Archive for category Chapter 3:Movement of Substances Across the Plasma Membrane

3.3 Appreciating the movement of Substances across the Plasma Membrane

1) The plasma membrane around each cell is a delicate and complex structure and its components have specific functions enabling substances to move in and out of the cell.

2)The proper functioning of the plasma membrane is vital to our health hence it is important that we take in enough nutrients and water in our diet.

3) Drinking enought water helps to maintain the osmotic concentration of our body fluid like blood in order for the cells to carry out the various living processes.


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3.2 The Movement of Substances Across the Plasma Menbrane in Everyday Life

The Effect of Hypotonic, Hypertonic and Isotonic Solutions on Animal and Plant Cells 

1) In comparing three solutions with different solute concentrations:



A hypertonic solution is a solution having a greater solute concentration than the cytosol. It contains a greater concentration of impermeable solutes on the external side of the membrane. When a cell’s cytoplasm is bathed in a hypertonic solution the water will be drawn into the solution and out of the cell by osmosis. If water molecules continue to diffuse out of the cell, it will cause the cell to shrink, or crenate.

A hypertonic solution is used in osmotherapy[1] to treat cerebral hemorrhage.


A hypotonic solution is a solution having a lesser solute concentration than the cytosol. It contains a lesser concentration of impermeable solutes on the external side of the membrane. When a cell’s cytoplasm is bathed in a hypotonic solution the water will be drawn out of the solution and into the cell by osmosis. If water molecules continue to diffuse into the cell, it will cause the cell to swell, up to the point that cytolysis (rupture) may occur. In plant cells, the cell will not always rupture. When placed in a hypotonic solution, the cell will have Turgor Pressure and proceed with its normal functions. 


A condition or property of a solution in which its solute concentration is the same as the solute concentration of another solution with which it is compared.

Osmosis in Everyday life

1) Under normal conditions,cells in living organisms rarely burst or crenet because the cytoplasmic fluid in the cells.

2) The rate at which water enters and leaves the cells by osmosis is the same.

Wilting in Plants

Wilting refers to the loss of rigidity of non-woody parts of plants. This occurs when the turgor pressure in non-lignified plant cells falls towards zero, as a result of diminished water in the cells. The process of wilting modifies the leaf angle distribution of the plant (or canopy) towards more erectophile conditions.

Lower water availability may result from:

  • drought conditions, where the soil moisture drops below conditions most favorable for plant functioning;
  • the temperature falls to the point where the plants vascular system can not function.
  • high salinity, which causes water to diffuse from the plant cells and induce shrinkage;
  • saturated soil conditions, where roots are unable to obtain sufficient oxygen for cellular respiration, and so are unable to transport water into the plant; or
  • bacteria or fungi that clog the plant’s vascular system.

Wilting diminishes the plant’s ability to transpire and grow. Permanent wilting leads to plant death. Symptoms of wilting and blights resemble one another.

In woody plants, reduced water availability leads to cavitation of the xylem.

Wilting occurs in plants such as Balsam and tulasi.

7 reasons why plants wilting and how to fix them

  1. Overwatering – this is a common mistake with growing indoor plants. We often water them the same as we would those growing outdoors but forget that evaporation is much lower inside. So plants end up sitting in very moist soil and their roots begin to struggle.
    Overwatering is also a common mistake early in Spring as gardeners adjust to their plant’s requirements. We’re eager to see them grow that we begin watering a little too early.
  2. Lack of water – the flip side of the first problem is not watering them enough. If your plants are wilting because the soil has become too dry then the obvious solution is to begin watering them and keep this constant until the plant picks up again.
    Container plants have a knack of drying out quicker than those growing in the ground. So, the best way to resuscitate your pot plants is to plunge them into a bucket of water and hold until all the air bubbles have subsided. Note: this is only for extreme cases.
  3. Too much sun – plant wilt often happens when you’re growing them in the wrong position or if indoors, the plant is too close to a window. Too much sun for a shade loving plant is like too much social activity for an introvert.
    If outdoors, try moving your plant to another garden bed where it is less likely to be scorched by the sun’s rays. Indoor plants may need to be moved away from the window but still where it can receive some indirect sunlight.
  4. Not enough sun – and this ties in with the overwatering idea. Plants wilt sometimes because they’re not receiving enough sunlight. Picture an extrovert confined to a cubicle office space every day and you’ll understand the problem. The answer, again, is to move them.
  5. Rootbound plants – often plants can outgrow their containers if they’re not transplanted very year or two. Once a plant gets too large for its pot it struggles to draw nutrients and moisture from the soil – if there is any left, that is.
    The answer is to repot your plant into a larger container and use some quality potting mix as its growing medium.
  6. Too much fertiliser – overzealous gardeners can cause plant wilt just by feeding it too much. When adding fertiliser to a plant’s growing medium, whether it be soil or potting mix, take into account the size of the plant and when you last fed it. Plants don’t usually become obese, they just die.
    Try using slow release fertilisers where possible and usually they should only be added at the start of the growing season and again during flowering times.
  7. Disease – plants can often wilt as a result of an infection as well. There are a few main types of plant wilt related to disease, namely – Fusarium wilt which is a fungal disease common to cotton, tomatoes and palms. This type of wilt can be controlled via a fungicide which should be used as per the directions. Other forms are Bacterial wilt and Verticillium wilt.

