Homeostasis - biology

Unit 4

What is homeostasis?

The ability of an organism to maintain a relatively stable environment despite changes in their external environment

It is essential for organisms to function properly and essential for life

What is a stimulus model?

It describes how living organisms react to changes in their environment

It allows the body to correct any changes that may be detrimental

What is the positive feedback loop (NOT IN SYLLABUS)

It works to amplify a change in a variable

E.g. during childbirth, the release of oxytocin causes the uterus to contract, causing more oxytocin to be released. This helps to ensure the continuation of contractions until the baby is born

What is the negative feedback loop?

They work to keep a variable within a certain range

Its main purpose is to counteract the stimulus

E.g. if the body temperature rises, the hypothalamus sends signals to the body to sweat and increase blood flow to the skin, helping to lower the temperature

What does the negative feedback loop consist of?

• Stimulus

• Receptor

• Control center

• Effector

• Response

What is the stimulus?

The change in the environment that triggers the homeostatic response

E.g. a change in body temperature

What is the receptor?

The part of the body that detects the stimuli

E.g. the hypothalamus (for body temperature)

What is the control centre?

The part of the body that receives information from the receptor and sends signals to the effector

E.g. the hypothalamus (for body temperature)

What is the effector?

The part of the body that carries out the homeostatic response

E.g. sweat glands are effectors for body temperature

What is the response?

The specific action or physiological change initiated by the effector to counteract the deviation from the set point - tries to reverse the effect and restore balance

What is the main purpose of the negative feedback model?

It is to counteract the stimulus

It allows the body to correct any changes that may be detrimental

Give an example

• fill this in once Dr Ruoss replies

Provide some examples of homeostasis:

• Body temperature: kept within narrow range of 36.5-37.5. If it gets too high, the hypothalamus sends signals to the body to sweat and increase blood flow to the skin. If it gets too low, the hypothalamus sends signals to the body to shiver and increase muscle activity

• Blood glucose levels: kept within narrow range of 70-100 mg per decilitre. If it gets too high, the pancreas releases insulin, helping the body to use glucose for energy. If it gets too low, the pancreas releases glucagon, helping the body to release stored glucose into the bloodstream

• Blood pressure: kept within narrow range of 120/80 mm of mercury. If it gets too high, the heart has to work harder to pump blood, which can lead to heart disease. If it gets too low, the body may not get enough oxygen and nutrients, which can lead to organ damage.

Why is homeostasis important?

• Essential for life, and allows organisms to function properly and survive

• Imbalances/disruptions can lead to various health problems and diseases

• Studying homeostasis helps us understand how organisms maintain stability in their internal environment and adapt to different conditions

What are the receptors?

• Photoreceptors

• Olfactory receptor cells

• Gustatory receptors

• Auditory receptor cells

• Thermoreceptors

• Mechanoreceptors

• Interoceptors

• Mechanoreceptors

• Nociceptors

• Peripheral chemoreceptors

What are photoreceptors

They react to light stimuli

Cells in the retina of the eye

What are olfactory receptor cells?

Located in the olfactory epithelium (at the roof of the nose) and react to odors/smells

What are gustatory receptors

Located in the taste buds and responsible for taste

What are auditory receptor cells?

Located in the inner ear, and reacts to sound stimuli

What are thermoreceptors

Receptors in a sensory cell that are sensitive to changes in temperature

What are mechanoreceptors?

Receptors in the skin that react to various mechanical stimuli (e.g. touch, pressure, vibration, etc.)

What are interoceptors?

They respond to stimuli inside of the body (e.g. blood pressure, pain, etc.)

What are nociceptors?

They are responsible for sensing/detecting/responding to pain

What are peripheral chemoreceptors?

They respond to chemical changes in the blood, (e.g. oxygen concentration)

What is a tolerance limit?

A set range in which an organism can tolerate the temperature, water balance, and different levels of organic and inorganic materials

Cellular functions fail is it is below minimum tolerance limit, and cellular damage will occur if it is above the maximum tolerance

What is thermoregulation?

The process by which animals maintain a stable body temperature, regardless of the external temperature variations

What is an ectotherm?

An organism whose body temperature is determined by the external environment

What is an endotherm?

An organism that uses metabolic processes to generate its own heat in order to maintain its internal temperature within their tolerance range - modify their own internal body processes

What are the different strategies that animals use to regulate their body temperature?

• Conduction

• Convection

• Radiation

• Evaporation

What is conduction?

