Plasma Membrane and Transport Practice Questions

Correct answer: Fluid mosaic model

The fluid mosaic model describes the plasma membrane as a fluid lipid bilayer with proteins embedded within it. This model explains membrane flexibility and lateral movement of components.

Correct answer: Lipids and proteins

The plasma membrane mainly consists of a phospholipid bilayer with proteins embedded in it. Carbohydrates are present but in smaller amounts, usually attached to proteins or lipids.

Correct answer: Phospholipid bilayer

The phospholipid bilayer forms the core structure of the plasma membrane and restricts the free passage of polar and charged molecules. This property makes the membrane selectively permeable.

Correct answer: Carrier protein

Carrier proteins bind specific solutes and help transport them across the membrane down their concentration gradient. This process does not require energy.

Correct answer: Passive transport

Passive transport allows molecules to move across the membrane along their concentration gradient without energy expenditure. Simple diffusion and facilitated diffusion are both examples of passive transport.

Correct answer: Small oxygen molecules

Small non-polar molecules like oxygen can diffuse freely through the lipid bilayer. Charged ions and large polar molecules such as glucose require transport proteins to cross the membrane.

Correct answer: Osmosis of water

Osmosis refers specifically to the movement of water molecules across a semipermeable membrane from a region of higher water potential to lower water potential.

Correct answer: Aquaporin channel

Aquaporins are specialized channel proteins that increase membrane permeability to water. They are essential for maintaining water balance in cells.

Correct answer: Requirement of metabolic energy

Active transport moves substances against their concentration gradient and requires energy, usually in the form of ATP. It is mediated by specific carrier proteins.

Correct answer: Sodium-potassium pump

The sodium-potassium pump uses ATP directly to transport sodium and potassium ions against their concentration gradients, making it a clear example of primary active transport.

Correct answer: Embedded within the lipid bilayer

According to the fluid mosaic model, proteins float within or on the lipid bilayer. This arrangement allows lateral movement and functional diversity of membrane proteins.

Correct answer: Membrane cholesterol

Cholesterol modulates membrane fluidity by preventing phospholipids from packing too tightly or becoming too fluid. It is a key structural component of animal cell membranes.

Correct answer: Uptake of material via membrane invagination

Endocytosis involves the inward folding of the plasma membrane to form vesicles that bring materials into the cell. It is an energy-dependent process used for nutrient uptake and receptor internalization.

Correct answer: Phagocytosis

Phagocytosis is the process by which cells engulf large solid particles such as bacteria. It is commonly performed by specialized immune cells like macrophages and neutrophils.

Correct answer: Carrier-mediated diffusion

Carrier-mediated (facilitated) diffusion uses specific carrier or channel proteins that bind particular molecules. Because of limited protein availability, it shows both specificity and saturation kinetics.

Correct answer: Functional specialization

Asymmetry in membrane components allows different surfaces of the membrane to perform distinct functions. This is essential for processes like cell recognition, signaling, and directed vesicle trafficking.

Correct answer: Coupled transport using ion gradients

Secondary active transport uses the energy stored in ion gradients rather than ATP directly. The movement of one ion (typically Na+) down its gradient drives another substance against its own gradient.

Correct answer: Selective permeability

Selective permeability allows the plasma membrane to regulate the entry and exit of substances. This control is essential for maintaining a stable internal environment and cellular homeostasis.

Correct answer: Cell recognition and adhesion

Glycoproteins and glycolipids form the glycocalyx on the cell surface. They play a crucial role in cell recognition, immune response, cell signaling, and adhesion.

Correct answer: Phospholipids

The Fluid Mosaic Model, proposed by Singer and Nicolson, describes the lipid bilayer as a viscous fluid where individual lipid molecules can move rapidly in the plane of the membrane.

Correct answer: Cholesterol

Cholesterol prevents the membrane from becoming too fluid at high temperatures by restraining lipid movement and prevents it from freezing at low temperatures by disrupting the tight packing of phospholipids.

Correct answer: 3 Na+ out and 2 K+ in

The pump uses ATP to transport three sodium ions out of the cell against their gradient while bringing two potassium ions into the cell, maintaining the electrochemical gradient.

Correct answer: Secondary active transport

Secondary active transport (cotransport) uses the energy stored in an electrochemical gradient, created by primary active transport, to move a different substance against its own gradient.

Correct answer: Transmembrane protein

Integral proteins are embedded within the hydrophobic core of the lipid bilayer. Transmembrane proteins are a subset that pass all the way through from one side to the other.

Correct answer: Plasmolysis

In a hypertonic solution, the solute concentration is higher outside the cell than inside, so water leaves the cell by osmosis. In a plant cell, this causes the protoplast to shrink away from the rigid cell wall, a process called plasmolysis. Osmotic lysis is the opposite outcome, occurring only when a cell in a hypotonic solution absorbs too much water and bursts.

Correct answer: Two substances moving in opposite directions

Antiport is a type of coupled transport where two different molecules or ions are pumped in opposite directions across a membrane.

Correct answer: Receptor-mediated endocytosis

Receptor-mediated endocytosis allows cells to concentrate and take in bulk quantities of specific substances that bind to specialized receptors on the cell surface.

Correct answer: Glycolipids

Glycolipids are found exclusively on the outer face of the plasma membrane, where they contribute to the glycocalyx and play roles in cell-cell recognition.

Correct answer: It requires a specific carrier or channel protein

Facilitated diffusion is a passive process that uses transport proteins to move polar or charged substances across the membrane down their concentration gradient.

Correct answer: Lipid bilayer

Gorter and Grendel extracted lipids from red blood cells and found the surface area of the lipids was exactly double the surface area of the cells, suggesting a bilayer structure.

Correct answer: Oxygen gas

Small, non-polar, uncharged molecules like O2, CO2, and N2 can dissolve in the lipid bilayer and pass through the membrane without the help of proteins.

Correct answer: Cell recognition and protection

The glycocalyx is a carbohydrate-rich coating formed by the sugar chains of glycoproteins and glycolipids. It is essential for immune response and cell adhesion.

Correct answer: All transport proteins are occupied

Because carrier proteins have a limited number of binding sites, the rate of transport reaches a maximum (Vmax) when all carriers are busy. This is called saturation.

Correct answer: Davson-Danielli Model

The Davson-Danielli model proposed a lipid bilayer coated on both sides with globular proteins, resembling a sandwich. It was later replaced by the Fluid Mosaic Model.

Correct answer: Symport

The sodium-glucose linked transporter (SGLT) is a symporter because both sodium and glucose move in the same direction into the cell.

Correct answer: Potassium (K+)

The Na+/K+ pump maintains high internal K+ and high external Na+, which is vital for electrical signaling and osmotic balance.

Correct answer: It increases

Higher temperatures increase the kinetic energy of molecules, leading to faster movement and an increased rate of diffusion across the membrane.

Correct answer: Synaptic exocytosis

Exocytosis is the process by which vesicles fuse with the plasma membrane to release their contents outside the cell. At the synapse, an influx of calcium ions triggers neurotransmitter-filled vesicles to undergo this synaptic exocytosis, releasing their contents into the cleft; endocytosis moves material into the cell and would not release neurotransmitters outward.

Correct answer: Symmetry of the two leaflets

The plasma membrane is highly asymmetrical; the lipid composition and the types of proteins/carbohydrates attached differ significantly between the inner and outer leaflets.