I. Maintaining Balance
• How do cells maintain balance?
– Cells need to maintain a balance by controlling material that move in and out of the cell
–> HOMEOSTASIS.

I. Maintaining balance
• Small molecules like water, oxygen, and carbon dioxide can move in and out of the cell freely.
• Large molecules like proteins and carbohydrates cannot.
• Eliminating wastes.

I. Maintaining balance

Structure of the Plasma
(cell) membrane
• All cells are surrounded by a plasma membrane
• Functions like a GATE, controlling what
ENTERS and LEAVES the cell.
• The cell membrane is semipermeable or selectively permeable.

Structure of the Plasma
(cell) membrane
• A semipermeable membrane only allows
certain molecules to pass through.
– Some substances easily cross the membrane, while others cannot cross at all.

Structure of the Plasma
(cell) membrane
• Made of a thin layer of
lipids and proteins
– Made mostly of
phospholipid molecules
(phosphate + lipid).
– Phospholipids are a kind of lipid that consists of 2
FATTY ACIDS (tails), and
PHOSPHATE GROUP
(heads).

Structure of the Plasma
(cell) membrane
• Cell membranes consist of TWO phospholipid layers called a LIPID
BILAYER.

Structure of the Plasma
(cell) membrane

Structure of the Plasma
(cell) membrane
• Water molecules surround both sides of the cell membrane.
– Polar phosphate heads sticking
TOWARD the water (hydrophilic)
– Nonpolar lipid tails pointing AWAY from the water (hydrophobic)

Structure of the Plasma
(cell) membrane
• The cell membrane is constantly being
formed and broken down in living cells.

III. Lipid bilayer
• Moving with and among the phospholipids are cholesterol, proteins, and carbohydrates.
– Cholesterol:
• Nonpolar, found among the phospholipids to help prevent the fatty acid tails from sticking together
• Helps with structure and homeostasis.

III. Lipid bilayer

III. Lipid bilayer
• Proteins:
– Found on the surface of the plasma membrane = transmit signals to the inside of the cell.
– Embedded in the plasma membrane = structure and support of cells shape, and move large substance in and out of the cell.

III. Lipid bilayer

III. Lipid bilayer
• Carbohydrates:
– Attached to proteins, helps cells identify chemical signals.
– Ex.: help disease fighting cells recognize and attack a potentially harmful cell.