Gaseous Exchange, Grade 11: Carbonic Acid Is Not a Transport Form, and Five More Corrections

Carbon dioxide is carried in the blood in three ways, and one of the three on the printed list is not a transport form at all.

The list given is: in solution in the plasma as bicarbonate ions, combined with haemoglobin, and as carbonic acid in the plasma.

Carbonic acid is the step in between. It forms and immediately falls apart into bicarbonate and hydrogen ions, which is why bicarbonate is on the list in the first place. Naming it as a third method leaves out the one that is genuinely missing: carbon dioxide simply dissolved in the plasma.

This page gives all three units, six places where the printed account is wrong or incomplete, a free model of the breathing system you can build from a cool drink bottle, and an honest statement that we have nothing in stock to sell you for this topic.

Where this fits in the curriculum

Subject Life Sciences
Grade 11
Term 3
Topic Topic 8, Gaseous exchange
Units Unit 1 the need for gas exchange. Unit 2 gas exchange in humans. Unit 3 respiratory disorders
Formal assessment None. The smoking graph analysis is a 10 mark activity
Marks Our worksheet is 75

Finding 1: carbonic acid is not one of the three ways

What is printed What it should be
In solution in the plasma as bicarbonate ions Dissolved as carbon dioxide in the plasma, the smallest share
Combined with haemoglobin Combined with haemoglobin, as carbaminohaemoglobin
As carbonic acid in the plasma As bicarbonate ions in the plasma, much the largest share

Two things have gone wrong in one sentence. The first form has had bicarbonate attached to it, which collapses two separate methods into one. The third form is carbonic acid, which is not a method, and in its place the plain dissolved gas has disappeared.

What actually happens, in four steps

  1. Carbon dioxide diffuses from the tissues into the red blood cell
  2. Carbonic anhydrase joins it to water to make carbonic acid
  3. The carbonic acid immediately splits into a hydrogen ion and a bicarbonate ion
  4. The bicarbonate moves out into the plasma, and that is the form it travels in

Carbonic acid exists for a moment inside the red blood cell. It is not what the blood carries. If an exam asks for the three ways carbon dioxide is transported, the answer is dissolved, as bicarbonate, and bound to haemoglobin. Write carbonic acid and you lose the mark.

Finding 2: the activity asks for a number of people from a graph of rates

The smoking graph activity is worth 10 marks and its arithmetic is sound, which is worth saying because a lot of printed tasks are not. Questions 2.1 and 2.2 are the problem.

They ask "how many people who stopped smoking 10 years ago are likely to die from lung cancer", and again for 30 years.

The y-axis of that graph reads "Lung cancer deaths per 1 000".

What the learner reads off About 0,55
What that number is A rate: 0,55 deaths per thousand ex-smokers
What the question asks for A number of people
What a learner writes "0,55 people", which is not a thing

The question cannot be answered as asked, because no population size is given anywhere. Three words fix it: "how many deaths per thousand".

This is worth doing deliberately with a class. Reading a rate as a count is one of the most common data-handling errors in the subject, and here is a printed task that invites it. Ask them what a population of 0,55 people would look like.

And a smaller point about Graph A

Graph A is a straight dashed line from the origin, with no plotted points on it. Question 1.3 then asks what conclusion can be reached "from the data in Graph A".

There is no data in Graph A. There is a trend line. The conclusion is still available, that risk rises roughly in proportion to the number smoked, but a learner should be told that a line without points is somebody's summary of data, not the data.

Finding 3: tuberculosis gets one word

Unit 3.1 is tuberculosis. It is the first and largest part of the respiratory disorders unit.

In the unit summary, TB appears once, as the first item in a list of six diseases, and nothing else is said about it.

What the summary says about TB What a Grade 11 learner here needs
It is a disease that affects the respiratory system It is caused by a bacterium, so it is treated with antibiotics, unlike most of the other five
The treatment course runs about six months and has to be finished, because stopping early is how drug-resistant TB is made
South Africa carries one of the heaviest TB burdens in the world. This is not a disease from somewhere else
It is spread through the air, which is exactly why it belongs in a topic about the breathing system

Listing TB alongside hay fever, as though the two were the same kind of problem, is the part that does not work. One is an allergy. The other killed more South Africans than anything else for years.

Finding 4: respiration defined so as to exclude the previous topic

"Respiration uses oxygen to break down sugars, to provide the body with energy."

That is aerobic respiration. The topic immediately before this one teaches anaerobic respiration, which breaks down sugar without oxygen and has its own assessed practical.

Aerobic Anaerobic
Oxygen Required Not required
Products in humans Carbon dioxide and water Lactic acid
Products in yeast Carbon dioxide and water Ethanol and carbon dioxide
Energy released A great deal Much less
In the printed definition Yes No

The fix is one word. Respiration is the release of energy from food in cells. Say "aerobic respiration uses oxygen" and both halves of the course survive.

Finding 5: the breathing system without the muscles that drive it

"The human breathing system consists of the lungs and the air passages."

