Form 4- Chapter 3 : Movement of Substances Across the Plasma Membrane1. 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 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

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. 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:

CO2 excess H2O

nitrogenous wasteSubstances that are allowed to move into the cell:

O2 amino acids

mineral salts

glucoseMaterials 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.

Type of Solution1. 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:

Type of SolutionHypotonicIsotonicHypertonic

Animal CellThe cell inflates due to the water molecules enter the cell. Eventually it bursts (thin plasma membrane).Example: red blood cell in distilled water.No change in the size of cell. Net movement of water is zero.Example: red blood cell in 0.85% sucrose solution.The cell shrinks and becomes soft and dehydrated due to the water molecule leave the cell.Example: red blood cell in 5% sodium chloride solution.

Plant CellThe cell expands and becomes firm / turgid due to the water molecules enter the cell. The rigid cellulose cell wall expands slightly and prevents cell from bursting.Example: strip of potato in distilled water.No change in the size of cell. Net movement of water is zero.Example: strip of potato in 5% sucrose solution.The cell becomes flaccid (plasmolysis occurs), vacuole and cytoplasm shrink due to the water molecules leave the cell.Example: strip of potato in 30% sucrose solution.

Application1. Food is soaked in a concentrated salt solution to prevent bacteria and fungus to survive.

2. Chemical fertilizer (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.

Inferring the concentration of the cell sap and the movement of substances across the plasma membrane

Observation of potato strips placed in sucrose solution of different concentrationsInference as to the concentrationExplanations

MassLengthSurcose solutionCell sap

Increased Increased Hypotonic

Concentration of water molecules higher Hypertonic

Concentration of water molecules lower Water molecule diffuse from sucrose solution across the plasma membrane into the cell sap

The movement is down the concentrated gradient of the water molecules

No changeNo change Isotonic Isotonic No net water movement

Concentration gradient of water molecules is zero

Decreased Decreased Hypertonic

Concentration of molecules lower Hypotonic

Concentration of water molecules higher Water molecules diffuse from cell sap across the plasma membrane into the sucrose solution. Movement is down the concentrated gradient of water mooolecules

The phenomenon of Witting in plants

Witting of plants is caused by the excessive use of chemical fertilizers.

A plant wilts if bends towards the ground because of heat or a lack of water.

Chemical fertilizers scattered on the soil around plants easily dissolve, forming ions or solute.

The excessive use of chemical fertilizers release huge amount of ions such as phosphates, nitrates, calcium, magnesium and sulphur into the soil water.

Dissolved ions increase solute concentration but decrease water molecules concentration in soil water, around the roots and root hairs. Compared to the soil water, the vacuole sap now has a higher concentration, but a lower solute concentration. This results water molecules diffusing from the cell sap into soil water by osmosis. With water drawn out of the plant cell, the cytoplasm shrinks away from the cellulose cell wall causing plasmolysis. Plasmolysed cells lose turgidity and support, causing the whole plant to wilt.The preservation of food

Food goes bad due to bacterial and fungi activities.

To make food lasts for a long time, preservation is needed by using salt or sugar.

A concentrated salt solution salt solution has a high concentration of solute (Na+ and Cl- ions) and very low concentration of water molecules.

If we leave food ( such as fish or vegetable) in a concentrated salt solution, all the water molecules within the food are drawn out by osmosis, making the food dry.

Without water fungi and bacteria cannot survive.

In a concentrated sugar solution, sugar molecules from the high concentration of solute, with a very low concentration of water molecule. Examples of food that uses sugar for preservations are longan, lychee and rambutan.

Explain the Necessity of movement of substances across the plasma membrane

Cells form the basic units of life. To sustain life, the materials needs of the cells must be fulfilled. This means that raw material needed by cells must be brought in and waste materials must be removed. All these life-sustaining metabolic activities take place within the plasma membrane. The fluid mosaic model is the current accepted model that helps to describe and explain the working wonders of the plasma membrane. The ingenious molecular design of plasma membrane ensures that the movement of substance across it takes place in a continuous and controlled manner. Learning about diffusion helps us to realize that the exchange of respiratory gases- oxygen and carbon dioxide happens continuously with an inbuilt control that we call the diffusion gradient. Then, there is the membrane-bound carrier protein which helps to facilitate the continuous diffusion of needed substances into the cell energy-free, but always under control. Osmosis helps in the intake and the removal of water molecules in a continuous and controlled manner. The active transport mechanism, with the membrane-bound carrier proteins, make sure that the difficult to handle substances required by cells are brought in or removed continuously, but in a controlled manner.DefinitionAmodelconceived by S.J. Singer and Garth Nicolson in 1972 to describe the structural features of biological membranes.

SupplementTheplasma membraneis described to befluidbecause of itshydrophobicintegralcomponentssuch aslipidsand membraneproteinsthat move laterally or sideways throughout the membrane. That means the membrane is notsolid, but more like a 'fluid'.

The membrane is depicted asmosaicbecause like amosaicthat is made up of many different parts theplasma membraneis composed of different kinds ofmacromolecules, such as integralproteins, peripheralproteins,glycoproteins,phospholipids, glycolipids, and in some casescholesterol,lipoproteins.

According to themodel, theplasma membraneis alipidbilayer (interspersed withproteins). It is so because of itsphospholipidcomponentthat can fold in itself creating a double layer - or bilayer - when placed in apolarsurrounding, likewater. This structural feature of the membrane is essential to its functions, such as cellulartransportandcell recognition.

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