Dr Pradeep Kumar Professor, Physiology KGMU

The Plasma Membrane – a Phospholipid Bilayer

Components of Plasma Membrane Lipids Proteins 1) Phospholipids 2) Cholesterol 3) Glycolipds 1) Peripheral (Associated) 2) Integral (Membrane Spanning ) 3) Glycoproteins ~50% (75%) (20%) (5%)

 Isolate the cell’s contents from the external environment  Regulate traffic in and out of the cell  Communicate with other cells

The phospholipid bilayer 1.Impermeable to water-soluble and polar molecules, ions 2.Permeable to small and nonpolar molecules 3.Lipids oriented with polar heads facing out

tails (hydrophobic) head (hydrophilic)

hydrophobic tails hydrophilic heads hydrophilic heads extracellular fluid (watery environment) cytoplasm (watery environment) phospholipid bilayer

 Membranes are “fluid mosaics” with proteins embedded in or attached to the membrane  Proteins can move within the fluid lipid bilayer

extracellular fluid (outside) cytoplasm (inside) protein filaments recognition proteinreceptor proteintransport protein binding site phospholipid bilayer phospholipid cholesterol carbohydrate

1. Transport proteins ◦ regulate the movement of water-soluble molecules across the membrane  Ion Channel proteins, Pumps, Receptors  Carrier proteins, Enzymes, Cell adhesion Molecules

2. Receptor Proteins ◦ trigger cellular response when specific molecules bind to them Nervous system Endocrine system

 Passive transport is a function of molecular size, lipid solubility, and size of the concentration gradient 1. Simple diffusion

1 A drop of dye is placed in water. 2 Dye molecules diffuse into the water; water molecules diffuse into the dye. drop of dye pure water 3 Both dye molecules and water molecules are evenly dispersed.

(a) simple diffusion (extracellular fluid) (cytoplasm)

Passive transport…(cont.) ◦ 2. Osmosis a. Isotonic b. Hypertonic c. Hypotonic

(a) isotonic solution(b) hypertonic solution(c) hypotonic solution 10 micrometers equal movement of water into and out of cells net water movement out of cells net water movement into cells

Passive transport…(cont.) ◦ 3. Facilitated diffusion

ions (b) facilitated diffusion through a channel channel protein proteins forming permanent hydrophilic channel

(extracellular fluid) (cytoplasm) Carrier protein has binding site for molecule. amino acids, sugars, small proteins (c) facilitated diffusion through a carrier carrier protein Molecule enters binding site. Carrier protein changes shape, transporting molecule across membrane. Carrier protein resumes original shape.

Energy-requiring transport 1. Active transport Ion gradients and energy production 2.Endocytosis 3.Exocytosis

Primary Active Transport: The Na + /K + Pump Antiport

Secondary Active (Indirect): e.g., Na + /Glucose transporter Symport

(extracellular fluid) Transport protein binds ATP and Ca 2+. Transport protein uses energy from ATP to change shape and move ion across membrane. Transport protein resumes original shape. (cytoplasm) transport protein ATP binding site recognition site ATP Ca 2+

(extracellular fluid) (cytoplasm) food particle particle enclosed in vesicle phagocytosis vesicle containing extracellular fluid cell pseudopod pinocytosis (a) (b)

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