2.9.1 Excretion

Question 1

Many of the necessary metabolic reactions that take place within the cells of organisms produce, what?

Answer 1

waste products

Question 2

What is excretion

Answer 2

Excretion is the removal of the waste substances of metabolic reactions, toxic materials and substances in excess of requirements

Question 3

Metabolic wastes produced by the human body include:

Answer 3

Carbon dioxide and water from aerobic respiration in cells

Urea produced by the breakdown of excess proteins (amino acids) in the liver

Other substances in excess such as medical drugs (e.g. antibiotics) and dietary minerals (e.g. sodium)

Question 4

If waste products are allowed to build up they can have a range of negative effects on the body, such as?

Answer 4

1. Toxicity – waste products can have toxic effects if they are allowed to reach high concentrations

• Concentrated body fluids can cause water to move out of cells, changing their water potential and preventing them from carrying out essential reactions

Question 5

Exam Tip

Answer 5

Be careful not to get confused between excretion and egestion, they are two very different things!

Egestion is the expulsion of food that has never been absorbed from the body (as faeces) via the anus.

Question 6

Within plant cells there is a range of metabolic reactions taking place producing, what?

Answer 6

waste products

Question 7

Some of these waste products can be used up in other processes in the plant while some must exit the plant via, what?

Answer 7

the leaf organ

Question 8

Waste products or substances in excess within a plant can include, what?

Answer 8

Oxygen
Carbon dioxide
Water/water vapour
Other unwanted chemical substances

Question 9

Name 2 reactants and waste products in plants

Answer 9

Oxygen and carbon dioxide

Question 10

Name something that affects the amount of waste products in plants

Answer 10

The amount or intensity of light

Question 11

During the day, when there is sufficient light:

Answer 11

1. The rate of photosynthesis is higher than the rate of respiration

Question 12

During the night, when there is insufficient light

Answer 12

1. There is no photosynthesis, only respiration

Question 13

Whichever gas is in excess diffuses out of the plant via, what?

Answer 13

the leaf organ

• The gases exit through the stomata

Question 14

As the excretion of gases in plants occurs via diffusion it is technically, what?

Answer 14

not an active process

Question 15

The majority of water vapour lost from a plant is not a waste product of metabolism, but instead, what?

Answer 15

water that has been drawn up from the roots in the transpiration stream

Question 16

Plant cells can break down molecules into, what?

Answer 16

chemical substances no longer required by the plant

Question 17

Some of these substances cannot be converted into another useful compound and so what must happen?

Answer 17

must be removed from the plant

• Chemical waste materials such as this can be stored in the dying tissues of a plant

Question 18

When the dying tissue falls off the plant the substances are removed, give an example.

Answer 18

in autumn the leaves of deciduous trees turn a variety of colours due to the presence of chemical waste products