gas exchange

GAS EXCHANGE IN SINGLE-CELLED ORGANISMS

SIMPLE DIFFUSION OF GASES THROUGH CELL SURFACE MEMBRANE

adaptions of gas exchange in single celled organisms

• They have a relatively large surface area 

• They have a thin surface

• They have a short diffusion pathway

GAS EXCHANGE IN DICOTYLEDONOUS PLANTS

DIFFUSION OF OXYGEN AND CARBON DIOXIDE DURING PHOTOSYNTHESIS AND RESPIRATION 

At times the gases produced in one process can be used for the other. 

• This reduces gas exchange with the external air. 

• Overall, this means that the volumes and types of gases that are being exchanged by a plant leaf change. 

• This depends on the balance between the rates of photosynthesis and respiration.

The main gas exchange surface is the surface of the mesophyll cells in the leaf.

There is no specific transport system for gases, which simply move in and through the plant by diffusion.

Diffusion takes place in the gas phase (air), which makes it more rapid than if it were in water.

ADAPTATIONS FOR EFFICIENT GAS EXCHANGE in plants

large surface area

short diffusion pathway

maintained concentration gradient

LARGE SURFACE AREA TO VOLUME RATIO

Air spaces inside a leaf have a very large surface area compared with the volume of living tissue.

Large surface area of mesophyll cells for rapid diffusion

SHORT DIFFUSION PATHWAY 

Many stomata so no living cell is far from the external air, and therefore a source of oxygen and carbon dioxide.

Numerous interconnecting air-spaces that occur throughout the mesophyll so that gases can readily come in contact with mesophyll cells

MAINTAINED CONCENTRATION GRADIENT 

When stomata are open, gases enter and exit so there is always a favourable diffusion gradient for CO₂ and O₂. 

Stomata

• OPEN: in bright light when photosynthesis and respiration occurs, so gas exchange is needed. Water moves into the guard cells  by osmosis making them turgud and they open. 

• CLOSED: In dim light -when photosynthesis cannot occur therefore no gases need to be exchanged, the stomata close to limit transpiration and save water.  They also close when the plant is dehydrated and water moves out of the guard cells and they become flaccid, causing them to close.

XEROPHYTIC PLANTS

Xerophytes have a limited access to water so they are specially adapted for life in warm, dry or windy habitats, where water loss is a problem. They need to reduce the rate at which water can be lost through evaporation. As the vast majority of water loss occurs through the leaves, it is these organs chat usually show most modifications

adaptations of xerophytic plants

Thicker waxy, waterproof cuticles

• on leaves and stems to reduce evaporation.

A layer of ‘hairs’ on the epidermis to trap water vapour round the stomata.

• The water potential gradient between the inside and the outside of the leaves is reduced and therefore less water is lost by evaporation.

Stomata sunk into pits or grooves to trap still, moist air/ water vapour,

• reducing the concentration gradient of water (water potential gradient) between the leaf and the air. This reduces evaporation of water from the leaf.

A reduced number of stomata,

• so there are fewer places for water to escape.

A reduced surface area to volume ratio. having leaves that are small and roughly circular in cross-section the rate of water loss can be considerably reduced.

• This reduction in surface area is balanced against the need for a sufficient area for photosynthesis to meet the requirements of the plant.

GAS EXCHANGE IN TERRESTRIAL INSECTS

DIFFUSION OF GASES USING A TRACHEAL SYSTEM 

structure of the tracheal system

• Spiracles - round, valve like openings on abdomen, oxygen and carbon dioxide diffuse in/out. Spiracles attach to trachea

• Tracheae - a  network of internal tubes with rings to strengthen and prevent collapse and to keep them open so gases can constantly move in and out. They branch into tracheoles. 

• Tracheoles - microscopic tubules which extend to every respiring tissue to provide oxygen to every cell and to take carbon dioxide away.

