How Transport Media Change Around the Organism

HSC Biology | Free Study Notes

In this lesson

• how blood composition changes in animals

• how transport changes in plants

• how nutrients move around the organism

• how gases move around the organism

• how wastes are carried and removed

What is a transport medium?

A transport medium is the substance that carries materials from one part of an organism to another.

Examples

• In animals, the main transport medium is blood.

• In plants, substances move in xylem sap and phloem sap.

Why transport media change

The composition of the transport medium changes because materials are:

• added in some parts of the organism

• removed in other parts

• used by cells

• produced as wastes

This means the transport medium does not stay exactly the same all the way around.

Blood composition changes in animals

In animals, blood changes as it passes different exchange surfaces and organs.

Nutrient movement in blood

After digestion, nutrients are absorbed into the blood from the small intestine.

What is added

Blood leaving the small intestine contains increased levels of:

• glucose

• amino acids

• other absorbed nutrients

• water and mineral ions

What happens next

These nutrients are then transported to body cells, where they may be:

• used in respiration

• used for growth and repair

• stored

So blood reaching tissues contains nutrients, but blood leaving active tissues may contain lower concentrations because the cells have taken some up.

Gas movement in blood

Blood composition also changes as it passes gas exchange surfaces and body tissues.

At the lungs

In mammals:

• oxygen enters the blood

• carbon dioxide leaves the blood

So blood leaving the lungs is:

• higher in oxygen

• lower in carbon dioxide

At body tissues

As blood passes through tissues:

• oxygen leaves the blood and diffuses into cells

• carbon dioxide enters the blood from respiring cells

So blood leaving tissues is:

• lower in oxygen

• higher in carbon dioxide

Waste movement in blood

Blood also carries wastes away from cells.

Examples of wastes carried in blood

• carbon dioxide

• nitrogenous wastes such as urea

• excess water in some contexts

How this changes blood composition

As blood passes body tissues:

• wastes are added to the blood

As blood passes organs involved in waste removal:

• some wastes are removed from the blood

For example:

• the lungs remove carbon dioxide

• the kidneys remove urea, excess salts and water

So blood leaving excretory organs has a different composition from blood entering them.

Blood does not stay chemically identical

The key idea is that blood is always moving substances:

• into tissues

• out of tissues

• to exchange surfaces

• away from exchange surfaces

Because of that, its composition changes continuously around the circulation.

Plant transport changes

Plants do not use blood, but their transport media also change as substances move through the organism.

Xylem transport changes

Xylem transports:

• water

• dissolved mineral ions

Where substances enter

Water and mineral ions enter through the roots.

So xylem near the roots contains water and minerals absorbed from the soil.

What happens as xylem moves upward

As water moves up through the plant:

• some is used by cells

• some is lost by transpiration from the leaves

So the amount of water in the xylem changes as it moves through the plant.

Key point

Xylem mainly carries raw materials needed by the leaves, especially water for photosynthesis.

Phloem transport changes

Phloem transports:

• sugars, mainly sucrose

• other dissolved organic substances

Where substances enter

Sugars are added to the phloem in the leaves, where photosynthesis occurs.

Where substances leave

Sugars are removed from the phloem in places where they are:

• used in respiration

• used for growth

• converted to storage products

These areas may include:

• roots

• stems

• fruits

• growing tissues

Key point

This means phloem composition changes as sugars are loaded into it in leaves and unloaded from it in other parts of the plant.

Nutrient movement

In animals

In animals, nutrients:

• enter the blood after absorption in the digestive system

• are transported to body cells

• are removed from the blood as cells take them up

So blood nutrient levels change depending on where the blood is in the body.

In plants

In plants, nutrients move differently:

• water and mineral ions move mainly in xylem

• sugars produced by photosynthesis move mainly in phloem

So plants use two main transport pathways rather than one main transport medium like blood.

Gas movement

In animals

In animals:

• oxygen is added at respiratory surfaces

• oxygen is removed by body tissues

• carbon dioxide is added by body tissues

• carbon dioxide is removed at respiratory surfaces

In plants

Plants mainly exchange gases by diffusion rather than transporting oxygen widely in vascular tissues in the same way animals do with blood.

The main transport changes in plants are more focused on:

• water and mineral ions in xylem

• sugars in phloem

Waste movement

In animals

In animals, wastes such as carbon dioxide and urea are added to the blood by cells and later removed at organs such as the lungs and kidneys.

In plants

In plants, waste handling is different. For this Module 2 page, the main focus is on:

• water loss through transpiration

• movement of useful materials through xylem and phloem

Comparing plant and animal transport changes

Feature	Animals	Plants
Main transport medium	Blood	Xylem sap and phloem sap
Main nutrient additions	Small intestine	Roots and leaves
Main gas changes	Lungs and tissues	Mostly gas exchange by diffusion
Main waste changes	Tissues, lungs, kidneys	Water loss through transpiration, sugar movement and usage

Why this idea matters

This topic helps students see that transport systems are not just pipes carrying the same fluid everywhere.

Instead, transport media are dynamic. Their composition changes depending on:

• where substances enter

• where substances are used

• where wastes are produced

• where wastes are removed

That is the central idea in this Module 2 inquiry question.  

Worked example

Exam-style question

Explain why the composition of blood changes as it moves from the lungs to body tissues.

Worked answer

At the lungs, oxygen diffuses into the blood and carbon dioxide diffuses out, so blood leaving the lungs has more oxygen and less carbon dioxide. As the blood moves through body tissues, oxygen is taken up by cells for respiration and carbon dioxide is added as a waste product, so its composition changes.

Why this works

This answer:

• identifies where the change happens

• explains what is added and removed

• links the change to respiration

Common mistakes

• Saying blood has the same composition everywhere in the body.

• Forgetting that nutrients are added to blood after absorption in the small intestine.

• Saying xylem transports sugars.

• Saying phloem transports only wastes.

• Treating plant and animal transport changes as identical.

Quick quiz

1. What is a transport medium?

2. Why does blood leaving the lungs differ from blood leaving body tissues?

3. Where are nutrients added to blood in mammals?

4. What is added to phloem in leaves?

5. Why does the composition of transport media change as they move around an organism?

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