Adaptations For Gas Exchange

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Notas de aula

Surface Area to Volume Ratio

Surface area is the total area of an organism exposed to the external environment; volume is the total internal volume.
As the overall size of an organism increases, its surface area to volume ratio decreases.
This is because volume increases more rapidly than surface area as size increases.
The SA:V ratio can be calculated for objects of different shapes; different shapes may represent different organisms, e.g. a bacterial cell as a cylinder.
You must be able to calculate surface area and volume for a cube, cuboid, cylinder and sphere.
SA:V ratio is normally notated as x : 1.

Agar Blocks Practical

Agar blocks containing indicator can be used to investigate the effect of different factors on the rate of diffusion of an acid or alkali.
The effect of surface area to volume ratio on diffusion can be investigated using agar cubes of different sizes.
The effect of concentration gradient on diffusion can be investigated by placing agar cubes into solutions at a range of concentrations.
Method: cut agar containing indicator into cubes of different dimensions, e.g. sides of 0.5 cm, 1 cm and 2 cm.
Purple agar can be made using very dilute sodium hydroxide solution and universal indicator; alternatively, agar can be made with universal indicator only.
Calculate and record the SA:V ratio of each agar block.
Place the cubes into boiling tubes containing a solution that will affect the pH of the agar, e.g. dilute hydrochloric acid; use the same volume and concentration in each tube.
Measure either the time taken for the agar blocks to entirely change colour or the distance travelled into the block by the colour change in a given time.
The measurements can be converted to diffusion rates and a graph plotted of SA:V ratio against diffusion rate.
The graph shows that rate of diffusion increases as SA:V ratio increases.

Adaptation to Facilitate Exchange

Single-celled organisms have a high SA:V ratio, meaning simple diffusion at the cell surface is sufficient to meet the needs of the cell.
The large surface area allows for maximum diffusion of nutrients and gases; the small volume means the diffusion distance to all parts of the cell is short.
As organisms increase in size, their SA:V ratio decreases; there is less surface area for diffusion compared to the size of the organism.
The greater volume results in a longer diffusion distance to all parts of the organism, e.g. there may be many layers of cells.
Large, multicellular animals and plants have evolved adaptations to facilitate exchange, e.g. the gas exchange system and digestive system in mammals, and the leaves of plants.
Leaves have a large internal surface area, an adaptation that facilitates gas exchange by diffusion in plants.
Do not confuse overall surface area with SA:V ratio: an elephant has a larger surface area than an amoeba, but its SA:V ratio is smaller, so it needs specialised exchange surfaces.

SA:V Ratio & Metabolic Rate

The metabolic rate of an organism is the energy expended by that organism within a given period of time.
The SA:V ratio of an organism is related to its metabolic rate because of the relationship between SA:V ratio and heat loss.
Heat is lost to the environment at the body's surface, so having a large body surface in relation to volume allows more heat to be lost.
Small animals, with a higher SA:V ratio, lose more heat to their surroundings, so they need a relatively high metabolic rate to maintain body temperature.
Large animals, with a lower SA:V ratio, lose less heat, so they can maintain body temperature at a relatively low metabolic rate.
Basal metabolic rate per unit mass decreases as mass increases.
BMR per unit mass is different from total BMR: an elephant has a greater total BMR than a mouse, but a lower BMR per unit mass.

Adaptations of Gas Exchange Surfaces

Effective gas exchange supplies oxygen for respiration and removes waste carbon dioxide from respiration.
The features of exchange surfaces ensure that gas exchange can take place at a sufficient rate.
The key features of an exchange surface are: large surface area, short diffusion distance and steep concentration gradient.

Single-celled Organisms and Insects

Single-celled organisms, e.g. amoeba, carry out gas exchange at the cell surface by simple diffusion.
Their high SA:V ratio means diffusion occurs at a high rate over their relatively large surface area, and the diffusion distance from the surface to all parts of the cell is short.
Gas exchange in insects occurs via the tracheal system.
Air enters the bodies of insects via openings in the exoskeleton known as spiracles.
Air flows into tracheae tubes, and then into narrower tubes called tracheoles.
Many tracheoles lead to the muscle fibres, where their endings provide a large surface area for gas exchange.
Movement of gases in the tracheal system mainly relies on diffusion gradients: oxygen moves down its concentration gradient from the air into the respiring muscle cells; carbon dioxide moves down its concentration gradient from the respiring muscle cells into the air.
Active insects may need a more rapid supply of oxygen, which they gain using rapid contractions of the abdominal muscles to draw oxygen into the tracheae down a pressure gradient.

