Membranes,Transport,and Signaling

Glucagon is released by the pancreas when blood glucose levels are low to stimulate the liver to release stored glucose. Insulin is released when blood glucose levels are high to lower them.

Negative feedback loops work to counteract a change in a variable, bringing it back to a set point. This is the primary mechanism for maintaining homeostasis in blood glucose regulation.

Homeostasis is the process by which living organisms maintain a stable internal environment despite external changes. Equilibrium is a physical state, while homeostasis is a biological regulatory process.

Hormones are chemical messengers that only affect target cells. Target cells possess specific receptors that bind to the hormone, allowing it to trigger a response.

The pituitary gland is often called the master gland because it secretes hormones that control the activity of several other endocrine glands. The thyroid, adrenal, and pancreas glands have more specific regulatory roles.

Lipid-soluble (steroid) hormones can pass directly through the hydrophobic lipid bilayer of the plasma membrane. Protein hormones are water-soluble and require membrane receptors.

The hypothalamus acts as the primary control center for the endocrine system by receiving signals about body conditions and regulating the pituitary gland. It links the nervous and endocrine systems.

Hormones are chemical messengers secreted into the blood that travel long distances to reach target cells. Impulses refer to electrical signals in neurons.

Turgor pressure is the force exerted by the cell contents against the cell wall in plants, which helps maintain structural rigidity. Osmotic pressure drives the water movement that creates this force.

Co-transport (or secondary active transport) uses the energy from an existing electrochemical gradient to move other molecules across the membrane. This is essential for nerve cell function, such as the sodium-glucose symporter.

Active transport is the movement of molecules against their concentration gradient, which requires energy (ATP). Diffusion and facilitated diffusion are passive processes.

A hypertonic solution has a higher solute concentration than the cell or another solution, causing water to move out of the cell. Hypotonic is lower, and isotonic is equal.

Osmosis is specifically the facilitated diffusion of water across a semi-permeable membrane, often involving channel proteins called aquaporins.

A concentration gradient is the difference in the concentration of a substance between two regions. Molecules naturally move down this gradient during diffusion.

Equilibrium is reached when the concentration of a substance is uniform throughout a space, meaning there is no net movement of molecules.

Small, nonpolar molecules can pass directly through the lipid bilayer of the plasma membrane via simple diffusion. Large, polar, or charged molecules require transport proteins.

The plasma membrane is primarily composed of a phospholipid bilayer, which provides the structure and selective permeability of the cell.