NOTES: Movement of Substances across the Plasma Membrane

•Living cells carry out the following life processes:

–absorb water and nutrients (minerals, ions,glucose, amino acids) from the surroundings.

–Excrete waste products (urea, uric acid)

–Exchange respiratory gases during resp.

•These processes are important to a cell because they help the cell to:

–maintain the optimum pH value and concentration gradient of ions that are suitable for the activities of the cell (muscle and nerve activities)

–obtain nutrients for metabolism

–eliminate toxic waste products from the cell

–secrete useful substances (hormones,enzymes)

•The movement of substances in and out of the cell occurs across the plasma membrane.

•The plasma membrane regulates the exchange of substances between the content of the cell and the external environment.

The structure of the plasma membrane

•Plasma membrane, according to the fluid mosaic model (S. Singer and G. Nicolson, 1972) are composed mainly of phospholipids and proteins.

•Each phospolipid molecule consist of two parts:

–a polar head that gives it a hydrophilic property (attracted to water)

–a pair of non-polar fatty acid tails  that give it a hydrophobic property (repelled by water)

•At the plasma membrane, phospholipids are arranged in a double layer, called phospholipid bilayer.

•

•According to this model,

–the phospholipid, proteins and other components of the membrane are not rigid or static, but form a dynamic and fluid structure.

Phospholipid Bilayer

–The protein molecules float about freely in the phospholipid bilayer.

•The protein and phospholipids are free to move sideway within the membrane, and this causes the membrane to have a fluid characteristic.

•The various proteins built into the plasma membrane form a mosaic pattern

Fig. A model of the plasma membrane (fluid- mosaic  structure)

Fluid Mosaic Model

•Cell-cell recognition proteins 

- identify type of cell and identify a cell as “self” versus foreign

–Most are glycoproteins

•-Carbohydrate chains vary between species, individuals, and even between cell types in a given individual.

•-Glycolipids also play a role in cell recognition

Fluid Mosaic Model of the PM

•A membrane is a mosaic

–-Proteins and other molecules are embedded in a framework of phospholipids

•

•A membrane is fluid

–-Most protein and phospholipid molecules can move laterally

The permeability of the Plasma Membrane

•-The plasma membrane is selectively permeable/ semi- permeable/partially permeable.

•-This means that certain substances can move across the membrane freely while  others cannot.

•The selective permeability of the plasma membrane depends on the:

–- the size of molecules or ions, or

–- the polarity of the molecules.

•The phospholipid bilayer is permeable to:

–-non-polar molecules (lipid-soluble)

–-small, uncharged molecules

•The phospholipid bilayer is not permeable to:

–-charged ions

–-polar molecules (lipid-insoluble)

•Due to hydrophobic core of the bilayer. The plasma membrane is only permeable to:

–- small hydrophobic solutes such as fatty acids, glycerol, steroids and all the fat soluble vitamins (A,D,E,K).

–- non- polar molecules such as oxygen and carbon dioxide

–- water as it is small enough to slide between the phospholipid bilayer or move through the pores.

•- Pore proteins allow small water-soluble molecules and ions to pass through the membrane.

•- Charged and large molecules such as glucose, amino acids, water soluble vitamins and polysaccharides cannot pass across the plasma membrane and therefore they are transported by either facilitated diffusion or active transport (aided by carrier proteins)

•There are two ways of transporting substances or solutes across the plasma membrane:

–-Passive transport (without the involvement of chemical energy)

–- Active transport (use energy and carrier proteins)

Diffusion

•-Movement of molecules from an area of high concentration to an area of low concentration.

•

•-Movement from one side of a membrane to another, un-facilitated

•A. Passive Transport

•i) Simple Diffusion

•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

Osmosis

•Osmosis – diffusion of water across a selectively permeable membrane

•- Water moves from an area of high water concentration (dilute solution) to an area of low water concentration(concentrated solution).

– Is energy required ?

•Type of Solution

•-Hypotonic

•-Isotonic

•-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.

Osmosis and Animal Cells

Osmosis and Plant Cells

Osmosis Summary

•When a cell is placed in a Hypotonic solution:

–-Cell gains water through osmosis

–-Animal cell lyses; plant cell becomes turgid (firm)

–

–

•When a cell is placed a Hypertonic solution:

–-Cell loses water through osmosis

–-Animal cell shrivels; plant cell plasmolyzes

•Facilitated Diffusion:

•very specific: glucose, nucleic acids, amino acids and mineral ions.

•control by cell.

•transport of molecules (only certain molecules) across the outer membrane of living cell aided by a  carrier protein 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

•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

Active Transport

•Active transport proteins move substances across the PM against their concentration gradient.

–Requires energy (ATP)

–Active transport proteins are highly selective

–Active transport is needed for proper functioning of nerves and muscles

•Application

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

•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.