Complete notes for Chapter 16 Support and Movements. Covers support in plants, skeleton systems, human skeleton, joints, muscles, and locomotion for FSc Part 2 Biology.
Animals and plants show variety of physical and biochemical activities. The main difference between the plants and animals is of locomotion. The animals show locomotion but plants do not.
Both plants and animals need support against gravity:
Thus animals can move towards food, away from danger, and for shelter.
The parts of plants are stem, root, and leaves. The most important function of the stem is to give support to plants. It also acts as a supply line between root and aerial parts. Different types of cells are involved for support in the stem.
In living cells of epidermis, cortex and pith, the internal hydrostatic pressure is called turgor pressure. This condition is called "turgidity" and in this condition the cells are said to be "turgid". It keeps them rigid and resistant to bending.
Water enters into the living cell by osmosis. Sometimes plants lose water due to exosmosis. Such plants lose their turgor. Thus some parts of plant will wilt. Therefore maintenance of turgor pressure is an important phenomenon in plants.
Generation of Turgor Pressure: Turgor pressure is generated by high osmotic pressure of vacuole. The membrane that bounds vacuoles called tonoplast which contains a number of active transport systems that pump ions into the vacuoles or compartments despite the high concentration than that of extra cellular fluid. Because of higher ionic concentration, water enters the vacuole and hence provides turgidity, mechanical support to soft tissues of plant.
They have protoplast, hence they are living cells. These cells form strands or continuous cylinders beneath the epidermis of stems or leaf stalk and along veins in leaves. They are usually elongated with unevenly thickened primary walls but they lack secondary wall. Strands of collenchyma provide much of support for plant organs in which secondary growth has not yet taken place.
These cells have tough, thick secondary walls; they usually do not contain living protoplast when they are mature. Their secondary cell walls are often impregnated with lignin (an organic substance that makes the wall tough and hard). Their primary function is to provide support to plant tissues.
Types of Sclerenchymatous Cells:
In most terrestrial plants, the major mechanical stress is imposed by wind so that stem must be able to resist bending. The vascular bundle containing the xylem is tough and inextensible to perform the same function as steel rods perform in reinforced concrete. This arrangement as a ring within the stem provides very effective resistance to wind stress and weight bearing ability of plant. The vascular bundle of some plants is strengthened by additional sclerenchyma fibers. They form bundle cap.
An increase in plant girth due to activity of vascular cambium is called secondary growth.
Stem and root often begin to thicken after their apical meristem has produced embryonic or primary tissue. After this the stem and roots become thick by secondary growth.
Secondary growth occurs due to cell division in:
The vascular cambium differentiates from parenchyma cells within the vascular bundles of stem, between primary xylem and primary phloem. The cylindrical form of vascular cambium consists of elongated flattened cells with large vacuoles. Vascular cambium divides to form secondary xylem and secondary phloem.
The secondary xylem causes most of the increase in stem thickness. Over the years the woody stem gets thicker and thicker as its vascular cambium produces layer upon layer of secondary xylem. These layers are visible as rings. Since cambium growth is seasonal, one ring is formed in one year.
Wood: In most trees the conduction of water and dissolved substances by secondary xylem becomes limited to the outer or younger portion of that tissue. As trees grow older, only few annual growth rings are active in conduction at one time. The active portion is called sapwood. The inactive non-conducting wood is called heartwood.
In most species the heartwood accumulates a variety of chemicals such as resins, oils, gums and tannins. These provide a resistance to decay and insect attack, for example cedar and conifers.
Callus Formation: Another important function of cambium is to form callus or wood tissue on or over the wood. Soft parenchymatous tissues are rapidly formed on or below the damaged surface of stems and roots. Callus unites the branches during budding and grafting.
Cork cambium develops in outer layer of stem. The cork cambium usually consists of plates of dividing cells. The inner layers of cork cambium contain large amounts of fatty substance suberin, which makes the layers of cork nearly impermeable to water. Cork cells are dead at maturity.
The response on receiving the stimuli is different in animals and plants. Animals change their location in response to stimulus while plants remain fixed. The change in their growth pattern is a kind of response.
On the basis of causes of movement there are two types:
These movements are spontaneous movements which have internal causes. They are further divided into three types:
On receiving the external stimulus the whole cell or organism shows movement. This is called tactic movement. It may be either positive if it is towards the stimulus or negative if it is away from stimulus.
Turgor movement is due to differential changes in turgor and size of cells as result of gain or loss of water. Examples: leaflets of touch-me-not and sleep movement of plants.
