesophyll for rapid diffusion.
77
How do mesophyll help diffusion in plants?
Large surface area for rapid diffusion
78
What are stomata?
Minute pores that occur mainly on the leaves, especially the underside.
79
What surrounds each stomata?
Pair of special cells called guard cells.
80
What do guard cells do?
Can open and close the stomatal pore.
81
What does terrestrial mean?
Live on land. Most insects are terrestrial.
82
What requirements are needed for efficient gas exchange but conflicts with the need to limit water loss?
A thin, permeable surface with a large area.
83
What three evolutions have insects made to reduce water loss?
Small surface area to volume ratio Waterproof covering Spiracles
84
How does a small surface area to volume ratio reduce water loss in insects?
Minimises the area over which water is lost.
85
How does a waterproof covering reduce water loss in insects?
In the case of insects this covering is a rigid outer skeleton of chitin that is covered with a waterproof cuticle.
86
How does spiracles reduce water loss in insects?
Can open and close. This conflicts with the need for oxygen and so occurs largely when the insect is at rest.
87
What does tracheae do in insects?
Carry air containing oxygen directly to the tissues
88
Why cant plants have a small SA:V ratio?
They photosynthesis which requires a large leaf surface area for the capture of light and for the exchange of gases.
89
How do terrestrial plants reduce water loss?
Have a waterproof covering over parts of the leaves and have the ability to close stomata when necessary.
90
What adaptations have xerophytes got that normal plants don't?
Plants that have evolved a range of adaptations to limit water loss through transpiration.
91
What are xerophytes?
Plants that are adapted to living in areas where water is in short supply.
92
Give examples of how xerophytes have adapted to reduce the rate of water being lost through evaporation?
Thick cuticle Rolling up of leaves Hairy leaves stomata in pits or grooves Reduced surface area to volume ratio of the leaves
93
Explain how a thick cuticle helps prevent water loss in xerophytes
Although the waxy cuticle on leaves forms a waterproof barrier up to 10% of water loss can still occur by this route. The thicker the cuticle, the less water can escape by this mean.
94
Explain how rolling up leaves helps prevent water loss in xerophytes
Most leaves have their stomata largely confined to the lower epidermis. Rolling leaves protects the lower epidermis from outside helps to trap a region of still air within the rolled leaf. This region becomes very saturated with water vapour and has a high water potential.
95
Give an example of a plant that rolls its leaves
Marram grass
96
Why does hairy leaves prevent water loss in xerophytes?
Traps still, moist air next to the leaf surface. The water potential gradient between the inside and outside of the leaves is reduced and therefore less water is lost by evaporation.
97
Give an example of a plant with hairy leaves
One type of heather plant
98
Why does stomata in pits of grooves help prevent water loss in xerophytes?
This traps still, moist air next to the leaf and reduces the water potential gradient.
99
Give an example of a plant which has stomata in pits or grooves
Pine trees
100
How does a reduced surface area to volume ration of the leaves help xerophytes retain their water?
Leaves that are small and roughly circular in cross-section rather than broad and flat has a reduced rate of water loss.
101
How can you maintain diffusion of gases across the alveolar epithelium?
Air is constantly moved in and out of the lungs. This is called breathing
102
What is another word for breathing?
Ventilation
103
What is inspiration?
Inhalation - When the air pressure of the atmosphere is greater than the air pressure inside the lungs, air is forced into the lungs.
104
What is expiration?
Exhalation - When the air pressure in the lungs is greater than that of the atmosphere, air is forced out of the lungs.
105
What three sets of muscles that cause the pressure to change within the lungs?
Diaphragm, internal and external intercostal muscles
106
What are the two sets of intercostal muscles?
Internal and external
107
What are internal intercostal muscles?
Contraction leads to expiration
108
What are external intercostal muscles?
Whose contraction leads to inspiration
109
Explain the process of inspiration
External intercostal muscles contract - internal intercostal muscles relax Ribs pull upwards and outwards, increasing the volume of the thorax. Diaphragm muscles contract, flattening and increasing the volume of the thorax. Increased volume of the thorax results in reduction of pressure in the lungs Atmospheric pressure is now greater than pulmonary pressure and air is forced into the lungs
110
What do the intercostal muscles do in inspiration?
External contract and internal relax
111
What do the ribs do during inspiration?
