Topic 3: Organism Exchange substances with their Environment

Question 1

What ability do single-celled/ Small organisms have to ‘diffuse’ substances through their outer surface?

Answer 1

They have a Large Surface Area to Volume ratio, which provides them with a short diffusion pathway to all parts of the cell.

(As the size of an organism increases, the surface area to volume ratio decreases)

Question 2

How do larger organisms perform gas exchange?

Answer 2

They have developed specialised exchange structures such as Lungs and Gills to maintain adequate rates of gaseous exchange.

Question 3

How do larger organisms maintain a constant body temperature?

Answer 3

–>Changes in body shape/ specialised structures have evolved e.g. large ears in elephants to increase the surface area for heat loss.
+
–>The large amount of body cells release a lot of heat from metabolic processes (mainly cellular respiration)

Question 4

How do smaller organisms maintain a constant body temperature?

Answer 4

Because smaller organisms have a large SA-to-volume ratio, they lose a large amount of heat (relative to their mass) due to this:

–>Have to respire at a high rate to help maintain a constant body temperature (respiration produces heat), therefore smaller organisms have a high metabolic rate

–>High oxygen uptake relative to their mass i.e. per gram of tissue.

–>Consume a lot of food, relative to their mass.

Question 5

How do insects perform Gas exchange?

Answer 5

They perform gas exchange through a system of pipes called the ‘Tracheal System’:

1) Oxygen diffuses from the air, through the spiracles along the trachea and tracheoles to the cells.
–> Tracheoles are thin, providing a short diffusion + they are numerous, providing a large surface area.

2) They penetrate all body tissues and the oxygen then diffuses into the cells through the cell membrane of the tracheole (this is the gas exchange surface)

3) Carbon dioxide leaves the insect’s cells and travels to the atmosphere by the reverse pathway

Question 6

Which factor determines the opening and closing of Spiracles?

Answer 6

Carbon dioxide levels:

-When carbon dioxide levels rise due to respiration, the spiracles open wider, increasing the rate of diffusion of gases in the tracheal system.

-During rest, the spiracles close to help reduce water loss via evaporation.

Question 7

How do Insects increase the rate of diffusion?

Answer 7

1) (When flying)–> Muscles contract to compress the trachea, forcing air out of them.–>When the muscles relax, the trachea springs back into shape and fresh oxygen rich air is drawn into them
—>all of this in order to speed up the rate of diffusion.

2) Anaerobic Respiration–> lactic acid is produced which lowers the water potential of the cells and some of the water in the ends of the tracheoles moves into the cells by osmosis (enabling more air to move in)–> Therefore increasing the rate f diffusion.

Question 8

How are gills adapted for Gaseous Exchange?

Answer 8

1) Large surface area–> provided by a large number of gill filaments.
–>These filaments possess lamellae (thin plates) increasing the SA further

2) Short Diffusion Pathway–> as blood and water are separated by a thin barrier of cells consisting of 2 layers:

-Epithelial Layer (of the gill lamellae)
-Endothelial layer of the blood capillaries.

3) Extensive Network Of Blood Capillaries–> Once oxygen diffuses into the blood it is carried away, maintaining a steep concentration gradient for oxygen .

4) The Counter-current System –> Blood and water flowing in opposite directions, which ensures that blood continually meets water with a higher oxygen concentration so that a high diffusion gradient is maintained along the whole length of the lamellae.–>

meaning around 80% of the oxygen in the water diffuses into the bloodstream (instead of 50%) as equilibrium between the water and the blood is never reached (because they flow in opposite directions)

Question 9

When do Plants perform Respiration and Photosynthesis?

Answer 9

-Plants respire 24 hours a day but only photosynthesise during daylight.
(during daylight the rate of photosynthesis is greater than the rate of respiration so there is a net uptake of carbon dioxide and loss of oxygen)

Question 10

How are Plants adapted for gas exchange?

Answer 10

–>Respiration and photosynthesis maintain diffusion gradients by using and producing oxygen and carbon dioxide.

