Membrane Structure and Function

Membrane Structure and Function
Chapter 5 Membrane Structure and Function

Overview: Life at the Edge
The plasma membrane is the boundary that separates the living cell from its surroundings The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Figure 7.1 Figure 7.1 How do cell membrane proteins help regulate chemical traffic?

Cellular membranes are fluid mosaics of lipids and proteins
Phospholipids are the most abundant lipid in the plasma membrane Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it For the Cell Biology Video Structure of the Cell Membrane, go to Animation and Video Files. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

WATER Hydrophilic head Hydrophobic tail WATER
Fig. 7-2 WATER Hydrophilic head Figure 7.2 Phospholipid bilayer (cross section) Hydrophobic tail WATER

In 1935, Hugh Davson and James Danielli proposed a sandwich model in which the phospholipid bilayer lies between two layers of globular proteins In 1972, J. Singer and G. Nicolson proposed that the membrane is a mosaic of proteins dispersed within the bilayer, with only the hydrophilic regions exposed to water

Phospholipid bilayer Hydrophobic regions of protein Hydrophilic
Fig. 7-3 Phospholipid bilayer Figure 7.3 The fluid mosaic model for membranes Hydrophobic regions of protein Hydrophilic regions of protein

Freeze-fracture studies of the plasma membrane supported the fluid mosaic model
Freeze-fracture is a specialized preparation technique that splits a membrane along the middle of the phospholipid bilayer Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Inside of extracellular layer Knife
Fig. 7-4 TECHNIQUE RESULTS Extracellular layer Proteins Inside of extracellular layer Knife Figure 7.4 Freeze-fracture Plasma membrane Cytoplasmic layer Inside of cytoplasmic layer

The Fluidity of Membranes
Phospholipids in the plasma membrane can move within the bilayer Most of the lipids, and some proteins, drift laterally Rarely does a molecule flip-flop transversely across the membrane

(a) Movement of phospholipids
Fig. 7-5 Lateral movement (~107 times per second) Flip-flop (~ once per month) (a) Movement of phospholipids Fluid Viscous Unsaturated hydrocarbon tails with kinks Saturated hydrocarbon tails (b) Membrane fluidity Figure 7.5 The fluidity of membranes Cholesterol (c) Cholesterol within the animal cell membrane

The temperature at which a membrane solidifies depends on the types of lipids
Membranes rich in unsaturated fatty acids are more fluid that those rich in saturated fatty acids Membranes must be fluid to work properly; they are usually about as fluid as salad oil

Membrane Proteins and Their Functions
A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer Proteins determine most of the membrane’s specific functions

Fibers of extracellular matrix (ECM) Glyco- Carbohydrate protein
Fig. 7-7 Fibers of extracellular matrix (ECM) Glyco- protein Carbohydrate Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Figure 7.7 The detailed structure of an animal cell’s plasma membrane, in a cutaway view Cholesterol Microfilaments of cytoskeleton Peripheral proteins Integral protein CYTOPLASMIC SIDE OF MEMBRANE

Peripheral proteins are bound to the surface of the membrane
Integral proteins penetrate the hydrophobic core Integral proteins that span the membrane are called transmembrane proteins The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices

Six major functions of membrane proteins:
Transport Enzymatic activity Signal transduction Cell-cell recognition Intercellular joining Attachment to the cytoskeleton and extracellular matrix (ECM)

(b) Enzymatic activity (c) Signal transduction
Fig. 7-9ac Signaling molecule Enzymes Receptor Figure 7.9a–c Some functions of membrane proteins ATP Signal transduction (a) Transport (b) Enzymatic activity (c) Signal transduction

(d) Cell-cell recognition (e) Intercellular joining (f) Attachment to
Fig. 7-9df Glyco- protein Figure 7.9d–f Some functions of membrane proteins (d) Cell-cell recognition (e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM)

The Role of Membrane Carbohydrates in Cell-Cell Recognition
Cells recognize each other by binding to surface molecules, often carbohydrates, on the plasma membrane Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins) Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual

Membrane structure results in selective permeability
A cell must exchange materials with its surroundings, a process controlled by the plasma membrane Plasma membranes are selectively permeable, regulating the cell’s molecular traffic Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Permeability of the Lipid Bilayer
Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly Polar molecules, such as sugars, do not cross the membrane easily Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Transport Proteins Transport proteins allow passage of hydrophilic substances across the membrane Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel Channel proteins called aquaporins facilitate the passage of water Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