VENTILATION SYSTEM IN INSECTS

1. MASS TRANSPORT 

Insects also use rhythmic abdomen movements where the muscles within the insect’s abdomen contract and relax to pump air in and out of the spiracles to speed up gas exchange. It increases the amount of CO₂ released while raising the pressure 

2. WATER IN ENDS OF TRACHEOLES 

When in flight, insects respire anaerobically and produce lactate (soluble). This lowers the water potential of muscle cells, so water moves into cells from tracheoles by osmosis. This lowers the pressure and volume in tracheoles and forces air into the trachea.   

ADAPTATIONS FOR EFFICIENT GAS EXCHANGE of the tracheal system

LARGE SURFACE AREA TO VOLUME RATIO

• Lots of tracheoles which are highly branched

SHORT DIFFUSION PATHWAY 

• Tracheoles have thin walls (single-celled). 

• Short distance between spiracles and tracheoles 

MAINTAINED CONCENTRATION GRADIENT 

• Respiring cells use up oxygen and produce carbon dioxide producing a steep concentration gradient with the environment.

CONTROL OF WATER LOSS IN INSECTS

Water evaporates off insects, gas exchange adaptation increase this. There needs to be a balance gas exchange and water loss. 

Adaptations to control water loss: 

1. Small SA:V where water can evaporate from. 

2. Lipid layer on their exoskeleton which is waterproof, so water can’t evaporate 

3. Spiracles can open and close to prevent water loss. If insects are losing too much water, they close their spiracles using muscles.

4. Tiny hairs around their spiracles help reduce water loss

GAS EXCHANGE IN FISH

DIFFUSION OF GASES OVER LAMELLAE, USING A COUNTER-CURRENT SYSTEM. 

In a fish, the gas exchange surface is the gills.

• The gills are located within the body of the fish, behind the head, 4 layers on each side.  

• The operculum is a bony flap that protects the gills from getting damaged, it opens and closes to allow water to pass over the gills. 

GILL STRUCTURE 

Gill filaments - The gill filaments are stacked up in a pile. 

Lamellae - on the surface of gill filaments and are at right angles to the gill filaments. 

Gill rakers - cartilage extension that carry out food acquisition, they trap food particles to feed the fish. 

Gill bar/arch - supports the gills and blood vessels. 

GAS EXCHANGE OVER LAMELLAE

1. Diffusion of gases happens on lamellae. 

2. Water is taken in through the mouth and forced over the gills and out through an opening on each side of the body.

3. Blood enters lamellae with low oxygen, leaves with high oxygen concentration. 

4. Water flowing towards lamellae has high oxygen concentration and leaves with low oxygen. 

ADAPTATIONS FOR EFFICIENT GAS EXCHANGE

LARGE SURFACE AREA TO VOLUME RATIO

Many gill filaments. 

Each gill filaments has many lamellae on its surface which further increases the surface area 

SHORT DIFFUSION PATHWAY 

The lamellae have lots of blood capillaries. 

They also have a thin surface layer of cells. 

MAINTAINED CONCENTRATION GRADIENT 

Counter-current system. 

COUNTER-CURRENT SYSTEM

Blood and water flow in opposite directions through/over lamellae

2. So oxygen concentration always higher in water (than blood near)

3. So maintains a concentration gradient of O2 between water and blood

4. For diffusion along whole length of lamellae

GAS EXCHANGE IN HUMANS

Humans and all aerobic organisms require a constant supply of oxygen into the blood to release energy in the form of ATP during respiration. They also need to get rid of carbon dioxide (made by respiring cells) as this could build up and be harmful to the body.

VENTILATION

is the movement of air in and out of the lungs (breathing).

RESPIRATION

is a chemical reaction to release ATP (energy). 

GAS EXCHANGE

• Diffusion of oxygen from the air in the alveoli into the blood. 

• Diffusion of carbon dioxide from the blood into the air in the alveoli. 

STRUCTURE OF HUMAN GAS EXCHANGE SYSTEM

RIBCAGE

LUNGS

TRACHEA  

BRONCHI

BRONCHIOLES

ALVEOLI

ribcage

protect and support the lungs, they can be moved by intercostal muscles between them. 

lungs

the site of gas exchange, a pair of lobed structures made up of a series of highly branched tubules, called bronchioles, which end in tiny air sacs called alveoli.

