Cell Membranes

1
Cell Membranes

• AP Biology
• Chapter 7

2
Selective Permeability

• Barrier that allows SOME substances to cross more
  easily than others
• Key aspect of membrane structure and function
• Remember selectively permeable membranes are
  important in many organelles as well as the
  plasma membrane of the cell itself.

3
Membrane Structure

• Primary components
• Lipids
• Foundation molecules
• Proteins
• Embedded within the lipids of the membrane
• Important in carrying out many membrane functions

4
Phospholipids

• Phospholipids are the specific lipids that make
  up the cell membrane
• Hydrophilic AND hydrophobic regions
• Phosphate (POLAR) head hydrophilic
• Fatty acid (NON POLAR) tails - hydrophobic

5
Phospholipid Bilayer foundation of membranes

• Because of the polar and nonpolar regions of the
  phospholipids, they tend to form a bilayer when
  placed in aqueous solution (water).
• Polar parts orient TOWARDS water
• Non polar parts orient AWAY from water (point
  toward the center of the membrane)
• To view the formation of a phospholipid bilayer,
  click HERE.

6
Proteins are Embedded in the Phospholipid Bilayer

• Transmembrane proteins
• Polar at the top and bottom
• Non polar in the middle
• Allows protein to be securely placed in the
  membrane

7
Fluid Mosaic Model

• 1972
• Singer and Nicolson
• The cell membrane is a mosaic of protein
  molecules bobbing in a fluid bilayer of
  phospholipids
• Replaced the old idea (Davson-Danielli model)
  that that the proteins were stuck onto the
  surface of the membrane
• Click HERE for an animation

8
Movements of the Phospholipid Bilayer

• FLUID Mosaic Model implies that membranes are not
  static, but have some fluidity
• Phopholipids can move around each other
  laterally, but seldom flip-flop from one side
  of the layer to the other
• Click HERE for an animation

9
Unsaturated phospholipids tend to be more fluid
than saturated ones.

• Kinks in fatty acid tails of unsaturated
  phospholipids make them unable to pack closely
  together.
• Caused by double bonds between carbons

10
Cholesterol and the Phospholipid Bilayer

• Reduces membrane fluidity by reducing
  phospholipid movement at moderate temperatures
• ALSO hinders solidification at lower temperatures
• antifreeze

11
Integral Proteins

• Transmembrane proteins with hydrophobic regions
  spanning the inner part of the bilayer
• Hydrophilic regions stick out at either side
• Causes protein to be locked into the bilayer
• That means integral to the bilayer
• See earlier slide on transmembrane proteins for
  another diagram

12
Peripheral Proteins

• Not embedded in the bilayer
• Loosely bound to the surface of a membrane

13
Overall membrane structure - diagram

• In addition to the bilayer and proteins, this
  diagram also shows
• Cytoskeleton
• Helps hold proteins in place on cytoplasmic side
  of membrane
• Extracellular Matrix
• Outside the cell
• Carbohydrates
• Most often used in cell-cell recognition
• Oligosaccharides bound to lipid or protein -
  Glycolipids or glycoproteins
• Vary from species to species individual to
  individual and cell type

14
Membrane Proteins - Functions
15
Importance of Cell-Cell Recognition

• Allows immune cells to distinguish between your
  own cells and invading cells
• Also, allows cells to determine placement during
  embryonic development
• Glycoproteins and glycolipids are important in
  cell-cell recognition
• Different according to species, individual and
  cell type
• Oligosaccharide short polysaccharide
• Binds to protein or lipid to make glycoprotein or
  glycolipid

16
Traffic Across Membranes

• Remember Selective Permeability
• Hydrophobic core of the bilayer is critical in
  determining what can cross a membrane
• Molecules that are IMPEDED by the hydrophobic
  core of the plasma membrane
• Ions
• Polar molecules
• Both are hydrophilic and will cling to water and
  other polar molecules rather than mingle with the
  inner part of the bilayer
• Molecules that can CROSS the hydrophobic core of
  the plasma membrane
• Hydrocarbons

