5.1.1 Homeostasis and Communication

What is homeostasis

The regulation of the internal conditions of a cell or organism, to maintain optimum conditions for function, in response to internal and external changes

What is the need for communication systems in organisms

• Animals & plants need to respond to changes in their internal/external environment

• Also need to coordinate the activity of different organs

What are the principles of homeostasis

• Most homeostatic control mechanisms operate using negative feedback

  • Help maintain a normal range or balance within an organism - reducing the effect of the stimulus

  • Typical negative feed back loop contains a receptor, a coordination system and an effector

What is positive feedback

The original stimulus produces a response that enhances the effect of the original stimulus - essentially amplifies it

What is cell signalling

• Cells communicating with each other (adjacent cells and disant cells)

• Allows multicellular organisms to co-ordinate their bodies

How does cell signalling work

• Stimulus is received by a receptor cell

• Stimulus is converted to a (chemical signal) that is passed on via transduction

• Signal reaches a target cell (effector) that can detect it (via receptors)

• Response is made

What are the 2 cell signalling pathways in animals

• Paracrine signalling:

  • Signalling between cells that are close together

  • Involves signalling molecules travelling via the circulatory system - proteins, glycoproteins, amino acids, lipids and phospholipids

• Endocrine signalling:

  • Signalling between cells that are far apart

  • Involves hormones travelling via the circulatory system

Why is thermoregulation important

• Lower temperatures reduce kinetic energy available for molecules - slows down chemical reactions

• Higher temperatures speed up reactions (up to a point) then the R.O.R drop sharply as the enzymes denature

What are endotherms

• Animals that have physiological mechanisms to maintain their internal body temperature

• E.g.mammals, birds

How do endotherms thermoregulate themselves

• These animals detect external temperatures via peripheral receptors - thermoreceptors found in skin and mucus membranes

  • They detect heat and cold - send this info to the hypothalamus

• Hypothalamus contains receptors that monitor the internal temperature of the blood passing through it

  • Processes the information

    • Initiates response to lower or raise the body temperature

How do endotherms react to high body temperatures

• Vasodilation: muscles (effectors) in the walls of the arterioles relax

  • Dilation

    • More blood flows into skin capillaries

      • Heat is lost to the environment by radiation

• Sweating: secreted by sweat glands (effectors) in the skin

  • Heat energy from the body is used to convert liquid water into water vapour

• Flattening of hairs: hair erector muscles relax

  • Hairs lie flat

    • No insulating layer of air

      • Air can freely circulate over the skin

How do endotherms react to low body temperatures

• Vasoconstriction: muscles (effectors) in the walls of arterioles contract

  • Constriction

    • Less blood flows into skin capillaries

      • Heat loss is reduced

        • Blood is directed through shut vessels deep in the skin

• Shivering: muscles (effectors) contract and relax repeatedly in quick succession

  • Heat energy is released as a byproduct of this exothermic reaction

    • Warms & raises the core body temp

• Erection of hairs: hair erector muscles (effectors) contract

  • Hairs stand up

    • Insulating layers of air created

      • Reducing heat loss by radiation

• Increased metabolic rate: Thyroxine released by the thyroid gland (effectors)

  • Increases our basal metabolic rate

    • Increases heat production in the body

What is an ectotherm

• Animals that rely on behavioural mechanisms to ensure their internal body temperature is maintained

  • E.g. basking in the sun, huddling together for warmth

  • E.g. reptiles and amphibians

How do ectotherms thermoregulate themselves

• Rely on behavioural mechanisms to regulate their body temperature

• To warm up they may:

  • Bask in the sun or on warmer surfaces

  • Huddle together to retain heat that may have been gained from the sun earlier

• To cool down they may:

  • Seek shade

  • Move their bodies into water

What are the advantages of how ectotherms thermoregulate themselves

• Save a lot of energy by not regulation their body temperature internally - thus can survive in environments where food is limited

• Can also grow faster than endotherms as energy they aren’t spending on thermoregulation, can be spent on growth

What are the disadvantages of how endotherms thermoregulate themselves

• Ectotherms are restricted by environmental temperature - so can’t easily colonise extreme environments

Examples of homeostasis mechanisms

• Core body temperature

• Metabolic waste

• Blood pH

• Concentration of glucose in the blood

• Water potential of the blood

• Concentration of respiratory gases (carbon dioxide and oxygen in the blood)

What is the nervous system

Consists of:

• Central nervous system (CNS): brain & spinal cord

• Peripheral nervous system (PNS): all other nerves in the body

Info is sent through the system as nerve impulses

What is the endocrine system

• Endocrine glands produce and secrete hormones

• A hormone transmits useful information to target organs

• Target organs that respond with an action

• Hormonal coordination is slower, so is only used when our bodies do not need instant responses

