Cell Biology: Membranes, Transport, Organelles, and Stem Cells

Comprehensive vocabulary flashcards detailing cell membrane composition, transport mechanisms, organelle compartmentalization, and stem cell differentiation.

Amphipathic

A property describing a molecule that contains both hydrophilic (water-loving, polar) and hydrophobic (water-fearing, non-polar) regions, such as a phospholipid.

Phospholipid

A membrane molecule consisting of a hydrophilic polar head (composed of glycerol and a phosphate group) and two hydrophobic non-polar tails (composed of fatty acid hydrocarbon chains).

Phospholipid Bilayer

A two-layered sheet of phospholipids held together by weak hydrophobic interactions, where hydrophobic tails orient inward away from fluids and hydrophilic heads face outward to associate with aqueous cytosolic and extracellular fluids.

Semi-permeability

A membrane characteristic whereby only certain materials are capable of freely crossing the phospholipid bilayer.

Selectivity

The capacity of a cell membrane to regulate and coordinate the transport of materials that cannot freely cross the bilayer, mediated by embedded membrane proteins.

Integral Proteins

Transmembrane proteins that penetrate and remain permanently embedded in one or both lipid layers of the bilayer, which cannot be isolated without disrupting the membrane using detergents.

Peripheral Proteins

Proteins temporarily associated with one surface of the lipid bilayer by attaching to integral proteins, polar lipid heads, or the cytoskeleton/extracellular matrix.

Simple Diffusion

The passive, non-selective movement of small or non-polar molecules (such as $O_2$ and $CO_2$) directly between membrane phospholipids down a concentration gradient without energy or protein assistance.

Osmosis

The net passive movement of water molecules from a region of low solute concentration to a region of high solute concentration across a semi-permeable membrane.

Aquaporins

Integral channel proteins embedded in cell membranes that selectively permit the rapid passage of water molecules.

Facilitated Diffusion

The passive movement of large, polar molecules or ions across a membrane down their concentration gradient mediated by transmembrane proteins (channels or carriers) without energy expenditure.

Channel Proteins

Ion-selective, pore-forming transport proteins that may be gated to regulate the passive movement of specific ions down a concentration gradient when open.

Carrier Proteins

Integral glycoproteins that bind a specific solute and undergo a conformational change to translocate the solute across the membrane, operating with a slower transport rate than channel proteins.

Active Transport

The directional movement of materials across a biological membrane against a concentration gradient, requiring cellular energy expenditure via direct ATP hydrolysis or indirect coupling.

Protein Pumps

Membrane transport proteins that utilize metabolic energy from adenosine triphosphate ($ATP$) hydrolysis to move specific particles across membranes against a concentration gradient.

Glycosylation

The biochemical attachment of carbohydrate chains to membrane lipids or proteins, producing glycolipids or glycoproteins.

Glycolipid

A membrane lipid with a carbohydrate chain attached on the extracellular surface, functioning in cellular recognition (such as ABO blood group antigens) and tissue cohesion.

Glycoprotein

A membrane protein covalently linked to an extracellular carbohydrate chain, functioning in cell adhesion (e.g., sperm-egg binding) and intercellular recognition.

Ovum Glycocalyx

A prominent extracellular carbohydrate coat surrounding an unfertilized egg cell that mediates protection and sperm binding during fertilization.

Extracellular Matrix

A complex network of external molecules providing structural and biochemical support to surrounding cells, anchored by cell surface carbohydrates.

Fluid-Mosaic Model

A conceptual model describing the plasma membrane as a two-dimensional viscous fluid of phospholipids embedded with a mosaic of proteins, cholesterol, and carbohydrates.

Organelles

Discrete cellular subunits or structures that are adapted to perform specialized functions, including nuclei, vesicles, ribosomes, and the plasma membrane.

Non-organelle Structures

Cellular structures that are not classified as organelles, specifically including the cell wall, cytoskeleton, and cytoplasm.

Compartmentalization

The structural separation of the eukaryotic interior into distinct membrane-bound spaces, facilitating enzyme/metabolite concentration, isolation of hazardous reactions, and optimization of biochemical processes.

Post-transcriptional Modification

The processing and modification of pre-mRNA (e.g., 5' capping, poly-A tailing, intron removal) within the eukaryotic nucleus prior to interacting with ribosomes in the cytoplasm.

Stem Cells

Unspecialized cells that possess the capacity to divide endlessly (self-renewal) and differentiate into diverse specialized cell lineages.

Cell Differentiation

The developmental process by which unspecialized stem cells selectively regulate gene expression to form specialized cells with distinct structures and functions.

Morphogen

A diffusible signaling chemical distributed in a concentration gradient across an early embryo whose local concentration switches specific genes on or off, directing cellular differentiation.

Retinoic Acid

An embryonic morphogen that forms signaling gradients to guide cell differentiation during the development of organs including forelimbs, pancreas, lungs, and kidneys.

Stem Cell Niche

The precise anatomical microenvironment within adult tissues (such as bone marrow or hair follicles) that maintains stem cells in an undifferentiated state or stimulates their proliferation and differentiation.

Hematopoietic Stem Cells

Multipotent stem cells located in red bone marrow niches that proliferate and differentiate to produce all types of blood cells.

Epidermal Stem Cells

Adult stem cells residing in the hair follicle bulge that participate in hair growth, skin innervation, vascularization, and cutaneous wound repair.

Totipotent Stem Cells

Stem cells that can form any cell type, including embryonic and placental tissues, and can develop into an entire organism (e.g., zygote, earliest embryo cells).

Pluripotent Stem Cells

Embryonic stem cells, such as those of the blastocyst inner cell mass, that can differentiate into any body cell type from all three germ layers, but cannot form placenta or a whole organism.

Multipotent Stem Cells

Adult stem cells that can only differentiate into a limited number of closely related cell types (e.g., hematopoietic stem cells in bone marrow).

Induced Pluripotent Stem Cells ($iPSCs$)

Adult somatic cells reprogrammed back into an embryonic-like pluripotent state by artificially activating specific regulator genes.

Surface Area-to-Volume Ratio ($SA:Vol$)

The mathematical relationship between the outer membrane area of a cell and its internal volume; as cell size increases, volume expands faster than surface area ($volume \propto r^3$ vs. $area \propto r^2$), lowering the ratio and constraining metabolic exchange.