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When a cell decreases in size does the diffusion pathway get longer or shorter?



If the diffusion pathway of a cell is longer will substances that diffuse across the cell membrane diffuse quickly or slowly?

Slowly, as it has further to travel.


What substances are transported through diffusion?

Nutrients such as oxygen and glucose, waste products such as carbon dioxide and urea (from liver cells) are transported out of cells by diffusion.


Gas exchange

The process by which oxygen reaches cells and removing carbon dioxide from them.



The process of bringing air or another medium to and from an exchange surface.



A series of chemical reactions i.e. hydrolysis of glucose/ the formation of ATP/ phosphorylation (adding a phosphate) to ADP.


Which organism (multicellular or unicellular) has the largest surface area to volume ratio?



What is the surface area to volume ratio of a cube with:
Width: 1cm
Height: 1cm
Length: 1cm

surface area: 1x1x1=1
Volume: 1x1x1=1


As size increases does the surface area to volume ratio:
A) Increase
B) Decrease

B) Decrease


Unicellular organisms e.g. Amoeba

Extremely large SA:V ratio.
Gas exchange happens across whole surface.
Permeable membrane allows diffusion of gases.
Specialised gas exchange organs are not needed.


Simple multicellular organisms e.g. Flatworm -aquatic.

These organisms have evolved to have a flattened shape to over come the problem of size increase.
This increases their SA:V therefore no cell in the body isn't far from the surface (short diffusion pathway)- so there is no need for specialised gas exchange organs.
They exchange gases directly with the environment via diffusion, permeable membrane allows diffusion of gases.


Simple multicellular organisms e.g. Earthworm

Developed a tubular shape and is restricted to damp environments.
Worms secrete mucus to keep cells of the body surface moist (allows gases to dissolve and diffuse).
Elongated shape provides a large SA:V compared with a compact organism (e.g.spherical shape) of similar volume.
They exchange gases directly with the environment by diffusion across the moist surface- blood vessels are close to the body surface so gases can diffuse in/out of the blood and then across he cells covering the body surface.
Blood circulates in the vessels, this maintains a concentration gradient for diffusion of oxygen into the cells and carbon dioxide out.
Blood contains the respiratory pigment haemoglobin to carry oxygen to the body cells.


Requirements for all respiratory surfaces.

Thin (short diffusion pathway).
Concentration gradient is maintained.
Large SA:V


Why do insects need the trachaeal system?

Due to the fact insects fly they require lots of energy and for that they need a good supply of oxygen. So it developed the tracheal system.


What is the trachaeal system?

The tracheal system is a transport system that insects use to transport oxygen to respiring tissues in their body to keep up with the high oxygen demand whilst flying.


What is the trachaeal system made up of?

Spiracles- which are paired holes running along either side of the insects body.
These then lead into chitin lined tubes called tracheae, that then branch out into tracheoles that feed directly into the muscles.


Why do spiracles open and close?

To allow for the exchange of gases
To reduce water loss


At rest do insects need to use the trachaeal system at rest?

No, they don't at rest they can rely on simple diffusion.


Why is there fluid at the end of the tracheoles and how does it help?

Fluid helps with the exchange of gases as it allows them to dissolve and diffuse.


What are the advantages of the trachaeal system?

Oxygen is supplied directly to the tissues.
No respiratory pigment is needed.
Oxygen moves faster in air (gas) than it does blood (liquid).
Spiracles close to reduce water loss.



The structural feature keeping the tracheae open.


In all land living (terrestrial) organisms their gas exchange organs are found inside their body, why?

It reduces water and heat loss, and are well protected by the ribs or exoskeleton in insects.


What problems is there with living in water?

It has less oxygen than air.
It's denser.
The rate of diffusion is slower.


What are the row different types of fish?

Cartilaginous fish and bony fish


Cartilaginous fish e.g. Shark

Skeletons are fully made of cartilage.
Nearly all live in sea water
They have five gill clefts which open at gill slits just behind their head.
They take water into their mouth and force it through the gill slits by raising the floor of its mouth.
for gas exchange they use parallel flow (blood in the gill capillaries flows in the same direction as the water that is forced over the gills.


