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Chapter 20 Unifying Concepts of Animal Structure and Function Overview: Hierarchy of Structural Organization Exchanges with the environment Climbing the Walls • The function of any part of an animal results from its unique structure • Consider the gecko, a small lizard commonly found in the tropics – A gecko can walk up a wall and across ceilings – How does it do that? • The explanation relates to hairs, called setae, on the gecko’s toes – They are arranged in rows – Each seta ends in many split ends called spatulae, which have rounded tips – The ability to “stick” to surfaces results from attractions between molecules on the spatulae and molecules on the surface – The structure of the gecko’s feet leads to a unique function THE HIERARCHY OF STRUCTURAL ORGANIZATION IN AN ANIMAL Structure fits function in the animal body • The correlation between structure and function is one of biology’s most fundamental concepts Animal structure has a hierarchy • Structure and function are correlated at each level in the structural hierarchy of an animal’s body • Life is characterized by hierarchical levels of organization • In animals – – – – Individual cells are grouped into tissues Tissues combine to form organs Organs are organized into organ systems Organ systems make up the entire organism • Biologists distinguish anatomy from physiology – Anatomy is the study of the structure of an organism – Physiology is the study of the function of an organism’s structural equipment Tissues • In most multicellular animals, cells are grouped into tissues – A tissue is a collection of many structurally similar cells that act cooperatively to perform a specific function Tissues are groups of cells with a common structure and function • A tissue is a cooperative of many similar cells that perform a specific function • Animals have four major categories of tissue – – – – Epithelial tissue Connective tissue Muscle tissue Nervous tissue Epithelial tissue covers and lines the body and its parts • Epithelial tissue occurs as sheets of closely packed cells – It covers surfaces and lines internal organs and cavities – Examples: epidermis, stomach lining • The structure of each type of epithelium fits its function Connective tissue binds and supports other tissues • Connective tissue is characterized by sparse cells – The cells manufacture and secrete an extracellular matrix – The matrix is composed of fibers embedded in a liquid, solid, or gel • Connective tissues have a sparse population of cells scattered through an extracellular matrix – The matrix consists of a web of protein fibers embedded in a uniform foundation • The structure of connective tissue correlates with its function – It binds and supports other tissues • Loose connective tissue is the most widespread connective tissue – It binds epithelia to underlying tissues – It holds organs in place • Adipose tissue stores fat – It stockpiles energy – It pads and insulates the body • Blood is a connective tissue with a matrix of liquid – Red and white blood cells are suspended in plasma • Fibrous connective tissue has a dense matrix of collagen – It forms tendons and ligaments • The matrix of cartilage is strong but rubbery – It functions as a flexible, boneless skeleton – It forms the shock absorbing pads that cushion the vertebrae of the spinal column • Bone is a rigid connective tissue with a matrix of rubbery fibers hardened with deposits of calcium Muscle Tissue • Muscle tissue consists of bundles of long, thin, cylindrical cells called muscle fibers • Each cell has specialized proteins that contract when the cell is stimulated by a nerve • Skeletal muscle is responsible for voluntary body movements • Cardiac muscle pumps blood • Smooth muscle moves the walls of internal organs such as the stomach • Skeletal muscle is attached to bones by tendons – It is responsible for voluntary movements – The contractile apparatus forms a banded pattern in each cell or fiber – It is said to be striated, or striped • Cardiac muscle is found only in heart tissue – Its contraction accounts for the heartbeat – Cardiac muscle cells are branched and joined to one another • Smooth muscle is named for its lack of obvious striations – It is found in the walls of various organs – It is involuntary Nervous Tissue • Nervous tissues makes communication of sensory information possible – Sensory input is received and processed – Motor output is then relayed to make body parts respond • Nervous tissue is found in the brain and spinal cord • The basic unit of nervous tissue is the neuron, or nerve cell – Neurons can transmit electrical signals rapidly over long distances Nervous tissue forms a communication network • The branching neurons of nervous tissue transmit nerve signals that help control body activities Organs and Organ Systems • The next level in the structural hierarchy after tissue is the organ – An organ consists of two or more tissues packaged into one working unit that performs a specific function – Examples: heart, liver, stomach, brain, and lungs The body is a cooperative of organ systems • The level of organization higher than an organ is an organ system • Each organ system has one or more functions • The organs of humans and most other animals are organized into organ systems – Organ systems are teams of organs that work together to perform a vital bodily function • The digestive system gathers food • The respiratory system gathers oxygen • The circulatory system, aided by the lymphatic system, transports the food and oxygen • The immune system protects the body from infection and cancer • The excretory system disposes of certain wastes • The endocrine and nervous systems control and coordinate body functions • The integumentary system covers and protects the body • The skeletal system supports and protects the body • The muscular system enables movement • The reproductive system perpetuates the species Connection: New imaging technology reveals the inner body • New technologies enable us to see body organs without surgery – Computed tomography (CT) – Magnetic resonance imaging (MRI) – Positron-emission tomography (PET) EXCHANGES WITH THE EXTERNAL ENVIRONMENT • Every organism is an open system – This means that organisms exchange chemicals and energy with their surroundings – Organisms must do this to survive • Animals are not closed systems – An animal must exchange materials and heat with its environment – This exchange must extend to the cellular level Body Size and Shape • An animal’s size and shape affect how it exchanges energy and materials with its surroundings – All living cells must be bathed in water so that exchange of materials may occur (e.g. hydra) • Small animals with simple body construction have enough surface to meet their cells’ needs – Hydras can exchange materials with the environment though direct diffusion • Exchange with the environment is easy for single-celled organisms – The entire surface area of an amoeba is in contact with the environment • Animals with complex body forms face the same basic problem – Every living cell must be bathed in fluid – Every cell must have access to essential nutrients from the outside environment • Complex animals have extensively folded or branched internal surfaces – These maximize surface area for exchange with the environment • Larger, complex animals have specialized internal structures that increase surface area • Lungs exchange oxygen and carbon dioxide with the air – The epithelium of the lungs has a very large total surface area for this purpose Animals regulate their internal environment • In response to changes in external conditions, animals regulate their internal environment – They must do this to achieve homeostasis, an internal steady state Homeostasis • Homeostasis is the body’s tendency to maintain relatively constant conditions in the internal environment even when the external environment changes Negative and Positive Feedback • Most mechanisms of homeostasis depend on a common principle called negative feedback – The results of some process inhibit that very process • Negative feedback mechanisms keep fluctuations in internal conditions within the narrow range compatible with life • Less common is positive feedback – The results of a process intensify that same process – Example: uterine contractions during childbirth