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  Description Example
Simple Diffusion The movement of molecules along the concentration gradient (no energy required) across a partially permeable membrane 1)      Alveoli

2)      Gut

3)      Leaves (movement of CO2)

Facilitated diffusion Transport of substances across a membrane by trans-membrane proteins 1)      Absorbing of glucose

2)      Movement of Ions

Active Transport Movement of molecules against the concentration gradient requiring a protein pump (Required energy) 1)      Root hair cells (absorbing minerals from the soil)
Osmosis The movement of water across a selective membrane towards an area of high water potential (positive area) to an area of low water potential (negative area) 1)      Roots (absorbing water from the ground)





  • Water is a polar molecule and therefore many molecules can dissolve in it. These dissolved substances are called solutes
  • Water diffuses by Osmosis from a region of high Water Potential to a region of low Water Potential through the Water Potential Gradient.
  • Hypertonic – More Concentrated solute
  • Isotonic – Equal Concentration
  • Hypotonic – Less concentration solute
  • Osmosis is the diffusion of water across a cell membrane
  • Cell contains solutions of different solutes
  • Water molecules can diffuses freely across the phospholipid bilayer, but always down the concentration gradient
  • Water therefore diffuses from a dilute to a concentrated solution


Active Transport


  • Active transport is the pumping of substances across a membrane by a trans-membrane protein pump molecule
  • The protein bonds to a molecule of the substance to be transported on one side of the membrane, change shape and released onto the opposite side.
  • The protein pumps are specific so there is a different protein pump for each molecule to be transported


Transport through the membrane

The cell membrane is primarily made up of phospholipids bilayers with various channels to allow molecules into the cells through integral proteins.

Simple Diffusion

Simple Diffusion is the random movement of particles from an area of high concentration to an area of low concentration.

The movement of gas is random as it moves away from its source using its own kinetic energy they travel across an area until an equal amount is found throughout the area.

Facilitated diffusion

Facilitated diffusion is the transport of substances across a membrane by a transmembrane protein molecule.

Transport proteins tend to be specific to one molecule that can cross a membrane if it contains the appropriate protein, this is a passive diffusion process as it uses the kinetic energy of the molecules to change the shape of the transmembrane protein.

The molecules can only move along their concentration gradient

Protein Channels

They form water filled hydrophilic channels across the membrane. They allow specific water soluble ions to pass through. The channels are selective, only opening to specific ions . They are intrinsic proteins, so span across the whole membrane

The channel that the channel proteins make, are full of water. This means only water soluble substances can pass through.

Facilitated diffusion happens here. Some channels are also gated and/or selective. Gated means it opens only when appropriately stimulated.

Carrier Proteins

Spanning across the plasma membrane, when a molecules, such as glucose, is specific to the protein that is present it binds to the protein.  This Changes the shape which allows the molecule to be released to the inside of the membrane.

No extra energy is required as it follows the concentration gradient, using only the kinetic energy of the molecules.

Na+ K+ Pump


  • This transported protein is present in the cell membrane of all animal cells and is the most abundant and important of all the membrane pumps.
  • They are found in neurons as they require a positive (+) charge, so there it pumps Sodium (Na) out of the cell and K+
  • 3 Naions are removed, to 2K+ which gives the internal cell a +1 charge and the outside of the cell a relative -1 charge




  • Protein pumps are also ATPase as they split ATP into ADP + phosphate
  • The energy released in them is then used to charge the shape of the pump molecule
  • Therefore pumping is an active process and is only transported mechanism that can transport substances against (up) the concentration gradient


Water Potential


  • Osmosis can be quantified using water potential (Ψ) so therefore you can calculate which way water molecules will move and how quickly
  • Water potential is a measure of the water molecules potential for movement in a solution
  • Water potential is measured in Pascal
  • Water always moves by osmosis from less negative to more negative water potential
  • 100% pure water has a water potential (Ψ) of 0. This is the highest water potential so all solutions have Ψ less than 0. No water potential can be greater than 0.


Osmosis in Cells


  • Water molecules diffuses into a cell because there if a less/lower water potential of water molecules within the cell
  • Different biological molecules have different solutes in them, whether they be salts or sugars, therefore they have lower water potentials than that is within the cell
  • In animal cells, as they do not contain a cell wall if the water potential is higher on the outside than inside the cell, they will expand until they burst open; becoming Haemolysed. If the water potential is greater on the inside then the water will diffuse out, thus the cell will look wrinkled and become crenated.
  • In plant cells, as they do contain a cell wall if the water potential is higher on the outside than inside of the cell, it will expand and become turgid. If the water potential is greater on the inside then the water will diffuse out, thus the cytoplasm will pull away from the cell wall. The cell will become Plasmolysed.


