Cell Biology Final

What are the main functions of the plasma membrane?

It gives structure, acts as a selective barrier, controls transport, helps with signaling, lets the cell interact with the environment, anchors proteins for reactions, and helps with energy processes.What does “fluid mosaic model” mean?

What does “fluid mosaic model” mean?

“Mosaic” = many components (lipids, proteins, carbs, cholesterol).
“Fluid” = these components move and aren’t locked in place.

What experiment showed the membrane is fluid?

FRAP — bleach a spot and the fluorescence comes back because lipids move in.

What experiment showed proteins are embedded in the membrane?

Freeze-fracture — splitting the membrane reveals protein bumps.

What does “amphipathic” mean?

A molecule has both hydrophilic and hydrophobic parts.

Why do phospholipids form bilayers?

The heads like water, the tails don’t, so the heads face out and the tails hide inside.

What are the three major lipid types in membranes?

Phosphoglycerides, sphingolipids, and sterols (like cholesterol).

How does cholesterol affect membrane fluidity?

It stops the membrane from getting too rigid in the cold and too fluid in the heat.

Saturated vs. unsaturated fatty acids: which increases fluidity?

Unsaturated fatty acids (because the kinks prevent tight packing).

What does membrane asymmetry mean?

Each leaflet has different lipids and proteins arranged in different ways.

Which lipid is mostly on the inner leaflet?

Phosphatidylserine (PS) — it has a negative charge.

Which lipids are mostly on the outer leaflet?

Phosphatidylcholine (PC), sphingomyelin (SM), and glycolipids.

How do we know carbohydrates face outward?

Labeling and enzyme experiments only modified the outside of cells.

What are the three types of membrane proteins?

Integral, peripheral, lipid-anchored.

What is the key trait of integral proteins?

They span the bilayer and have hydrophobic parts that sit in the membrane.

How can you remove peripheral proteins?

High salt (because they attach through non-covalent interactions).

What is special about lipid-anchored proteins?

They are covalently attached to a lipid on either the cytosolic side or by a GPI anchor outside.

What structure do most transmembrane proteins use?

Alpha helices with ~20 hydrophobic amino acids.

Which membrane proteins use β-barrels?

Porins in mitochondria, chloroplasts, and gram-negative bacteria.

What does a hydrophobicity plot show?

Places in the protein sequence that are hydrophobic enough to be transmembrane regions.

Where are carbohydrates found on membrane molecules?

Only on the exoplasmic (outer) side.

What do membrane carbohydrates do?

Help with recognition, signaling, and interactions (e.g., blood groups).

What molecules cross the membrane MOST easily?

Gases (O₂, CO₂, N₂) — they diffuse right through.

What small molecules can cross but slower?

Water, ethanol, glycerol (small uncharged polar).

What molecules CANNOT cross without help?

Large polar molecules (like glucose) and all ions (Na⁺, K⁺, Ca²⁺, Cl⁻).

When do molecules need ATP?

Only when moving against their gradient (low → high concentration).

What are the 3 main types of transport proteins?

Channels, transporters, pumps.

What makes channels selective?

The size + charge of the pore formed by specific amino acids.

What makes transporters selective?

Their binding pocket — they must bind the molecule to move it.

What makes pumps different?

They use ATP to move ions against their gradient.

Do channels need ATP?

No — they only move things down the gradient.

Are channels fast or slow?

Very fast (millions of ions per second).

What are the 3 types of gated channels?

Voltage-gated, ligand-gated, mechanosensory-gated.

How do transporters move molecules?

They bind, change shape, and release (conformational cycling).

Can transporters be passive?

Yes — facilitated diffusion (no ATP, moves with gradient).

Can transporters be active?

Yes — when coupled to ATP or another gradient (symport/antiport).

What are the 3 modes of transporter movement?

Uniport, symport, antiport.

What pump sets up the main ion gradients?

The Na⁺/K⁺ ATPase (3 Na⁺ out, 2 K⁺ in).

Why is the Na⁺/K⁺ pump important?

It establishes gradients used for electrical signals and active transport.

What is an example of a P-type pump?

Na⁺/K⁺ ATPase or H⁺/K⁺ pump (stomach).

What is an ABC transporter example?

MDR1 — pumps out toxic drugs (chemotherapy resistance)

What does a patch clamp experiment measure?

Open/closed states of single ion channels by measuring current.

What does an upward current trace mean?

The channel opened.

What does a downward trace mean?

The channel closed.

What transporter is defective in cystic fibrosis?

CFTR (a Cl⁻ channel)

Why does CF cause thick mucus?

