Chapter 7: Mass transport

Question 1

What is haemoglobin?

Answer 1

A large protein found in red blood cells

Question 2

structure of haemoglobin

Answer 2

• quaternary structure of 4 polypeptide chains

- each with a haem group which contains an iron ion

Question 3

Haemoglobin has a high affinity for oxygen. What does this mean?

Answer 3

It means that each molecule of haemoglobin can carry 4 oxygen molecules

Question 4

When there is a high partial pressure of oxygen what happens?

Answer 4

Haemoglobin’s affinity to oxygen increase so oxygen readily loads onto it to form oxyhaemoglobin. The opposite happens when there is a low partial pressure of oxygen.

Question 5

Why does haemoglobin unload its oxygen at respiring cells?

Answer 5

respiring cells use up oxygen which decreases the partial pressure of oxygen (pO2) so that haemoglobin has a lower affinity for oxygen and releases oxygen which can be used up by the cell

Question 6

When there is a high partial pressure of carbon dioxide what happens?

Answer 6

Haemoglobin’s affinity to oxygen decreases and it readily unloads its oxygen

Question 7

Why do multicellular organisms such as mammals need a specialised transport system to supply their cells with raw materials

Answer 7

because they have a low surface area to volume ratio

Question 8

What is the circulatory system?

Answer 8

The circulatory system is a specialised transport system made up of the heart and blood vessels that carries raw materials from specialised exchange organs to the body cells

Question 9

The heart has its own blood supply. Which vessels supply the heart itself with blood?

Answer 9

The left and right coronary arteries supply the heart with blood.

Question 10

there are two circuits to the circulatory system. describe them

Answer 10

one circuit takes blood from the heart to the lungs and then back to the heart

the other circuit supplies blood to the rest of the body

Question 11

Xylem tissue transports…

Answer 11

water and mineral ions up from the roots to the leaves

Question 12

Phloem tissue transport…

Answer 12

organic substances in solution up and down the plant

Question 13

Xylem and phloem are both mass transport systems. What does this mean?

Answer 13

They transport substances over long distances

Question 14

Structure of xylem vessels

Answer 14

parts of the xylem tissue that actually move the water and ions
very long tube-like structures
made up of dead cells joined end to end
with no end walls
so uninterrupted/unbroken tube

Question 15

What is the cohesion-tension theory of water transport

Answer 15

water evaporates from the leaves
due to heat from the sun
this creates tension (suction)
which pulls more water into the leaf
cohesion
where water molecules form hydrogen bonds between themselves
means that water molecules stick together
so that a column of water is pulled up the xylem

Question 16

transpiration

Answer 16

Transpiration is the evaporation of water from a plant’s surface esp. leaves

It is a side effect of photosynthesis:
plants need to open their stomata to let in CO2 which is needed for glucose production
but this also lets water out

Question 17

Name the 4 main factors that affect transpiration rate.

Answer 17

light
humidity
temperature
wind

Question 18

Explain how light affects transpiration rate.

Answer 18

more light = faster transpiration rate
positive correlation
stomata open to let in CO2
when light is present
usually close at night

Question 19

Explain how humidity affects transpiration rate.

Answer 19

less humidity = faster transpiration rate
negative correlation
air around the plant is dry
increases concentration gradient

Question 20

Explain how temperature affects transpiration rate.

Answer 20

higher temp = faster transpiration rate
positive correlation
water evaporates from cells faster
increases concentration gradient
diffusion is faster

Question 21

Explain how wind affects transpiration rate.

Answer 21

more wind = faster transpiration rate
positive correlation
more water vapour being removed
steeper diffusion gradient/increases conc. gradient

Question 22

Describe how transpiration works.

Answer 22

water evaporates from the moist cell walls
accumulates in the spaces between cells in the leaf
when the stomata open
water vapour moves out of the leaf
down a concentration gradient

Question 23

translocation

Answer 23

Translocation is the process by which organic molecules such as sugars like sucrose, and amino acids and some mineral ions are transported from one part of a plant to another

Question 24

The Mass Flow Hypothesis

Answer 24

The best supported theory that explains how solutes are transported around a plant by translocation

Question 25

Structure of the phloem

Answer 25

Phloem tissue is made up of sieve tube elements and companion cells

Sieve tube elements:
living cells
no nucleus
few organelles
arranged end to end
ends walls are perforated
form sieve plates

Companion cells:
carry out the living functions for sieve tube elements
e.g. providing energy ATP needed for active transport
one for each sieve tube element

Question 26

Is transpiration a passive process. Explain why?

Answer 26

Yes, transpiration is a passive process because the only energy required is heat energy to evaporate water at the surface of leaves - it does not require metabolic energy.

Question 27

sources

Answer 27

Sources are locations where sugars are made i.e. sucrose is made in the leaves via photosynthesis

Question 28

sinks

Answer 28

Sinks are locations where the sugar will be used directly or stored for later

Question 29

Give 2 objections against the Mass Flow hypothesis of translocation.

Answer 29

The sieve plates would create a barrier to mass flow

Sugar travels to lots of different sinks not just the ones with the highest water potential

Question 30

Briefly describe the stages of the mass flow hypothesis of translocation.

Answer 30

Sucrose is actively transported into the sieve tubes using hydrogen ions/protons and co-transport proteins

Mass flow of sucrose through sieve tube elements occurs down a hydrostatic pressure gradient

Sucrose is then transferred from the sieve tubes into the sink cells

Question 31

Describe how sucrose is transferred into the sieve tube elements.

Answer 31

sucrose is produced in photosynthesising cells
it diffuses down a conc. gradient into companion cells
via facilitated diffusion
hydrogen ions are actively transported from companion cells into the spaces within cell walls using ATP
H+ ions diffuse down their conc. gradient through carrier proteins in the sieve tube elements
sucrose molecule are transported along with the H+ ions by co-transport using co-transport proteins

Question 32

Describe the mass flow of sucrose.

Answer 32

at the sources
sucrose produced during photosynthesis is actively transported into the sieve tubes
this lowers the water potential in the sieve tubes
the xylem has a higher water potential
so water moves from the xylem into the sieve tubes via osmosis
this creates a high hydrostatic pressure inside the sieve tubes

at respiring cells (sinks)
sucrose is either used or converted into starch
the respiring cells therefore have a lower conc. of sucrose
so sucrose is actively transported into the cells from the sieve tubes
this lowers the water potential of the cells
so that water moves into them from the sieve tubes via osmosis
the hydrostatic pressure inside the sieve tubes is lowered

the hydrostatic pressure at the source is higher than at the sink
sucrose solution moves down the hydrostatic gradient in the sieve tube at once
so there is mass flow of sucrose down the sieve tube

Question 33

Describe how sucrose is transferred from the sieve tubes into sink cells.

Answer 33

Sucrose is actively transported out of the sieve tubes and into the sink cells by the companion cells