Active Transport: Movement against a concentration gradient; requires energy (ATP).

Primary Active Transport: Uses ATP directly (e.g., Na+/K+ pump).
Secondary Active Transport (Co-transport): Uses energy from an electrochemical gradient established by primary active transport.
Bulk Transport:
- Endocytosis: Cell takes in substances by forming vesicles.
  - Phagocytosis ("cell eating"): Engulfs large particles.
  - Pinocytosis ("cell drinking"): Engulfs extracellular fluid and solutes.
  - Receptor-mediated endocytosis: Specific uptake of molecules using receptors.
- Exocytosis: Cell releases substances by fusing vesicles with the plasma membrane.

Structure:
- Nuclear Envelope: Double membrane (inner and outer) perforated by nuclear pores. Continuous with the ER.
- Nuclear Pores: Regulate the passage of macromolecules (proteins, RNA) between the nucleus and cytoplasm.
- Nuclear Lamina: A net-like array of protein filaments (intermediate filaments) lining the inner surface of the nuclear envelope; maintains nuclear shape.
- Nucleoplasm: The viscous fluid filling the nucleus.
- Chromatin: Complex of DNA and proteins (histones). DNA is organized into chromosomes during cell division.
- Nucleolus: A dense, non-membranous structure within the nucleus, responsible for ribosomal RNA (rRNA) synthesis and assembly of ribosomal subunits.
Function:
- Houses and protects the cell's genetic material (DNA).
- Controls gene expression, cell growth, metabolism, and reproduction.
- Site of DNA replication and transcription (synthesis of mRNA, tRNA, rRNA).

Structure: Small particles composed of ribosomal RNA (rRNA) and ribosomal proteins. Consist of a large subunit and a small subunit.
Location:
- Free Ribosomes: Suspended in the cytosol; synthesize proteins that function within the cytosol (e.g., enzymes, structural proteins).
- Bound Ribosomes: Attached to the ER or nuclear envelope; synthesize proteins destined for secretion, insertion into membranes, or delivery to certain organelles (e.g., lysosomes, Golgi).
Function: Protein synthesis (translation) – translating messenger RNA (mRNA) into polypeptide chains.

Structure: Extensive network of interconnected membranes forming sacs (cisternae) and tubules throughout the cytoplasm. Continuous with the outer nuclear membrane.
Types:
- Rough ER (RER):
  - Appearance: Studded with ribosomes on its cytosolic surface.
  - Function:
    - Synthesis of proteins destined for secretion, insertion into membranes, or delivery to organelles like the Golgi, lysosomes, and vacuoles.
    - Involved in protein folding, modification (e.g., glycosylation), and quality control.
    - Membrane factory for the cell.
- Smoo