Biology Quiz: Cell Transport Mechanisms

Cell Membrane Structure & Function

• Phospholipids are the most abundant lipids in the plasma membrane.
• Phospholipids are amphipathic, meaning they have both hydrophobic (water-fearing) and hydrophilic (water-loving) regions.
• The fluid mosaic model describes membranes as a fluid structure with various proteins embedded in the lipid bilayer.

Fluidity of Membranes

• Phospholipids can move within the bilayer.
• Most lipids and some proteins drift laterally or rotate within the membrane.
• Flip-flop (transverse movement) across the membrane is rare.
• Lateral movement occurs at a rate of ~10⁷ times per second.
• Flip-flop occurs approximately once per month.

Temperature and Membrane Fluidity

• As temperature increases, membranes transition from a solid (gel) state to a more fluid state.
• Membranes with shorter fatty acid chains are more fluid than those with longer chains.
• Shorter chains have less surface area, reducing hydrophobic/van der Waals interactions between neighboring phospholipid molecules.

Saturated vs. Unsaturated Fatty Acids and Membrane Fluidity

• Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids.
• Kinks (or bends) in unsaturated fatty acid tails prevent tight packing of phospholipids.

Cholesterol and Membrane Fluidity

• In animal cells, cholesterol acts as a fluidity buffer against temperature extremes.
• At warm temperatures, cholesterol restrains phospholipid movement, decreasing membrane fluidity.
• At cool temperatures, cholesterol prevents tight packing, increasing membrane fluidity.

Membrane Proteins and Their Functions

• Membranes are composed of various proteins embedded in the lipid bilayer.
• Proteins determine most of the membrane's specific functions.
• Peripheral proteins are bound to the membrane surface.
• Integral proteins penetrate the hydrophobic core.
• Transmembrane proteins span the entire membrane.
• Hydrophobic regions of integral proteins consist of one or more stretches of non-polar amino acids, often coiled into alpha helices.

Six major functions of membrane proteins

• Transport
• Enzymatic activity
• Signal transduction
• Cell-cell recognition
• Intercellular joining
• Attachment to the cytoskeleton and extracellular matrix

Role of Membrane Carbohydrates

• Cells recognize each other by binding to surface molecules, often carbohydrates, on the plasma membrane.
• Membrane carbohydrates are covalently bonded to lipids (forming glycolipids) or to proteins (forming glycoproteins).
• Carbohydrate chains on the external side of the plasma membrane vary among species, individuals, and even cell types within an individual.

Selective Permeability

• Cells exchange materials with their surroundings via the plasma membrane.
• Plasma membranes are selectively permeable, regulating the cell's molecular traffic.
• Hydrophobic (non-polar) molecules readily dissolve in the lipid bilayer and pass through the membrane quickly. Water, a polar molecule, also crosses easily.
• Polar molecules (e.g., sugars) do not cross the membrane easily.
• Charged substances (ions) are virtually unable to pass through the lipid bilayer.

Transport Proteins

• Transport proteins allow passage of hydrophilic substances across the membrane.
• Channel proteins form hydrophilic channels that certain molecules or ions can use as a tunnel, spanning the entire membrane.
• Aquaporins facilitate the majority of water passage.
• Carrier proteins bind to molecules and change shape to shuttle them across the membrane.
• Transport proteins are specific to the substance they transport.

Passive Transport

• Diffusion is the tendency for molecules to spread out evenly into the available space.
• Molecules diffuse down their concentration gradient (from high concentration to low).
• Diffusion across a biological membrane is passive transport, requiring no energy investment from the cell.

Osmosis

• Osmosis is the diffusion of water across a selectively permeable membrane.
• Water diffuses from a region of lower solute concentration to a region of higher solute concentration.

Water balance in Cells Without Cell Walls

• Tonicity describes a solution's ability to cause a cell to gain or lose water.
• Isotonic solution: solute concentration is the same as inside the cell; no net water movement.
• Hypertonic solution: solute concentration is greater than inside the cell; the cell loses water.
• Hypotonic solution: solute concentration is less than inside the cell; the cell gains water.

Water balance in Cells with Cell Walls

• Cell walls help maintain water balance. A plant cell in a hypotonic solution swells until the wall opposes uptake; the cell is now turgid (firm)
• If a plant cell and its surroundings are isotonic, there is no net water movement, and the cell is flaccid (limp).
• In a hypertonic environment, plant cells lose water; the membrane pulls away from the wall, a usually lethal effect called plasmolysis.

Facilitated Diffusion

• Transport proteins speed the passive movement of molecules across the plasma membrane.
• Channel proteins provide facilitated diffusion corridors.
• Aquaporins facilitate water diffusion through ion channels.
• Carrier proteins change shape, translocating the solute-binding site across the membrane

Active Transport

• Active transport moves substances against their concentration gradient (from low to high).
• Active transport requires energy, usually in the form of ATP. The sodium-potassium pump is an example.

Co-transport

• Co-transport is the simultaneous Movement of two distinct molecules across a biological membrane by one membrane transport protein.
• Symport: two molecules transported in the same direction.
• Antiport: two molecules transported in opposite directions.

Bulk Transport

• Exocytosis: transport vesicles fuse with the membrane and release their contents.
• Endocytosis: the cell takes in macromolecules by forming vesicles from the plasma membrane.
• Types of endocytosis: Phagocytosis, Pinocytosis, Receptor-mediated endocytosis.