Review

PART 1 — CHEMISTRY

1. PASSIVE DIFFUSION / PASSIVE TRANSPORT

Passive transport is the movement of substances across a cell membrane without consuming adenosine triphosphate ($ATP$). This process occurs naturally as substances move down their concentration gradient, transferring from an area of high concentration to an area of low concentration. Because movement occurs down the gradient, no cellular energy input is required. The main types of passive transport include simple diffusion, facilitated diffusion, osmosis, and filtration.

2. SIMPLE DIFFUSION

Simple diffusion involves the unassisted movement of molecules directly through the hydrophobic phospholipid bilayer from a region of high concentration to a region of low concentration. It requires neither $ATP$ nor a transport protein. Molecules that typically cross via simple diffusion include oxygen ($O_2$), carbon dioxide ($CO_2$), and steroid hormones. These molecules can pass directly through because the inner region of the membrane is nonpolar, allowing small nonpolar molecules to diffuse freely across. In summary, simple diffusion is defined by movement down a concentration gradient without energy or transport proteins.

3. FACILITATED DIFFUSION

Facilitated diffusion is the movement of substances from high concentration to low concentration through a transmembrane protein. Like simple diffusion, it does not require $ATP$ because movement follows the concentration gradient. However, it requires a transport protein because the substances passing through are either polar, charged, or too large to pass through the hydrophobic membrane interior independently. Crucially, facilitated diffusion relies on a protein carrier or channel without consuming $ATP$.

4. ACTIVE TRANSPORT

Active transport is the movement of substances across a cell membrane from an area of low concentration to an area of high concentration. Because the substance is moving against its concentration gradient, the process requires both a transport protein and cellular energy in the form of $ATP$. Active transport is distinguished by its movement against a gradient using energy and membrane proteins.

5. SECONDARY ACTIVE TRANSPORT

Secondary active transport uses the energy stored in an existing ion concentration gradient rather than consuming $ATP$ directly. In this process, a carrier protein or cotransporter moves one substance against its concentration gradient by coupling it with another substance moving down its concentration gradient. While secondary active transport uses no direct $ATP$, it is classified as active transport because the driving ion gradient was initially established by primary active transport, which consumed $ATP$. Thus, primary active transport directly uses $ATP$ and a protein, whereas secondary active transport uses an established ion gradient and a protein without direct $ATP$ breakdown.

6. OSMOSIS

Osmosis is the specialized passive diffusion of water across a selectively permeable membrane. Water moves down its concentration gradient toward the side with a higher solute concentration (or lower water concentration). Osmosis occurs spontaneously without requiring cellular energy ($ATP$).

7. FILTRATION

Filtration is the movement of water and small solute molecules across a membrane driven by physical pressure, specifically hydrostatic pressure. Unlike diffusion driven by solute concentration gradients, filtration relies on pressure differences and does not require $ATP$.

8. PHAGOCYTOSIS

Phagocytosis, commonly referred to as "cell eating," is an active transport mechanism in which a cell engulfs large solid particles by extending its membrane to form a vesicle. Because membrane remodeling and vesicle formation are involved, phagocytosis moves material into the cell and requires $ATP$.

9. PINOCYTOSIS

Pinocytosis, often described as "cell drinking," is a form of endocytosis where the cell takes in extracellular fluid along with dissolved substances into small vesicles. Like phagocytosis, pinocytosis is an active process that consumes $ATP$.

10. HYPERTONIC, HYPOTONIC & ISOTONIC

Tonicity describes how a solution affects cellular volume through osmosis. In a hypertonic solution, the fluid outside the cell contains a higher concentration of solutes than the cell interior, causing water to exit the cell and leading it to shrink. Conversely, a hypotonic solution has a lower solute concentration outside the cell than inside, causing water to enter the cell and leading to cell swelling. In an isotonic solution, solute concentrations are equal on both sides of the membrane, resulting in no net water movement and allowing the cell to maintain a constant size.

11. SOLVENT & SOLUTE

In a solution, a solvent is the substance that does the dissolving, whereas a solute is the substance that becomes dissolved. For example, in a salt water solution, water acts as the solvent and salt acts as the solute.

12. ORGANIC & INORGANIC COMPOUNDS

Biological compounds are categorized as either organic or inorganic based on their chemical composition. Organic compounds contain carbon bonded to hydrogen, and the major biological classes include carbohydrates, lipids, and proteins. In contrast, inorganic compounds generally lack a carbon-hydrogen framework; common examples include water, mineral salts, as well as many acids and bases.

