TRANSPORT OF MATERIALS IN LIVING THINGS I

TRANSPORT OF MATERIALS IN LIVING THINGS I

To stay alive all living organisms need to transport materials like food, water and oxygen to various parts of the body. Food and oxygen are transported to all cells in the body for respiration and growth. Waste products are transported from the cells to organs that excrete them.

All plants need water. The wilted leaves recover when water is added to the soil, which means that water has been conducted upward into the leaves. You have also learnt that the leaves for photosynthesis need water. Likewise, the food produced in the leaves has to be transported to other parts of the plant including the stem, the roots flowers and fruits etc. All this transportation is the function of conducting tissues.

Similarly, in animals, the food absorbed by the gut has to be carried to all the body parts, oxygen absorbed in the lungs has to be transported to every cell of the body, the carbon dioxide produced in the cells has to be carried to the lungs for elimination, and the poisonous body wastes like urea has to be transported to the kidneys for elimination in urine, and so on. All such functions are the outcome of a transport system.

WAYS OF TRANSPORTATION OF MATERIALS

Molecules move in and out of a cell through the cell membrane, which forms the boundary of each cell. The cell membrane is selectively permeable to substances, which means that it permits entry and exit of certain molecules only. There are various methods by which materials are transported in plants and animals. The movement of molecules takes place by passive transport (simple diffusion, facilitated diffusion, osmosis, and mass flow) and active transport. 

1. PASSIVE TRANSPORT

Passive transport is a movement of ions and other atomic or molecular substances across cell membranes without need of energy input. Passive transport is naturally occurring phenomenon and does not require the cell to expend energy to accomplish the movement.

2. ACTIVE TRANSPORT

In active transport, molecules have to move (against concentration gradient) i.e., from a region of their lower concentration to a region of their higher concentration. Energy is required in active transport.

The four main kinds of passive transport are simple diffusion, facilitated diffusion, osmosis, and mass flow.

A) DIFFUSION

Diffusion is a passive process of transport in which substances tend to move out from their region of higher concentration to the region of lower concentration until concentration is equal across the space. For example, during respiration, oxygen-laden air in lungs being at a higher concentration moves into blood capillaries having lower concentration of oxygen in them. Such movement of particles or molecules from a region of their higher concentration to a region of their lower concentration is termed diffusion.

Factors that affect the rate of diffusion

i) Extent of the concentration gradient: the greater the difference in concentration, the more rapid the diffusion. The closer the distribution of materials gets to equilibrium, the slower the rate of diffusion becomes.

ii) Mass of the molecules diffusing: more massive molecules move more slowly, because it is more difficult for them to move between the molecules of the substance they are moving through; therefore, they diffuse more slowly.

iii) Size of the molecule: the smaller the molecule such as gas, the faster the rate of diffusion while the larger the molecule (liquid) the slower the rate of diffusion. 

iv) Temperature: higher temperatures increase the energy and therefore the movement of the molecules, increasing the rate of diffusion. 

v) State of matter: diffusion in gas molecules is faster than the diffusion in liquids and it is not possible in solids. 

vi) Surface area to volume ratio: this is the ratio of total surface area exposed by an organism compared to its body volume. Small sized living organisms have a large surface area to volume ratio. The larger the surface area to volume ration, the high the rate of diffusion and vice versa. Small organisms like amoeba and paramecium can hence rely on diffusion for transport of substances into and within its body and removal of waste products. 

vii) Thickness of membranes (distance): molecules take longer to diffuse across thick membranes than across thin membranes hence the thin the membrane the higher the rate of diffusion. (distance over which diffusion takes place) 

B) FACILITATED DIFFUSION

In facilitated transport also called facilitated diffusion, material moves across the plasma membrane with the assistance of transmembrane proteins down a concentration gradient (from high to low concentration) without expenditure of cellular energy. 

C) OSMOSIS

Osmosis is the movement of water molecules from a region having more water molecules to a region having less water molecules when separated by a semi permeable membrane. Semi permeable membrane means a membrane, which allows some molecules (e.g. water molecules) to pass through it but not some other larger molecules. No energy is spent during osmosis.

Whereas diffusion transports material across membranes and within cells, osmosis transports only water across a membrane and the membrane limits the diffusion of solutes in the water.