Fish

Fish are adapted to extract oxygen from water and have gills to maximise surface area for gas exchange.
There are a 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.
Gas exchange in the gills is maximised by a counter-current system: blood in the capillary system flows in the opposite direction to the flow of water as it passes over the gills.
This ensures that the concentration gradient is maintained along the whole length of the capillary.
The water that enters the capillary has the highest oxygen concentration and flows adjacent to blood that is already partially oxygenated; the water that exits has the lowest oxygen concentration and is adjacent to the most deoxygenated blood.

Dicotyledonous Plants

Plants need carbon dioxide for photosynthesis and oxygen for respiration, and the leaves are adapted to maximise exchange of these gases.
Spongy mesophyll layer: air flows into and around the air spaces; the surfaces of the spongy mesophyll cells come into contact with the air spaces, creating a large surface area for gas exchange.
Stomata: pores on the underside of most leaves which allow air to enter and exit the leaf; guard cells control the opening and closing of the stomata.
Shape of leaves: leaves are flat and thin, reducing the diffusion distance for gases.
Gases move in and out of the cells of the leaf by diffusion down their concentration gradients.
Gas exchange in leaves occurs at the surface of spongy mesophyll cells in the spongy mesophyll layer.

Gas Exchange vs Water Loss

Adaptations that aid gas exchange tend to increase the potential for water loss, meaning that organisms need to compromise between maximising gas exchange and minimising water loss.
Examples of this compromise can be seen in terrestrial insects and xerophytic plants.
Insects have a waterproof exoskeleton that prevents water loss by evaporation, but the spiracles provide openings through which water vapour can be lost.
Insect features that minimise water loss include the ability to close spiracles and hairs around the spiracles to reduce diffusion of water vapour.
Plants need to keep their stomata open to allow gas exchange, but open stomata also allow the loss of water vapour.
Plants that live where fresh water is limited have evolved adaptations to conserve water, including: few stomata, stomata in pits, hairs surrounding stomata, needle-shaped leaves with a reduced surface area, and a thickened waxy cuticle.
Plants with these adaptations are described as xerophytic.

Xerophyte Examples: Cacti and Marram Grass

Adaptations in cacti that reduce water loss include: leaves reduced to spines to reduce surface area for water loss; the stem has a thick cuticle to prevent water loss.
Sunken stomata trap water vapour, reducing water loss while allowing stomata to remain open for gas exchange.
Marram grass is commonly found on sand dunes and its leaves are well adapted to minimise water loss.
Marram grass leaves can roll up to reduce the exposure of surfaces to the wind.
The rolling of the leaf provides deep grooves which protect the stomata.
The exposed surface has no stomata and a thick cuticle.
The inner surface of the leaf possesses a large number of hairs.

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Questões de prática

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1.In an experiment using agar blocks containing sodium hydroxide and universal indicator solution, which solution should the blocks be immersed in to examine diffusion?
Easy
- ADilute hydrochloric acid
- BDistilled water
- CSodium hydroxide solution
- DUniversal indicator solution
2.Basal metabolic rate per unit mass decreases as the mass of an organism increases.
Easy
True or false?
3.Which of the following are features of an effective gas exchange surface? (select all that apply)
Medium
- ALarge surface area
- BShort diffusion distance
- CSteep concentration gradient
- DThick, impermeable outer layer
- ELong diffusion pathway
4.Match each organism or structure to its correct feature for gas exchange.
Medium
- Single-celled organism
- Insect
- Fish
- Leaf
- Simple diffusion at the cell surface
- Tracheae and tracheoles
- Gill lamellae with counter-current flow
- Spongy mesophyll and stomata
5.A mouse has a much higher resting heart rate than a human at the same body temperature. Which statement best explains this?
Medium
- AThe mouse has a higher surface area to volume ratio, so it loses heat faster and needs a higher metabolic rate.
- BThe mouse has a lower surface area to volume ratio, so it retains more heat.
- CThe mouse has a smaller total surface area than the human.
- DThe mouse has a larger volume relative to its surface area.
6.Place the structures of the insect tracheal system in the order air passes through them, from entry to the muscle cells.
Medium
- Spiracle
- Trachea
- Tracheole
- Muscle cell
7.Which feature of rolled marram grass leaves most directly reduces water loss by trapping water vapour?
Medium
- ASunken stomata in deep grooves
- BA thin waxy cuticle
- CStomata on the exposed outer surface
- DA large number of stomata per cm²
8.Which of the following are adaptations of xerophytic plants that reduce water loss? (select all that apply)
Medium
- AFew stomata
- BStomata in pits
- CHairs surrounding stomata
- DNeedle-shaped leaves with reduced surface area
- EA thin, permeable cuticle