13. METABOLISM

Metabolism encompasses the sum total of all chemical reactions occurring within a cell or whole organism.

14. KINETIC & POTENTIAL ENERGY

Energy exists in two primary states within biological systems. Kinetic energy is the energy of motion, performing work through particle movement. Potential energy is stored energy held for later release.

15. LIGAND

A ligand is any molecule that specifically binds to a receptor or target protein to trigger a biological response.

16. ISOTOPE

Isotopes are atoms of the same chemical element that share identical numbers of protons but differ in their number of neutrons.

17. ABSORPTION & SECRETION

Absorption and secretion describe opposing directional movements of substances in tissues. Absorption refers to the process where a substance is taken into a cell, tissue, or the bloodstream. Secretion refers to the release of a substance produced by a cell or gland.

18. pH

The $pH$ scale measures the concentration of free hydrogen ions ($H^+$) in a solution. A higher concentration of $H^+$ results in a lower $pH$, indicating an acidic solution. Conversely, a lower concentration of $H^+$ results in a higher $pH$, indicating a basic solution, with a $pH$ of $7$ representing neutral conditions.

19. BUFFER

A buffer is a substance that helps maintain homeostatic $pH$ levels by resisting rapid changes in $pH$.

20. WATER

Water is a polar molecule essential for life because oxygen exerts a stronger pull on shared electrons than hydrogen, generating an uneven charge distribution. This polarity makes water an excellent solvent for polar and ionic substances, facilitates chemical reactions, and helps regulate temperature.

21. SODIUM (Na⁺) & POTASSIUM (K⁺)

Sodium ($Na^+$) and potassium ($K^+$) play major roles in fluid balance, nerve impulses, muscle contraction, and maintaining membrane potential. The sodium-potassium pump actively maintains these electrical differences across the membrane by pumping $3\,Na^+$ out of the cell and $2\,K^+$ into the cell using $ATP$.

22. PROPERTIES OF GASES

Gases lack both a fixed shape and a fixed volume, expanding freely to fill whatever space is available. Because their particles move freely, gases are highly compressible.

23. CHEMICAL BONDS

Chemical bonds hold atoms together through various electron interactions. Ionic bonds form when electrons are transferred from one atom to another. Covalent bonds occur when atoms share electrons; nonpolar covalent bonds feature equal electron sharing, whereas polar covalent bonds involve unequal electron sharing. Hydrogen bonds are weak attractions involving a partially positive hydrogen atom and an electronegative atom. Peptide bonds are specific covalent bonds that join amino acids together.

24. ORGANIC COMPOUNDS

The major classes of biological organic molecules serve distinct functional roles. Carbohydrates, composed mainly of carbon, hydrogen, and oxygen ($C, H, O$), are generally polar molecules that serve as immediate sources of energy. Lipids consist mainly of glycerol and fatty acids, are largely nonpolar and hydrophobic, and function in long-term energy storage, cell membrane structure, and hormone signaling. Proteins are made from amino acids joined by peptide bonds and perform diverse functions including enzymatic reactions, molecular transport, structural support, and cellular signaling.

25. PROTEIN STRUCTURE

Proteins exhibit four levels of structural organization. Primary structure is the linear sequence of amino acids in a polypeptide chain. Secondary structure involves localized folding such as alpha helices and beta sheets, held together mainly by hydrogen bonds. Tertiary structure represents the overall three-dimensional shape of a single polypeptide chain. Quaternary structure occurs when multiple distinct polypeptide chains come together.

26. ATP

Adenosine triphosphate ($ATP$) serves as the cell's main immediate energy-transfer molecule. $ATP$ is synthesized by combining adenosine diphosphate ($ADP$), a phosphate group, and energy ($ADP + \text{phosphate} + \text{energy} \rightarrow ATP$). Energy is stored in the bonds involving $ATP$'s phosphate groups, and when $ATP$ is broken down ($ATP \rightarrow ADP + \text{phosphate}$), energy is released for cellular work.

