exchange surfaces

Question 1

the need for exchange surfaces

Answer 1

Single-celled organisms have a high SA:V ratio which allows for the exchange of substances to occur via simple diffusion

The large surface area allows for maximum absorption of nutrients and gases and secretion of waste products

The small volume means the diffusion distance to all organelles is short

As organisms increase in size their SA:V ratio decreases
There is less surface area for the absorption of nutrients and gases and secretion of waste products

The greater volume results in a longer diffusion distance to the cells and tissues of the organism

Large multicellular animals and plants have evolved adaptations to facilitate the exchange of substances between their environment

They have a large variety of specialised cells, tissues, organs and systems
Eg. gas exchange system, circulatory system, lymphatic system, urinary system, xylem and phloem

Question 2

the need for a specialised system for gas exchange

Answer 2

Supply of Oxygen:
Organisms require ATP in order to carry out the biochemical processes required for survival. The majority of ATP is produced through aerobic respiration which requires oxygen

Removal of Carbon Dioxide:
Carbon dioxide is a toxic waste product of aerobic respiration
If it accumulates in cells/tissues it alters the pH

Question 3

diffusion for single celled organisms vs multicellular organisms

Answer 3

Chlamydomonas is a single-celled organism that is found in fresh-water ponds. It is spherical in shape and has a diameter of 20μm. Oxygen can diffuse across the cell wall and membrane of the Chlamydomonas

The maximum distance that oxygen molecules would have to diffuse to reach the centre of a Chlamydomonas is 10μm, which would only take 100 milliseconds

If the diffusion distance increased to 15cm the diffusion time would increase substantially to 7 hours

This demonstrates how diffusion is a viable transport mechanism for single-celled organisms but not for larger multicellular organisms
The time taken for oxygen to diffuse from the cell-surface membrane to the tissues would be too long

Question 4

the relationship between surface area: volume ratio and metabolic rate

Answer 4

The metabolic rate of an organism is the amount of energy expended by that organism within a given period of time

The basal metabolic rate (BMR) is the metabolic rate of an organism when at rest. The BMR is significantly lower than when an organism is actively moving

During periods of rest, the body of an organism only requires energy for the functioning of vital organs such as the lungs, heart and brain

The metabolic rate of an organism can be measured/estimated using different methods and apparatus:
Oxygen consumption (respirometers)
Carbon dioxide production (carbon dioxide probe)
Heat production (calorimeter)

Question 5

body mass

Answer 5

Experiments conducted by scientists have shown that the greater the mass of an organism, the higher the metabolic rate

Therefore, a single rhino consumes more oxygen within a given period of time compared to a single mouse

Although metabolic rate increases with body mass the BMR per unit of body mass is higher in smaller animals than in larger animals

Smaller animals have a greater SA:V ratio so they lose more heat, meaning they have to use up more energy to maintain their body temperature

Question 6

features of exchange surfaces

Answer 6

Effective exchange surfaces in organisms have a:
Large surface area
Short diffusion distance (thin)
Good blood supply
Ventilation mechanism

Question 7

increased surface area (of a root hair cells)

Answer 7

Many exchange surfaces within organisms have adaptations that increase their surface area

A larger surface area provides more space over which the exchange of substances with the environment can occur

Root hair cells are specialised cells found in the roots of plants. They play an important role in the absorption of water and mineral ions from the soil
Root hair cells have a root hair that increases the surface area (SA) so the rate of water uptake by osmosis is greater (can absorb more water and ions than if SA were lower)

Question 8

short diffusion distance in the alveoli

Answer 8

The exchange of oxygen and carbon dioxide occurs between the alveoli and the capillaries in the lungs

Oxygen and carbon dioxide are exchanged in a process of simple diffusion; (passive movement from high to low concentration)
The air in the alveoli contains a high concentration of oxygen.