This type of movement occurs due to unequal growth on two sides of plant organs like stem, root, tendrils, bud etc.
The causes in this type of movement are external. These are of following types:
The word tropic is derived from Greek word "Tropos" meaning turn. It is the movement in curvature of whole organ towards or away from stimuli such as light, gravity and touch.
These are the non-directional movement of parts of plant in response to external stimuli.
The movement of the organs of plant is controlled by plant hormone called auxin.
The skeleton is a tough and rigid framework of body of animals which provides protection, shape and support to body organs. It is composed of inorganic or organic substances or both. In protozoa it is secreted by single cell; while in multicellular animals it consists of specialized cells.
There are essentially three types of skeleton:
Such type of skeleton is found in those animals which do not have hard skeleton. They have fluid-filled gastrovascular cavity or coelom which works as hydrostatic skeleton. It is mostly found in certain soft-bodied animals such as cnidarians (sea anemone), flat worms (planaria) and annelids (earthworm).
An exoskeleton is a hardened outer surface to which internal muscles are attached. It is non-living. The ectodermal cells are responsible to form exoskeleton.
Layers of Exoskeleton:
Exoskeleton of Molluscs: Molluscs have shells which enclose or surround the body. Some have one piece and others have two pieces of shell. Some marine bivalves have crystals of calcium carbonate on their shells. Snails have very soft shells as they lack hard mineral. The soft part of molluscan body acts as hydrostatic skeleton also.
Exoskeleton of Arthropods: Arthropods have muscles that are attached to rigid chitin. Exoskeleton enables them to swim, to walk and to fly. The joints on this skeleton are thin, soft and flexible. It also contains sensory receptors called sensilla that are in form of bristles and lenses.
Advantages: The exoskeleton in arthropods protects the animal against their enemy and rough environment. It also protects from drying.
Disadvantages: However, it has one disadvantage and that is animals cannot grow larger. The animal needs to shed its exoskeleton periodically and replace it with a larger one. This process is known as "ecdysis" or "moulting".
Ecdysis: Ecdysis is divided into four stages:
All these changes are controlled by the nervous system and the hormone ecdysone.
The skeleton in which hardened surface is inner to outer soft muscles is called endoskeleton. It consists of tissues, bones and cartilage.
Bone: Bone is a special form of connective tissue in which collagen fibers are coated with calcium phosphate salt. Bones supporting the human arms and legs consist of an outer shell of compact bone with spongy bone in the interior.
Bone Cells: Bone is a dynamic, living tissue that is constantly being reconstructed throughout the life of individual.
Formation of Bone: When the bone is forming, when it is replacing the cartilage, the osteoclasts invade and dissolve the cartilage. Then osteoblasts replace it with bone. As bones grow, the matrix of bone is hardened and osteoblasts are gradually entrapped within it.
Cartilage: Cartilage is a specialized connective tissue in which collagen matrix between cells is formed at position of mechanical stress. It is much softer than bone. It covers ends of bone at joints and also supports the flexible portion of nose and external ears. The living cells of cartilage are called chondrocytes. These cells secrete flexible, elastic, nonliving matrix collagen that surrounds the chondrocytes. No blood vessels penetrate into this cartilage.
Types of Cartilage:
The skeleton of human is made up of 206 individual bones. These are grouped according to their function. The human skeleton can be divided into two parts:
The axial skeleton is made up of skull, backbone (vertebral column) and ribcage.
There are three types of bones found in skull:
(i) Cranium: Skull consists of total 28 bones in which 8 forms cranium (4 unpaired and 2 paired) which encases the brain. Of the bones of cranium, parietal and temporal are paired bones, whereas frontal, occipital, sphenoid and ethmoid are unpaired bones.
(ii) Facial Bones: Besides that, there are 14 facial bones of which 6 are paired and 2 are unpaired. The paired facial bones are maxilla, zygomatic, nasal, lacrimal, palatine, and inferior concha. The unpaired facial bones are mandible and vomer.
(iii) Hyoid Bone: The skull also contains the hyoid bone. It is suspended at the back of jaw by muscles and a form of connective tissue called ligament and supports the base of tongue.
Vertebral column extends from skull to pelvis to form backbone which protects the spinal cord. Normally the vertebral column has 4 curvatures which provide more strength than a straight column. The vertebral column consists of 33 vertebrae.
