Topic 3: Exchange of substances Flashcards
What is tissue fluid?
How is it kept constant and why?
The environment around the cells of multicellular organisms.
Most cells are too far from exchange surfaces or the external environment for diffusion to supply and remove materials to keep its composition constant. Mass transport systems are used for this instead.
What do organisms need to exchange with their environment? Give examples
- Respiratory gases (e.g O2/CO2)
- Nutrients (e.g glucose, fatty acids, amino acids)
- Excretory products (e.g urea, faeces, CO2)
- Heat
What is the difference between passive and active exchange?
Active exchange requires metabolic energy, passive exchange does not
How do very small organisms exchange substances with their environment? Why?
Simple diffusion of substances across the outer surface is sufficient to meet their needs because they have a very large surface area to volume ratio.
Single-celled organisms have a surface only covered by a cell-surface membrane or cell wall so there is a short diffusion pathway.
Why do large organisms not exchange substances with their environment via diffusion?
As organisms get larger, their volume increases at a faster rate than surface area, decreasing surface area to volume ratio.
Metabolic rate is proportional to volume but diffusion is proportional to surface area, so the rate of diffusion would not be sufficient enough to support their needs.
It would also take too long to reach the centre of the organism as most cells aren’t in contact with the environment.
How have organisms evolved to exchange substances?
- A flattened shape so no cell is far from the surface for simple diffusion
- Specialised exchange surfaces with large areas to increase surface area to volume ratio
What is the equation for the surface area of a sphere?
What is the equation for the volume of a sphere?
SA = 4 x pi x r^2
V = 4/3 x pi x r^3
Give some features of exchange surfaces and state how they are an advantage
- Large SA:V - increases rate of exchange
- Very thin - short diffusion pathway to increase rate of exchange
- Selectively permeable - allows selected materials to cross
- Movement of environmental medium - maintains concentration gradient to increase rate of exchange
- Transport system - maintains internal medium movement to maintain concentration gradient to increase rate of exchange
Why are exchange surfaces usually located inside the organism?
They are very thin, so are easily damaged and dehydrated. They would dessicate
How have insects evolved to reduce water loss?
- Small SA:V to minimise the area over which water is lost
- Waterproof cuticle over their rigid chitin exoskeleton
- Spiracles can be closed to reduce water loss. Conflicts with the need for oxygen so usually occurs at rest
- Spiracle hairs trap water being evaporated, maintaining a humid environment in the spiracle, reducing water loss. Also minimises bulk air movement through the gap, reduces the area of the spiracle and allows for filtering of the air
Describe the structure of the insect gas exchange system
Tracheal system:
- Spiracles - tiny pores on the body surface that can be open/closed by a valve
- Tracheae - internal network of tubes attached to spiracles. Supported by rings of chitin ( a nitrogen-containing polysaccharide) to prevent them from collapsing at low pressures
- Tracheoles - smaller, dead-end tubes that branch from tracheae into body tissues - atmospheric air is brought directly to respiring tissues, providing a short diffusion pathway to any body cell
Give three ways that respiratory gases move in/out the insect tracheal system
- By concentration gradients
- Using mass transport (abdominal pumping)
- Using the water-filled ends of tracheoles
Describe how the tracheal system in insects uses concentration gradients to exchange gases
- Oxygen is used in respiring cells, creating a concentration gradient in the system
- Oxygen diffuses from the atmosphere, through the tracheae and tracheoles into the cells
- The inverse happens with carbon dioxide
Describe how the tracheal system in insects uses abdominal pumping to exchange gases
- Creates mass transport
- Muscles pull segments of the skeletal plates together
- This squeezes air into sacs deeper in the tracheal system
- Reduces pressure in the system to lower than atmospheric pressure
- Spiracles open and air moves in
Describe how the tracheal system in insects uses the water-filled ends of tracheoles to exchange gases
- In periods of activity, muscles respire partially anaerobically, producing soluble lactate
- Lowers the water potential of cells
- Water moves from the tracheole into the cell by osmosis
- Air is drawn into the space in the tracheole vacated by water
- Increases the rate of diffusion as there is a smaller diffusion pathway and diffusion occurs more quickly in the gas phase than liquid.
Why do fish need gills for gas exchange?
Need a specialised exchange surface as they can’t do gas exchange just by diffusion because:
- Fish have a waterproof, gas-tight outer covering
- They have a small SA:V
- Oxygen is not very concentrated in water and warm water has less oxygen dissolved in it than cold water
Describe the location of the gills and the flow of water through them
Located just behind the head and are protected by the operculum as they are very delicate.
Water passes through the mouth, past the gills and out the operculum opening, maintaining a water current across the gills.
Describe the structure of the gills in fish
- Made from gill filaments stacked in a pile (increases surface area)
- Gill lamellae sit at 90 degrees to the filaments (increases surface area)
- Thin epithelium (short diffusion pathway)
- Good blood supply (maintains a concentration gradient)
- Water and blood flow in opposite directions = countercurrent flow (maintains concentration gradient across whole gill lamellae)
What is the countercurrent exchange principle?
