Cell Membrane

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• Cell Membrane

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Cell Size

• Most cells are very small.
• Why dont we have huge cells?
• Whats the limiting factor?

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Cell Size

• Cell parts cannot be too far from the cell
  membrane.
• All substances (food, waste, information) must
  cross the cells surface in order to enter or
  leave.
• If the cell were too large, this food and
  information could not reach all the important
  components of the cell.
• Small cells work more efficiently than larger
  cells.
• There are roughly 100 trillion cells in the human
  body.

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Volume vs. Surface Area

• As a cell grows, the volume increases at a much
  faster rate than surface area.
• If the surface area becomes too small, the cell
  cannot take enough food into support its size.
• It cannot remove enough waste products for its
  size either.
• Small cells exchange substances much faster than
  larger cells.
• Need High Surface to Volume Ratio

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Cell Size

• Smaller IS better!
• Cells want a HIGH surface to volume ratio.

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Membrane Overview

• Cell membrane separates living cell from
  nonliving surroundings
• thin barrier
• Controls traffic in out of the cell
• selectively permeable
• allows some substances to cross more easily than
  others
• Made of phospholipids, proteins other
  macromolecules

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Cell Membranes A.K.A.s

• The cell membrane has a lot of names!
• Plasma membrane
• Phospholipid Bilayer
• Cytoplasmic Membrane

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Phospholipids
Phosphate

• Fatty acid tails
• Nonpolar hydrophobic
• Phosphate group head
• Polar hydrophilic
• Arranged as a bilayer

Fatty acid
Aaaah, one of thosestructurefunction examples
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Phospholipid Arrangement

• The phosphate heads that are hydrophilic are on
  the outside
• The lipid tails that are hydrophobic get pushed
  to the inside

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Phospholipid bilayer
polar hydrophilic heads
nonpolar hydrophobic tails
polar hydrophilic heads
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Membrane is a collage of proteins other
molecules embedded in the fluid matrix of the
lipid bilayer
Extracellular fluid
Phospholipids
Cholesterol
Cytoplasm
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Cell Membrane Proteins

1. Transport Proteins - allow molecules to cross
   through the membrane
2. Receptor Protein - Transfer information from one
   side to the other
3. Marker Protein - Identification for the cell

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Marker Protein
Transport Protien
Receptor Protein
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Bilayer Characteristics

• Two important characteristics
• Selectively Permeable - regulates the molecules
  that enter and leave, thereby helping to maintain
  homeostasis
• Has Fluidity - Flexible, able to move, and to
  change cell shape (due to lipids)
• Fluid Mosaic Model!

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Bilayer Stops Polar Molecules

• Ions and most polar molecules (sugars proteins)
  are attracted to the water inside and outside of
  cell.
• BUT, they are repeled by the nonpolar interior of
  the lipid bilayer.
• Therefore, the interior of the cell membrane acts
  as a hydrophobic barrier.

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Bilayers are Fluid

• Phospholipids and the proteins that make up the
  bilayer are not fixed in place.
• The phospholipids and protein molecules can move
  from one region of the cell membrane to another.
• Fluid-Mosaic Model - current model for the
  composition of the cell membrane.
• Cells can alter their cell membranes to fit the
  specific needs of different cell types.