Chapter 3Movement of

Substances across the Plasma Membrane

Originally by : Lee Chee Teik

The Structure of Plasma Membrane Plasma membrane, according to the

fluid mosaic model, are composed mainly of phospholipids and proteins.

Each phospholipid molecule consists of two parts:

a. A polar head that gives it a hydrophilic property

b. A pair of non-polar fatty acid tails that give it a hydrophobic property

• At the plasma membrane, phospholipids are arranged in a double layer, called the phospholipid bilayer

The fluid mosaic model proposed by S.J.Singer and G.L Nicholson is currently the accepted view of the plasma membrane

• The plasma membrane is selectively permeable ( semi-permeable / partially permeable)

a) It only allows certain substances to enter or leave the organelles.

b) Other examples – visking tubings, cellophane tape, egg membrane

c) The phospholipid bilayer is permeable to

The permeability of the plasma membrane

Non-polar molecules ( lipid-

soluble)

Small , uncharged molecules

• Movement of membrane across plasma membrane

Passive Transport

Active Transport

Molecules transported

down the concentration

gradient

Molecules transported against the

concentration gradient

Simple diffusion Osmosis Facilitated

diffusion

• In passive transport, no energy is used by the cell during the movement of substances.

1. Simple diffusion Simple diffusion is the net

movement of molecules or ions from a region of high concentration to a region of lower concentration, that is, down the concentration gradient.

The movement of substances across plasma membrane : passive transport

A concentration gradient is the difference in concentration of a

substance between two regions

Diffusion of substances occurs until equilibrium is achieved.

Molecules which move across plasma membrane by simple diffusion include

Lipid-soluble molecules such as fat-soluble vitamins (A, D, E, K) , glycerol, steroid compounds and fatty acids.

small, uncharged molecules such as oxygen and carbon dioxide

Simple Diffusion

Lipid-soluble ( non-polar molecules / hydrophobic molecules)

Small , uncharged molecules

Fatty acids

GlycerolSteroid

compounds Fat-

soluble vitamins (A,D,E,K)

OxygenCarbon dioxide

2. Osmosis Osmosis is the diffusion of water molecules

across a semi-permeable membrane. It is the net movement of water molecules from a

region of higher water concentration( lower solute concentration ) to a region of lower water concentration ( higher solute concentration) until a dynamic equilibrium is achieved.

The process of osmosis does not require energy as water moves across a semi-permeable membrane according to its concentration gradient .

Similarities in simple diffusion and osmosisDoes not require the cell to use

energy.Molecules move down the

concentration gradient.Molecules will continue to move from

a region of higher concentration to a region lower concentration until a dynamic equilibrium is achieved. At this point, the molecules are evenly dispersed in the liquid and the concentration gradient no longer exist.

Differences between simple diffusion and osmosis

Simple diffusion

Osmosis

Involves the movement of any molecules

Involves only the movement of water molecules

Does not require a semi-permeable membrane

Occurs through a semi-permeable membrane

3. Facilitated diffusion Facilitated diffusion is the movement of molecules or

ions across the plasma membrane with the help of transport proteins ( carrier proteins and pore proteins ).

Does not require energy because the transport proteins move the molecules down the concentration gradient.

Molecules which move across plasma membrane by facilitated diffusion include:

Charged ions such as K+, Na+, Ca2+, Mg2+ Large molecules such as glucose and amino acid.

Facilitated

DiffusionLipid-insoluble ( polar molecules / hydrophilic molecules )

Charged Ions

Large molecule

s

K+, Na+, Mg 2+, Ca 2+

Glucose, Amino acid