The oxygen diffuses from the alveoli and into the blood capillaries, before being carried away to the rest of the body for aerobic respiration

The blood in the capillaries has a relatively low concentration of oxygen and a high concentration of carbon dioxide. The carbon dioxide diffuses from the blood and into the alveoli and is then exhaled
The walls of the alveoli are only one cell thick and these cells are flattened

This means that gases have a very short diffusion distance so gas exchange is quick and efficient

Question 9

alveoli adaptions

Answer 9

Large number of alveoli
The average human adult has around 480 – 500 million alveoli in their lungs. This equals a surface area of 40 – 75 m2
The large number of alveoli increases the surface area available for oxygen and carbon dioxide to diffuse across

Extensive capillary network
The walls of the capillaries are only one cell thick and these cells are flattened, keeping the diffusion distance for gases short
The constant flow of blood through the capillaries means that oxygenated blood is brought away from the alveoli and deoxygenated blood is brought to them
This maintains the concentration gradient necessary for gas exchange to occur

Question 10

good blood supply in the gills

Answer 10

In order for the diffusion of a substance across an exchange site to continue for a prolonged period of time, the concentration gradient must be maintained

An adequate blood supply helps to maintain a concentration gradient as it is continuously flowing, bringing substances that have just entered the blood away from the exchange site

Fish gills are adapted to directly extract oxygen from water as they have a large capillary network
The extensive capillary system that covers the gills ensures that the blood flow is in the opposite direction to the flow of water - it is a counter-current system

The counter-current system ensures the concentration gradient is maintained along the whole length of the capillary

The water with the lowest oxygen concentration is found adjacent to the most deoxygenated blood

Question 11

ventilation mechanism in mammalian lungs

Answer 11

A ventilation mechanism also helps to maintain a concentration gradient across an exchange surface

Ventilation (mass flow of gases) in the lungs helps to ensure that there is always a higher concentration of oxygen in the alveoli than in the blood

The movements involved in breathing causes the air in the alveoli to change. Breathing removes air with low amounts of oxygen and high amounts of carbon dioxide and replaces it with air that has high amounts of oxygen and low amounts of carbon dioxide

Question 12

tissues of the gas exchange system

Answer 12

There are a number of different tissue types present in the mammalian gas exchange system
Each tissue is structurally adapted to perform a very specific purpose

Ciliated epithelial cells, goblet cells and mucous glands play vital roles in maintaining the health of the gas exchange system
Cartilage, smooth muscle, elastic fibres and squamous epithelial tissue all play important structural roles in maintaining the gas exchange system

Question 13

cartilage

Answer 13

Cartilage is a strong and flexible tissue found in various places around the body
One place is in rings along the trachea, called Tracheal rings
These rings help to support the trachea and ensure it stays open while allowing it to move and flex while we breathe

Question 14

ciliated epithelium

Answer 14

Ciliated epithelium is a specialised tissue found along the trachea down to the bronchi
Each cell has small projections of cilia which sweep mucus, dust and bacteria upwards and away from the lungs and the epithelium itself

Question 15

goblet cells

Answer 15

Goblet cells can be found scattered throughout the ciliated epithelium in the trachea

They are mucus-producing cells that secrete viscous mucus which traps dust, bacteria and other microorganisms and prevents them from reaching the lungs

The mucus is then swept along by the cilia of the ciliated epithelium upwards and is swallowed
The mucus and any microorganisms will then be destroyed by the acid in the stomach

Question 16

squamous epithelium

Answer 16

The alveoli have a lining of thin and squamous epithelium, that allows for gas exchange
The squamous epithelium forms the structure of the alveolar wall and so is very thin and permeable for the easy diffusion of gases

Question 17

smooth muscle

Answer 17

Smooth muscle can be found throughout the walls of the bronchi and bronchioles
It helps to regulate the flow of air into the lungs by dilating when more air is needed and constricting when less air is needed

Question 18

elastic fibres

Answer 18

Elastic fibres are present in all lung tissues. They are very important as they enable the lung to stretch and recoil. This ability to recoil is what makes expiration a passive process

