Cellular Level of Organization: General Structure and Function of Cellular Components

Comprehensive vocabulary flashcards detailing the cellular level of organization, including cell theory, plasma membrane dynamics, organelle structures and functions, and nuclear organization.

Cell

The smallest living unit in the body, averaging approximately $0.1\,\text{mm}$ in diameter, which could not be examined until the invention of the microscope in the 17th century.

Cell Theory

The biological concept stating that cells are the building blocks of all plants and animals, all new cells arise from the division of pre-existing cells, and cells are the smallest units that perform all vital physiological functions.

Differentiation

The developmental process whereby daughter cells produced by repeated divisions of a single fertilized ovum become specialized in structure and function by receiving different parcels of cytoplasm.

Four Primary Tissue Types

The fundamental tissues that differentiate from the fertilized ovum: epithelial tissue, connective tissue, muscle tissue, and neural tissue.

Major Structural Divisions of the Cell

The three primary parts that constitute a cell: the plasma membrane (cell membrane), cytoplasm (subdivided into cytosol and organelles), and nucleus.

Plasma Membrane

A selectively permeable boundary that isolates the cytoplasm, provides physical support and sensitivity to the environment, and controls the entry of ions and nutrients, the elimination of wastes, and the release of secretions.

Extracellular Fluid

The watery medium surrounding cells, commonly referred to as interstitial fluid in most body tissues.

Glycocalyx

A superficial carbohydrate coating on the cell surface composed of proteoglycans, glycoproteins, and glycolipids that functions in cell recognition, binding to extracellular structures, and lubrication.

Integral Proteins

Membrane proteins embedded within the lipid bilayer that cannot be removed without damaging the membrane; they often span the entire membrane as transmembrane proteins to transport water or solutes.

Peripheral Proteins

Membrane proteins attached to the inner or outer surface of the plasma membrane that are removable and fewer in number than integral proteins.

Phospholipid Bilayer

A double layer of phospholipid molecules featuring hydrophilic heads facing the fluid surfaces and hydrophobic tails directed toward the interior, isolating the cytoplasm from the extracellular fluid.

Cholesterol (Membrane)

A lipid constituent intercalated within the phospholipid bilayer that is essential for maintaining plasma membrane fluidity.

Anchoring Proteins

Membrane proteins that attach the plasma membrane to other extracellular structures and stabilize its position, often binding internally to the cytoskeleton.

Recognition Proteins

Cellular surface proteins detected and recognized by cells of the immune system.

Receptor Proteins

Membrane proteins that bind specific extracellular molecules termed ligands to initiate cellular responses.

Ligand

Any specific extracellular molecule that binds to a receptor protein, ranging from small ions such as calcium to large and complex hormones.

Carrier Proteins

Proteins in the plasma membrane that bind solutes and transport them across the membrane.

Channels

Integral proteins with a central pore forming a passageway completely across the plasma membrane, permitting the transit of water and small solutes that cannot cross lipid layers.

Cytoplasm

The material located between the cell membrane and the nuclear membrane, consisting of a protein-rich colloid divided into intracellular fluid (cytosol) and intracellular structures (organelles).

Nonmembranous Organelles

Organelles that are not enclosed by membranes and remain in direct contact with the cytosol, including the cytoskeleton, microvilli, centrioles, cilia, and ribosomes.

Membranous Organelles

Organelles enclosed by a phospholipid membrane that isolate their internal contents from the cytosol, including mitochondria, the nucleus, endoplasmic reticulum, Golgi apparatus, lysosomes, and peroxisomes.

Microfilaments

Cytoskeletal elements measuring $<6\,\text{nm}$ in diameter, typically composed of the protein actin, located commonly at the cell periphery where they form the terminal web.

Intermediate Filaments

Durable cytoskeletal filaments measuring $7\text{--}11\,\text{nm}$ in diameter that provide the cell with substantial mechanical strength.

Microtubules

The largest components of the cytoskeleton, measuring approximately $25\,\text{nm}$ in diameter, which extend outward from the centrosome near the nucleus.