Blood + water flow in opposite directions through the gills.
Maintains a concentration gradient across the whole length of the lamellae as the concentration of oxygen in the water is always slightly higher than the concentration in the blood.
Circulation replaces oxygen-rich blood, ventilation replaces oxygen-poor water.
Oxygen never reaches equilibrium so about 90% of oxygen from the water can be absorbed.
What is parallel flow?
Blood and water flow in the same direction through the gills.
There is no net diffusion once the concentrations of oxygen equalise so diffusion only occurs at the first part of the filament.
How does gas exchange in plants change with the activity of the plant?
- When more photosynthesis is happening, CO2 from the air is used and O2 produced. Some CO2 comes from respiration but most diffuses in. Some O2 is used in respiration but most diffuses out
- When photosynthesis slows down/stops, O2 diffuses in as it is used in respiration, CO2 produced by respiration diffuses out
Describe the adaptations of leaves for gas exchange
- Flat shape = no living cell is far from external air (short diffusion pathway)
- Diffusion occurs in the gas phase (quicker than as a liquid)
- Air spaces in the spongy mesophyll tissue increases the surface area
What are stomata and where are they located?
Small pores on leaves surrounded by a pair of guard cells which open/close the stomatal aperture (the opening).
They are small and mostly on the underside of leaves as it is shaded there, reducing water loss by evapotranspiration
Describe how stomata help to regulate water levels in plants
They open/close to balance the need to reduce water loss with the need to maximise gas exchange.
When the plant has a lot of water, guard cells become turgid and open, allowing gas exchange.
When the plant is short of water, guard cells become flaccid and close, preventing water loss.
Sensitive to light (close at night to reduce water loss as photosynthesis can’t occur so there is no need for gas exchange)
The opening/closing is possible due to the thickened inner walls + thin outer walls
How do guard cells work?
- Actively transport K+ ions inside to control water movement
- Concentration of K+ ions inside guard cell reduces water potential
- Water moves into guard cell by osmosis
- Guard cell expands and becomes turgid
- Rigid inner wall resists expansion + it becomes more curved
- Stomata open because the inner wall is less flexible than the outer wall
- When they lose water, they become flaccid and collapse, closing the stomata
What is a xerophyte?
Plants adapted to living in areas with a limited water supply
Name some adaptations of xerophytes
- Thick waterproof waxy cuticle
- Rolled leaves
- Hairy leaves
- Sunken stomata in pits on the epidermis
- Stomata at the bottom of ridges
- Low stomatal density
- Small SA:V
- Barrel-like stems
- Extensive roots
Explain these adaptations of xerophytes and give an example:
- Thick waterproof waxy cuticle
- Rolled leaves
- Increases length of diffusion pathway and makes leaf impermeable to water, e.g holly
- Reduces surface area of exposed leaf. Traps still air inside which becomes saturated with water vapour so has a high water potential. No water potential gradient between the inside and outside of leaf so no water loss, e.g marram grass
Explain these adaptations of xerophytes and give an example:
- Hairy leaves
- Sunken stomata in pits on the epidermis
- Especially on the lower epidermis, traps still moist air next to the leaf, reduces the water potential gradient, reducing water loss, e.g heather
- Traps moist air next to the leaf, lowering the water potential gradient. Shelters stomata from conditions that increase transpiration, e.g pine trees
Explain these adaptations of xerophytes:
- Stomata at the bottom of ridges
- Low stomatal density
- Sun only shines directly on stomata in brief periods of the day, reducing transpiration
- Reduces water loss by evaporation
Explain these adaptations of xerophytes:
- Small surface area to volume ratio
- Barrel-like stems
- Extensive roots
- Small and roughly circular cross sections of leaves reduce rate of evaporation
- Lets plant hold a lot of water
- Very deep roots can reach and absorb more water. Very shallow, widespread roots easily absorb any rainfall from the surface. Some plants can store water in their roots
Give some places that xerophytes can be found
- Deserts (low rainfall + high temperatures)
- Sand dunes (high winds on coast increase transpiration, rain drains through sand quickly out of reach from roots)
- Cold regions (water is frozen so very difficult to absorb)
- Salt marshes (the water has a low water potential due to the high salt concentration so is difficult to absorb)
Why do aerobic organisms need to exchange gases?
They need a constant supply of oxygen to release energy in the form of ATP during respiration. The carbon dioxide must be removed as its build-up could be harmful for the body
What is the relative volume of gases exchanged for mammals and why?
How have they adapted to cope with this?
The volume of oxygen absorbed and carbon dioxide removed is large because:
- They’re relatively large organisms with a large volume of living cells
- They maintain a high body temperature so have high metabolic and respiratory rates
They have evolved lungs as a specialised gas exchange surface