Mais perguntas

9.Cacti have leaves reduced to spines to reduce the surface area available for water loss.
Easy
True or false?
10.Match each part of the fish gill with its correct description.
Medium
- Gill arch
- Filament
- Lamella
- Structure with rows of lamellae on its surface
- Structure to which two stacks of filaments are attached
- Thin plate of flattened cells covering a capillary network
11.Place the events of gas exchange in a fish gill in the correct sequence, starting with water entering the fish.
Medium
- Water flows in through the mouth and over the gills
- Oxygen in the water diffuses into the blood in the lamellae
- Blood in the capillaries flows in the opposite direction to the water
- The concentration gradient is maintained along the whole length of the capillary
12.Which of the following are adaptations of marram grass leaves that reduce water loss? (select all that apply)
Medium
- ALeaves roll up to reduce exposure to wind
- BThe exposed surface has a thick cuticle and no stomata
- CThe inner surface of the leaf has many hairs
- DStomata are sunken in grooves
- ELeaves are broad and flat to increase surface area
13.In the agar block practical, blocks of different sizes are placed in dilute hydrochloric acid. Which statement correctly describes the expected result?
Medium
- AThe block with the highest surface area to volume ratio changes colour fastest
- BThe block with the lowest surface area to volume ratio changes colour fastest
- CAll blocks change colour at the same rate regardless of size
- DThe rate of colour change depends only on the concentration of the acid, not on block size
14.Which of the following best explains why single-celled organisms can rely on simple diffusion for gas exchange?
Medium
- AThey have a large surface area to volume ratio and a short diffusion distance
- BThey have a small surface area to volume ratio and a long diffusion distance
- CThey have specialised gas exchange surfaces such as gills
- DThey actively pump gases across their cell surface
15.As the overall size of an organism increases, its surface area to volume ratio decreases.
Easy
True or false?
16.In fish gills, blood flows in the same direction as the water passing over the gills.
Easy
True or false?
17.One of these organisms is a mouse and the other is a human. Which animal is represented by Organism A, and why?
Medium
- AThe human, because it has the lower surface area to volume ratio
- BThe mouse, because it has the lower surface area to volume ratio
- CThe human, because it has the higher surface area to volume ratio
- DThe mouse, because it has the higher surface area to volume ratio
18.A scientist measured the rate of oxygen uptake of toad eggs, tadpoles and adult toads and expressed each as μmol g⁻¹ h⁻¹. Which pattern would be expected?
Medium
- AEggs have the highest rate per gram and adult toads the lowest
- BAdult toads have the highest rate per gram and eggs the lowest
- CAll three stages have equal rates per gram
- DEggs have the highest rate per gram and tadpoles the lowest
19.Figure 3 shows the number of times per minute the spiracles of a louse open at different oxygen concentrations. At low oxygen concentration the spiracles open most frequently, and at high oxygen concentration they open least frequently. Which explanation is correct?
Medium
- AAt low oxygen the insect needs more gas exchange, so spiracles open more often; high oxygen means less frequent opening is needed
- BAt low oxygen the insect closes its spiracles to conserve oxygen, so they open less often
- CAt high oxygen the insect opens its spiracles more to get rid of excess oxygen
- DSpirable opening is unrelated to oxygen concentration and varies randomly
20.The agave (Agave americana) lives in hot, arid regions and has very few stomata per cm² of leaf surface compared with other plants. Which statement describes a disadvantage of having few stomata?
Medium
- AThe rate of gas exchange, and therefore photosynthesis, may be limited
- BWater loss by transpiration will be increased
- CThe leaf will be unable to absorb light for photosynthesis
- DThe plant will be unable to transport water up the stem
