3.3.1 and 3.3.2 - surface area to volume ratios and gas exchange

What happens to relationship between volume and surface area as the size of the organism increases, and how does this affect being able to diffuse?

Surface area and volume both increase, but the sa:v ratio decreases, this means simple diffusion of substances across the outer surface can only meet the needs of relatively organisms, and it would take too long for substances from the outer surface to reach the middle of the organism if only diffusion was used

What is used in larger organisms to transport substances?

Movement of the environmental medium (maintains diffusion gradient), and a transport system to ensure the movement of the internal medium, eg blood. Has specialised gas exchange surfaces, and body surfaces that increase sa relative to volume

What are spircacles?

Tiny holes on the surface of the insect which can open and close.

What are trachae and tracheoles?

Tubes from the spiracle which lead into the body of an insect - supported by chitin rings.

Tracheoles: smaller tubes with dead ends that extend into body tissue bringing O2 directly to respiring cells.

What is the purpose of air sacs in insects?

Storage area for gases so the spiracles don’t need to open as much which conserves water.

How are gases exchanged via a diffusion gradient in the tracheal system?

Oxygen is used up in cells, which decreases the concentration on the ends of the tracheoles, which creates a diffusion gradient that allows O2 to diffuse from the tracheoles to the cells. CO2 has a diffusion gradient in the opposite direction.

How are tracheoles adapted for efficient gas exchange and prevention of water loss?

Highly branched which increases the surface area, and thin walls so the diffusion distance is shorter. Abdominal pumping also maintains a concentration gradient. Spiracles are often surrounded by hairs to trap water vapour. This minimises water loss by reducing the water potential gradient between the trachea and the air surrounding the spiracles. Exoskeleton is waterproof - made of chitin.

How does it benefit the insect having the ends of tracheoles filled with water?

In periods of major activity, the muscles respire anaerobically which produces lactate (soluble in water) so lowers cell water potential. Water then moves into the cells by osmosis which decreases the water in the ends of the tracheoles, drawing more air into them. This increases rate of diffusion.

How does abdominal pumping benefit the insect?

• Larger or more active insects use rhythmic body movements to actively ventilate their tracheal system. It increases the volume of the abdomen, lowering the pressure so that air is forced in with a higher concentration of oxygen into the tracheoles. An increase in pressure forces air with a higher concentration of carbon dioxide out of the tracheoles, maintaining the concentration gradient for oxygen and carbon dioxide.

Outline the cuticle and its adaptations:

Cuticle - waxy layer on epidermal cells, prevents water loss to stop plant drying out and is a protective layer.

Function of upper epidermis:

Upper epidermis - rectangular cells that are thin and transparent to allow light to enter

Function of palisade mesophyll:

Palisade mesophyll - rectangular cells with many chloroplasts to absorb more light and maximise photosynthesis. Tightly packed so more light is absorbed.

Function of spongy mesophyll:

Spongy mesophyll - increases sa:v ratio for gas diffusion due to air spaces

Function of lower epidermis:

Lower epidermis - protects the cell above and allows gas exchange and regulation of flow of water between internal leaf structures and the environment.

Function of stomata:

Tiny pores that occur mainly on the underside of leaves. Each hole is surrounded by guard cells that can open and close, so can control the rate of gaseous exchange, and also prevent water loss.

What adaptations do leaves have for rapid gas exchange?

Many stomata so the diffusion distance is short.

Large surface area of palisade mesophyll cells

Intercellular air space system allows for gases to readily come into contact with mesophyll cells

Photosynthesis establishes a concentration gradient

Leaf is thin so diffusion distance is short

How do plants limit water loss?

They have a waterproof covering, and the ability to close stomata when necessary.

What are xerophytes?

Plant adapted to reduce water loss so they can live in arid environments like deserts.

How does a swollen stem and thick cuticle help xerophytes?

• Swollen stem - reduces surface area that water is lost through, and is a storage of water.

• Thick cuticle - prevents water loss

How do rolled leaves help xerophytes>

• Most leaves have stomata mostly on the underside, so rolling the leaves protects the stomata and traps a layer of air within the rolled leaf. This are gets saturated with water vapour and has a high water potential. This reduces the w.p. gradient between leaf and air so less water is lost.

How do sunken stomata, hairy leaves and reduced s.a help xerophytes?

• Stomata in pits - these trap moist air next to the leaf so less water is lost

• Hairy leaves - traps moist air next to the leaf so the water potential gradient is reduced and less water is lost by evaporation.