Divisions of Vertebrae: The vertebrae are named according to their location in body:
Curving forward from vertebrae are 12 pairs of ribs that articulate with thoracic vertebrae. Ten of them connect anteriorly with sternum either directly or through the costal arch. The lower two pairs of ribs are called floating ribs because they do not attach with sternum. The rib cage provides support for semi-vacuum chamber called chest cavity.
The appendicular skeleton is composed of pectoral and pelvic girdle along with their appendages.
The pectoral girdle consists of two large, flat shoulder blades called scapulae, suprascapula and clavicle. The clavicle connects scapula with sternum.
Forelimbs: Forelimb is appendage of pectoral girdle.
Bones: The forelimb consists of 1 humerus, 2 radius and ulna, 8 carpals, 5 metacarpals and 14 phalanges.
Joints: The different bones of fore limbs form joints such as humerus with scapula forms ball and socket joint. While humerus at distal end forms hinge joint with radius and ulna. The radius and ulna at their distal end form multistage joint with eight wrist bones called carpals. Five metacarpals form framework of palm of hand. Five rows of phalanges are attached to metacarpals. They support the fingers.
Pelvic girdle attaches the hind limb to the vertebral column. It is composed of coxal bones. Each coxal bone is formed by the fusion of three bones: ilium, ischium, and pubis. The pelvic girdle supports the pelvic region.
Hind Limbs: Hind limbs are composed of 1 femur, tibia and fibula, 8 tarsals, 5 metatarsals and 14 phalanges.
Joints: Femur is the proximal bone which forms a hip joint with hip bone. It is a ball and socket joint. At the distal end, the femur forms knee joint with proximal end of two parallel bones called tibia and fibula. The distal end of tibia and fibula form a joint with eight tarsals, which are also distally attached with five metatarsal bones of ankle. Five rows of fourteen phalanges of toes are attached with metatarsals.
Joints: Joints are sites where one bone meets another.
There are three types of joints on the basis of movements:
Joints are also classified on basis of structure:
These are immovable joints connected by thin fibers embedded in connective tissue. Such joints are found in skull which fix teeth in jaws.
These are slightly movable joints. Hyaline cartilage forms joint between growing bone. The vertebral bodies of your spine are separated by pads of cartilage called intervertebral disks. The disk allows some movement while acting as shock absorber.
These joints consist of a cavity which is filled with fluid called synovial fluid which reduces the friction between joints. This joint consists of two layers: an outer fibrous layer (fibrous membrane) and an inner layer called synovial membrane. Some parts of capsule may be modified to form distinct ligament holding the bones together.
Synovial joints are further divided into two major categories:
These joints are present at elbow and knee which allow the movement in two directions. At these joints, muscles are arranged at same plane. One end of each muscle, the origin, is fixed to immovable bone on one side of joint and other end of muscles, the insertion, is attached to movable bone.
This type of joint allows the movement in several directions. Such joints have at least two pairs of muscles present perpendicular to each other. They provide maximum flexibility. Hip joint is one of the examples.
The skeleton of human supports its body. The causes of deformations of the skeleton are various:
Osteoporosis: It is a type of disease in which bone deposition outpaces bone deposit. In this case, bone mass is reduced and chemical composition of the matrix remains normal. Osteoporosis usually occurs in aged women which is related to decreased estrogen level. Other factors which may contribute include insufficient exercise, diet poor in calcium and protein, smoking, etc. Estrogen Replacement Therapy (ERT) is the best protection against osteoporotic bone fractures.
Osteomalacia (soft bones): It includes a number of disorders in which the bones receive inadequate minerals. In this disease, calcium salts are not deposited and hence bones soften and become weaker. Weight-bearing bones of legs and pelvis bend and deform. The main symptom is the pain when weight is put on affected bones.
Rickets: It is the disease which is caused in children. The legs get bowed and pelvis is deformed. The cause of this disease is poor calcium level in diet or deficiency of vitamin D. It is treated by vitamin D fortified milk and exposing skin to sunlight.
Intervertebral Discs: The intervertebral discs are found in between the vertebrae. Each disc consists of an outer tough connective tissue ring, the annulus fibrosus, and a gelatinous nucleus, the nucleus pulposus. The annulus fibrosus holds together successive vertebrae.
Function: The main function of intervertebral discs is shock absorption during walking, jumping, running, and to lesser extent to bend laterally. They are compressed under load. In the movement of vertebral column, the intervertebral disc acts as stiff deformable elements.
