Cell Membranes Structure and Function Building cell parts

Cell Membranes Structure and Function

Building cell parts • The biological molecules are the basic building blocks of all cell parts. • Cell membranes are composed of proteins, lipids, and some complex carbohydrates.

• Phospholipids are the most abundant lipid in the plasma membrane • Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions

Plasma Membrane • Boundary that separates the living cell from it’s non-living surroundings. • Phospholipid bilayer • Amphipathic - having both: hydrophilic heads hydrophobic tails • ~8 nm thick Phospholipid

Membrane components: Lipid bilayer extracellular fluid (watery environment) hydrophilic heads phospholipid hydrophobic tails bilayer hydrophilic heads cytosol (watery environment)

The Fluid Mosaic Model of the cell membrane Which classes of biomolecules are represented here? extracellular fluid (outside) carbohydrate receptor protein recognition protein cholesterol phospholipid binding site cytosol (inside) glycoprotein phospholipid bilayer protein transport pore protein filaments

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

Membrane components: proteins The final shape of the protein determines its function.

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

Some Functions of Membrane Proteins

Plasma Membrane Proteins PROTEINS CAN MOVE IN THE MEMBRANE, TOO!

• 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

Transport of Materials • Passive transport: • Simple diffusion • Facilitated diffusion • Osmosis • Energy-consuming transport: • Active transport • Exo/Endocytosis

• Passive transport is diffusion of a substance across a membrane with no energy investment

Diffusion 1 A drop of dye is placed in water. 2 Dye molecules diffuse into the water; water molecules diffuse into the dye. 3 Both dye molecules and water molecules Are evenly dispersed. drop of dye water molecule The movement of molecules in solution from a high concentration of that molecule in the direction of a low concentration of that molecule until equilibrium is reached.

Passive transport by facilitated diffusion carrier protein amino acids, sugars, small proteins (extracellular fluid) (cytosol) 1 Carrier protein has binding site for a specific molecule. 2 Molecule enters binding site. 3 Carrier protein changes shape, transporting molecule across membrane. 4 Carrier protein resumes original shape.

Facilitated Transport

• 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 until the solute concentration is equal on both sides

Osmosis The diffusion of water through a semipermeable membrane. Which direction will water move here? Water moves from its own high concentration to its own low concentration.

Tonicity If the contents of a cell and the fluid around the cell have the same concentration of dissolved solutes. . . the fluid is isotonic compared with the cell. Water moves in and out in equal amounts.

Tonicity If the fluid around the cell contains less dissolved solute than the fluid in the cell. . . the fluid is hypotonic compared with the cell. More water enters the cell than leaves. The cell may swell to the point of bursting.

Tonicity If the fluid around the cell contains more dissolved solute than the fluid inside the cell. . . the fluid is hypertonic compared with the cell. More water leaves the cell than enters. The cell shrinks.

What is the tonicity of the solutions that these red blood cells have been dropped into? (a) Isotonic solution Equal movement of water into and out of cells. (b) Hypertonic solution Net water movement out of cells. (c) Hypotonic solution Net water movement into cells. W O R K T O G E T H E R

Tonicity and plant cells cytosol central vacuole When water is plentiful, it fills the central vacuole, pushes the cytosol against the cell wall, and helps maintain the cell's shape.

Tonicity and plant cells cell wall plasma membrane When water is scarce, the central vacuole shrinks and the cell wall is unsupported.

Active Transport 1 The transport protein binds both ATP and Ca 2+. 2 Energy from ATP Changes the shape of the transport protein and moves the ion across the membrane. 3 The protein releases the ion and the remnants of ATP (ADP and P) and closes. (extracellular fluid) ATP recognition site Ca 2+ ATP binding site (cytosol) ADP P

Active Transport

Passive/Active Transport

• 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

• 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

• An electrogenic pump is a transport protein that generates voltage across a membrane • The sodium-potassium pump is the major electrogenic pump of animal cells • The main electrogenic pump of plants, fungi, and bacteria is a proton pump • Electrogenic pumps help store energy that can be used for cellular work

Facilitated diffusion Cl– H 2 O, ions Cl– Proteins form a hydrophilic channel. Cl– channel protein Cl– (cytosol) Why are channel proteins necessary to move hydrophilic ions through the membrane?

• Exocytosis and Endocytosis Exocytosis---Cellular secretion • Endocytosis— • Phagocytosis— “Cell eating” • Pinocytosis– “Cell drinking”

ENDOCYTOSIS • Phagocytosis: “Cell eating” solid particles involved. Cell engulfs them with pseudopods. The vessicle then fuses w/ a food vacuole • Pinocytosis: “Cell drinking” fluid droplets involved

Exocytosis secreted material plasma membrane (extracellular fluid) vesicle (cytosol) Material is enclosed in a vesicle that fuses with the plasma membrane, allowing its contents to diffuse out. 0. 2 micrometer

Endocytosis Phagocytosis is one example of endocytosis. food particle (cytosol) pseudopods (extracellular fluid) food vacuole

Phagocytosis Phagocytos

Exocytosis

Pinocytosis

Exocytosis/Endocytosis Transport el Moleculo LARGO Exocytosis • Exportation of macromolecules by the fusion of vessicles w/membrane • Vessicle comes from ER or Golgi • Used by secretory cells to export products Endocytosis • Importation of macromolecules into a cell by forming vessicles from membrane • Used by cells to incorporate extracellular substances

Factors Contributing to Membrane Potential: • Neg. charged proteins in the cells interior • Plasma membrane’s selective permeability to various ions • The Sodium-Potassium Pump is an ELECTROGENIC PUMP: a transport protein which generates voltage across a membrane. Na+/K+ ATPase is the major one in animals, a Proton pump is the major one in Plants, bacteria, fungi (also Mitochondria, Chloroplasts use it to make ATP)

The sodium-potassium pump

Na, Na, K, K, !

Recap • Cell parts, such as cell membranes, are made up of biological molecules. • The form and properties of the molecules determine their functions. • Cell membranes actively and passively control what materials enter and leave the cell.

Why do the fatty acid tails of phospholipids point toward each other in the membrane? 1. They are non-polar, so they are hydrophilic. 2. They are non-polar, so they are hydrophobic. 3. They are polar, so they are hydrophilic. 4. They are polar, so they are hydrophobic.

Why are channel proteins necessary to move hydrophilic ions through the cell membrane? 1. Ions are charged; they cannot go through a non-polar lipid layer. 2. Ions are not charged; they cannot move through a non-polar lipid layer.