- Transport of materials i.e nutrients and gaseous exchange is basic necessity of living organisms
- Unicellular organisms transport materials via active and passive transport
- Multicellular organisms require a complete system of transportation called circulatory system
- Passive transport is carried out either by diffusion or by osmosis process
- Passive transport does not require energy
- Active transport requires energy to occur
- Active transport is carried out by different carriers
- Circulatory system in multicellular organisms is complex system having specialized organs and tissues
In all living beings nutrients and the gases are transported to all parts of the body. This transportation is necessary for many vital life activities such as digestion, respiration, immunity and ultimately growth. There are different methods of transport of materials in different organisms.
In unicellular organisms and some simple multicellular the transport of materials is taken place by simple processes of active and passive transport however in complex multicellular organisms there are complex systems of transport of materials. In these organisms this system is called circulatory system which comprises of a network of blood vessels and specialized organs and the blood. On a molecular level in multicellular organisms transfer of nutrients and gases also takes place because of active or passive transport.
There are two types of transport by which substances enter or leave the cell .
A ) passive transport B ) active transport
It is the type of transport in which materials moves in and out of the cell according to the concentration gradient without the use of energy. Molecules move using their own kinetic energy. The passive transport allows cells to get water, oxygen and other molecules which are required by the cell. It also allows cells to get rid of waste molecules like carbon dioxide. Passive transport can be further categorized into three groups :
1 ) Diffusion 2 ) Osmosis 3 ) Facilitated diffusion
The movements of molecules from higher to lower concentration to attain equilibrium. The molecules in liquids and gases are in continuous motion due to which them posses kinetic energy. As the movement of these molecules is random and finally the concentration of molecules on both sides becomes equal, thus an equilibrium is attained. After equilibrium the motion of molecules doesn’t stop but they move continuously so that there is equal number of molecules everywhere in the system. This is called dynamic equilibrium. There are certain factors on which the rate of diffusion depends which are as follows:
The concentration gradient
The more is the difference in concentration the higher will be the rate of diffusion
Distance within the system
Smaller is the distance between the two regions of system the higher will be the rate of diffusion.
Surface area also effects the rate of diffusion; the larger the surface area the faster is the rate of diffusion. This can be expressed by The Fick’s Law which is a:
Diffusion is proportional to: (surface area x difference in concentration) / Length of diffusion path
The higher the temperature, the higher the rate of diffusion
Size of molecule
The bigger the size of the molecules, the slower the rate of diffusion
Examples of diffusion in animal cells
Diffusion is very important to all living organisms including humans
- The cells in our body need glucose and oxygen for respiration. Both these processes are carried out in blood. When blood reaches the cell the molecules of glucose and oxygen diffuses out of the blood into the cell.
- Gases exchange at the alveoli in which oxygen from air to blood and carbon dioxide diffuse out from blood to air.
- As cells uses glucose and oxygen they also produce waste chemicals and carbon dioxide which can poison the cell therefore they have to diffuse out of the cell.
This is the type of passive transport that does not require energy to move molecules down their concentration gradient and it uses a membrane protein to move molecules across the membrane that are larger or charged ions. The material to be transported is first attached to a protein or glycoprotein receptors on the surface of plasma membrane. The process is facilitated by two type of transport protein
a) Channel protein b) Carrier protein
- These are integral lipoprotein contains pore through which cross from one side of the membrane to the other
- Channel proteins are ion selective
- Channel protein only moves molecules along the concentration gradients
- Channel protein are either open or gated. The attachment of a specific ion to channel protein may control the opening or other mechanisms involved.
This type of protein binds with the substance and in doing so changes its own shape and this moves the bound molecule from outside of the cell to its interior.
Osmosis is the kind of diffusion of water molecules from an area of higher water concentration to an area of lower water concentration through a partly permeable membrane. Water is a universal solvent it can dissolve polar and charged molecules (solutes). The large solute molecules cannot pass through semi permeable membrane so the water molecule move to equalize the two solutions.
Some examples of osmosis:
- Absorption of water by plant roots
- Reabsorption of water by proximal and distal tubules of nephron
- Absorption of water by alimentary canal stomach and small intestine etc
It is the tendency of water to move from one place to other. Water always is downhill that is towards the more negative number its values are always negative. Units are pressure (kpa)
Water potential = pressure potential + solute potential
Pressure potential = force of cell walls on contents
So for animal cell this is zero so water potential = solute potential
It is the measure of solute concentration i.e the number of solute molecules per liter of solution (osmol/L)
Based on this solution may be hypertonic, hypotonic or isotonic
Hypertonic solution are those with higher osmolarity (high solute concentration)
Hypotonic solution are those with low osmolarity ( low solute concentration)
Isotonic Solution are those with the same osmolarity ( with same solute and solvent concentration)
In hypertonic solution water will leave the cell causes it to shrink (crenation). In hypotonic solution water enters the cell causing it to swell and burst (lysis)
Osmosis in animal cells
Animal cells have a cell membrane which is partially permeable; if these are placed in pure water the water will pass into the cell by osmosis because the cytoplasm is a stronger solution than water. As a result cells swell and burst called lysis of cell. Red blood cell are the examples as they contains variety of solutes dissolve in their water cytoplasm.
Transport in amoeba
In amoeba water enter to osmosis and extra water enters in the contractile vacuole due to the contractile vacuole which swells and move towards the edge of cell membrane, then the contractile vacuole burst and water is expelled out of the cell.
Animal and plant cells cannot only depend only on diffusion alone to get the molecules they need. Sometimes they need some molecules that are in low concentration so they have to move inside the cell by a special mechanism called active transport.
Active transport is the type of movement of molecules in and out of a cell through a semi permeable membrane from a region of lower concentration to higher concentration. As this movement is against the concentration gradient so it requires a energy, this energy is utilized in the form of ATP which is provided by respiration. Active transport involves the use of some carrier proteins called PUMP PROTIENS.
MECHANISM OF ACTIVE TRANSPORT
- The carrier protein span the surface of cell membrane and accept ions to be transported on one side of it
- The ions binds itself to the binding site of carrier protein
- On the inside of cell ATP is attached
- ATP is hydrolyzed by protein carrier into ADP and phosphate group
- As a result, the carrier protein changes its shape and re-orients itself towards the exterior of membrane
- The ions are then released on other side of membrane
- The phosphate group released from carrier protein and recombine with ADP to form ATP for reutilization
- The protein returns to its original shape
One example of this is the sodium potassium pump:
Endocytosis & Exocytosis
Materials may be transported to the cell either as molecule or in bulk; for a large molecule this process is called cytosis. This process involves the fusion of vesicles with the cell membrane. This may be endocytosis or exocytosis.
It is the process by which large substances enters the cell by invagination of cell membrane to form a flask like structure which encloses the external substance and then a vesicle is formed with that material endocytosis may be:
Phagocytosis in which solid substances are ingested
Pinocytosis in which liquid /dissolved substances are ingested
The process by which large molecules/substances are transported out of the cell by vesicles which fuses with plasma membrane and expel their material into extracellular environment. Vesicles are budded off from Golgi bodies within cell