When water moves across a semi permeable membrane by osmosis into another solution, a pressure builds up to stop the flow of pure water into the solution. This pressure is called osmotic pressure .Thus water moves from a region of high osmotic pressure to a region of low osmotic pressure.

Terms used in connection with osmosis 

i. Hypotonic solution: This is used to describe a solution containing less solute and more water molecules compared to another. A hypotonic solution has lower osmotic pressure and is generally termed as less concentrate. 

ii. Isotonic solution: These are solutions with same concentration of solute and water i.e. solutions with same osmotic pressure. 

iii. Hypertonic solution: This is used to describe a solution with more solutes and less water molecules than another .A hypertonic solution has a higher osmotic pressure and is generally termed as more concentrated solution. 

iv. Haemolysis: refers to the destruction of red blood cells (RBCs), which is broadly defined as a reduction in the life span below 100 days (normal 110-120 days). 

v. Crenation is the contraction (shrinkage) of a cell after exposure to a hypertonic solution, due to the loss of water through osmosis. 

vi. Plasmolysis is when plant cells lose water after being placed in a solution that has a higher concentration of solutes than the cell does. This is known as a hypertonic solution. Water flows out of the cells and into the surrounding fluid due to osmosis. This causes the protoplasm, all the material on the inside of the cell, to shrink away from the cell wall. 

vii. Turgidity is the fully expanded condition of a cell with its wall stretched due to excessive accumulation of water. The outward pressure exerted by the cell fluid on the cell wall is called turgor pressure. The inward pressure exerted on the cell contents by the stretched cell wall is called wall pressure.

OSMOSIS AND CELLS

a) Animal cells

Unlike the plant cells animal cells lack cell wall, possess only a cell membrane which is weak and non – resistant to high internal pressure.

            Osmosis and Red Blood Cells

When red blood are placed in a dilute solution (hypotonic solution) e.g. distilled water, the cells swell up and eventually burst i.e. haemolyse. This is because water moves from the surrounding solution (distilled water) via the semi permeable cell membrane into the cell.

When red blood cells are placed in a more concentrated solution (hypertonic solution) e.g. a strong sugar or salt solution, water moves out of the cells to the surrounding solution by osmosis. As a result, the cells shrink a process called crenation.

However, when red blood cells are placed in isotonic solution they neither gain water and burst nor lose water and shrink. This indicates that the blood plasma is isotonic to solution in red blood cells.

b) Osmosis and plant cells

Plant cells are surrounded by an in-extensible, resistant and completely permeable cellulose cell wall. The centre of cells contains a vacuole, which contains sap. Sap is a solution of salt, sugars and organic acid. Sap is surrounded by a semi-permeable tonoplast membrane.

When a plant cell is placed in hypotonic solution e.g. distilled water, it swells up hence increasing in size due to osmotic flow of water from the solution into the cell. As the cell gains water osmotically it reaches a time when no more water enters it because the cell wall resist further expansion .At this stage, the cell is said to be at full turgor or fully turgid. At full turgidity the sap vacuole enlarges and pushes the cytoplasm against the cell wall. The pressure exerted out wards by vacuole is called turgor pressure.

When the cell (or tissue) is placed in an isotonic solution, there is no net flow of water towards the inside or outside. If the external solution balances the osmotic pressure of the cytoplasm it is said to be isotonic. When water flows into the cell and out of the cell and are in equilibrium, the cells are said to be flaccid. The plant cell becomes flaccid because the cell contents are no longer pushing against the cell wall.

When a plant cell is in a hypertonic solution e.g. strong sugar solution .the cell shrinks and decreases in size .this is because water moves out of plant cells to surrounding solution by osmosis and cell vacuole shrinks causing the cell membrane to pull away from the cell wall.

Therefore, plasmolysisis the shrinkage of the protoplasm away from the cell wall due to loss of water from the plant cell by osmosis to the surrounding hypertonic solution.

Significance of osmosis

• Osmosis is important in absorption of water by root hairs from the soil.
• It enhances the movement of water from roots hairs via the root cortex to the xylem.
• It facilities the opening and the closure of the stomata by guard cells.
• It enables movement of water from gut into blood streams via gut walls.
• It enables reabsorption of water into blood streams via kidney tubules.

