Ways of movement of material across the cell

Diffusion 

 "Diffusion is the net movement of a substance from an area of higher concentration to the area of lower concentration i.e. along concentration gradient." Since the molecules of any substance (solid, liquid, or gas) are in motion when that substance is above 0 degrees Kelvin or -273 degrees C, energy is available for movement of the molecules from a higher potential state to a lower potential state. 
The molecules move from higher to lower concentration, a though there will be some that move from low to high. The overall (or net) movement is us from high to low concentration. Eventually, if no energy is input into the system the molecules will reach a state of equilibrium where they will be distributed equally throughout the system.
 Because a cell does not expend energy when molecules diffuse across its membrane, the diffusion is type of passive transport. Diffusion is one principle method of movement of substances within cells, as well as the method for essential small molecules to cross the cell membrane. Carbon dioxide and oxygen are among the few simple molecules that can cross the cell membrane by diffusion. Gas exchange in gills and lungs operates by this process. 

Facilitated diffusion

 Many molecules do not diffuse freely across cell membranes because of their size or charge. Such molecules are taken into or out of the cells with the help of transport- proteins present in cell membranes. When one of these transport proteins makes it possible for a substance to move it down its concentration gradient (from higher to lower concentration), the process is called facilitated diffusion. The rate of facilitated diffusion is higher than simple diffusion. Facilitated diffusion is also a type of passive transport because there is no expenditure of energy in this process. 

Osmosis

Osmosis is the movement of water across a selectively permeable membrane from a solution of lesser solute concentration to a solution of higher solute concentration. The rules of osmosis can be best understood through the concept of tonicity of the solutions. The term tonicity refers to the relative concentration of solutes in the solutions being compared. 

Hypertonic solutions 

These are those in which more solute is present. 

Hypotonic solutions 

 These are those with less solute. Isotonic solutions have equal concentrations of solutes. 
 In a hypertonic solution the solute molecules attract clusters of water molecules, so that fewer water molecules are free to diffuse across the membrane. On the other hand in a hypotonic solution with fewer solute molecules, there are more free water molecules and there is net movement of water from hypotonic solution to the hypertonic solution. 

Water balance problems: 

When an animai cell, such as the red blood cell, is placed in an isotonic solution, the cells volume remains constant because the rate at which water is entering the cell is equal to the rate at which it is moving out. When a cell is placed in a hypotonic solution (which has lower salt concentration than the cell) water enters and the cell swells and may rupture like an over-filled balloon. 
Similarly an animal cell placed in a hypertonic solution (which has higher salt concentration than the cell) will lose water and will shrink in size. So in hypotonic environments (freshwater) animal cells must have ways to prevent excessive entry of water and in hypertonic environments (seawater) they must have ways to prevent excessive loss of water. 

Turgor

 Turgor of the cells is responsible for maintaining shapes of non-woody plants and soft portions of trees and shrubs. Water balance problems are somewhat different for plant cells because of their rigid cell walls. Most plant ceils live in hypotonic environment because there is low concentration of solutes in extracellular fluids than in their cells. 
As a result water tends to move first inside the cell and then inside the vacuole. When vacuole increases in size the cytoplasm presses firmly against the interior of the cell wall, which expands a little. Due to strong cell wall, plart cell does not rupture but instead becomes rigid. 

Turger pressure

The internal pressure of such a rigid cell is known as turgor pressure and this phenomenon is known as turgor.
 In isotonic environment the plant cell is flaccid (loose / not firm), because the net uptake of water is not enough to make it turgid. In a hypertonic environment a plant cell loses water, causing the cytoplasm to shrink within the cell wall. 
The shrinking of cytoplasm is called plasmolysis. Stomata (openings) in leaf epidermis are surrounded by guard cells. During daytime guards cells are making glucose and so are hypertonic (have higher concentration of glucose) than their nearby epidermis cells. 
Water enters them from other cells and they swell. In this form they assume a rigid bowed shape, creating a pore between them. At night when there is low solute concentration in guard cells water leaves them and they become flaccid. In this form both guard cells rest against one another, closing the opening. 

Semi-permeable membrane

The knowledge about semi-permeable membranes is applied for various purposes. We know that semi-permeable membrane is capable of separating substances when a driving force is applied across it. Artificially synthesized semi-permeable membranes are used for separation of bacteria from viruses, because bacteria cannot cross a semi- permeable membrane. In advanced water-treatment technologies membrane-based filtration systems are used. 
Semi-permeable membranes efficiently separate salts from water under pressure.

 Filtration 

 Filtration is a process by which small molecules are forced to move across semi- permeable membrane with the aid of hydrostatic (water) pressure or blood pressure. For example in the body of an animal, blood pressure forces water and dissolved molecules to move through the semi-permeable membranes of the capillary wall cells.
 In filtration the pressure cannot force large molecules, such as proteins, to pass through the membrane pores . 

Active transport 

Active transport is the movement of molecules from an area of lower concentration to the area of higher concentration. This movement against the concentration gradient requires energy in the form of ATP. In this process, carrier proteins in the cell membrane use energy to move the molecules against the concentration gradient.
 For example the membranes of nerve cells have carrier proteins in the form of “sodium-potassium pump”. In a resting (not conducting nerve impulse) nerve cell, this pump spends energy (ATP) to maintain higher concentrations of K+ and lower concentrations of Na+ inside the cell. For this purpose, the pump actively moves Na+ to the outside of the cell where they are already in higher concentration. Similarly this pumps moves K+ from outside to inside the cell where they are in higher concentration molecules. Outside of cell In diffusion and filtration, only small molecules can pass across membrane. It is a special type of movement of material across cell membranes. 

Endocytosis

In endocytosis bulky materials, rather than individual molecules, are moved across the cell membrane. Endocytosis occurs in following steps: Aportion of cell membrane invaginates (depressed inward). The material from outside is taken inside the invagination. The open ends of the invagination seal and form a small vesicle. The vesicle detaches from the cell membrane and moves into the cytoplasm. The two forms of endocytosis are 
phagocytosis (cellular eating) 
 pinocytosis (cellular drinking).

Phagocytosis (cellular eating)

 In phagocytosis cell takes in solid material 

  Pinocytosis (cellular drinking).

 Pinocytosis cell takes in liquid in the form of droplets. 

Exocytosis

 It is the process through which bulky material is exported. Exocytosis occurs in following steps: 
The bulky material is packed inside a membrane and a vesicle is formed. The vesicle moves to the cell membrane. The vesicle fuses with the membrane and releases its contents into the extracellular environment. This process adds new membrane which replaces the part of cell membrane lost during endocytosis. 
Bulky Cell material membrane are familiar with the levels of structural organization of life and recognize tissue as a qroup of similar cells specialized for the performance of a common function. In this topic we will leam about the major types of animal and plant tissues, with reference to their cell specificities, locations and functions.
 In a colony of cells there are many cells and each cell performs all general functions on its own. Such a group does not get tissue level of organization because cells are not specific and there is no coordination among them.