ap bio cell membrane study guide notes

phospholipid bilayer

• 2 layers of phospholipids

• Heads = hydrophilic (water-loving)

• Tails = hydrophobic (water-fearing)

• Makes the plasma membrane

• Selectively permeable

• Holds proteins, lipids, and carbs

fluid mosaic model

• Membrane is fluid with mobile proteins embedded in a phospholipid bilayer

• Phospholipids form a hydrophobic barrier that repels water and hydrophilic molecules

selective permeability of cell membrane

• The membrane controls which molecules enter and exit

• Phospholipid layer forms a hydrophobic barrier

• Transport proteins regulate which molecules can pass through

simple diffusion

• moves high to low concentration

• no energy is required

• possible bc small, nonpolar molecules can pass through phospholipid bilayer

transport of polar molecules

• can’t pass through the bilayer by simple diffusion

• polar or hydrophilic, so the membrane repels them

• rely on transport molecules and proteins to enter/exit cell

integral proteins

• Permanently embedded in the phospholipid bilayer

• Have hydrophobic regions to stay within the membrane

• Made by ribosomes on the rough ER

• Structure includes α-helices (spiral) and β-barrels (pores)

• Adapted to serve specific roles: pore, pump, receptor, or carrier

transmembrane proteins

• Span the entire cell membrane, from extracellular to intracellular sides

• Often integral proteins or closely affiliated with them

• Function in transport, signaling, and cell adhesion

channel proteins

• Proteins that allow materials to move into/out of the cell

• Can function via passive or active transport

aquaporins

integral channel proteins that facilitate the rapid transport of water across a cell membrane 

g-protein

• Molecular switches: GTP = ON, GDP = OFF

• Activated by GPCR

• Change shape to start signaling inside the cell

• GTPase turns them off

sodium potassium pump

• Uses ATP to move 3 Na⁺ out and 2 K⁺ in

• Maintains ion concentration gradients

• Creates membrane potential needed for nerve signals and muscle contractions

glucose transponders

proteins that facilitate glucose across the membranes 

anchor proteins

• Attach proteins to the inner or outer membrane (often via lipid anchor)

• Ensure proper positioning of membrane proteins

• Help with signal transduction, cell-cell adhesion, membrane protection, and protein trafficking

• Can be integral or peripheral proteins

peripheral proteins

• Loosely attached; do not enter lipid bilayer

• Extracellular: cell recognition/communication

• Cytoplasmic: link cytoskeleton, enzymes, signaling

• Functions: signaling, structure, enzymatic activity, vesicle transport

• Easily detached

Carbohydrate Receptors

• Bind carbs on cell surfaces (“sugar sensors”)

• Functions:, cell recognition & communication, Immune recognition of pathogens, Pathogen entry , and Intracellular trafficking, Development & differentiation

• Important for: immune response and tissue differentiation

ligands

• Bind specific protein receptors

• Can be proteins or non-proteins (e.g., hormones, neurotransmitters)

• Induce conformational changes → control enzymatic activity, cell signaling, drug action

• Examples: insulin, oxygen (erythrocytes), neurotransmitters, antibodies

endocytosis

• the cell forms new vesicles from the plasma membrane

• allows for the cell to take in macromolecules

exocytosis

bulk transport out of the cell through the membrane

phagocytosis

• “cell eating,” brings in large particles or cells. 

• cell wraps a pseudopodia around a solid particle to bring into the cell

pinocytosis

•  “cell drinking,” brings in small droplets of extracellular fluid.

• takes in small droplets of extracellular fluid within small vesicles

Receptor-Mediated Endocytosis

• A specific process where ligand proteins bind to receptors on the cell surface.

• Usually occurs in coated pits that pinch off into the cytoplasm.

Cell surface receptors

Binds specific extracellular molecules to transmit signals into the cell to trigger an intracellular response.

Cell surface identity markers

• They’re usually glycoproteins that are on the outer membrane.

• Help cells recognize each other (important for the immune system).

• Allow the body to distinguish self from non-self.

enzyme

Carry out chemical reactions at the membrane surface.

-ase suffix

Cell adhesion proteins

• proteins that help cells stick to each other or to the extracellular matrix.

• Maintain tissue structure, cell positioning, and cell communication.

Attachments to the cytoskeleton

• Anchor the cell membrane to the internal cytoskeleton fibers.

• Help maintain cell shape, stability, and organization of membrane proteins.

Selective transport channel

• Membrane proteins that allow only specific molecules or ions to pass through.

Anchoring Proteins (phosphatidylinositol )

• Membrane proteins are attached to phosphatidylinositol (a lipid in the inner membrane).

• Anchor the membrane to the cytoskeleton or extracellular structures.

• Help position proteins and transmit signals inside the cell.

Anchors – receptors for signals

• Membrane proteins that bind signaling molecules (like hormones or neurotransmitters)

• Act as anchors by linking to the cytoskeleton or other membrane structures

• Transmit signals from outside the cell to the inside.

Pores – B pleated sheets

• Made of β-pleated sheets.

• Create openings for molecules to move across the membrane.

Diffusion of ions through channels

• Ions move through ion channels, which are protein tunnels in the membrane.

• These channels let ions pass because they are hydrophilic, unlike the lipid bilayer.

facilitated diffusion

• helps large, polar, or charged molecules move across membrane

• Passive: no ATP required

• Can saturate: transport rate maxes out when all carriers are occupied

• Example application: adding phosphate to blood donations helps RBCs produce ATP and prolong shelf life

active transport

• low to high concentration

• Requires ATP energy

• Transfer proteins are used to pump particles from low to high concentration.

Proton Pump

pumps H+ (hydrogen ions) across the membrane

Coupled Channels

 one molecule moves down its gradient to help another move against its gradient

osmosis

diffusion of water through membrane

• moves from high to low concentration

maintains cell homeostasis

isotonic

when a solution is balanced

hypotonic

more water than solutes

hypertonic

more solutes than water

concentration gradient

a boundary with varying levels of concentration 

active transport

• Inside of cell: negatively charged

  • outside: more positive → membrane potential

• Ions move due to:

  • Chemical force: concentration gradient

  • Electrical force: voltage gradient attracts opposite charges

• Drives ion movement (e.g., Na⁺, K⁺) across the membrane

electrogenic pump

• Use ATP to move ions → create electrochemical gradient (concentration + charge)

• Inside cell: negative → attracts positive ions, repels negative ions

• Examples: Sodium-Potassium Pump, Calcium Pump, Proton Pump

cotransport

• Indirect active transport: ATP pump moves one solute → drives movement of another solute against its gradient

• Analogy: like water pumped uphill doing work as it flows back down

• Generates: electrochemical gradient → source of cellular potential energy