How does homeostasis occur in plants

• Regulating the opening/closing of stomata balanced carbon dioxide uptake with water loss

• Environmental stimuli causing stomata to open:

  • Low co2 concentration in the air spaces within the leaf

• Environmental stimuli causing stomata to close:

  • Darkness

  • High carbon dioxide concentrations in the air spaces within the leaf

  • Low humidity

  • High temperature

  • Water stress - when the supply of water from the roots is limited and/or there are high rates of transpiration

What is an advantage + disadvantage of the stomata being open during the day

• ADV: Leaves gain carbon dioxide for photosynthesis

• DISADV: Leaves lose large amount of water by transpiration

What is an advantage + disadvantage of the stomata being closed during the day

• Water is retained inside the leaf, which is important in times of water stress

• Supply of carbon dioxide decreases the rate of photosynthesis decreases

What are first and second messengers

• First messenger: normally the hormone form the endocrine gland - binds to a receptor on the cell surface membrane

• Second Messenger: inside of the cell - causes an effect

• e.g.

  • The Adrenaline binds to specific receptors on the surface of liver cells

  • Adenylyl cyclase (enzyme) undergoes a confirmational shape change - becoming activated

  • Activated adenyll cyclase converts ATP to cyclic AMP (cAMP) - the second messenger molecule

  • cAMP binds to protein kinase A = activates further enzymes that result in the breakdown of glycogen, into glucose

What does the cortex of the adrenal glands do

• The cortex produces steroid hormones such as:

  • Aldosterone: regulates level of salts (sodium and potassium) and water concentration in the blood

  • Cortisol: the primary stress hormone, which regulates the metabolism of glucose, proteins and fats, to release usable energy

What does the medulla of the adrenal glands do

• Produces:

  • Adrenaline: produces at times of stress & excitements; preparing the body to respond in emergency situations - the “fight or flight” response

What is the structure and function of the pancreas

Structure:

• Endocrine:

  • Islets of langerhans (alpha/beta cells) - Produce hormones and secretes them into the bloodstream

  • Alpha cells produce glucagon

  • Beta cells produce insulin

• Exocrine:

  • Majority of the tissue

  • Acinar cells - produce NaHCO3 & digestive enzymes which are connected to ducts, which drain into the small intestine

    • Acinus - singular

    • Acini - plural

Function:

• Endocrine and exocrine gland

  • Exocrine: function to produce pancreatic juice (containing digestive enzymes) to be delivered to the small intestine - aids digestion

    • Exocrine glands secrete substance via a duct

  • Endocrine: produce glucagon and insulin

    • Endocrine glands secrete hormones directly into the blood

What happens when there is a decrease in blood glucose concentration

• Drop in blood glucose concentration is detected primarily by Alpha cells but also Beta cells

• Alpha cells respond by secreting glucagon and beta cells respond by stopping or reducing the secretion of insulin

1) Glucagon released by the pancreas - travels in the bloodstream - binds to Receptors on liver cells

2) Receptor undergoes a conformational shape change - activates intracellular G protein

3) Activated G protein activates the enzyme adenylyl cyclase

4) Adenylyl cyclase converts ATP to cAMP (second messenger)

5) cAMP binds to and activates protein kinase A enzymes, by phosphorylating them

6) Activated protein kinase enzymes activate glycogen phosphorylase enzymes

7) Activated glycogen phosphorylase enzymes catalyse the breakdown of glycogen into glucose

What happens when there is an increase in blood glucose concentration (Insulin being released)

• Beta cells take in excess glucose by facilitated diffusion

• Beta cells use this glucose in respiration, producing ATP

• High concentrations of ATP cause potassium channels on the surface of the bet cells to close - potassium is trapped in the cell - changes the membrane potential (electrical charge) - it becomes more positive (-70mV to -30mV)

• Change in the membrane potential causes voltage-gates calcium channels to open (as they are sensitive to a change in voltage)

• Influx of calcium ions causes beta cells to secrete insulin…travels in the blood

• Glucose is uptaken by muscle, fat and liver cells

• Insulin binds to receptors on the surface of muscle, liver and fat cells

What does insulin do when released

• Helps to increase the uptake of glucose in the liver by stimulating glycogenesis

• Glucose enters the liver cells - enzyme converts it to glucose phosphate

• Enzyme cascade follows, that leads to glucose phosphate being converted to glycogen

What is the role of the liver

• The liver plays a vital role in the conversion between glycogen and glucose

  • Insulin and glucagon interact with liver cells and trigger several processes

• Glycogenesis - the synthesis of glycogen from glucose molecules removed from the bloodstream - (triggered by insulin but by buildup by