DE Bio Chapter 5 & 6

Johnson, 2025-2026

Fluid Mosaic Model

The model describing cell membrane structure as a flexible layer with embedded proteins that can move laterally

Selectively permeable

Property of membranes allowing only certain substances to pass through while blocking others

Phospholipid bilayer

Two layers of phospholipids forming the basic structure of cell membranes

Integral proteins

Membrane proteins that penetrate the hydrophobic interior of the lipid bilayer

Peripheral proteins

Membrane proteins loosely bound to the membrane surface without embedding in the lipid bilayer

Glycoproteins

Proteins with carbohydrate chains attached that function in cell recognition and signaling

Glycolipids

Lipids with carbohydrate chains attached found on the extracellular surface of membranes

Aquaporins

Membrane channel proteins that transport water molecules exclusively allowing billions per second

Hydrophilic

Water-loving property of phosphate heads that interact with aqueous environments

Hydrophobic

Water-fearing property of fatty acid tails that avoid water and form the membrane interior

Amphipathic

Having both hydrophilic and hydrophobic regions like phospholipids and most membrane proteins

Lateral movement

The rapid shifting of lipids and some proteins sideways within the membrane layer

Membrane fluidity

The ability of membrane components to move and flow affecting membrane function

Unsaturated fatty acids

Fatty acids with kinks that prevent tight packing and maintain membrane fluidity at lower temperatures

Cholesterol effects on fluidity

Cholesterol restrains phospholipid movement at high temperatures and prevents solidification at low temperatures

Transmembrane proteins

Integral proteins that span the entire membrane from one side to the other

Channel proteins

Transport proteins providing hydrophilic passages for specific solutes across membranes

Carrier proteins

Transport proteins that shuttle substances across membranes by changing shape

Signal transduction

Process where membrane receptors relay external signals to the cell interior

Cell-to-cell recognition

Function of glycoproteins serving as identification tags recognized by other cells

Intercellular joining

Membrane proteins connecting adjacent cells through gap junctions or tight junctions

Cytoskeleton attachment

Connection of membrane proteins to internal cytoskeleton for structural support

Extracellular matrix

Network of proteins and carbohydrates outside cells that some membrane proteins attach to

Membrane carbohydrates

Short branched sugar chains of fewer than 15 units on the extracellular membrane surface

Blood type markers

Variations in carbohydrate parts of glycoproteins on red blood cells determining A B AB O types

Membrane synthesis

Process where membrane proteins and lipids are made in ER and modified in Golgi

Exocytosis

Fusion of vesicles with cell membrane releasing contents outside and adding membrane components

Nonpolar molecules

Hydrophobic molecules like O2 and CO2 that easily dissolve in and cross lipid bilayers

Transport proteins

Proteins helping hydrophilic molecules cross membranes including channels and carriers

Diffusion

Movement of molecules from high to low concentration down a concentration gradient

Osmosis

Diffusion of water molecules only through membranes via aquaporins

Hypertonic solution

Solution with higher solute concentration than cells causing water to leave cells

Hypotonic solution

Solution with lower solute concentration than cells causing water to enter cells

Isotonic solution

Solution with equal solute concentration to cells resulting in no net water movement

Plasmolysis

Shrinking of cells when placed in hypertonic solutions as water leaves

Passive transport

Movement of substances down concentration gradients through proteins without ATP

Facilitated diffusion

Passive transport of large or charged molecules through channel or carrier proteins

Ion channels

Channel proteins specifically transporting ions across membranes

Gated channels

Ion channels that open or close in response to specific stimuli

Active transport

Movement of substances against concentration gradients through carrier proteins requiring ATP

Sodium-potassium pump

Active transport protein pumping 3 Na+ out and 2 K+ in using ATP

Membrane potential

Voltage across membrane from unequal distribution of ions ranging from -50 to -200 millivolts

Electrochemical gradient

Combined effect of concentration gradient and electrical force on ion movement

Electrogenic pump

Transport protein generating voltage across membrane like sodium-potassium pump

Proton pump

Main electrogenic pump in plants fungi and bacteria actively transporting H+ out

Cotransport

Coupling of downhill diffusion of one solute to uphill transport of another substance