Cl⁻ can’t leave → salt stays inside cells → less water secreted.

How do drugs like digoxin affect the heart?

They inhibit the Na⁺/K⁺ pump → Na⁺ increases → Ca²⁺ increases → stronger

What two things maintain resting potential?

Na⁺/K⁺ pump + K⁺ leak channels.

What ion contributes MOST to resting potential?

K⁺ leaving the cell through leak channels.

Where is Na⁺ high?

Outside the cell.

Where is K⁺ high?

Inside the cell.

What channel opens first in an action potential?

Voltage-gated Na⁺ channels (Na⁺ rushes in → depolarization).

What channel opens second?

Voltage-gated K⁺ channels (K⁺ rushes out → repolarization).

What resets the gradients after firing?

Na⁺/K⁺ ATPase pump.

What is the central dogma of molecular biology?

DNA → RNA → protein.

Where does transcription happen?

In the nucleus.

Where does translation happen?

In the cytoplasm on ribosomes.

What is the role of tRNA?

It brings the correct amino acids to the ribosome by matching codons.

Where is genetic information stored in eukaryotic cells?

In DNA inside the nucleus.

How is DNA packaged?

DNA wraps around histones and condenses into chromosomes.

Why does DNA need to be tightly packed?

To fit inside the nucleus and to help control gene access.

What enzyme reads DNA to make RNA?

RNA polymerase.

What is the genetic code?

Sets of 3 nucleotides (codons) that each code for an amino acid.

Where are proteins made?

On ribosomes in the cytoplasm.

How is DNA replication regulated?

DNA polymerase proofreading, p53, and cell cycle checkpoints.

How is transcription regulated?

Transcription factors and chromatin structure (whether DNA is open or packed).

Name one RNA processing regulation mechanism.

Alternative splicing or controlling mRNA stability.

How is translation regulated?

Cells control whether ribosomes can bind to the mRNA.

Do different eukaryotic cells in one organism have different DNA?

No — they all have the same DNA.

If cells have the same DNA, why do they look and act different?

Because they turn on different genes (different gene expression).

What stays constant in all eukaryotic cells?

Same genome, same chromosome number, same basic transcription/translation machinery.

What does the first law of thermodynamics say?

Energy can’t be created or destroyed, only transformed.

Why is the first law important for cells?

Cells survive by converting energy (ATP, food, heat), not creating it.

What does the second law of thermodynamics say?

Systems tend toward disorder (entropy increases).

Why is the second law important in cell biology?

Cells use energy to stay ordered, and their reactions release heat to increase overall entropy.

What equation do we use to calculate free energy change?

ΔG = ΔG° + RT ln(Q).

What does ΔG tell you?

Whether a reaction is favorable under actual cellular conditions.

What does it mean if ΔG < 0?

The reaction is favorable/spontaneous.

What does Q represent in the ΔG equation?

The ratio of products to reactants in the cell right now.

How can a cell turn an unfavorable reaction into a favorable one?

Change concentrations so ΔG becomes negative.

What is the main strategy cells use to drive unfavorable reactions?

Coupling to a favorable reaction like ATP hydrolysis.

How does manipulating concentrations help?

Keeping reactants high and products low makes ΔG negative.

Do enzymes change ΔG?

No — they lower activation energy to make reactions faster.

Why do pathways use multiple small steps?

Breaking one big reaction into smaller ones makes each step easier to make favorable.

How do enzymes affect reactions?

They lower activation energy and speed up reactions.

What is allosteric regulation?

Molecules bind away from the active site to turn enzymes on or off.

What is feedback inhibition?

The end product of a pathway shuts down the first enzyme.

What is covalent enzyme regulation?

Adding/removing phosphate groups (phosphorylation).

What do competitive inhibitors do?

Compete for the active site; increase Km; Vmax stays the same.

What do noncompetitive inhibitors do?

Bind elsewhere; decrease Vmax; Km stays the same.

What do uncompetitive inhibitors do?

Bind only to ES complex; decrease both Vmax and Km.

What is Vmax?

The maximum rate when the enzyme is fully saturated.

What is Km?

Substrate concentration at ½ Vmax; measures affinity.

What does a low Km mean?

High affinity for substrate.

What is the Lineweaver–Burk plot used for?

Linear form of enzyme kinetics; helps identify inhibition types.

What happens to Vmax and Km with competitive inhibition?

Km ↑, Vmax stays the same.

What happens with noncompetitive inhibition?

Vmax ↓, Km stays the same.

What happens with uncompetitive inhibition?

Both Vmax ↓ and Km ↓.