Evolution: Size, Shape, and Exchange

Evolution of Size and Shape

• Physical laws, including strength, diffusion, movement, and heat exchange govern all life
• Water properties limit the physical shapes of fast-swimming animals
• Larger animals require thicker skeletons for support
• Convergent evolution leads to similar adaptations in diverse organisms facing the same challenge

Materials Exchange With the Environment

• Nutrients, waste products, and gases are exchanged across animal cell membranes
• Exchange rate is proportional to a cell's surface area
• The amount of exchange material is proportional to a cell's volume
• Single-celled organisms in water have sufficient surface area for necessary exchange
• Multicellular animals with sac-like body plans have thin body walls to facilitate material diffusion

Adaptations to Increase Surface Area

• Alveoli in lungs and microvilli in the small intestine increase surface area for exchange
• Diffusion occurs between cells and capillary beds
• Glomeruli in kidneys also contribute to increased surface area

Thermoregulation

• Thermoregulation is how animals maintain an internal temperature within a tolerable range

Endothermy vs. Ectothermy

• Endothermic animals generate heat through metabolism; birds and mammals are examples
• Ectothermic animals gain heat from external sources; most invertebrates, fishes, amphibians, and nonavian reptiles are ectotherms
• Endotherms maintain a stable body temperature despite environmental fluctuations
• Endothermy is more energetically expensive than ectothermy
• Ectotherms tolerate greater internal temperature variation

Balancing Heat Loss and Gain

• Organisms exchange heat through four physical processes: radiation, evaporation, convection, and conduction

Thermoregulation Adaptations

• Mammalian heat regulation often involves the integumentary system: skin, hair, and nails
• Five adaptations that help animals thermoregulate are insulation, circulatory adaptations, evaporative heat loss, behavioral responses, and adjusting metabolic heat production

Insulation

• Insulation is a major thermoregulatory adaptation for birds and mammals
• Skin, feathers, fur, and blubber reduce heat flow with the environment
• Insulation is vital for marine mammals like whales and walruses

Circulatory Adaptations

• Thermoregulation is significantly affected by regulation of blood flow near the body surface
• Many endotherms and ectotherms can alter blood flow between the core and skin
• Vasodilation increases skin blood flow and facilitates heat loss
• Vasoconstriction decreases skin blood flow and reduces heat loss
• Marine mammals and birds use countercurrent exchange through the arrangement of blood vessels
• Countercurrent heat exchangers transfer heat between fluids flowing in opposite directions, reducing heat loss
• Some bony fishes and sharks use countercurrent heat exchanges
• Many endothermic insects have countercurrent heat exchangers to maintain high thoracic temperatures

Cooling by Evaporative Heat Loss

• Many animals lose heat through water evaporation
• Sweating or bathing cools the skin
• Panting increases cooling in birds and many mammals

Behavioral Responses

• Both endotherms and ectotherms use behavioral responses to control body temperature
• Some terrestrial invertebrates use postures to minimize or maximize solar heat absorption
• Honeybees huddle to conserve heat in cold weather

Adjusting Metabolic Heat Production

• Thermogenesis adjusts metabolic heat production to maintain body temperature
• Thermogenesis is increased by muscle activity like moving or shivering
• Nonshivering thermogenesis involves hormones to increase mitochondrial activity
• Some ectotherms shiver to increase body temperature

Metabolic Rates Continued

• Thermogenesis is the adjustment of metabolic heat production to maintain body temperature
• Thermogenesis is increased by muscle activity such as moving or shivering
• Nonshivering thermogenesis occurs when hormones cause mitochondria to increase activity
• Some ectotherms can also shiver to increase body temperature
• A graph represents O2 consumption against contractions per minute

Acclimatization in Thermoregulation

• Birds and mammals adjust insulation for seasonal temperature changes
• Some ectotherms produce "antifreeze" compounds at subzero temperatures to prevent cell ice formation

Physiological Thermostats and Fever

• The hypothalamus controls thermoregulation in mammals
• The hypothalamus triggers heat loss or generation
• Fever is an increased biological thermostat range in response to infections
• Some ectotherms seek warmer environments during infections

Homeostasis

• An internal thermostat in the hypothalamus activates cooling mechanisms when body temperature increases

Feedback Mechanisms

• Responses to increased body temperature: blood vessels in skin dilate and sweating occurs
• Decreased body temperature triggers the thermostat in hypothalamus to activate warming mechanisms
• Responses to decreased body temperature: blood vessels in skin constrict, and shivering occurs

Bioenergetics

• Bioenergetics involves the overall flow and transformation of energy in an animal
• An animal's food needs are determined by its size, activity, and environment

Energy Allocation and Use

• Organisms are classified by how they obtain chemical energy
• Autotrophs harness light to build energy-rich molecules
• Heterotrophs harvest chemical energy from food
• Food molecules are used make ATP, which powers cellular work
• Biosynthesis, after the needs of staying alive are met, involves body growth, repair, and storage material synthesis like fat, and production of gametes
• A diagram shows the pathway of organic molecules to heat, work, waste etc

