topic 3 - exchange & transport systems

What is digestion?

Breaking down large, insoluble molecules into small, soluble molecules which can be absorbed into the bloodstream

Which molecules don't need to go through digestion to be absorbed?

Already small molecules e.g. monosaccharides and minerals

What is the structure of the digestive system?

mouth, salivary glands ,esophagus, stomach, duodenum,jejuneum, illeum, large intestine, rectum, anus, liver, gallbladder

What is the structure of the small intestine?

duodenum - top part where enzymes from the pancreas & bile from the bile duct secrete into
jejunum - middle part where absorption begins of digestive material
ileum - last part where digested matter is absorbed

How is the polymer starch digested?

Amylase : first breaks down starch into maltoses by hydrolysing the alternate glycosidic bonds
Then membrane-bound disaccharidases : they hydrolyse the bonds in maltose to relase alpha glucose
Amylase is found in the salivary glands & pancreas where as the membrane-bound disaccharidases are found in the membrane of the epithelial cells in the illeum

What are the names of the different disacchardiases for the different disaccharides?

Sucrose - Sucrase
Maltose - Maltase
Lactose - Lactase

How are monosaccharides absorbed?

glucose and galactose are absorbed via co-transport with sodium ions; fructose passes via facilitated diffusion - all this happens across the epithelial membranes in the ileum and into the blood

What is the first step of lipid digestion?

The lipids are emulsified by bile salts to increase surface area.

Then lipase enzymes hydrolyse the ester bonds in triglycerides to form a monoglyceride and fatty acids.

This occurs in the small intestine but lipases are made in the pancreas.

What is the second step of lipid digestion?

Once the lipids are broken down into monoglycerides and fatty acids these molecules stick to bile salts.
This forms mielles which are tiny structures that constantly break up and reform releasing the monoglyerides etc.
This means they can be absorbed across the epithelial cell membrane through simple diffusion.

What doesn't happen in lipid digestion?

The mielles aren't absorbed! Only the molecules they release are absorbed.

What is the first step of protein digestion?

The proteins are broken down by 2 different peptidases.
- Endopeptidases hydrolyse the peptide bonds in a protein forming poly-peptide fragments
- Exopeptidases then hydrolyse the peptide bonds at the end of protein molecules removing one amino acid at a time
---> Dipeptidases are a type of exopeptidase that works on dipeptides specifically and breaks them into 2 amino acids

Where can the different types of peptidases be found in the digestive system?

Some are produced/found in the stomach - pepsin
Some are made in the pancreas and then secreted into the small intestine - tryprin & ohymotyprin
Dipeptidases are found on the cell surface of epithelial cells in the small intestine.

What is the second step of protein digestion?

The newly formed amino acids are absorbed in a similar way to glucose & galactose - co-transport with a sodium ion

How do single celled organisms carry out gas exchange?

They’re able to have substances diffuse in and out directly across the cell membrane as that’s the only “barrier”.

• This allows for a fast diffusion rate because of the short diffusion distance

Why can multicellular organisms not use the same method of gas exchange as single celled organisms?

Diffusion across the outer membrane would be too slow:

• Some cells are deep in the body - large distance between them & outside environment

• Low surface area to volume ratio meaning there is too small of an outer membrane for all the cells in the body

This means multicellular organisms have to use specialized exchange organs & an efficient system to transfer substances to all cells.

What are the factors required for a fast rate of diffusion?

• Large surface area - more substances at one time

• Short diffusion pathway

• Steep concentration gradient

What is the structure of the gas exchange system for insects?

Spiracles - small holes on either side of an insects body/exoskeleton through which gases leave or enter the body

Tracheae - large impenetrable system of tubes connected to a spiracle

Tracheoles - very small tubes which grow into & between body cells - where exchange actually happens

How can insects “ventilate”?

Insects can ventilate to a small extent through abdomen compressions which squeeze air out of the tracheoles etc

How do insects maintain a high rate of gas exchange when most active?

The tracheoles have fluid in their ends so when an insect is very active & lactic acid builds up in their cells the water/fluid is drawn into their cells. This is because the lactic acid build up decreases water potential.

This means there’s less water in the tracheoles so more air is in contact in the tracheoles with the tissues so more oxygen is diffused in & more CO₂ can be diffused out.

How do insects reduce water loss?

The spiracles have muscular valves around them which can close to reduce water loss and hairs around the spiracles can trap moist air also reducing water loss.

How does an insects gas exchange system meet the factors for a high rate of diffusion?

• Large surface area - many microscopic tracheoles which go all the way into all the cells

• Short diffusion pathway - short distance from air in tracheoles to cells & from the spiracles to tracheoles - however limits size of insect

• Steep concentration gradient - maintained through cells doing respiration & through ventilation of tracheae by abdominal compressions

What is the gas exchange surface for fish?

