Cytology Membrane Structure and Function Overview Life at

Cytology Membrane Structure and Function

Overview: Life at the Edge • The plasma membrane – Is the boundary that separates the living cell from its nonliving surroundings – is about 8 nm in diameter – surrounds the cell and controls chemical traffic into and out of the cell. – Has a unique structure which determines its function. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Membranes inside a cell • • Endomembranes system and membrane-bound organelles Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2021/12/14

Various roles of membranes in cell PM define the boundaries of the cell and organelles. Compartmentalization: membranes form continuous sheets that enclose intracellular compartments. Transporting solutes: membrane proteins facilitate the movement of substances between compartments. Responding to external signals: membrane receptors transduce signals from outside the cell in response to specific chemical signals Intercellular interaction: membrane mediate recognition and interaction between adjacent cells by cell-to-cell communication and junction. Locus for biochemical activities: membrane provide a scaffold that organizes enzymes for effective interaction. Energy transduction: membranes transduce photosynthetic energy, convert chemical energy to ATP, and store energy in ion and solute gradients. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Synthesis of Membranes • Synthesis of molecules starts out in the inside face of the ER ends up on the outside face of the plasma membrane • Synthesis of membrane proteins and lipids in ER • Inside Golgi apparatus, lipids and proteins become glycolipids and glycoproteins. • glycoproteins and glycolipids are transported in vesicles to the plasma membrane. • Vesicle fuses with the membrane releasing secretory proteins from the cell and depositing glycoproteins and glycolipids on the outer surface of the cell. • Asymmetry in the two surfaces of membrane Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

1 Synthesis Transmembrane glycoproteins and transport Of Membranes Secretory protein Glycolipid Golgi apparatus ER 2 Via Endomembrane system Vesicle 3 Plasma membrane: Cytoplasmic face 4 Secreted protein Figure 7. 10 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Extracellular face Transmembrane glycoprotein Membrane glycolipid

Membrane Models: Scientific Inquiry Observations on membrane properties: Differential permeability--lipid soluble molecules can pass rapidly through the membrane. Polar molecules and charged ions can pass through membrane more slowly. Chemical analysis: membranes are comprised almost entirely of proteins and lipds. The lipids are mainly phospholipids. Appearance: Characteristic 3 -layered trilaminar appearance when viewed with the electron microscope. Any membrane model proposed must be able to explain these observations. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Membrane Models: Scientific Inquiry Gorter and Grendel 1925 – Reasoned that it must be a phospholipid bilayer WATER Hydrophilic head Hydrophobic tail Figure 7. 2 WATER Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Polar vs nonpolar molecule Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2021/12/14

Oil - immiscible with water Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2021/12/14

Soap molecule Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2021/12/14

Membrane – Monolayer & micelle Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2021/12/14

Phospholipid Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2021/12/14

Cellular membranes are made up of lipids and proteins • Phospholipids – Are the most abundant lipid in the plasma membrane – Are amphipathic, containing both hydrophobic and hydrophilic regions Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Membrane lipid bilayer Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Membrane Models: Scientific Inquiry The Davson-Danielli sandwich model of membrane structure (1935) – Stated that the membrane was made up of a phospholipid bilayer sandwiched between two protein layers – Was supported by electron microscope pictures of membranes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Tri-lamellar layer model of membrane Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2021/12/14

There are problems with this model: 1. a membrane with an outside layer of proteins would be an unstable structure because: Both phospholipids and proteins are amphipathic and ideally their hydrophilic parts should be in contact with water and their hydrophobic parts away from water. 2. Evidence from Freeze fracturing shows that proteins do NOT form a continuous layer. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Membrane Models: Scientific Inquiry Evidence from Freeze-fracturing • Tissue is rapidly frozen (-100 C. ) • Fractured with a knife --- along planes of weakness • Ice sublimes under high vacuum leaving an etched surface. • A replica is made by depositing a layer of carbon over it. • This carbon replica is shadowed with heavy metal. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2021/12/14

Freeze fracturing Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2021/12/14

Freeze-fractureing show particles embedded in Membrane Further reading: http: //www. hillstrath. on. ca/moffatt/bio 3 a/cellbio/phase 1. htm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2021/12/14

Myelin sheath membrane (metabolically inactive) shows little particles embedded Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2021/12/14