17
Molecules that are IMPEDED by the hydrophobic
core of a plama membrane

• Ions
• H
• Na
• Polar molecules
• Large polar molecules, especially
• Glucose
• Basically any molecule that is charged or large
  and polar will find it difficult to cross the
  hydrophobic core
• Ions tend to collect water molecules around them
  (a hydration shell) This makes the particle large
  AND it will tend to be attracted to the polar
  heads of the bilayer NOT the hydrophobic core

18
Molecules that CAN cross a plasma membrane

• Anything nonpolar (hydrophobic)
• Hydrocarbons
• CO2 molecules
• Oxygen molecules
• Even really small polar molecules
• Though it is thought now that there are probably
  channels that help with these
• Aquaporin
• A transmembrane channel that allows water (a
  polar molecule) to pass through a cell membrane.

19
How do molecules that are impermeable to the
phospholipid bilayer manage to pass through it?

• Proteins play a key role in transport of
  molecules across cell membranes
• For example hydrophilic molecules that could
  not ordinarily pass through the phospholipid
  bilayer CAN pass through specially designed
  protein channels.
• Such channels are SPECIFIC for particular
  molecules

20
Types of Transport Across a Membrane

• Two major types
• Passive
• Does NOT require energy from the cell
• Substances move WITH their concentration gradient
• High to Low
• Active
• DOES require energy from the cell
• Substances move AGAINST their concentration
  gradient
• Low to High

21
Types of Passive Transport - Diffusion

• Diffusion click here for animation
• Tendency for molecules of ANY substance to spread
  out into available space.
• Substances move DOWN their concentration
  gradients
• From areas of HIGH concentration to LOW
  concentration
• NO Energy input from the cell is required
• Diffusion continues until equilibrium is reached
• Movement does not stop when equilibrium is
  reached, but no net gain or loss is detectable

22
Types of Passive Transport - Osmosis

• Osmosis
• Diffusion of WATER across a selectively permeable
  membrane
• High to Low concentration
• Hypertonic
• Water with a HIGH concentration of SOLUTES
• Hypotonic
• Water with a LOW concentration of SOLUTES
• Water moves from HYPOTONIC to HYPERTONIC
  solutions
• Click HERE, HERE or HERE for animation

23
Osmosis

• Isotonic Solutions
• Aqueous solutions that are in equilibrium
• Equal amounts of solute in water on either side
  of a membrane
• Water moves across the membrane at the same rate
  in both directions

24
Osmosis in Cells

• A cell placed in a HYPERtonic environment will
  LOSE water.
• Cell will become FLACCID
• Plasmolysis
• Plant cell membrane pulls away from the cell wall.

25
Osmosis in Cells

• A cell placed in a HYPOtonic environment will
  GAIN water
• If it is an animal cell, it will BURST (lysis)
• If it is a plant cell it will not burst, but will
  become TURGID due to presence of a cell wall.
• Cells in an isotonic environment will undergo no
  net changes
• Click HERE or HERE for tutorials

26
Osmoregulation

• Cells that lack cell walls AND that live in a
  hypotonic environment MUST CONTROL the amount of
  water in their cells in order to prevent rupture
  (lysis). This is known as OSMOREGULATION.
• Contractile Vaculoles
• Organelles that continually pump water out of a
  cell
• This is actually ACTIVE transport because it
  requires energy and moves water AGAINST its
  concentration gradient.

27
Types of Passive Transport - Facilitated Diffusion

• For years it was thought that at least some
  hydrophilic molecules could simply diffuse
  between the individual phospholipids of the
  bilayer if they were small enough, only weakly
  polar, etc.
• It is now thought that most hydrophilic
  molecules, no matter how small, have some sort of
  protein channel that makes their movement across
  the membrane possible.
• Diffusion of a SPECIFIC molecule through some
  protein channel facilitated diffusion

28
Types of Passive Transport - Facilitated Diffusion

• PASSIVE transport
• NO energy from cell is required
• Protein channels allow only a SPECIFIC molecule
  to pass through
• Molecules pass through by DIFFUSION High to Low
• Allows the cell to very precisely control what
  enters and leaves
• Molecules CANNOT flow AGAINST the concentration
  gradient.

