Biology unit 3

What is the surface area to volume ratio of small single celled organisms?

small organisms have a surface area that is large enough compared with their volume, to allow efficient exchange across their body surface. There diffusion distance is smaller and therefore diffusion is faster

What is the surface area to volume ratio of larger organisms?

larger organisms have a smaller surface area to volume ratio, this means there is a longer diffusion distance and the organisms cannot get the substances they need fast enough and so have specialised features

What are the features of specialised exchange surfaces?

• larger surface area relative to the volume of the organism which increases the rate of exchange

• very thin so that the diffusion distance is short and therefore materials cross the exchange surface rapidly

• selectively permeable to allow selected materials to cross

• movement of the environment medium for example air to maintain a diffusion gradient

• a transport system to ensure the movement of the internal medium (for example blood) in order to maintain a diffusion gradient

What is gas exchange in insects?

Insects have evolved an internal network of tubes called trachea.

• The trachea are supported by strengthen rings (chitin) to prevent them from collapsing

these trachea then decide into smaller dead end tubes called tracheoles. These extend throughout all the body tissues of the insect. In this way atmospheric air, with the oxygen it contains is brought directly to the respiring tissues as there is a short diffusion pathway from a tracheole to any body cell

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

Along a diffusion gradient - when cells are respiring, oxygen is used up and so its concentration towards the ends of the tracheoles falls. This creates a diffusion gradient that causes gaseous oxygen to diffuse from the atmosphere along the trachae and the tracheoles to the cells. Carbon dioxide is produced by cells during respiration, this causes gaseous carbon dioxide to diffuse along the tracheoles and trachea from the cells to the atmosphere

Mass transport - the contraction of muscles in insects can squeeze the trachea enabling mass movements of air in and out, this further speeds up the exchange of respiratory gases

The ends of the tracheoles are filled with water - during periods of major activity the muscle cells around the tracheoles respire carry out some anaerobic respiration. This produces lactate which is soluble and lowers the water potential of the muscle cells. water therefore moves into the cells from the tracheoles by osmosis. The water in the ends of the tracheloes decreases in volume and in doing so draws air further into them. This means the final diffusion pathway is in a gas rather than a liquid phase and therefore diffusion is more rapid

How do insects limit water loss?

small surface area to volume ratio - to limit the area over which water is lost

waterproof coverings over their body surfaces - this is a rigid exoskeleton of chitin that is covered with waterproof cuticle

spiracles - opening of the trachea at the body surface and these can be closed to reduce water loss. this conflicts with the need for oxygen and so occurs largely when insects are at rest

How does gas enter and leave insects?

gases enter and leave trachea though tiny pores called spiracles on the body surface. the spiracles may be opened and closed by a valve. When spiracles are open water vapour can evaporate from the insect. however for much of the time insects keep their spiracle closed to prevent this water loss and only open them to allow gas exchange

What is the counter current flow?

This is important for ensuring maximum possible gas exchange is achieved.

• blood that is already well loaded with oxygen meets water which has its maximum oxygen concentration. Therefore diffusion of oxygen occurs from the water to the blood takes place

• Blood with little oxygen in it meets water which has had most but not all of its oxygen removed. Diffusion of oxygen from the water to blood takes place

As a result a diffusion gradient for oxygen uptake is maintained across the entire length / width of the gill lamellae. If the flow of water and blood has been in the same direction the diffusion gradient would only be matinee across part of the length of the gill lamellae and only 50% of the valuable oxygen would be absorbed by the blood

What is the structure of fish gills?

the gills are located within the body of the fish, behind the head.

• These are made up of gill filaments which are stacked up in a pile like pages in a book.

• At right angles to the gill filaments are gill lamellae which increase the surface area of the gills.

The flow of water over the gill lamellae and the flow of blood within them are in opposite directions. This is known as the counter current flow

What reduces gas exchange with the external air for a plant?

at times the gases produced in one process of either photosynthesis or respiration can be used for the other process. This reduced gaseous exchange with the external air. Overall this means that the volumes and types of gases that are being exchanged by a plant leaf change. This depends on the balance between the rates of photosynthesis and respiration

• when the plant is photosynthesising, although some carbon dioxide comes from respiration of cells most of it is obtained from the external air. In the same way some oxygen from photosynthesis is used in respiration but most of it diffuses out of the plant

• when the plant is respiring oxygen diffuses into the leafs because it is constantly being used by cells during respiration, in the same way carbon dioxide produced during respiration diffuses out

What is the structure of a plant leaf and gas exchange?

• no living cell is far from the external air and therefore a source of oxygen and carbon dioxide

• diffusions takes place in the case phase, which makes it more rapid than in if it were in water

• airspaces between mesophyll cells and large surface area of mesophyll cells for rapid diffusion

• many small pores called stomata and so no cell is far from stomata and therefore the diffusion pathway is short

what are the stomata?