Freeze-fracture studies of plasma membrane – Supported the fluid mosaic model of membrane structure APPLICATION TECHNIQUE A cell membrane can be split into its two layers, revealing the ultrastructure of the membrane’s interior. A cell is frozen and fractured with a knife. The fracture plane often follows the hydrophobic interior of a membrane, splitting the phospholipid bilayer into two separated layers. The membrane proteins go wholly with one of the layers. Extracellular layer Proteins Knife RESULTS Plasma Cytoplasmic membrane layer These SEMs show membrane proteins (the “bumps”) in the two layers, demonstrating that proteins are embedded in the phospholipid bilayer. Extracellular layer Figure 7. 4 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cytoplasmic layer

In 1972, Singer and Nicolson – Proposed that membrane proteins are dispersed and individually inserted into the phospholipid bilayer Hydrophobic region of protein Phospholipid bilayer Figure 7. 3 Hydrophobic region of protein Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

The fluid mosaic model of membrane structure – States that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Phospholipids in the plasma membrane – Can move within the bilayer Lateral movement (~107 times per second) (a) Movement of phospholipids Figure 7. 5 A Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Flip-flop (~ once per month)

Proteins in the plasma membrane – Can drift within the bilayer EXPERIMENT Researchers labeled the plasma mambrane proteins of a mouse cell and a human cell with two different markers and fused the cells. Using a microscope, they observed the markers on the hybrid cell. RESULTS Membrane proteins + Mouse cell Human cell Hybrid cell Figure 7. 6 Mixed proteins after 1 hour CONCLUSION The mixing of the mouse and human membrane proteins indicates that at least some membrane proteins move sideways within the plane of the plasma membrane. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Evolution of membrane models Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Summary of structure of cell membrane according to the Fluid mosaic model • Membranes are mainly composed of lipid and protein, with carbohydrate portions attached to the external surfaces of some lipid and protein molecules. • The lipids spontaneously form a bilayer owing to their polar heads and non-polar tails. • the proteins that are integral to the membrane arranged in an amphipathic structure, that is, with the ionic and highly polar groups protruding from the membrane into the aqueous phase, and the nonpolar groups largely buried in the hydrophobic interior of the membrane. • The proteins are variable in position and function. Some attach to the surfaces -peripheral proteins, while some penetrate through (or embedded in) the membrane -integral proteins, • The two sides of a membrane may differ in composition and properties—Asymmetrical. • Both lipids and proteins show rapid lateral diffusion in the plane of the membrane unless anchored or restricted in some way. • The inner surface is supported by the cytoskeleton; some membrane proteins attach to the cytoskeleton Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

The Fluidity of Membranes • Membranes are held together by weak hydrophobic interactions • Most membrane lipids and proteins can move laterally within the membrane • Molecules rarely flip-flop transversely across the membrane • Phospholipids move quickly along membrane plane (2 μm/second). • Membrane proteins move slowly than lipids. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Membranes must be fluid to work properly Why? Because when solidifies its permeability and enzymatic proteins in the membrane may become inactive Factors affecting fluidity of membrane: 1. Unsaturated hydrocarbon tails enhance membrane fluidity 2. Cholesterol in plasma membranes of eukaryotes maintain fluidity by: Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Unsaturation in hydrocarbon tails of phospholipids – affects the fluidity of the plasma membrane Fluid Unsaturated hydrocarbon tails with kinks Viscous Saturated hydro. Carbon tails 1. Membranes solidify if temperature decreases to critical point 2. Cells can alter lipid composition according to temp; cold tolerant plants such as winter wheat increase unsaturated phospholipid concentration in autumn (Fall) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

The steroid cholesterol within the animal cell membrane – Has different effects on membrane fluidity at different temperatures (reference only) Cholesterol 1. Less fluid at warmer temperature (by reducing phospholipids movement) 2. More fluid at lower temperature (by preventing close packing of phospholipids) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Importance of membrane fluidity – signal transduction • Allow chemical messages from outside to be translated into intracellular actions Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2021/12/14

Signal Transduction – has cascading effect (reference only) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2021/12/14

Membrane Proteins and Their Functions • A membrane is mosaic of different proteins embedded in the phospholipid bilayer (>50 kinds) • Phospholipids forms the main fabric of the membrane, but proteins determine most of the membrane's functions Fibers of extracellular matrix (ECM) Glycoprotein Carbohydrate Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Microfilaments of cytoskeleton Cholesterol Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Peripheral protein Integral CYTOPLASMIC SIDE protein OF MEMBRANE