So, finding your favourite plant wilt doesn’t mean it’s the end of it. There are some things you can do to try and save all your effort and hopefully turn your plant around so that it blooms another day.

Food Preservation

1) Food preservation is based on the concepts of osmosis and plasmolysis to ensure that foods like fruits, vegetables, fish and meat can last longer without turning bad.

2) The addition of salt or sugar to the surrounding solution makes it hypertonic to the food contents. Water moves out from the food cells which become plasmolysed and preservatives move into the food.

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3.1 Movement of Substances Across the Plasma Membrane

Living cells require various substances(absorb water and nutrients) to carry out life processes.

They also produce various substances as  a result of these processes.

Example:  i) Excrete waste products such as urea and uric acid

ii) Exchange respiratory gases (oxygen ang carbon dioxide) during respiration.

These substances need to enter the cell or move out of the cell.

The Permeability of The Plasma  Membrane

 A plasma membrane is the selectively permeable or semi-permeable that some substances can move across the membrane freely while other not.It permeable to the lipid-soluble molecules(fatty acids and glycerol),non-polar molecules(oxygen and carbon dioxide) and water(it’s molecules are small although basically it shouldn’t allow to pass through).On the other hand,it impermeable to the large water-soluble molecules(glucose and amino acids) and ion that charged.These substances cannot easily pass through the plasma membrane and require the help of pore protein and carrier protein.

Movement of Substances

The movement of substances can be classified into:

(a)Passive Transport

-no energy is used by the cell

-Substances move down the concentration gradient ,that is ,from a region of low concentration to a region of low.

Example: Simple diffusion, osmosis, and faciliated diffusion.

1: Diffusion: the net movement of a substance from an area of high concentration to low concentration. So, say if water wanted to get into the cell, and there’s lots of water outside and little inside, then the water will just slip through the membrane – easily.

2: Facilitated diffusion: the movement of substances across the plasma membrane with the help of a carrier molecule. In this case, it’s when the substance is too large, or doesn’t fit across the membrane, so a certain molecule (usually a protein) that is embedded in the membrane helps the substance across.

3: Osmosis: Osmosis is defined as the movement of water molecules down a water potential gradient across a semi-permeable membrane. A solution with many water molecules (dilute) has a high water potential while a solution with few water molecules (concentrated) has a low water potential.

(b)Active Transport

-require the use of both carrier proteins and cellular energy

-substances move against the concentration gradient, that is, from a region of low concentration to a region of high concentration.

1: Active transport: (the opposite of diffusion) the movement of a substance from an area of low to high concentration. Say water is scarce outside the cell, and there’s already heaps inside, if the cell wants to gain more water it will have to expend energy to help the water across (thus ACTIVE transport).

Example :

(a) the absorption of mineral ions from soil by root hairs

(b)the sodium-potassium pump in kidneys and nerves

(c)the accumulation or elimination of substances in sea water organisms.

Differences between Passive and Active Tranport  

Active Transport vs Passive Transport

As minute as they are, cells in the body carry some very important processes deep within. These processes are all vital to the overall growth and development of every organism, may it be an animal or a plant. But every internal process must have some unique mechanisms done to make it successful. In this regard, nutrients, chemicals and other substances are flowing to and fro the cells with the use of certain transport systems. These transport mechanisms are classified into two, namely active and passive transport systems.

In the simplest terms, active transport is termed ‘active’ because of the inclusion of one vital component and that is the use of energy. This energy is being utilized by the cell, in the form of ATP (Adenosine Triphosphate) for it to be able to move most substances in and out of its cellular membranes. On the contrary, passive transport is regarded as such because it is just a plain old ‘passive’ mechanism. It does not use any energy (ATP) from the cell for it to carry out the said processes.

Another distinct characteristic that separates active from passive transport system is the difference in the concentration gradients. It must be made known that the concentration of substances that are partitioned by cell membranes are relatively different. For example, the inside of the cell has a concentration gradient that is higher (more concentrated) than the outside of the cell (less concentrated) or it can also be the other way around depending on various biological factors. Hence, in active transport, it tries to achieve a more difficult task of opposing the concentration gradient. If the cell wants to transport certain substances towards itself (in this situation, it so happens to be more concentrated) then it needs much energy for its protein or sodium pumps to operate and transfer the said substances.

In the case of passive transport, it is not against but along the concentration gradient. Because the cell sees that the same ions or molecules can be transferred to the other side immediately due to a ‘favorable’ concentration gradient, it no longer expends any energy. The word ‘favorable’ simply means that it follows the rules of normal diffusion. When the substances from the more concentrated internal environment of the cell are to be transported outside, that is for example the outside happens to be less concentrated, then the substances can easily flow out.

In brief, active and passive transport differ because:
1.Active transport makes use of energy in the form of ATP whereas passive transport does not utilize any.
2.Active transport involves the transfer of molecules or ions against a concentration gradient whereas passive transport is the transfer along a concentration gradient