Dr Pradeep KumarProfessor, PhysiologyKGMU

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% ~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)

hydrophobictails

hydrophilicheads

hydrophilicheads

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 protein receptor protein transport 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 waterinto 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

channelprotein

proteins formingpermanenthydrophilic 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 changesshape, transporting moleculeacross 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 Ca2+.

Transport protein uses energy from ATP to change shape and moveion across membrane.

Transport protein resumes original shape.

(cytoplasm)

transport protein

ATP binding

site

recognitionsite

ATP

Ca2+

(extracellular fluid)

(cytoplasm)

food particle

particle enclosed in vesicle

phagocytosis

vesicle containing extracellular fluid

cell

pseudopod

pinocytosis

(a)

(b)

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