Integral proteins – Penetrate the hydrophobic core of the lipid bilayer, may be: – unilateral: reaching only partway across the membrane. – transmembrane: exposed on both sides of the membrane – The hydrophobic regions consists of one or more stretches of non polar amino acids -- usually coiled into α-helices N-terminus EXTRACELLULAR SIDE C-terminus Figure 7. 8 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings a Helix CYTOPLASMIC SIDE

Peripheral proteins – loosely bound to the surface of the membrane – In contrast to integral membrane proteins, peripheral membrane proteins tend to collect in the water-soluble component – may be held by cytoskeleton filaments in the cytoplasmic side Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

The Role of Membrane Carbohydrates in Cell Recognition • Is a cell’s ability to distinguish one type of neighboring cell from another • Cell-cell recognition is very important in the functioning of an organism. It is the basis for: – sorting of an animal embryo’s cells into tissues and organs. – Rejection of foreign cells by the immune system Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

– Cell-cell recognition is made possible by certain molecules on the external side of the plasma membrane – Membrane carbohydrates usually branched oligosaccharides ( 15 monomers) – some covalently bonded to lipids: glycolipids. – Most covalently bonded to proteins: glycoproteins Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Membrane carbohydrates – Interact with the surface molecules of other cells, facilitating cell-cell recognition – Vary from species to species and between individuals of the same species. – These membrane carbohydrates are the identification tags recognized by other cells – The blood grouping A, B, AB and O are based on oligosaccharides found on the RBC’s membrane. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

An overview of six major functions of membrane proteins (a) Transport. (left) A protein that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. (right) Other transport proteins shuttle a substance from one side to the other by changing shape. Some of these proteins hydrolyze ATP as an energy ssource to actively pump substances across the membrane. (b) (c) Enzymatic activity. A protein built into the membrane ATP Enzymes may be an enzyme with its active site exposed to substances in the adjacent solution. In some cases, several enzymes in a membrane are organized as a team that carries out sequential steps of a metabolic pathway. Signal transduction. A membrane protein may have Signal a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone. The external messenger (signal) may cause a conformational change in the protein (receptor) that relays the message to the inside of the cell. Figure 7. 9 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Receptor

(d) (e) Cell-cell recognition. Some glyco-proteins serve as identification tags that are specifically recognized by other cells. Intercellular joining. Membrane proteins of adjacent cells may hook together in various kinds of junctions, such as gap junctions or tight junctions (see Figure 6. 31). (f) Attachment to the cytoskeleton and extracellular Matrix (ECM). Microfilaments or other elements of the cytoskeleton may be bonded to membrane proteins, a function that helps maintain cell shape and stabilizes the location of certain membrane proteins. Proteins that adhere to the ECM can coordinate extracellular and intracellular changes (see Figure 6. 29). Figure 7. 9 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Glycoprotein

Plasma membrane exhibits selective permeability – It allows some substances to cross it more easily than others Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Membrane structure results in selective permeability • A cell must exchange materials with its surroundings, a process controlled by the plasma membrane • Plasma membrane is selectively permeable: allowing some substances to cross easier than others. Examples; sugars, amino acids, ions (Na+, K+, Ca+2, Cl–) cross the membrane on either direction of the plasma membrane Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

The Permeability of the Lipid Bilayer • Non-polar hydrophobic molecules: • dissolve in the membrane (lipid soluble) and cross easily (e. g. hydrocarbons, CO 2 and O 2). • Polar hydrophilic molecules: • small molecules like H 2 O and glucose can pass through the membrane. • Larger polar uncharged molecules like glucose cannot pass easily. • All ions have difficulty penetrating the hydrophobic layer of the membrane Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Transport proteins • Hydrophilic substances which can not pass through plasma membrane are transported by transport proteins that span the membrane • may provide a hydrophilic tunnel through the membrane e. g. movement of water in certain cells by Aquaporins • may bind to a substance and physically move it across the membrane like a shuttle. • In both cases the transport protein is specific for the substance they transport. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Thus, the selective permeability of a membrane depends on both the discriminating barrier of the lipid bilayer and the specific transport proteins built into the membrane Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Passive transport is diffusion of a substance across a membrane with no energy investment • diffusion of substances across biological membranes • Passive process which does not require the cell to expend energy • Rate of diffusion is regulated by the permeability of the membrane, some molecules diffuse more easily than others • Water diffuse freely across most cell membranes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Diffusion – Is the tendency for molecules of any substance to spread out evenly into the available space (a) Diffusion of one solute. The membrane has pores large enough for molecules of dye to pass through. Random movement of dye molecules will cause some to pass through the pores; this will happen more often on the side with more molecules. The dye diffuses from where it is more concentrated to where it is less concentrated (called diffusing down a concentration gradient). This leads to a dynamic equilibrium: The solute molecules continue to cross the membrane, but at equal rates in both directions. Molecules of dye Membrane (cross section) Net diffusion Figure 7. 11 A Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Equilibrium

