Biology 12: Cell Structure and Membrane Transport

Cell Theory

The biological generalization stating that the cell is the fundamental building block of all organisms and that all cells arise from pre-existing cells.

Schleiden and Schwann

The German botanist and zoologist who, in the late 1830s, formulated the foundational principle that all living organisms are composed of cells.

Rudolph Virchow

The biologist who expanded the cell theory by contributing the observation that cells originate exclusively from pre-existing cells.

Prokaryotic Cells

Simple cells that lack a true nucleus and internal membrane-bound organelles (containing only ribosomes), resulting in metabolic reactions occurring throughout the cytoplasm.

Kingdom Monera

The biological kingdom comprising all prokaryotic organisms.

Eukaryotic Cells

Complex cells characterized by a membrane-bound nucleus, internal compartmentalizing membranes, and specialized organelles such as mitochondria and endoplasmic reticulum.

Chloroplasts

Large, double membrane-bound organelles in plant cells containing chlorophyll that absorb sunlight to conduct photosynthesis.

Cell Wall

A rigid, protective outer layer surrounding plant cells, composed of cellulose, that provides structural rigidity and stability.

Plant Vacuole

A large, single-membraned, liquid-filled organelle occupying up to $90\%$ of a plant cell's volume that stores water, nutrients, and wastes, and performs digestive functions.

Nucleus

The largest organelle within eukaryotic cells, acting as the control center that houses hereditary chromatin and directs cellular activities.

Nuclear Pores

Protein-lined openings across the double-layered nuclear envelope that regulate the passage of macromolecules between the nucleus and the cytoplasm.

Nucleolus

A dense, dark region located within the nucleoplasm that synthesizes and stores ribosomal RNA ($\text{rRNA}$) and assembles ribosome subunits.

Chromatin

The uncoiled genetic material within the nucleus composed of DNA and associated proteins, which condenses into chromosomes during cellular division.

Chromosomes

Rod-shaped structures of tightly condensed chromatin visible during cell division that carry genes (human body cells contain $46$, while sex cells contain $23$).

Cytoplasm

A colloidal intracellular substance that suspends and supports organelles, capable of shifting between a semi-solid gel state and a fluid sol state.

Plasma Membrane

A selectively permeable boundary surrounding the cell composed of a phospholipid bilayer and embedded proteins, regulating the entrance and exit of substances.

Smooth Endoplasmic Reticulum (SER)

A network of interconnected membranous tubular canals devoid of ribosomes that synthesizes lipids, forms steroid hormones, and detoxifies drugs and alcohol in the liver.

Blebbing

The physical process in which small sections of the endoplasmic reticulum or Golgi apparatus pinch off to form membrane-bound transport vesicles.

Rough Endoplasmic Reticulum (RER)

A membranous organelle studded with ribosomes on its outer surface that synthesizes, folds, and transports proteins primarily destined for export or membrane integration.

Golgi Apparatus

A stack of flattened membranous sacs that receives protein-filled vesicles from the ER, modifies, sorts, and packages the contents, and dispatches them for secretion or intracellular use.

Vesicles

Small, membrane-enclosed sacs formed by blebbing from the Golgi body, ER, or cell membrane that store and transport materials throughout the cell.

Lysosomes

Membrane-bound digestive vesicles produced by the Golgi apparatus containing hydrolytic enzymes used to digest phagocytosed materials or break down damaged organelles.

Autodigestion

The controlled cellular breakdown and recycling of a cell's own aging or damaged organelles by lysosomal hydrolytic enzymes.

Ribosomes

Non-membrane-bound complexes of $\text{rRNA}$ and proteins that catalyze the formation of peptide bonds during protein synthesis.

Polyribosome

A linear cluster of multiple ribosomes simultaneously translating the same mRNA transcript to produce multiple copies of a polypeptide.

Mitochondria

Double-membraned cellular organelles known as the powerhouse of the cell that convert glucose into adenosine triphosphate ($\text{ATP}$) through cellular respiration.

Cristae

Shelf-like folds of the inner mitochondrial membrane that significantly increase surface area for cellular respiration reactions.

Cytoskeleton

An internal framework composed of microfilaments and microtubules that maintains cell shape, anchors organelles, and facilitates intracellular movement.

Cilia

Short, numerous, hair-like membrane-bound projections composed of microtubules that beat in an oar-like motion to provide locomotion or move fluids.

Flagella

Long, whip-like appendages constructed from microtubules that provide motility to unicellular organisms and specialized cells such as sperm.

Fluid Mosaic Model

The structural model describing the plasma membrane as a semi-fluid phospholipid bilayer with embedded, floating proteins.

Glycoproteins and Glycolipids

Membrane proteins and lipids bonded to external carbohydrate chains that function as cell identification markers for recognition and tissue compatibility.

Diffusion

The passive net movement of solute molecules down a concentration gradient from an area of high concentration to low concentration without requiring a membrane, carrier, or $\text{ATP}$.

Osmosis

The passive net movement of water molecules across a selectively permeable membrane from an area of higher water concentration to an area of lower water concentration.

Hypertonic Solution

A surrounding solution with a higher concentration of solute than inside the cell, causing water to leave the cell and resulting in cellular shrinkage (crenation).

Hypotonic Solution

A surrounding solution with a lower concentration of solute than inside the cell, causing water to enter the cell and leading to swelling or lysis.

Isotonic Solution

A surrounding environment having an equal concentration of solute compared to the inside of the cell, resulting in no net movement of water.

Facilitated Transport

The passive passage of specific molecules (such as glucose) across a membrane along their concentration gradient with the assistance of a carrier or channel protein, consuming no $\text{ATP}$.

Active Transport

The carrier-mediated movement of solutes (such as $\text{Na}^+$ and $\text{K}^+$) across a membrane against their concentration gradient, requiring metabolic energy in the form of $\text{ATP}$.

Surface Area to Volume Ratio

The geometric relationship where internal cell volume increases faster than surface area, restricting maximum cell size to ensure adequate nutrient uptake and waste removal.

Phagocytosis

A form of endocytosis colloquially termed 'cell eating' where the cell engulfs large solid particles, cellular debris, or intact pathogens within vesicles.

Pinocytosis

A form of endocytosis known as 'cell drinking' in which the cell membrane invaginates to take in fluids containing dissolved solutes.

Exocytosis

An energy-dependent process by which membrane-bound vesicles fuse with the plasma membrane to secrete wastes or macromolecules out of the cell.