5.1.1. communication and homeostasis

why is communication needed between cells in a multicellular organism

to coordinate all the individual cell functions for the survival and proper development of the entire organism

what does plant cell communication involve

short range, direct contact via plasmodesmata and long distance transport of hormones and other signalling molecules like RNA and peptides

what does animal cell communication involve

short range and long distance cell signalling

what is cell signalling

process by which cells communicate, one cell produces a signalling molecule that can be detected by another cell and induce a response

what is the method for communication systems

cell signalling

what is the structure for cell signalling to take place

signalling molecule binds to cell surface receptor with complementary binding site for signal molecule on a target cell, second messenger within the cell will enable the cellular response

what are the forms of cell signalling in the body

endocrine, paracrine, autocrine, direct cell-cell signalling

what is endocrine cell signalling

signalling molecules called hormones are secreted into the bloodstream for distribution throughout the body, hormones reach all living cells in the body

what is paracrine cell signalling

a secreting cell acts on nearby target cells by discharging signalling molecules into the extra-cellular fluid

what are examples of paracrine cell signalling

• growth factors that stimulate proliferation of local cells e.g. the effect of oestrogen on the developing ovarian follicle

• nerve cell releases neurotransmitter into the synapse stimulating the target cell

  • muscle cells stimulated to contract by neural innervation

  • synapses connect nerve cells throughout the nervous system

what is autocrine cell signalling

signalling molecule is released by the cell and binds to receptors on that same cell leading to changes in the cell

what are examples of autocrine cell signalling

• T lymphocytes produce interleukins that stimulate their own cell division

• cancer cells release chemicals that promote their own mitosis

what is direct cell-cell signalling

also called juxtacrine signalling, occurs by transferring signalling molecules across gap junctions between adjacent cells

what are examples of direct cell-cell signalling

• plasmodesmata allows for communication between adjacent cells

• early embryonic development depends on this type of signalling

what does homeostasis rely on

a continuous flow of information between different parts of the organism to regulate key physiological features

define homeostasis

maintaining a constant internal environment around a norm/set value despite external and internal changes

define negative feedback

a mechanisms that reverses a change in a variable bringing the level back to the norm/set value

define communication systems

systems which allow multicellular organisms to control and coordinate their bodies and respond to their environments, made up of the nervous system and the endocrine system

define autonomic nervous system

a branch of the nervous system that carries nerve impulses to muscles and glands, it controls involuntary activities and has two divisions: sympathetic and parasympathetic

define hypothalamus

region of the brain that serves as the control centre for the autonomic nervous system

what is the region of the brain that serves as the control centre for the autonomic nervous system

hypothalamus

define endocrine system

a system of glands that secrete hormones into the blood

define pituitary gland

small, hormone producing gland located at the base of the brain, known as the master gland

define receptor

specialised structure that detects a specific type of stimulus

define effector

an organ, tissue or cell that produces a response to a stimulus

define internal environment

the tissue fluid or extracellular fluid

what is the cell signalling molecules produced by endocrine glands

hormones

what is the physiological value around which the normal range fluctuates

norm/set value

what must be maintained constant in the internal environment

temperature, water, pH, blood glucose

why does temperature change in the internal environment

external value changes, physical activity, heat generation

why is it important to keep temperature regulated

optimum temp is required for enzyme activity

why does water potential change in the internal environment

intake of water, loss of water e.g. sweating

why is it important water is kept regulated in the internal environment

water potential maintained for metabolism, blood pressure, tissue formation

why does pH of the internal environment change

production of CO2/lactic acid

why is it important pH is regulated in the internal environment

optimum pH required for enzyme activity

why does blood glucose change

intake or usage of glucose

why is it important blood glucose is regulated

supply of respiratory substrate especially to the brain

what is the principle process of a homeostatic control mechanism

change in internal/external environment → variable → sensory receptor → (cell signalling) control/coordination centre in the brain: hypothalamus, knows ideal value/norm → (cell signalling) communication systems: autonomic nervous system, endocrine system → (cell signalling) effector/target cell → (negative feedback) variable

what are the features of the endocrine system

• collection of glands that produce hormones to control the functions of the body

• effect is widespread

• controls blood glucose levels, blood water potential, growth, production of gametes

