Exchange surfaces

Question 1

Gas exchange in insects

Answer 1

Network of tubes called tracheae and tracheoles, pores called spiracles on the surface

Extensive network means there is a short diffusion pathway to every cell
Oxygen is used up at the ends of the tracheoles which maintains a concentration gradient
Insects can contract some muscles to force air in and out of spiracles
The ends of the tracheoles are filled with water and in periods of major activity lactate is produced in anaerobically respiring cells, so the water moves into the cells by osmosis and the air can move faster can it doesn’t have to travel through water anymore
Spiracles are closed much of the time to prevent water loss

Question 2

Gas exchange in fish

Answer 2

Long sticky-out things called gill filaments which have little lumps called lamellae on them.

Lamellae increase the surface area
Countercurrent flow makes sure as much gas is exchanged as possible

Countercurrent flow:
Blood with already quite a lot of oxygen meets water at maximum concentration of oxygen, so oxygen diffuses from the water to the blood.
Blood with little oxygen meets water which has had most of its oxygen removed, so oxygen diffuses from the water into the blood.
Parallel flow only allows for 50% of the oxygen to diffuse but countercurrent flow means around 80% can diffuse.

Question 3

Gas exchange in plants

Answer 3

Structure of a leaf from top to bottom: waxy cuticle, upper epidermis, palisade mesophyll, spongy mesophyll and air spaces, lower epidermis and guard cells and stomata.

Leaf has a large surface area to volume ratio
Leaf is thin so short diffusion pathway
Air spaces encourage diffusion and increase the surface area of the spongy mesophyll
Lots of stomata so short diffusion pathway

Stomata:
Pores mostly on the lower epidermis of a leaf
Controlled by a pair of guard cells which have a thicker inner wall and a thinner outer wall; when they have water they swell and push against the outer wall which opens the stomata, the stomata closes again when the don’t have much water.
This reduces water loss.

Question 4

Limiting water loss

Answer 4

Insects:
Waterproof cuticle over their outer surface
Small surface area to volume ratio
Spiracles mostly closed

Plants:
Waxy cuticle
Stomata can close when necessary
Xerophytic plants can have hairy or rolled up leaves, a thicker cuticle, stomata in pits, or a reduced surface area.

Question 5

The human gas exchange system

Answer 5

Trachea is supported by rings of cartilage to stop it from collapsing when the pressure falls from breathing in, it has walls made of muscle and lined with cilia and mucus.
Bronchi are similar to the trachea, the amount of cartilage gets smaller as the bronchi gets smaller.
Bronchioles have muscular walls lined with epithelial cells to control the flow of air in and out of the alveoli.
Alveoli are lined with epithelium and between alveoli are collagen and elastic fibres; the elastic fibres allow the alveoli to stretch when breathing in and spring back to expel carbon dioxide-rich air.

Question 6

The mechanism of breathing

Answer 6

Breathing in (active process, uses energy)
External intercostal muscles contract, internal intercostal muscles relax
Ribs are pulled up and out, increasing the volume of the thorax
Diaphragm muscles contract, diaphragm flattens, also increasing the volume of the thorax
Pulmonary pressure decreases to lower than atmospheric pressure so air is forced into the lungs

Breathing out (largely passive process, uses not much energy)
Internal intercostal muscles contract, external intercostal muscles relax
Ribs move down and in, decreasing the volume of the thorax
Diaphragm muscles relax, diaphragm pushed up by the (previously compressed during inspiration) contents of the abdomen, also decreasing the volume of the thorax
Pulmonary pressure increases to higher than atmospheric pressure so air is forced out of the lungs

Question 7

Gas exchange in lungs

Answer 7

Pulmonary capillaries are so narrow red bloods cells have to squeeze through, slowing them down so more time for gas exchange
This, along with the thin alveolar and capillary walls, reduces the diffusion pathway
Alveoli and network of capillaries have a large surface area
Breathing constantly ventilates the lungs and the heart constantly circulates blood so a steep concentration gradient is maintained

Question 8

Enzymes and digestion

Answer 8

Carbohydrates
Salivary amylase is found in saliva
The pancreas makes pancreatic amylase found in the small intestine
Maltase is found in the cell membranes of the epithelial cells of the ileum

Lipids
Lipids are split into tiny droplets called micelles by bile salts produced by the liver (emulsification)
Surface area to volume ratio increased
Lipases in the pancreas can digest the lipids much faster

Proteins
Proteins are hydrolysed by peptidases
Endopeptidases hydrolyse peptide bonds in the middle of the protein
Exopeptidases hydrolyse peptide bonds at the ends of the protein
Dipeptidases break up dipeptides