Chapter 6

BiologySixth Edition

Raven/Johnson

(c) The McGraw-Hill Companies, Inc.

Membranes

• Allows a cell to interact with its environment

• Delicate skin of proteins embedded in a thin sheet of lipid

Phospholipid Bilayer• Membranes are only two molecules (not atoms)

thick• The lipid that makes up the membrane is a

phospholipid

Hydrophilic polar heads lie on the outward-facing surfaces.

Hydrophobic nonpolar tails extend to the interior.

-a fluid consistency and a mosaic pattern of embedded proteins.

Components of the Cell Membrane• Lipid Bilayer• Transmembrane proteins• Network of supporting fibers• Exterior proteins and glycolipids

Figure 6.7

Non-Polar sections of protein force the molecule to stay within in the membrane.

Polar ends extend on either side of membrane

A single nonpolar segment is adequate to anchor a protein

Attach to the spectrin network of the cytoskeleton

Receive signals

Channel and carrier proteins have several non-polar helical segments

Allows passage of water soluble molecules or ions

Beta – pleated sheets form a large open tunnel

Allows water and other materials to pass through

Movement from an area of high concentration to an area of low concentration

Ion channels allow movement of ions by diffusion

Most channels are specific only for a certain type of ion

Carriers transport ions as well as other solutes

Physically binds with the solute

Movement is determined by the concentration gradient 3 characteristics of facilitated diffusion

1. It is specific

2. It is passive

3. It saturates

Water molecules move through specialized channels called aquaporins along a concentration gradient

Concentration of all solutes in a solution determines the osmotic concentration

Cell membrane is a differentially permeable membrane

Bulk Passage Into and Out of the Cell

Endocytosis – 3 types: Phagocytosis, Pinocytosis, and receptor-mediated endocytosis

- brings items into the cell

Exocytosis – discharges materials from the cell

Phagocytosis – particulate (solid) matter

Pinocytosis – material is liquid

Receptor-mediated endocytosis – specific molecules

Exocytosis – discharges material from the cell

Requires energy (ATP)

Involves a highly selective carrier protein

Moves substance against a concentration gradient

Works through a series of conformational changes – the first is energized by ATP

(active transport)

This process can occur up 100 times per second!

Moves protons against their gradient

Down gradient – high low concentration (no energy required)

Up gradient – low high concentration (energy required)

The Down gradient for Na+ is greater than the Up gradient for glucose

Protein here is a symport – both items move in same direction

Antiport – items move in opposite directions Na+/Ca++

(counter transport)

The End.