–> Numerous mesophyll cells lining the inter-cellular air spaces in the leaf, providing a large surface area (for gaseous exchange).

–> Gases diffuse in through the stomata between the guard cells, down a concentration gradient rapidly through the intercellular air spaces.

–> The gases dissolve in the moist cell walls of the mesophyll cells.

–> Gases diffuse across the thin cell wall and cell membrane and of mesophyll cells.

Question 11

What is ‘Transpiration’?

Answer 11

-The evaporation of water from a plant’s surface, particularly through the stomata when they open to allow the entry of carbon dioxide for photosynthesis

Question 12

What are some environmental factors which affect the rate of transpiration?

Answer 12

Light:
during daylight, stomata open to allow carbon dioxide to enter for photosynthesis
–> This increases the rate of transpiration, as water evaporates from the mesophyll cells and diffuses out of the leaf.

Temperature:
–> An increases the rate of transpiration by providing water molecules with more kinetic energy, allowing them to evaporate more easily.

Humidity:
–> Increases the water potential of the air, this will lead to a decrease in the rate of transpiration, as the water potential gradient for the diffusion of water decreases.

Air Movement:
–>Removes water vapour from the leaf surface, increasing the water potential gradient and the rate of transpiration

(In still air, water vapour builds up around the leaf, decreasing the water potential gradient and the rate of transpiration.

Question 13

What are ‘Xherotypes’ and what adaptations do they have?

Answer 13

plants that possess a range of adaptations to limit water loss via transpiration:

-A thick cuticle–> provides a short diffusion pathway, reducing the rate of evaporation

-Hairs on the leaf surface–> trap a layer of still air, which becomes saturated with water vapour, reducing the water potential gradient for water loss

-Rolling up of leaves–>traps a layer of still air which becomes saturated with water vapour, reducing the water potential gradient for water loss, therefore reducing the rate of transpiration (e.g. marram grass)

-Reduced surface area to volume ratio of leaves (e.g. pine needles reduce the surface area for water loss)

-Stomata positioned in ‘epidermal pits/grooves’ beneath the leaf surface, reducing exposure to air currents. The trapped air becomes saturated with water vapour and reduces the water potential gradient for evaporation.

Question 14

What does the gas exchange system in mammals consist of?

Answer 14

1) The trachea is supported by incomplete rings of cartilage that prevent it from collapsing during pressure changes that occur in ventilation.

2) The trachea divides into 2 bronchi that repeatedly divide into smaller tubes–> The bronchioles

3) The alveoli at the ends of the bronchioles provide a large surface area where gaseous exchange occurs.

Question 15

How are Alveoli adapted as a gaseous exchange surface?

Answer 15

1) Shape and large number–>produces a large surface area

2) Fluid lining in the alveolus allows gases to dissolve and diffuse across

3) Two cell layer –> provides a short diffusion pathway:
1) Flattened epithelial cells (alveolar wall)
2) Endothelial cells (capillary wall)

4) An extensive network of blood capillaries surrounds each alveolus –> providing a large surface area for absorbing oxygen and releasing carbon dioxide

5) A circulatory system–> maintains a high concentration gradient for gaseous exchange by transporting deoxygenated blood to the lungs and removing oxygenated blood from the lungs

6) A ventilation system–> maintains a high concentration gradient for gaseous exchange by continually providing air with a high oxygen concentration and removing air with a high carbon dioxide concentration.

Question 16

How does gas exchange occur in the alveoli?

Answer 16

1) Oxygen in the alveolar air space dissolves in the fluid lining in the epithelium of the alveolus

2) It then diffuses across from the flattened epithelial cells of the alveolus to the endothelial cells of the capillary wall
(Both layers are single-cell layers–> short diffusion pathway)

3) The oxygen combines with haemoglobin in the red blood cells to form oxyhaemoglobin, the oxygenated blood is then carried away from the lungs.

