SCNC1112 Chap.11 Cells, Organ Systems and Feedback Control
Cells, Organ Systems and Feedback Control
Cell theory makes three claims: every living organism is built from at least one cell, the cell is the smallest unit in which structure and organisation are assembled, and new cells come only from cells that already exist. The third clause was the hard-won one historically, replacing the idea that organisms could appear from non-living matter.
Every cell carries a plasma membrane, a way of reading its sequence record and machinery for building proteins, and eukaryotic cells add membrane-bound compartments that let incompatible chemistry run at the same time in one place. Specialisation is a change of emphasis rather than of parts list.
A muscle cell carries the machinery for contraction, a stomach lining cell produces acid together with the mucus that protects it, and a nerve cell is electrically excitable, while all three carry the same genome. They differ in which proteins they are making, which is the point Chapter 10 established and the reason structure and function match. The membrane is where a cell meets the rest of the body.
Ion channels let specific ions pass down their gradient and open and close quickly, which is what makes nerve signalling possible. Transporters carry particular molecules across, sometimes against a gradient. Receptors bind a signalling molecule and trigger a response inside without the signal itself entering.
A molecule that binds a receptor and produces the response is an agonist; one that binds and produces nothing while blocking the site is an antagonist. Both bind and only one activates, which is why many medicines and several natural toxins are antagonists, and why an animal whose receptor shape differs slightly can be insensitive to a venom. Homeostasis is the organising principle at the next level up.
Body temperature, blood pH, blood pressure and total body water are each held near a set point by a loop with the same shape: a sensor detects a departure, a control centre compares it with the set point, and an effector acts to correct it. Because the correction removes the signal that triggered it, the loop switches itself off, which is what negative feedback means.
Hormonal control uses the same logic across organs, with a cascade from the hypothalamus to the pituitary to a target gland and an inhibitory return from the final product. Positive feedback reverses the sign and is deliberately rare.
In a positive loop the product promotes more of itself, which is useful when a fast decisive response is needed rather than a measured one: blood clotting, the hormonal surge that triggers ovulation and the contractions of childbirth all work this way, and each has a definite stopping condition built into the situation.
Where no stopping condition exists, the same amplification becomes the pathology, which is the shape of heat stroke and of circulatory shock.
What this chapter covers
- 01
Cell Theory and Its Third Clause
- 02
Organelles and Compartmentation
- 03
Channels, Transporters and Receptors
- 04
Agonists Against Antagonists
- 05
Sensor, Control Centre and Effector
- 06
Positive Loops and Their Stopping Conditions
Locating the failure in a hormone cascade from two symptoms
- 2Set out the cascade. The hypothalamus signals the pituitary, the pituitary signals the adrenal cortex, and the cortex releases cortisol, which inhibits both levels above it.
- 2Locate the fault. Both symptoms are consequences of too little cortisol rather than of too little signalling, so the most likely failing level is the final effector, the adrenal cortex.
- 2Derive the symptoms. Cortisol promotes the breakdown of stored glycogen and the making of glucose from non-carbohydrate sources, so too little of it makes blood glucose hard to hold between meals, and cortisol also contributes to blood pressure regulation.
- 1Predict the state of the levels above. With little cortisol returning, the inhibition is lifted, so the hypothalamus and pituitary keep signalling hard and their hormones run high. That pattern is how the site of the fault is confirmed.
Key terms
- Plasma Membrane
- The lipid boundary with embedded proteins that separates a cell from its surroundings and controls what crosses in each direction.
- Ion Channel
- A membrane protein that opens to let specific ions pass down their gradient, whose fast switching underlies nerve and muscle signalling.
- Agonist
- A molecule that binds a receptor and produces the response the receptor normally triggers.
- Antagonist
- A molecule that binds a receptor without activating it, blocking the site so the natural signalling molecule cannot act.
- Homeostasis
- The maintenance of an internal variable near a set point by a loop of sensor, control centre and effector.
- Negative Feedback
- A loop in which the correction opposes and removes the departure that triggered it, so the response switches itself off.
- Set Point
- The value a regulated variable is held near, which can itself be reset, as it is during a fever.
Cells, Organ Systems and Feedback Control FAQ
What is the difference between an agonist and an antagonist?
Binding and activating are separate properties. An agonist binds a receptor and produces the response; an antagonist binds the same site and produces nothing, while preventing the natural signal from acting. Many medicines work by blocking rather than stimulating, which is why describing a drug only as binding tightly says nothing about what it will do.
Why is positive feedback described as rare and also as important?
Because it is the right design only when a fast, committed response is wanted and something in the situation will stop it. Clotting, ovulation and childbirth all qualify. Where no stopping condition exists the same amplification runs away, which is what makes heat stroke and circulatory shock dangerous rather than merely severe.
Is a fever a failure of homeostasis?
Usually not. In a fever the regulated temperature has been reset to a higher value and the body then defends the new value, shivering and constricting surface vessels to reach it. Recognising that a set point has moved rather than that control has been lost is the difference between a diagnosis and a description.
Exam move
Draw the three-stage loop once and then fill it in for three different regulated variables, naming the sensor and the effector each time. For the cascade, practise reading the level of each hormone as a diagnostic rather than as a fact, since that is what turns the diagram into an answer.