29
Facilitated Diffusion Example

• Aquaporin is a protein channel that allows water
  to pass through the cell membrane by osmosis
• For years, it was thought that water could simply
  pass between phospholipids, but channels made of
  a protein called aquaporin were found to be vital
  in allowing water to pass through the membrane.
• In fact, without aquaporin, it was found that
  almost no osmosis occurs.
• For more, click HERE

30
ACTIVE Transport

• The PUMPING of some molecule AGAINST its
  concentration gradient
• LOW to HIGH
• Requires ATP
• Involves energy to change the shape of the
  channel proteins that pump the molecules across
• Conformational change in shape of the
  transmembrane protein (the pump)moves molecules
  AGAINST their gradient
• For an animation of active transport, click HERE

31
Active Transport - Types of Pumps

• Sodium Potassium Pump
• Na/K Pump
• Classic example
• Important in nerve cell and nerve impulse
  transmission (among others)
• Generates voltage difference across the membrane
  of the neuron
• For animations click
• HERE, HERE or HERE

32
Active Transport Types of Pumps

• Proton Pumps
• Pump Hydrogen ions (H) across a membrane
• Can also generate membrane potential (like Na/K
  pump)
• Difference in voltage from one side of the
  membrane to the other
• Proton pumps are VERY important in mitochondrion
  cell respiration

33
Cotransport

• A membrane protein couples the transport of one
  solute to another
• A combination of both active transport and
  facilitated diffusion (passive).
• Pump (active)
• Transport channel (passive)

34
Cotransport Example in plants

• Proton Pumps pump H against the concentration
  gradient and OUT of the cell.
• Creates a very hi concentration of H outside the
  cell.
• ACTIVE transport
• That means, if given an opportunity, H would
  flow WITH its concentration gradient back INSIDE
  the cell.

35
Cotransport Example in plants

• In plants, sucrose is in HIGH concentration
  inside the cells, but plants need to get even
  even MORE sucrose into cells for storage.
• Thus, sucrose must move into a plant cell AGAINST
  its concentration gradient.

36
Cotransport Example in plants

• One specific transport protein can actually
  couple the downhill (WITH concentration)
  movement of H back into the cell with the
  uphill (AGAINST concentration) movement of
  sucrose into the cell.
• Sucrose MUST bind to an H ion. Then sucrose can
  ride with the H as H flows INTO the cell
  through the cotransport channel WITH its
  concentration gradient.
• Remember, the H concentration gradient is
  maintained by the proton pump
• The only way that sucrose can enter is if it is
  bound to H. Then the very specific cotransport
  channel will allow both to diffuse through.
• PUMP to maintain H conc. ACTIVE
• Cotransport channel facilitated diffusion
  PASSIVE

37
Exocytosis and Endocytosis

• Movement of very large molecules or groups of
  molecules into or out of a cell
• bulk
• Both require energy from the cell

38
Exocytosis

• Large molecules or groups of molecules are
  secreted from a cell
• Transport vesicles fuse with the plasma membrane
  to release particles
• Examples
• Secretion of insulin by the pancreas
• See animation here

39
Endocytosis

• Cell takes in particles/macromolecules by forming
  new vesicles from the plasma membrane

40
Types of Endocytosis

• Phagocytosis
• Taking in a large food particle by wrapping a
  pseudopod around it and enclosing it in a food
  vacuole.
• See animation here
• Pinocytosis
• Taking droplets of liquid in using vesicles
• See animation here
• Both are NONspecific

41
Types of Endocytosis

• Receptor-Mediated Endocytosis
• SPECIFIC
• Proteins with specific receptor sites are
  embedded in the cells membrane
• In regions called coated pits
• Molecules capable of binding to the receptors are
  called ligands
• Example Cholesterol
• Allows the cell to acquire bulk amounts of a
  SPECIFIC substances
• Even when the substance may not be highly
  concentrated in the cellular fluid

42
Receptor-Mediated Endocytosis and
Hypercholesterolemia

• Cholesterol travels in particles called
  low-density lipoproteins (LDLs).
• LDLs bind to LDL receptors on membranes and enter
  cells by endocytosis
• In people with inherited hypercholesterolemia
  (high cholesterol in the blood), the LDL
  receptors are defective and LDLs cannot enter
  cells
• LDLs then accumulate in blood, where it builds up
  in blood vessels