Biological Membranes

These flashcards cover key concepts related to biological membranes, including their structure, function, and transport mechanisms.

What is the primary function of biological membranes?

To provide a boundary for cells and organelles, regulating the passage of substances into and out of the cell and its compartments.

What model describes the structure of biological membranes?

The Fluid Mosaic Model, which describes the membrane as a fluid structure with a 'mosaic' of various proteins embedded in or associated with a double layer of phospholipids.

What components make up the phospholipid bilayer?

It is composed of two layers of phospholipids, each with a hydrophilic (water-loving) phosphate head and two hydrophobic (water-fearing) fatty acid tails.

What type of movement is common for phospholipids within the membrane?

Lateral diffusion, where phospholipids rapidly move sideways within the same layer of the membrane.

What does 'amphipathic' mean in relation to phospholipids?

It means phospholipids have both a hydrophilic (water-loving) head and hydrophobic (water-fearing) tails, allowing them to form a stable bilayer in aqueous environments.

What happens to phospholipids during a 'flip-flop' movement?

They move from one leaflet of the phospholipid bilayer to the other, a relatively rare event that requires enzymatic assistance (flippases).

What are integral proteins?

Proteins that are firmly embedded in or span across the entire phospholipid bilayer (transmembrane proteins), often having both hydrophilic and hydrophobic regions.

What are peripheral proteins?

Proteins that are loosely bound to the surface of the membrane, often attached to integral proteins or the hydrophilic heads of phospholipids, without penetrating the hydrophobic core.

How does cholesterol affect membrane fluidity?

It acts as a 'fluidity buffer;' at low temperatures, it prevents phospholipids from packing too closely (increasing fluidity), and at high temperatures, it restrains phospholipid movement (decreasing fluidity).

Define osmosis.

The net diffusion of water molecules across a selectively permeable membrane from an area of higher water concentration (lower solute concentration) to an area of lower water concentration (higher solute concentration).

What types of transport proteins are used in facilitated diffusion?

Channel proteins, which provide hydrophilic pores for specific ions or molecules, and carrier proteins, which bind to specific molecules and change shape to shuttle them across the membrane.

What is the difference between active transport and facilitated diffusion?

Active transport requires energy (often from ATP hydrolysis) to move substances against their concentration gradient, from an area of lower concentration to higher concentration, whereas facilitated diffusion moves substances down their concentration gradient without direct energy input, utilizing transport proteins.

How do ions move across membranes?

Ions, due to their charge, cannot easily pass through the hydrophobic core of the lipid bilayer and primarily move across membranes through specific transport proteins, such as ion channels or ion pumps.

What mechanism allows large molecules to cross membranes?

Endocytosis (bringing substances into the cell) and exocytosis (releasing substances out of the cell), both involving the formation and fusion of vesicles with the plasma membrane.

What is pinocytosis?

Often called 'cell drinking,' pinocytosis is a type of endocytosis where the cell non-specifically engulfs small droplets of extracellular fluid, along with any dissolved solutes, by forming small vesicles.

Describe phagocytosis.

Often called 'cell eating,' phagocytosis is a type of endocytosis where the cell engulfs large particles, such as bacteria, cellular debris, or other microorganisms, by extending pseudopods to form a large vesicle called a phagosome.

What is receptor-mediated endocytosis?

A highly specific type of endocytosis where specific receptor proteins on the cell surface bind to target molecules (ligands), triggering the formation of clathrin-coated vesicles to internalize the bound molecules.

What role do ATP and ion pumps play in cellular transport?

ATP (adenosine triphosphate) provides the direct energy through hydrolysis for primary active transport, enabling ion pumps (like the sodium-potassium pump) to move specific ions against their electrochemical gradients across the membrane.

What is co-transport?

Co-transport, also known as secondary active transport, is an indirect active transport mechanism where the downhill movement of one solute (typically an ion like Na^+) down its electrochemical gradient is coupled to the uphill movement of another solute against its gradient by the same transport protein.

What does the term 'selectively permeable' mean regarding biological membranes?

It means that the biological membrane allows certain substances to pass through more easily than others, actively regulating which molecules can enter or exit the cell or organelle, thus maintaining specific internal environments.

What happens during simple diffusion of solutes?

During simple diffusion, small, nonpolar molecules (like O\text{2} or CO\text{2}) passively move directly across the lipid bilayer from a region of higher solute concentration to a region of lower solute concentration, without the aid of transport proteins and without energy input.

What is the difference between diffusion of solutes and diffusion of water?

Solutes diffuse directly down their individual concentration gradients (from high to low solute concentration). In contrast, water (osmosis) moves from an area of higher water potential (lower solute concentration) to an area of lower water potential (higher solute concentration) across a selectively permeable membrane to equalize solute concentration.