Cell membranes Stef Elorriaga 4/11/2016 BIO102
Announcements Lab report 2 is due now Quiz 2 is on Wednesday on cells, part of the cells, plasma membrane, and enzymes
Outline of the day Activity on the parts of the cells Lab write-ups are graded for lab 1 Lecture on the plasma membrane Activity on osmosis Lecture on reactions and enzymes
Learned so far Introduction Matter Macromolecules Cells Parts of the cells
Plasma membrane is a phospholipid bilayer embedded with other components Alpha-helix protein
Plasma membrane is fluid and dynamic Fluid mosaic model Model was introduced in 1972 by S.J. Singer and Garth L. Nicolson, and still stands Alpha-helix protein
Cell membrane functions 1. Isolation 2. Regulation 3. Sensitivity 4. Attachment
Cell membrane functions 1. Physical isolation Separates inside of cell from extracellular fluid
Cell membrane functions 2. Regulates intracellular-extracellular exchange Controls ions, nutrients, waste, and secretory product exchange
Cell membrane functions 3. Sensitivity to the extracellular environment Receptors allow cell recognition/response to molecules in the environment
Cell membrane functions 4. Attachment (within cell) Cytoskeleton Microfilaments (actin) Microtubules (tubulin) Intermediate filaments microtubules (red) intermediate filaments microtubules nucleus microfilaments microfilaments (blue) Cytoskeleton Light micrograph showing the cytoskeleton
Cell membrane functions 4. Attachment (outside cell) Cells don t live floating in fluid must attach to surface and to each other Provide structural and biochemical support, stiffness, and elasticity Examples: fibronectin, cadherins
Cell membrane structure Membrane separates inside/outside Inside cell = aqueous Outside cell = aqueous Solutes are mostly polar How do we keep molecules where they need to be?
Cell membrane structure Solution: cell membrane must be fundamentally non-polar, but able to exist in aqueous environment Molecule that makes this possible = phospholipid
Cell membrane structure Polar heads face both outside and inside Hydrophilic heads face aqueous environments Non-polar tails protected in between phospholipid extracellular fluid (watery environment) hydrophilic heads Plasma membrane hydrophobic tails hydrophilic heads cytoplasm (watery environment)
Cell membrane structure Membrane must be very fluid Fluidity adjusted by changing saturation of fatty acid tails More saturated = less fluid phospholipid extracellular fluid (watery environment) hydrophilic heads Plasma membrane hydrophobic tails hydrophilic heads cytoplasm (watery environment)
Cell membrane structure Problem: system is too effective; cell membranes isolate the cell from the outside Like a room with no doors and no windows How does anything get in or out? Selective permeability
Cell membrane structure Solution: membranes not 100% phospholipid Cell membranes have: Phospholipid Cholesterol Carbohydrates Proteins
Cholesterol 50% dry weight of membrane Adds stiffness Straightens phospholipid tails Prevents small polar molecules from passing through membrane
Membrane carbohydrates Carbohydrates attach to other molecules in the membrane Attach to protein = glycoprotein Replace phosphate in phospholipid = glycolipid
Membrane carbohydrates Carbohydrate functions Multiple functions Allow cell-to-cell interactions, ex. Platelet aggregation Cell recognition - fingerprint
Membrane proteins Proteins = major functional component Connection proteins
Types of membrane proteins Two configurations Integral Span entire width of membrane Part of membrane structure Peripheral Bind to inner or outer surfaces Distinct from membrane
Integral membrane proteins Hydrophilic surface region Hydrophobic transmembrane segments made of alpha-helices or beta-sheets
How do the cells get nutrients? Diffusion allows molecules dissolved in liquids to move from a highly concentrated region to a lesser concentrated region The interior of the cell must be close to the external environment
Cells are small! Most cells range in size from about 1 to 100 micrometers in diameter Because cells need to exchange nutrients and wastes with the environment through the plasma membrane
Why are cells so small? Reactants needed for metabolism are present in low concentrations Low concentration means reactants don t collide often This makes chemical reactions slow
Cell size and reactant concentration Concentration gets lower as cells get bigger What happens to chemical reaction rate in cells as cells get bigger?
Reaction rate and cell size Concentration gets lower as cells get bigger What happens to chemical reaction rate in cells as cells get bigger?
How are eukaryotic cells larger than prokaryotic ones? Eukaryotic cells are 10 to 100X larger than prokaryotic ones Eukaryotic cells have found a way around this: membrane-bound organelles Serve to concentrate reactants in appropriate compartments Improves cell efficiency
Cell size difference This means eukaryotic cells can be larger than prokaryotic cells
Cell size exercise Still, being small has some advantages Solutes taken into cells through membrane Consider 2 cubes (even though most cells are spherical) 1 m 2 m
Cell size exercise Consider the following calculations: Surface Area: length x width x 6 Volume: length x width x height Surface Area/Volume Cell 1 Cell 2 1 m 2 m
Cell size exercise Which cell has the greater surface area? Which cell has the greater volume? Which cell has the greater ratio of surface area to volume? Surface Area: length x width x 6 Volume: length x width x height Surface Area/Volume Cell 1 Cell 2 6 m 2 24 m 2 1 m 3 8 m 3 6 3
Cell size exercise Volume increases faster than surface area (x 3 vs x 2 ) So as cells get bigger, the proportion of surface area decreases Keeps cells small Cells need surface area to absorb solutes Less surface area = fewer reactions
Diffusion Leads to the Even Distribution of Molecules 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
Types of passive transport (diffusion) across membranes (extracellular fluid) O 2 Cl water glucose phospholipid bilayer (cytoplasm) channel protein aquaporin carrier protein (a) Simple diffusion through the phospholipid bilayer (b) Facilitated diffusion through channel proteins (c) Osmosis through aquaporins or the phospholipid bilayer (d) Facilitated diffusion through carrier proteins Facilitated diffusion or facilitated transport allows for the transport of specific molecules
Osmosis and Cells Osmosis is the diffusion of water across selectively permeable membranes Water diffuses from a region of high water concentration to one of low water concentration across a membrane Dissolved substances (solutes) reduce the concentration of free water molecules (solvent)
Water moves in or out of cells depending on the relative tonicity of the solution Isotonic No net movement of water across the membrane Hypertonic Water moves across a membrane toward the hypertonic solution Hypotonic Water moves across a membrane away from the hypotonic solution
Osmosis and cells 10% salt 90% water isotonic 10% salt 90% water hypertonic 30% salt 60% water 10% salt 90% water hypotonic 0% salt 100% water 10% salt 90% water
Osmosis and plants cells Plasmolysis Turgor pressure
Active Transport: Using Energy to Move Against the Gradient (extracellular fluid) 1 The transport 2 Energy from ATP 3 The protein protein binds both ATP and Ca 2 changes the shape of the transport protein and moves the ion across the membrane releases the ion and the remnants of ATP (ADP and P) and closes recognition site ATP ATP binding site ATP ADP P Ca 2 (cytoplasm) Cells use active transport to concentrate molecules in places they are needed
Electrochemical gradient across plasma membrane
Types of transporters Examples: Na + -K + ATPase, H + -K + ATPase, Ca 2+ ATPase, and H + ATPase
Primary active transport
Secondary active transport
Bulk transport for movement of large molecules Endocytosis Phagocytosis Pinocytosis Potocytosis Receptor-mediated
Bulk transport for movement of large molecules Exocytosis Transcytosis