• Substances diffuse down their concentration gradient, the difference in concentration of a substance from one area to another (b) Diffusion of two solutes. Solutions of two different dyes are separated by a membrane that is permeable to both. Each dye diffuses down its own concentration gradient. There will be a net diffusion of the purple dye toward the left, even though the total solute concentration was initially greater on the left side. Net diffusion Figure 7. 11 B Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Net diffusion Equilibrium

Effects of Osmosis on Water Balance • Osmosis – Is the movement of water across a semipermeable membrane – Is affected by the concentration gradient of dissolved substances Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

– is affected by the concentration gradient of dissolved substances Lower concentration of solute (sugar) Higher concentration of sugar Same concentration of sugar Selectively permeable membrane: sugar molecules cannot pass through pores, but water molecules can Water molecules cluster around sugar molecules More free water molecules (higher concentration) Fewer free water molecules (lower concentration) Osmosis Figure 7. 12 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Water moves from an area of higher free water concentration to an area of lower free water concentration

Water Balance of Cells Without Walls • Tonicity – Is the ability of a solution to cause a cell to gain or lose water – Has a great impact on cells without walls Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• If a solution is isotonic – The concentration of solutes is the same as it is inside the cell – There will be no net movement of water • If a solution is hypertonic – The concentration of solutes is greater than it is inside the cell – The cell will lose water • If a solution is hypotonic – The concentration of solutes is less than it is inside the cell – The cell will gain water Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Water balance in cells without walls Hypotonic solution (a) Animal cell. An animal cell fares best in an isotonic environment unless it has special adaptations to offset the osmotic uptake or loss of water. H 2 O Lysed Figure 7. 13 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Isotonic solution Hypertonic solution H 2 O Normal H 2 O Shriveled

• Animals and other organisms without rigid cell walls living in hypertonic or hypotonic environments – Must have special adaptations for osmoregulation contractile vacuole in protists Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Water Balance of Cells with Walls • Cell walls – Help maintain water balance • If a plant cell is turgid – It is in a hypotonic environment – It is very firm, a healthy state in most plants • If a plant cell is flaccid – It is in an isotonic or hypertonic environment Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Water balance in cells with walls (b) Plant cells are turgid (firm) and generally healthiest in a hypotonic environment, where the uptake of water is eventually balanced by the elastic wall pushing back on the cell. H 2 O Turgid (normal) Figure 7. 13 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings H 2 O Flaccid H 2 O Plasmolyzed

Facilitated Diffusion: Passive Transport Aided by Proteins • In facilitated diffusion – Transport proteins speed the movement of molecules across the plasma membrane Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Channel proteins – Provide corridors that allow a specific molecule or ion to cross the membrane EXTRACELLULAR FLUID Channel protein Solute CYTOPLASM (a) A channel protein (purple) has a channel through which water molecules or a specific solute can pass. Figure 7. 15 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Carrier proteins – Undergo a subtle change in shape that translocates the solute-binding site across the membrane Carrier protein Solute (b) A carrier protein alternates between two conformations, moving a solute across the membrane as the shape of the protein changes. The protein can transport the solute in either direction, with the net Figure 7. 15 movement being down the concentration gradient of the solute. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Active transport uses energy to move solutes against their gradients • Active transport – moves substances against (or along) their concentration gradient – requires energy, usually in the form of ATP Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• The sodium-potassium pump – Is one type of active transport system EXTRACELLULAR FLUID 1 Cytoplasmic Na+ binds to the sodium-potassium pump. 2 Na+ binding stimulates phosphorylation by ATP. [Na+] high [K+] low Na+ Na+ Na+ CYTOPLASM [Na+] low [K+] high ATP P ADP Na+ Na+ 3 K+ is released and Na + sites are receptive again; the cycle repeats. 4 Phosphorylation causes the K+ protein to change its conformation, expelling Na + to the outside. P K+ K+ K+ 5 Loss of the phosphate K+ restores the protein’s Figure 7. 16 original conformation. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings P K+ Pi 6 Extracellular K + binds to the protein, triggering release of the Phosphate group.