Pulled upwards and outwards increasing the volume of the thorax
112
What does the diaphragm do during inspiration?
Muscles contract causing them to flatten and increasing the volume of the thorax.
113
Fully explain the process of expiration
Internal intercostal muscles contract and external relax Ribs move downwards and inwards decreasing volume of thorax Diaphragm muscles relax and pushed up again by the contents of the abdomen that were compressed during inspiration. Volumes of thorax is further decreased. Decreased volume of thorax increases pressure in the lungs Pulmonary pressure is now greater than that of the atmosphere so the air is forced out the lungs.
114
What type of process is inspiration?
Mainly active
115
What type of process is expiration?
Largely passive
116
What does the intercostal muscles do during expiration?
Internal contract and external relax
117
What do the ribs do during expiration?
Move downwards and inwards decreasing the volume of the thorax.
118
What does the diaphragm do during expiration?
Diaphragm muscles relax and pushed up again by the contents of the abdomen that were compressed during inspiration. Volumes of thorax is further decreased.
119
During normal breathing what is the main cause for air being forced out?
Recoil of elastic tissue in the lungs.
120
What is the pulmonary ventilation rate?
The total volume of air that is moved into the lungs during 1 minute.
121
How do you calculate the pulmonary ventilation rate?
multiply tidal volume and breathing rate
122
What is the normal tidal volume?
0.5 dm^3
123
What is the normal breathing rate?
12-20 breaths in a healthy adult
124
What is the equation for pulmonary ventilation rate?
pulmonary ventilation rate = tidal volume x breathing rate
125
Where is the site of gas exchange in mammals?
The **epithelium** in the alveoli
126
What is an **alveoli** and what is its approximate diameter?
Minute air sacs situated in the lungs.
Some 100-300 um
127
How can a constant supply of oxygen to the body be ensured?
A diffusion gradient must be maintained at the alveolar surface.
128
What three things must there be to enable the efficient transfer of materials across an alveolar surface?
1. exchange surfaces are thin
2. partially permeable
3. large surface area
129
How can you maintain a diffusion gradient between alveolar surfaces?
Must be movement of both the environmental medium (eg. air) and the internal medium (eg. blood)
130
What is the downside of specialised exchange surfaces?
How can this be prevented?
They are thin where means they are easily damaged - therefore are often located inside an organism for protection.
131
Why must lungs in a mammal need ventilating?
Diffusion alone is not fast enough to maintain adequate transfer of oxygen and carbon dioxide along the trachea, bronchi and bronchioles.
Breathing is basically a form of mass transport
132
How many alveoli are in each human lung?
300 million.
133
What is the surface area of the total alveoli's in the lungs?
70m^2 - half the area of a tennis court.
134
What are each alveolus lined with?
Epithelial cells only 0.05 um to 0.3 um.
135
What are around each alveolus?
A network of pulmonary capillaries so narrow (7-10pm) that red blood cells are flattened against the thin capillary walls in order to squeeze through.
136
Give at least 3 reasons why diffusion of gases between the alveoli and the blood is rapid
* red blood cells are **slowed** as they pass through pulmonary capillaries, allowing **more time for diffusion**
* **Distance** between the alveolar air and 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 surface area**
* breathing movements constantly **ventilate** the lungs and the action of the heart **constantly circulated** **blood** around the alveoli. Together these ensure that a **steep concentration gradient** of the gases to be exchanged is maintained.
* Blood flow through the pulmonary capillaries maintains a concentration gradient.
137
What does an enzyme do for digestion?
Hydrolyses large molecules into small ones ready for absorption.
138
Give, in order, the organs of the human digestive system
* tougue
* salivary glands
* oesophagus
* lobe of liver
* stomach
* pancreas
* Transverse limb of the large intestine
* descending limb of the large intestine (colon)
* ascending limb of the large intestine (colon)
* small intestine (ileum)
* rectum
* anus
139
What does the oesophagus do?
Carries food from the mouth to the stomach
140
What does the stomach do?
It 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 proteins.
141
What is the ileum and what does it do?
A long muscular tube.
Food is further digested in the ileum by enzymes that are produced by its walls and by glands that pour their secretions into it.
The inner walls of the ileum are folded into villi, which gives them a large surface area.