Table 5.1 Effects of Osmosis on Living Cells when Placed in Different Solutions

SOLUTION TYPE	Concentration of dissolved substances (solutes)	Concentration gradient	Results in animal cells	Results in plant cells
ISOTONIC	Same as living cell	Zero	No change	No change
HYPOTONIC	Lower than the living cell	Net water movement into the cell	Cell swells and bursts	Pressure created by excess water store in central vacuole (tugor pressure)
HYPERTONIC	Higher than the living cell	Net water movement out of the cell	Cell shrinks (plasmolysis)	Central vacuole collapses and plasmolysis occurs

Practical Activity on Osmosis

Living tissues for example Irish potatoes, raw pawpaw can be used to determine the levels of solutions. Strips or cylinders of known length and width are cut and at least two pieces soaked in each solution for 30 minutes. They are then removed and measured to get the final length. They are also squeezed to feel their firmness.

Expected observations and conclusions

• Some strips would be flexible and soft to touch, while the length and the width would have reduced. This would mean that the tissue lost water to surrounding solution and the solution is therefore hypertonic to cell sap.
• Others would be rigid length and width would have increased .This would mean that the tissue gained water from the surrounding solution by osmosis and the solution therefore was hypotonic to the cell sap.
• There would be no noticeable change in firmness and length of other tissues. This would mean that the tissue neither gained nor lost water to the surrounding solution was isotonic to cell sap.

Osmosis and Unicellular Organisms

In aquatic unicellular organisms such as protozoa and algae, the cell is hypertonic relative to its surrounding, i.e. the inside of the cell has higher concentration of solutes than the environment. As a result, the water tends to flow into the cell through osmosis. The contractile vacuole helps prevent excessive water influx that could harm and cause rupture (lysis) to the cell. The contractile vacuole contracts to expel water out of the cell (thus, the name).

A contractile vacuole is a specialized type of vacuole in eukaryotic cells, particularly protozoa and certain unicellular algae. It is involved in osmoregulation. Osmoregulation is the process of regulating water potential in order to keep fluid and electrolyte balance within a cell or organism relative to the surrounding.

D) MASS FLOW

Mass flow refers to the movement of substance in bulk from one point to another as a result of pressure differences between the two points.

In living organisms this usually means the bulk movement of water (the solvent) together with all its dissolved solutes and suspended objects. So mass flow is like a river carrying everything with it. Mass flow always requires a source of energy to pump the fluid, but it has the advantage of being much faster than diffusion, especially over large distances. Mass flow is completely independent of concentration differences.

Examples of mass flow include: circulatory systems in animals, xylem and phloem systems in plants, filter feeder currents, and ventilation.

Differences between diffusion, osmosis and mass flow

Characteristic	Diffusion	Osmosis	Mass flow
Substance transported	Molecules/ions	Water molecules	Water and solutes together
Condition (cause of movement)	High concentration to low concentration (Movement due to concentration gradient)	High concentration to low concentration (Movement due to concentration gradient)	High pressure to pressure (Movement due to pressure gradient)
Additional requirements	Down concentration gradient	Across a partially permeable membrane	Through vessels (tubes) and cytoplasm

NB:

While diffusion is the intermingling of molecules as a result of their inherent kinetic energy based on random motion, whether in a gas, liquid or solid, osmosis is the flow of liquid between two sections separated by a semi-permeable or permeable membrane. Osmosis usually refers to the flow of water.

Diffusion is based on random flow of molecules and is much more common in gases while, osmosis is based on the inherent solvent capacity of the molecules of a substance in water. It is the membrane, in osmosis, that allows the flow of certain types of molecules while restricting the flow of other types.

In scientific terms, both diffusion and osmosis are categorized as means of ‘passive transport’ as no external energy is required for the flow of molecules. Osmosis is an important biological concept.

In both osmosis and diffusion, the molecules necessarily flow from an area of higher concentration to the lower one. A practical example of diffusion is when you spray a room freshener in a corner and the entire room soon becomes full with the scent. A practical example of osmosis is when you start feeling thirsty after eating something salty because the excess salt draws water to the cells in the body.

Diffusion and osmosis both have an important role to play in living organisms for maintaining homeostasis ‘“an internal balanced condition or equilibrium to regulate various mechanisms through cellular functioning.

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