Cell physiologyBDS Lecture2 Dr Pradeep Kumar Professor Physiology

Cell physiology-BDS Lecture-2 Dr Pradeep Kumar Professor, Physiology KGMU

Cell Membrane: Structure and Function The Plasma Membrane – a Phospholipid Bilayer

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

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

Plasma membrane structure and functions 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)

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

Membrane Structure and Function �Membranes are “fluid mosaics” with proteins embedded in or attached to the membrane �Proteins can move within the fluid lipid bilayer

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

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

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

Transport across membranes � 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. drop of dye pure water 2 Dye molecules diffuse into the water; water molecules diffuse into the dye. 3 Both dye molecules and water molecules are evenly dispersed.

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

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

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

Transport across membranes Passive transport…(cont. ) ◦ 3. Facilitated diffusion

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

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

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

(a) pinocytosis (extracellular fluid) 1 2 3 vesicle containing extracellular fluid (cytoplasm) cell (b) phagocytosis food particle pseudopod 1 2 3 particle enclosed in vesicle

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