The surface area of these villi is further increased by millions of tiny projections called microvilli on the epithelial cells of each villus.
This adapts the ileum for its purpose of absorbing the products of digestion into the bloodstream.
142
What does the large intestine do?
Absorbs water.
Most of the water that is absorbed is water from the secretion of the many digestive glands.
143
What does the rectum do?
This is the final section of the intestines.
the faeces are stored here before periodically being removed via the anus in a process called egestion.
144
What does the salivary glands do?
Situated near the mouth.
They pass their secretions via a duct into the mouth.
These secretions contain the enzyme amylase which hydrolyses starch into maltose.
145
What does the amylase do?
Hydrolysed starch into maltose.
146
What does the pancreas do?
A large gland situated below the stomach.
It produces a secretion called pancreatic juice.
This secretion contains protease to hydrolyse proteins, lipase to hydrolyse lipids and amylase to hydrolyse starch.
147
what does protease hydrolyse?
proteins
148
What does lipase hydrolyse?
lipids
149
What are the two stages of digestion?
1. Physical breakdown
2. Chemical digestion
150
Explain the physical breakdown stage of digestion
If food is large, it is broken down into smaller pieces by means of structures such as the teeth.
This not only makes it possible to ingest the food but also provides a large surface area for chemical digestion
Food is churned by the muscles in the stomach wall and this also physically breaks it up.
151
Explain the chemical digestion stage of digestion
Hydrolysed large, insoluble molecules into smaller soluble ones.
This is carried out by enzymes which are produces by hydrolysis.
Eg. Carbohydrases, Lipases and proteases
152
What do carbohydrates break down into?
Monosaccharides
153
What do lipids break down into?
glycerol and fatty acids
154
What do proteins break down into?
amino acids
155
What is the order of enzymes used in carbohydrate digestion?
* **amylase** is produced in the mouth and the pancreas - hydrolyses the alternate glycosidic bonds of the starch molecule to produce disaccharide **maltose**.
* **Maltose** - to monosaccharide a-glucose by a second enzyme, a disaccharidase called **maltase**. Produced by the lining of ileum.
156
Explain the hydrolysis of the enzyme amylase
hydrolyses the alternate glycosidic bonds of the starch molecule to produce disaccharide **maltose**.
157
Explain the process of carbohydrate digestion
* Saliva enters the mouth from the salivary glands and it thoroughly mixed with the food during chewing
* Saliva contains **salivary amylase**. This starts hydrolysing any starch in the food to **maltose**. It also contains mineral salts that help to maintain the **pH** at around neutral. This is the optimum pH for salivary amylase to work.
* The food is swallowed and enters the stomach, where the conditions are **acidic**. This acid denatures the amylase and prevents further hydrolysis of the starch.
* After a time the food is passed into the small intestine, where it mixes with the secretion from the pancreas called **pancreatic juice**
* This contains **pancreatic amylase**. This continues the hydrolysis of any remaining starch to maltose. **Alkaline salts** are produced by both the pancreas and the intestinal wall to maintain the pH at around neutral so that the amylase can function.
* **Muscules in the intestine wall** push the food along the ileum. Its epithelial lining produces the **disaccharidase maltase.** This is not released into the lumen of the ileum but is part pf to the cell-surface membranes of the epithelial cells that line the ileum. It is therefore referred to as a **membrane-bound disaccharidase.** The maltase hydrolyses the maltose from starch breakdown into a-glucose.
158
In addition to maltose, what are two common disaccharides in the diet that are hydrolysed?
* **Sucrose** - Found in many natural foods like fruits.
* **lactose** - in milk and hence in milk products such as yoghurt and cheese.
159
What does sucrase do?
Hydrolyses the **single glycosidic bond** in the **sucrose** molecule.
This hydrolysis produces the two monosaccharides glucose and fructose.
160
What does lactase do?
Hydrolyses the **single glycosidic bond** in the **lactose** molecule.
This hydrolysis produces the two monosaccharides glucose and galactose.
161
What are lipases?
**Lipids hydrolysed by enzymes.**
They are enzymes produced in the pancreas that hydrolyse the ester bond found in triglycerides to form fatty acids and monoglycerides.
162
What is a monoglyceride?
A glycerol molecule with a single fatty acid molecule attached.
