Chapter 5
Membrane Structure & Function
Concept 5.1 Cellular Membranes are Fluid Mosaics
● The plasma membrane is selectively
permeable, allows some substances to cross it more easily than others.
● The ability of the cell to discriminate in its chemical exchanges with its environment is critical to life.
● A phospholipid is amphipathic or having both a hydrophilic and hydrophobic region.
● Phospholipids and proteins are arranged according to the fluid mosaic model, the membrane is a fluid structure of proteins
embedded in a double or bilayer of phospholipids.
● The plasma membrane is held together by hydrophobic interactions , allowing the phospholipids to shift laterally.
Concept 5.1 Cellular Membranes are Fluid Mosaics
● Plasma membranes stay fluid until temperature decreases.
● Cholesterol embedded in the plasma membrane hinders solidification of the membrane.
● Integral proteins penetrate the hydrophobic interior and peripheral
proteins are more like appendages loosely bound to the interior or exterior surface.
● Membrane carbohydrates, such as
glycolipids or glycoproteins, vary from species to species which act as markers, ie A,B, AB,O blood types.
Concept 5.2 Membrane Structure Results in Permeability
● The plasma membrane has the ability to
regulate transport across cellular borders which is an essential function of the plasma
membrane due to its structure.
● Nonpolar substances which are hydrophobic can cross the membrane easily.
● Polar molecules, including water, do not cross very rapidly.
● Hydrophilic substances can avoid contact with the interior by passing through transport
proteins, ie channel proteins and aquaporins.
● Carrier proteins change shape with their transport substances to shuttle them through the membrane.
Concept 5.3 ATP Passive Transport is Diffusion
● Diffusion is the movement of molecules of any substance so that they spread out evenly into the available space.
● The law of diffusion is that unless acted on by another force, substances move from high to low concentration or down its concentration gradient, region along which the density of a chemical increases or decreases.
● Diffusion of a substance across a biological membrane is passive transport because the cell does not expend chemical energy.
● Osmosis is the movement of water across a plasma membrane which leads to tonicity, the ability of a
surrounding solution to cause a cell to gain or lose water.
Concept 5.3 ATP Passive Transport is Diffusion
● Cells, without cell walls, immersed in isotonic environments will have no net movement of water across the membrane.
● Cells, without cell walls, immersed in hypertonic solutions will lose water and shrivel.
● Cells, without cell walls, immersed in hypotonic solutions will gain water and expand.
● Some cells, without cell walls, have adaptations to control solute concentration and water balance called osmoregulation.
● In cells with cell walls, the cell wall can better maintain shape in hypertonic and hypotonic solutions.
● The cell wall of cells in hypotonic solutions will become turgid and in hypertonic solutions undergo plasmolysis.
Concept 5.4 Active Transport Uses Energy
● In active transport, a membrane protein or pump moves solutes against the concentration gradient by using cellular energy.
● Active transport enables cells to maintain internal concentrations of small solutes that differ from concentrations in its environment, ie
sodium-potassium pump.
● Two forces drive the diffusion of ions across the membrane: a chemical force or the ion’s
concentration gradient and an electrical force of the effect of membrane potential.
● Cotransport is the transport of a substance due to the action of a single-ATP powered pump.
Concept 5.5 Bulk Transport Uses Vesicles
● Large molecules are less easily transported across the membrane.
● A transport vesicle that secretes certain biological molecules across the membrane is exocytosis.
● In endocytosis, a cell takes in biological molecules by forming vesicles.
● Ligands, chemicals that bond to
cell-surface receptors, drive vesicle
transport.
Concept 5.6 Converting Signals
● Cell communication has some universal mechanisms in cellular regulation.
● Yeast cells identify mates through chemical signaling which cause the two cells to grow towards each other.
● A signal transduction pathway is a process from which a cell responds to a chemical signal after reception.
● Local regulators are chemical signals that influence cells in the local vicinity, ie growth factors or synaptic signaling.
● Long distance signaling in plants and animals involve chemicals called hormones.
● The ability of a cell to respond to a hormone requires a specific receptor.
Concept 5.6 The Three Stages of Cell Signaling
● Cell Signaling involves 3 stages:
○ Reception involves the signaling molecule binding to a receptor protein inside or outside the cell.
○ Transduction binding of signaling molecule to receptor causes a change in the receptor protein initiating transduction.
○ Response is the transduced signal that triggers a specific cellular response
Concept 5.6 Receptors and Reception
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● A ligand is a molecule that specifically binds toa receptor protein causes a change in the protein’s shape, in many cases activating the receptor.
● Upon activation the receptor protein may interact with other cellular molecules or
aggregation of two or more receptors molecules that induce signal transduction, ie G
protein-coupled receptors and tyrosine-kinase receptors.
● Most water soluble ligands bind to surface
receptors whereas water insoluble ligands bind to intracellular receptors ie cytoplasmic or nuclear receptors.
Concept 5.6 G protein-coupled receptor
Concept 5.6 tyrosine kinase receptors
Concept 5.6 ion channel receptor
Concept 5.6 intracellular receptors
Concept 5.6 Transduction is a Party Event
● One benefit of multiple steps in a transduction pathway is the possibility of amplifying the signal.
● In a transduction pathway the signal activated receptor activates a molecule which activates
another molecule until the protein that produces the final cellular response is activated.
● Many relay molecules in transduction pathways are protein kinases that initiate phosphorylation
cascades.
● Protein phosphatases help remove phosphate groups or dephosphorylate.
● Second messengers are small water-soluble molecules that easily spread through the cell, ie cyclic AMP, Ca2+
Concept 5.6 One Complicated Transduction Pathway
Concept 5.6 Apoptosis
● During apoptosis or “cell suicide” cellular agents chop up DNA and fragment
organelles leading to vesicles digesting the cell.
● Two key genes ced-3 and ced-4 code for Ced-3 and Ced-4 code which are two
important apoptosis proteins also known as caspases.
● Ced-9 is a protein in the outer
mitochondrial membrane serves as a master apoptosis regulator.
● The apoptosis signal can come from outside the cell or inside the cell.
