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Osmosis is a kind of diffusion in which water molecules move through a partially permeable membrane from a more dilute solution to a more concentrated solution.

Isotonic Solutions
Isotonic means the concentration of the solute is the same as that of the cell. In these solutions no change occurs to the cell. The diagram below shows the effect of isotonic solutions on cells.




Hypotonic/Weak Solutions
A Hypotonic solution is one in which the concentration of the solute in the solution is less than that of the cell. Examples of these include dilute sugar solutions or water. In these solutions the plant cell will absorb water by osmosis to become turgid (stiff), the cytoplasm and vacuole will also increase in volume and the cell wall will stretch. The diagram below shows a plant cell in a hypotonic solution.





Hypertonic/Strong Solutions
A Hypertonic solution is one in which the concentration of the solute in solution is greater than the concentration in the cell. Here the cells have a higher water potential than the solute. The plant cell loses water by osmosis to become flaccid, its vacuole decreases in volume and the plasma membrane shrinks from the cell wall. The diagram below shows this effect:




Other Resources:

To perform a lab/experiment involving plant cells immersed in different solutions click here
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Substances can enter or leave the plant cell through two known processes these are active and passive transport. Active processes involve the use of energy while passive processes do not involve the use of energy but depends mainly on the permeability of the membrane. Active processes deal with the transfer of molecules and particles whereas passive processes involve diffusion, facilitated diffusion and osmosis.

What is Diffusion?
Diffusion can be defined as the movement of particles from an area where they are at a high concentration to areas where they are at a lower concentration. This will continue until the particles' concentration is uniformed throughout.

What is facilitated Diffusion?
This involves the movement of specific molecules down a concentration gradient (difference in concentration). This is done using a carrier protein, which binds to the molecule allowing it to pass through the membrane. Examples of these include amino acids and glucose.

What is Osmosis?
This is the movement of water molecules across a partially permeable membrane from an area of high water potential to an area of lower water potential. Having a partially permeable membrane means it only allows for some molecules to pass through but not all usually it does not allow for larger solute molecules to pass through.

Active Transport
This is the movement of molecules against a concentration gradient (from lower to higher concentration) with the use of energy. The energy supplied comes from ATP of the mitochondria.

The table below shows examples where the processes mentioned above are operative:


Process
Cases where process is taking place
Diffusion
Gas exchanges in photosynthesis:
·         More carbon dioxide is outside the leaf than inside so carbon dioxide will diffuse into the leaf while the opposite happens for oxygen.
Facilitated Diffusion
Glucose is too large to pass through pores of the membrane therefore it has to bind to a specific carrier protein in order to pass through.
Osmosis
In the roots of plants water is absorbed from the soil.
Active Transport
Re-absorption of salts in the proximal convoluted tubule.


Other Resources:

   Click here to see a PDF containing a more detailed explanation.
   Read Exocytosis | Endocytosis
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These are cellular organelles found in animals they have many functions including breaking down waste materials as well as cellular debris using acid hydrolase enzymes. The digestive enzymes of a lysosome works around a pH of 4.5, it is the membrane surrounding this organelle that allows for these enzymes to work at this pH. Lysosomes contains a variety of enzymes namely, protease, amylase, phosphoric acid and lipase.

Below are some common functions of lysosomes:

  • Lysosomes as stated above, contains numerous amounts of enzymes, some of these enzymes are capable of digesting a wide variety of substances.
  • They are capable of digesting membranes and organelles, this function is of great importance because it allows cells to remodel or replace old organelles.
  • They are often called “suicide bags” because of their ability to rapidly digest an entire old cell.
  • Digesting foreign bacteria and other waste substances.
  • Digesting macromolecules.

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Endoplasmic Reticulum
What is the Endoplasmic Reticulum?


In its simplest form these are membranes forming channels within the cytoplasm, they are continuous with the nuclear membrane and enclose the Cisternae (cellular spaces).


Types of Endoplasmic Reticulum(ER):
  • Rough Endoplasmic Reticulum(RER)
  • Smooth Endoplasmic Reticulum(SER)
Rough Endoplasmic Reticulum


RER is covered with ribosomes; these are tiny granules which help in the synthesis of proteins. It is because of these ribosomes why the RER has its name. Rough endoplasmic reticulum is found mainly in cells that are growing rapidly or that secrete proteins, some are also found in the pancreas which secretes insulin.

Smooth Endoplasmic Reticulum


SER has its name because it contains no ribosomes. Unlike RER it is mainly found in cells that secrete Steroids and lipid substances and serves its functions in many metabolic processes.

Functions of the Endoplasmic Reticulum:

  • It manufactures proteins and enzymes.
  • It manufactures Steroids as well as lipids.
  • It has a large surface area which allows for biological and chemical reactions to take place.
  • Collects and stores synthesized materials.
  • The cell carries out many processes including the transport and exchange of materials; therefore it is the ER which acts as a pathway for these transport and exchanges.
  • Maintenance of cellular shape by forming a structural skeleton.
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 The late scientists Schwann and Schleiden devised a theory known as the Cell Theory which posed that a cell is the basic unit of structure and function in living organisms. A cell is surrounded by a membrane known as the Cell Surface Membrane which is a thin membrane acting as a barrier for the cell contents, its function is to control what molecules enter and leave the cell. All cells contain a structure known as the nucleus which comprises the chromatin, which is a coiled form of a chromosome, and the nucleolus. Next is the cytoplasm which contains the organelles, and is a jelly-like structure found between the cell surface membrane and the nucleus. Despite these common structures present in both animal and plant cells, they also have distinguishing features.

The table below analyzes the common differences between plant and animal cells.
    Plant Cell                                    Animal Cell                            
      Has a rigid cell wall
 Has no cell wall
          Has Chloroplasts
Has no chloroplast
       Has a Large Central vacuole   
       May have small vacuoles      


The image below shows more differences and similarities as well as the structures of  the plant and animal cell.

Image taken from evolution.berkeley.edu
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Vessels are very long, tubular looking structures and are the characteristic conducting units of angiosperm xylem. There are several cells that make up this structure and they are all connected end to end in a row. The vessel element represent each of the cells of a xylem vessel which is equivalent to a tracheid. How are vessels formed? well vessels are formed when vessel elements that are close bond as a result of the disintegration of their walls
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