21.A scientist counted stomata on the top and underside of 12 privet leaves. The top surface had 0 stomata in every field of view, while the underside had a mean of about 41. Which statement best explains why the top surface has no stomata?
Medium
- AThe top surface is exposed to more direct sunlight and wind, so stomata there would cause excessive water loss
- BThe top surface is where most photosynthesis occurs, so it does not need gas exchange
- CStomata on the top surface would block light from reaching the chloroplasts
- DThe top surface has a thicker cuticle that prevents stomata from forming
22.A student investigates diffusion using agar cubes of side 0.5 cm, 1 cm and 2 cm placed in dilute hydrochloric acid. Which statements about this practical are correct? (select all that apply)
Hard
- AThe 0.5 cm cube has the highest surface area to volume ratio
- BThe 0.5 cm cube will change colour completely in the shortest time
- CThe 2 cm cube has the highest surface area to volume ratio
- DThe rate of diffusion increases as the surface area to volume ratio increases
- EThe same volume and concentration of acid must be used for each cube
23.Which change occurs in guard cells when stomata close at night?
Medium
- AThe guard cells lose water and become less turgid
- BThe guard cells take up water and become more turgid
- CThe guard cells photosynthesise more rapidly
- DThe guard cells become more permeable to carbon dioxide
24.Place the stages of the agar block diffusion experiment in the correct order, starting with preparing the agar.
Medium
- Cut agar containing indicator into cubes of different side lengths
- Calculate and record the surface area to volume ratio of each cube
- Place the cubes into boiling tubes containing the same volume and concentration of dilute hydrochloric acid
- Measure the time taken for each cube to change colour completely
- Plot a graph of surface area to volume ratio against diffusion rate
25.A fish gill has a counter-current system. Which statement correctly describes the oxygen concentrations along the gill?
Hard
- AWater with the highest oxygen concentration flows next to blood that is already partially oxygenated, while water leaving with the lowest oxygen concentration flows next to the most deoxygenated blood
- BWater with the highest oxygen concentration flows next to the most deoxygenated blood, while water leaving with the lowest oxygen concentration flows next to partially oxygenated blood
- CWater and blood flow in the same direction so oxygen concentrations equalise along the gill
- DWater and blood flow in opposite directions but oxygen concentration is the same at all points along the gill
26.In the agar block practical, a student wants to investigate the effect of concentration gradient on the rate of diffusion. Which change should they make?
Medium
- APlace identical agar cubes into hydrochloric acid solutions of different concentrations
- BPlace agar cubes of different sizes into the same hydrochloric acid solution
- CUse agar cubes made with different amounts of indicator
- DUse the same size agar cube in the same acid but change the temperature
27.As the surface area to volume ratio of an organism increases, its rate of diffusion increases.
Easy
True or false?
28.Match each feature of an exchange surface to the way it increases the rate of gas exchange.
Medium
- Large surface area
- Short diffusion distance
- Steep concentration gradient
- More molecules can diffuse across at once
- Molecules have less distance to travel to reach cells
- More molecules move in one direction than the other
29.Which of the following is a feature of an effective gas exchange surface?
Medium
- AA large surface area
- BA long diffusion distance
- CA shallow concentration gradient
- DA thick, impermeable barrier
30.In the agar block practical, purple agar cubes of side 0.5 cm, 1 cm and 2 cm are placed in dilute hydrochloric acid. Which statement correctly describes the expected result?
Medium
- AThe 0.5 cm cube decolourises first because it has the highest surface area to volume ratio
- BThe 2 cm cube decolourises first because it has the largest surface area
