Gas exchange in insects: trachea

Gas exchange in insects: trachea • Insects have a series of air-filled tubes, trachea, that provide oxygen to living cells. These trachea make up the respiratory system of insects. • Air enters the system via various inlets called spiracles and move into the main trachea that continue to branch, getting smaller each time, with tracheoles being the smallest into the insect cells. • Tracheole ends make direct contact with the body cell, and oxygen diffuses from tracheoles across the membrane • Gas exchange occurs similarly to vertebrates and fish, whereby CO2 is exchanged for O2 via the process of diffusion.

Comparing positioning of respiratory systems • Complete Quick-check questions 11-16 on page 150.

Transport of wastes • Metabolic activities produce wastes that cannot be utilised by the body – in fact, they have to be eliminated. This process is known as excretion. • The main wastes that are generated by metabolic activities and need to be excreted are: • Carbon dioxide – excreted via the lungs as a waste product of cellular respiration • Nitrogenous wastes such as urea – excreted via the kidneys in the urinary system

Nitrogenous wastes in animals • When animals metabolise protein, nitrogen-containing compounds are produced as wastes – called nitrogenous wastes. These need to e excreted as they can cause damage to tissues if they are left to accumulate. • Different animals excrete different forms of nitrogenous waste, depending on the environment they live in. • When protein is metabolised, it initially produces ammonia, a very toxic form, which must be excreted immediately and can only be excreted in this form if the animal lives in water – fish excrete ammonia. • In organisms without a ready supply of water, the ammonia must be converted to a less toxic form of nitrogenous waste – called urea. Mammals excrete urea. This conversion requires energy. • Another form, uric acid, is excreted by birds and reptiles. It requires lots of energy but very little water to excrete this form.

The urinary system • Most nitrogenous wastes are removed from the mammalian body by the urinary system. • Water is the most common compound in the body, and the urinary system plays a significant role in maintaining water balance in mammals. • The urinary system is comprised of two kidneys, two ureters, a bladder and a urethra. • Figure 6.38 – Components of the human urinary system.

Kidneys - function • Humans have two kidneys, flattened, bean-shaped organs located on each side of the spine, partially protected by the rib cage and embedded in a mass of fat for protection. • Kidneys play a major role in the the balance of water in mammals. • Kidneys are adapted for filtering wastes (including nitrogenous wastes) from the blood. They also excrete hormones and other substances such as vitamins that would otherwise build up in the body. • Kidneys also maintain a correct balance of ions in the blood by excreting those that are in excess, which helps maintain the pH of the blood.

Kidneys - structure • The outer-most layer of kidney tissue is called the cortex, a granular-looking layer that extends into the second, striated layer, the medulla. The functional units of the kidney, nephrons, are found in these regions. • Each kidney contains about one million nephrons. It is the process of filtration and reabsorption that regulates the body fluid composition. • The artery that enters the kidney, the renal artery, branch into smaller and smaller arteries and then eventually into capillaries. These capillaries form small clusters, called glomerulus. • Each glomerulus is surrounded by a double walled cup, known as a Bowman’s capsule, which forms part of the tubule. The remainder of the tubule coils away from the glomerulus and eventually forms a collecting tubule, part of which is the Loop od Henle.

The arteriole that leaves the capillary network in the glomerulus winds around the tubule before it joins with the other vessels to form a vein that leaves the kidneys.

Formation of urine

Formation of urine - task • Read through the information under the sub heading ‘Formation of urine’ on pages 153 -154 as well as the information in Figure 6.41 on page 154. • Construct a table which summarises the three main stages of the formation of urine: • Filtration • Reabsorption • Excretion • Pay particular attention to explaining the parts of the nephron, the movement of molecules and the way molecules are moving.

Hypertonic vs Hypotonic • Mammals produce hypertonic urine – urine that contains a higher concentration of solutes than do the body fluids of the same animal. • Other animals, for example freshwater bony fish and amphibians, produce hypotonic urine – urine that contains a lower concentration of salts than do the body fluids of that same animal. • Hypertonic urine is said to be more concentrated, whereas hypotonic urine is said to be more dilute. • After it is formed, urine is carried away from the kidneys via the ureters. Two ureters, one from each kidney, transport the urine to the bladder, which stores the urine until it leaves the body. Urine is transported from the bladder to outside the body by the urethra.

Excretion in fish Summarise excretion in fish: - compare saltwater fish to freshwater fish.