• composed of glands

• signals are transmitted through the blood

• signals take more time to reach the effector organ

• whole system is not physically connected

• uses hormones to transmit the signal to the effector

what are the features of the nervous system

• network of nerve cells that coordinate the functions of the body by transmitting nerve impulses

• effect is localised

• controls muscle movement, heartbeat, digestion, breathing, speech, memory

• composed of neurones that are arranged in the brain, spinal cord and peripheral nerves

• signals are transmitted through neurones

• signals are transmitted within a short period of time

• whole system is physically connected

• uses neurotransmitters to transmit the signals to the effector organs

what is positive feedback

change in a variable/internal environment is detected, effectors are stimulated to enhance the change and deviate it further from the norm

examples of necessary positive feedback loops in the body

• uterine contraction and childbirth- oxytocin

• suckling and let down of milk

• nervous transmission

• clotting mechanism

• fever

examples of dangerous positive feedback loops

• very high fever (hyperthermia)

• carbon dioxide levels increasing in an airlock

• severe blood loss

• hypercytokinaemia

• hypothermia

why does very high fever cause positive feedback

increased temp increases rate of collisions with enzymes, heat released

why does carbon dioxide levels increasing in an airlock cause positive feedback

loss of oxygen increases breathing rate

why does severe blood loss cause positive feedback

decreased blood volume decreases blood pressure, causes heart to pump more

why does hypercytokinaemia cause positive feedback

over production of cytokines destroys healthy cells, causes more cytokines to be released

define thermoregulation

the ability of an organism to keep its body temperature within certain boundaries even when surrounding temperature is very different

what are the mechanisms for thermoregulation

physiological mechanisms- homeostatic control, behavioural mechanisms- maximise or minimize heat transfer with the environment

what does heat transfer between the body of an organism and its environment depend on

magnitude and direction of the temperature gradient

what are the ways heat may be lost or gained

conduction, convection, radiation, evaporation, exothermic metabolic reactions

what is conduction

heat transfer by physical contact of adjacent particles

what is convection

heating or cooling by currents of air or water

what is radiation

heat transfer in the form of long-wave, infrared electromagnetic waves

what is evaporation

heat loss during the conversion of water to water vapour

what are the types of animals with relation to thermoregulation

endotherms, ectotherms

what are endotherms

animals that primarily generate their own heat

what are ectotherms

animals that primarily gain heat through their environments

what are the strategies animals have for thermoregulation

homeotherms, poikilotherms

what are homeotherms

animals that have a steady body temperature

what are poikilotherms

animals whose body temperature adjusts depending on the environment

what animals are endotherms and homeotherms

mostly birds and mammals

what animals are endotherms and poikilotherms

some birds and mammals- those that allow their body temps to vary during certain time periods, many insects and some invertebrates

what is torpor

like hibernation but temporary

what is the temporary version of hibernation

torpor

what animals are ectotherms and homeotherms

some tropical reptiles, organisms deep in the ocean

what animals are ectotherms and poikilotherms

most fish, amphibians, and reptiles, most invertebrates

what are the features of ectotherms

• body temperature fluctuates with, although generally not as much as, the environment

• need less food than endotherms to supply their metabolic needs

• behavioural and physiological mechanisms important in limiting variation in body temp

• found in a limited range of environments

what are the features of endotherms

• body temp more or less constant and largely independent of the environment

• need more food than ectotherms to supply their metabolic needs

• physiological mechanisms important in maintaining body temp

• found in an extremely wide range of environments

how do ectotherms maintain body temp

behavioural rather than physiological methods

what are the thermoregulation methods of ectotherms

• reorientation

• thermal gaping

• colour changes

• body raising

• burrowing

• bradycardia

what is reorientation

thermoregulation method for ectotherms, reorientation of the body with respect to solar radiation can vary the surface area exposed to heating, a terrestrial ectotherm may gain heat rapidly by aligning at right angles to the suns rays but as its body temperature rises it may reduce the exposed surface by reorientating itself parallel to the sun’s rays

what is thermal gaping

thermoregulation method for ectotherms, used by larger ectotherms e.g. alligators, open mouth allows heat loss by evaporation from moist mucous surfaces, tortoises spread saliva over neck and front legs to act as evaporative surface

what is colour changes thermoregulation method

method for ectotherms, colour changes of skin may alter ability of the body to absorb radiation heat energy, dark-bodied individual will absorb heat more rapidly than a light-bodied one, some ectotherms begin the day with a dark body to warm up and lighten as temp rises