Phagocytosis

Cell eating form of endocytosis where cell membrane engulfs large particles

Pinocytosis

Cell drinking form of endocytosis where cell membrane engulfs liquid droplets

Receptor-mediated endocytosis

Specific endocytosis where ligands bind receptors before membrane invagination

LDL receptors

Membrane receptors binding low-density lipoproteins for cholesterol uptake

Local signaling

Cell communication between nearby cells through direct contact or paracrine signaling

Long-distance signaling

Cell communication using hormones traveling through circulatory system

Reception

First stage of cell signaling where target cell detects signaling molecule

Transduction

Second stage converting signal to form that produces cellular response through relay molecules

Response

Third stage producing specific cellular activity from signal transduction pathway

Ligand

Molecule that specifically binds to receptor protein during cell signaling

G-protein-coupled receptor

Cell surface receptor working with G protein that binds GTP

G protein

Protein binding energy-rich GTP molecule that activates enzymes in signaling

Ligand-gated ion channel

Receptor with gate opening or closing to allow ion passage when ligand binds

Intracellular receptors

Receptor proteins in cytoplasm or nucleus for hydrophobic signaling molecules

Protein kinase

Enzyme transferring phosphate groups from ATP to proteins during signal transduction

Protein phosphatase

Enzyme removing phosphate groups from proteins to reverse phosphorylation

Second messengers

Small water-soluble molecules like cAMP and Ca2+ spreading signals through cell by diffusion

Cyclic AMP

Common second messenger molecule spreading signals rapidly through cells

Signal amplification

Increase in signal strength through cascades of molecular interactions

Transcription regulation

Response to signaling involving changes in gene expression

Concentration gradient

Difference in concentration of substance between two areas driving diffusion

ATP requirement

Energy from ATP hydrolysis needed for active transport against gradients

Membrane solidification

Loss of fluidity when phospholipids pack closely at low temperatures

Alpha helix structure

Coiled structure of nonpolar amino acids in hydrophobic regions of integral proteins

Hydrophilic channels

Passages in membrane proteins allowing hydrophilic substances to cross

Enzymatic activity

Function of membrane proteins catalyzing specific chemical reactions

Vesicle fusion

Process where vesicle membrane merges with cell membrane during exocytosis

Endocytosis

Process where cell membrane invaginates to bring substances into cell

Cholesterol in blood

Transported as low-density lipoproteins that bind receptors for cell uptake

Receptor activation

Change in receptor shape when signaling molecule binds

Relay molecules

Molecules in signal transduction pathway passing signal from one to next

Phosphorylation cascade

Series of protein phosphorylation events amplifying and transducing signals

Dephosphorylation

Removal of phosphate groups by phosphatases to deactivate proteins

Calcium ions

Common second messengers involved in muscle contraction and other responses

Cytoplasmic activities

Cellular responses regulated by signaling pathways without affecting transcription

Membrane protein mobility

Movement of proteins within membrane some mobile others held immobile

Motor proteins

Proteins driving directed movement of membrane proteins along cytoskeletal fibers

Membrane adaptations

Evolutionary changes in lipid composition for different temperature environments

Glucose transporter

Highly specific carrier protein that accepts glucose but rejects fructose

Voltage across membrane

Electrical potential energy from separation of charges creating membrane potential

Anion distribution

Unequal distribution of negatively charged ions across membrane

Cation distribution

Unequal distribution of positively charged ions across membrane

Chemical force

Driving force from ion concentration gradient

Electrical force

Driving force from membrane potential on charged particle movement

Proton gradient

Concentration difference of H+ ions used for ATP synthesis and cotransport

Chemiosmosis

ATP synthesis driven by proton gradient across membrane

Coupled transport

Active transport using energy from one substance moving down gradient

Amino acid uptake

Active transport of amino acids into plant cells driven by proton gradient

Sugar uptake

Active transport of sugars into cells using cotransport mechanisms

Vesicle contents

Molecules inside transport vesicles released during exocytosis

Membrane invagination

Inward folding of cell membrane during endocytosis

Coated pit

Depression in membrane with receptors for receptor-mediated endocytosis

Cholesterol synthesis

Use of cholesterol from LDL for making membranes and steroids

LDL particles

Low-density lipoproteins carrying cholesterol through bloodstream