The very next bullet says: "Breathing is brought about by contraction and relaxation of the diaphragm and intercostal muscles."

So the thing that makes breathing happen is not part of the breathing system.

Part What it does In the printed definition
Nose, trachea, bronchi, bronchioles Carry, warm, moisten and filter the air Yes
Lungs and alveoli Where the exchange happens Yes
Diaphragm Contracts and flattens to pull air in No
Intercostal muscles Raise the rib cage No
Rib cage and pleural membranes Protect the lungs and seal the chest cavity No

The lungs have no muscle of their own. They cannot inflate themselves. Leave the diaphragm out of the system and there is no mechanism left in it.

Finding 6: smoking damages the alveoli, and that is the whole sentence

The entire account of smoking in the summary is: "Smoking damages the alveoli and affects gaseous exchange."

True, and it does not explain anything. The mark is in the mechanism, and the mechanism is a surface area argument the learner has already met in Unit 1.

Step What happens
1 Smoke damages the very thin walls between neighbouring alveoli
2 The walls break down and small alveoli merge into fewer, larger spaces
3 Total surface area for gas exchange falls sharply. This is emphysema
4 Less oxygen can diffuse into the blood per breath, so the person is breathless on the smallest effort

Unit 1 says a gas exchange surface must be large. Unit 3 is the case where it stops being large. Teach them as the same idea and emphysema stops being a word to memorise.

Smoke also paralyses the cilia that sweep mucus and trapped particles out of the airways, which is why a smoker's cough exists and why infections settle more easily.

The part the material gets exactly right

"Breathing is under homeostatic control of the respiratory centre in the brain stem, which regulates the levels of carbon dioxide in the blood."

Carbon dioxide, not oxygen. This is the single most misremembered fact in the topic, and the summary states it correctly and plainly.

When you hold your breath, what finally forces you to breathe is rising carbon dioxide, not falling oxygen. The body monitors the waste gas because its concentration changes far more sharply and far sooner. Learn it with the breath-holding example and it never goes away.

What the three units cover

Unit What is in it
1. The need for gas exchange Cellular respiration against breathing against gas exchange, the requirements of an efficient exchange surface, surface area to volume ratio, and exchange in different organisms
2. Gas exchange in humans The breathing system, inhalation and exhalation, exchange at the alveoli and at the tissues, transport of both gases, altitude, and artificial respiration
3. Respiratory disorders Tuberculosis, other disorders, and the effects of smoking

The four requirements of a gas exchange surface, which everything else hangs off

  1. Large, so that enough gas can cross at once
  2. Thin, so the diffusion distance is short
  3. Moist, because gases must dissolve before they can diffuse across
  4. Well ventilated and well supplied with blood, so a steep concentration gradient is maintained

Every alveolus question is one of these four. So is the emphysema question, and so is the reason an earthworm needs no lungs.

Build the breathing system for nothing

You do not need a model and we do not have one to sell you. This one takes ten minutes and demonstrates the pressure argument better than a plastic replica does.

You need Stands for
A 2 litre cool drink bottle with the bottom cut off The chest cavity and rib cage
A drinking straw pushed through the cap, sealed with tape or glue The trachea
A small balloon tied over the inside end of the straw A lung
A second balloon cut open and stretched over the cut bottom The diaphragm

Pull the bottom balloon down and the lung inflates. Let it go and the lung empties. Nobody blows into anything, which is the point.

Ask the class the question the model is for: did the lung fill because you pushed air in, or because you made the space round it bigger? Air was pushed in by atmospheric pressure, because the pressure inside dropped. That is the whole of inhalation, and a learner who builds it rarely gets it wrong again.

The model has one honest limitation and you should say so: a real diaphragm is a dome that flattens, not a flat sheet that is pulled down, and the real rib cage moves as well. The pressure argument is right; the geometry is simplified.

What this topic costs

Nothing, and we are not going to pretend otherwise.

What a school might want Our position
A heart, lungs and larynx model We list one at R2 800,00 and it is out of stock. We are not linking you to it
A torso model showing the lungs in place All six torso models we list are out of stock
Lung tissue under a microscope Our histology slide sets may carry a section. We have not verified the slide list, so we are not claiming it
The bottle model above A bottle, a straw, two balloons and some tape. R0,00

Microscopes at microscopes, which is the only link on this page that leads to something we can actually ship.

What you should be able to do by the end

  • Separate breathing, gas exchange and cellular respiration in one sentence each
  • Give the four requirements of a gas exchange surface and use them to explain any example
  • Explain surface area to volume ratio and why a small organism needs no lungs
  • Describe inhalation and exhalation in terms of muscles, volume and pressure
  • Describe an alveolus and explain every feature in terms of the four requirements
  • Give the three ways carbon dioxide is carried, without carbonic acid among them
  • Explain why the respiratory centre monitors carbon dioxide
  • Explain emphysema as a surface area problem
  • Read a rate off a graph without turning it into a count of people

Free worksheet and marking memo

Both free, no sign up, straight to the PDF.

Related practicals