Question 19

capillaries

Answer 19

Each alveolus is surrounded by an extensive network of capillaries
Carbon dioxide diffuses out of the capillaries and into the alveoli to be exhaled, while oxygen diffuses the other way from alveoli and into the capillaries to be carried around the body

These capillaries have a diameter of around 3-4µm, which is only wide enough for one red blood cell to travel through at any one time
This ensures that there is sufficient time and opportunity for gas exchange to occur

Question 20

trachea

Answer 20

Trachea
The trachea is the channel that allows air to travel to the lungs

C-shaped rings of cartilage ensure that this air channel remains open at all times
They are C-shaped to prevent any friction from rubbing with the oesophagus located close behind

The trachea is lined with ciliated epithelium
There is a substantial covering of mucus inside the trachea (produced by goblet cells and mucous glands) that helps to trap dust and bacteria to prevent them from entering the lungs

The wall of the trachea contains smooth muscle and elastic fibres

Question 21

bronchi

Answer 21

Bronchi have a similar structure to the trachea but they have thinner walls and a smaller diameter

The cartilage in the bronchi does not form a c-shape, but can form full rings, and can also form irregular blocks

Question 22

bronchioles

Answer 22

Bronchioles are narrow self-supporting tubes with thin walls
They are not usually supported by cartilage, though a few bronchioles may contain some cartilage

A large number of bronchioles are present in the gas exchange system

Bronchioles are lined with ciliated epithelium in the same way as the trachea and bronchi, though the usually do not contain any goblet cells

Bronchioles vary in size and structure, getting smaller as they get closer to the alveoli

The larger bronchioles possess elastic fibres and smooth muscle that adjust the size of the airway to increase or decrease airflow

The smallest bronchioles do not have any smooth muscle but they do have elastic fibres

Question 23

alveoli

Answer 23

Groups of alveoli are located at the ends of the bronchioles
The alveolar wall consists of a single layer of epithelium
Elastic fibres are located in the extracellular matrix
There is an extensive capillary network
A watery fluid lines the alveoli, facilitating the diffusion of gases

Question 24

ventilation in mammals

Answer 24

Gas exchange in the lungs requires a concentration gradient

Ventilation (mass flow of gases) in the lungs and the continuous flow of blood in the capillaries helps to ensure that there is always a higher concentration of oxygen in the alveoli than in the blood

The movements involved in breathing causes the air in the alveoli to change, which supplies fresh oxygen and takes away carbon dioxide
The oxygen in the alveoli diffuses into the red blood cells which are rapidly carried away in the blood and replaced by oxygen-depleted red blood cells

Exercise causes oxygen demands to increase which can be facilitated by an increased rate of ventilation

Question 25

passage of air

Answer 25

Nose / mouth
Trachea (windpipe)
Bronchi
Bronchioles
Alveoli

Question 26

breathing in inspiration

Answer 26

The breathing-in process causes the volume in the chest to increase and the air pressure in the lungs to decrease until it is slightly lower than the atmospheric pressure

As a result, air moves down the pressure gradient and rushes into the lungs

Mechanism when at rest:
The diaphragm contracts and flattens, increasing chest volume

Mechanism when exercising
In addition to the flattening of the diaphragm the external intercostal muscles contract, causing the ribcage to move upwards and outwards

Question 27

breathing out expiration

Answer 27

When at rest breathing out occurs mostly due to the recoil of the lungs after they have been stretched

Volume in the chest decreases and pressure increases, causing air to be forced out

Mechanism when at rest:
External intercostal muscles relax
The recoil of elastic fibres surrounding alveoli causes the air to be forced out
Diaphragm becomes dome-shaped

Mechanism when exercising:
Internal intercostal muscles contract to pull the ribs down and back
Abdominal muscles contract to push organs upwards against the diaphragm, increasing the internal pressure
This causes forced exhalation

Question 28

measuring breathing

Answer 28

There are four main ways that breathing can be scientifically measured. These include:

Vital capacity - this is the maximum volume of air that can be breathed in or out in one breath

