BIOLOGY THE FUNDAMENTAL UNIT OF LIFE

Transport of Materials Across Cells

 Plasma membrane or the cell membrane forms the outermost covering of the cell that separates the content of the cell from its external environment.  It is called a selectively permeable membrane because it permits the entry or exit of some materials in and out of the cell. It also prevents the movement of some other materials.  Different processes such as diffusion and osmosis are responsible for the transport of materials in and out of the cell.

Transport of materials across cells

Passive Active Bulk transport transport transport

Simple Facilitated Osmosis Pinocytosis Endocytosis Exocytosis diffusion diffusion

Receptor- Endosmosis Exosmosis Phagocytosis Potocytosis mediated endocytosis

Passive Transport

 Passive transport is a kind of diffusion in which an ion or a molecule crossing the cell membrane moves against its electrochemical or concentration gradient.

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BIOLOGY THE FUNDAMENTAL UNIT OF LIFE

 In passive transport, no metabolic energy is consumed.  Passive transport occurs by three processes—simple diffusion, facilitated diffusion and osmosis.

Simple Diffusion  In simple diffusion, molecules of gases such as oxygen and carbon dioxide enter the cell without the help of transport proteins such as permeases.

Facilitated Diffusion  In facilitated diffusion, ions or molecules cross the membrane rapidly by using specific proteins called transport proteins or permeases which are present in the membrane. The transport proteins form a small passageway through the membrane allowing the solute molecules to cross the phospholipid bilayer.

 Facilitated diffusion occurs only in the direction of the concentration gradient. It does not require any metabolic energy.  In several animal cells, facilitated diffusion helps in the transport of glucose or blood sugar into the body cells. The carrier system or mediated transport is responsible for an increase in the inward flow of glucose. www.topperlearning.com 3

BIOLOGY THE FUNDAMENTAL UNIT OF LIFE

Osmosis  The spontaneous passage of water molecules from a region of high water concentration through a selectively permeable membrane to a region of low water concentration is called osmosis.  Osmosis occurs due to a difference in the concentration of solutes on either side of the membrane.  If the medium surrounding the cell has exactly the same water concentration as that of the cell (isotonic solution), then there will be no overall movement of water molecules across the plasma membrane. The cell remains the same size.  If the medium surrounding the cell has higher water concentration than that of the cell (hypotonic solution), then water molecules will freely pass across the plasma membrane in both the directions. The cell will gain water by osmosis. It will swell up and become inflated or turgid.  If the medium surrounding the cell has lower water concentration than that of the cell (hypertonic solution), then water will pass across the membrane in both the directions. Water leaving the cell will be more than that entering it. The cell will shrink.

Endosmosis  The process by which water molecules enter a cell is called endosmosis.

Exosmosis  The process by which water molecules move out of the cell is called exosmosis.  In plant cells, when excess of exosmosis occurs, the cytoplasm and plasma membrane shrink away from the cell wall. This is known as plasmolysis.

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BIOLOGY THE FUNDAMENTAL UNIT OF LIFE

Active Transport

 Active transport is the movement or transport of substances through a biological membrane such as the cell membrane. This process requires energy.  The molecules move uphill against the forces of passive diffusion. They always move in a direction opposite to that of the concentration gradient, i.e. from a lower concentration to a higher concentration. Because materials are pumped against the concentration gradient, the process always involves the expenditure of energy in the form of ATP.

 Certain membrane proteins act as carrier molecules and transport the solute to the other side of the membrane.  Active transport occurs in only one direction. This results in accumulation of materials within the cell.  The important active transport systems in animals maintain sodium potassium gradients between cells and their surrounding extracellular fluid or external environment.

Differences between Active Transport and Diffusion

ACTIVE TRANSPORT DIFFUSION 1. Rapid process 1. Slow process 2. Materials move through a biological 2. Materials move across a biological membrane against the concentration membrane down the concentration gradient gradient 3. Occurs in one direction 3. Occurs in both the directions 4. Needs carrier (transport) proteins 4. Does not need carrier (transport) proteins 5. Uses energy in the form of ATP 5. Does not use energy 6. Brings about selective uptake of materials 6. Allows transmissible molecules to pass through the membrane 7. Leads to accumulation of materials 7. Does not cause accumulation of materials

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BIOLOGY THE FUNDAMENTAL UNIT OF LIFE

Bulk Transport

 Large molecules are continuously imported or exported into the cells across the plasma membrane. The process where the cells either release or absorb fluids and particles through their outer membrane is called bulk transport.

Exocytosis  Materials enter a cell by invagination and formation of vesicles.  As the materials leave the cell, the membrane of a vesicle fuses with the plasma membrane and extrudes its contents to the surrounding medium. This outward transport of materials by using carrier molecules is called exocytosis.  Undigested residues of substances brought in by endocytosis are removed from the cell by exocytosis.  Exocytosis also helps to excrete substances or transport a substance completely across a cellular barrier. Example: Immunoglobulins or antibodies which are sometimes picked up on one side of the blood vessel wall by phagocytosis are then moved across the cell and released by exocytosis.

Process of Exocytosis

The food vacuole containing undigested waste or the secretory vesicle which has budded off from the Golgi complex is moved with the help of cytoskeleton from within the cell to the cell surface.

There is a rearrangement of lipid molecules between the two bilayers and the two membranes fuse together.

A passage forms in the fused membrane and the vesicle discharges its contents outside the cell.

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BIOLOGY THE FUNDAMENTAL UNIT OF LIFE

Endocytosis  Endocytosis is the intake or ingestion of materials by cells through the plasma membrane.  It occurs by infolding or extension of the plasma membrane to form either a vesicle or a vacuole.

 Endocytosis collectively describes three very similar processes—phagocytosis, potocytosis and receptor-mediated endocytosis.

Phagocytosis  Phagocytosis, also known as cell eating, is a common method in which substances are taken up in a solid form.  Cells which are involved in phagocytosis are called phagocytes. White blood cells or leucocytes engulf cellular debris and microbes such as viruses and bacteria in the blood through this process.

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BIOLOGY THE FUNDAMENTAL UNIT OF LIFE

Process of Phagocytosis

An area of the plasma membrane which was initially coated with actin-myosin forms a pocket that engulfs the solid material.

The membrane-bound vesicle or phagosome detaches itself from the plasma membrane and enters into the cytoplasm, where the contents are digested by lysosomal enzymes.

A type of white blood cell or phagocyte engulfs the infectious agent or particle by pseudopodial movement.

The plasma membrane grows around the particle as pseudopodia and an intracellular vacuole is formed.

Lysosomes join with the vacuole which contains the ingested particle and pour their contents or the digestive enzymes into the vacuole.

The particle gets destroyed.

Potocytosis  In potocytosis, small molecules or ions are specifically internalised into the cell.  Small areas of the plasma membrane bearing specific receptors for small molecules or ions invaginate to form a caveolae.

Receptor-mediated Endocytosis  Receptor-mediated endocytosis is a pathway for selective uptake of large molecules such as ligands in clathrin-coated pits.  The binding of the ligand to the receptor on the surface of the plasma membrane stimulates invagination of the pits.

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BIOLOGY THE FUNDAMENTAL UNIT OF LIFE

Pinocytosis  In pinocytosis, also known as cell drinking, substances are taken up by the cell in a fluid form.

 Pinocytosis is seen in amoeboid protozoans and certain kidney cells. It also occurs in plants.

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