Surface area to volume ratio & gas exchange

Why can simple transport work for single celled organisms?

• Large surface area : volume ratio, so large surface available for exchange

• Short diffusion path/distances

• Substances are rapidly and efficiently exchanged

• Hence simple transport can meet the metabolic demands of the cell (no need for exchange systems)

Why can’t multi-cellular organisms rely on simple transport?

• Small surface area : volume ratio, so small surface available for exchange

• Results in long diffusion distances

• Substances are slowly and inefficiently exchanged

• Simple transport cannot meet the metabolic demands of the cell (hence specialised transport systems are needed)

What is the relationship between the metabolic rate and surface area?
(larger organisms)

• As organisms size increases, the SA:VR decreases

• This increases diffusion distances

• So takes longer for heat to be lost across the surface of organism

• Therefore metabolism isn’t relatively high as they do not need to generate lots of heat to stay warm

What is the relationship between the metabolic rate and surface area?
(smaller organisms)

• As organisms size decreases, the SA:VR increases

• This decreases the diffusion distances

• So heat is lost more quickly from the surface of organism

• Hence metabolic rate must be high to generate enough heat to replace lost heat and stay warm

What are adaptations of exchange systems?

• Large surface area to volume ratio

• They are thin (one layer of epithelial cells) providing a short diffusion pathway

• Organisms maintain a steep concentration gradient

Describe adaptations in the gills

• Fish have thin plates called gill filaments which give the gills a large surface area to volume ratio

• Smaller structures called lamallae which further increase the SA

• Lamallae have many blood capillaries which remove O₂ in the blood and help keep a steep blood concentration gradient

• Counter current flow of water maintains a steep concentration gradient

Describe the counter current flow of water

• Water and blood flow in opposite directions

• Water with a high oxygen concentration flows past blood with a lower oxygen concentration

• Because water and blood move in opposite directions a steep concentration gradient is maintained along whole length of the gill

• So water continuously diffuses from the water into the blood and equilibrium is never reached

How do fish ventilate their gills?

• When mouth opens, the opercular valve shuts, lowering the buccal cavity floor

• This causes the volume of the buccal cavity to increase and the pressure to decrease

• So water enters the buccal cavity down a pressure gradient

• When mouth closes, the opercular valve opens, raising the buccal cavity floor

• This causes the volume of the buccal cavity to decrease and pressure to increase

• This forces water over and out of the gills down a pressure gradient

What are the features of the exchange systems in insects?

• Spiracles

• Tracheae

• Tracheoles

• Tracheole fluid

Describe the structure and function of the spiracles

• Spiracles are openings in the exoskeleton of the insect which allow gases to enter the tracheal system

• They contain valves which can open and close the spiracles to reduce water loss by evaporation and control gas exchange

• They contain hairs which trap water vapour to prevent water loss by evaporation and to prevent entry of dust and pathogens.

Describe the structure and function of the tracheae

• Large networks of tubes branch off from the spiracles which allow air to enter and leave the tracheal system easily for ventilation

• They have a wide internal diameter reduces resistance to airflow, causing air to move quickly through the system

• The walls are lined with chitin to prevent collapsing, keeping an open pathway for airflow

Describe the structure and function of the tracheoles

• Small, thin-walled tubes branch from the trachea

• Many of these tubes increase the surface area for diffusion of oxygen and carbon dioxide

• Thin walls create shorter diffusion distances, which increase the rate of gas exchange

• Penetrate directly into tissues and lie very close to respiring tissues to reduce diffusion distances for oxygen

Describe the structure and function of the tracheole fluid (during rigorous activity)

• The rate of respiration rapidly increases and produces lactic acid

• Lactic acid lowers the water potential of the muscle cells

• The water moves in from the tracheoles into the muscle cells by osmosis to replace the lost water

• This reduces the volume of fluid in the tracheoles, leading to more of the tracheoles to be filled with air

• This means a shorter diffusion distance between the O₂ and the tracheoles and in the respiring cells

• This creates a steep concentration gradient and hence increases the rate of oxygen diffusion

How can insects cause ventilation within themselves?