PART 2 — CELLS

27. CELL ORGANELLES

Cell organelles perform specialized functions necessary for cellular survival. The nucleus, usually located near the center of the cell, contains $DNA$ and controls gene expression and cell activities. Inside the nucleus, the nucleolus produces ribosomal components. Ribosomes, located free in the cytoplasm or attached to the rough endoplasmic reticulum ($ER$), carry out protein synthesis. The rough $ER$ processes proteins intended for specific destinations and is named for its attached ribosomes. The smooth $ER$ lacks ribosomes and functions in lipid synthesis, detoxification, and calcium storage. The Golgi apparatus modifies, sorts, and packages proteins and lipids. Mitochondria produce the majority of the cell's $ATP$. Lysosomes handle intracellular digestion and recycling of worn-out cellular components, while peroxisomes break down certain fatty acids and detoxify harmful substances. The cytoskeleton provides cell shape, internal support, movement, and organelle organization. The centrosome organizes microtubules during cell division, and vesicles transport or store materials within the cell.

28. TRANSCRIPTION

Transcription is the process of synthesizing $RNA$ from a $DNA$ template, typically taking place within the nucleus.

29. TRANSLATION

Translation is the process of using messenger $RNA$ ($mRNA$) instructions to synthesize a protein at the ribosomes.

PART 3 — PLASMA MEMBRANE

30. PLASMA MEMBRANE

The plasma membrane forms the cell's outer boundary and regulates the passage of materials in and out. Its main structure is a phospholipid bilayer composed of individual phospholipids with hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

31. PLASMA MEMBRANE COMPOSITION

The main components of the plasma membrane include phospholipids, proteins, cholesterol, and carbohydrates. Cholesterol helps regulate membrane fluidity and stability, while carbohydrates aid in cell recognition and cellular interactions.

32. PLASMA MEMBRANE MODIFICATIONS

Specialized plasma membrane modifications enhance cell function. Microvilli increase cell surface area to facilitate absorption. Cilia help move materials across the cell surface. A flagellum is used primarily for whole-cell movement.

33. CELL JUNCTIONS

Cell junctions connect adjacent cells in tissues. Tight junctions seal spaces between cells to prevent leakage. Desmosomes anchor cells firmly together. Gap junctions allow cells to communicate directly by passing chemical or electrical signals.

34. PLASMA MEMBRANE PROTEINS

Plasma membrane proteins serve specialized transport and signaling roles. Channel proteins provide a passageway through the membrane, whereas carrier proteins bind a substance and change shape to move it across. Receptor proteins bind signaling molecules to trigger a cellular response, and enzymatic proteins help catalyze chemical reactions. Cell-identity proteins allow cells to recognize one another, and gated ion channels open or close in response to specific signals.

PART 4 — TISSUE TERMINOLOGY

35. LUMEN

A lumen is the open space or cavity inside a tube or hollow organ.

36. LACUNA

A lacuna is a small space containing a cell, found especially within cartilage or bone tissue.

37. BLAST, CLAST & CYTE

Tissue cell naming conventions indicate cell activity: a cell ending in "-blast" is actively building or forming tissue, a cell ending in "-clast" is breaking down tissue, and a cell ending in "-cyte" is a mature tissue cell that maintains the tissue.

QUICK TEST COMPARISONS

Transport mechanisms differ based on energy requirements and transport proteins: simple diffusion moves substances down a concentration gradient without energy or proteins; facilitated diffusion moves substances down a gradient using a transport protein without $ATP$; active transport moves substances against a gradient using $ATP$ and transport proteins; secondary active transport uses an established ion gradient without direct $ATP$; osmosis is the passive movement of water; filtration is pressure-driven movement; phagocytosis brings large particles into the cell using $ATP$; and pinocytosis brings extracellular fluid into the cell using $ATP$.

Tonicity dictates water movement relative to the cell: hypertonic solutions cause water exit and cell shrinkage; hypotonic solutions cause water entry and cell swelling; and isotonic solutions result in no net water movement, maintaining stable cell volume.

In chemical bonding, ionic bonds involve electron transfer, covalent bonds involve electron sharing (polar covalent share unequally while nonpolar covalent share equally), hydrogen bonds are weak electrostatic attractions, and peptide bonds join amino acids. Protein structure progresses from primary (amino acid sequence) to secondary (local folding like alpha helices and beta sheets), tertiary (overall 3D shape of one chain), and quaternary (assembly of multiple chains).

Key cellular terminology summarizes essential processes and tissue concepts: transcription converts $DNA$ to $RNA$, translation converts $mRNA$ to protein, a lumen is an open inner space, a lacuna is a space containing a cell, "-blast" cells build tissue, "-clast" cells break down tissue, "-cyte" cells maintain mature tissue, ligands bind to receptors, and isotopes share proton counts while differing in neutron counts.