Membrane Structure & Function

These flashcards cover key vocabulary related to the structure and function of cell membranes, including components, transport mechanisms, and solutions.

Fluid Mosaic Model

A model that describes the structure of cell membranes as a mosaic of various components, including phospholipids, cholesterol, and proteins.

Glycoprotein

A protein with a carbohydrate attached, important for cell recognition and signaling.

Glycolipid

A lipid with a carbohydrate attached, involved in cell recognition and membrane stability.

Phospholipid bilayer

A double layer of phospholipids that makes up the cell membrane, with hydrophilic heads facing outward and hydrophobic tails facing inward.

Integral membrane protein

Proteins that are embedded within the phospholipid bilayer and can span the membrane.

Peripheral membrane protein

Proteins that are attached to the exterior or interior surfaces of the membrane but do not penetrate the bilayer.

Cholesterol

A lipid that helps to stabilize the fluidity of the cell membrane by preventing packing of phospholipids.

Transport proteins

Proteins that assist in the movement of molecules across the cell membrane.

Passive transport

The movement of molecules across the cell membrane without the use of energy, moving from high to low concentration.

Active transport

The movement of molecules across the cell membrane using energy, moving from low to high concentration.

Osmosis

The diffusion of water across a selectively permeable membrane.

Hypertonic solution

A solution that has a higher concentration of solutes compared to another solution, causing cells to lose water.

Hypotonic solution

A solution that has a lower concentration of solutes compared to another solution, causing cells to gain water.

Isotonic solution

A solution that has the same concentration of solutes as another solution, resulting in no net water movement across a membrane.

Cotransport

A type of active transport that involves the coupling of the transport of one molecule with another.

Endocytosis

The process by which cells internalize substances from the external environment by engulfing them in a membrane.

Exocytosis

The process by which cells expel substances to the external environment by vesicles fusing with the plasma membrane.

Fluid Mosaic Model

A biological model describing the cell membrane as a dynamic, fluid structure where lipids and proteins can move laterally. The 'mosaic' refers to the diverse arrangement of:

• Proteins (integral and peripheral)
• Lipids (phospholipids and cholesterol)
• Carbohydrates (glycoproteins and glycolipids)

Glycoprotein

A protein covalently bonded to one or more carbohydrates (oligosaccharides). They play critical roles in:

• Cell-cell recognition: Acting as molecular 'ID tags'.
• Signal transduction: Serving as receptors for chemical messengers.
• Immune response: Sorting of cells into tissues and organs.

Glycolipid

A lipid molecule with a carbohydrate attached, found on the outer surface of the plasma membrane. Their primary functions include:

• Membrane stability: Maintaining the structural integrity of the bilayer.
• Cellular markers: Assisting in tissue formation by facilitating cell-to-cell interactions.

Phospholipid bilayer

The fundamental structure of the cell membrane, consisting of two layers of phospholipids. Features include:

• Amphipathic nature: Possessing both a hydrophilic (polar) phosphate head and two hydrophobic (nonpolar) fatty acid tails.
• Permeability: Creates a semi-permeable barrier that regulates the passage of ions and molecules.

Integral membrane protein

Proteins that are permanently attached to the biological membrane. Many are transmembrane proteins, which:

• Span the entire bilayer.
• Contain hydrophobic regions consisting of nonpolar amino acids (often coiled into \alpha-helices) that interact with the fatty acid tails.

Peripheral membrane protein

Proteins that are not embedded in the lipid bilayer but are loosely bound to the surface of the membrane. They often:

• Associate with integral proteins or the polar heads of phospholipids.
• Function as enzymes, anchors for the cytoskeleton, or components of cell signaling pathways.

Cholesterol

A steroid lipid wedged between phospholipid molecules in animal cell membranes. It acts as a fluidity buffer:

• At high temperatures (37^{\circ}C), it restrains phospholipid movement, reducing fluidity.
• At low temperatures, it prevents tight packing of phospholipids, hindering solidification.

Transport proteins

Specific proteins that facilitate the movement of hydrophilic substances across the hydrophobic core of the membrane. Major types include:

1. Channel proteins: Provide a hydrophilic tunnel (e.g., aquaporins for water).
2. Carrier proteins: Undergo a shape change to translocation-bound molecules across the membrane.

Passive transport

The diffusion of a substance across a biological membrane without the expenditure of metabolic energy (ATP). Movement occurs:

• Along the concentration gradient (from high concentration to low concentration).
• Includes simple diffusion, osmosis, and facilitated diffusion.

Active transport

The movement of a substance across a membrane against its concentration gradient, requiring the expenditure of energy (ATP). This process:

• Is mediated by specific carrier proteins (pumps).
• Allows cells to maintain internal concentrations of small molecules that differ from concentrations in the environment.

Osmosis

The specific diffusion of free water (H_{2}O) across a selectively permeable membrane. Water moves from a region of:

• Lower solute concentration (higher free water) to a region of higher solute concentration (lower free water).
• This continues until the solute concentrations on both sides are nearly equal.

Hypertonic solution

A solution with a higher concentration of non-penetrating solutes compared to the inside of the cell (C{out} > C{in}). Effects include:

• Animal cells: Lose water and shrivel (crenation).
• Plant cells: Undergo plasmolysis, where the plasma membrane pulls away from the cell wall.

Hypotonic solution

A solution with a lower concentration of non-penetrating solutes compared to the cell interior (C{out} < C{in}). Effects include:

• Animal cells: Gain water and may burst (lyse).
• Plant cells: Become turgid (firm), which is the healthy state for most plants due to the mechanical support of the cell wall.

Isotonic solution

A solution where the solute concentration is equal to that inside the cell (C{out} = C{in}). Highlights:

• There is no net movement of water across the plasma membrane.
• Animal cells are most stable in this environment, while plant cells become flaccid (limp).

Cotransport

A mechanism where a single -ATP powered pump that transports a specific solute can indirectly drive the active transport of several other solutes. It involves:

• Symport: Two substances moving in the same direction.
• Antiport: Two substances moving in opposite directions.

Endocytosis

A process for the bulk transport of matter into the cell via the formation of new vesicles from the plasma membrane. Three types include:

1. Phagocytosis: 'Cell eating' (large particles).
2. Pinocytosis: 'Cell drinking' (extracellular fluid).
3. Receptor-mediated endocytosis: Specific uptake of ligands.

Exocytosis

The cellular process of secreting biological molecules by the fusion of vesicles with the plasma membrane. Examples include:

• Release of neurotransmitters from neurons.
• Secretion of insulin from pancreatic cells.
• Export of cell wall materials in plant cells.