Cell Biology and Cell Cycle: Key Structures, Processes, and Transport Mechanisms

Cell

The smallest living unit of life.

Nucleus

Control center of the cell; contains DNA, RNA, proteins, enzymes.

Cytoplasm

Fluid inside the cell that surrounds organelles.

Cytoskeleton

Network of fibers providing structure, support, and transport.

Microfilaments

Actin filaments; maintain shape and aid in movement.

Microtubules

Tubulin filaments; form spindle fibers, transport vesicles.

Intermediate filaments

Provide mechanical support to cell structure.

Ribosome

Organelle responsible for protein synthesis.

Rough Endoplasmic Reticulum (RER)

Synthesizes proteins for export or membrane insertion.

Smooth Endoplasmic Reticulum (SER)

Synthesizes lipids, detoxifies chemicals.

Golgi Apparatus

Modifies, sorts, and packages proteins for secretion or transport.

Mitochondria

Powerhouse of the cell; produces ATP via oxidative phosphorylation; contains its own DNA.

Lysosome

Contains digestive enzymes to break down waste or cellular debris.

Plasma Membrane

Selectively permeable barrier separating the cell from its environment.

Interphase

Phase where the cell grows and DNA is replicated (G1, S, G2).

G₀ phase

Resting phase; cell is metabolically active but not dividing.

S phase

DNA synthesis occurs.

Checkpoint

Control mechanisms in the cell cycle ensuring proper division.

Mitosis

Division of the nucleus; phases: prophase, metaphase, anaphase, telophase.

Cytokinesis

Division of the cytoplasm following mitosis.

Chromatid

One half of a duplicated chromosome.

Centromere

Region where sister chromatids are attached.

Spindle fibers

Microtubules that move chromosomes during mitosis.

Tumor

Abnormal growth of cells; may be benign or malignant.

Cancer

Uncontrolled cell growth and division due to mutations in cell cycle regulation.

Diffusion

Passive movement of molecules from high to low concentration.

Osmosis

Diffusion of water across a selectively permeable membrane.

Facilitated Diffusion

Passive movement of molecules via protein carriers or channels.

Active Transport

Movement of molecules against their gradient using ATP.

Primary Active Transport

Uses ATP directly (e.g., Na⁺/K⁺ pump).

Secondary Active Transport

Uses gradient created by another molecule's transport (e.g., SGLT).

Vesicular Transport

Materials carried into or out of the cell via vesicles (endocytosis/exocytosis).

Endocytosis

Process of taking substances into the cell via vesicles.

Exocytosis

Process of exporting substances out of the cell via vesicles.

Phagocytosis

"Cell eating" type of endocytosis.

Pinocytosis

"Cell drinking" type of endocytosis.

Tonicity

Relative concentration of solutes in solution compared to the cell.

Hypertonic

Solution has higher solute → cell shrinks.

Hypotonic

Solution has lower solute → cell swells.

Isotonic

Solution has equal solute → cell stays the same.

Membrane Potential

Difference in charge across the plasma membrane.

Electrochemical Gradient

Combined effect of concentration and electrical gradients on ion movement.

K⁺ leak channels

Primary contributors to resting membrane potential.

Na⁺/K⁺ pump

Maintains gradients of Na⁺ and K⁺; minor contributor to resting potential.

DNA

Stores genetic information.

RNA

Single-stranded nucleic acid involved in transcription and translation.

mRNA

Messenger RNA; carries coding instructions from DNA to ribosome.

tRNA

Transfer RNA; carries amino acids to the ribosome.

rRNA

Ribosomal RNA; structural and catalytic part of ribosomes.

Transcription

DNA → RNA, occurs in nucleus, requires RNA polymerase.

Translation

RNA → polypeptide chain, occurs on ribosomes.

Pre-mRNA

Initial RNA transcript that must be processed (spliced) before translation.

Splicing

Removal of introns from pre-mRNA.

Codon

Three-nucleotide sequence on mRNA coding for an amino acid.

Anticodon

Three-nucleotide sequence on tRNA complementary to mRNA codon.

MicroRNA (miRNA)

Small RNA molecules that regulate gene expression post-transcriptionally.

Post-transcriptional regulation

Control of gene expression after RNA is made but before translation.

Metabolism

Sum of catabolic (breakdown) and anabolic (build-up) reactions.

Catabolism

Breaking down molecules to release energy.

Anabolism

Building molecules, requiring energy.

Glycolysis

Glucose → pyruvate; occurs in cytoplasm.

Link Reaction

Pyruvate → Acetyl-CoA; connects glycolysis to TCA cycle.

Citric Acid Cycle (TCA/Krebs)

Acetyl-CoA → NADH, FADH₂, GTP; occurs in mitochondria.

Electron Transport Chain

Series of protein complexes in mitochondria; uses electrons to create proton gradient for ATP production.

Oxidative Phosphorylation

Production of ATP using proton gradient.

Beta-Oxidation

Breakdown of fatty acids → Acetyl-CoA.

Fermentation

Anaerobic process converting pyruvate → lactate or ethanol.

Nutrient Pool

Body's collection of macromolecules (carbs, fats, proteins) available for metabolism.

Absorptive State

Period after eating; nutrients absorbed and stored.

Postabsorptive State

Fasting period; stored nutrients mobilized.

Oxidation

Loss of electrons from a molecule.

Reduction

Gain of electrons by a molecule.

Resting Membrane Potential (RMP)

Membrane potential at rest (~−70 mV).

Threshold Potential

~−55 mV; depolarization needed to trigger action potential.

Depolarization

Na⁺ enters cell → membrane potential becomes less negative.

Repolarization

K⁺ exits → membrane potential returns toward −70 mV.

Hyperpolarization

Membrane potential dips below −70 mV temporarily.

Action Potential

All-or-none electrical signal along axon.

Continuous Conduction

AP propagates step-by-step in unmyelinated axons.

Saltatory Conduction

AP jumps node-to-node in myelinated axons → faster.

Absolute Refractory Period

No AP possible.

Relative Refractory Period

AP possible with stronger stimulus.

Synapse

Junction between neurons where neurotransmitters transmit signals.

EPSP (Excitatory Post-Synaptic Potential)

Depolarizes postsynaptic neuron.

IPSP (Inhibitory Post-Synaptic Potential)

Hyperpolarizes postsynaptic neuron.

Neurotransmitter Release

Triggered by Ca²⁺ influx into presynaptic terminal.

Axon Hillock

Trigger zone for action potentials.

Voltage-gated Channels

Open in response to membrane potential changes.

Ligand-gated Channels

Open in response to chemical signals.

Nodes of Ranvier

Gaps in myelin sheath for saltatory conduction.

Summation

Integration of multiple EPSPs/IPSPs at axon hillock.