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A-Level Biology > 6 Exchange > Flashcards

6 Exchange Flashcards

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1
Q

Why can larger organisms not only rely on simple diffusion?

A

Smaller surface area to volume ratio
Substances taken in or produced in one area need to be transported somewhere else for use or removal
Distances are too larger for diffusion or active transport
Larger animals might be more active (higher metabolic rate) –> respiring cells have a greater demand for oxygen and other substances

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2
Q

Why do small organisms have efficient gas exchange?

A

Small organisms have a SA that is large enough, compared with their volume, to allow efficient exchange across their body surface

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3
Q

How do organisms evolve for a larger surface area to volume ratio?

A

Flattened shape so that no cell is ever far from the surface
Specialised exchange surfaces with large areas to increase SA: vol

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4
Q

What are the 5 features of exchange surfaces?

A
  1. Thin - shorter distance for diffusion
  2. Movement of internal medium
  3. Movement of external medium (maintain a steep concentration gradient)
  4. Larger SA
  5. Selectively permeable membrane to control what enters or leaves the cell
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5
Q

True or false: all cell-surface membranes are always the same thickness

A

True (at a-level)

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6
Q

How is diffusion, surface area, difference in concentration and length of diffusion path all linked?

A

Diffusion directly proportional to surface area x difference in concentration/length of diffusion path

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7
Q

Why are specialised exchange surfaces found inside an organism?

A

Thin so can be easily damaged and dehydrated
Therefore, organism needs to have a means of moving the external medium over the surface eg ventilating lungs in a mammal

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8
Q

How does gas exchange work in single-celled organisms?

A

Small = larger SA: vol
Oxygen is absorbed by diffusion across their body surface (covered only by a cell-surface membrane)
Carbon dioxide from respiration diffuses out across their body surface
No extra barrier to diffusion if cell has a cell wall

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9
Q

Why do insects not have a very high SA:vol?

A

Lose too much water as will evaporate from it

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10
Q

What network do insects have for gas exchange?

A

Internal network of tracheae
Divide into smaller dead-end tubes called tracheoles
Tracheoles extend throughout all the body tissues of the insect
Atmospheric air brought directly to respiring tissues with short diffusion pathway from tracheole to any body cell

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11
Q

How do respiratory gases move in and out of the insects tracheal system?

A

Along a diffusion gradient
Mass transport
Ends of tracheoles are filled with water

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12
Q

How do respiratory gases move along a diffusion gradient in the tracheal system?

A

Cell respiring so oxygen is used up and so concentration towards the ends of the tracheoles falls
Diffusion gradient that causes oxygen to diffuse from the atmosphere along the tracheae and tracheoles to the cells
Carbon dioxide produced in respiration - diffusion gradient in opposite direction
Carbon dioxide diffuse along tracheoles and tracheae from cells to the atmosphere
Diffusion in air is more rapid that in water

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13
Q

How do respiratory gases move via mass transport in the tracheal system?

A

Contraction of muscles in insects can squeeze the trachea enabling mass movements of air in and out
Further speeds up exchange

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14
Q

How do respiratory gases move due to the ends of the tracheoles being filled with water in the tracheal system?

A

During major activity, muscle cells around tracheoles respire carry out some anaerobic respiration
Produces lactate, soluble and lower water potential of muscle cells
Water moves into the cells from tracheoles by osmosis
Water in ends of tracheoles decreases in volume so draws air in
Final diffusion pathway is in a gas rather than a liquid phase, diffusion more rapid
Increases the rate at which air is moved in the tracheoles but leads to greater water evaporation (trade off)

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15
Q

What are the tiny pores on the insect body surface and what is their purpose?

A

Spiracle
Gases enter and leave tracheae via spiracles
When open, water vapour can evaporate from the insect (mostly closed)

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16
Q

What open and close spiracles?

A

Valves

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17
Q

What structure reduces water loss on spiracles?

A

Chitinous hairs
- Increase the humidity around the spiracle
- Reduces concentration gradient for water vapour and reduces the rate of evaporation

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18
Q

What is the function of air sacs?

A

Act as bellows
When the muscles of the insect contract, they force air in and out of the insect

19
Q

True or false: Tracheae have chitin

A

True
Supported by strengthened rings to prevent them from collapsing

20
Q

True or false: Tracheoles have chitin

A

False
Chitin is waterproof which we don’t want in the tracheoles

21
Q

What are some limitations of the tracheal system?

A

Relies on diffusion to exchange gases
Diffusion pathway needs to be short so insects of small size - limits size insects can attain

22
Q

What is the fish specialised internal gas exchange surface?

A

Gills - water taken in through the mouth and force over the gills and out through an opening on each side of the body

23
Q

What is the structure of the gills (what are they made up of)?

A

Located behind the head
Made up of gill filaments - stacked up in a pile

24
Q

What are at right angles to gill filaments and what is their function?

