Cell and its environments

                Cell and its environment

 The cells of organisms are surrounded by a watery medium or environment.

Materials flow into the cell from its environment by any of the five(5) procecess.

1. Diffusion
2. Osmosis
3. Active transport
4. Endocytosis
5. Exocytosis

Diffusion and Osmosis are referred to as 'passive transport' as they do not require energy for materials to be transported between the cell and its environment.

 Diffusion can take place in any of the three states of matter( solid,liqui and gas). However,it is very fast in gases , quite slow in liquids and extremely slow in solids.

   Above are  diagrams that explain the process of diffusion

It is noticed that the particles are initially concentrated at the top corner of the glass. If the particles randomly move around (diffuse) in the water, they eventually become evenly/uniformly distributed randomly from an area of high concentration to an area of low concentration.

 So, what's diffusion?

Diffusion is the process by which molecules or ions of a substance migrate from a region of higher concentration to a region of lower concentration until dynamic equilibrium is attained.

Note: The difference in concentration of the substance in the two regions before diffusion is the concentration gradient or diffusion gradient.

Factors affecting the rate of diffusion include

1. State of matter  
2. Concentration gradient.
3. Temperature
4. Molecular size 

1. States of matter : The rate of diffusion varies with the three states of matter. It is fastest in gases(this is because they are free to move randomly owing to the neglible force of attraction between the molecules of gas), faster in liquids(the molecules of liquids are less tightly packed together ,freedom of movement is less restricted) fast in solids(free of movement is restricted).

 Below are diagrams showing how molcules are packed in each states of matter.

2. Temperature: Temperature is the measure of the average kinetic energy of the molecules of substance and thus an increase in its speed. The higher the temperature, the faster the rate of diffusion.

3. Molecular size: The molecular size also affects the rate of diffusion. The larger the molecules, the slower the rate of diffusion while the smaller the  molecules,the higher the rate of diffusion.

4. Concentration gradient: This is necessary for difusion to take place , the  higher the concentration gradient , the faster the rate of diffusion and vice versa.

      Importance of Diffusion to animals

—Gaseous exchange in the lungs
—Gaseous exchange from the surface of unicleuilar organisms e.g Amoeba
—The intake of oxygen or nutrients from mother to foetus(embryo) through the placenta

     Importance of diffusion to plants

—The intake of carbon(iv) oxide and removal of oxygen gas through the stomata during photosynthesis.
—The removal of water vapour from leave surfaces during TRANSPIRATION.
—The Intake of oxygen gas and the removal of carbon(iv)oxide through the stomata during respiration.

Now,consider two equal volume of a strong and weak solution separated by a selectively permeable membrane, there is a movement of  water molecules from the weak solution to the strong solution till the concentration of both solution is equal. This is a special type of diffusion called 'Osmosis'.

What is osmosis? Osmosis is the spontaneous movement of water molecules from a weak(less concentrated) solution to a strong solution(high concentrated) solution across a selectively permeable  membrane.

Types of solution and effects on animal cells

1. Hypertonic  solution: A hypertonic solution has a greater concentration of solutes than another solution. There is an outward movement of water molecules from the cell to the surrounding. This is known as Exosmosis

2. Hypotonic solution: A hypotonic solution has a less concentration of solutes relative to the contents of the cell. There is a net  movement of water molecules from the  surrounding fluid into the cell. This is Endosmosis.

3. Isotonic solution: Here the concentration of the solute particles an that of the surrounding fluid are equal. No net flow of water molecules because the concentration gradient is zero. For a living cell to survive or function well,its surrounding fluid that bathe it  must be Isotonic. Hypertonic and Hypotonic fluid may cause death of  living cells.

Plant cells are made up of cellulose cell wall which is tough and slightly elastic. When plant cells are placed in a solution hypotonic to its cell sap(a complex mixture of high concentration contained in vacuole),endosmosis occurs i.e water flows from the surrounding fluid into the cell sap which causes the cell to become turgid.

Turgidity makes the plants firm and give supports to herbaceous plants.

Now consider plant cell placed in a solution Hypertonic to its cell sap, exosmosis occurs I.e water flows from the cell sap into the surrounding fluid.
 This causes the vacuole to shrink resulting in  pulling away of the cytoplasm away from the cell. The process explained above is called Plasmolysis.

A biological process may occur where molecules or ions are moved from a region of lower concentration to a region of higher concentration against the concentration gradient.

A spectacular example of this Is the sodium-potassium pump, where sodium ions are pumped from the blood cells to plasma and potassium ions are transported from plasma to the blood cells.
 This process is against the concentration gradient and requires a great amount of energy. This process is called the Active transport.

 Cells carrying out this process must have a large numbers of mitochondria. The cell's energy comes from the hydrolysis of adenosine triphosphate(ATP).


The endocytotic and exocytotic processes are both required for the intake of bulky materials into the cell. Certain materials are engulfed by the cell of an amoeba, this process is phagocytosis.

 This is an endocytotic process as there is an intake of food materials lnto the  cell by the invagination of the plasma membrane which forms a depression enclosing food particles forming a phagocytotic vesicle or food vacuole as shown by the diagram below.

This process is also used by white blood cells for engulfing foreign invaders.

Liquid materials may also be  transported into the cell in large quantities. This is the Pinocytotic process. It is same thing with the phagocytotic process, but only the materials taken in differs.


Another process may occur where a membrane bound vesicle is carried out of the cell membrane and contents are discharged to the extracellular environment.

This process is called the exocytotic process. Below is a diagram to explain this process?