Cell Structure, Membranes, Signaling, and Viruses: Key Concepts for Biology

Cell size

As a spherical cell gets larger, volume increases faster than surface area, so surface area-to-volume ratio decreases and exchange becomes less efficient.

Surface area-to-volume ratio

As cell size increases, surface area-to-volume ratio decreases.

Universal cell features

All cells have DNA, ribosomes, cytosol, and a plasma membrane.

Prokaryotic cell

Cell without a nucleus; DNA is located in a nucleoid region.

Eukaryotic cell

Cell with a nucleus and membrane-bound organelles.

Nucleoid

Region where DNA is located in a prokaryotic cell.

Plant-only structures

Chloroplasts, cell wall, and plasmodesmata.

TEM

Transmission electron microscope; used to see detailed internal structures at very high resolution.

SEM

Scanning electron microscope; used mainly to view surface structures.

Magnification

How much larger an image appears.

Resolution

Ability to distinguish two nearby objects as separate.

Nucleus

Stores most cellular DNA and is the site of transcription.

Mitochondrion

Site of aerobic cellular respiration and major ATP production.

Chloroplast

Site of photosynthesis in plant cells.

Lysosome

Contains digestive enzymes that break down and recycle cellular materials.

Smooth ER

Major site of phospholipid synthesis.

Rough ER

Has ribosomes attached; synthesizes proteins destined for secretion, membranes, or certain organelles.

Golgi apparatus

Modifies, sorts, and packages proteins and lipids.

Transmembrane protein

Protein that spans the entire phospholipid bilayer.

Peripheral protein

Protein associated with one surface of the membrane without spanning the bilayer.

Lipid-anchored protein

Protein attached to a lipid that inserts into the membrane.

Glycoprotein

Protein with a carbohydrate attached.

Glycolipid

Lipid with a carbohydrate attached.

Secreted/transmembrane protein pathway

Rough ER → vesicle → cis-Golgi → Golgi processing/sorting → vesicle → plasma membrane.

Intermediate filaments

Cytoskeletal elements that resist tension and mechanical stress.

Microfilaments

Cytoskeletal elements involved in cell shape changes and movement.

Microtubules

Cytoskeletal elements involved in movement, organization, and transport.

Endosymbiotic theory

Explains how mitochondria and chloroplasts likely originated from ancient bacteria living inside ancestral cells.

Strong evidence for endosymbiosis

Mitochondria and chloroplasts contain their own DNA and bacterial-like ribosomes.

Fluid mosaic model

Describes the plasma membrane as a fluid phospholipid bilayer containing proteins and other molecules.

Membrane fluidity

The ability of membrane components to move within the bilayer.

Cis double bond

Creates a kink in a fatty-acid tail, prevents tight packing, and increases membrane fluidity.

Saturated fatty-acid tails

Pack tightly and decrease membrane fluidity.

Shorter fatty-acid tails

Increase membrane fluidity.

Professor's cholesterol rule

For this exam, increasing cholesterol is treated as decreasing membrane fluidity.

Diffusion

Net movement of particles from high concentration to low concentration.

Osmosis

Diffusion of water across a selectively permeable membrane.

Simple diffusion

Movement directly through the phospholipid bilayer without a transport protein.

Facilitated diffusion

Movement down a concentration gradient through a membrane protein without directly using ATP.

Primary active transport

Uses ATP directly to move substances against their concentration gradient.

Secondary active transport

Uses energy stored in an ion gradient to move another substance against its gradient.

Symport

Two substances move in the same direction through a transporter.

Antiport

Two substances move in opposite directions through a transporter.

Aquaporin

Membrane channel that increases water permeability.

Hypotonic solution

Solution with lower solute concentration outside the cell; water enters the cell.

Hypertonic solution

Solution with higher solute concentration outside the cell; water leaves the cell.

Isotonic solution

Solution with equal solute concentration; no net water movement.