Factors that affect the rate of diffusion:


  • Surface Area -temperature
  • Length of Diffusion pathway
  • Concentration of molecules


Simple vs Facilitated Diffusion

The key differences between Simple and Facilitated Diffusion is diffusion in a general term describes the movement of any particle from an area of high concentration to low concentration. Facilitated diffusion is more specific to the movement of materials in and out through the cell membrane. The use of protein channels and carrier proteins allow for only specific molecules into the cell. Facilitated diffusion is a passive process as no additional energy is required, rather is uses the kinetic energy of the molecule to travel into the cell

Movement of Glucose

Glucose is transported into cells by pores in the proteins that span across a cell phospholipid bilayer. This is due to them not being a soluble lipid.

Circulatory System

Circulatory System

The general pattern of blood circulation is:

  • Large Surface Area: Decrease in the surface area:volume ratio
  • Suitable median: A method of transporting oxygen around the body i.e. blood
  • A form of mass transport in which the median is moved over a large distance
  • Mechanism for moving the medium around the vessels i.e. hearts in mammals or passive process such as transpiration
  • Mechanisms to maintain the mass flow in one direction
  • Controlling the flow of the transport median

Single Circulation

  • Used in smaller organisms such as fish
  • The heart pumps the blood to the exchange surface, to the cells and back to the heart in one contraction

Double Circulation

  • Used in larger organisms such as mammals
  • The heart is comprised of two pumps, the blood leaves the heart to the lungs and then returns back to the heart. The heart then pumps the blood again for it to journey around rest of the body
  • This is required as blood will lose too much pressure after the lungs alone
  • The high pressure allows the mammals to have warm body temperatures as well as a fast-metabolic rate


Human Circulatory System


  • The transport system is used to move substance large distances, the final part of the journey to the cells is via diffusion.
  • The final exchange from blood vessels into the cells is rapid because of the large surface area, short diffusion pathway and the steep concentration gradient.
  • The main reason for a gas exchange system is to allow for large organisms with large surface areas:volume ratios to have a method of supplying the required gases to all of its cells



Vessels of the Heart

  • The vessels which supply the heart with oxygenated blood are called the coronary arteries
  • In the Vena cava, blood is deoxygenated after it has circulated the body and travels into the right atrium
  • The pulmonary artery has deoxygenated blood in it which travels from the right ventricle to the lungs
  • The left atrium has oxygenated blood straight from the pulmonary vein
  • The blood travels from:

Vena cava -> Pulmonary Artery -> Pulmonary Vein -> Aorta

  • Deoxygenated and oxygenated blood should not mix as they have varying concentrations of oxygen in them. If oxygenated blood reaches the lung it will reduce the concentration gradient, therefore less oxygen is absorbed.

Chambers of the Heart

  • The heart is made up of 4 chambers, the left and right atrium and the left and right ventricles
  • Blood enters the right atrium through the superior and inferior vena cava during diastole.
  • In atrial systole, the atria contract to push the blood down into the right ventricle
  • In ventricular systole, the blood is pushed out of the right ventricle into the pulmonary artery which leads to the lungs for the blood to be deoxygenated. The blood then returns through the pulmonary veins back into the left atrium of the heart
  • During diastole, the blood flows from the left atrium down into the left ventricle
  • In ventricular systole, the blood is then pushed out of the left ventricle through the aorta and out to the rest of the body

Coronary Arteries and veins

  • Coronary arteries leave from the aorta into the heart muscle tissues via capillaries before leaving via the coronary veins
  • If one of the arteries gets blocked, then the heart tissue is starved of oxygen. Cardiac tissue is unable to respire anaerobically and therefore will die. Resulting in a heart attack.

