Glossary
Active Transport
The movement of substances across a cell membrane against their concentration gradient, from an area of low concentration to high concentration, requiring the expenditure of cellular energy (ATP).
Example:
The sodium-potassium pump uses active transport to maintain the electrochemical gradient necessary for nerve impulse transmission.
Chloroplasts
Organelles found in plant cells and other eukaryotic photosynthetic organisms, responsible for photosynthesis by converting light energy into chemical energy (sugars).
Example:
The green color of leaves comes from the abundant chloroplasts within their cells, capturing sunlight for energy.
Citric Acid/Krebs Cycle
A central metabolic pathway occurring in the mitochondrial matrix, which completes the breakdown of glucose derivatives, generating ATP, NADH, and FADH2.
Example:
During intense exercise, the Citric Acid Cycle works overtime in your muscle cells to keep up with the demand for energy precursors.
Endocytosis
A process by which cells take in substances from their external environment by engulfing them in a portion of the plasma membrane, forming a vesicle.
Example:
A macrophage uses endocytosis to engulf and internalize bacteria, initiating the immune response.
Endoplasmic Reticulum (ER)
A network of membranes found throughout the cytoplasm of eukaryotic cells, involved in mechanical support, intracellular transport, and compartmentalization.
Example:
The Endoplasmic Reticulum acts like a cellular highway system, moving molecules to different destinations within the cell.
Endosymbiotic Theory
The widely accepted theory proposing that eukaryotic organelles like mitochondria and chloroplasts originated from free-living prokaryotic cells that were engulfed by a host cell and established a symbiotic relationship.
Example:
The presence of their own circular DNA and prokaryote-like ribosomes in mitochondria and chloroplasts provides strong evidence for the Endosymbiotic Theory.
Exocytosis
A process by which cells release substances from their internal environment to the outside by fusing vesicles containing the substances with the plasma membrane.
Example:
Pancreatic cells release insulin into the bloodstream through exocytosis.
Fluid mosaic model
A model describing the structure of the plasma membrane as a dynamic, fluid arrangement of phospholipids and various proteins embedded within or associated with the bilayer.
Example:
According to the fluid mosaic model, membrane proteins are not rigidly fixed but can move around, much like icebergs floating in a lipid sea.
Glycolysis
The first step in cellular respiration, occurring in the cytoplasm, where glucose is broken down into two molecules of pyruvate, producing a small amount of ATP and NADH.
Example:
Even without oxygen, a cell can still generate some energy through glycolysis, which is why it's considered an ancient metabolic pathway.
Golgi Complex
An organelle consisting of flattened membrane sacs called cisternae, responsible for modifying, sorting, and packaging proteins and lipids for secretion or delivery to other organelles.
Example:
After proteins are made in the ER, they often travel to the Golgi Complex for final processing and 'shipping labels' before reaching their cellular destination.
Hydrophilic heads
The phosphate-containing, water-attracting portions of phospholipid molecules that face the aqueous environments inside and outside the cell membrane.
Example:
The hydrophilic heads of the phospholipids are crucial for the membrane's interaction with the watery cytoplasm and extracellular fluid.
Hydrophobic tails
The fatty acid chains of phospholipid molecules that are water-repelling and face inward, forming the nonpolar core of the cell membrane.
Example:
The barrier created by the hydrophobic tails prevents most water-soluble molecules from freely passing through the cell membrane.
Light-dependent reactions
The first stage of photosynthesis, occurring in the thylakoid membranes of chloroplasts, where light energy is captured to produce ATP and NADPH.
Example:
Without sunlight, the light-dependent reactions cannot occur, halting the entire process of photosynthesis.
Light-independent reactions (Calvin Cycle)
The second stage of photosynthesis, occurring in the stroma of chloroplasts, where ATP and NADPH from the light reactions are used to fix carbon dioxide and produce sugars.
Example:
The sugar molecules that plants use for energy and growth are synthesized during the light-independent reactions.
Lysosomes
Membrane-bound organelles containing hydrolytic enzymes, responsible for breaking down waste materials, cellular debris, and worn-out organelles through intracellular digestion.
Example:
When a white blood cell engulfs bacteria, lysosomes fuse with the vesicle containing the bacteria to digest and destroy them.
Membrane Transport
The movement of substances across the cell membrane, essential for maintaining cellular homeostasis and acquiring necessary resources.
