Cells and Their Environment

Ch. 4

Biology

Membrane Structure

Phospholipid Bilayer 2 layers of phospholipids Proteins

Transport Receptors

Cholesterol Maintains fluidity

Fluid-Mosaic Model

Plasma membrane contains a variety of molecules

Molecules move laterally

Selectively Permeable

Because of polar and nonpolar regions of the phospholipid bilayer, the membrane allows certain materials in and certain materials out of the cell Small molecules Small ions Nonpolar molecules

Passive Transport

Diffusion caused by the random movement of particles across a membraneMovement due to concentration gradient Moving from a higher concentration to a lower

concentration No energy used

Movement continues until equilibrium reached Concentration is the same on both sides of the

membrane

Osmosis

Diffusion of water across a selectively permeable membrane

Does not use energy

Direction of Water Movement

Hypertonic solution Solution contains more solute than the solution

it is compared with

Hypotonic solution Solution contains less solute than the solution it

is compared with

Isotonic solution Solution contains same amount of solute as the

solution it is compared with

Facilitated Diffusion

Passive transport—no energy used

Uses transport proteins embedded in the plasma membrane (ion channels) Potassium (K+) Sodium (Na+) Calcium (Ca+) Chloride (Cl-)

Pores—always open

Gated channels—open/close in response to stimuli

Active Transport

Substances are transported across a membrane, against the concentration gradient

Use carrier proteins embedded in the membrane

Use energy (ATP)

Sodium-Potassium Pump

Sodium-Potassium Pump

1. 3 Na+ ions inside the cell bind to the pump. ATP donates a phosphate to the pump

2. The pump changes shape, transporting 3 Na+ across the membrane, and are released

3. 2 K+ ions outside the cell bind to the pump and are transported across the membrane

4. The phosphate group is released and the 2 K+ are released

Sodium-Potassium Pump

Vesicles Move Substances

Substances that are too large for carrier proteins (proteins and polysaccharides)

Endocytosis—movement of substances into the cell (a.k.a. phagocytosis)

Exocytosis—movement of substance out of the cell

Receptor Proteins

Protein that binds a specific signal molecule, allowing the cell to respond to the signal moleculeSome receptor proteins are attached to ion channels Changes permeability to a specific ion

Some may cause the formation of a second messenger, which acts as a signal molecule in the cytoplasmSome may act as enzymes to speed up chemical reactions by lowering the activation energy