163
Explain the process of lipid digestion
* Lipids are firstly split up into tiny droplets called micelles by bile salts these are produced by the liver
This process is called emulsification and increases the surface area of the lipids so that the action of lipases is speeded up.
164
What enzymes are used to hydrolyse proteins?
peptidases (proteases)
165
What are the three types of peptidases?
1. Endopeptidases
2. Exopeptidases
3. Dipeptidases
166
What does endopeptidases do?
hydrolyses the peptide bonds between amino acids in the central region of a protein molecule forming a series of peptide moleulces.
167
What do exopeptidases do?
hydrolyse the peptide bonds on the terminal amino acids of the peptide molecules formed by endopeptidases.
In this way, they progressively release dipeptides and single amino acids.
168
What do dipeptidases do?
Hydrolyse the bond between the two amino acids of a dipeptide.
Dipeptidases are membrane-bound, being part of the cell-surface membrane of the epithelial cells lining the ileum.
169
What does the ileum do?
It is adapted to the function of absorbing the products of digestion.
170
How is the ileum adapted to its function?
The wall of the ileum is folded and possessed find=ger-like projections about 1mm long called villi.
Thin walls, lined with epithelial cells on the other side of which is a rich network of blood capillaries.
The villi considerably increase the surface area of the ileum and therefore accelerate the rate of absorption.
171
Where is the villi situated?
At the interface between the lumen (cavity) of the intestines (in effect outside the body) and the blood and other tissues inside the body.
Part of a specialised exchange surface adapted for the absorption of the products of digestion.
172
In what way did the properties of the lumen increase the efficiency of absorption?
* **Increase the surface area** for diffusion
* They are very **thin-walled,** thus reducing the distance over which diffusion takes place.
* They **contain muscle** and so are able to move. This helps to **maintain diffusion gradients** because their movement mixes the contents of the ileum. This ensures that, as the products of digestion are absorbed from the food adjacent to the villi, new material rich in the products of digestion replaces it.
* They are **well supplied with blood vessels** so that blood can carry away absorbed molecules and hence maintain a diffusion gradient.
* The **epithelial cells lining the villi possess microvilli.** These are finger-like projections of the cell-surface membrane that further increase the surface area for absorption.
173
What happens once triglycerides is formed during digestion?
Monoglycerides and fatty acids remain in association with the bile salts that initially emulsified the lipid droplets.
These structures formed are called micelles.
174
What is the size of the micelles ?
Very tiny around 4-7 um in diameter.
175
What do the micelles come in contact with through the movement of material within the lumen of the ileum?
The epithelial cells lining the villi of the ileum.
176
Where do micelles break down?
When they come into contact with the epithelial cells lining the villi of the ileum.
177
When broken down what do micelles produce?
Release **monoglycerides** and **fatty acids.**
As these are non-polar molecules they easily diffuse across the cell-surface membrane into the epithelial cells.
178
Once inside epithelial cells what do monoglycerides and fatty acids do?
They are transported to the endoplasmic reticulum where they are recombined to form **triglycerides**.
Start in ER and onto Golgi apparatus.
179
When the triglycerides reach the Golgi apparatus what they do?
Associate with cholesterol and lipoproteins to form structures called **chylomicrons**.
180
List the stages of triglyceride absorption
1. Monoglycerides and fatty acids associate with bile salts.
2. Emulsified lipid droplets form micelles
3. Micelles contact epithelial cells and broken down
4. monoglycerides and fatty acids released.
5. diffuse into epithelial cells
6. go to ER then Golgi Apparatus
7. Associate with cholesterol and lipoproteins form chylomicrons
8. move out of epithelial cells by exocytosis.
9. enter lymphatic capillaries called lacteals found in the centre of each villus.
10. chylomicrons pass via lymphatic vessels to the blood system.
11. triglyceride in chylomicrons hydrolysed by enzymes in endothelial cells of blood capillaries
12. Diffuse into cells.
181
What is chylomicron?
They are special particles adapted for the transport of lipids.
Move out of epithelial cells by exocytosis.
182
What process means the chylomicrons can move out of the epithelial cells?
exocytosis.
183
What is the name of the lymphatic capillaries that are found at the centre of each villus?
lacteals
184
Where is the enzyme found that helps hydrolysed chylomicrons?
In the Endothelial cells of blood capillaries.
