BIOLOGY- EXAM TESTING

4.3 HOMEOSTATIC MECHANISMS & STIMULUS-RESPONSE MODELS

What is homeostasis:
Homeostasis is defined as the constant maintenance of the body’s internal environment.
All the liquids in the body fall in a certain range of things this is called a set point, so our metabolism functions correctly.
Our body has a set point, which is a set value.
Two key systems involved homeostasis: nervous system and endocrine system.
The nervous system uses electrical messages, and it’s very quick, but the response is short-lived.
The endocrine system uses chemical messages in the form of hormones and is slower in acting, but the response is longer-lived.
Our Body keeps an eye on the set point, and if the variable moves outside the range they will do something to bring it back into the range, this os called stimulus-response model, which contains 5 steps (SRCER):
It stands for stimulus, receptor, control centre, effector, and response.

There’s two types of stimulus-response models: open and closed.

Receptors:
A receptor is a structure that can detect any stimulus and give a signal through neurone to the brain.
Examples of receptors are:
Chemoreceptors: (Chemical receptors, chemo for chemical) detects changes in dissolved chemicals.
Mechanoreceptors: detect physical forces like touch, pressure, stretch, and vibration in your skin.
Photoreceptors: detects changes in light.
Thermoreceptors: detects changes in temperature.
Osmoreceptors: detects changes in the osmotic concentration of the body fluids.

Open stimulus-response model:
In the open model, the response has no effect on the stimulus. They’res three structural components, a receptor, a control centre, and an effector, additionally a stimulus and response on either end.
Receptors in the ear detect sound and send an electrical message to the brain.
The brain is the control centre and processes the message from the receptor, then it transmits an appropriate message to an effector.
The effector is the skeletal muscles in the arms and legs of the sprinter.
The response is the contraction of those muscles, which push the sprinter off the starting blocks.
The response in this situation has no effect on the stimulus. There’s no feedback loop.

Many other variables are manages by a closed model, which is known as the homeostatic model.
It has the same components as SRCER but in a loop arrangement.
The response has an impact on the stimulus and feedback is always monitored to see what the effect is, this is called feedback loop.

Negative feedback loops:
In generative feedback loops, if the stimulus is an increase in a variable, the response will be a decrease in the same variable.
If the stimulus is a decrease in a variable, the response will be an increase in the same variable.

Positive feedback loops:
In a positive feedback loop, the body acts to increase the magnitude of the original stimulus. An example is childbirth.
The hormone oxytocin, realised from the pituitary gland (in the brain) signals the walls of the uterus to begin contracting.

4.7 MALFUNCTIONS IN HOMEOSTATIC MECHANISMS

Hyperthyroidism:
In hyperthyroidism and hypothyroidism the thyroid gland in the throat is important in maintaining basal metabolic rate.
Basal metabolic rate: It is the base line of energy you’re consuming.
With some people the thyroid gland can produce too much thyroxine, this is called hyperthyroidism.
Symptoms of hyperthyroidism:

  • Increased resting heart rate

  • High body temperature

  • Bigger appetite

  • Hard to gain weight

  • Always too hot, for ex: if you go into a warmish room, you’ll feel boiling, but if you go into a cold room you’ll be fine.

It’s caused by physical issues.

Hypothyroidism:
Hypothyroidism is where you produces way too much thyroxine, but it’s not functional thyroxine.
The common results from a diet is that there is not enough iodine. Which is usually found in salt.
When your body produces too much thyroid, it literally starts growing in your body, so it causes a big bump on your neck called goitre.

2.2 CELL REPLICATION

The cell cycle:
SOMETIMES!! There’s G0- which is resting phase.
When most cells are entering G1, some others will enter G0.
The difference is that cells in G0 dot change their internal structure or size. Most cells enter G0 temporarily but for some (like neurons) it’s permanent. This means neurons never undergo mitosis.