• Review: Passive and active transport compared Passive transport. Substances diffuse spontaneously down their concentration gradients, crossing a membrane with no expenditure of energy by the cell. The rate of diffusion can be greatly increased by transport proteins in the membrane. Active transport. Some transport proteins act as pumps, moving substances across a membrane against their concentration gradients. Energy for this work is usually supplied by ATP Diffusion. Hydrophobic molecules and (at a slow rate) very small uncharged polar molecules can diffuse through the lipid bilayer. Facilitated diffusion. Many hydrophilic substances diffuse through membranes with the assistance of transport proteins, either channel or carrier proteins. Figure 7. 17 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Maintenance of Membrane Potential -- Ion Pumps • Membrane potential – Is the voltage difference across a membrane • An electrochemical gradient – Is caused by the concentration electrical gradient of ions across a membrane Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• An electrogenic pump – Is a transport protein that generates the voltage across a membrane – ATP EXTRACELLULAR FLUID + – + H+ H+ Proton pump H+ – + H+ H+ + – CYTOPLASM – Figure 7. 18 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings + + H+

Cotransport: Coupled Transport by a Membrane Protein • Cotransport – Occurs when active transport of a specific solute indirectly drives the active transport of another solute Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Cotransport: • active transport driven by a concentration gradient – + H+ ATP H+ + – H+ Proton pump H+ – + Sucrose-H+ cotransporter H+ Diffusion of H+ H+ – – Figure 7. 19 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings + + Sucrose

Bulk transport across the plasma membrane occurs by exocytosis and endocytosis • Large proteins – Cross the membrane by different mechanisms Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• In exocytosis Transport vesicles migrate to the plasma membrane, fuse with it, and release their contents • In endocytosis The cell takes in macromolecules by forming new vesicles from the plasma membrane Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Three types of endocytosis In phagocytosis, a cell engulfs a particle by Wrapping pseudopodia around it and packaging it within a membraneenclosed sac large enough to be classified as a vacuole. The particle is digested after the vacuole fuses with a lysosome containing hydrolytic enzymes. PHAGOCYTOSIS EXTRACELLULAR CYTOPLASM FLUID Pseudopodium 1 µm Pseudopodium of amoeba “Food” or other particle Bacterium Food vacuole An amoeba engulfing a bacterium via phagocytosis (TEM). In pinocytosis, the cell “gulps” droplets of extracellular fluid into tiny vesicles. It is not the fluid itself that is needed by the cell, but the molecules dissolved in the droplet. Because any and all included solutes are taken into the cell, pinocytosis is nonspecific in the substances it transports. PINOCYTOSIS 0. 5 µm Plasma membrane Figure 7. 20 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM). Vesicle

Receptor-mediated endocytosis enables the cell to acquire bulk quantities of specific substances, even though those substances may not be very concentrated in the extracellular fluid. Embedded in the membrane are proteins with specific receptor sites exposed to the extracellular fluid. The receptor proteins are usually already clustered in regions of the membrane called coated pits, which are lined on their cytoplasmic side by a fuzzy layer of coat proteins. Extracellular substances (ligands) bind to these receptors. When binding occurs, the coated pit forms a vesicle containing the ligand molecules. Notice that there are relatively more bound molecules (purple) inside the vesicle, other molecules (green) are also present. After this ingested material is liberated from the vesicle, the receptors are recycled to the plasma membrane by the same vesicle. RECEPTOR-MEDIATED ENDOCYTOSIS Coat protein Receptor Coated vesicle Ligand Coated pit A coated pit and a coated vesicle formed during receptormediated endocytosis (TEMs). Coat protein Plasma membrane 0. 25 µm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Membrane fusion also works in Intercellular transport in endomembrane sysetm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Intracellular transport