• Contraction of muscles in the thorax and abdomen changes the volume of the body cavity

• The volume changes create pressure differences inside the tracheal system

• Decrease in volume increases pressure inside the tracheal system, forcing air out through the spiracles

• An increase in volume decreases pressure, drawing air in through the spiracles

• This all happens down the pressure gradient

• This maintains the steep concentration gradient, and ensures the rapid removal of carbon dioxide

What are adaptations of xerophytes?

Thick Waxy Cuticle:

• Reduces water loss by evaporation.

• The waterproof cuticle decreases diffusion of water vapour from the epidermis.

Rolled Leaves

• Traps moist air inside the roll, reducing the water potential gradient between the leaf and the atmosphere

• This reduces transpiration rate

Sunken Stomata:

• Stomata sit in pits that trap humid air

• This reduces the water potential gradient and slows diffusion of water vapour out of the leaf

Hairy Leaves (Trichomes):

• Trap still, humid air near the leaf surface

• Reduces air movement and therefore reduces transpiration

Extensive Root System:

• Deep tap roots which grow deeply in the soil so can access groundwater

• Widespread shallow roots rapidly absorb surface water after rainfall

What are adaptations of the human gas exchange system?

Short Diffusion Distance:

• Alveolar epithelium is one cell thick

• Capillary endothelium is one cell thick

• Very thin overall diffusion pathway

• Reduces diffusion distance → increases diffusion rate

Dense Capillary Network:

• Each alveolus surrounded by many capillaries

• Good blood supply maintains a steep concentration gradient

Large Surface Area:

• Millions of alveoli provide a very large surface area (~70 m²)

• Increases rate of diffusion of oxygen and carbon dioxide

Continuous blood flow:

• Oxygen is rapidly removed from alveoli

• Carbon dioxide is constantly delivered

• Maintains concentration gradient

Moist Surface:

• Thin layer of fluid lines alveoli

• Oxygen dissolves before diffusing

Elastic Fibres:

• Allow alveoli to stretch and recoil

• Helps force air out during exhalation

Ventilation (Breathing Movements):

• Diaphragm and intercostal muscles ventilate lungs

• Refreshes air in alveoli

• Maintains high oxygen and low carbon dioxide concentration

What is the adaptations and functions of goblet cells?

Adaptation:

• Specialised epithelial cells that secrete mucus

Function:

• Mucus traps dust, bacteria, and other pathogens

• Prevents particles reaching the alveoli

• Protects delicate gas exchange surface

Why this matters:

• Keeps alveoli clean so diffusion can occur efficiently

What is the adaptations and functions of cilia cells?

Adaptation:

• Cells lining the trachea and bronchi have hair-like projections called cilia

Function:

• Cilia beat rhythmically upwards toward the throat

• Move mucus (with trapped pathogens) out of the airways

Why this matters:

• Prevents infection and blockage → maintains efficient airflow to alveoli

Describe the mechanism of breathing in

• During inhalation, the diaphragm contracts and flattens

• The external intercostal muscles contract while the internal intercostal muscles relax

• This causes the ribs to move upwards and outwards

• As a result, the volume of the thoracic cavity increases

• The increase in lung volume causes the pressure inside the lungs to decrease below atmospheric pressure

• Air then moves into the lungs down the pressure gradient from a region of higher pressure to lower pressure

Describe the mechanism of breathing out

• During exhalation, the diaphragm relaxes and returns to its dome shape

• The external intercostal muscles relax

• The ribs move downwards and inwards

• This decreases the volume of the thoracic cavity

• The elastic fibres in the lungs recoil, causing the lungs to decrease in volume

• The pressure inside the lungs increases above atmospheric pressure

• Air moves out of the lungs down the pressure gradient from higher pressure to lower pressure.

Describe the mechanism of forced exhalation

• During forced exhalation, such as during exercise, the internal intercostal muscles and abdominal muscles contract

• This further reduces the volume of the thoracic cavity

• This increases the pressure inside the lungs

• This forces air out more rapidly