The gills which are made up of the gill arch, gill filaments & lamellae.

Why is it not advantageous for fish to have water as their respiratory medium?

Air is roughly 21% oxygen but water is only 1% oxygen so fish have to pass large quantities of water over their gills.

How do fish ventilate?

The fish can open its mouth increasing volume & so decreasing pressure so water moves in.

The fish then closes its mouth and squeezes water over the gills so diffusion occurs & out of the fish.

This movement keeps the concentration gradient high.

How does the structure of the gills make for an efficient gas exchange system?

• Many gill filaments which increases surface area

• Lamellae at right angles to filaments further increases surface area

• Gills have a rich blood supply over the lamellae so look pink

What is the counter current system of the gills?

Blood & water flow over lamella in opposite directions.

The water flows across the lamellae from highly oxygenated blood to less oxygenated blood & them to deoxygenated blood

Why is the counter current system of the gills important?

• Blood high in O₂ meets water - max O₂ conc - so O₂ diffuses into the blood

• Blood with little/no O₂ meets water with most of the water removed O₂ so O₂ still diffuses into the blood

This doesn’t maximise the conc gradient but it does maintain it across the length of the gill plate so almost all of the O₂ from the water diffuses into the blood via the lamellae.

What would happen if there wasn’t a counter current system?

The blood & water would eventually reach equilibrium while flowing across the gill plate etc so only 50% of the O₂ in the water diffuses into the blood.

What is the structure of the human gas exchange system?

What are the lungs?

Pair of lobed structures made up of a series of branched tubules that end in tiny sacs called alveoli.

What are the bronchioles?

A series of subdivisions of the bronchi whoes walls are made with muscle and lined with epithelium cells. The muscle allows contraction and thus air flow into the alveoli.

What are the bronchi?

Bronchi = 2 bronchus where each one goes into each lung with a similar structure to the trachea. They also produce mucus to trap dust & dirt, cilia to move this towards the throat.

What is the trachea?

A flexible airway supported by rings of cartilage that prevent collapsing when breathing in. The walls are made up of muscle and lined with ciliated epithelial calls & goblet cells.

What are the alveoli?

Minute air sacs lined with epithelium with collagen & elastic fibres. These allow them to streach & recoil as ventilation occurs. Their membrane is the gas exchange surface.

How do alveoli achieve a very large surface area?

There are millions of microscopic air sacs - singular is an alveolus.

An alveolus also has a bumpy/ uneven shape which also increases the surface area compared to a perfectly spherical alveolus.

How is a steep concentration gradient maintained in the alveoli?

They have a very rich blood supply from the many capillaries surrounding the alveoli.

This means there is always a flow of blood transporting O₂ away so deoxygenated blood is next to the alveoli.

The constant ventilation of the lungs also maintains a steep concentration gradient.

How do the alveoli ensure a short diffusion pathway?

Alveoli are made out of squamous epithelial cells.

• Squamous epithelial cells are the most flattened/thinnest type of epithelial cell

This means the gases have a shorter distance to travel over so faster rate of diffusion.

What are expiration & inspiration (simple definitions)?

Expiration = breathing out

Inspiration = breathing in

How does ventilation occur in humans?

Inspiration - when atmospheric pressure is greater than the pressure in the lungs.

Expiration - when atmospheric pressure is less than the pressure in the lungs

What happens in inspiration?

• External intercostal muscles contract

• Internal intercostals relax

• The ribs pull up and out

• The diaphragm contracts causing it to flatten

Overall this causes the space in the thorax/chest cavity to increase so a reduction of the pressure in the lungs due to the increased space.

Air is forced into the lungs & avleoli expand and directly after this occurs the alveoli recoil in order to bounce back to their original shape forcing air & CO₂ out.

What happens in expiration?

• External intercostal muscles relax

• The diaphragm relaxes returning to a dome shape

• Unlike in inspiration

When and how is expiration an active process?

When someone is exercising, expiration becomes an active process as the internal intercostal muscles contract forming the ribs down. This reduces the volume of the thorax and increases pulmonary pressure.

This is done to increase breathing rate.

What are forced expiratory volume(FEV) and forced vital capacity?

Forced Expiratory Volume - maximum volume of air that can be breathed out - FEV1 is this but measured in one second

Forced Vital Capcity - maximum volume of air that you can force out of the lungs after a large breath in

What are the effects of tuberculosis?

The damage to the alveoli reduces their surface area and so tidal volume.

This can also cause scarring - fibrosis - which further decreases tidal volume and increases the diffusion distance.Overall the rate of diffusion.

What are the symptoms of tuberculosis?

Someone with tuberculosis would experience a reduction of tidal volume so less air can be