Bony fish e.g. Herring

Skeleton made of bone.
Gills are cover with a flap called the operculum.
Counter current flow is how gas exchange is done, this is when the flow of blood and the flow of water go in opposite directions.


How many gills are on either side of a bony fishes head?



Where is the buccal cavity?

The buccal cavity is the space just behind its lips/mouth.


How does water flow through the gills?

it flows in through the buccal cavity and passes over the gill arches and out through the operculum. Buccal cavity floor and operculum allow for one way movement.


What is the ventilation mechanism?

Water flows in:
Mouth: open
Operculum: closed
Floor of buccal cavity: low
Volume: Increases
Pressure: Decreases

Water flows out:
Mouth: closed
Operculum: open
Floor of the buccal cavitiy: rises
Volume: decreases
Pressure: increases


What are the thin microscopic filaments found along each gill arch?

Lamellae also known as gill plates.


How do gill lamellae help with gas exchange?

As there are many gill lamellae it increases the surface area of the gill arch allowing maximum gas exchange to occur.
Lamellae also have a good blood supply as they each contain an extensive network of capillaries within them.
Blood circulates through the gill plates creating a concentration gradient causing oxygen to diffuse into the blood and carbon dioxide to diffuse out.


How is counter current flow achieved?

Counter current flow is when the blood circulates in the opposite direction to the water flowing along the gill lamellae.


Which gas exchange method is more efficient (counter current or parallel flow) and why?

Counter current flow because the blood always meets water with a higher oxygen concentration. This means the diffusion gradient of oxygen is maintained across the whole length of the gill lamellae. Causing it to be more efficient than parallel flow as the blood has a higher oxygen saturation.


Parallel flow

When water is taken into the mouth and water is pushed in the same direction that the blood is flowing gas exchange starts off very efficiently as there is a high concentration gradient however around halfway along the gill plates equilibrium is reached and this defeats the concentration gradient so diffusion of oxygen and carbon dioxide will not diffuse anymore.


What are the five classes of vertebrates?

Amphibians, birds, fish mammals and reptiles.


At what stage of an amphibians live do they have gills?

When they are tadpoles/larvae as they live in water how ever as they become older they live on land relying on diffusion through their moist skin (at rest) and their lungs for more active activities like mating.


What is the structure of an amphibians lungs?

Simple structure with little folding of gas exchange tissues.


What are the structures involved in the human breathing system?

Larynx, trachea, bronchus, bronchioles, alveoli, diaphragm pleural membranes, the ribs and the intercostal muscles.


What are the rings made of that support the trachea, bronchi and bronchioles and what is its purpose?

the rings are made of cartilage and prevent the collapse of these features.



The airtight compartment of the body that encloses the lungs.


What are the layers of the trachea?

1. Cilia
2. Lymphocyte
3. Goblet cells
4. Basement membrane
5. Connective tissue
a) Fibrocytes
b) Collagen Fibres.


How does the lung tissue change when someone has emphysema?

The walls of the alveoli have broken down and the surface area for gas exchange has reduces drastically.


How does the human ventilation system work during inspiration/negative pressure breathing?

The intercostal muscles contract pulling the ribs upward and outwards, this then pulls on the pleural membranes causing them to be pulled outwards, in turn the lungs are pulled out and this allows the alveoli to expand decreasing the pulmonary pressure below that of the atmospheric pressure causing air to flow into the lungs. Diaphragm contracts.


Human ventilation- expiration

The intercostal muscles relax causing the ribs to move down and in, this is followed by the pleural membranes, this cause both the lungs and alveoli decrease in volume , which increases the pressure above that of the atmosphere causing the air to move out. Diaphragm relaxes.


Why are alveoli suitable for gas exchange?

They are moist so gasses can diffuse.
They have a short diffusion distance as the air sacs are only one cell thick.
They have a large surface area to volume ratio as there are many in the lungs.
They have a short diffusion pathway as they have a dense network of capillaries.
They also maintain a concentration gradient as there is a high circulation and high air ventilation.