Plasma Membranes: Structure and Function

Flashcards covering the structure and function of plasma membranes, including components, fluidity, passive transport, active transport, and bulk transport mechanisms.

What is the most basic function of the plasma membrane?

To define the cell's borders and keep the cell functional.

What does it mean for the plasma membrane to be selectively permeable?

The membrane allows some materials to freely enter or leave the cell, while other materials require specialized structures or energy for crossing.

According to the fluid mosaic model, what are the primary components of the plasma membrane?

Phospholipids, cholesterol, proteins, and carbohydrates.

What type of molecules form the main fabric of the plasma membrane?

Amphiphilic phospholipid molecules.

Describe the two regions of a phospholipid molecule and their interaction with water.

It has a hydrophilic ('water-loving') head that interacts with aqueous fluid and hydrophobic ('water-hating') tails that avoid water and interact with other non-polar molecules.

What role do integral proteins play in the plasma membrane?

They are integrated completely into the membrane structure, often spanning it, and serve functions like transmitting signals or facilitating transport.

Where are peripheral proteins located and what are some of their functions?

They are on the membrane's exterior and interior surfaces, attached to integral proteins or phospholipids, and can serve as enzymes, structural attachments, or cell recognition sites.

What is the primary function of carbohydrates attached to the exterior of cell membranes?

Cell recognition, which is vital for tissue formation, embryonic development, and immune response ('self' versus 'non-self' distinction).

What is the glycocalyx?

A collective term for the carbohydrates (glycoproteins and glycolipids) on the cell's exterior surface, which is highly hydrophilic and aids in cell interaction with its watery environment.

How do unsaturated fatty acids contribute to membrane fluidity, especially in cold environments?

The double bonds in unsaturated fatty acids create 'kinks' or bends in their tails, which prevent phospholipids from packing too tightly, thus maintaining fluidity at lower temperatures.

What role does cholesterol play in maintaining membrane fluidity in animal cells?

Cholesterol acts as a buffer, preventing lower temperatures from inhibiting fluidity and preventing increased temperatures from increasing fluidity too much, thus extending the functional temperature range.

What is passive transport?

A naturally occurring phenomenon where substances move from an area of higher concentration to an area of lower concentration without the cell exerting any energy.

What is a concentration gradient?

A physical space in which there is a single substance concentration range, from high concentration to low concentration.

Which types of molecules can easily pass through the plasma membrane by simple diffusion?

Non-polar, lipid-soluble materials with a low molecular weight, such as fat-soluble vitamins, hormones, oxygen, and carbon dioxide.

How does facilitated transport differ from simple diffusion?

In facilitated transport, materials diffuse across the plasma membrane with the help of membrane proteins (channels or carriers), but still down their concentration gradient, without expending cellular energy.

What are aquaporins?

Channel proteins that allow water to pass through the membrane at a very high rate.

How do carrier proteins facilitate transport across the membrane?

A carrier protein binds to a specific substance, changes its own shape, and moves the bound molecule from one side of the membrane to the other along its concentration gradient.

What is osmosis?

The movement of water through a semipermeable membrane according to the water's concentration gradient, from an area of higher water concentration (lower solute) to an area of lower water concentration (higher solute).

Define tonicity.

Tonicity describes how an extracellular solution can change a cell's volume by affecting osmosis, often correlating with the solution's osmolarity.

What happens to an animal cell in a hypotonic solution?

Water enters the cell because the extracellular fluid has lower osmolarity than the cell's cytoplasm, causing the cell to swell and potentially lyse (burst).

What happens to a plant cell in a hypertonic solution?

Water leaves the cell, causing the cell membrane to detach from the cell wall and the cytoplasm to constrict, a process called plasmolysis, which results in wilting.

Why is active transport necessary for a cell?

It allows the cell to move substances against their concentration or electrochemical gradient, requiring cellular energy (usually ATP), to maintain necessary internal concentrations.

What is an electrochemical gradient?

The combined effect of a concentration gradient (difference in substance concentration) and an electrical gradient (difference in charge) across the plasma membrane, which affects an ion's movement.

Describe the main function of the sodium-potassium pump (Na+-K+ ATPase).

It maintains the electrochemical gradient in animal cells by moving three sodium ions out of the cell for every two potassium ions moved into the cell, using ATP and contributing to the cell's negative interior charge.

What is the difference between primary active transport and secondary active transport?

Primary active transport directly uses ATP to move ions and create an electrochemical gradient, while secondary active transport uses the energy from an already established electrochemical gradient (created by primary active transport) to move other substances.

What are uniporters, symporters, and antiporters?

These are types of carrier proteins: a uniporter carries one specific ion/molecule, a symporter carries two different ions/molecules in the same direction, and an antiporter carries two different ions/molecules in different directions.

What is endocytosis?

A type of active transport that moves particles, such as large molecules, parts of cells, or even whole cells, into a cell by engulfing them with its plasma membrane to form an intracellular vesicle.

What is phagocytosis?

A variation of endocytosis, meaning 'cell eating,' where a cell takes in large particles, such as other cells or relatively large particles, by surrounding and engulfing them.

What is pinocytosis?

A variation of endocytosis, meaning 'cell drinking,' where the cell takes in small volumes of extracellular fluid and dissolved molecules, forming much smaller vesicles than in phagocytosis.

In receptor-mediated endocytosis, what ensures the uptake of specific substances?

It employs receptor proteins in the plasma membrane that have a specific binding affinity for certain target substances, often forming clathrin-coated pits for uptake.

What is exocytosis?

The process of moving material from inside the cell to the extracellular fluid by enveloping the substance in a membrane-bound vesicle that fuses with the plasma membrane and releases its contents to the exterior.