Biology Unit 2 (Chapters 4-6)

Plasma Membrane Structure and Function

The protective barrier that isolates a cell from its external environment and maintains homeostasis within the cell.

Fluid-Mosaic Model

The model describing the plasma membrane as a fluid lipid bilayer with a light-oil consistency, embedded with a mosaic of various proteins.

Phospholipid Bilayer Structure

A membrane composition where hydrophilic (water-loving) glycerol and phosphorylated alcohol heads face the inner and outer surfaces, and non-polar hydrophobic (water-fearing) fatty acid tails face the interior.

Peripheral Proteins

Membrane proteins that often have a structural role to stabilize and shape the plasma membrane, or function in signaling pathways.

Integral Proteins

Membrane proteins that span or integrate into the lipid bilayer and perform diverse specific functions (e.g., channel, carrier, or receptor proteins).

Channel Proteins

Integral proteins with a channel that allows specific substances or ions to freely move down a concentration gradient across the membrane without energy.

Carrier Proteins

Specific integral proteins that combine with a certain molecule or ion to assist its transport across the membrane during active or passive transport.

Cell Recognition Proteins

Membrane glycoproteins that help the body recognize when it is being invaded by pathogens and distinguish self from non-self.

Receptor Proteins

Transmembrane proteins shaped specifically to bind with particular external signaling molecules (chemical messengers) to trigger a cellular response.

Enzymatic Proteins

Membrane proteins that directly catalyze specific metabolic reactions at the cell surface.

Glycolipids

Lipids in the plasma membrane that have attached carbohydrate chains.

Glycoproteins

Proteins in the plasma membrane that have attached carbohydrate chains.

Selectively Permeable

A property of the plasma membrane that allows some substances (like gases) to cross freely while restricting others (like ions and macromolecules) unless assisted.

Passive Transport

Ways of crossing a plasma membrane (such as diffusion and facilitated transport) that do not require an expenditure of chemical energy (ATP).

Active Transport

Ways of crossing a plasma membrane that require chemical energy (ATP) to pump molecules against their concentration gradient (from low to high concentration).

Diffusion

The passive process in which molecules move randomly down their electrochemical or concentration gradient from an area of high concentration to an area of low concentration until evenly distributed.

Osmosis

The diffusion of water (solvent) across a selectively permeable membrane toward an area of higher solute (lower water) concentration.

Isotonic Solution

A solution with an equal solute concentration compared to the inside of a cell, causing the cell to neither gain nor lose water.

Hypotonic Solution

A solution with a lower concentration of solute (higher concentration of water) than the inside of a cell, causing water to enter the cell and make it swell or burst.

Hypertonic Solution

A solution with a higher concentration of solute (lower concentration of water) than the inside of a cell, causing water to leave the cell and make it shrink or shrivel.

Osmotic Pressure

The pressure that develops in a system due to osmosis and differences in tonicity.

Cytolysis

The bursting or disruption of an animal cell resulting from the buildup of pressure when placed in a hypotonic solution.

Hemolysis

The specific bursting or disruption of red blood cells when placed in a hypotonic solution.

Turgor Pressure

The internal pressure created in a plant cell when placed in a hypotonic solution as the central vacuole gains water and pushes the plasma membrane against the rigid cell wall.

Crenation

The shriveling or wrinkling of an animal cell when placed in a hypertonic solution.

Plasmolysis

The shrinking of a plant cell's cytoplasm due to osmosis, causing the plasma membrane to pull away from the cell wall in a hypertonic environment.

Facilitated Transport / Diffusion

The passive movement of a molecule down its concentration gradient through a membrane, accelerated by a specific carrier or channel protein without requiring energy.

Sodium-Potassium Pump

An active transport mechanism that uses energy from ATP to move three sodium ions ($Na^+$) out of the cell and two potassium ions ($K^+$) into the cell, creating an electrical gradient across the membrane.

Cotransport (Symport)

A coupled transport process where two substances are moved simultaneously in the same direction by a single transmembrane protein; the passive movement of one ion down its gradient drives the active transport of another molecule up its gradient.

Countertransport (Antiport)

A coupled transport process where two different substances bind to the same transport protein on opposite sides of the membrane and are moved simultaneously in opposite directions.

Exocytosis

An active process where a membrane-bound secretory vesicle migrates to the plasma membrane, fuses with it, and discharges its contents outside of the cell.

Endocytosis

An active process where the plasma membrane surrounds, engulfs, and ingests fluid, food, large molecules, or other cells into a vesicle.

