Membrane Structure & Function (Outline) • Basic function, biochemical composition , and physical fluid state • Role of cholesterol for animal cells membranes. • Summarize the functions of membrane proteins. • Traffic of small and large molecules across the plasma membrane. • Chemical and physical basis of selective membrane permeability • Passive (diffusion and facilitated diffusion) and active transport. • Osmosis (passive transport of water) • Role of protein transporters in passive and active transport • Role of ATP and phosphorylation in active transport. • Movement of macromolecules and particles in and out of the cell

MEMBRANE STRUCTURE AND FUNCTION

• Membranes divide the cell into compartments • The plasma membrane:

• outer boundary of the cell • selectively permeable (controls the flow of

substances into or out of the cell)

Membranes are made of phospholipids bilayer

One hydrophilic head Two hydrophobic tails

CH2

CH2 CH2 CH2 CH2 CH2

CH2 CH2 CH2

CH2 CH2

CH2

CH2 CH2

CH2 CH3

CH2

CH2

CH2 CH2 CH2 CH2 CH2

CH2 CH

CH

CH2 CH2

CH2

CH2 CH2

CH2 CH2

CH3

CH2 CH2

CH3

CH3

CH3 N +

O O O–

P O CH2 CH CH2

C O C O

O O

Phosphate group

Symbol

Hydrophilic head

Hydrophobic tails

Water

Water

Hydrophilic heads

Hydrophobic tails

In addition to phospholipids, membranes contain proteins that determine the function of the membrane

Hydrophilic region of protein

Hydrophobic region of protein

Phospholipid bilayer

Cholesterol is a steroid lipid with a carbon skeleton consisting of four fused carbon rings present in plasma membranes of animal cells

• Cholesterol is wedged between phospholipid molecules in the plasma membrane of animals cells.

• At warm temperatures, it restrains the movement of phospholipids and reduces fluidity.

• At cool temperatures, it maintains fluidity by preventing tight packing.

Cholesterol Cholesterol within the animal cell membrane

A fluid mosaic of phospholipids and proteins Membrane fluidity affects activity of membrane-bound

enzymes. http://www.insidecancer.org/

Figure 5.12

Fibers of the extracellular matrix Carbohydrate (of glycoprotein)

Glycoprotein

Microfilaments of cytoskeleton

Phospholipid

Cholesterol Proteins

Plasma membrane

Glycolipid

Cytoplasm

Functions of plasma membrane proteins

Enzymes Signal

Receptor ATP

Transport Enzymatic activity Signal transduction

Receptors receive chemical messages from other cells

Messenger molecule

Receptor

Activated molecule

Traffic Across Membranes

A. Non-polar, polar, ions and monomers (small molecules) Move physically through the membrane

B. Macromolecules and large particles move across inside vacuoles & vesicles

Selective permeability depends on interaction of that molecule with the hydrophobic core and presence of specific proteins

• non-polar molecules can pass unassisted

• polar and ionic molecules and inorganic ions are assisted by membrane proteins

Traffic of ions and monomers

1

2

4

7

6

8

9 10

Identify, by number, the non-polar molecules or structures. Do they pass through the plasma membrane unassisted?

What about the remaining molecules?

Na+

11

3

5

Selective permeability depends on interaction of that molecule with the hydrophobic core and presence of specific proteins

• non-polar molecules can pass: hydrocarbons, CO2, and O2

• polar and ionic molecules and inorganic ions are assisted by membrane proteins – nutrients (monomers of sugars and amino acids) and metabolic

waste products

– Na+, K+, Ca2+, and Cl-

Traffic of ions and monomers

Both diffusion and facilitated diffusion are forms of passive transport of molecules down their concentration gradient, while active transport requires an investment of energy to move molecules against their concentration gradient.

Movement across membranes

I. Passive transport From an area of high concentration to one

with lower concentration, down a concentration gradient, no ATP required

II. Active transport From an area of low concentration to one

with higher concentration, requires ATP

I. Passive transport is diffusion across a membrane - without work by the cell - Spreading from areas of high concentration to

areas of low concentration

Equilibrium Membrane Molecules of dye

Equilibrium

Figure 5.14B

Figure 5.14A

I. Passive Transport

1. Simple diffusion (gases & hydrocarbons)

2. Osmosis diffusion of solvent (H2O)

3. Facilitated diffusion (via protein transporters)

Solvation of ionic compounds in water http://programs.northlandcollege.edu/biology/

Biology1111/animations/dissolve.html

Osmosis is the diffusion of water across a membrane Water travels from a solution of lower solute

concentration to one of higher solute concentration

Figure 5.16

Lower concentration

of solute

Higher concentration

of solute

Equal concentration

of solute

H2O Solute molecule

Selectively permeable membrane

Water molecule

Solute molecule with cluster of water molecules

Net flow of water

Water balance between cells and their surroundings is crucial to organisms

Osmosis causes cells to shrink in hypertonic solutions

swell in hypotonic solutions not change in isotonic solutions

In isotonic solutions Animal cells are normal, but plant cells are limp

Figure 5.17

Plant cell

H2O

H2O H2O

H2O

H2O

H2O

H2O

H2O Plasma membrane

(1) Normal (2) Lysed (3) Shriveled

(4) Flaccid (5) Turgid (6) Shriveled (plasmolyzed)

Isotonic solution Hypotonic solution Hypertonic solution

Animal cell

Transport proteins may facilitate diffusion across membranes by providing passages or channels

Figure 5.15

Solute molecule

Transport protein

P P P Protein changes shape

Phosphate detaches

ATP ADP Solute

Transport protein

Solute binding 1 Phosphorylation 2 Transport 3 Protein reversion 4

II. Active transport uses cellular energy – Transport proteins can move solutes against a

concentration gradient – ATP provides a phosphate group to change the shape of

the transporter protein

Figure 5.18

B. Transport of large macromolecules across plasma membrane (Exocytosis and Endocytosis)

• Uptake of macromolecules is known as endocytosis

• Secretion or excretion of macromolecules is known as exocytosis e.g. insulin (protein hormone) by pancreatic cells.

Endocytosis and exocytosis http://highered.mcgraw-

hill.com/sites/0072437316/student_view0/chapter6/animations.html#

Fluid outside cell

Cytoplasm

Protein Vesicle

In exocytosis, a vesicle fuses with the membrane and expels its contents

Figure 5.19A

In endocytosis, a membrane vesicle folds inward enclosing material from the outside

Figure 5.19B

Vesicle forming

Endocytosis can occur in three ways • Phagocytosis • Pinocytosis • Receptor-mediated endocytosis

Pseudopodium of amoeba Food being ingested

Phagocytosis Pinocytosis Receptor-mediated endocytosis

Material bound to receptor proteins

PIT

Cytoplasm

Plasma membrane

TEM

54,

000×

TEM

96,

500

×

LM 2

30×

Figure 5.19C

Faulty membranes can overload the blood with cholesterol – Harmful levels of cholesterol can accumulate in the

blood if membranes lack cholesterol receptors

LDL particle

Protein

Phospholipid outer layer

Cytoplasm Receptor protein

Plasma membrane

Vesicle

Cholesterol

Figure 5.20