3.5 ENDOCRINE SYSTEM IN ANIMALS

What is the endocrine system?
Endocrine systems job is to send chemical messages around the body in the form of hormones. It has endocrine glands, like thyroid gland and adrenal glands.
The nervous system also sees messages, but it shouldn’t be confused for the nervous system. Nervous system sends electrical messages, they travel through nerves, its super quick but short in duration.
The chemical messages goes through the blood stream so it takes a while for the message to reach target, they are longer in duration.

Hormones:
A hormone is a chemical message produced by an endocrine gland.
Hormones are either - Amino acid based, which are hydrophilic, which means they cannot cross the plasma membrane. Receptors for them are found on the surface of a cell.

  • Steroid-based, which are hydrophobic and can diffuse through the phospholipid bilayer. Receptors for them are found inside cells, in their cytoplasm.

Hormones regulate many body functions, like growth, metabolism, blood sugar levels and sexual development.
Their job is to carry a message to a target cell like the instruction ‘release T3 and T4’ being carried by TSH to the thyroid gland.
Hormones have receptors on their target cells.
Hormones and their receptors have a complementary shape which means they are specific to each other.
This means each hormone only acts on a specific cell.

Endocrine vs exocrine glands:
An endocrine gland releases its messages (hormones) directly to the blood stream, which carries them to the target cell(s).
Whereas the exocrine glands release their secretions to the surface
(Ends for interior, exo for outside.)

Components of plasma membrane:

  1. Carbohydrates allows cells to communicate to each other.

1) Molecules & plasma membrane:

Three different things that determine what can go through the plasma membrane:

  • The type of molecule, either hydrophobic ( hates water, its oil based, it will go through the phospholipid lipophilic.

  • Hydrophilic is water based and it will not go through the phospholipid its also known as lipophilic

  1. The size molecule, the bigger it is the harder it is to get through the membrane.

There’s a positive end and negative end in the molecule, this is a polar molecule, it has poles. Water is polar. This means if other molecules (like salt) it will be attached to the poles and the water molecules and it will dissolve in the water. Positive is attracted to negative.

  1. Ions have an overall charge, some can dissolve in water like sodium or potassium, not all ions can.

  2. Polar molecules can also pass the membrane if they are small. Water when its by itself (free-water) is also small enough to pass between the phospholipid molecules, only if its pure water, whole being polar

Crossing the plasma membrane: Diffusion

Particles in any solution always move from an area of high concentration to areas of low concentrations. This is called diffusion, it’s very slow. Diffusion does not require energy, this means its a passive process. Diffusion can be seen with gases and liquids.

If we put a membrane in the middle of the solution, diffusion will continue to across the membrane if the molecules will cross it. There’s 3 different types of diffusion:

  • Simple diffusion: only occurs if a membrane is freely permeable to the molecules.

  • The molecules will be constantly moving from a side of the membrane to the other until its the same concentration on both ends. This is called equilibrium. Once equilibrium is reached, the molecules will still be constantly moving around, but there’s no more net movement.

Things that can affect the rate of diffusion is:

  • Concentration: the higher the concentration gradient, the higher rate of diffusion.

  • Temperature: higher temperatures increase the rate of diffusion, bc the particles have more kinetic energy.

  • Particle size: smaller particles diffuse more quickly.

Membrane can choose to open its protein channels, if it wants salt or sugar, it occurs on concentration gradient.

1. The facilitated diffusion,

There are two types of protein channels:

  • The first one is a channel protein (DO NOT BE CONFUSED WITH PROTEIN CHANNEL), this means it simply opens up to provide water tunnel for any polar molecule or ion to pass through. It is a straight tunnel that can let anything in.

  • The second one is a carrier protein: it only lets specific molecules in like glucose and amino acids letting the cell choose what they let in. Is specific to one molecule. If a molecule can come in the protein, a conformational change occurs. The protein changes its shape to let the molecule in.

CROSSING THE PLASMA MEMBRANE

If there are large molecules that are too big to fit in through protein channels, waste products nutrients and water, can be transported in bulk, this is active process.