Phagocytosis

A type of endocytosis used to ingest large, solid particulate material, such as a bacterial cell or organic fragment.

Pinocytosis

A type of endocytosis that occurs continuously as the plasma membrane forms vesicles around liquids or liquids containing very small particles.

Receptor-Mediated Endocytosis

A selective and efficient type of pinocytosis where specific target molecules bind to receptors concentrated within a coated pit in the plasma membrane, which then pinches off into a coated vesicle.

Coated Pit

An indented location in the plasma membrane layered with protein on its cytoplasmic side where specific receptors accumulate for receptor-mediated endocytosis.

Extracellular Matrix (ECM)

A meshwork of proteins and polysaccharides in close association with and secreted by animal cells that influences cell shape, behavior, and tissue formation.

Collagen

A structural protein fiber found within the extracellular matrix that functions to resist stretching.

Elastin

A structural protein fiber within the extracellular matrix that provides the matrix with resilience and flexibility.

Adhesion Junctions

Junction proteins that mechanically join and hold cells of the same tissue together to allow stretching and stability.

Tight Junctions

Junction proteins between cells that connect plasma membranes tightly, creating an impermeable barrier to hold cells together and prevent leaking.

Gap Junctions

Junction proteins that form narrow channels between adjacent animal cells, allowing the direct passage of small molecules and ions between cells.

Cell Wall

A freely permeable, rigid structure surrounding plant cells, prokaryotes, fungi, and most algae composed primarily of cellulose in plants, providing support and protection.

Plasmodesmata

Numerous narrow, membrane-lined channels spanning plant cell walls that contain strands of cytoplasm, allowing direct exchange of water, small solutes, and materials between adjacent cells.

Chemical Messengers

Signaling molecules utilized by prokaryotic and eukaryotic cells to deliver signals and communicate with other cells.

Signal Transduction Pathway

A cascade of molecular events triggered by the interaction of a signaling molecule and a receptor that alters, amplifies, or relays the signal to elicit a specific cellular response.

Phosphorylation

The activation or modification of a protein or molecule accomplished by the addition of a phosphate group, typically driven by enzymes called kinases.

Dephosphorylation

The deactivation or modification of a protein or molecule accomplished by the removal of a phosphate group, typically driven by enzymes called phosphatases.

Kinases

Enzymes that conduct phosphorylation reactions by adding phosphate groups to target molecules.

Phosphatases

Enzymes that conduct dephosphorylation reactions by removing phosphate groups from target molecules.

G Protein

A membrane-associated protein that assists in signal transduction by interacting between an activated receptor and an effector protein to initiate a cellular response.

Effector Protein

A membrane-bound protein activated by a G protein that directly initiates a localized cellular response in a signal transduction pathway.

Second Messenger

A small molecule or ion within a cell (such as cAMP or calcium ions) used to relay, amplify, and accelerate the signal received from an external chemical messenger.

cAMP (Cyclic AMP)

One of the two most common second messenger molecules used in signal transduction pathways to accelerate an enzyme cascade and prompt cell responses.

Synaptic Signaling

A form of localized cell signaling involving neurotransmitters (e.g., epinephrine, serotonin) to transfer information within the nervous system.

Endocrine Signaling

A form of long-distance cell signaling utilizing hormones (e.g., insulin, sex hormones) produced by one organ or tissue to alter the function of a distant organ or tissue.

Paracrine Signaling

A form of localized cell signaling involving molecules released over short distances to coordinate localized tasks like blood clotting and tissue repair.

Direct Contact Signaling

A form of cell signaling resulting from immediate, physical contact between receptors on interacting cells.

Cell Cycle

The process of cell growth and division in eukaryotes, divided into phases based on cellular activities.

Interphase

The period of the cell cycle during which the cell engages in growth, metabolic activities, and prepares for division.

G1 Phase (Gap 1)

The first and longest phase of interphase where normal cell functions occur, organelles and cytoplasmic components (including centrioles) replicate, and the cell grows.

S Phase (Synthesis)

The phase of interphase during which DNA replication occurs, producing two sister chromatids for each chromosome.

G2 Phase (Gap 2)

The final phase of interphase where the cell continues to grow and synthesizes proteins and enzymes needed for division.

M Phase (Mitosis)

The stage of the cell cycle involving nuclear division, where duplicated copies of chromosomes are divided equally into two new nuclei.

Cytokinesis (C Phase)

The division of the cell cytoplasm and organelles, which overlaps with later parts of mitosis, resulting in two distinct daughter cells.