Centrioles

Cylindrical organelles composed of nine groups of microtubule triplets (two per centrosome) that control DNA movement during cell division; they are absent in non-dividing red blood cells and skeletal muscle cells.

Cilia

Slender plasma membrane extensions composed of nine microtubule pairs surrounding a central pair anchored to a basal body, which beat rhythmically to propel fluids or secretions across cell surfaces in the respiratory and reproductive tracts.

Ribosomes

Nonmembranous organelles consisting of a large and small subunit made of proteins and ribosomal RNA (rRNA) that assemble together to conduct protein synthesis.

Free Ribosomes vs. Fixed Ribosomes

Free ribosomes are scattered throughout the cytoplasm and release manufactured proteins into the cytosol, whereas fixed ribosomes are attached to the endoplasmic reticulum and synthesize proteins for export or organelle use.

Endoplasmic Reticulum (ER)

A network of intracellular membranes attached to the nucleus that forms hollow tubes, flattened sheets, and fluid-filled chambers known as cisternae.

Smooth Endoplasmic Reticulum (SER)

A division of the ER lacking ribosomes that possesses tubular cisternae; responsible for synthesizing phospholipids, cholesterol, steroid hormones (androgens and estrogens), and synthesizing/storing glycerides and glycogen.

Rough Endoplasmic Reticulum (RER)

A division of the ER studded with fixed ribosomes where newly synthesized polypeptides enter cisternae, fold into secondary and tertiary structures, and become packaged into transport vesicles for shipment to the Golgi apparatus.

Golgi Apparatus

An organelle typically consisting of 5–6 flattened membranous discs (cisternae) located near the nucleus that renews the plasma membrane, modifies secretions for exocytosis, and packages digestive enzymes into lysosomes.

Forming Face and Maturing Face

The polarity of the Golgi apparatus where transport vesicles from the RER arrive at the cisterna of the forming face and modified products depart from the maturing face oriented toward the cell surface.

Membrane Renewal Vesicles

Golgi-derived vesicles that fuse with the plasma membrane to replenish surface area, alter membrane properties, and regulate cellular sensitivity.

Secretory Vesicles

Vesicles packaged by the Golgi apparatus containing secretions that fuse with the plasma membrane to discharge their contents into the extracellular environment.

Lysosomes

Membranous vesicles containing inactive digestive enzymes that provide an isolated environment for breaking down organic matter, destroying pathogens, and recycling nutrients.

Autolysis

The destructive process occurring following cell injury or death where lysosomal membranes rupture and active enzymes destroy the cytoplasm, leading to lysosomes being called 'suicide packets.'

Mitochondria

Double-membrane organelles that produce cellular energy (ATP) by breaking down carbohydrates (glucose); completely absent in red blood cells and occupying $30\%$ of cardiac muscle cell volume.

Cristae and Matrix

Structural components of the mitochondrion: cristae are internal folds of the inner membrane that increase reaction surface area, and the matrix is the fluid enclosed within.

Nucleus

The primary control center of cellular operations containing genetic instructions in nucleotide sequences to direct the synthesis of $>100{,}000$ different proteins.

Nuclear Envelope and Nuclear Pores

The double membrane enclosing the nucleus, separated by the perinuclear space and perforated with nuclear pores that permit the regulated transit of ions and small molecules.

Nucleoplasm

The fluid interior of the nucleus containing fine filaments, ions, enzymes, RNA, DNA, and nucleotides.

Nucleoli

Transient, clear nuclear organelles composed of RNA, enzymes, and histone proteins that assemble ribosomal subunits.

Histones

Nuclear proteins around which DNA strands wrap to produce structural complexes called nucleosomes.

Chromatin

The loosely coiled, diffuse arrangement of DNA and histones present in the nucleus of a nondividing cell.

Chromosomes

Tightly coiled, condensed DNA structures present during cell division, consisting of paired copies held together at a centromere; somatic cells contain $23$ pairs.