Heart Valves

  • Semi-lunar valves link the ventricles to the pulmonary artery (pulmonary valve) and the aorta (aortic valve) . They prevent blood from re-entering the heart after the ventricular systole
  • Atrioventricular valves (tricuspid on the right, bicuspid (mitral) on the left) like the atria to the ventricles. They prevent blood from being pushes back into the atria after ventricular systole

Tissue Fluid

Tissue Fluid


  • Tissue fluid is the liquid that surrounds the cells
  • It allows for transport of molecules such as respiratory gases between the blood and the cells.
  • Capillaries are one cell thick wall, they are highly branched to increase their surface area. The capillaries walls are partially permeable.
  • Tissue fluid is the result of interplay of:
  • Blood pumped from the heart reaches the capillaries and as pressure still remains high, hydrostatic pressure occurs at the arterial end of the capillary.
  • Hydrostatic pressure results in the tissue fluid to leave the blood plasma.
  • However, the outwards pressure is opposed by:
  • Hydrostatic pressure of the tissue fluid outside the capillary, which resists the outward of liquids
  • Lower Water Potential of the blood due to the plasma proteins causing water to move back into the blood within the capillaries


  • However, the overall force is to create and overall pressure that pushes the tissue fluid out of the capillaries at the arterial end
  • At the Venous end, the osmotic pressure and the net pressure is higher than the pressure within the capillary, therefore molecules diffuse enter the blood plasma
  • The overall net fluid movement is greater out of the artery end compared to the movement of fluid in. The net pressure is 10mm Hg at the arterial end compared to -7mm Hg at the venous end.
  • Any remaining tissue fluid is removed through the lymphatic system which restores the fluid back into the blood.


Gases Exchange in Humans

Gases Exchange in Humans

Pathway of Air

Air enters through the nostrils and mouth passing through:


  • Pharynx
  • Glottis
  • Trachea (protected by cartilaginous rings)
  • Bronchi (splits the air into either lung)
  • Bronchioles
  • Alveoli


Adaptations of the Lungs


  • Large Surface Area: Allows for more places for the oxygen and carbon dioxide can diffuse in/out of the blood
  • Movement of Blood: The removal of oxygenated blood to make room for deoxygenated blood, thus maintaining a concentration gradient
  • Lungs are well ventilated: The removal of carbon dioxide and replenishing with oxygen results in maintain a concentration gradient
  • Thin Walls: The alveoli and the capillaries that surround that are 1 cell thick each, thus there is a short diffusion pathway


Purpose of the Respiratory System

Humans require an efficient, large gas exchange system as they are large in size with a, in comparison, small surface area. Humans have trillions of cells (37.2 trillion to be precise) making it impossible to gases to simple diffuse through their membranes, like a single celled organism. The high demand for oxygen is the result of our high metabolism that needs to fuel, for example our 37.5oc body temperature and other measures to ensure homeostasis.

Features of the airways

The trachea and the bronchioles are lined with cartilage which prevents the airways from collapsing when under pressure, such as when the person lies flat. Lined on the walls of the airways are goblet cells which produces mucus which traps any foreign dirt/pathogens from getting into the lungs. The cilia then move the debris out of the body or down into the stomach to avoid further infection.

Exchange of Gases in the Lungs

The site of gas exchange in a mammal is the epithelium of the alveoli. The alveoli air scars are 100-300µm in diameter

Pulmonary Ventilation

  • The measure of pulmonary ventilation is how much air is taken in and out of the lungs in a given time at rest
  • Pulmonary ventilation rate is the total volume of air moved into the lungs in one minute:

Pulmonary Ventilation rate:        Total Volume of Air Taken     x   Breathing rate

dm-3min-1                                       dm-3                                    min-1

e.g.  “A person has pulmonary ventilation rate of 10.2dm-3 min-1 and a tidal volume of 0.6dm-3.

Calculate the persons breathing rate




  • Alveoli are thin to increase the rate of the diffusion of oxygen, however the result of this is that they are fragile. As a result, alveoli are found deep into the lungs
  • There are 300 million alveoli per human lung
  • Fully spread out, the two human lungs have a surface area of 70m2
  • The thin capillary walls have the advantage of pushing the red blood cells to the walls, thus reducing the diffusion pathway further
  • The large network of capillaries around each alveoli also increases their surface area
  • Breathing Constantly ventilates the lungs replacing the air in the lungs with new air and removing the CO2
  • The heart constantly circulates the blood which maintains the concentration gradient across the alveoli as there is always readily available unsaturated blood.



Disease of the Respiratory System

What is the cause of Pulmonary Tuberculosis?

Pulmonary tuberculosis is the result of a bacterial infection; Mycobacterium tuberculosis.

There are two branches of tuberculosis:

Latent TB – the bacteria remain in the body in an inactive state. They cause no symptoms and are not contagious, but they can become active.

Active TB – the bacteria do cause symptoms and can be transmitted to others.