- CAll three cubes decolourise at the same time because they are made of the same agar
- DThe 2 cm cube decolourises first because it has the highest surface area to volume ratio
31.Which of the following are features of the leaves of dicotyledonous plants that maximise gas exchange? (select all that apply)
Medium
- ASpongy mesophyll cells in contact with air spaces
- BA flat, thin leaf shape
- CA thick waxy cuticle covering the whole leaf
- DStomata on the underside of the leaf
- EA single layer of flattened cells over a dense capillary network
32.Match each organism or structure to the feature that aids gas exchange.
Medium
- Insect tracheal system
- Fish gill lamellae
- Dicotyledonous leaf
- Spongy mesophyll layer
- Many tracheoles in contact with muscle cells
- A single layer of flattened cells covering a capillary network
- Air spaces contacted by the surfaces of mesophyll cells
- Flat and thin to reduce the diffusion distance
33.Place the stages of the agar block practical in the correct order, starting with preparing the agar.
Medium
- Cut agar containing indicator into cubes of different dimensions
- Calculate and record the surface area to volume ratio of each agar block
- Place the cubes into boiling tubes containing dilute hydrochloric acid
- Measure the time taken for the agar blocks to entirely change colour
34.A scientist counted stomata in the field of view for the top surface and underside of 12 privet leaves. The top surface had 0 stomata in every field of view, while the underside had a mean of about 40 per field of view. Suggest the best explanation for this difference.
Medium
- AStomata on the underside are shaded and cooler, so less water vapour is lost by evaporation
- BThe top surface has no stomata because it is the only surface exposed to light
- CThe underside has more stomata because it is closer to the soil and takes up water vapour
- DStomata are found on the underside because carbon dioxide is heavier than air
35.Insects have a waterproof exoskeleton that prevents water loss by evaporation, but the spiracles provide openings through which water vapour can be lost.
Easy
True or false?
36.In the agar block practical, a student wants to investigate the effect of surface area to volume ratio on the rate of diffusion. Which variable must be kept constant between the boiling tubes?
Medium
- AThe volume and concentration of the dilute hydrochloric acid
- BThe mass of each agar cube
- CThe surface area to volume ratio of each cube
- DThe time each cube is left in the acid
37.A student places three agar cubes with side lengths 0.5 cm, 1 cm and 2 cm into separate boiling tubes of dilute hydrochloric acid. They record the time taken for each cube to completely change colour. Which statement correctly describes the expected result?
Medium
- AThe 2 cm cube changes colour quickest because it has the largest surface area
- BThe 0.5 cm cube changes colour quickest because it has the highest surface area to volume ratio
- CAll three cubes change colour at the same time because they are made of the same agar
- DThe 1 cm cube changes colour quickest because it has an intermediate surface area to volume ratio
38.A student investigates diffusion using agar cubes containing universal indicator and dilute sodium hydroxide. Which of the following are valid improvements to the method? (select all that apply)
Hard
- AUse a ruler to measure the distance the colour change has moved into the block after a fixed time
- BUse the same volume and concentration of acid in each boiling tube
- CUse a different acid concentration for each cube to speed up the experiment
- DRepeat the experiment and calculate a mean time for each cube size
- EOnly use one cube size so that the results are more reliable
39.In the agar block practical, which solution is used to make the agar blocks so that a colour change can be observed as acid diffuses in?
Medium
- AVery dilute sodium hydroxide solution and universal indicator
- BDilute hydrochloric acid and universal indicator
- CDistilled water and iodine solution
- DSodium hydrogencarbonate solution and limewater