Excretion in insects (and spiders) • Insects have specialised excretory organs called Malpighian tubules. They extend into the body cavity from the intestine and float freely in the blood in the open circulatory system. • Wastes and fluid pass from the blood into the tubules where nitrogenous wastes are converted into uric acid crystals. • Material from the tubules is then emptied into the intestine. • Fluids are reabsorbed by the intestine and the uric acid crystals are excreted through the anus.

Comparing systems in animals Complete Quick-check questions 17-22 on page 157.

Transport in plants • Plants transport materials in vascular tissue, which is made of xylem and phloem. • Xylem transports water and dissolved minerals. Some minerals are found in relatively large amounts (macronutrients) for normal growth. Other nutrients are required in very small amounts, these are called micronutrients or trace elements. • Phloem transports sucrose, produced by photosynthetic tissue, to other regions of the plant that are non-photosynthetic. Hormones and any other organic materials made by the plant also travel through the phloem.

Water is absorbed by the root hairs and moves up the xylem vascular (called xylem parenchyma cells) tissue via the process of cohesion – the water molecules tend to stick together, being pulled up through the hollow xylem cells towards the leaves, where they are required for photosynthesis. Transpiration refers to the movement of water throughout the plant. Xylem

Phloem • Soluble organic substances move through the phloem tissue, mainly in the form of sucrose. • These substances are moved from the leaves where they are synthesised, to other parts of the plant where they are used or stored. • The transport of organic substances through the plant is called translocation. • Photosynthetic leaves produce sugars, so are the source of organic matter for the rest of the plant. • Sucrose is actively transported from phloem tissue into cells where it is used in different ways. It can be: • Stored as starch • Stored as subunits for building structural components of cells • Used as an energy source in cellular respiration (glucose)

Figure 6.48 – Summary of the processes of transpiration in xylem tissue and translocation in phloem tissue. • Complete Quick-check questions 23 and 26 on page 163.

Gaseous exchange in plants • Many plants have specialised structures for gas exchange. • Although some CO2 is produced by plant cells as they carry out respiration, the amounts produced are far too insufficient for adequate rates of photosynthesis – they require CO2 from the air. • Stomata in the leaves are the main site for gas exchange in the plant – comprised or a stoma and two guard cells. • Each stoma (pore) is surrounded by two guard cells, that control whether the stoma is open or closed. • The shape of the guard cell is influenced by the amount of water vapour in the leaves. • When the guard cells have a high amount of water, they become full or turgid, and the stoma opens. • When the guard cells lose water, they become more flaccid and the pore closes.

Gases are not the only substance to move into and out of the stomata – water vapour also leaves through the stoma. This occurs at a greater rate on dry days where the humidity (water vapour content of the air) is low. • Some plants have specialised features to help reduce water loss. • Lenticels are specialised openings to allow gas exchange in stems – to much less extent than stomata in leaves. • Gaseous exchange also occurs in the roots, aided by the root hairs – oxygen which is required for cellular respiration diffuses in to the root hairs and carbon dioxide diffuses out.

Excretion in plants • Unlike animals, plants have no specialised excretory systems. • They either use their waste or store the waste in a situation that poses no problems or harm to the cells. • In plants, the term ergastic substances is used instead of excretory products. These substances include stored food materials such as starch granules. • Unwanted ions such as sodium and chloride remain in the leaves that are shed by deciduous plants. • Some plants have special features that enable them to eliminate substances that have been absorbed from the soil along with the water – demonstrating that plants can remove unwanted substances.

Comparing plants with mammals • The transport systems of plants and mammals have some similarities – both are made up of a network of fine tubes that penetrate tissues and come into close contact with every cell and they each supply their tissues with nutrients, but there are some important differences: • Mammals require blood to carry and a heart to pump O2 to all cells; plants lack an equivalent organ. • Mammals have specialised pigmented cells, e.g. haemoglobin to handle the increased O2 demands in animal cells; plants do not have the same demands and therefore lack these. Most of the O2 required by plants is obtained as a product of photosynthesis. • Bulk intake of food for energy in mammals produces large amounts of toxic nitrogenous waste that must be eliminated through a specialised excretory system; plants either use or store any ergastic substance produced at no harm to cells.

Questions and revision • Complete Quick-check questions 27-30 on page 167. • Complete Quick-check questions 31-32 on page 169. • Biochallenge question 1 on page 170. • Complete the following Chapter Review questions on pages 171-173. • 3, 6, 7, 11, 13 and 14

Gas exchange in insects: trachea