Cell Biology: Membrane Transport and Structure

Cell Membrane Structure & Function

• Phospholipids are the most abundant lipids in the plasma membrane
• Phospholipids are amphipathic, meaning they have both hydrophobic and hydrophilic regions.
• The fluid mosaic model describes the plasma membrane as a fluid structure with various proteins embedded in it.

Fluidity of Membranes

• Phospholipids can move within the bilayer.
• Most lipids and some proteins drift laterally or rotate.
• Flip-flop (transverse movement) across the membrane is rare.

Temperature and Membrane Fluidity

• As temperatures increase, 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 fatty acid chains result in less surface area for van der Waals or hydrophobic interactions between neighboring phospholipids.
• Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids due to kinks introduced by the unsaturated carbons. The kinks prevent tight packing of the phospholipids.

Cholesterol in Animal Cell Membranes

• Cholesterol affects membrane fluidity at different temperatures, acting as a fluidity buffer.
• 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 a collage of different proteins embedded in the fluid matrix of 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, and transmembrane proteins span the membrane.
• Hydrophobic regions of integral proteins consist of stretches of nonpolar amino acids, often coiled into alpha helices.
• Membrane proteins have six major functions: transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment to the cytoskeleton and extracellular matrix.

Role of Membrane Carbohydrates in Cell-Cell Recognition

• Cells recognize each other by binding to surface molecules, often carbohydrates on the plasma membrane.
• Membrane carbohydrates may be covalently bonded to lipids (glycolipids) or more commonly to proteins (glycoproteins).
• Carbohydrate chains vary among species, individuals, and even cell types.

Membrane Structure and Selective Permeability

• Cells exchange materials with their surroundings through the plasma membrane, which is selectively permeable.
• Hydrophobic molecules can dissolve in the lipid bilayer and pass rapidly through the membrane.
• Water molecules can also pass easily.
• Polar molecules and charged substances (ions) do not cross the membrane easily.

Transport Proteins

• Transport proteins allow hydrophilic substances to pass across the membrane.
• Channel proteins have a hydrophilic channel that certain molecules or ions can use as a tunnel.
• Aquaporins are channel proteins that facilitate the passage of water.
• Carrier proteins bind to molecules and change shape to shuttle them across the membrane.
• Transport proteins are specific for the substance they move.

Passive Transport

• Diffusion is the tendency of molecules to spread out evenly into the available space.
• Diffusion of a population of molecules may exhibit a net movement in one direction.
• Substances diffuse down their concentration gradients (from high to low concentration).
• Diffusion across a biological membrane is passive transport as it doesn't require energy.
• Dynamic equilibrium happens when an equal number of molecules cross the membrane in each direction.

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.
• Osmosis is specifically the movement of water, not other molecules.

Tonicity and Water Balance in Cells Without Cell Walls

• Tonicity describes the ability of a solution to cause a cell to gain or lose water.
• Isotonic: solute concentration is the same inside and outside the cell, no net water movement.
• Hypertonic: solute concentration is greater outside the cell, water moves out, cell loses water.
• Hypotonic: solute concentration is less outside the cell, water moves in, cell gains water (and can potentially lyse).

Water Balance in Cells With Cell Walls

• Cell walls help maintain water balance.
• In hypotonic solutions, plant cells swell until the wall opposes uptake, becoming turgid (firm).
• In isotonic solutions, there's no net water movement, and cells become flaccid (limp).
• In hypertonic solutions, plant cells lose water, and the membrane pulls away from the wall, a usually lethal effect called plasmolysis.

Facilitated Diffusion

• Transport proteins speed passive movement of molecules across the membrane.
• Channel proteins provide corridors for specific molecules or ions.
• Examples include aquaporins (for water) and ion channels (that open or close in response to a stimulus.)
• Carrier proteins change shape to shuttle solutes across the membrane.

Active Transport

• Active transport moves substances against their concentration gradients, requiring energy (usually ATP).
• The sodium-potassium pump is an example of active transport.

Cotransport

• Cotransport is the simultaneous movement of two distinct molecules across a biological membrane by a single protein.
• Symport moves the molecules in the same direction.
• Antiport moves the molecules in opposite directions.

Bulk Transport

• Exocytosis occurs when transport vesicles migrate to the membrane, fuse with it, and release their contents.
• Endocytosis is when the cell takes in macromolecules by forming vesicles from the plasma membrane.
• Endocytosis involves phagocytosis ("cellular eating").
• Endocytosis includes pinocytosis ("cellular drinking").
• Receptor-mediated endocytosis is also a form of endocytosis.

This quiz focuses on cell membrane transport proteins and the various solutions that affect cell water movement. Test your knowledge on concepts such as osmosis, passive transport, and phospholipid functions in the plasma membrane. Perfect for students studying cell biology topics.