Chromatin

The diffuse, uncoiled form of genetic material located within the nucleus surrounded by the nuclear envelope during interphase.

Chromosomes

Completely condensed structures of DNA and protein formed as the cell enters mitosis, ensuring safe distribution of genetic material.

Centrosome

An organelle that functions as the main microtubule-organizing center, formed from a pair of centrioles in animal cells, which replicates during interphase.

Centrioles

Cylindrical structures within animal cell centrosomes that replicate during G1/interphase and migrate to opposite ends of the cell to define the poles of the mitotic spindle.

Mitotic Spindle

An apparatus composed of microtubules extending from centrosomes that assembles during prophase to move and separate chromosomes.

Centromere

The specialized region where two replicated sister chromatids are held together to form a duplicated chromosome.

Kinetochore

A specialized protein complex located on either side of a centromere where spindle fibers attach to the chromosome.

Kinetochore Microtubules

Spindle fibers extending from a pole that attach directly to the kinetochores of chromosomes to pull them toward the poles.

Polar Microtubules

Non-retracted spindle fibers that extend from opposite poles, overlap at the equator, and push against each other to elongate the cell.

Metaphase Plate

The spindle equator midway between the poles where chromosomes align horizontally during metaphase.

Cleavage Furrow

An indentation in the plasma membrane that forms and deepens around the circumference of an animal cell during cytokinesis to separate it into two cells.

Prophase

The first stage of mitosis where chromatin condenses into distinct chromosomes, the nuclear envelope breaks down, centrosomes migrate to opposite poles, and the mitotic spindle forms.

Prometaphase

The mitotic transition period during which the nuclear envelope completely disintegrates and spindle fibers attach to kinetochores.

Metaphase

The mitotic stage characterized by the alignment of chromosomes along the central metaphase plate.

Anaphase

The mitotic stage where sister chromatids separate into individual daughter chromosomes and are pulled toward opposite poles as kinetochore microtubules shorten.

Telophase

The final stage of mitosis where chromosomes arrive at opposite poles, decondense back into chromatin, spindle fibers depolymerize, and new nuclear envelopes reform.

Somatic Cells

The general body cells of an organism whose numbers are increased by cell division and regulated by apoptosis.

Apoptosis

A highly regulated program of controlled cell destruction resulting in a rounded-up cell with a fragmented nucleus and blistered plasma membrane, which is eventually engulfed by white blood cells.

Checkpoints

Chemical/regulatory "stop signs" within the cell cycle (notably at G1/S, G2/M, and late Mitosis/Spindle) that assess whether processes have occurred accurately before allowing progression.

G1/S Checkpoint

The primary checkpoint that responds to growth factors, cell size, nutritional states, and DNA integrity to determine if the cell will divide, delay, or enter resting.

G2/M Checkpoint

The chemical checkpoint near the end of the G2 phase that assesses the success of DNA replication before allowing a cell to begin mitosis.

Spindle Checkpoint

The mitotic checkpoint ensuring that all chromosomes are properly attached to spindle fibers prior to the onset of anaphase.

Tumor Suppressor Genes

Genes that encode proteins to slow down cell division, inhibit the cell cycle, or promote apoptosis at checkpoints, acting as recessive "loss-of-function" controls.

Recessive Mutation (Cell Cycle)

A mutation requiring both copies of a gene to be altered in order to exhibit a phenotype; typical of tumor suppressor genes because one functional copy can still properly regulate the cycle.

Loss-of-Function Mutation

A mutation that eliminates or reduces the normal function of a gene product, such as a mutated tumor suppressor failing to inhibit cell division.

Retinoblastoma (Rb) Gene

A vital tumor suppressor gene that controls the G1 checkpoint; when unphosphorylated, it ties up regulatory proteins like E2F or Myc to block cell proliferation.

E2F

A transcription factor that promotes gene expression to advance the cell cycle once freed from the inhibitory binding of the Rb protein.

Growth Factors

External signaling proteins/hormones that promote tissue growth and repair by binding to surface receptors, accelerating the progression of the cell cycle.

Cyclins & CDKs

Cyclin-dependent protein kinases and their associated cyclins which form complexes required to phosphorylate target proteins, driving the cell through checkpoints.

p53 Protein

A tumor suppressor protein acting at the G1 checkpoint as a "quality control" that checks for damaged DNA, halting the cycle for repairs or triggering apoptosis if unrepairable.

Proto-oncogenes

Normal genes encoding positive regulatory proteins that stimulate synthesis of proteins to accelerate progression through the cell cycle or prevent apoptosis.