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Thursday, 3 January 2019

Gaseous Exchange, Digestion and Absorption

Size and Surface Area
Cells need to take in oxygen and nutrients, as well as excrete waste products such
as carbon dioxide and urea
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Single-celled organisms have a large surface area : volume ratio
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Smaller animals have higher surface area : volume ratios, therefore they lose a large
amount of heat relative to their mass
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Larger animals have lower surface area : volume ratios
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Gaseous Exchange In Humans
As you breathe in, air enters the trachea, which is supported by incomplete rings of
cartilage that prevent it collapsing during the pressure changes which occur in
ventilation
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Each bronchus then divides into smaller tubes called bronchioles, which
end in air sacs called alveoli
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The alveoli are adapted as a gaseous exchange surface as:

• their shape and number provide a large surface area

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Thursday, 3 January 2019
• they have a short diffusion pathway by being only two cell layers


- a single layer of flattened epithelial cells (alveolar wall)

- a single layer of endothelial cells (capillary wall)

An extensive network of blood capillaries surrounds each alveolus, providing a large
surface area for absorbing oxygen and releasing carbon dioxide
...
It then combines with haemoglobin in the red
blood cells to form oxyhemoglobin
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Blood flow ensures deoxygenated blood continually reaches the alveoli so that
oxygenated blood is removed, maintaining a high diffusion gradient for gaseous
exchange
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Ventilation Mechanism
A ventilation mechanism maintains a high diffusion gradient by continually
providing air with a high oxygen concentration and removing air with a high carbon
dioxide concentration
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This cause the rib cage to move upwards and outwards, and the diaphragm to
flatten, which increases the volume of the thoracic cavity

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Thursday, 3 January 2019
1
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This cause the rib cage to move downwards and inwards, and the diaphragm to
move up towards its dome shape, which decreases the volume of the thoracic
cavity

3
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Elastic recoil of the lung tissue helps to force air out of the lungs during
expiration


Pulmonary Ventilation
Pulmonary ventilation is the total volume of air that is normally moved into the lungs
in one minute
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Air
moves into the tracheae through pores on the surface called spiracles
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Oxygen diffuses from the air, through the spiracles along the trachea and the
tracheoles to the cells
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Carbon dioxide leaves the cells down its own concentration gradient towards the
spiracles to be released into the atmosphere by the reverse pathway
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Carbon dioxide levels rise due to respiration, causing the spiracles to open
wider, increasing the rate of diffusion of gases in the tracheal system
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Thursday, 3 January 2019

In large or very active insects, body muscles may contact to compress the trachea
forcing air out of them
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During increased activity, anaerobic respiration often occurs, producing lactic acid
in cells, lowering the water potential of the cells and so some of the water in the
ends of the tracheoles moves into the cells by osmosis
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Gaseous Exchange In Fish
In bony fish gas exchange occurs over the surface of the gills which consists of four
pairs of gill arches, consisting of many gill filaments, protected by the operculum
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The thin barrier consists of two cell layers:


- the epithelial layer of the gill lamellae

- the endothelial layer of the blood capillaries

• a circulatory system which ensures a continual flow of blood through the
respiratory surface to absorb oxygen and remove carbon dioxide and so
maintaining a high diffusion gradient

• a ventilation mechanism which provides a continual flow of water over the gills
bringing more oxygen and removing carbon dioxide and so maintaining a Hugh
diffusion gradient

• a countercurrent system which ensures that blood continually meets water with a
higher oxygen concentration so a high diffusion gradient is maintained along the
whole length of the lamellae


Gaseous Exchange In Plants
Plants need carbon dioxide for photosynthesis, which produces oxygen as a waste
gas
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Photosynthesis and respiration maintain diffusion gradient by using and
producing oxygen and carbon dioxide
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Gases diffuse down through the stomata in the
epidermis of the leaves between the guard cells, down a concentration gradient and
rapidly through the intercellular air spaces
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Stomata can open to allow the exchange of gases and can close if the plant is
losing too much water
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Digestion of Carbohydrates In Humans
Digestion is the process in which large molecules are hydrolysed by enzymes to
produce smaller molecules that can be absorbed and assimilated
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Saliva contains
salivary amylase, which hydrolyses starch into maltose
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Chewing breaks the food into smaller pieces increasing the surface area for more
rapid hydrolysis
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The complete hydrolysis of starch occurs in the ileum by maltase
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The cell-surface
membrane is folded to form microvilli that increase the surface area of the ileum for
absorption
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Digestion of Lipids In Humans
Lipids are hydrolysed by lipase into glycerol, fatty acids and monoglycerides
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Thursday, 3 January 2019
Bike salts are produced by the liver and emulsify lipids causing them to form small
droplets, which increases the surface area of the lipids and speeds up the
hydrolysis by lipase
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Once the lipid has been hydrolysed monoglycerides and fatty
acids stick with the bile salts to form micelles
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Endopeptidases in the stomach hydrolyse internal peptide bonds between the
amino acids of proteins to form small polypeptides and peptides
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The pancreas excretes exopeptidases, which hydrolyse the terminal peptide bonds
at either end of a polypeptide, which produces a dipeptide
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They ac to
separate the two amino acids by hydrolysing the peptide bond between them
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Absorption In the Ileum
The ileum is adapted for absorption of digested products by:

• its long length and the presence of villi and microvilli provide a large surface area

• maintaining a high diffusion gradient as the villi contain blood capillaries that
absorb monosaccharides and amino acids

• maintaining a high diffusion gradient as the lacteals in the villi absorb digest lipids

• having a short diffusion pathway for absorption as the wall of each villus villi
consists of a single layer of epithelial cells

• having a large number of mitochondria present to supply ATP for active transport

• having carrier and channel proteins in the cell-surface membrane for absorption of
specific molecules by active transport and facilitated diffusion

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Absorption Of Digested Lipids
Micelles are necessary as they transport the poorly soluble monoglycerides and
fatty acids to the surface of the epithelial cell so they can be absorbed
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Short chained fatty acids diffuse from the epithelial cells directly into the blood,
whereas longer-chained fatty acids, monoglycerides and glycerol are recombined in
the epithelial cell in the smooth endoplasmic reticulum to form triglycerides
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Absorption Of Monosaccharides And Amino
Acids
Initially monosaccharides and amino acids are at a higher concentration in the
lumen of the ileum than in the epithelial cells, and so they are absorbed down a
concentration gradient by facilitated diffusion
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Glucose is absorbed via co-transport
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The sodium ions can then diffuse from the
lumen of the ileum into the epithelial cells through sodium-dependent transporter
proteins known as symports, carrying glucose with them
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