chapter 7 p1

Question 1

two main reasons why diffusion alone is enough to supply the needs of single-celled organisms:

Answer 1

The metabolic activity of a single-celled organism is usually low, so the oxygen demands and carbon dioxide production of the cell are relatively low.
The surface area to volume (SA: V) ratio of the organism is large

Question 2

Cellular Complexity and Metabolic Challenges in Larger Organisms:

Answer 2

As organisms get larger they can be made up of millions or even billions of cells arranged in tissues, organs, and organ systems.
Their metabolic activity is usually much higher than most single-celled organisms.
For example in a dolphin, the amount of energy used in moving through the water means the oxygen demands of the muscle cells deep in the body will be high and they will produce lots of carbon dioxide.
The distance between the cells where the oxygen is needed and the supply of oxygen is too far for effective diffusion to take place.
the bigger the organism, the smaller the SA: V ratio.
So gases can’t be exchanged fast enough or in large enough amounts for the organism to survive.

Question 3

Surface area: volume ratio - modelling an organism

Answer 3

A sphere is a useful shape for modelling cells or organisms.
A series of simple calculations shows clearly how the SA:V ratio changes as the organism gets bigger, and why size matters so much.

Question 4

worked example of SA:V ratio

Answer 4

Question 5

Specialised exchange surfaces:

Answer 5

Large, multicellular organisms have evolved specialised systems for the exchange of the substances they need and the substances they must remove.
All effective exchange surfaces have certain features in common.

Question 6

summary of the characteristic features of effective exchange surfaces, along with some examples:

Answer 6

Increased surface area:
Thin layers
Good blood supply:
Ventilation to maintain diffusion gradient

Question 7

. Increased surface area:

Answer 7

provides the area needed for exchange and overcomes the limitations of the SA: V ratio of larger organisms.
Examples include root hair cells in plants and the villi in the small intestine of mammals.

Question 8

Thin layers:

Answer 8

these mean the distances that substances have to diffuse are short, making the process fast and efficient.
Examples include the alveoli in the lungs and the villi of the small intestine.

Question 9

Good blood supply:

Answer 9

• the steeper the concentration gradient, the faster diffusion takes place.
• Having a good blood supply ensures substances are constantly delivered to and removed from the exchange surface.
• This maintains a steep concentration gradient for diffusion.
• For example the alveoli of the lungs, the gills of a fish and the villi of the small intestine.

Question 10

Ventilation to maintain diffusion gradient:

Answer 10

for gases, a ventilation system also helps maintain concentration gradients and makes the process more efficient
for example the alveoli and the gills of a fish where ventilation means a flow of water carrying dissolved gases

Question 11

Gaseous exchange surfaces are…

Answer 11

moist, so oxygen dissolves in the water before diffusing into the body tissues.
As a result the conditions needed to take in oxygen successfully are also ideal for the evaporation of water.
Mammals have evolved complex systems that allow them to exchange gases efficiently but minimise the amount of water lost from the body.

Question 12

The human gaseous exchange system:

Answer 12

Mammals are relatively big - they have a small SA: V ratio and a very large volume of cells.
They also have a high metabolic rate because they are active and maintain their body temperature independent of the environment.
As a result they need lots of oxygen for cellular respiration and they produce carbon dioxide, which needs to be removed.
This exchange of gases takes place in the lungs.

Question 13

structure of the human gaseous exchange system diagram

Answer 13

Question 14

Key structures of the human gaseous exchange system

Answer 14

Nasal cavity
Trachea:
Bronchus:
Bronchioles:
Alveoli:

Question 15

Nasal cavity

Answer 15

• a large surface area with a good blood supply, which warms the air to body temperature
• a hairy lining, which secretes mucus to trap dust and bacteria, protecting delicate lung tissue from irritation and infection
• moist surfaces, which increase the humidity of the incoming air, reducing evaporation from the exchange surfaces.
• After passing through the nasal cavity, the air entering the lungs is a similar temperature and humidity to the air already there.

Question 16

Trachea:

Answer 16

• The trachea is the main airway carrying clean, warm, moist air from the nose down into the chest.
• It is a wide tube supported by incomplete rings of strong, flexible cartilage, which stop the trachea from collapsing.
• The rings are incomplete so that food can move easily down the oesophagus behind the trachea.
• The trachea and its branches are lined with a ciliated epithelium, with goblet cells between and below the epithelial cells
• Goblet cells secrete mucus onto the lining of the trachea, to trap dust and microorganisms that have escaped the nose lining.
• The cilia beat and move the mucus, along with any trapped dirt and microorganisms, away from the lungs.
• Most of it goes into the throat and is swallowed and digested.
• One of the effects of cigarette smoke is that it stops these cilia beating.

Question 17

Trachea picture

Answer 17

Question 18

Bronchus:

Answer 18

In the chest cavity the trachea divides to form the left bronchus (plural bronchi), leading to the left lung, and the right bronchus leading to the right lung.
They are similar in structure to the trachea, with the same supporting rings of cartilage, but they are smaller.

Question 19

Bronchioles:

Answer 19

In the lungs the bronchi divide to form many small bronchioles.
The smaller bronchioles (diameter 1 mm or less) have no cartilage rings.
The walls of the bronchioles contain smooth muscle. When the smooth muscle contracts, the bronchioles constrict (close up).
When it relaxes, the bronchioles dilate (open up).
This changes the amount of air reaching the lungs.
Bronchioles are lined with a thin layer of flattened epithelium, making some gaseous exchange possible.

Question 20

Alveoli:

Answer 20

The alveoli (singular alveolus) are tiny air sacs, which are the main gas exchange surfaces of the body. Alveoli are unique to mammalian lungs.
Each alveolus has a diameter of around 200-300 pm and consists of a layer of thin, flattened epithelial cells, along with some collagen and elastic fibres (composed of elastin).
These elastic tissues allow the alveoli to stretch as air is drawn in.
When they return to their resting size, they help squeeze the air out.
This is known as the elastic recoil of the lungs.