The Plasma (Cell) Membrane What is the function?

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Why must cells control materials that enter and leave the cell? Cells need to maintain a balance (homeostasis)

Why do organisms have many small cells instead of a few large cells? Cells are limited by surface-area-to-volume ratio Smaller cells – higher ratio Larger cells – lower ratio Hint : Think of a pile of wet laundry

Relationship btwn surface area & volume Surface area = 6 mm2 Volume = 1 mm3 Surface area = 24 mm2 Volume = 8 mm3 1 mm 2 mm 4 mm Surface area = 96 mm2 Volume = 64 mm3 Not enough membrane to allow materials in/out

Then how can eukaryotic cells be bigger than prokaryotic cells? Organelles bound by membranes - increases surface area Organelles help divide jobs for the cell

Why is the Plasma Membrane so Important? Allows cells to be alive Separates life from non life

What is the main component of the cell membrane? Lipids (aka Fats, oils, and waxes)

Monomers and Polymers Cells build macromolecules (aka polymers) by bonding small molecules (monomers)

Monomers-2 parts: glycerol & 3 fatty acids Elements: Carbon, Hydrogen, Oxygen Do not mix w/water Different types of fatty acids make lipids different

Function Long term energy storage Insulation Protection

Specialized Lipid- phospholipid Glycerol Backbone Two Fatty Acid Chains Phosphate Group

Phospholipid bilayer Phosphate head (glycerol +phosphate) hydrophilic – “water-loving” 2 Fatty acid tails hydrophobic – “water-fearing”

Inside of cell Outside of cell protein carbohydrate cholesterol phospholipid

Fluid Mosaic Model Molecules freely move sideways w/in each layer Flexible Proteins – communication & control movement of large molecules in/out Cholesterol – increase stability & strength Carbohydrates – identification & communication

Selectively Permeable aka semi-permeable Allows some smaller materials to pass (ex. water, glucose) Keeps out bigger materials (ex. starch, polymers)