2. Cell Structure: Key Terms

• Active immunity**:** Immunity that develops when the body produces its own immune response after exposure to a pathogen.
• Active transport**:** The energy-dependent movement of substances from an area of low concentration to an area of higher concentration against the concentration gradient, using ATP.
• Agglutination**:** The clumping of cells or particles caused by antibodies, aiding phagocytosis.
• Antibody**:** A protein produced by plasma cells that binds specifically to antigens as part of the immune response.

• Antigen**:** Molecules on the surface of pathogens that can trigger an immune response by being recognised by antibodies.
• Binary fission**:** A form of asexual reproduction in prokaryotes where the circular DNA and plasmids replicate, followed by division of the cytoplasm to produce two cells.
• Cell cycle**:** The sequence of stages in a cell's life, including interphase and mitosis, leading to division and replication.
• Cell-surface membrane**:** A phospholipid bilayer embedded with proteins that surrounds the cell, controlling interactions with its environment.
• Cell vacuole**:** A membrane-bound structure in plant cells containing cell sap, which helps maintain cell structure and storage.
• Cell wall**:** A structural layer made of polysaccharides (e.g., cellulose in plants) that provides strength and support to plant, algae, and fungal cells.
• Chloroplast**:** An organelle in plant and algal cells that carries out photosynthesis.
• Clonal expansion**:** The rapid division of activated B or T lymphocytes to produce identical cells during an immune response.
• Clonal selection**:** The process where specific antigens are matched with complementary receptors on B or T lymphocytes, initiating their activation.
• Co-transport**:** Simultaneous transport of two substances across a membrane, either in the same direction (symport) or opposite directions (antiport).
• Cytokinesis**:** The division of the cytoplasm to form two separate daughter cells after mitosis or meiosis.

• Facilitated diffusion**:** Passive transport of substances down their concentration gradient through specific carrier or channel proteins, without energy input.
• Flagella**:** Long, whip-like structures enabling movement in certain cells, like bacteria.
• Fluid-mosaic model**:** A description of the cell membrane structure where a fluid bilayer of phospholipids is embedded with mobile proteins.
• Golgi apparatus**:** A eukaryotic organelle that modifies, sorts, and packages proteins and lipids for transport.
• Helper T cell**:** A type of T cell that activates B cells, cytotoxic T cells, and phagocytes in the immune response.
• Herd immunity**:** Protection from a disease that occurs when a large proportion of a population is vaccinated, reducing its spread.
• HIV (Human Immunodeficiency Virus)****: A virus that targets T cells, weakening the immune system and potentially leading to AIDS.
• Lysosomes**:** Membrane-bound vesicles containing enzymes that break down waste and cellular debris.
• Magnification**:** The ratio of the size of an image to the size of the actual object, calculated as:

$$\text{Magnification} = \frac{\text{Image size}}{\text{Real size}}$$

• Mitochondrion**:** An organelle that produces ATP through aerobic respiration in eukaryotic cells.
• Mitosis**:** A type of cell division producing two genetically identical daughter cells from a single parent cell.

• Monoclonal antibodies**:** Identical antibodies derived from a single parent cell used for specific medical or experimental purposes.
• Nucleus**:** A membrane-bound organelle in eukaryotic cells that houses genetic material.
• Osmosis**:** The passive movement of water molecules from an area of high water potential to an area of low water potential across a selectively permeable membrane.
• Passive immunity**:** Immunity gained through the transfer of ready-made antibodies into the body.
• Phagocytosis**:** The engulfing and digestion of pathogens or debris by specialised cells like phagocytes.
• Plasmids**:** Small, circular DNA molecules found in bacterial cells, often carrying antibiotic resistance genes.
• Primary immune response**:** The body's initial response to a pathogen, involving the activation of B and T cells.
• Resolution**:** The ability to distinguish between two points as separate in an image.
• Ribosomes**:** Organelles responsible for protein synthesis, found freely in the cytoplasm or attached to the RER.
• Rough endoplasmic reticulum (RER)****: A membrane-bound organelle with ribosomes attached, involved in protein synthesis and transport.
• Secondary immune response**:** A faster, stronger immune reaction upon re-exposure to a previously encountered pathogen.
• Simple diffusion**:** The passive movement of molecules down their concentration gradient, without requiring energy.
• Smooth endoplasmic reticulum (SER)****: A membrane-bound organelle involved in lipid synthesis and detoxification.
• Vaccine**:** A substance containing inactive or weakened pathogens that stimulates the immune system to produce immunity.