A transport protein is specific for the substance it moves
Other transport proteins, called carrier proteins, bind to molecules and change shape to shuttle them across the membrane A transport protein is specific for the substance it moves Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Passive transport is diffusion of a substance across a membrane with no energy investment
Diffusion is the tendency for molecules to spread out evenly into the available space At dynamic equilibrium, as many molecules cross one way as cross in the other direction Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Membrane (cross section)
Molecules of dye Membrane (cross section) WATER Net diffusion Net diffusion Equilibrium (a) Diffusion of one solute Figure 7.11 The diffusion of solutes across a membrane Net diffusion Net diffusion Equilibrium Net diffusion Net diffusion Equilibrium (b) Diffusion of two solutes

Substances diffuse down their concentration gradient, the difference in concentration of a substance from one area to another No work must be done to move substances down the concentration gradient The diffusion of a substance across a biological membrane is passive transport because it requires no energy from the cell to make it happen

Concentration gradient

(b) Diffusion of two solutes
Fig. 7-11b Net diffusion Net diffusion Equilibrium Figure 7.11b The diffusion of solutes across a membrane Net diffusion Net diffusion Equilibrium (b) Diffusion of two solutes

Effects of Osmosis on Water Balance
Osmosis is the diffusion of water across a selectively permeable membrane Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Higher concentration Lower Same concentration concentration of sugar
Fig. 7-12 Lower concentration of solute (sugar) Higher concentration of sugar Same concentration of sugar H2O Selectively permeable membrane Figure 7.12 Osmosis Osmosis

Water Balance of Cells Without Walls
Tonicity is the ability of a solution to cause a cell to gain or lose water Isotonic solution: Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane Hypertonic solution: Solute concentration is greater than that inside the cell; cell loses water Hypotonic solution: Solute concentration is less than that inside the cell; cell gains water Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Hypotonic Isotonic Hypertonic H2O H2O H2O H2O (a) Animal cell Lysed
Figure 7.12 Hypotonic Isotonic Hypertonic H2O H2O H2O H2O (a) Animal cell Lysed Normal Shriveled Cell wall Plasma membrane H2O Plasma membrane H2O H2O H2O Figure 7.12 The water balance of living cells (b) Plant cell Turgid (normal) Flaccid Plasmolyzed

Hypertonic or hypotonic environments create osmotic problems for organisms
Osmoregulation, the control of water balance, is a necessary adaptation for life in such environments The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Water Balance of Cells with Walls
Cell walls help maintain water balance A plant cell in a hypotonic solution swells until the wall opposes uptake; the cell is now turgid (firm) If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell; the cell becomes flaccid (limp), and the plant may wilt Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

In a hypertonic environment, plant cells lose water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis

Facilitated Diffusion: Passive Transport Aided by Proteins
In facilitated diffusion, transport proteins speed the passive movement of molecules across the plasma membrane Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane Channel proteins include Aquaporins, for facilitated diffusion of water Ion channels that open or close in response to a stimulus (gated channels) For the Cell Biology Video Water Movement through an Aquaporin, go to Animation and Video Files. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Channel protein Solute (a) A channel protein Solute Carrier protein
Fig. 7-15 EXTRACELLULAR FLUID Channel protein Solute CYTOPLASM (a) A channel protein Figure 7.15 Two types of transport proteins that carry out facilitated diffusion Solute Carrier protein (b) A carrier protein

Active transport uses energy to move solutes against their gradients
Facilitated diffusion is still passive because the solute moves down its concentration gradient Some transport proteins, however, can move solutes against their concentration gradients Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Need for Energy in Active Transport
Active transport moves substances against their concentration gradient Active transport requires energy, usually in the form of ATP Active transport is performed by specific proteins embedded in the membranes The sodium-potassium pump is one type of active transport system Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Facilitated diffusion
Fig. 7-17 Passive transport Active transport ATP Diffusion Facilitated diffusion Figure 7.17 Review: passive and active transport

1 2 3 6 5 4 EXTRACELLULAR FLUID [Na+] high Na+ [K+] low Na+ Na+ Na+
Fig EXTRACELLULAR FLUID [Na+] high Na+ [K+] low Na+ Na+ Na+ Na+ Na+ Na+ Na+ [Na+] low ATP P Na+ P CYTOPLASM [K+] high ADP 1 2 3 K+ Figure 7.16, 1–6 The sodium-potassium pump: a specific case of active transport K+ K+ K+ K+ P K+ P 6 5 4

Bulk transport by exocytosis and endocytosis
Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteins Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles Bulk transport requires energy Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Exocytosis In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents Many secretory cells use exocytosis to export their products Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

How Substances Cross Membranes: Endocytosis and Exocytosis

Endocytosis In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane Endocytosis is a reversal of exocytosis, involving different proteins There are three types of endocytosis: Phagocytosis (“cellular eating”) Pinocytosis (“cellular drinking”) Receptor-mediated endocytosis Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

