BSC1010 Chapter 7

Passive transport

Movement of small molecules across the membrane without energy input, may require transport proteins

Active transport

Movement of small molecules across the membrane that requires both energy and a transport protein

Bulk transport

Movement of large molecules using exocytosis or endocytosis

Phospholipids

Molecules with hydrophobic and hydrophilic regions

Phospholipid bilayer

Structure formed by phospholipids with hydrophobic tails inside and hydrophilic heads outside

Amphipathic proteins

Most membrane proteins with both hydrophilic and hydrophobic regions

Hydrophilic protein regions

Oriented toward cytosol and extracellular fluid

Fluid mosaic model

Describes membrane as a mosaic of proteins in a fluid bilayer of phospholipids

Membrane bonding

Held together mainly by weak hydrophobic interactions

Lateral movement

Most lipids and some proteins can move sideways in membrane

Flip-flop movement

Rare lipid movement across the bilayer

Cholesterol

A membrane component in animal cells that affects fluidity based on temperature

Membrane fluidity

Required for proper function, affects permeability and protein movement

Peripheral proteins

Bound to the membrane surface

Integral proteins

Penetrate the hydrophobic membrane core

Transmembrane proteins

Integral proteins that span the membrane

Hydrophobic regions of integral proteins

Consist of nonpolar amino acids often coiled into α helices

Membrane protein functions

Transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, attachment to cytoskeleton and ECM

Glycolipids

Carbohydrates bonded to lipids

Glycoproteins

Carbohydrates bonded to proteins

Selective permeability

Membrane property where some substances cross more easily than others

Hydrophobic molecules

Dissolve in the lipid bilayer and pass through the membrane rapidly

Examples of hydrophobic molecules

Hydrocarbons, CO2, and O2 pass easily through the membrane

Hydrophilic interior

Impairs passage of polar molecules through the membrane

Transport proteins

Help hydrophilic substances cross the membrane faster

Channel proteins

Provide a hydrophilic tunnel for molecules or ions to cross

Carrier proteins

Bind to molecules and change shape to shuttle them across the membrane

Aquaporins

Channel proteins that greatly increase the rate of water passage

Transport protein specificity

Transport proteins only move specific substances

Glucose carrier proteins

Transport glucose but not fructose (a structural isomer)

Diffusion

Movement of particles so they spread out evenly in available space

Concentration gradient

Region where the density of a chemical substance increases or decreases

Passive transport

Diffusion across a membrane that requires no energy from the cell

Osmoregulation

Control of solute concentration and water balance in organisms

Turgor pressure

Pressure exerted by a plant cell wall against incoming water in a hypotonic solution

Turgid

Very firm plant cell in a hypotonic solution, healthy state for most plant cells

Flaccid

Limp plant cell in an isotonic solution; plant wilts

Plasmolysis

Cell shrivels and membrane pulls away from wall in hypertonic solution

Facilitated diffusion

Passive transport where proteins help move molecules across membrane

Ion channels

Proteins that facilitate transport of ions

Gated channels

Ion channels that open or close in response to a stimulus

Carrier proteins in passive transport

Move substances down concentration gradient with no energy input

Active transport

Movement of solutes against concentration gradient using energy

ATP

Energy source used in active transport

Carrier proteins in active transport

All active transport proteins are carrier proteins

Membrane potential

Voltage across a membrane caused by ion distribution

Charge difference

Inside of the cell is negative relative to the outside

Electrochemical gradient

Combined forces of ion concentration and membrane potential driving diffusion

Electrogenic pump

Transport protein that generates voltage across a membrane

Sodium-potassium pump

Main electrogenic pump in animals

Proton pump

Main electrogenic pump in plants, fungi, and bacteria

Cotransport

Active transport of one solute indirectly drives transport of another

Exocytosis

Transport vesicles fuse with membrane to release contents outside the cell

Exocytosis example

Pancreatic cells secrete insulin via exocytosis

Endocytosis

Process of taking macromolecules into the cell using vesicles

Phagocytosis

"Cellular eating" type of endocytosis

Pinocytosis

"Cellular drinking" type of endocytosis

Receptor-mediated endocytosis

Endocytosis triggered by binding of specific molecules to receptors

Phagocytosis

Cell engulfs a particle by extending pseudopodia and enclosing it in a food vacuole

Pinocytosis

Cell "gulps" extracellular fluid into tiny vesicles

Receptor-mediated endocytosis

Vesicle formation is triggered by solute binding to specific receptors