Herniated Disc: One of the main deformity is herniation of one or more discs. This is caused by suddenly bending forward while lifting a heavy object. In this case the disc is slipped. Actually annulus fibrosus is ruptured followed by protrusion of spongy nucleus pulposus. If protrusion presses on spinal cord or on spinal nerves, it generates severe pain or even destruction of these nervous structures. The treatment of this disease is complete bed rest and use of pain killer. If this fails then disc may be removed surgically.
Bone is a special form of connective tissue in which collagen fibers are coated with calcium phosphate salt. It is a rigid structure but it may break.
In youth, the reasons of fracture may be trauma such as that may twist or break the bones. Sports injury or automobile accidents etc. are the reasons of fracture. In old age, bones become thin and weak and thus fractures occur.
The treatment of the bones is reduction followed by realignment of broken bones. There are two types of reduction:
Healing Time: Healing time is 8-12 weeks, but it is much longer for large weight-bearing bones and for bones of elderly people.
There are four stages in the repairing process:
Many multicellular animals have evolved specialized cells for movement called muscle cells. These cells contain numerous filaments of special protein actin and myosin.
Vertebrate muscle can be classified into three types:
They are also called visceral muscles. Smooth muscle cells are long, spindle-shaped and tapered. Each cell has one nucleus. The cells commonly occur in flattened sheets and are found throughout the body.
Smooth muscle is considered involuntary meaning that there is little or no conscious control over its functioning. Smooth muscle tends to contract slowly and in wave-like manner. These muscles are found in walls of gut, blood vessels and at the bases of hairs. Smooth muscles were earliest form of muscle to evolve and is found throughout animal kingdom.
Key characteristics: Non-striated, involuntary, smooth.
Cardiac muscles are the muscles of heart. They have traits in common with smooth and skeletal muscles. For example, like smooth muscle it is involuntary and like skeletal muscle it is striated. The difference lies in its fibers which are branched. It has chains of single cells, each with its own nucleus.
Key characteristics: Striated, involuntary, branched fibers.
The muscles that are attached with skeleton and associated with movement of bones are called skeletal muscles. They control the voluntary actions. Skeletal muscles are often called striated because of prominent light and dark bands. Triceps and biceps are the examples of skeletal muscles. Generally each end of entire muscle is attached to bone by bundle of collagen fibers known as tendons.
Key characteristics: Striated, voluntary, attached to bones.
Muscles are held together by connective tissue composed largely of extracellular material. Skeletal muscle is supplied by blood vessels and nerves.
Each muscle consists of muscle bundles. Its multinucleate cells are referred to as muscle fibers. Groups of fibers with the same action are called muscle bundles.
Each muscle fiber is a long cylindrical cell with multiple oval nuclei arranged just beneath its sarcolemma (the equivalent of plasma membrane). Skeletal muscle fibers are huge cells having diameter from 10-100 μm.
Sarcoplasm of muscle fiber is similar to cytoplasm of other cells but it contains large amount of stored glycogen and a unique oxygen-binding protein myoglobin, a red pigment that stores oxygen.
Lying just beneath the sarcolemma is the sarcoplasmic reticulum which, like the endoplasmic reticulum of other cells, is a membranous hollow structure. Somewhat larger than sarcoplasmic reticulum are transverse tubules (T-tubules).
The actual contractile parts of muscle fibers are rod-shaped myofibrils. The diameter of myofibrils ranges from 1-2 μm. Bundles of myofibrils are enclosed by muscle cell membrane or sarcolemma. The myofibrils consist of smaller contractile units called sarcomere.
In each sarcomere, a series of dark and light bands are seen:
This gives the cell as a whole its striped appearance. Each A band has a lighter stripe in its midsection called H zone (H stands for "helle" means bright). The H zone is bisected by a dark line called M line. The I bands have mid-line called Z line.
A sarcomere is the region of myofibril between two successive Z lines and is the smallest contractile unit of muscle fiber.
Thick and thin filaments combine to form myofilaments.
The central thick filaments extend the entire length of A band. The diameter of thick filament is 16 nm. It is composed of myosin. Myosin molecule has a tail terminating in two globular heads. Myosin tail consists of two long polypeptide chains coiled together. The heads are called cross bridges.
It is composed of actin molecules which are 7-8 nm thick. It is composed of three proteins:
Each myosin filament is surrounded by six actin filaments on each end.
H. Huxley and A.F. Huxley and their colleagues in 1954 gave the sliding filament theory which explains muscle contraction.
According to this theory, the thin filaments slide past the thick ones so that actin and myosin filaments overlap to a greater degree. Actin from both sides brings the Z line closer together and shortens the entire sarcomere. This movement continues until the actin filaments touch or overlap. At that time, H zone becomes smaller.
