Cell Membrane

Selectively Permeable

Basic Structure

• Double layer of phospholipids

• Referred to a bilayer• A phospholipid has a

head and two tails• The phospholipids

face away from each other in the bilayer

Phospholipids

• Phosphate “heads” are water-soluble

• They like water and are attracted to water

• Fatty acid “tails” are not soluble in water

• They repel water and want to stay away from it

Permeability

• Small molecules that dissolve in lipids can pass right through the phospholipid bilayer

• Oxygen, carbon dioxide and some water can also pass through

Permeability

• Glucose, amino acids, and other large molecules that do not dissolve in lipids cannot pass through

• Most water molecules, and ions such as H+, NA+, K+, and Ca+ cannot pass through

Some molecules need help

• We use proteins embedded in the membrane to help get these molecules across the cell membrane

Phospholipid bilayer plus

Notice that there are many structures stuck in the phospholipid bilayer

Proteins scattered throughout

A Gallery of Membrane Proteins

• Adhesion proteins: Help cells of the same type stick together in tissues

• Cell to cell communication proteins: match up with another cell to pass on a signal

A Gallery of Membrane Proteins

• Receptor proteins: stick out of the membrane to bind hormones.

• Recognition proteins: Identify each

person’s cells as their own

• Must be matched in transplants, blood transfusions, etc.

A Gallery of Membrane Proteins

• Passive Transporter proteins: allow molecules to move through them without requiring energy

• Active transporters: use energy to pump molecules “uphill” across the membrane

Transport Across Cell Membranes

Passive or Active

Movement

• Is either passive or active• Passive does not require cell energy• Active uses cell energy

Passive Mechanisms of Transport

• Diffusion: molecules spread or scatter from regions of higher concentration toward regions of lower concentration

• Molecules will travel down the concentration gradient

Passive Mechanisms of Transport

• Facilitated Diffusion: molecules cannot

pass through the lipid bilayer without the help of a membrane protein. The molecule being transported combines with a carrier molecule which moves it to other side

Passive Mechanisms of Transport

• Osmosis: the diffusion of water. Water moves from a region of higher concentration to a region of lower concentration

• In this example, the membrane is NOT permeable to the solute

Passive Mechanisms of Transport

• Filtration: forces molecules through membranes

• Often used to separate solids from liquids

• Kidneys filter blood, the force coming from blood pressure

Active Mechanisms of Transport

• Active transport: uses energy and carrier proteins to move molecules from areas of LOW concentration to areas of HIGH concentration

• Against the concentration gradient; a pump

Active Mechanisms of Transport

• Endocytosis: uses cellular energy to form a vesicle from a section of cell membrane to bring IN substances

• Exocytosis: uses energy to secrete a substance stored in a vesicle