Patterns In Nature
Part 4
Gaseous exchange and transport systems transfer chemicals through the internal and between the external environments of plants and animals
· compare the roles of respiratory, circulatory and excretory systems
Respiratory Systems:
- All organisms respire to produce energy; this requires oxygen
- Plants photosynthesise to produce food; this requires carbon dioxide
- These processes require gases from the external environment
- Respiratory systems are the systems that allow gas exchange for the organism
- Respiratory systems must be thin and moist, and have a large surface area, so that gases can DIFFUSE freely
- They also bring together the internal and the external environment
- Aquatic organisms expose their respiratory systems to the environment
- Terrestrial organisms have their respiratory systems enclosed, to reduce water loss by evaporation
Circulatory Systems:
- In unicellular organisms, all nutrients needed can be diffused from the external environment over their surface area
- Wastes can just be removed from cells by diffusion as well
- However, in multicellular organisms, the surface area is not great enough to provide nutrients for all the organisms cells
- Circulatory systems are used to carry nutrients to all the body’s cells, and to carry wastes away
- The flow of materials is usually maintained by a pump (such as the heart)
- Circulatory systems can be open or closed
Excretory Systems:
- Excretion is the removal of metabolic waste products from an organism
- The metabolic processes constantly produce wastes. If they were allowed to accumulate, the organisms would be poisoned by its’ own wastes and die.
- The main waste products of organisms are: carbon dioxide made in respiration (excreted by respiratory system) and nitrogenous wastes made during the breakdown of proteins and nucleic acids (excreted as ammonia, urea or uric acid).
§ identify and compare the gaseous exchange surfaces in an insect, a fish, a frog and a mammal
- All organisms respire; that is, they take in oxygen from their external environment and release carbon dioxide. To obtain the oxygen required by cells in respiration and to get rid of unwanted carbon dioxide produced, multicellular animals have developed specialised gas exchange or respiratory surfaces.
- Respiratory surfaces must be thin and moist with a large surface area so that the gases can diffuse through. They must also bring together the internal and external environments so that gases can be exchanged between the cells and the organism’s surroundings.
- The respiratory surfaces of aquatic animals are more exposed to the external environment than those of land animals. Land animals have internal respiratory surfaces that they can keep moist without too much loss of water from evaporation.
Insects:
- Insects have a system of interconnecting tubes called tracheae within their body.
- These tracheae are lined with cuticle and open to the external environment through pores or spiracles on the segments of the abdomen.
- In many insects, especially those that fly, the tracheal system includes inflatable air sacs that hold a reserve of air that can be pumped to tissues needing an extra supply, such as wing muscles.
The spiracles are divided into air-intake spiracles and air-expiration spiracles, and can be closed to prevent water loss.
- The cuticle lining the tracheae lacks the cement and wax layers of normal cuticle, and it is therefore permeable to water.
- The tracheae branch throughout the tissues of the insect, bringing air directly to the cells. The endings of the tracheae, called tracheoles, are normally filled with fluid.
- The volume occupied by the tracheal system is relatively large (up to half the total body volume), so the surface area of the tracheae is sufficient to supply all the body cells with oxygen and remove carbon dioxide.
- Aquatic insects, including the aquatic larval stage of many terrestrial insects, usually have a modified system of spiracles or gas gills that take in air directly, either at the water surface or from bubbles of air trapped against the body. In some groups, such as dragonflies and mayflies, the aquatic larvae have feathery gills that take oxygen from the water by diffusion.
Fish
- In fish, the respiratory surfaces are usually gills over which water flows.
- Some animals have exposed gills (e.g. sharks) and others have gills covered by an operculum (e.g. bony fish).
- Gills are usually finely divided, and the incoming water flows over a large surface area at the one time. There is a plentiful blood supply to transport the gases to and from the gills.
(a) Water flows across fish gills in one direction, through the mouth and pharynx, past the gills and out under the operculum. Each gill arch has many finger-like gill filaments and each of these has a row of closely packed, flat, leaf-like lamellae across which gas exchange occurs. The lamellae provide a large surface area for exchange and are visibly red because they contain many blood vessels. Water and blood flow in opposite directions through the lamellae, which increases the amount of gas exchanged.
(b) A row of lamellae along the gill filament of a rainbow trout.
Frogs
- Frogs have two respiratory surfaces: the lungs and the skin.
- There is a very well developed blood supply to the skin.
- Oxygen from the air diffuses into the moist skin (subcutaneous breathing) and is transported by the blood to the heart, from where it is sent directly to the body.
- The lungs are simpler structures with a smaller surface area than those of mammals. They hang in the frog’s abdominal cavity and air is passed in and out by the pumping movement of the floor of the mouth (known as the buccal pump) and the opening and closing of the nostrils.
Mammals
- In mammals, gases are exchanged in the lungs.
- These surfaces are protected from desiccation by being inside the body’s waterproof covering.
- The surface area of contact between the blood and air is increased by the convolution of the lungs into lobes, by the branching of the bronchioles into smaller and smaller tubules, and by the division of the tubules into clusters of tiny air sacs called alveoli.
- There is a plentiful blood supply to transport gases to and from the lungs.
The human respiratory system. Air in small volumes into and out of the lungs. Gas exchange occurs across thin alveoli walls.
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§ explain the relationship between the requirements of cells and the need for transport systems in multicellular organisms
- In unicellular organisms, all nutrients needed, can be diffused from the external environment over their surface area.
- Wastes can just be removed from cells by diffusion as well.