In this process of contraction, the cross bridges of thick filament become attached to binding sites on actin filament. The cross bridges then contract to pull the actin filament towards center of sarcomere.
Actin does the actual moving during contraction. It provides binding sites for myosin bridges. Tropomyosin and troponin are able to control muscle contraction because they can block actin-myosin binding sites and inhibit contraction.
The binding sites will be blocked until the appearance of calcium ion. When Ca2+ is present in contractile unit, it binds to troponin, causing it to release its hold on binding site of actin. This allows the tropomyosin to slide the troponin off the binding sites.
Actin's myosin binding sites are often exposed and myosin head attaches and becomes active. Upon the release of energy from ATP, the bridges draw the actin filaments inward and muscle contracts. It is found that each complete actin movement by myosin head requires two ATPs. In resting muscle, calcium ions are stored in sarcoplasmic reticulum.
Neural impulses elicit muscle contraction. When an impulse arrives, all the fibers served by motor neuron will react simultaneously. This acting group is called motor unit.
When a neural impulse from a motor neuron reaches the neuromuscular junction of sarcolemma, it travels along sarcolemma and then deep into elongated hollow tubes (T-tubules) to the vicinity of membranous sarcoplasmic reticulum within which calcium ions are stored. The disturbance causes calcium channels in sarcoplasmic reticulum to open and as ions suddenly leak out, they diffuse rapidly in surrounding contractile units.
The calcium ions combine with troponin, causing tropomyosin-troponin complex to move, exposing the myosin binding sites. The muscle then contracts. The muscle will contract as long as calcium ions are present and calcium remains as long as neural impulses continue. When impulses cease, calcium ion pumps quickly clear the ions from contractile units; the tropomyosin-troponin complexes block the myosin binding sites of actin and muscle relaxes.
The contraction of each muscle is based on all or none principle. All of its fibrils participate in contraction. The degree of contraction depends upon number of fibers that participate in contraction.
Energy for muscle contraction comes from ATP. Supply of ATP is maintained by aerobic breakdown of glucose in muscle cell, which comes from stored glycogen in cell. When more energy is required due to high metabolism, it is provided by another energy-storing substance called creatine phosphate. Sometimes oxygen deficiency or very high lactic acid accumulation causes muscle fatigue.
It is a state of physiological inability to contract. Due to deficiency of ATP, muscles get fatigue. In this state, there is a continuous contraction which makes the cross bridges unable to detach. In addition to ATP, excess accumulation of lactic acid and ionic imbalances are also the reasons of muscle fatigue. Lactic acid which causes pH to drop, causes extreme fatigue by breaking glucose.
Low calcium ions in the blood is its cause. During this condition, neurons get greatly excited which results in loss of sensations. Muscle twitches and convulsions occur.
It is also known as tetanic contraction of entire muscle. It lasts for just few seconds or several hours, causing the muscles to become taut and painful. It is most common in thigh and hip muscles. It usually occurs at night or after exercise. It reflects low blood sugar level, electrolyte depletion, dehydration, irritability of spinal cord and neuron caused by anaerobic bacterium.
Connective tissue serves primarily to bind organs together. Connective tissue includes two specialized kinds:
The skeletal muscles produce movements by pulling on tendons through cords of connective tissues that anchor muscles to bones. The tendons then pull on bones. Most muscles pass across a joint and are attached to bones that form joints. When such muscle contracts, it draws one bone towards or away from bone with which it articulates.
Muscles occur in pairs and at joints these work against each other by contraction. This relationship is called antagonistic.
Example: The movement of elbow by biceps and triceps is the best example. The biceps bends the arm at elbow joint and triceps straightens it.
The biceps brachii muscle arises from the two heads of scapula and is inserted into medial surface of radius bone. The other two muscles lie below the biceps brachii: brachialis (inserted in ulna) and brachioradialis (inserted in radius).
When these muscles contract, they pull radius and ulna and bend the arm at elbow. In the antagonistic pair, one muscle reverses the effect of other but they do not contract simultaneously.
Euglena moves with the help of flagellum. Flagellum is a whip-like structure. As the flagellum whips backward, the organism moves forward. However, when flagellum moves forward, the Euglena does not move backward.
Euglenoid Movement: Euglena does not move in straight line but in zigzag manner and at the same time rotates on its own axis. Such type of movement is called Euglenoid movement. A wave of activity is created which passes in spiral fashion from its base to its tip. They increase amplitude and velocity. Euglena is able to change its direction by active contractile myonemes which run along its body. On their contraction, the body changes its shape and direction.