In phagocytosis a cell engulfs a particle in a vacuole
In pinocytosis, molecules are taken up when extracellular fluid is “gulped” into tiny vesicles In receptor-mediated endocytosis, binding of ligands to receptors triggers vesicle formation

Figure 7.22 Phagocytosis Pinocytosis Receptor-Mediated Endocytosis
EXTRACELLULAR FLUID Solutes Pseudopodium Receptor Plasma membrane Ligand Coat proteins Coated pit “Food” or other particle Coated vesicle Figure 7.22 Exploring: Endocytosis in Animal Cells Vesicle Food vacuole CYTOPLASM 47

Local and Long-Distance Signaling
Cells in a multicellular organism communicate by chemical messengers Animal and plant cells have cell junctions that directly connect the cytoplasm of adjacent cells In local signaling, animal cells may communicate by direct contact, or cell-cell recognition This type of local signaling in animals is also called paracrine signalling Another more specialized type of local signalling called synaptic signaling in animals. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Gap junctions between animal cells Plasmodesmata between plant cells
Fig. 11-4 Plasma membranes Gap junctions between animal cells Plasmodesmata between plant cells (a) Cell junctions Figure 11.4 Communication by direct contact between cells (b) Cell-cell recognition Communication by direct contact between the cells

In many other cases, animal cells communicate using local regulators, messenger molecules that travel only short distances In long-distance signaling, plants and animals use chemicals called hormones Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

(a) Paracrine signaling (b) Synaptic signaling
Fig. 11-5ab Local signaling Target cell Electrical signal along nerve cell triggers release of neurotransmitter Neurotransmitter diffuses across synapse Secreting cell Secretory vesicle Figure 11.5 Local and long-distance cell communication in animals Local regulator diffuses through extracellular fluid Target cell is stimulated (a) Paracrine signaling (b) Synaptic signaling

Long-distance signaling
Figure 11.5b Long-distance signaling Endocrine cell Blood vessel Hormone travels in bloodstream. Target cell specifically binds hormone. Figure 11.5 Local and long-distance cell signaling by secreted molecules in animals. (c) Endocrine (hormonal) signaling 52

The Three Stages of Cell Signaling: A Preview
Earl W. Sutherland discovered how the hormone epinephrine acts on cells Sutherland suggested that cells receiving signals went through three processes: Reception Transduction Response Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Plasma membrane 1 Reception Receptor Signaling molecule 1
Fig EXTRACELLULAR FLUID CYTOPLASM Plasma membrane 1 1 Reception Receptor Figure 11.6 Overview of cell signaling Signaling molecule

Plasma membrane 1 Reception Transduction Receptor Signaling molecule 1
Fig EXTRACELLULAR FLUID CYTOPLASM Plasma membrane 1 1 Reception 2 Transduction Receptor Relay molecules in a signal transduction pathway Figure 11.6 Overview of cell signaling Signaling molecule

Plasma membrane 1 Reception Transduction Response Receptor Activation
Fig EXTRACELLULAR FLUID CYTOPLASM Plasma membrane 1 Reception 2 Transduction 3 Response Receptor Activation of cellular response Relay molecules in a signal transduction pathway Figure 11.6 Overview of cell signaling Signaling molecule

Most signal receptors are plasma membrane proteins
Reception: A signal molecule binds to a receptor protein, causing it to change shape The binding between a signal molecule (ligand) and receptor is highly specific A shape change in a receptor is often the initial transduction of the signal Most signal receptors are plasma membrane proteins Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Receptors in the Plasma Membrane
Most water-soluble signal molecules bind to specific sites on receptor proteins in the plasma membrane There are three main types of membrane receptors: G protein-coupled receptors Receptor tyrosine kinases Ion channel receptors Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

A G protein-coupled receptor is a plasma membrane receptor that works with the help of a G protein
The G protein acts as an on/off switch: If GDP is bound to the G protein, the G protein is inactive Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Figure 11.7 Membrane receptors—G protein-coupled receptors, part 2
Fig. 11-7b Plasma membrane G protein-coupled receptor Inactive enzyme Activated receptor Signaling molecule GDP G protein (inactive) Enzyme GDP GTP CYTOPLASM 1 2 Activated enzyme Figure 11.7 Membrane receptors—G protein-coupled receptors, part 2 GTP GDP P i Cellular response 3 4

A ligand-gated ion channel receptor acts as a gate when the receptor changes shape
When a signal molecule binds as a ligand to the receptor, the gate allows specific ions, such as Na+ or Ca2+, through a channel in the receptor Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