• Reduced surface area to volume ratio of the leaves - small leaves reduce water loss.

Describe the structure of the gills:

There are 4 overlapping gill arches (bones), and each arch supports a double row of many gill filaments (primary lamellae). Each lamella has a row of secondary lamellae (gill plates), and this is the gas exchange surface.

How does a large number of the secondary lamellae benefit the gas exchange efficiency?

Provides a large surface area, which maximises the rate of diffusion of gases

How do thin epithelial cells and rich blood supply benefit the fish?

Provides a shorter diffusion distance, and a rich blood supply maintains a steep concentration gradient.

How do circulation and ventilation maximise gas exchange?

Replaces blood saturated with oxygen, and ventilation replaces water once oxygen is removed.

How does a counter current flow work in the gills, and how is this an advantage?

It increases the efficiency of diffusion of oxygen from water to the blood. Low oxygen levels in the blood meet a higher oxygen concentration in the water. This means equilibrium is never reached along the whole length of the secondary lamellae, and the concentration gradient is maintained. Water is always next to blood with an even lower oxygen concentration. Therefore 80% of oxygen available is absorbed.

Why does a large volume of 02 and CO2 have to be absorbed and removed in large mammals?

They are large organisms with many living cells, and also maintain a high body temperature related to high metabolic

Why are lungs inside the body?

Air is not dense enough to support these delicate organs, and the body as a whole otherwise would lose a lot of water.

What is the trachea?

Flexible airways supported by cartilage rings that prevent the trachea from collapsing as the pressure inside falls when breathing in. Walls made of muscle lined with epithelial cells and goblet cells.

What are the bronchi?

2 divisions of the trachea each leading to one lung, and have a similar structure to the trachea with cilial cells, and the amount of cartilage reduces as the size of the bronchi reduces.

What are the bronchioles?

Series of branching subdivisions of the bronchi that are made of muscle that can contract to control air flow.

What are the alveoli?

Tiny air sacs that are thin and flat lined with epithelial cells. In between the alveoli are elastic fibres and collagen that allow the alveoli to stretch and expand as they fill with air when breathing in.

What are the features of the alveoli that help it adapt for gas exchange?

The epithelial cells that make up the walls support a rich capillary network. These capillaries flatten the blood cells against the thin walls, which slows down the cells to allow more time for diffusion. Flattening the cells also reduces the diffusion distance.

Blood flow maintains a steep concentration gradient

The alveolar and capillary walls are very thin so reduce diffusion distance.

Alveoli have a huge surface area to maximise rate of diffusion

Breathing constantly ventilates the lungs which maintains a steep concentration.

What is inspiration?

When air pressure inside the lungs is less than the air pressure in the outside atmosphere so air is forced in.

What is expiration?

When the air pressure in lungs is greater than the outside atmosphere, so air is forced out

Explain the process of inspiration:

• It is an active process. External intercostal muscles contract and internal intercostal muscles relax.

• Ribs are pulled upwards and outward which increases volume of the thoracic cavity.

• The diaphragm contracts (flattens it) which also increases the volume.

• This lowers the pressure in thoracic cavity, so atmospheric pressure is greater than the lungs, forcing air in.

Explain the process of expiration:

• External intercostal muscles relax, internal intercostal muscles relax.

• Ribs move downwards and in, reducing thoracic cavity volume.

• Diaphragm relaxes to further decrease volume

• Pressure in the thorax increases, so it is higher than pressure outside the body, which forces air out down the pressure gradient.

• Elastic lung tissue recoils to resting position

How can more air be forced out of the lungs during exercise?

Contraction of the internal intercostal muscles to cause further lung and rib compression.

Contractions of muscles of the abdominal wall forcing intestines up, which forces the diaphragm up more.

What is Pulmonary ventilation rate (PVR)?

Total volume of air moved into the lungs in one minute

What is tidal volume?

Volume of air breathed in or out during a normal breath.

What is ventilation rate?

Number of breaths per minute

What is a spirometry?

Device used to measured volume of air/ oxygen consumption rate during ventilation

What is the PVR equation?

PVR = tidal volume x ventilation rate

What are the risk factors for lung disease?

Smoking, air pollution, genetics, infections, and occupations

What is correlation, and does it always mean a causation?

When 2 factors are linked so a change in one variable is reflected in a change in another, and does not always mean one causes the other.