cell membranes - linn–benton community...
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Cell membranesStef 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
Cytoskeleton
intermediatefilaments
Light micrograph showing the cytoskeleton
microtubules
microfilaments
microtubules (red)
microfilaments (blue)
nucleus
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
hydrophilicheads
hydrophobictails
hydrophilicheads
extracellular fluid(watery environment)
cytoplasm(watery environment)
Plasma membrane
Cell membrane structure
• Membrane must be very fluid
• Fluidity adjusted by changing saturation of fatty acid tails• More saturated = less fluid
phospholipid
hydrophilicheads
hydrophobictails
hydrophilicheads
extracellular fluid(watery environment)
cytoplasm(watery environment)
Plasma membrane
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)…
2 m1 m
Cell size exercise
• Consider the following calculations:
2 m1 m
Cell 1 Cell 2
Surface Area: length x width x 6
Volume: length x width x height
Surface Area/Volume
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
Volume: length x width x height
Surface Area/Volume
6 m2 24 m2
1 m3 8 m3
6 3
6 m2
1 m3
6
24 m2
8 m3
3
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
A drop of dye isplaced in water
Dye moleculesdiffuse into the water;water molecules diffuseinto the dye
Both dye moleculesand water molecules areevenly dispersed
drop of dye
water molecule
2
13
Types of passive transport (diffusion) across membranes
water glucose
carrierprotein
aquaporinchannelprotein
phospho-lipidbilayer
(cytoplasm)
(extracellularfluid)
Cl–
O2
(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
hypertonicisotonic hypotonic
10% salt90% water
10% salt90% water
10% salt90% water
10% salt90% water
0% salt100% water
30% salt60% water
Osmosis and plants cells
Plasmolysis Turgor pressure
Active Transport: Using Energy to Move Against the Gradient
The transportprotein binds bothATP and Ca2
Energy from ATPchanges the shape of thetransport protein and movesthe ion across the membrane
The protein releases the ion andthe remnants of ATP(ADP and P) and closes
ATPbindingsite
recognitionsite
ATP
P
ADP
Ca2
(extracellular fluid)
(cytoplasm)
ATP
1 2 3
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, Ca2+ 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