TRACHEA

a flexible airway that is supported by rings of cartilage (C shaped) that keeps it open.

The cartilage prevents the trachea collapsing as the air pressure inside falls when breathing in.

The tracheal walls are made up of muscle, lined with ciliated epithelium (helps in mobilising the mucus and keeping the respiratory tract clear and unobstructed) and goblet cells (secrete mucin and create a protective mucus layer which traps particles).  

BRONCHI

two divisions of the trachea, each leading to one lung.

They are similar in structure to the trachea, they also produce mucus to trap dirt particles and have cilia that move the dirt-laden mucus towards the throat.

The larger bronchi are supported by cartilage, although the amount of cartilage is reduced as the bronchi get smaller.

BRONCHIOLES

 a series of branching subdivisions of the bronchi. Their walls are made of muscle lined with epithelial cells. This muscle allows them to constrict so that they can control the flow of air in and out of the alveoli.

ALVEOLI

tiny air-sacs, with a diameter of between 100 µm and 300 µm, at the end of the bronchioles. Between the alveoli there are some collagen and elastin - protein fibres that allow the alveoli to stretch as they fill with air when breathing in, the elastic fibres also mean they spring back (recoil) in order to expel the carbon dioxide-rich air.  The alveoli are lined with epithelium. The alveolar membrane is the gas-exchange surface.

INTERCOSTAL MUSCLES AND DIAPHRAGM

The intercostal muscles fill the space between the ribs. There are internal, innermost and external which are in layers.They are ANTAGONISTIC.

The diaphragm is a flat sheet muscle, when it relaxes it domes upwards, when it contracts it flattens. 

VENTILATION MECHANISM 

 

	INHALATION	EXPIRATION	FORCED EXPIRATION
Internal intercostals	Relax	Contract	Contract
External intercostals	Contract	Relax	Relax
Diaphragm	Contract / flatten / down	Relax / dome / up	Relax / dome / upwards
Ribs	Up and out	Down and in	Further down and in
Volume in chest cavity	Increase	Decrease	Decreases more
Pressure in thoracic cavity	Decrease (rises as air in)	Increase (falls as air out)	Increases more
Air (moving down pressure gradient)	Draws air IN	Forces air OUT	More OUT
Active or passive	Active	Passive	Active

GAS EXCHANGE OVER ALVEOLI

ALVEOLI are the gas exchange surface. 

Alveoli are made up of a single layer of epithelium cells which are flat and permeable to allow diffusion of gases. 

Capillaries are made up of a single layer of endothelium cells.

They fill with air during inhalation. 

DIFFUSION (down concentration gradient)

O₂ = from alveoli (high conc of O₂) across the alveolar epithelium and the capillary endothelium, and into a compound called haemoglobin in the blood (deoxygenated/ low as oxygen has been  transported to respiring cells in the body). 

CO₂ = from blood (high conc of CO₂) across the capillary endothelium and the alveolar epithelium, to the alveoli (low conc of CO₂)

ADAPTATIONS FOR EFFICIENT GAS EXCHANGE 

LARGE SURFACE AREA TO VOLUME RATIO

SHORT DIFFUSION PATHWAY 

MAINTAINED CONCENTRATION GRADIENT 

MOIST 

LARGE SURFACE AREA TO VOLUME RATIO

Many alveoli - 300 million in each lung. 

The total surface area is around 70 m².

SHORT DIFFUSION PATHWAY 

The alveoli are surrounded by a network of blood capillaries. 

Alveoli have a thin single layer of flat epithelial cells - alveolar epithelium. 

Capillary membranes  are also made of a single layer of cells - endothelium. 

MAINTAINED CONCENTRATION GRADIENT 

Circulatory system constantly moving blood. Each alveoli is surrounded by a network of capillaries. The blood flowing has returned to the lungs so has a low concentration of oxygen and a high concentration of carbon dioxide following respiration. This maintains a steep concentration gradient as the oxygen transported in is taken away quickly. 