Quantifying Energy Use

• Metabolic rate measures the energy used by an animal per unit of time
• Metabolic rate determined by: heat loss, oxygen consumption/carbon dioxide production, and measuring energy content of consumed food vs. waste products

Metabolic Rate Terms

• Basal metabolic rate (BMR) is an endotherm's metabolic rate at rest at a "comfortable" temperature
• Standard metabolic rate (SMR) is an ectotherm's metabolic rate at rest at a specific temperature
• Both rates assume non-growing, fasting, non-stressed animals
• Ectotherms have much lower metabolic rates than endotherms of comparable size

Influences on Metabolic Rate

• Metabolic rates are affected by factors beyond whether an animal is an endotherm or ectotherm
• Key factors that affect metabolic rate are age, sex, size, activity, temperature, and nutrition

Size and Metabolic Rate

• Metabolic rate is proportional to body mass to the power of three quarters (m³/4)
• Smaller animals have higher metabolic rates per gram than larger animals
• This leads to higher oxygen delivery, breathing rate, heart rate, and blood volume compared to larger animals

Activity and Metabolic Rate

• Endotherm and ectotherm metabolic rate is greatly affected by activity
• The maximum sustainable metabolic rate is related inversely to the activity duration
• The average daily energy consumption rate is 2-4 times BMR (endotherms) or SMR (ectotherms)
• The fraction of energy budget depends on environment, behavior,size and thermoregulation

Torpor and Energy Conservation

• Torpor is a physiological state with low activity and decreased metabolism
• Torpor enables animals to save energy while avoiding tough conditions
• Hibernation is long-term torpor adapting to winter's cold temperatures and food shortages

Estivation

• Summer torpor, or estivation, enables animals to survive periods of high heat or scarce water
• Daily torpor by small mammals and birds seems adapted to feeding patterns

Adaptations

• There are relationships between structure and function in animals
• There are also similarities in adaptations of plants and animals

Feedback Control

• Faced with environmental fluctuations, animals manage their internal environment by either regulating or conforming
• A regulator uses internal control mechanisms to control internal change in the face of external fluctuation
• A conformer allows its internal condition to vary with certain external changes
• Animals may regulate some environmental variables while conforming to others

Homeostasis Mechanisms

• Homeostasis is used to maintain a steady state internally, regardless of the external environment
• Human body temperature, blood pH, and glucose concentration are maintained at a constant level
• Homeostasis changes are regulated by internal mechanisms
• A stimulus detected by a sensor triggers a response
• Feedback returns the variable to its set point

Feedback Control in Homeostasis

• Homeostasis in animals relies largely on negative feedback, which helps to return a variable to a normal range
• Positive feedback amplifies a stimulus and does not usually contribute to homeostasis in animals

Alterations in Homeostasis

• Set points and normal ranges can change with age or show cyclic variation
• In animals and plants, a circadian rhythm governs physiological changes for 24 hour periods
• Homeostasis can adjust to external environments, called acclimatization
• Humans and animals, and what happens in different circumstances are provided as examples

Animal Tissues

• Animal tissues have structures suited to their functions
• There are four tissue categories: epithelial, connective, muscle, and nervous

Epithelial Tissue

• Epithelial tissue covers the outside of the body and lines the organs and cavities
• Cells tightly joined
• Epithelial cell shapes: cuboidal, columnar, or squamous
• Arrangements are simple, stratified, or pseudostratified

Connective Tissue

• Connective tissue binds and supports other tissues
• Includes sparsely packed cells scattered throughout an extracellular matrix
• The matrix consists of fibers in a liquid, jellylike, or solid foundation
• Three types of connective tissue fibers:
  • Collagenous fibers provide strength and flexibility
  • Reticular fibers join connective tissue to adjacent tissues
  • Elastic fibers stretch and snap back to their original length
• Connective tissue contains cells, including:
  • Fibroblasts secrete extracellular fibers
  • Macrophages are involved in the immune system
• Vertebrates have six connective tissues:
  • Loose connective tissue binds epithelia to underlying tissues
  • Fibrous connective tissue is found found in tendons and ligaments
  • Bone is mineralized to form the skeleton
  • Adipose tissue stores fat for insulation and fuel
  • Blood is composed of blood cells and cell fragments in blood plasma
  • Cartilage is a strong and flexible support material

Muscle Tissue

• Responsible for body movements
• Muscle cells consist of actin and myosin filaments and enable contraction
• Three types of muscle tissue:
  • Skeletal (striated) muscle enables for voluntary movement
  • Smooth muscle enables involuntary body activities
  • Cardiac muscle responsible for contraction of the heart

Nervous Tissue

• Nervous tissue is used to receive, process, and transmit information
• Contains neurons, or nerve cells, that transmit nerve impulses
• Contains glial cells, or glia, support cells

Coordination and Control

• Bodily control and coordination rely on the endocrine and nervous systems
• The endocrine system sends chemical signals called hormones to cells throughout blood
• A single hormone can have many results
• Hormones have relatively slow action, but have long-lasting effect
• The nervous system transmits information between locations
• The information depends on the signal's pathway, not the signal type
• Nerve signal transmission is very fast