1 Signaling molecule (ligand) Gate closed Ions Plasma membrane
Fig. 11-7d 1 Signaling molecule (ligand) Gate closed Ions Plasma membrane Ligand-gated ion channel receptor 2 Gate open Cellular response Figure 11.7 Membrane receptors—ion channel receptors 3 Gate closed

Intracellular Receptors
Some receptor proteins are intracellular, found in the cytosol or nucleus of target cells Small or hydrophobic chemical messengers can readily cross the membrane and activate receptors Examples of hydrophobic messengers are the steroid and thyroid hormones of animals An activated hormone-receptor complex can act as a transcription factor, turning on specific genes Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Hormone (testosterone) Plasma membrane Receptor protein DNA NUCLEUS
Fig Hormone (testosterone) EXTRACELLULAR FLUID Plasma membrane Receptor protein DNA Figure 11.8 Steroid hormone interacting with an intracellular receptor NUCLEUS CYTOPLASM

Hormone (testosterone) Plasma membrane Receptor protein Hormone-
Fig Hormone (testosterone) EXTRACELLULAR FLUID Plasma membrane Receptor protein Hormone- receptor complex DNA Figure 11.8 Steroid hormone interacting with an intracellular receptor NUCLEUS CYTOPLASM

Fig Hormone (testosterone) EXTRACELLULAR FLUID Plasma membrane Receptor protein Hormone- receptor complex DNA Figure 11.8 Steroid hormone interacting with an intracellular receptor mRNA NUCLEUS CYTOPLASM

Fig Hormone (testosterone) EXTRACELLULAR FLUID Plasma membrane Receptor protein Hormone- receptor complex DNA Figure 11.8 Steroid hormone interacting with an intracellular receptor mRNA NUCLEUS New protein CYTOPLASM

Transduction: Cascades of molecular interactions relay signals from receptors to target molecules in the cell Signal transduction usually involves multiple steps Multistep pathways can amplify a signal: A few molecules can produce a large cellular response Multistep pathways provide more opportunities for coordination and regulation of the cellular response Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Signal Transduction Pathways
The molecules that relay a signal from receptor to response are mostly proteins Like falling dominoes, the receptor activates another protein, which activates another, and so on, until the protein producing the response is activated At each step, the signal is transduced into a different form, usually a shape change in a protein Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Protein Phosphorylation and Dephosphorylation
In many pathways, the signal is transmitted by a cascade of protein phosphorylations Protein kinases transfer phosphates from ATP to protein, a process called phosphorylation Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Protein phosphatases remove the phosphates from proteins, a process called dephosphorylation
This phosphorylation and dephosphorylation system acts as a molecular switch, turning activities on and off Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Phosphorylation cascade
Fig. 11-9 Signaling molecule Receptor Activated relay molecule Inactive protein kinase 1 Active protein kinase 1 Inactive protein kinase 2 ATP Phosphorylation cascade ADP Active protein kinase 2 P PP P i Figure 11.9 A phosphorylation cascade Inactive protein kinase 3 ATP ADP Active protein kinase 3 P PP P i Inactive protein ATP ADP P Active protein Cellular response PP P i

Small Molecules and Ions as Second Messengers
The extracellular signal molecule that binds to the receptor is a pathway’s “first messenger” Second messengers are small, nonprotein, water-soluble molecules or ions that spread throughout a cell by diffusion Cyclic AMP and calcium ions are common second messengers Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

First messenger Adenylyl cyclase G protein GTP G protein-coupled
Fig First messenger Adenylyl cyclase G protein G protein-coupled receptor GTP ATP Second messenger cAMP Figure cAMP as second messenger in a G-protein-signaling pathway Protein kinase A Cellular responses

Calcium Ions and Inositol Triphosphate (IP3)
Calcium ions (Ca2+) act as a second messenger in many pathways Calcium is an important second messenger because cells can regulate its concentration Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Nuclear and Cytoplasmic Responses
Ultimately, a signal transduction pathway leads to regulation of one or more cellular activities The response may occur in the cytoplasm or may involve action in the nucleus The cell’s response to an extracellular signal is sometimes called the “output response” Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Nuclear responses to a signal Growth factor Reception Receptor
Fig Growth factor Reception Receptor Phosphorylation cascade Transduction CYTOPLASM Inactive transcription factor Active transcription factor Figure Nuclear responses to a signal: the activation of a specific gene by a growth factor Response P Nuclear responses to a signal DNA Gene NUCLEUS mRNA

Membrane Structure and Function

Similar presentations

Presentation on theme: "Membrane Structure and Function"— Presentation transcript:

Similar presentations

About project

Feedback

Log in

Auth with social network:

Membrane Structure and Function

Similar presentations

Presentation on theme: "Membrane Structure and Function"— Presentation transcript:

Similar presentations

About project

Feedback