The Cell Membrane

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The Cell Membrane
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Overview

• The cell membrane separates a living cell from
  its nonliving surroundings
• thin barrier 8nm thick (nnano10-9)
• Controls traffic in out of the cell
• selectively permeable
• allows some substances to cross more easily than
  others- choosy
• Made of phospholipids, proteins , cholesterol,
  and carbohydrates.

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Phospholipids
Phosphate

• Fatty acid tails
• Hydrophobic
• Water fearing
• Phosphate group head
• Hydrophilic
• Water loving
• Arranged as a bilayer

Fatty acid
Inside cell
Why do the phospholipids arrange themselves like
this?
Outside cell
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More than lipids

• In 1972, S.J. Singer G. Nicolson proposed that
  membrane proteins are inserted into the
  phospholipid bilayer
• The composition of the cell membrane is called
  the fluid mosaic model because the phospholipid
  bilayer and the embedded proteins can move around
  like a fluid to let compounds into and out of
  the cell.

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Cholesterol also makes up the cell membrane
structure. It is between the tails of the
phospholipids.
Fluid outside the cell
Phospholipids
Cholesterol
Cytoplasm
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Membrane Proteins

• There are 2 types of membrane proteins
• peripheral proteins
• loosely bound to the surface of the membrane
• integral proteins
• Pass through the lipid bilayer

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Why areproteins the perfect molecule to build
structures in the cell membrane?
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Protein Structure

• Proteins are molecules that are composed of amino
  acids linked together by peptide bonds.
• 20 different amino acids exist
• Proteins have complex shapes that include various
  folds, loops, and curves.
• Chemical bonding between portions of the protein
  chain aid in holding the protein together and
  giving it its 3-dimensional shape.

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Some amino acids are hydrophobic and some are
hydrophilic
nonpolar hydrophobic
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polar hydrophilic
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Many Functions of Membrane Proteins
Outside
Plasma membrane
Inside
Transporter
Enzymeactivity
Cell surfacereceptor
Cell adhesion
Cell surface identity marker
Attachment to thecytoskeleton
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Membrane carbohydrates

• Play a key role in cell-cell recognition
• ability of a cell to distinguish one cell from
  another
• basis for rejection of foreign cells by immune
  system

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Sketch the structure of the cell membrane- label
the phospholipid bilayer, cholesterol, integral
protein, peripheral protein, and carbohydrate
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Movement across the Cell Membrane
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• What is the natural movement of molecules into
  and out of the cell?

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Simple Diffusion

• Movement from HIGH to LOW concentration
• passive transport
• no energy needed

diffusion
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Diffusion across cell membrane

• The cell membrane is the boundary between the
  inside outside

cell needs food and materials in products or
waste out
IN food sugars proteins lipids salts O2 H2O
OUT waste salts CO2 H2O products
OUT
IN
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Simple diffusion through phospholipid bilayer

• What molecules can get through directly?
• fats other lipids

• What molecules can NOT get through directly?
• H2O
• Salts
• Sugars

lipid
salt
NH3
aa
H2O
sugar
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How do the non-fat compounds get in/out?

• Membrane becomes selectively-permeable with
  protein channels formed by integral proteins.
• This is called facilitated diffusion

inside cell
sugar
aa
H2O
salt
outside cell
NH3
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Facilitated Diffusion

• no energy needed
• still moving molecules from high to low
  concentration

facilitate to help
The Bodyguard
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Osmosis

• Water is very important to cell function
• Diffusion of water from high concentration of
  water to low concentration of water is osmosis
• across a selectively-permeable membrane

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Concentration of water

• The direction of osmosis is determined by
  comparing solute concentrations on each side of
  the membrane. What is a solute?
• Hypertonic - more solute, less water
• Hypotonic - less solute, more water
• Isotonic - equal solute, equal water

water
net movement of water
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For each cell, label the solutions as hypertonic,
hypotonic or isotonic and draw an arrow to show
the direction of water movement.
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Managing water balance

• Cell survival depends on balancing water uptake
  loss

freshwater
balanced
saltwater
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Managing water balance

• A cell in fresh water
• example Paramecium
• What type of environment?
• problem gains water, swells can burst
• solution contractile vacuole
• pumps water out of cell which
• requires ATP (energy)

ATP
freshwater
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Water regulation

• Contractile vacuole in Paramecium

ATP
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Managing water balance

• Another example
• Plant cells
• When in a hypotonic (freshwater) environment they
  are constantly taking up water.
• Do they burst?
• No!
• Plant cells have a cell wall that prevents
  bursting.
• They build up turgor pressure which makes the
  plant stand tall.
• Plants lose water out of their leaves
  transpiration

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Managing water balance

• Another example
• Plant cells
• How do they deal with hypertonic environment?

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Getting through cell membrane

• Passive Transport
• diffusion with the concentration gradient
• high ? low
• Simple diffusion
• diffusion of hydrophobic molecules
• Lipids (fats)
• Facilitated transport
• diffusion of hydrophilic molecules
• through a protein channel
• high ? low concentration gradient
• Active transport
• diffusion against the concentration gradient
• low ? high
• uses a protein pump
• requires ATP

ATP
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Transport summary
simplediffusion
facilitateddiffusion
ATP
activetransport
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Active Transport

• Why is active transport necessary?
• The Na/K pump is the way that our nervous
  system works. By pumping ions up their
  concentration gradients, electricity is generated.

conformational change
ATP
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Na/K Pump

• The steps of the pump are as follows
• 3 Na ions inside the cell bind to the carrier
  protein (pump).
• A phosphate from ATP attaches to the pump and the
  pump changes shape.
• The 3 Na ions are dumped outside the cell.
• 2 K ions outside the cell bind to the pump.
• The phosphate detaches from the pump and the pump
  goes back to its original shape.
• The 2 K ions are dumped inside the cell.

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How about large molecules and fluids?

• Endocytosis- Active Transport
• The taking in of large molecules (too big to pass
  through the membrane) or a large amount of a
  fluid by engulfing. To engulf means that the
  cell membrane pinches in and surrounds what is
  being ingested. The molecules are then digested
  in a pouch called a vesicle.
• phagocytosis Ingesting large molecules
• pinocytosis Ingesting large amounts of a fluid
• Draw endocytosis

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• Exocytosis- Active Transport
• The opposite of endocytosis. The vesicle fuses
  with the cell membrane and it then opens up to
  release the contents.