Cell Membrane and Trasnport

Homeostasis

The maintenance of a stable internal environment within an organism or cell despite changes in external conditions.

Cell membrane

The flexible, selectively permeable outer boundary of a cell that regulates what enters and exits.

Phospholipid

A lipid molecule composed of a hydrophilic phosphate head and two hydrophobic fatty acid tails, forming the bilayer of cellular boundaries.

Selective permeability

A property of a cellular barrier that allows only certain substances to pass through while blocking others.

Passive transport

The movement of substances across a cellular barrier without using energy, moving down a concentration gradient.

Concentration gradient

A difference in the amount of a substance across a distance or space.

Solution

A uniform, homogeneous mixture formed when one substance is evenly distributed within another.

Solute

The substance that dissolves in a liquid to form a mixture.

Solvent

The liquid capable of dissolving another substance to create a uniform mixture.

Diffusion

The passive movement of particles from an area of higher concentration to an area of lower concentration.

Facilitated diffusion

The passive movement of specific molecules across a cellular boundary through transport proteins without cellular energy.

Osmosis

The movement of water molecules across a selectively permeable barrier from high water concentration to lower water concentration.

Hypotonic solution

An external environment having a lower solute concentration and higher water concentration than inside a cell, causing water to enter.

Hypertonic solution

An external environment having a higher solute concentration and lower water concentration than inside a cell, causing water to leave.

Isotonic solution

An external environment with an equal solute concentration compared to the cell interior, resulting in no net water movement.

Protein channel

A membrane-spanning pore that allows specific ions or small polar molecules to pass passively across the lipid bilayer.

Active transport

The movement of substances across a cellular barrier against a concentration gradient, requiring cellular energy ($\text{ATP}$).

Protein pump

A membrane transporter that uses cellular energy to force molecules or ions from areas of lower concentration to higher concentration.

Glycoprotein

A membrane molecule consisting of carbohydrate chains bonded to an amino acid chain, functioning in cellular identification and communication.

Cholesterol

A steroid lipid embedded within the bilayer that regulates membrane fluidity and stability during temperature changes.