4) Carbon dioxide then diffuses from the blood in the opposite direction into the alveolar space

–>Blood flow ensures that deoxygenated blood continually reaches the alveoli SO that oxygenated blood is removed (Therefore maintaining a high concentration
gradient)

–>The ventilation mechanism continually replenishes the air in the alveolar space, ensure it has a high concentration gradient and low carbon dioxide concentration 9=(therefore maintaining a high concentration gradient)

Question 17

What is Ventilation?

Answer 17

Ventilation is the method by which air (or water in fish) is passed over the respiratory surface to ensure a high concentration gradient is maintained

Question 18

What is ‘Inspiration’ and how does it work?

Answer 18

-Inspiration (breathing in) is an active process:
–> External intercostal muscles contract (pulling the ribs upwards and outwards), whilst the internal intercostal muscles relax

–>The diaphragm muscles contract, pulling it down so it flattens

-(Both these actions increase the volume of the thoracic cavity)

-(The pressure inside the lungs decreases below atmospheric pressure and air enters the lungs down a pressure gradient)

Question 19

How do the external and internal intercoastal muscles work?

Answer 19

They work antagonistically-when one of them contracts the other relaxes and vice versa.

Question 20

What is ‘Expiration’ and how does it work?

Answer 20

-Expiration (breathing out) is mainly a passive process:
–> The external intercostal muscles relax

–> The diaphragm muscles relax and the diaphragm moves upwards to its dome shape
(These actions decrease the volume of the thoracic cavity)

–>The pressure inside the lungs increases above atmospheric pressure and air is forced out of the lungs

–>Elastic recoil of the lung tissue helps to force air out of the lungs during expiration

–> The internal intercostal muscles may also contract pulling the ribcage downwards and inwards.

Question 21

What is Pulmonary Ventilation?

Answer 21

Pulmonary ventilation is the total volume of air that is moved into the lungs in one minute.

Question 22

How do you calculate Pulmonary Ventilation?

Answer 22

Two factors are needed:
1) Tidal volume–> The volume of air normally taken in at each breath (about 0.5 dm cubed)

2) Ventilation rate–> The number of breaths per minute (normally around 12-30 bpm)

Pulmonary Ventilation =
Tidal Volume x Ventilation rate

Question 23

What is a risk factor?

Answer 23

Anything which increases the chance of getting a disease (e.g. Diet/Smoking/Age/Occupation etc.)

(Occupation–> working with harmful chemicals/gases, or radiation)

Question 24

What is ‘Correlation’?

Answer 24

Where a change in 1 of 2 variables is reflected by a change in the other variable.

–>(Some factors may show a correlation with a disease but there may be no actual evidence to prove it is the cause of the disease)

Question 25

What is ‘Cause’?

Answer 25

A factor which is directly a cause of a disease

–>(This can only be established when scientists have produced compelling experimental evidence to show a particular factor is causing a disease)

Question 26

What is ‘Relative Risk’?

Answer 26

Risk measured by comparing the likelihood of harm occurring in those exposed to a hazard with those who are not exposed to it

–>(e.g. smokers may be 15x more likely to develop lung cancer than non-smokers)

Question 27

What is ‘Digestion’?

Answer 27

Digestion is the process in which large molecules are hydrolysed by enzymes to produce smaller molecules (that can be absorbed and assimilated)

Question 28

How is the digestion of ‘Carbohydrates’ carried out?

Answer 28

1) In the mouth:
The sight/smell/taste of food all stimulate the secretion of saliva from the salivary glands, saliva contains–> Salivary amylase–> Amylase hydrolyses starch into maltose.

–>Only a small amount of starch is broken down (due to the short time food remains in the mouth)
–>Chewing breaks the food into smaller particles (increased surface area for hydrolysis by enzymes)

2) In the small intestine:
The pancreas releases pancreatic juice into the small intestine.
(Pancreatic juice is an alkaline fluid containing several different enzymes including Pancreatic Amylase)–>Hydrolysing the remaining starch into the disaccharide maltose.