WELL VENTILATED Diffusion alone is not fast enough to maintain a concentration gradient.

MOIST

Dissolve gases so they can diffuse across. 

LUNG FUNCTION MEASURES 

• Tidal volume is the volume of air in each breath - it’s usually between 0.4dm³ and 0.5dm³ for adults.

• ƒƒVentilation rate is the number of breaths per minute. For a healthy person at rest it’s about 15 breaths

• Forced expiratory volume (FEV) is the maximum volume of air that can be breathed out in 1 second. 

• Forced vital capacity (FVC) is the maximum volume of air it is possible to breathe forcefully out of the lungs after a really deep breath in.

ASTHMA

 

CAUSE	Genetics, allergens, respiratory tract infections, airborne irritants
WHAT HAPPENS IN LUNGS	Inflamed bronchi when come into contact with a trigger
EFFECTS ON GAS EXCHANGE/ VENTILATION	Smooth muscle lining bronchioles contracts Constriction of airways Narrow diameter Airflow in/out of lungs reduced (FEV reduced) Less oxygen enters alveoli / blood Reduced rate of gas exchange in alveoli Less oxygen diffuse into blood Cells receive less oxygen Rate of aerobic respiration reduced Less energy released Fatigue, weakness etc.

FIBROSIS 

 

CAUSE	Smoking, pollutant exposure, genetic, autoimmune disease
WHAT HAPPENS IN LUNGS	Scar tissue in lungs
EFFECTS ON GAS EXCHANGE/ VENTILATION	Scar tissue is thicker and less elastic than normal Diffusion distance increased  Rate of diffusion decreased  Faster ventilation rate to get enough oxygen into lungs/blood  Lungs can expand and recoil less  Can’t hold as much air  Reduced tidal volume  Reduced forced vital capacity

EMPHYSEMA

 

CAUSE	Smoking and air pollution
WHAT HAPPENS IN LUNGS	Damage to alveoli walls
EFFECTS ON GAS EXCHANGE/ VENTILATION	Harmful particles trapped in alveoli.  Inflammatory response triggered.  Phagocytes are attracted to the area. Enzyme released which breaks down elastin (protein helping recoil) Alveoli can’t recoil Air remains trapped  Alveoli walls are damaged so there is a permanent increase in alveoli cavity  Surface area reduces.  Bronchioles narrow so ventilation affected

TUBERCULOSIS

 

CAUSE	Contagious bacterium: Mycobacterium tuberculosis.
WHAT HAPPENS IN LUNGS	Bacteria lodges in the lung tissue causing it to become thick and hard (fibrosis) then cells may die (necrosis) and tissues tear and cause cavitation.
EFFECTS ON GAS EXCHANGE/ VENTILATION	Lung tissue becomes hard  Exchange of oxygen is impossible  Cells in lung tissue die Tearing occurs  Cavitation occurs and more bacteria sit in there and destroy more tissue

CAUSE AND CORRELATION

CORRELATION is a link between two variables. a change in one of two variables is reflected by a change in the other variable.

A correlation does not mean CAUSE i.e one variable caused the other. 

Graphs may suggest that there is a causal relationship, however, experiments would have to be carried out to provide evidence for cause. 

INTERPRETING LUNG DISEASE DATA 

1. DESCRIBE THE DATA 

2. DRAW CONCLUSIONS 

3. PROBLEMS/ISSUES WITH DATA 

DESCRIBE THE DATA 

• general trend

• Linear / non linear 

• example

• anomaly 

DRAW CONCLUSIONS 

• ___ correlation

• not causal

• suggest other factors causing it 

• deaths from smoking decrease when people stop smoking but might be due to medical advance

PROBLEMS/ISSUES WITH DATA 

• Sample size 

• Sampling strategy 

• Gender 

• Age 

• Repeats to identify anomalies  

• Controls