Biology: Key Themes, Cell Structure, Membranes, and Signaling

Five fundamental themes of biology

Organization, Information, Energy & Matter, Interactions, Evolution.

Example of the theme 'Organization'

Cells form tissues → organs → organ systems → organism.

Features all cells share

Plasma membrane, cytoplasm, DNA, and ribosomes.

Surface area to volume ratio limit on cell size

As cells grow, volume increases faster than surface area → inefficient exchange of materials.

Prokaryotic cells

No nucleus, smaller, no organelles.

Eukaryotic cells

Nucleus, larger, have organelles.

Function of ribosomes

Synthesize proteins.

Function of rough ER

Protein synthesis and transport.

Function of smooth ER

Lipid synthesis, detoxification, calcium storage.

Function of Golgi apparatus

Modify, package, and ship proteins and lipids.

Function of lysosomes

Digest macromolecules and worn-out organelles.

Function of vacuoles

Storage; plant vacuole maintains pressure.

Function of mitochondria

Site of cellular respiration; produces ATP.

Function of chloroplasts

Photosynthesis.

Function of peroxisomes

Break down fatty acids and detoxify.

Function of cytoskeleton

Support, movement, and transport within the cell.

Protein secretion pathway

DNA → mRNA → ribosome (RER) → Golgi → vesicle → plasma membrane → exocytosis.

Endosymbiotic theory

Mitochondria and chloroplasts originated as free-living bacteria engulfed by eukaryotes.

Evidence for endosymbiotic theory

Double membranes, circular DNA, 70S ribosomes, self-division.

Cristae, matrix, stroma, and thylakoids

Mitochondria: cristae (folds), matrix (inner fluid). Chloroplast: thylakoids (discs), stroma (fluid).

Three parts of the cytoskeleton and functions

Microtubules (support, transport), Microfilaments (movement), Intermediate filaments (strength).

Protein that makes up microtubules

Tubulin.

Composition of microfilaments

Actin (works with myosin).

Movement of cilia and flagella

Dynein motor proteins cause microtubules to slide → bending motion.

Differences between plant and animal cells

Plants: cell wall, chloroplasts, large vacuole.

Extracellular structures and junctions

Coordinate cell activities and communication (e.g., gap junctions, plasmodesmata).

Phospholipids

Hydrophilic head + hydrophobic tails.

Cholesterol in membranes

Maintains fluidity and stability.

Integral proteins

Transport, signaling, enzymes.

Carbohydrates on membranes

Cell recognition (glycoproteins/glycolipids).

Regulation of membrane fluidity

Change lipid saturation or add cholesterol.

Lipid bilayer permeability

Small nonpolar molecules (O₂, CO₂) pass through easily.

Substances that cannot pass easily

Ions and large polar molecules.

Passive transport

Movement down concentration gradient; no energy.

Active transport

Movement against gradient using energy (ATP).

Osmosis

Diffusion of water across a selectively permeable membrane.

Aquaporins

Channel proteins that speed up water transport.

Animal cell in hypotonic solution

Swells and may burst (lysis).

Plant cell in hypotonic solution

Turgid (normal state).

Isotonic solution

Equal solute concentration; no net water movement.

Hypertonic solution

More solutes outside → cell shrinks.

Electrogenic pump

Active pump that generates voltage across membrane (e.g., Na⁺/K⁺ pump).

Na⁺/K⁺ pump

Pumps 3 Na⁺ out and 2 K⁺ in → creates membrane potential.

Co-transport

Coupled movement of substances (e.g., H⁺ gradient drives sucrose uptake).

Endocytosis and exocytosis

Endocytosis = intake of large molecules; exocytosis = release.

Types of cell signaling

Direct (gap junctions), local (paracrine/synaptic), long-distance (hormones).

Lipid-soluble vs water-soluble signals

Lipid-soluble enter cell → intracellular receptor. Water-soluble bind surface receptor → cascade.

Steps of cell signaling

Reception → Transduction → Response.

Second messengers

Small molecules that amplify signals (cAMP, Ca²⁺, IP₃).

Function of kinases and phosphatases

Kinases phosphorylate (activate), phosphatases dephosphorylate (deactivate).

Phosphorylation cascade

Chain of kinase activations amplifying a signal.

Purpose of signal amplification

One ligand triggers large cellular response.

GPCR pathway example

Epinephrine → GPCR → G protein → adenylate cyclase → cAMP → response.

Receptor tyrosine kinase (RTK) example

Growth factor receptor → dimerization → phosphorylation → multiple responses.

Difference between short-term and long-term responses

Short: enzyme activation; Long: gene expression.

Signaling regulation

Feedback inhibition, receptor degradation, or deactivation by phosphatases.

Metabolism

All chemical reactions in a cell.

Two laws of thermodynamics

1st: Energy conserved. 2nd: Entropy (disorder) increases.

Entropy

Measure of disorder or randomness.

Catabolic pathways

Break down molecules → release energy.

Anabolic pathways

Build molecules → require energy.

Exergonic vs endergonic reactions

Exergonic releases energy (−ΔG); endergonic requires energy (+ΔG).

ATP power in cellular work

Transfers phosphate to molecules (phosphorylated intermediate).

Phosphorylated intermediates

Molecules activated by receiving a phosphate group.

ATP regeneration

Energy from catabolism regenerates ATP from ADP + Pi.

Enzyme

Catalyst that speeds reactions by lowering activation energy.

How enzymes work

Bind substrates at active site → form product.

Factors affecting enzyme activity

Temperature, pH, and substrate concentration.

Competitive vs noncompetitive inhibition

Competitive = inhibitor binds active site; noncompetitive = binds elsewhere.

Allosteric regulation

Molecule binds at site other than active site to activate/inhibit enzyme.

Feedback inhibition

End product inhibits an earlier enzyme in the pathway.

Cooperativity

Substrate binding increases enzyme's affinity for more substrate.