SPM Biology Form 4 Notes

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 selectivepermeability 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):

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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 outwards Hydrophobic 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 o

Process of Passive Transport 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.

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: Type Hypotonic of Solut ion Anim The al

Cell to the water the cell.

cell. Net cell. Example: red

Eventually

moveme blood cell in 5% sodium

it bursts(thin

nt of

plama

water is

cell in distilled

blood cell

water.

in 0.85%

Plant The

becomes firm / t the size

lysis occurs), vacuole

urgid due to the of

and

e cell wall

leave the cell. Example:

water is strip of potato in 30%

expands slightly zero. Exa sucrose solution. and prevents cell mple: from

strip of

bursting. Examp potato in

the water concentration inside the cell.

the same.

moveme to the water molecules

The rigidcellulos nt of

Water concentration outside the cell is higher than

Water concentration inside and outside of the cell is

The cell

water molecules cell. Net cytoplasmashrink due

lesser than solute concentration inside the cell.



No

Cell cell expands and change in becomes flaccid(plasmo

Solute concentration in the external solution is

equal to the solute concentration inside the cell.

chloride solution.

solution.

1) Hypotonic

Solute concentration in the external solution is

molecule leave the

sucrose

3. Hypertonic



rated due to the water

mple: red blood mple: red

Hypotonic

2) Isotonic

The cell shrinks and

membrane). Exa zero. Exa

2. Isotonic



the size

molecules enter of

Type of Solution



No

cell inflates due change in becomes soft anddehyd

enter the cell. 1.

Isotonic Hypertonic

le: strip of

5%

potato in

sucrose

distilled water.

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.