Form 4- Chapter 3 : Movement of Substances Across the Plasma Membrane

Form 4- Chapter 3 : Movement of Substances Across the Plasma Membrane

1. Uniqueness of Plasma Membrane /cell membrane it is a semi-permeable 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 membranePhospholipid 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

Fluid Mosaic Model (Protein embedded in the bilayer)Carrier proteincarrier for some molecules (glucose, amino acids, proteins and nucleic acids)

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

Glycoprotein

Glycolipidcombination of lipids and polysaccharides

4. PermeabilityPermeable (allow to pass through)

small non-polar molecules (vitamins A, D, E, K, fatty acids, glycerol and steroids)

Impermeable (not allowed to pass through but with help of carrier protein and cellular energy, it is allowed to pass through)

large polar molecules (glucose, amino acids, nucleic acids and polysaccharides)

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

Substances that are allowed to move out of the cell:

CO2excess H2O

nitrogenous waste

Substances that are allowed to move into the cell:

O2amino acids

mineral salts

glucose

Materials must be able to move through the plasma membrane in order for the cell cytoplasm to interact with the external environment. Therefore, the movement of soluble substances can occur in several mechanisms:

A. Process of Passive Transport

B. Process of Active Transport

A. Passive Transporti) 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 and diffusion 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 a semi permeable 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 acids, 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.

controlled 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.