Tuberculosis (TB) bacteria can lie dormant in the body for weeks or even many years, causing no illness in 90 to 95 percent of infected people.


What are the symptoms of pulmonary tuberculosis?

While latent TB is symptomless, the symptoms of active TB include the following:


  • Coughing, sometimes with mucus or blood
  • Chills
  • Fatigue
  • Fever
  • Loss of weight
  • Loss of appetite
  • Night sweats


Tuberculosis usually affects the lungs, but can also affect other parts of the body.


  • TB infecting the bones can lead to spinal pain and joint destruction
  • TB infecting the brain can cause meningitis
  • TB infecting the liver and kidneys can impair their waste filtration functions and lead to blood in the urine
  • TB infecting the heart can impair the heart’s ability to pump blood, resulting in a condition called cardiac tamponade that can be fatal



How is pulmonary tuberculosis transmitted between individuals in the population?

Pulmonary tuberculosis is transmitted through water particles that someone with TB produces (e.g. When a person with TB of the lungs or throat coughs, sneezes, sings or talks, droplets containing the bacteria are released into the air.)

How does the disease develop within the body?

Tuberculosis usually strikes the lungs, and eventually a hole can develop in the patient’s lung. Air or fluid may accumulate between the chest wall and lungs, causing the patient to have chest pain and feel short of breath. In some cases, the bacteria may spread through the body and damage other organs. If not treated, tuberculosis can be fatal.

What is fibrosis, asthma and emphysema?

Pulmonary fibrosis scars and thickens the tissue around and between the air sacs (alveoli) in your lungs. This makes it more difficult for oxygen to pass into your bloodstream. The damage can be caused by many different factors — including long-term exposure to certain toxins, certain medical conditions, radiation therapy and some medications.

The 3 types of Fibrosis:


  • Replacement fibrosis – This occurs in response to lung damage caused by infarction or an infection such as pneumonia or tuberculosis.
  • Focal fibrosis – This occurs as a response to irritation by substances that are inhaled and then carried to nearby lymph tissue by macrophages. In the lymph tissue, the process of fibrosis begins. Occupational exposure to silica or asbestos are common examples of the substances that can cause this form of pulmonary fibrosis.
  • Diffuse parenchymal lung disease (DPLD) – This occurs in cases of fibrosing alveolitis, which manifests in idiopathic pulmonary fibrosis. DPLD also occurs in extrinsic allergic alveolitis, where there is diffuse inflammation of the lung tissue in response to inhalation of dust antigens such as animal dander.


Asthma is a common, long-term or chronic, disease. It affects about five million people in the UK. Asthma often starts in childhood, but it can happen for the first time at any age. Asthma affects the airways – the tubes that carry air in and out of your lungs. If you have asthma, you have very sensitive airways that become inflamed and tighten when you breathe in anything that irritates your lungs like smoke or allergens like pollen. This can cause chest tightness and wheezing and make it harder to breathe.

Emphysema is a long-term, progressive disease of the lungs that primarily causes shortness of breath due to over-inflation of the alveoli (air sacs in the lung). In people with emphysema, the lung tissue involved in exchange of gases (oxygen and carbon dioxide) is impaired or destroyed. Emphysema is included in a group of diseases called chronic obstructive pulmonary disease or COPD (pulmonary refers to the lungs). Emphysema is called an obstructive lung disease because airflow on exhalation is slowed or stopped because over-inflated alveoli do not exchange gases when a person breaths due to little or no movement of gases out of the alveoli.


How does Fibrosis, asthma and emphysema affect lung function?
Cystic fibrosis
 causes the mucus that coats the breathing tubes to become so thick and sticky that the cilia are unable to sweep the germs and other particles up and out of the lungs. The trapped bacteria lead to frequent, serious infections and permanent lung damage. In response to infections, the body’s immune system sends white blood cells to the lungs to attempt to destroy the infection. White blood cells release chemicals that kill both bacteria and surrounding normal cells. After attacking the bacteria, the white blood cells die, adding to the thickness of the mucus and destruction of the airways.

In the upper respiratory tract, thick, sticky mucus may also clog the nasal passages and sinuses. Small growths, or polyps, on the inner lining of the nose may develop from repeated infection and inflammation.

