Biological Membranes Quiz

Biological Membranes

• Membranes regulate the passage of molecules in and out of cell organelles.
• They divide cells into compartments with specialized metabolism.
• They act as surfaces holding enzymes.

Composition of Membranes

• Membranes are composed of proteins (60%), phospholipid molecules (40%), and sugars (8%).
• Intrinsic proteins (integral proteins) are partially immersed in the lipid bilayer.
• Extrinsic proteins (peripheral proteins) are located outside the membrane and have enzymatic functions.
• Glycoproteins are oligosaccharides/sugars attached to intrinsic proteins.
• Glycolipids are attached to membrane lipids.

Phospholipid

• A phospholipid molecule consists of a glycerol molecule, two fatty acids, and a phosphate group.
• The phosphate end is polar (hydrophilic).
• The polar end is attracted to water.
• Fatty acid chains are nonpolar (hydrophobic).
• The nonpolar end repels water.
• Hydrophilic heads face the watery surroundings inside and outside of the cell.
• Hydrophobic tails form the inside of the double layer.

Fluid Mosaic Model

• The current model for the structure of membranes.
• Protein molecules float in a fluid phospholipid bilayer.
• Characterizes plasma membranes and other cell membranes as consisting of a double layer (bilayer) of lipid molecules.
• Proteins resemble a mosaic pattern in the lipid bilayer.
• Membrane structure is fluid rather than static.
• Lipids and proteins move sideways within the membrane.

Properties of Membranes

• Membranes can grow molecule by molecule in certain regions of the cell.
• Whole pieces of membrane can move as vesicles or small bubbles to different sites in the cell.
• Vesicles fuse with the growing membrane allowing the transport of materials.

Permeability

• Impermeable membranes let nothing pass through.
• Freely permeable membranes let virtually anything pass through.
• Selectively permeable membranes allow some substances to pass rapidly, but others only slowly.

Properties of Membranes (Homeostasis)

• Membranes regulate the passage of materials to maintain homeostasis.
• Homeostasis is a relatively constant set of internal conditions in an organism.
• Membranes receive information from their surroundings using cell signaling, hormones, and chemicals.
• Membranes help cells respond to their environments.

Passage of Materials Across Membranes

• Materials move passively or actively.
• Passive transport: Diffusion is the movement of particles (atoms, molecules, or ions) from a region of higher concentration to a region of lower concentration.
• Examples include sugar in a beaker of water.

Osmosis

• Osmosis is a special type of diffusion, specifically the diffusion of water across a selectively permeable membrane.
• Water flows spontaneously from a region of lower solute concentration (more water) to a region of higher solute concentration (less water).

Plasmolysis (in Plant Cells)

• In a hypotonic solution, water diffuses into a plant cell until the cell wall prevents a further osmotic increase.
• Turgid is the maximum water quantity a plant cell can hold.
• Plasmolysis occurs when the cell membrane shrinks and detaches from the cell wall.
• Plasmolyzed cells lose turgidity and are unable to support the plant.
• Flaccid means wilting.

Facilitated Diffusion

• Materials diffuse from a higher concentration area to a lower concentration area through specialized passages.
• Membrane proteins allow hydrophilic and charged molecules to diffuse across.
• Passageways are called carrier proteins.
• They act like conveyor belts following the concentration gradient.

Active Transport

• Large intrinsic membrane proteins bind a molecule and force it through the membrane.
• This process requires energy (ATP).
• It moves substances against the concentration gradient.

Endocytosis and Exocytosis

• Exocytosis is the fusion of a vesicle with the cell membrane, releasing contents outside the cell.
• Endocytosis is the invagination of the membrane, forming a vesicle that pinches off to carry external material inside the cell.

Imbibition

• Imbibition is a specialized type of diffusion.
• Water is attracted to cellulose microfibrils causing them move apart, causing the cell wall to swell.
• Cellulose imbibes water, allowing the relatively free passage of water.

Water Potential in Plant Cells

• Water potential measures the tendency of water to flow.
• It is the combination of osmotic potential and pressure potential.
• Osmotic potential is the measurement of water tendency to cross a membrane due to solute concentration.
• Pressure potential is the pressure a cell wall exerts on the protoplast contents.

Water Potential in Plant Cells (continued)

• Tissues with lower water potential than pure water absorb water, increasing weight and volume.
• Osmotic potential is zero or negative; pressure potential is positive.
• Water potential can be +, -, or 0 (expanding, shrinking, or equilibrium)

Cell Wall Elasticity

• Cell wall elasticity is the ability of a cell wall to change shape in minimal volume.
• Young, actively growing cells have more elastic cell walls.
• Mature cells have thicker secondary thickenings making them tough and rigid.

Test your knowledge on biological membranes, their composition, and functions. This quiz covers the key concepts of membrane structure, including proteins, phospholipids, and their roles within cells. Dive into the intricacies of cell compartmentalization and enzymatic functions!