Plasma Membrane

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.