The locomotory organ in Paramecium is cilia.
Structure of Cilia: Cilia are thin, fine thread-like extensions of cell membrane. The length of cilia ranges from many microns to many hundred microns. Diameter varies from 0.1 to 0.5 μm. In cilium, there are two types of fibrils: long peripheral double fibrils which are nine in number and give the appearance of 9+2. The other is smaller, which are two, present in the center. All these fibrils run longitudinally through the cilium.
Mechanism of Movement: The exact mechanism of movement of cilia is not clear. However, Bradford in 1955 suggested that movement of cilia is due to simultaneous contraction or sliding of double fibrils in two groups one after the other:
As a result of bending and recovering strokes, the Paramecium swims against water. The energy for movement of cilia is provided by ATP.
Pseudopodia are the locomotory organ in Amoeba (Pseudo means false and podia means foot). Pseudopodia are finger-like projections thrown in the direction of movement in which the cytoplasm flows and body moves in that direction.
Jelly fish shows the movement called jet propulsion. As jelly fish has umbrella-like body called bell, so first water enters the bell, then bell contracts, water is forced out like a jet and animal flies forward.
In earthworm, movement takes place with the help of setae and muscles, and peristaltic contraction.
Mechanism:
In this way, earthworm moves from one place to another.
In cockroach, locomotion takes place through legs. It moves from one place to another through walking but it also takes to flight by its wings.
Mechanism of Walking: During walking, the legs are used on one side; the foreleg pulls the body forwards and hind leg pushes it in same direction. The middle leg of opposite side acts as prop. In mean time, the other three legs have been raised together and process is repeated.
Mechanism of Flight: The posterior pair of wings brings about flight. These beat tips in such a manner that they support body weight and drive it through air.
Snails and mussels are mollusks which crawl or move very slowly by foot.
Tube feet are the source of movement in starfishes. The tube feet are present on both sides of radial canal that extends up to the tip of arm.
Mechanism: The tube feet extend when water is pumped into them, then they fix themselves by suction cup with some object. Later on, they shorten and pull the body in this direction. In this way, starfish moves in any direction. Arms of starfish also help in swimming.
The fishes swim in water for movement from one place to another. As swimming in water creates many problems, fishes show many adaptations for this purpose:
The body plan of amphibians is like fish. As amphibians live on both land and water, so mode of locomotion is also different.
Mechanism: Salamanders wriggle along their belly on ground with the help of segmentally arranged muscles as they swim on land when moving deliberately. On the other hand, frogs raise up their body on legs which then propel them along as movable levers.
Anurans: In anurans (frogs and toads), the whole skeleton and muscular system has become specialized for peculiar swimming and jumping methods of locomotion by means of extensor thrusts of both hind limbs acting together. Frogs and toads also walk and hop on land due to their strong hind limbs.
Reptiles live in the terrestrial environment. Their skeleton is much evolved and they use the method of walking and running. Their skeleton evolved from ancient amphibians.
Adaptations:
The skeleton of a bird is modified for flight. The most obvious adaptations are:
Passive Flight (Gliding): When a bird glides, the wings act as aerofoils. An aerofoil is any smooth surface which moves through the air at an angle to the airstream. The air flows over the wing in such a way that the bird is given lift. The amount of lift depends on the angle at which the wing is held relative to the air stream.
Active Flight: When little or no support can be gained from upward air currents, the same effect can be achieved by flapping the wings. As the bird moves through the air, the air flows more quickly over the curved upper surface than over the lower surface. This reduces air pressure on the top of the wing compared with air pressure below the wing. There is, therefore, a net upward pressure on the wing; this gives lift to the bird.
The most efficient way of supporting the body is seen in mammals. The limbs of mammals have undergone further modifications to produce three following modes of locomotion:
The body plan of all vertebrates is common; their basic parts are same but many differences are found in them. The difference is due to the habitat where they live; for example, the mode of support and locomotion in sea is different from that of land.
Many of the fishes propel forward by means of muscle contraction which pass along the body from anterior to posterior producing characteristic S-band locomotion. Alternate contractions on both sides produce thrust through water. This type of motion is found in cartilaginous fish like dogfish and sharks.
Amphibians and reptiles:
Flight has evolved in three types of vertebrates: pterodactyls, birds and bats. Flying involves more muscular effort than swimming, walking or running.