Asthma causes airway obstruction, during normal breathing, the bands of muscle that surround the airways are relaxed, and air moves freely. But in people with asthma, allergy-causing substances, colds and respiratory viruses, and environmental triggers make the bands of muscle surrounding the airways tighten, and air cannot move freely. Less air causes a person to feel short of breath, and the air moving out through the tightened airways causes a whistling sound known as wheezing. Causes Inflammation. People with asthma have red and swollen bronchial tubes. This inflammation is thought to contribute greatly to the long-term damage that asthma can cause to the lungs. And, therefore, treating this inflammation is key to managing asthma in the long run.

Asthma also causes Airway irritability. The airways of people with asthma are extremely sensitive. The airways tend to overreact and narrow due to even the slightest triggers such as pollen, animal dander, dust, or fumes.

Emphysema changes the anatomy of the lung in several important ways. This is due to in part to the destruction of lung tissue around smaller airways. This tissue normally holds these small airways, called bronchioles, open, allowing air to leave the lungs on exhalation. When this tissue is damaged, these airways collapse, making it difficult for the lungs to empty and the air (gases) becomes trapped in the alveoli.


Blood Vessels

Blood Vessels



  • Carry blood from the heart to the rest of the body
  • Their cell walls are made up of layers, all of which are used to maintain the pressure of the blood
  • They an elastic layer which can stretch to ensure that the arteries do not burst under the high pressure of the heart
  • All arteries, except for the pulmonary artery, carries oxygenated blood


  • Carry blood from the arteries to more specific regions of the body
  • They an elastic layer which can stretch to ensure that the arteries do not burst under the high pressure of the heart
  • They can control the rate of blood going into certain regions through constriction or dilation of the muscle layers.


  • Carries blood from the body back to the heart
  • They are under low pressure with wider lumens compared to the arteries.
  • They do little elasticity or muscle layer.
  • Muscle contractions from muscles around the veins encourages deoxygenated blood to be pushed back towards the heart
  • The veins contain valves that prevent backflow


  • Carries blood from the arterioles to the cellular level to exchange the contents, i.e. oxygen, into the tissue fluid.
  • They have walls which are only one cell thick therefore decreasing the diffusion pathway
  • There is a large network of capillaries to increase the surface area for exchange. Lots of capillaries together in a network are called capillary beds

The Walls of Veins and Arteries

  • Tough Fibrous: Both of the veins and the arteries have equal sized outer layers. The particular presence of the outer layer maintains the pressure of the artery so it cannot rupture
  • Muscle Layer: The muscle layer is thicker in the arteries and can be constricted and dilated in order to control the volume of blood within
  • Elastic Layer: The Elastic layer is ticker in the artery and is used to deal with the high pressure from the heart.
  • Valves are only found in the veins to prevent backflow from the reduction in pressure
  • The lumen is larger in the veins compared to the arteries

Gas Exchange in Fish

Gas Exchange in Fish


  • Fish have thick skin that enable them to be waterproof however the result of this is gases cannot readily diffuse through the skin.
  • Fish also have a small surface area : volume ratio
  • Body surface therefore cannot supply adequate amount of respiratory gases


Structure of the Gills


  • The gills are located within the body of the fish behind the head
  • They are made of gill filaments. These gill filaments are stacked up in a pile
  • At a right angle to the filaments are the gill lamellae which increase the surface area of the gills
  • Water is taken through the mouth and is forced over the gills and out through an opening on each side.


The gills of a fish are efficient at gas exchange as they have a large surface area with each lamellae. They also have a short diffusion pathway which further increases the gas exchange efficiency. The countercurrent flow maintains a high concentration gradient throughout, with the circulatory system moving the blood saturated with oxygen away with the ventilated water being replaced with new water, all to ensure gas exchange is effective and efficient.

Countercurrent Flow

Countercurrent flow means the blood flows in the opposite direction to the flow of water

The concentration of oxygen in the water compared to the concentration of oxygen in the blood is always higher, therefore a concentration gradient is maintained right across the surface. As seen above, when there is 100% oxygen saturation in the water when it enters, the blood is almost as saturated at 90% so there is a concentration gradient. At the next stage where the oxygen has donated 10% to the blood, the water now has 90% but the blood has 80% so again there is a concentration gradient. This continues along the entire surface. Equilibrium is never reached and diffusion of oxygen from the water into the blood is constantly occurring

Concurrent Flow (parallel System)

Concurrent flow means the blood flows in the same direction to the flow of water

When the blood flows in the same direction as the blood the concentration gradient diminishes to a point where the percentage of oxygen is equal in the blood and the water. Equilibrium has been reached, and therefore there is no concentration gradient so there is no net movement of oxygen.