RBC in hypotonic solution

Water enters the RBC, causing it to swell and potentially burst.

Plant cell in hypertonic solution

Water leaves the cell and the plasma membrane pulls away from the cell wall; this is plasmolysis.

Threonine transport

Moving threonine from low to high concentration requires active transport and a transport protein.

Phagocytosis

Endocytosis of large particles, such as bacteria.

Pinocytosis

Endocytosis of extracellular fluid and dissolved solutes.

Receptor-mediated endocytosis

Endocytosis triggered by a specific ligand binding to a receptor.

Endocytosis

Movement of material into the cell using membrane vesicles.

Exocytosis

Vesicle fuses with plasma membrane and releases cargo outside the cell.

Exocytosis sequence

Packaging → vesicle movement → membrane fusion → cargo release.

Tight junctions

Seal neighboring cells and prevent substances from passing between them.

Desmosomes

Strongly attach neighboring cells and resist mechanical stress.

Gap junctions

Allow communication between animal cells.

Plasmodesmata

Allow communication between plant cells.

Primary vs secondary active transport

Primary active transport uses ATP directly; secondary active transport uses an ion gradient created by active transport.

Active transport

Moves against the gradient and requires energy.

Channel

Provides a passageway.

Carrier

Binds a substance and changes shape to transport it.

Cell signaling

Process by which cells communicate using signaling molecules.

Autocrine signaling

Cell releases a signal that acts on the same cell.

Paracrine signaling

Signal acts on nearby cells.

Juxtacrine signaling

Signaling requires direct contact between cells.

Endocrine signaling

Hormones travel through the bloodstream to distant target cells.

Target cell

Cell that has the appropriate receptor for a specific signaling molecule.

Receptor specificity

Only cells with the appropriate receptor can respond to a particular signal.

Ligand

Signaling molecule that binds to a receptor.

KD

Dissociation constant used to describe binding affinity.

Low KD

High receptor-ligand affinity.

High KD

Low receptor-ligand affinity.

Lipid-soluble signaling molecule

Can cross the plasma membrane and often binds an intracellular receptor.

Hydrophilic signaling molecule

Cannot easily cross the lipid bilayer and generally binds a membrane receptor.

Vitamin D receptor

Intracellular receptor because vitamin D is lipid-soluble.

Peptide hormone receptor

Usually a plasma membrane receptor because peptide hormones are hydrophilic.

Ligand-gated ion channel

Receptor that opens or closes a pore when a ligand binds, allowing ions to cross.

Receptor protein kinase

Receptor associated with pairing and phosphorylation.

GPCR

G protein-coupled receptor that activates a G protein when a ligand binds.

GPCR activation

Ligand binds → receptor changes shape → GDP leaves Gα → GTP binds Gα → Gα activates an effector.

GPCR shutoff

Gα hydrolyzes GTP to GDP, turning the G protein off.

GDP in GPCR signaling

Inactive state of Gα.

GTP in GPCR signaling

Active state of Gα.

Second messenger

Small intracellular molecule that helps relay a signal.

cAMP

Common second messenger.

cGMP

Common second messenger involved in several signaling pathways.

NO and cGMP

NO can stimulate cGMP production, which can promote smooth muscle relaxation/dilation.

Protein kinase cascade

Series of kinases activating one another; can amplify a signal.

Signal amplification

One signaling event activates many downstream molecules.

Crosstalk

Interaction in which one signaling pathway influences another.

Agonist

Drug or molecule that produces a response similar to the natural ligand.

Antagonist

Drug or molecule that blocks the normal response of a receptor.

Virus

Noncellular infectious particle containing hereditary information and requiring a host cell to reproduce.

Capsid

Protein coat surrounding a viral genome.

Viral genome

Hereditary information of a virus; can be DNA or RNA.

Viral envelope

Lipid membrane surrounding some viruses; optional.

Virion

Complete virus particle capable of infecting a cell.