40.A student wants to investigate the effect of concentration gradient on diffusion rate using agar cubes. Which change should they make to the practical?
Medium
- AUse agar cubes of different sizes
- BPlace identical agar cubes into hydrochloric acid solutions of different concentrations
- CChange the temperature of the water bath
- DUse a different indicator in the agar
41.Complete the following sentence about surface area to volume ratio.
Easy
The size of an organism is ____ proportional to its surface area to volume ratio.
42.A spherical toad egg has a surface area of 10.12 mm². Using surface area = 4πr², calculate the diameter of the egg in mm. Give your answer to 3 significant figures.
Medium
43.Name the small pores that have evolved on the surface of certain insect species that are used for breathing.
Easy
44.Complete the sentence about the agar block practical.
Easy
The rate of diffusion ____ as the surface area to volume ratio increases.
45.Complete the sentence about gas exchange in leaves.
Easy
Gas exchange in leaves occurs at the surface of ____ cells in the ____ layer.
46.An agar cube has sides of length 2 cm. Calculate its surface area to volume ratio in the form x : 1.
Medium
47.Name the openings in the exoskeleton of an insect through which air enters the tracheal system.
Easy
48.Complete Table 1 by calculating the missing surface area to volume ratio for Organism B. Do not include units. Give your answer to 2 decimal places.
Medium
49.Basal Metabolic Rate (BMR) is a measure of the amount of energy expended by an organism within a given period of time when at rest. Suggest one variable that can be measured as a way of determining BMR.
Medium
50.A researcher calculated the mean surface area of a large number of spherical toad eggs to be 10.12 mm². Using surface area = 4πr², calculate the mean diameter of a toad egg in mm. Give your answer to 3 significant figures.
Medium
51.Complete the sentence about the relationship between organism size and surface area to volume ratio.
Easy
The size of an organism is ____ proportional to its surface area to volume ratio.
52.A cube of agar has sides of length 1 cm. Calculate its surface area to volume ratio, expressed in the form x : 1.
Medium
53.Describe and explain one example of how the human breathing system is adapted to achieve sufficient gas exchange despite humans' low surface area to volume ratio.
Hard
54.Describe how oxygen in the tracheole reaches the muscle cells of an insect.
Hard
55.A molecule of oxygen would diffuse into a single-celled prokaryotic cell and reach the centre of the cell quicker than it would reach the centre of a single, eukaryotic cell. Explain why.
Hard
56.Some insects are very active flyers. After a period of time the flying muscles respire anaerobically and the products of anaerobic respiration accumulate in the cells. Explain how this affects the supply of oxygen to the flying muscles.
Hard
57.A student set up three boiling tubes each containing 10 cm³ of dilute hydrochloric acid. They placed a 0.5 cm, 1 cm and 2 cm agar cube into separate tubes and recorded the time for each cube to completely change colour. Explain how the student could use their results to calculate the rate of diffusion for each cube, and state the units.
Hard
58.A spherical toad egg has a surface area of 10.12 mm². Using surface area = 4πr², calculate the diameter of the egg in mm. Give your answer to 3 significant figures.
Medium
- A5.59 mm
- B3.59 mm
- C4.59 mm
- D2.59 mm
59.An agar cube has sides of length 2 cm. Calculate its surface area to volume ratio in the form x : 1.
Medium
- A2 :1
- B5 :1
- C4 :1
- D3 :1
60.Complete Table 1 by calculating the missing surface area to volume ratio for Organism B. Do not include units. Give your answer to 2 decimal places.
Medium
- A5040
- B4200
- C3780
- D4620
61.A researcher calculated the mean surface area of a large number of spherical toad eggs to be 10.12 mm². Using surface area = 4πr², calculate the mean diameter of a toad egg in mm. Give your answer to 3 significant figures.
Medium
- A1.79 mm
- B3.79 mm
- C2.79 mm
- D0.79 mm
62.A cube of agar has sides of length 1 cm. Calculate its surface area to volume ratio, expressed in the form x : 1.
Medium
- A8
- B6
- C5
- D7