A potometer is used in measuring the rate of transpiration of a plant



  1. A leafy shoot is cut whilst underwater to ensure that water does not touch the leaves
  2. The potometer is filled with water with no air bubbles inside
  3. Using a rubber tube, the leafy shoots are fitted to the potometer whilst underwater
  4. The potometer is removed from the water and all the joints are sealed with waterproof jelly
  5. An air bubble is introduced into the capillary tube
  6. The distance the bubble travels is then measured over a set time
  7. Using the mean result, the volume of water lost is calculated
  8. Once the air bubbler reaches the end of the rube, the reservoir tap can be opened to reset the apparatus
  9. The volume of water taken can be calculated using:

The potometer is only an estimation for the transpiration as it does not account for water used through photosynthesis, however only 1% of water used by a plant is the result of photosynthesis

To maintain fair conditions, ensure that either the same leave, or similar sized leave in reference to surface area or used.

e.g. In an experiment the distance the bubble moved was 15.28mm2 during 1mminute in a 0.5mm radius capillary tube. Calculate your answer in cm3 h-1

π x 0.52 x 15.28 = 12.001mm3

12.001 x 60 (minutes) = 720.01 mm3h-1

720.01 / 1000 = 0.72cm3 h-1

Transport of Organic Substances in the Phloem

Transport of Organic Substances in the Phloem

The phloem transports dissolved products of photosynthesis in various directions around the plant

Phloem Structure

  • The individual sieve tube elements that make up the phloem are made of cells; however, they do not have a nucleus and have few organelles. They are more just strands of cytoplasm.
  • The ends of the cell walls form sieve plates through which cytoplasm can pass through. This differs to the xylem as it has cell walls that disappear at the ends
  • As the sieve tubes have minimal elements to keep it alive, it is aided by the companion cells which respire, excrete etc on the cells behalf.
  • The cytoplasm of the companion cells and their sieve tube elements is joined through pores on the side walls.


  • Translocation is the transport of soluble organic substances (assimilate) within a plant

Mass Flow Hypothesis

  • Solute transport occurs in plants. Any part that produces sucrose is produced in plants is known as the source, and any area the consumes the sucrose is the sink. Sucrose is actively transported into the sieve tubes at the source (e.g. leaves/roots) lowering the water potential inside the sieve and so water enters the tubes through osmosis, thus creating a higher pressure inside the sieve tubes at the source At the sink sugar leaves the phloem to be used up increasing the water potential inside the sieve tubes. Therefore, water leaves via osmosis lowering the pressure inside the sieve tubes. A pressure gradient from the source to the think pushing sugar to where they are required.
For Against
Sap oozes out, showing there is a pressure gradient inside the plant Sugar travels to many different sinks
Equal water potential throughout the plant Sieve plates are barriers to mass flow
ATP is present in the phloem since it is an element for active transport The sieve plates are living, whereas there is no reason for them to be according to mass flow.
Phloem sap has a high pH, due to H+ ions being actively transported  

Gas Exchange in Insects

Gas Exchange in Insects


  • Insects have tracheae which allow for gas exchange but also prevents water loss
  • The gases enter through the open spiracles along a concentration gradient
  • The tracheae move the oxygen to call cells which are closely associated with cells
  • The tracheae divide into small dead-end tubes called tracheoles. The tracheoles due to their size are able to distribute oxygen throughout the body
  • Air is drawn into the insect as oxygen is moved into the cells, so there is an indifference in the concentration gradient; so, oxygen is drawn into the tracheae system. Carbon dioxide leaves through the same method as there is a high concentration of carbon dioxide inside the insect and low on the outside.
  • The spiracles are able to open and close as they need to control water loss
  • The end of each tracheole is filled with water, as when the muscles anaerobically respire lactose which is soluble and lowers the water potential. Water therefore moves into the cell from the tracheoles through osmosis. The water is then reduced, so more air can be in the tracheae. The rate of reaction is much faster, however more water is being used.


The lower internal amount of oxygen inside the tracheae causes the spiracles to open to allow for oxygen to enter and the Carbon Dioxide to leave. The spiracles are not constantly open to reduce water loss. The Carbon dioxide being too great in the tracheae causes the spiracles to open.

Oxygen concentration within the insect caused by the aerobic respiration of each cells causes oxygen to move into each cell leaving a low concentration inside the insect, so therefore oxygen is drawn in. This same process allows for carbon dioxide to leave

The spiracles open to their full size but only for a short period of time to maximise oxygen intake and carbon dioxide outtake but minimise water loss.