Minute pores that occur mainly on the leaves, especially on the underside. Each stomata is surrounded by a pair of specialised cells called guard cells.

Guard cells can open and close the stomatal pore. in this way they can control the rate of gaseous exchange, this is important as these organisms lose water by evaporation, plants need to balance their conflicting needs of gas exchange and control the loss of water, they do this by closing the stomata at different time when water loss would be excessive

How do plants limit water loss?

• ability to close stomata when necessary

• waterproof coverings over parts of their leaves - thick waxy cuticle

• rolling up of leaves, hairy leaves and stomata in pits and grooves - these trap still, moist air next to the leafs surface. The water potential gradient between the inside and outside of the leaves is reduced and therefore less water is lost by evaporation

• reduced surface area to volume ratio of leafs - the smaller the surface area to volume ratio, the slower the rate of diffusion. by having leaves that are small and rough circular in cross section as in pine needles rather than leaves that are broad and flat the rate of water loss can be reduced

What is the pathway of air in mammals?

The lungs are a pari of lobed structures made up of a series of highly branches tubes and tubules called bronchioles, which end in tiny air sacs called alveoli

• Trachea - flexible airway that is support by rings of cartilage, this prevents the trachea collapsing as the air pressure inside falls when breathing in. The tracheal walls are made up of muscles, lined with ciliated epithelium and goblet cells

• Bronchi - 2 divisions of the trachea, each leading to one lung. The longer bronchi are supported by cartilage, this cartilage is reduced as the bronchi get smaller. They also produce mucus to trap dirt particles and have cilia to move the dirt laden mucus towards the throat.

• Bronchioles - series of branching subdivisions of bronchi. Their walls are made of muscle lined with epithelial cells. this muscle allows then to constrict so that they can control the flow of air in and out of the alveoli

• Alveoli - are minute air sacs, at the end of the bronchioles. Between the alveoli there are some collagen and elastic fibres. The elastic fibres allow the alveoli to stretch as they fill with air when breathing in, they then spring back during breathing out in order to expel carbon dioxide rich air.

How does the body inspire?

• The external intercostal muscles contract while the internal intercostal muscles relax

• The ribs are pulled upwards and outwards, and the diaphragm muscles contract causing it to flatten this increases the volume of the thorax

• The increased volume of the thorax results in reduction of pressure in the lungs

• Atmospheric pressure is now greater than the pulmonary pressure and so air is forced into the lungs

How does the body expire?

• the internal intercostal muscles contract while the external intercostal muscles relax

• The ribs move downwards and inwards and the diaphragm muscles relax and so it domes up, this decreases the volume of the thorax

• this decreased volume of the thorax increases the pressure in the lungs

• the pulmonary pressure is now greater than that of the atmosphere and so air is forced out the lungs

What is the role of the alveoli in gas exchange?

• red blood cells are slowed as they pass through pulmonary capillaries, allowing more time for diffusion

• the distance between the alveolar air and red blood cells is reduced as the red blood cells are flattened against the capillary walls

• the walls of both alveoli and capillaries are very thin and therefore the distance over which diffusion takes place is very short

• alveoli and pulmonary capillaries have a very large total surface area

• blood flow through the pulmonary capillaries maintains a concentration gradient

what is the role of the salivary glands?

situated near the mouth. They pass their secretions via a duct into the mouth. These secretions contain the enzyme amylase which hydrolyses starch into maltose

What is the role of the pancreas?

large gland situated below the stomach. It produces a secretion called pancreatic juice. This secretion contains:

• protease to hydrolyse proteins

• lipase to hydrolyse lipids

• amylase to hydrolyse starch

What is physical break down of food?

food is large and broken down into smaller pieces by the means of structure such as the teeth. This not only makes it possible to ingest the food but also provides a large surface area for chemical digestion

food is also churned by the muscles in the stomach wall and this also physically breaks it up

what is chemical digestion of food?

chemical digestion hydrolyses large, insoluble molecules into smaller soluble ones. It is carried out by enzymes:

• carbohydrases - hydrolyses carbohydrates, intimately to monosaccharides

• lipases - hydrolyses lipids into glycerol and fatty acids

• proteases - hydrolyses proteins to amino acids

What is the process of carbohydrate digestion

• saliva contains salivary amylase, this hydrolyses any starch in the food to maltose. It also contains mineral salts that help to maintain the pH around neutral the optimum pH for salivary glands to work

• food is swallowed and enters the stomach where the conditions are acidic. The acid denatures the amylase and prevents further hydrolysis of starch

• the food is passed into the small intestine where is it mixed with pancreatic juices this contains amylase and this hydrolyses the remaining starch to maltose. Alkaline salts are produced by both pancreas and the intestinal wall to maintain the pH around neutral so that the amalyse can function

• muscles in the intestine wall push the food along the ileum. its epithelial lining produces the enzyme maltase, this hydrolysys’s the maltose from starch into alpha glucose

What is the process of lipid digestion?