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Home , Facilitated diffusion, Passive transport

Cell 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 , 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 • Lecture on reactions and enzymes Learned so far

• Introduction • Matter • Macromolecules • Cells • Parts of the cells Plasma membrane is a bilayer embedded with other components

Alpha-helix 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 membrane functions

1. Isolation 2. Regulation 3. Sensitivity 4. Attachment 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 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 • 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

extracellular fluid phospholipid (watery environment) hydrophilic heads

hydrophobic tails Plasma membrane hydrophilic heads

(watery environment) Cell membrane structure

• Membrane must be very fluid • Fluidity adjusted by changing saturation of fatty acid tails • More saturated = less fluid

extracellular fluid phospholipid (watery environment) hydrophilic heads

hydrophobic tails Plasma membrane 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 • 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?

• 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:

Cell 1 Cell 2 Surface Area: length x width x 6 Volume: length x width x height Surface Area/Volume

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?

Cell 1 Cell 2 Surface Area: length x width x 6 6 m2 2424 mm22 Volume: length x width x height 1 m3 88 mm33 Surface Area/Volume 6 33 Cell size exercise

• Volume increases faster than surface area (x3 vs x2) • 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

2 Dye molecules diffuse into the water; 3 Both dye molecules 1 A drop of dye is water molecules diffuse and water molecules are placed in water into the dye evenly dispersed

drop of dye water molecule Types of (diffusion) across membranes

(extracellular Cl– water fluid) O2

phospho- bilayer

channel aquaporin carrier protein protein (cytoplasm)

(a) Simple diffusion through (b) (c) Osmosis through (d) Facilitated diffusion through the phospholipid bilayer through channel proteins aquaporins or the carrier proteins phospholipid bilayer

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

isotonic hypertonic hypotonic 10% salt 30% salt 0% salt 90% water 60% water 100% water

10% salt 10% salt 10% salt 90% water 90% water 90% water Osmosis and plants cells

Plasmolysis Turgor : Using to Move Against the Gradient (extracellular fluid)

1 The transport 2 Energy from ATP 3 The protein protein binds both changes the shape of the releases the ion and ATP and Ca2 transport protein and moves the remnants of ATP the ion across the membrane (ADP and P) and closes

ADP ATP recognition binding ATP site P site ATP Ca2 (cytoplasm)

Cells use active transport to concentrate molecules in places they are needed across plasma membrane Types of transporters

• Examples: Na+-K+ ATPase, H+-K+ ATPase, Ca2+ ATPase, and H+ ATPase Primary active transport Secondary active transport Bulk transport for movement of large molecules • • • • • Receptor-mediated Bulk transport for movement of large molecules • •