(a)
How does an organism’s size relate to its surface area to volume ratio?
The larger the organism, the lower the surface area to volume ratio.
(a)
How does surface area to volume (SA/V) ratio affect transport of molecules?
The lower the SA/V ratio, the further the distance molecules must travel to reach all parts of the organism. Diffusion alone is not sufficient in organisms with small SA/V ratios.
(a)
Why do larger organisms require mass transport and specialised gas exchange surfaces?
- Small SA/V ratio
- Diffusion insufficient to provide all cells with the required oxygen and to remove all carbon dioxide
- Large organisms more active than smaller organisms
(a)
Name four features of an efficient gas exchange surface.
Large surface area
Short diffusion distance
Steep diffusion gradient
Ventilation mechanism
(b)
Gas exchange in insects
Insects have an impermeable cuticle to prevent water loss, so they exchange gases through spiracles on the thorax and abdomen. Spiracles lead to tracheae, which branch into tracheoles that deliver oxygen directly to muscle cells. Tracheoles contain fluid to aid oxygen diffusion. During flight, fluid levels decrease to shorten the diffusion path, and body contractions ventilate the tracheal system, increasing airflow.
(c)
Gas exchange mechanism in Amoeba
● Unicellular organism with a large SA/V ratio
● Thin cell membrane provides short diffusion distance
● Simple diffusion across the cell surface membrane is sufficient to meet the demands of respiratory processes
(c)
Gas exchange mechanism in Flatworm
● Multicellular organisms with a relatively small SA/V ratio (in comparison to the Amoeba)
● However, flat structure provides a large surface area and reduces the diffusion distance
● Simple diffusion is sufficient to meet the demands of respiratory processes
(c)
Gas exchange mechanism in Earthworm
● Cylindrical, multicellular organisms with a relatively small SA/V ratio (in comparison to the flatworm)
● Slow moving and low metabolic rate which require little oxygen
● Rely on external surface for gas exchange
● Circulatory system transports oxygen to the tissues and removes carbon dioxide, maintaining a steep diffusion gradient
(d)
Organ of gaseous exchange in fish
Gills
(d)
gill filaments
● Main site of gaseous exchange in fish, over which water flows
● They overlap to gain resistance to water flow - slows down water flow to maximise gaseous exchange
● Found in large stacks, known as gill plates, and have gill lamellae which provide a large surface area and good blood supply for exchange
(e)
Ventilation
The movement of fresh air into a space and stale air out of a space to maintain a steep concentration gradient of oxygen and carbon dioxide. Ventilation is needed for large active animals with high metabolic rates to have ventilating mechanisms to maintain gradients across respiratory surfaces.
(f)
Ventilation in bony fish
- Mouth opens, floor of buccal cavity lowers so volume increases, pressure decreases and water rushes in.
- Mouth closes, floor of buccal cavity raises, increasing pressure pushing water over the gills.
- Pressure in gill cavity increases and water forces operculum open and leaves through it.
(f)
Parallel flow
If water and blood flow in the same direction, equilibrium is reached and oxygen diffusion reaches no net movement halfway across the gill plate.
(f)
Counter current flow
If water and blood flow in opposite directions across the gill plate, the concentration gradient is maintained and oxygen diffuses into the blood across the entire gill plate.
(f)
How does counter current flow maintain a steep diffusion gradient? What is the advantage of this?
● Water is always next to blood of a lower oxygen concentration
● Keeps rate of diffusion constant and enables 80% of available oxygen to be absorbed
(f)
Comparison of counter current flow with parallel flow
Counter current flow:
1. Blood and water flow in opposite directions across the gill plate.
2. Steep diffusion gradient maintained, allowing diffusion of oxygen across the whole gill plate
3. High rate of diffusion
4. More efficient- more oxygen absorbed into the blood
5. Found in bony fish
Parallel flow:
1. Water and blood flow in the same direction across the gill plate.
2. Diffusion gradient not maintained ∴ diffusion of oxygen does not occur across the whole plate
3. Lower rate of diffusion
4. Less efficient- less oxygen absorbed into the blood
5. Found in cartilaginous fish, e.g. sharks
(g)
Human adaptation for gas exchange
Alveoli provide a large surface area and thin diffusion pathway, maximising the volume of oxygen absorbed from one breath. They also have a plentiful supply of deoxygenated blood, maintaining a steep concentration gradient.
(g)
Structure and function of the larynx
A hollow, tubular structure located at the top of the trachea involved in breathing and phonation.
(g)
Structure and function of trachea
The trachea is the primary airway that carries air from the nasal cavity to the chest. It is a wide tube supported by C-shaped cartilage rings, preventing collapse during pressure changes. Lined with ciliated epithelial cells, it moves mucus—produced by goblet cells—towards the throat for swallowing, helping to prevent lung infections.
(g)
Structure of bronchi
The bronchi are divisions of the trachea that lead into the lungs. They are narrower than the trachea and supported by rings of cartilage. Like the trachea, they are lined with ciliated epithelial cells and goblet cells to trap and remove mucus and debris.
(g)
Structure and function of bronchioles
Bronchioles are small branches of the bronchi that direct air to the alveoli. They contain smooth muscle to regulate airflow but lack cartilage. Lined with a thin layer of ciliated epithelial cells, they help remove mucus and debris, ensuring clean air reaches the alveoli.
(g)
Structure and function of alveoli
Alveoli are mini air sacs lined with epithelial cells, with walls just one cell thick for rapid gas exchange. A rich blood supply maintains a steep diffusion gradient. With around 300 million alveoli per lung, they provide a large surface area for efficient oxygen and carbon dioxide exchange.
(g)
Pleural membranes
Thin, moist layers of tissue surrounding the pleural cavity that reduce friction between the lungs and the inner chest wall.
(h)
Ventilation in humans
- External intercostal muscles contract and pull the rib cage up and out.
- Outer pleural membrane is pulled out. This reduces pressure in the pleural cavity and
the inner pleural membrane is pulled outward. - This pulls on the surface of the lungs and causes an increase in the volume of the alveoli.
- Alveolar pressure decreases to below atmospheric pressure and air is drawn into the
lungs.