3) The complete hydrolysis of starch into Alpha glucose occurs in the ileum by the disaccharidase enzyme maltase (part of the cell-surface membrane of the epithelial cells that line the ileum)–> Membrane-bound disaccharides.

–>The membrane-bound disaccharides include Maltase/Lactase/Sucrase

Question 29

What are the advantages of having membrane-bound disaccharides?

Answer 29

-Enzymes are not lost in the food (as they stay in one place)
-Enzymes can be used repeatedly
-Products of hydrolysis are taken up immediately through the membrane

Question 30

How is the digestion of ‘Lipids’ carried out?

Answer 30

1) Lipids are hydrolysed into glycerol, fatty acids, and monoglycerides (glycerol and one fatty acid) by the enzyme Lipase (which is secreted by the pancreas into the small intestines)

–>Lipase specifically hydrolyses the ester bonds of the triglycerides.

Question 31

How is the rate of hydrolysis of lipids increased?

Answer 31

It is increased by bile salts which:
(1) emulsify lipids, causing them to form small droplets
(2) This increases the surface area of the lipids which speeds up the hydrolysis by Lipase

(Emulsification is a physical process and does not cause chemical breakdown)

Question 32

How is the digestion of ‘Proteins’ carried out?

Answer 32

-Proteins are hydrolysed by: Endopeptidases, Exopeptidases, and Dipeptidases

1) Endopeptidases break bonds in the middle of the polypeptide chain to produce shorter polypeptide chains. (they do this by hydrolysing the internal peptide bonds between the amino acids of proteins)–>(they do this to produce ‘more ends’ for exopeptidases to act on to hydrolyse the terminal.)

–>The endopeptidase in the stomach of humans is ‘pepsin’ and has an optimum PH of 1-2

2) The Pancreas secretes Exopeptidases which hydrolyse the peptide bonds at either end of a polypeptide chain to produce dipeptides or individual amino acids.

3) The hydrolysis of the dipeptides into 2 single amino acids by Dipeptidase enzymes.
(Dipeptides —>(hydrolysis by dipeptidases)–> Amino Acids

–>Dipeptidases are membrane-bound enzymes in the folded cell-surface membrane (microvilli) of the epithelial cells of the ileum.

Question 33

How is the ‘Ileum’ adapted for absorption of digested products?

Answer 33

-Large Surface Area–> due to its long length + the presence of villi and microvilli (folds of the cell membrane)

-Villi contain blood capillaries that absorb monosaccharides and amino acids –> maintaining a high diffusion gradient for further absorption

-The wall of each villus consists of a single layer of epithelial cells–> providing a short diffusion pathway (for absorption)

-Large number of mitochondria present to supply ATP (for Active Transport)

-Carrier and Channel Proteins in the cell-surface membrane for–> absorption of specific molecules by Active Transport (carrier proteins) + Facilitated Diffusion (carrier & channel proteins)

Question 34

What are the steps in the ‘Co-transport of Monosaccharides and Amino Acids’?

Answer 34

1) Sodium ions are actively transported out of the epithelial cells and into the blood (–> this occurs via specific carrier proteins + requires ATP)

–> step 1 ensures that a low concentration of sodium ions is maintained within the epithelial cell
–>A higher concentration of sodium ions will be present in the lumen

2) Therefore, there is a concentration gradient for sodium ions to diffuse into the cell–> they diffuse into the epithelial cells from the lumen of the intestines through a carrier protein in the cell-surface membrane known as a (Co-transport Protein)
–>Glucose also moves through these co-transport proteins with the sodium ions

3) The Glucose molecules then move out of the cell towards the blood capillary by Facilitated Diffusion through another specific channel or carrier protein.

–>(The same mechanism applies to the uptake of other monosaccharides and amino acids)

Question 35

What is the Counter-Current System?

Answer 35

Blood and water flowing in opposite directions, which ensures that blood continually meets water with a higher oxygen concentration so that a high diffusion gradient is maintained along the whole length of the lamellae.