SPM Biology Form 4 Notes – Terminology and Concepts: Movement of Substances Across the Plasma Membrane (Part I)Berry Berry Easy is back with SPM Form 4 Biology notes for “Movement of substances across the plasma membrane“. The plasma membrane is a membrane of biological nature which forms the barrier between the interior of a cell against external environment. However, do not just think of it as a layer which do not have any function. Typically, movement of substances to and from cells to outer environment is controlled (due to its permeability) by the plasma membrane. The key concept here for this topic is to understand the selective-permeability nature of a plasma membrane. The rest of the topic are rather easy and intuitive. Do note that plasma membrane is also known as cell membrane and plasmalemma.

SPM Biology Form 4 – Terminology and Concepts:

Movement of Substances Across the Plasma

Membrane (Part 1)

1. Uniqueness of Plasma Membrane (also known as

cell membrane):- it is a semi-permeable cell membrane- it allows water and   certain substances    to move in 

and out of the cell.

2. Importance of Plasma Membrane:- cells obtain nutrients and gases cells excrete metabolic wastes cells can maintain pH for enzyme activity cells can maintain ionic concentration of the cells 

for enzyme activity control the types and the amount of substances allow useful substance (hormones/enzymes) to 

secrete from cells protect cells a boundary between the inside and outside of cell.

3. Structure of the basic unit of plasma membrane Phospholipid molecule:

‘Head’ – hydrophilic: a polar phosphate molecule (philic~loves water / attracted to water)‘Tail’ – hydrophobic: two non-polar fatty acids (phobic~hates water / repelled to water)

Formation:Hydrophilic heads pointing outwardsHydrophobic tails pointing inwards(Bilayer phospolipid)

Fluid Mosaic Model (Protein embedded in the

bilayer)

Carrier protein carrier for some molecules (glucose, amino acids, 

proteins and nucleic acids)

controls the movement of ions and particles (Na+, Ca2+ and K+)

Glycoprotein

Glycolipid combination of lipids and polysaccharides

4. Permeability

Permeable   (allow to pass through) small non-polar molecules (vitamins A, D, E, K, fatty 

acids, glycerol and steroids)

Impermeable   (not allow to pass through but with help of

carrier protein and cellular energy, it is allow to pass

through) large polar molecules (glucose, amino acids, 

mucleic acids and polysaccharides) charged ions (H+, Na+, K+, Cl- and Ca2+)

Substances that are allowed to move in the cell: CO2

O2

excess H2O waste: nitrogenous

Substances that are allowed to move out of the cell: CO2

O2

amino acids ionic salts glucose

Materials must be able to move through the

plasma membrane in order for the cell

cytoplasma to interact with the external

environment. Therefore, the movement of soluble

substances can occur in several mechanisms:

o Process of Passive Transporto Process of Active Transport

A.   Passive Transport

i) Simple Diffusion- not selective: lipid-soluble molecules, gases and 

water.- not control by cell.- movement of the molecules from a region 

of higher concentration to a region of lower concentration.

- Factors affecting the rate of diffusion are temperature, size of molecules/ions, diffusion gradient, surface area anddiffusion medium.

- example: diffusion of oxygen and carbon dioxide at the alveolus.

ii) Osmosis:- only water molecules.- not control by cell.- movement of water from a region

of higher concentration to one of lower concentration and often occurs across asemipermeable membrane.

- strong sucrose solution = less water molecule = low water potential.

- weak sucrose solution = more water molecule = high water potential.

- example: absorption of water by root hairs.

iii) Facilitated Diffusion: very specific: glucose, nucleic aicds, amino acids, 

protein and mineral ions. control by cell. transport of molecules (only certain molecules) 

across the outer membrane of living cell by a process of carrier protein(hydrophilic group) / channel protein (Ions: Na+, Ca2+, K+) within the cell membrane.

normally take place from a region with higher concentration of molecules to a region of lower concentration.

example: absorption of digested food in the villus.B.   Process of Active Transport

very specific: minerals ions and amino acids. control by cell. This process needs carrier proteins and energy 

(due to against concentration gradient) from a region of lower concentration to a region of higher concentration).

Cell must expend energy that derived from ATP (adenosine triphosphate)

example: human nerve cells (sodium ions are constantly transport out of the cell) / ions intake by root hairs of a plant.

Type of Solution

1. Hypotonic

2. Isotonic

3. Hypertonic

1) Hypotonic

Solute concentration in the external solution

is lesser than solute concentration inside the

cell.

Water concentration outside the cell is

higher than the water concentration inside

the cell.

2) Isotonic

Solute concentration in the external solution

is equal to the solute concentration inside the

cell.

Water concentration inside and outside of the

cell is the same.

3) Hypertonic

Solute concentration in the external solution

is greater than solute concentration inside

the cell.

Water concentration outside the cell is

lower than the water concentration inside the

cell.

Types of solutions:

Typ

e of

Solu

tion

Hypotonic Isotonic Hypertonic

Ani

mal

Cell

The

cell inflates du

e to the water

molecules

enter the cell.

Eventually

it bursts(thin

plama

membrane). E

xample: red

blood cell in

distilled

water.

No

change

in the

size of

cell. Ne

t

movem

ent of

water is

zero. Ex

ample:

red

blood

cell in

0.85%

sucrose

solution

.

The cell shrinks and

becomes soft an

ddehydrated due to

the water molecule

leave the

cell. Example: red

blood cell in 5%

sodium chloride

solution.

Plan

t

Cell

The

cell expands a

nd

becomes firm 

/ turgid due to

the water

molecules

enter the cell.

The rigi

No

change

in the

size of

cell. Ne

t

movem

ent of

water is

The cell

becomes flacci

d(plasmolysis occurs

), vacuole and

cytoplasmashrink du

e to the water

molecules leave the

cell. Example: strip

of potato in 30%

dcellulose cell

wall expands

slightly and

prevents cell

from

bursting. Exa

mple: strip of

potato in

distilled

water.

zero. Ex

ample:

strip of

potato

in 5%

sucrose

solution

.

sucrose solution.

Application

1. Food is soaked in a concentrated salt solution

to prevent bacteria and fungus to survive.

2. Chemical fertiliser (dissolved ions) increases

solute concentration (decrease water

molecules) in soil. Therefore, water leaves

from the cell sap of the plant which result the

plant wither.