The transfer of heat between objects in direct contact

Animals can absorb heat by sitting on warm surfaces, or they can lose heat by resting on cooler surfaces - behavioural adaptation

What is convection?

The transfer of heat through movement of a fluid (air or water)

Animals can increase heat loss by exposing themselves to cool air currents, or decrease heat gain by seeking shelter to warm air or water currents - behavioural adaptation

What is radiation?

The transfer of heat through electromagnetic waves

Animals can absorb heat from the sun or lose heat to their surroundings depending on the temperature gradient

What is evaporation?

The process of heat loss through conversion of liquid gas

Animals can dissipate excess heat by sweating (mammals) or panting (birds) helping them to cool their bodies through evaporative cooling - physiological process

What is enzyme optimisation?

It means that enzymes only function at their optimum temperature, making temperature regulation essential for cellular processes

What are temperature extremes?

This is when the body temperature deviates too far from the optimal range, making reaction rates slow down dramatically

What is enzyme denaturation?

It refers to how excessive heat can cause enzymes to lose their structure, leading to denaturation and potential cell death

What is excretion

The removal of waste from the body of metabolic activity - defecation is not excretion as feces are undigested remnants of food, not metabolic waste

What does the excretory system do?

• Removes nitrogenous wastes that may be harmful/toxic to the body in large concentrations

• Removes excess water to maintain suitable osmolality within the tissues and cells

Expand more on nitrogenous wastes?

They are produced from the breakdown of nitrogen-containing compounds, like amino acids and nucleotides

Nitrogenous wastes are toxic to the organism and excess levels must be eliminated from the body

The type of nitrogenous waste in animals is correlated with the evolutionary history of the animal and its habitat

How do aquatic animals eliminate their nitrogenous waste?

Most aquatic animals eliminate their nitrogenous waste as ammonia (NH3)

Ammonia is highly toxic but also very water soluble and can effectively be flushed in aquatic habitats (due to the environment)

It also costs minimal energy for the organism

How do terrestrial animals eliminate their nitrogenous waste?

They have less access to water and must package their nitrogenous waste in less toxic forms

Mammals eliminate nitrogenous waste as urea - less toxic, highly water soluble, and stored at higher concentrations

However it costs Higher energy for the organism

How do reptiles and birds eliminate their nitrogenous waste?

They eliminate wastes as uric acid, requiring more energy to make but relatively non-toxic for the organism and requires less water to flush - eliminated as a semi-solid paste

They have rather low water solubility and therefore requires an environment of less water.

What do the kidneys do?

They maintain a constant internal environment and plays an important role in osmoregulation

The nephrons filter the blood to regulate chemical concentrations and produce urine

Each of the various components of the nephrons are selectively permeable to different molecules and enable the complex regulation or water and ion concentration

What are the three roles of the kidneys?

1. Removal of nitrogenous wastes

2. Regulation of water concentration in the blood

3. Maintaining ion levels in the blood

What is filtration?

Fluid and solutes are filtered out of the blood to form a glomerular filtrate - glomerulus is the site in the nephron where filtration occurs

What is reabsorption?

When the filtrate (water, ions, and other substances) is absorbed back into the blood

In the mammalian nephron, filtration and reabsorption regulate body fluid concentrations

What are some behavioural ways that animals maintain water in terrestrial environments

• Avoiding the sun

• Avoiding having to drink

• Avoiding water loss through the body

Explain “avoiding water loss through the body”

Desert animals must keep cool, but to minimise water loss, they don’t have to do it in ways that involve evaporation

Reptiles like the Texas thorned lizard have tough, thick skin that doesn’t let water get out.

Explain “avoiding having to drink”:

Many desert animals become so good at water retention that they get all the water needed from their food

Kangaroo rats are vegetarians and only eat high carbohydrate seeds (seeds higher in fat take more water to process and are avoided). Their bodies take most of the water out of their urine so that they pass thick uric acid and their droppings are almost completely dry

Cactus mice eat fruits and insects for water and have a similar water retention system, while elf owls and kit foxes get all water needed from their prey

Explain “avoiding the sun”:

It is the best water retention adaptation for desert animals to avoid the sun and extreme heat

Water is used up in the cooling process and can dehydrate quickly

Adaptations include: nocturnal activity, crepuscular (active at dawn and dusk), seeking shade, burrowing, and aestivation (slowing down metabolic processes)

What are some structural ways that animals maintain water in terrestrial environments

