Cell Membranes and Transport

A Cell is a Container• A cell is the basic unit of life

• A cell and some organelles are containers formed by one or more membranes

• Membranes must be selectively permeable barriers to maintain specific internal conditions for life functions (homeostasis).

Phospholipid

← attracts water

← repels water

Phospholipids Form a Bilayer in Water

The Cell Membrane: Fluid Mosaic Model

Components of the Cell Membrane• Phospholipid Bilayer: serves as a barrier

separating water in (cytoplasm) from water out (extracellular fluid or environment)

• Cholesterol: stabilizes membrane at warm temperatures

• Membrane Proteins: proteins embedded in bilayer that serve various functions such as transporting substances across the membrane (transport proteins), receiving chemical signals (receptor proteins), cell-cell recognition (recognition proteins).

Diffusion

Diffusion Does Not Require Energy

Osmosis = diffusion of water

Osmosis

Osmosis

Blood cells in isotonic solution

(0.9% NaCl)

Blood cells in hypertonic solution

(1.01% NaCl)

Blood cells in hypotonic solution

(0.65% NaCl)

Some single-celled organisms must actively regulate water balance

Facilitated Diffusion vs. Active Transport