Resumos de revisão

## Surface Area to Volume Ratio - **Surface area**: the total area of an organism that is exposed to the external environment. - **Volume**: the total internal volume of an organism. - As the overall size of an organism increases, the **surface area to volume ratio decreases** — volume increases more rapidly than surface area as size increases. - SA:V ratio is notated as **x : 1** (divide both values by the volume). - Know how to calculate surface area and volume for a **cube, cuboid, cylinder and sphere**. - Surface area of a sphere = 4πr² - Volume of a sphere = 4/3 πr³ ## Agar Blocks Practical - Agar containing **universal indicator** (made up with very dilute sodium hydroxide) is cut into cubes of different sizes (e.g. 0.5 cm, 1 cm, 2 cm). - Calculate and record the SA:V ratio of each block. - Place cubes into **dilute hydrochloric acid** — same volume and same concentration in each tube. - Measure either the **time taken for the agar blocks to entirely change colour**, or the **distance travelled into the block by the colour change in a given time**. - Plot SA:V ratio against diffusion rate: **the higher the SA:V ratio, the faster the rate of diffusion**. ## Adaptations to Facilitate Exchange - Single-celled organisms have a **high SA:V ratio**: simple diffusion at the cell surface is sufficient. - Large surface area → maximum diffusion of nutrients and gases. - Small volume → short diffusion distance to all parts of the cell. - As organisms increase in size, SA:V ratio decreases: less surface area for diffusion, longer diffusion distance (e.g. many layers of cells). - Large multicellular organisms have evolved adaptations, e.g. the gas exchange and digestive systems in mammals, and leaves in plants. ## SA:V Ratio & Metabolic Rate - **Metabolic rate**: the energy expended by an organism within a given period of time. - Heat is lost at the body's surface, so a large body surface relative to volume allows more heat loss. - Small animals (higher SA:V) lose more heat → need a relatively **high metabolic rate** to maintain body temperature. - Large animals (lower SA:V) lose less heat → maintain body temperature at a relatively **low metabolic rate**. - **Basal metabolic rate per unit mass decreases as mass increases** (total BMR is different from BMR per unit mass). ## Adaptations of Gas Exchange Surfaces Features of an exchange surface: **large surface area, short diffusion distance, steep concentration gradient**. - **Insects**: gas exchange via the tracheal system. Air enters through **spiracles** in the exoskeleton → **tracheae** → narrower **tracheoles**, which run into muscle fibres. Gases move by diffusion gradients. Active insects use rapid contractions of the abdominal muscles to draw oxygen in down a pressure gradient. - **Fish**: gills maximise surface area. Each gill arch has two stacks of filaments; each filament has rows of **lamellae** — a single layer of flattened cells over a vast capillary network. The **counter-current system** (blood flows opposite to water) maintains the concentration gradient along the whole length of the capillary. - **Dicotyledonous plants**: spongy mesophyll air spaces give a large surface area; stomata (controlled by guard cells) let air in and out; leaves are flat and thin, reducing diffusion distance. ## Gas Exchange vs Water Loss - Adaptations that aid gas exchange increase the potential for water loss — organisms must **compromise**. - **Terrestrial insects**: waterproof exoskeleton, but spiracles allow water vapour loss; they can **close spiracles** and have **hairs around spiracles**. - **Xerophytes** (plants adapted to conserve water): few stomata, stomata in pits, hairs surrounding stomata, needle-shaped leaves with reduced surface area, thickened waxy cuticle. - **Cacti**: leaves reduced to spines, thick cuticle on the stem, sunken stomata trap water vapour. - **Marram grass**: leaves roll up to reduce exposure to wind, deep grooves protect stomata, exposed surface has no stomata and a thick cuticle, inner surface has many hairs.

Surface area: The total area of an organism that is exposed to the external environment.
Volume: The total internal volume of an organism.
Surface area to volume ratio: As the overall size of an organism increases, the surface area to volume ratio decreases, because volume increases more rapidly than surface area as size increases.
Metabolic rate: The energy expended by an organism within a given period of time.
Basal metabolic rate (BMR): A measure of the amount of energy expended by an organism within a given period of time when the organism is at rest.
Spiracles: Openings in the exoskeleton of insects through which air enters the body; they possess valves that allow insects to open and close their airways.
Tracheae: Tubes in the insect tracheal system that air flows into after entering via the spiracles.
Tracheoles: Narrower tubes in the insect tracheal system; many lead to the muscle fibres, where their endings provide a large surface area for gas exchange.
Gill lamellae: Rows of structures on the surface of each gill filament; their surface consists of a single layer of flattened cells that cover a vast network of capillaries.
Counter-current system: A system in fish gills in which blood in the capillaries flows in the opposite direction to the flow of water over the gills, maintaining the concentration gradient along the whole length of the capillary.
Spongy mesophyll layer: A leaf layer containing air spaces; the surfaces of the spongy mesophyll cells contact the air spaces, creating a large surface area for gas exchange.
Stomata: Pores on the underside of most leaves which allow air to enter and exit the leaf; guard cells control their opening and closing.
Xerophyte: A plant adapted to conserve water, living in conditions where fresh water is limited.
Guard cells: Cells that control the opening and closing of the stomata.

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CADASTRE-SE — GRÁTIS ENTRAR

Provas anteriores

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