Unit 2 – The cell membrane Biology

Plasma Membrane It protects and supports the cell and also controls everything that enters and leaves the cell. Selective permeability (semipermeability) – ability to allow only certain molecules in or out of the cell. *Remember: both plant and animal cells have a plasma membrane. The plant cell has a cell wall then the plasma membrane.

Phospholipid Bilayer Consists of fatty acids and glycerol. Each phospholipid molecule has a head and two tails. The head “loves” water (hydrophilic) and the tails “hate” water (hydrophobic).

What enters the cell? Molecules that are hydrophobic can easily pass through the plasma membrane, if they are small enough, because they are water- hating like the interior of the membrane. Hydrophilic molecules cannot pass through the plasma membrane. –they need help to pass through.

Other molecules in the plasma membrane Plasma membranes also contain other molecules: – Cholesterol (lipid) – helps the plasma membrane keep its shape. – Proteins – assist other substances in crossing the membrane.

Carbohydrates Carbohydrates (sugars) found attached to proteins or lipids on the outside of a cell membrane. Provides cushioning and protection for the plasma membrane. Also important in cell recognition – your body can recognize cells and determine if they should be there or not. Also acts as a glue to attach cells together.

Proteins Peripheral proteins – are located outside of the lipid bilayer. – act as enzymes, receptors for specific molecules, or transport materials. Integral proteins – embedded in lipid bilayer.

Transport Passive transport – does not require ATP, with concentration gradient – molecules move from high concentration to low concentration Active transport – requires ATP, against concentration gradient – molecules move from low concentration to high concentration.

Passive transport 1.Simple diffusion – movement of a substance across a membrane. 2.Osmosis – movement of water across membrane. 3.Facilitated diffusion – uses transport proteins to moves substances across the membrane. a.Channel proteins – tiny holes, allows water molecules and small ions to pass through. b.Carrier proteins – bind with specific ions or molecules, changes shape allowing them to carry ions or molecules across the membrane.

Active Transport Energy is needed because the substance is moving from an area of lower concentration to an area of higher concentration. 1.Sodium-potassium pump – sodium ions are pumped out of the cell, and potassium ions are pumped into the cell. -- both ions move from areas of lower to higher concentration.

Vesicle Transport Big molecules can be moved out or in cell. 1.Endocytosis – vesicle transport that moves a substance into the cell. a.Phagocytosis – taking in a solid. b.Pinocytosis – taking in a liquid. 2.Exocytosis – vesicle transport that moves a substance out of the cell.