B7 Mass Transport

Question 1

Relate the structure of arteries to their function.

Answer 1

Have thick muscular walls to handle high-pressure, and are elastic to control blood flow

Question 2

Describe how the structure of veins relate them to their function

Answer 2

Thinner walls and wider lumen due to lower pressure. Less muscular and elastic fibres as they don’t have to control blood flow. Contain valves

Question 3

Name the nodes involved in heart contraction

Answer 3

Sinoatrial Node (SAN)
Atrioventricular Node (AVN)

Question 4

Where is the SAN located?

Answer 4

Wall of right atrium

Question 5

Where is the Atrioventricular Node located?

Answer 5

Between the two atria.

Question 6

Explain how the heart contracts

Answer 6

SAN initiates and spreads impulse across the atria, so the atria contract
- AVN receives, delays, and then conveys impulse down the Bundle of His
- Impulse travels into the Purkyne fibres which branch across the ventricles

Question 7

Why does the impulse need to be delayed?

Answer 7

Needs time for all blood to pass through and for valves to close

Question 8

How is the structure of capillaries related to their function?

Answer 8

One cell thick - short diffusion pathway
Very narrow - RBC lie flat
Numerous and highly branched, Large SA

Question 9

How is water transported in plants?

Answer 9

Xylem: Long, continuous columns which also provide structural support

Question 10

What is the xylem made out of?

Answer 10

Lignin - waterproof polymer

Question 11

Explain the cohesion-tension theory.

Answer 11

Cohesion - water molecules form hydrogen bonds with each other, causing them to stick together
Tension - water molecules form Hydrogen bonds with the side of xylem vessel elements - draws water up the stem

Question 12

Describe sieve tube elements.

Answer 12

Form a tube to transport sucrose in the dissolved form of sap

Question 13

What are the sieve tube elements made from?

Answer 13

Cellulose

Question 14

Describe companion cells.

Answer 14

• Connected to sieve tube elements via plasmodesmata
• Have a nucleus

Question 15

What is plasmodesmata?

Answer 15

Allows the cytoplasm to be shared between companion cells and sieve tube elements.

Question 16

What are sieve plates?

Answer 16

At either end of the sieve tube elements and contain large pores to allow sap to move through

Question 17

How does sucrose in the leaf enter the phloem?

Answer 17

Sucrose enters companion cells via active transport. This involves the co-transport of hydrogen ions with sucrose. Sucrose then diffuses into the sieve tube elements via the plasmodesmata.

Question 18

How do phloem vessels transport sucrose around the plant?

Answer 18

The water potential is reduced in the phloem - water diffuses into the phloem via osmosis due to the water potential gradient.
Unloading of sucrose - sink cells

Question 19

What is the evidence for the mass flow theory?

Answer 19

• Sap is released when a stem is cut, therefore there must be pressure
• There is a higher concentration of sucrose in the leaves than in the roots
• Increasing sucrose levels in the leaves increases sucrose levels in the phloem

Question 20

What is the evidence against the mass flow theory?

Answer 20

The structure of sieve tubes seem to hinder mass flow