Lecture 6

BIOL 118 Exam 1

What is homeostasis?

the maintenance of a relatively stable internal environment (values fluctuate within a normal range)

normal range (homeostasis)

where the body functions well

tolerance limits (homeostasis)

boundaries beyond which cells are damaged

When is medical intervention needed?

when values go outside tolerance limits

Are all body values under homeostatic control? Why or why not?

Not all variables are under homeostatic control, because:

• Some things don’t threaten survival if they change

• Some values are controlled locally instead of system-wide

Why is homeostasis called a dynamic equilibrium?

because variables are always changing, but around a stable average

What are the benefits of a negative feedback response?

• Stability

• Protection of cells

• Prevents extreme changes

negative feedback

• Response moves the variable in the opposite direction of the stress

• Pushes the body back toward normal range

• Rate of change slows as normal is reached

• Stabilizes the system

When does a negative feedback response occur (in normal range, outside normal range, outside of the tolerance limits)?

A variable moves outside the normal range (before tolerance limits)

positive feedback

• Response moves the variable in the same direction as the stress

• Rate of change increases

• Drives system toward an endpoint

• Amplifies the change

• Must have a stop signal

In what direction does a variable change as a result of a negative feedback response to a stress?

the opposite direction of the stress

Parts of a homeostasis control loop

1. Stress: change in variable

2. Receptor: detects change

3. Integrator (controller): processes information (often brain or endocrine organ)

4. Effector: organ/tissue that carries out response

5. Response: brings variable back toward normal

After a meal (blood glucose homeostasis)

• Blood glucose rises → hyperglycemia

• Pancreas beta cells release insulin

• Cells increase glucose uptake

• Liver stores glucose as glycogen (anabolic)

• Blood glucose falls back to normal

before a meal (blood glucose homeostasis)

• Blood glucose drops → hypoglycemia

• Pancreas alpha cells release glucagon

• Liver breaks glycogen into glucose (glycogenolysis, catabolic)

• Glucose released into blood

• Blood glucose rises back to normal

What organs and organ systems are involved in blood glucose homeostasis?

• Pancreas (hormones)

• Liver (storage & release)

• Blood (transport)

• Muscle & fat (uptake)

• Endocrine system

hyperglycemia

abnormally HIGH blood glucose levels

hypoglycemia

abnormally LOW blood glucose levels

Type I diabetes

• Pancreas cannot make enough insulin

• Glucose stays in blood

• Cells are starved

• Treated with insulin

Type II diabetes

• Cells do not respond well to insulin (insulin resistance)

• GLUT4 transporters don’t move effectively

• Glucose stays in blood

• Pancreas overworks → may fail over time

Why does glucose have a transporter (GLUT4)?

because glucose is polar, so it cannot pass through lipid membrane

How do GLUT4 carriers transport glucose in muscle?

Insulin causes:

• GLUT4 to move to cell membrane

• Glucose to enter muscle and fat cells

What is insulin resistance?

• GLUT4 doesn’t respond properly

• Cells don’t get glucose

• Blood glucose remains high

How is insulin resistance harmful to cells?

it leads to oxidative stress and cell damage

What is a cell membrane?

a thin, flexible boundary that surrounds every cell and separates the inside of the cell from the outside environment

functions of cell membrane

• Protect the cell

• Control what enters and leaves

• Allow communication

• Maintain homeostasis

structure of cell membrane

is made of a phospholipid bilayer with proteins embedded in it (amphipathic: hydrophilic head and hydrophobic tails) → a selectively permeable barrier