Example:
Efficient membrane transport is vital for nerve cells to transmit signals by moving ions across their membranes.
Mitochondria
Organelles with a double membrane, known as the 'powerhouses' of the cell, primarily responsible for producing ATP through cellular respiration.
Example:
Muscle cells, which require a lot of energy for contraction, are packed with mitochondria to generate sufficient ATP.
Osmoregulation
The process by which living organisms maintain the balance of water and solutes within their bodies, controlling the internal environment.
Example:
Freshwater fish constantly engage in osmoregulation to prevent excessive water intake and maintain proper salt balance.
Osmosis
The specific diffusion of water molecules across a selectively permeable membrane from an area of higher water potential (lower solute concentration) to an area of lower water potential (higher solute concentration).
Example:
If you place a red blood cell in pure water, osmosis will cause water to rush into the cell, potentially causing it to burst.
Oxidative Phosphorylation
The final stage of cellular respiration, occurring in the inner mitochondrial membrane, where ATP is produced using the energy released from the electron transport chain and chemiosmosis.
Example:
The majority of ATP in aerobic organisms is generated through oxidative phosphorylation, making it a highly efficient energy production process.
Passive Transport
The movement of substances across a cell membrane down their concentration gradient, from an area of high concentration to low concentration, without the expenditure of cellular energy.
Example:
Oxygen diffuses into red blood cells via passive transport, moving from the higher concentration in the lungs to the lower concentration in the blood.
Phospholipid bilayer
The fundamental structure of the plasma membrane, consisting of two layers of phospholipid molecules with their hydrophilic heads facing outwards and hydrophobic tails facing inwards.
Example:
The fluidity of the phospholipid bilayer allows membrane proteins to move laterally, contributing to the dynamic nature of the cell membrane.
Plasma Membrane
The selectively permeable outer boundary of a cell, composed of a phospholipid bilayer with embedded proteins, regulating the passage of substances into and out of the cell.
Example:
The plasma membrane acts as a gatekeeper, ensuring that only necessary molecules enter the cell and waste products exit.
Pressure potential
A component of water potential that represents the physical pressure exerted on water, such as turgor pressure in plant cells.
Example:
The rigid cell wall of a plant cell allows for a positive pressure potential to build up, helping the plant stand upright.
Ribosomes
Cellular structures composed of ribosomal RNA (rRNA) and proteins, responsible for protein synthesis by translating mRNA into polypeptide chains.
Example:
When a cell needs to produce insulin, ribosomes will read the mRNA instructions and assemble the amino acids into the insulin protein.
Rough ER
A type of endoplasmic reticulum studded with ribosomes, primarily involved in the synthesis, folding, modification, and transport of proteins destined for secretion or insertion into membranes.
Example:
Pancreatic cells, which secrete digestive enzymes, have an abundance of Rough ER to handle the high volume of protein production.
Semi-permeable barrier
A property of the plasma membrane that allows certain molecules or ions to pass through it by means of active or passive transport, while blocking others.
Example:
The cell's ability to maintain a stable internal environment is largely due to its semi-permeable barrier, which controls what enters and leaves.
Smooth ER
A type of endoplasmic reticulum lacking ribosomes, involved in lipid synthesis, detoxification of drugs and poisons, and storage of calcium ions.
Example:
Liver cells contain extensive Smooth ER to efficiently detoxify harmful substances like alcohol and drugs.
Solute potential
A component of water potential that is influenced by the concentration of solutes; increasing solute concentration lowers the solute potential (makes it more negative).
Example:
Adding salt to water lowers its solute potential, making it more likely for water to move towards that solution.
Surface area-to-volume ratio
A critical concept in cell biology that describes the relationship between a cell's outer surface area and its internal volume, affecting its efficiency in exchanging materials with the environment.
Example:
Smaller cells have a higher surface area-to-volume ratio, which allows them to absorb nutrients and eliminate waste more efficiently than larger cells.
Vacuoles
Membrane-bound sacs within the cytoplasm that serve various storage and transport functions, particularly prominent in plant cells for water retention and maintaining turgor pressure.
Example:
A wilting plant often indicates that its central vacuoles have lost water, causing the cell to lose turgor pressure.
Water Potential
The potential energy of water per unit volume relative to pure water in reference conditions, indicating the tendency of water to move from one area to another.
Example:
Water always moves from an area of high water potential to an area of low water potential, which is why plants absorb water from the soil.