3.1 MOVEMENT OF SUBSTANCES ACROSS THE PLASMA MEMBRANE

• Metabolism : –all the chemical activities taking place in

the cell• Consists of two processes:

1. Anabolism 2. Catabolism

Anabolism

the building up of moleculesE.g. : synthesis of protein and

ATPs, which a cell needs

Catabolism

breaking down of large molecules to smaller simpler molecules.

E.g. : oxidation of glucose in cell respiration

Substances that leave the cell

Substance Reason for removal

Carbon dioxide (animal cell)

waste product of respiration

Oxygen (Plant cell) waste product of photosynthesis

Secretions cellular products for use in other parts of the body

Nitrogenous waste waste products from breakdown of excess proteins

Excess water Osmoregulation

Substances that enter the cell

Substance Reason for intakeOxygen (animal cell) For cell respiration

Carbon dioxide (plant cell)

As raw material in photosynthesis

Glucose For cell respiration

Amino acids As raw materials for protein synthesis

Ionic salts For cell metabolism and osmoregulation

The structure of plasma membrane

• Phospholipid bilayer consists of:

–A polar phosphate molecule head which is hydrophilic

– Two non-polar fatty acid tails which are hydrophobic

Permeability of the plasma membrane

Permeability of the phospholipids bilayer

• Plasma membrane : – semi-permeable– Allows only certain substances to pass

through but not other

Permeability of the phospholipids bilayer

• Phospholipid bilayer is permeable to:– Small non-polar (hydrophobic) molecules

that are lipid soluble, such as : fatty acids, glycerol, steroids, vitamin A, D, E and K

– Small uncharged molecules, such as: water, oxygen and carbon dioxide

• Phospholipid bilayer is not permeable to:– Large polar molecules that are not

soluble in lipid, such as glucose amino acids, nucleic acids and polysaccharides– Ions (charged), regardless of size, such

as: H+, Na+

TYPES OF TRANSPORT ACROSS THE PLASMA MEMBRANE

Types o transport across the plasma

membrane

Passive Transport

Simple Diffusion Facilitated Diffusion Osmosis

Active Transport

PASSIVE TRANSPORT

Simple Diffusion

Random movement of ions/molecules

from a region of their high concentration to a region of their low concentration

down a concentration gradient

until an equilibrium is achieved

Simple diffusion

Molecules that can cross are

Small non-polar (hydrophobic) molecules

Small uncharged molecules

Factors affecting the rate of diffusion:

Diffusion gradient

Size of molecules or ions

Temperature

Surface area

FACILITATED DIFFUSION

Movement of specific molecules (or ions) across the plasma membrane

Assisted by pore proteins / carrier proteins

• Charged ions

• Open up pore / channels across the membrane to allow entry or exit

• Specific

Pore Proteins

Carrier Protein• larger polar molecules

(such as sugar & amino acids)

• protein attaches itself to the binding site of a carrier protein

• the carrier protein changes shape and delivers the molecule across the plasma membrane

OSMOSIS

TYPES OF SOLUTION

HYPOTONIC

ISOTONIC

HYPERTTONIC

movement of water molecules

from a region of low solute concentration (high water concentration) to a region of high

solute concentration (low water concentration)

through semi-permeable membrane

Examples of osmosis:

• Absorption of water by root hairs

• Movement of water from one cell to another

• Absorption of water in the alimentary canal-stomach, small intestine and colon

Uses carrier proteins and energy

to move molecules or ions across the plasma membrane

against the concentration gradient

• Example of active transport:• Absorption of mineral salts by roots hairs• Absorption of glucose and amino acids

by cells in the small intestine

Gaseous exchange in the alveoli and blood capillaries by simple diffusion

Absorption of digested food in the villi

Absorption of water in root hairs of plants by osmosis