• Evaporation (avoiding the sun)

• Fatty deposits (avoiding having to drink)

• Specialised nasal passages (avoiding water loss)

Explain “evaporation”

Animals like antelope squirrels and camels are active during the hot days as they can accumulate heat without harm as they can cool themselves by evaporating body water

Squirrels lose excess heat to shaded surfaces

Camels lose heat to the cooler night air

Other examples include:

• Insulating fur

• Large ears

• Long legs

• Spines

Explain “fatty deposits”

Large lizards (Gila monsters) store the water in fatty deposits in their tails

Other examples include:

• Urinary bladders (tortoises storing water that can be reabsorbed if needed)

Explain “specialised nasal passages”:

Small rodents and birds (e.g. cactus wrens) have specialised nasal passages that cool the breath before it is exhaled. This condenses the water for reabsorption

Many desert lizards have nasal salt glands, excreting potassium and sodium chloride with very little water loss

What are some physiological ways that animals maintain water in terrestrial environments

• Aestivation (avoiding the sun)

• Transpiration (avoiding having to drink)

• Excretion (avoiding water loss)

Explain “aestivation”

It could be argued as a behavioural way but it is mainly physiological as the animals escape for months at a time by aestivating in their burrows

During this time period they have reduced breathing and heart rate, allowing them to escape from the high heat and to conserve water

Explain “transpiration”

This is where water moves through a plant and its evaporation from the leaves

The water vapour escapes from the leaves through the stomata (tiny pores)

Explain “excretion”:

This saves water as the animal has dry feces and concentrated urine

Some animals like the kangaroo rat has feces 5x drier than a regular rat, as well as urine 2x as concentrated

Other animals like lizards and snakes, excrete uric acid rather than liquid urine, to save water

What is osmosis

The diffusion of water across a semi-permeable membrane

Water will move from a low concentration of solutes to high (for balance)

What can an osmotic solution be?

• Hypertonic

• Isotonic

• Hypotonic

What is a hypertonic solution?

A solution where the outside of the cell is more concentrated than the inside

Water will move out of the cell through osmosis and cause it to shrink (plasmolysis)

What is an isotonic solution

A solution where the inside cell has the same concentration as the outside of the cell

Water will move in and out of the cell through osmosis at an equal rate

What is a hypotonic solution

A solution where the outside of the cell has a lower concentration than the inside

Water will move into the cell through osmosis and cause it to burst (cytolysis)

What is an osmoregulator?

Animals that regulate their salt and water changes, e.g. a mammal

They use physiological processes to maintain constant water balance in their body

FISH: Living in freshwater and saltwater will have different problems to face in terms of water balance

What is an osmoconformer?

Animals that can’t regulate their salt and water balance, their body fluid balance fluctuates with the environment, e.g. jellyfish

They use less energy to maintain their osmotic conditions

Differences between osmoregulators and osmoconformers?

Osmoregulators:

• Freshwater and marine

• Regulate internal osmolality independently from the external environment

• Control internal environment (osmoregulation)

• Actively regulate internal osmolarity

• Can survive in a narrow range of salinities

Osmoconformers:

• Maintain internal environment isotonic (same) to the outside environment

• Follow the environment (no osmoregulation)

• Actively/passively change internal environment

• Can survive in a wide range of salinities

Similarities between osmoregulators and osmoconformers?

• Animals can be marine

• They can live in unique environmental conditions

Problems that saltwater (marine) fish face with their physiological processes

• Loses too much water via osmosis across the skin

• Gains too much salt via drinking seawater and eating food

Problems that freshwater fish face with their physiological processes

• Gains too much water via osmosis across the skin and when eating food that contains water

• Loses too many salts via diffusion and in time

The adaptations that marine fish have for water balance:

1. Constantly drinking seawater

2. Eating food that contains water

3. High level of reabsorption in the kidneys

4. Excretes a low volume of highly concentrated urine

The adaptations that freshwater fish have for water balance:

1. Doesn’t drink water (fish swim with mouth open so that water passes the gills for gas exchange but they don’t swallow)

2. Low level of reabsorption in the kidneys

3. Excrete high volumes of dilute urine

The adaptations that marine fish have for salt balance:

1. Excretes highly concentrated urine, ridding the body of excess salts

2. Active transport of salts from salt-secreting cells in the gills to the seawater

The adaptations that freshwater fish have for salt balance:

1. Gain salts when eating food

2. Active uptake of salts from water across the gills