A

Gill lamellae - increase surface area of the gills

25
Q

How does the blood and water flow in a fish?

A

Countercurrent flow (blood and water flow opposite directions)
Ensures maximum possible gas exchange is achieved

26
Q

What does the countercurrent flow result in?

A

-Blood that is already well loaded with oxygen meets water with a maximum concentration of oxygen. Diffusion of oxygen from the water to the blood
- Blood with little oxygen in it meets water which has had most, but not all, of its oxygen removed. Diffusion of oxygen from water to blood takes place

Diffusion gradient for oxygen uptake is maintained across the entire width of the gill lamellae

27
Q

How is gas exchange in plants similar to insects?

A

No living cell far from external air - source of oxygen and carbon dioxide
Diffusion takes place in the gas phase (air) which is more rapid than water

28
Q

How is a leaf adapted for gas exchange?

A

Short, fast diffusion pathway
Air spaces inside a leaf have a very large surface area compared with the volume of living tissue
No specific transport system for gases - move in and through the plant by diffusion
Most gaseous exchange occurs

29
Q

How does gas exchange occur in a leaf?

A

Short, fast diffusion pathway
Air spaces inside a leaf have a very large surface area compared with the volume of living tissue
No specific transport system for gases - move in and through the plant by diffusion

30
Q

How is a leaf adapted for rapid diffusion?

A
  • Many small pores, called stomata, and so no cell is far from a stoma and therefore the diffusion pathway is short
  • Numerous interconnecting air-spaces that occur throughout the mesophyll so that gases can readily come in contact with mesophyll cells- steep concentration gradients
  • Thin-walled cells in the spongy mesophyll later
  • Large surface area of mesophyll cells for rapid diffusion
31
Q

What are stomata?

A

Minute pores on the leaves - especially underside
Guard cells open and close the stomatal pore - control rate of gas exchange
Balance needs of gas exchange and control of water loss

32
Q

Describe gas exchange in leaves during the night

A
  • Mesophyll cells respire, oxygen is taken up into the cells from the intercellular spaces and carbon dioxide is released
  • Low concentration of O2 is maintained within the intercellular spaces - concentration gradient that maintains a continual flow of oxygen
  • CO2 concentrations within the intercellular spaces, increase as respiration proceeds: CO2 diffuses out of the leaf
33
Q

Describe gas exchange in leaves during the day

A

Rate of photosynthesis > Rate of respiration
Carbon dioxide produced by respiring cells used in photosynthesis
Oxygen produced in photosynthesis is used in the process of respiration
Demand for CO2 greater than supplied by respiration - CO2 levels in the intercellular spaces is lower than that in the mesophyll cells and the gas diffuses into the cells

34
Q

What are xerophytes?

A

Plants with specific adaptations that enable them to survive in dry conditions

35
Q

What are the adaptations of insects that reduce water loss?

A

Small SA:vol - minimise the area over which water is lost
Waterproof coverings over body surfaces - insects have rigid outer skeleton of chitin covered with a waterproof cuticle
Spiracles - openings of the tracheae closed to reduce water loss (at rest as conflicts with demand for oxygen)

36
Q

How do plants reduce water loss?

A

Waterproof covering over parts of leaves and close stomata
Xerophytes - evolved to limit water loss through transpiration

37
Q

What are the adaptations of xerophytes to reduce water loss?

A

Thick cuticle
Rolling up leaves
Hairy leaves
Stomata in pits or grooves
Reduced surface area to volume ratio of the leaves

38
Q

How does a thick cuticle adaptation help reduce water loss in xerophytes?

A
  • Waterproof barrier but up to 10% of water loss can still occur by this route
  • Thicker = less water can escape
39
Q

How does the rolling up of leaves adaptation help reduce water loss in xerophytes?

A
  • Stomata mostly in lower epidermis
  • Rolling in a way that protects the lower epidermis from the outside helps to trap a region of still air
  • This region becomes saturated with water vapour = high water potential
  • No water potential gradient between inside and outside so no water loss eg marram grass
40
Q

How do hairy leaves help reduce water loss in xerophytes?

A
  • Traps still, moist air next to leaf surface, especially on the lower epidermis
  • Water potential gradient between inside and outside of the leaves is reduce = less water lost by evaporation eg heather plant
41
Q

How do stomata in pits or grooves help reduce water loss in xerophytes?

A
  • Traps still, moist air next to leaf surface and reduces water potential gradient eg pine trees
42
Q

How does a reduce SA: vol ration of the leaves help reduce water loss in xerophytes?

A
  • Smaller SA: vol = slower rate of diffusion
  • Leaves that have a small circular cross-section rather than broad and flat –> water loss can be considerably reduced
  • Reduction in SA is balanced against the need for a sufficient area for photosynthesis
43
Q

What are trichomes?

A

Hair like growths from epidermis

A-Level Biology (19 decks)

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