- However, in multicellular organisms, the surface area is not great enough to provide nutrients for all the organisms cells.
- Transport systems are used to carry nutrients to all the body’s cells, and to carry wastes away.
- Transports systems provide all the needs of organisms.
§ outline the transport system in plants, including:
- root hair cells
- xylem
- phloem
- stomates and lenticels
Transport Systems in Flowering Plants
The need for transport systems:
- To ensure that cells are supplied with the nutrients they need and that wastes do not accumulate, multicellular organisms have transport systems that enable substances to be moved to and from the internal body cells.
- These transport systems are specialised to exchange materials between the internal environment of the cells and the external environment.
- In flowering plants the transport system is called vascular or conducting tissue. It runs in long tubes through the roots, stems and leaves.
- It consists of two separate transport systems: xylem and phloem. These two systems run close to each other throughout the plant.
- In the root the xylem and phloem tubes form a central cylinder known as the stele. In young stems they form a ring of vascular bundles. In the leaf the vascular bundles are visible as veins. In flowering plants, no plant cell is far from vascular tissue.
Water transport
Water and mineral ions flow through plants in an upward stream. They enter via the roots and travel upwards in the xylem tubes. Water is lost from the plant by transpiration
Root hair cells
- Water is taken in by plants through the roots from the soil. The water in the soil has a low concentration of solutes (soluble substances) and is, therefore, a dilute solution. Inside the plant, water in the cytoplasm of cells has a high concentration of solutes and is, therefore, a more concentrated solution. Water moves by diffusion down its concentration gradient by osmosis from the soil into the root cells.
- The root hairs provide a large surface area for the uptake of water. The water moves through to the centre of the root and enters the xylem.
Xylem
The main conducting cells are known as xylem vessels. They consist of dead cells, thickened with woody material, whose cross-walls have broken down, forming a continuous system of tubes. Xylem tubes may be up to 1 metre long. Lying alongside the xylem vessels are strengthening fibres and other conducting cells. Xylem gives support, strength and rigidity to the stem. Water entering the xylem in the roots is transported upwards through the stem to the leaves.
Transpiration
When the stomates of a leaf are open, carbon dioxide, which is required for photosynthesis, can enter a leaf by diffusion.
At the same time, water molecules are lost through diffusion because of the higher water concentration inside the plant.
Water evaporates from the cell surfaces, diffuses through the intercellular spaces, and leaves through the stomates. This diffusion of water from a plant is called transpiration.
Water loss by transpiration is unavoidable when a plant has its stomates open. Water lost in this way needs to be replaced by water taken in through the roots. The constant upward flow of water through a plant is known as the transpiration stream.
If water loss exceeds water uptake, the stomates close and cells lose their turgidity. The stems and leaves wilt and the plant may die. When stomates are closed the transpiration rate drops and diffusion occurs at a much slower rate through the cuticle. Normally stomates are open during the day for the exchange of gases in photosynthesis and closed at night.
Some plants have special features (adaptations) to reduce the transpiration rate. These may include a very thick cuticle, sunken stomates, hairs, or reduced leaf size. In plants in hot dry environments, it may include the closure of the stomates during the middle of the day.
Guard cells control the opening and closing of the stomates. When stomates are open, the guard cells are distended with water and are said to be turgid.
Lenticels
Lenticels are pores in the woody stems of plants. Gases needed for respiration by the cells of the stem are exchanged by diffusion. Oxygen diffuses in and carbon dioxide diffuses out.
Phloem
Sugars produced in photosynthesis, mostly in the leaves, are stored as starch in the leaf during the day.
At night they are converted back to sugars and transported to other plant tissues in the phloem.
Phloem consists of long columns of living sieve tube cells with perforated end walls, so that the cytoplasm is continuous between one cell and the next.
Lying alongside the sieve-tube cells are companion cells and other supporting tissue.
Organic materials in the phloem are transported both upwards and downwards within the plant. This movement is called translocation
.
Companion Cell
A specialized parenchyma cell, located in the phloem of flowering plants and closely associated with the development and function of a sieve-tube element. Companion cells probably provide ATP, proteins, and other substances to the sieve-tube elements, whose cytoplasm lacks many structures necessary for cell maintenance.
· compare open and closed circulatory systems using one vertebrate and one invertebrate as examples
Open circulatory systems
Invertebrates such as arthropods and molluscs have open circulatory systems.
- Invertebrates such as molluscs and arthropods have open systems
- This involves the movement of body fluids (or haemolymph) around the whole body by a simple pumping system
- Haemolymph bathes the tissues and accumulates in large spaces in the animal
- The fluid is pumped to the front of the animal and slowly flows to the back
- The pressure is very low and fluids circulate slowly
- Open systems suit smaller animals
o EG:
o A insect, such as a fly, has an open circulatory system
o The ‘heart’ (called a dorsal longitudinal vessel) contracts and fluid flows to the front of the insect
o The fluid flows through the tissues of the body and enters the ‘heart’ again through a series of holes
o The fluid is then pumped again to the rest of the body
Closed circulatory systems
Large active animals such as vertebrates and squids have closed circulatory systems.
In humans, the circulatory system consists of a muscular pump (the heart) that forces a fluid (blood) through a closed system of tubes (blood vessels), which carry materials rapidly throughout the body.
No cell in the body is very far from a blood vessel. Nutrients, wastes and gases exchanged in respiration are carried in the blood.
To reach the body cells, materials in the blood must pass into the surrounding body fluid before reaching the cells.