Cells Their Environment Cell Membrane Every cell is

Cells & Their Environment

Cell Membrane **Every cell is surrounded by a cell membrane! - protects cell - helps move substances in & out of the cell

I. Homeostasis A. Definition – maintenance of stable internal conditions in a changing environment B. Cell Membrane & Homeostasis 1. Cell Membrane controls the movement of substances in & out of the cell 2. Provides structural support 3. Recognizes foreign material 4. Communicates with other cells

II. Lipid Bilayer A. Phospholipid – cell membrane is a “sea” of phospholipids in which proteins “float” 1. Phosphate head is polar, hydrophilic, waterloving 2. Two fatty acid tails are nonpolar, hydrophobic, water-fearing

B. Membrane Structure 1. Lipid Bilayer – hydrophobic tails are inside, protected by hydrophilic heads on outside C. Barrier – phospholipids form a barrier through which only small, nonpolar substances can pass

C. Cell Membranes are Fluid 1. Phospholipids are the most abundant lipid in the plasma membrane 2. The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it

Figure 7. 3 Phospholipid bilayer Hydrophobic regions of protein Hydrophilic regions of protein

Figure 7. 4 TECHNIQUE Extracellular layer Knife Plasma membrane Proteins Cytoplasmic layer RESULTS Inside of extracellular layer Inside of cytoplasmic layer

3. Phospholipids in the plasma membrane can move within the bilayer 4. Most of the lipids, and some proteins, drift laterally • Rarely does a molecule flip-flop transversely across the membrane

Figure 7. 6 Lateral movement occurs 107 times per second. Flip-flopping across the membrane is rare ( once per month).

III. Membrane Proteins A. Proteins in Lipids 1. Integral Proteins – proteins stretch across the bilayer & face both inside and outside 2. Peripheral Proteins – proteins that are only on one side (face inside or outside) of the membrane

B. Types of Proteins 1. Receptor Proteins – sense the cell’s surroundings by binding to substances outside the cell 2. Enzymes – proteins that help biochemical reactions in a cell 3. Transport Proteins – aid the movement of certain substances in & out of the cell

Types of Proteins in the Membrane