what is body raising

thermoregulation method for ectotherms, minimize heat gains by conduction from hot surfaces such as rocks and sand, whole body may be lifted, reduce area of contact to absolute minimum by balancing on alternate diagonal pairs of feet

what is burrowing

thermoregulation method for ectotherms, avoids temperature fluctuations on the surface, temperature in a shallow burrow may fluctuate 5’ over 24 hrs whereas surface temperature may range over 40’, alligators or crocodiles may return to water rather than burrow due to high heat capacity of water means temp is relatively constant

what is the bradycardia thermoregulation technique

used by marine iguanas, when basking on rocks body temp is 37’, during feeding in the sea it’s exposed to environmental temp of 22-25’, to avoid losing heat rapidly by conduction and convection the iguana reduces flow of blood between core tissues and skin by decreasing heart rate- bradycardia

what are the endotherms mechanisms for thermoregulation

• hairs raised

• vasoconstriction/vasodilation

• subcutaneous fat accumulation/reduction

• brown fat thermogenesis

• increased metabolic rate

• shivering

• sweating

• behavioural mechanisms

• countercurrent mechanism of blood in limbs

• use of ears

what cell in the skin detects changes in environmental temperature

peripheral thermoreceptor

how does raising hairs help with thermoregulation

contraction of erector pili muscles controlled by sympathetic nervous system raises hairs almost vertically, in furry animals it traps a thick layer of stationary air next to the skin, air is a poor conductor so heat loss from skin is reduced, in birds feathers fluff up

what controls erector pili muscles

sympathetic nervous system

how does vasoconstriction thermoregulate

blood transports heat around the body in the circulatory system as it is warmed in organs so warms tissues at low temps, narrowing arterioles supplying skin capillaries and widening shunt vessels reduces heat flow to skin

what controls vasoconstriction/vasodilation

sympathetic nerves from vasomotor centre in the brain under commands from thermoregulatory centre in hypothalamus

what controls vasodilation

parasympathetic nervous system controlled by hypothalamus

how does subcutaneous fat accumulation help with thermoregulation

mammals store fat in adipose tissue below skin surface, tissue has limited blood supply and poor conductor, aquatic mammals have thick layer of fat (blubber) under skin, animals that remain active through cooler season accumulate fat as a survival mechanism and food reserve

what are the types of adipose tissue

white and brown

how does brown fat thermogenesis help with thermoregulation

brown fat cells contain many mitochondria, well supplied with blood capillaries, occurs under skin of upper back especially in young mammals, produces considerable amounts of heat as lipids are respired with little ATP formation- most energy released as heat, heat is transported in blood to body’s organs

what triggers thermogenesis in brown fat

sympathetic nervous system

how does shivering help thermoregulation

muscle contractions generate heat

what does prolonged vasoconstriction cause

necrosis of tissues

how does increased metabolic rate help thermoregulation

under persistent cooler conditions basal metabolic rate is raised, increasing amount of heat generated by the body

what causes metabolic rate to be raised in cooler conditions

short term by secretion of adrenaline, longer term by secretion of thyroxine

what does secretion of adrenaline do for thermoregulation

short term increase in basal metabolic rate

what does secretion of thyroxine do

long term increase in basal metabolic rate

what behavioural mechanisms do endotherms have to increase temperature

moving to a warmer place e.g. in the sun, huddling, putting on clothes

how does countercurrent mechanism of blood in limbs of endotherms in cold environments help with thermoregulation

artery and vein in close proximity, some heat lost from arterial blood coming from trunk transfers to the vein coming away from extremities by conduction, raises temperature of the venous blood as it re-enters trunk

how does sweating help with thermoregulation

water has high latent heat of vaporisation due to hydrogen bonds, takes lot of energy to change state from liquid to vapour, evaporation from surface of skin removes heat energy from skin lowering its temperature

how does the use of ears help with thermoregulation

ears are radiators of heat, elephants fan themselves, increase blood flow, skin on outer ear for heat loss

what are the behavioural mechanisms to reduce temperature in endotherms

moving into the shade, decreasing physical activity to lower metabolic rate, keeping away from other warm animals, panting, wear fewer clothes, having lots of sweat glands

what regulates thermoregulation

hypothalamus