Tidal volume - this is the volume of air that is breathed in or out during normal breathing (at rest)

Breathing rate - this is the number of breaths taken in one minute (one breath = taking air in and breathing it back out again)

Oxygen uptake - this is the volume of oxygen used up by someone in a given time

Question 29

spirometers

Answer 29

The breathing measurements described above can all be made using a piece of apparatus known as a spirometer

The person (subject) being examined breathes in and out through the spirometer

Carbon dioxide is absorbed from the exhaled air by soda lime in order to stop the concentration of carbon dioxide in the re-breathed air from getting too high, as this can cause respiratory distress

As the subject breathes through the spirometer, a trace is drawn on a rotating drum of paper or a graph is formed digitally, which can be viewed on a computer

From this trace, the subject’s vital capacity, tidal volume and breathing rate can all be calculated
Oxygen uptake can also be calculated using a spirometer

Carbon dioxide is removed from the exhaled air, meaning that the total volume of air available in the spirometer gradually decreases, as oxygen is extracted from it by the subject’s breathing
This change in volume is used as a measure of oxygen uptake

Question 30

analysing data from a spirometer

Answer 30

The results from a spirometer (either in the form of a trace drawn on graph paper or a digital graph created by a computer) can be used to calculate vital capacity, tidal volume and breathing rate. This is shown in the image below
A small amount of air, known as the residual volume, is always retained in the lungs

Question 31

tracheal system of an insect

Answer 31

All insects possess a rigid exoskeleton with a waxy coating that is impermeable to gases
Insects have evolved a breathing system that delivers oxygen directly to all the organs and tissues of their bodies

A spiracle is an opening in the exoskeleton of an insect which has valves
It allows air to enter the insect and flow into the system of tracheae

Tracheae are tubes within the insect respiratory system which lead to tracheoles (narrower tubes)
There are rigid rings of cartilage that keep the tracheae open

A large number of tracheoles run between cells and into the muscle fibres - the site of gas exchange
For smaller insects, this system provides sufficient oxygen via diffusion

Question 32

ventilation mechanism of insects

Answer 32

Very active, flying insects need a more rapid supply/intake of oxygen.
They create a mass flow of air into the tracheal system by:
Closing the spiracles
Using abdominal muscles to create a pumping movement for ventilation

Also, during flight the fluid found at the narrow ends of the tracheoles is drawn into the respiring muscle so gas diffuses across quicker (due to the diffusion distance being shorter)

Question 33

gills of a fish

Answer 33

Oxygen dissolves less readily in water
A given volume of air contains 30 times more oxygen than the same volume of water
Fish are adapted to directly extract oxygen from water

Structure of fish gills in bony fish:
Series of gills on each side of the head
Each gill arch is attached to two stacks of filaments
On the surface of each filament, there are rows of lamellae
The lamellae surface consists of a single layer of flattened cells that cover a vast network of capillaries

Mechanism:
The capillary system within the lamellae ensures that the blood flow is in the opposite direction to the flow of water - it is a counter-current system
The counter-current system ensures the concentration gradient is maintained along the whole length of the capillary
The water with the lowest oxygen concentration is found adjacent to the most deoxygenated blood

Question 34

ventilation mechanism in fish

Answer 34

The ventilation mechanism in fish constantly pushes water over the surface of the gills and ensures they are constantly supplied with water rich in oxygen (maintaining the concentration gradient)

When the fish open their mouth they lower the floor of the buccal cavity. This causes the volume inside the buccal cavity to increase, which causes a decrease in pressure within the cavity

The pressure is higher outside the mouth of the fish and so water flows into the buccal cavity
The fish then raises the floor of the buccal cavity to close its mouth, increasing the pressure within the buccal cavity

Water flows from the buccal cavity (high pressure) into the gill cavity (low pressure)

As water enters pressure begins to build up in the gill cavity and causes the operculum (a flap of tissue covering the gills) to be forced open and water to exit the fish

The operculum is pulled shut when the floor of the buccal cavity is lowered at the start of the next cycle