transport across membrane

OBJECTIVE
To study the effects of hypotonic, hypertonic and isotonic solutions on plant and animal cells.

INTRODUCTION
In cellular biology the term membrane transport refers to the collection of mechanisms that regulate the passage of solutes such as ions and small molecules through biological membranes, which are lipid bilayers that contain proteins embedded in them. The regulation of passage through the membrane is due to selective membrane permeability – a characteristic of biological membranes which allows them to separate substances of distinct chemical nature. In other words, they can be permeable to certain substances but not to others. The movements of most solutes through the membrane are mediated by membrane transport proteins which are specialized to varying degrees in the transport of specific molecules.

As the diversity and physiology of the distinct cells is highly related to their capacities to attract different external elements, it is postulated that there is a group of specific transport proteins for each cell type and for every specific physiological stage. This differential expression is regulated through the differential transcription of the genes coding for these proteins and its translation, for instance, through genetic-molecular mechanisms, but also at the cell biology level: the production of these proteins can be activated by cellular signaling pathways, at the biochemical level, or even by being situated in cytoplasmic vesicles. Hypotonic refers to a lesser concentration. A hypotonic solution has a lower concentration of solutes in its surroundings, so in an attempt to balance concentrations, water will rush into the cell, causing swelling. Some organisms have evolved intricate methods of circumventing hypotonicity. For example, saltwater is hypertonic to the fish that live in it. They need a large surface area in their gills in contact with seawater for gas exchange, thus they lose water osmotically to the sea from gill cells. They respond to the loss by drinking large amounts of saltwater, and actively excreting the excess salt. This process is called osmoregulation.

An isotonic solution is one in which its effective osmole concentration is the same as the solute concentration of another solution with which it is compared. This occurs, for example, when the concentration of both water and total solute molecules are the same in an external solution as in the cell content. Water molecules diffuse through the plasma membrane in both directions, and as the rate of water diffusion is the same in each direction that cell will neither gain nor lose water. For example, the osmolarity of Normal saline, 9 grams NaCl dissolved in water to a total volume of one litre, is a close approximation to the osmolarity of NaCl in blood, i.e. Normal saline is almost isotonic to blood plasma. Hypertonic refers to a greater concentration. In biology, a hypertonic solution is one with a higher concentration of solutes on the outside of the cell. When a cell is immersed into a hypertonic solution, the tendency is for water to flow out of the cell in order to balance the concentration of the solutes.

When plant cells are in a hypertonic solution, the flexible cell membrane pulls away from the rigid cell wall, but remains joined to the cell wall at points called plasmodesmata. The cell takes on the appearance of a pincushion, and the plasmodesmata almost cease to function because they become constricted: a condition known as plasmolysis. In plant cells the terms isotonic, hypotonic and hypertonic cannot strictly be used accurately because the pressure exerted by the cell wall significantly affects the osmotic equilibrium point. A hypertonic solution is used in osmotherapy to treat cerebral hemorrhage

APPARATUS AND MATERIALS

Materials
Apparatus.
Onion
potato
table sugar/sucrose
table salt/NaCl2
distilled water
70% ethanol

small knife
glass slide
cover slip
microscope
test tubes
beakers (250 ml & 500 ml)
filter paper
cotton
lancet
benchtop digital balance (2)
magnetic stirrer/stirring rod
spatula

RESULTS
Experiment 1

Onion cell in distilled water

Fig 1.1

Onion cell in 5% (w/v) sucrose solution

Fig 1.2

Onion cell in 30% (w/v) sucrose solution

Fig 1.3
Experiment 2

Salt conc.
(w/v)
Initial wt.
of potato stick (g)
Final wt.
of potato stick (g)
Change in
Weight (g)
Change in
Weight (%)
Rank relative
Water loss or gain.

Thick
Thin
Thick
Thin
Thick
Thin
Thick
Thin
Thick
Thin
10% NaCl
2.00
0.52
1.68
0.45
-0.32
-0.07
-16.00
-13.46
Water loss
Water loss
3.5% NaCl
1.97
0.52
1.80
0.47
-0.17
-0.05
-8.63
-9.62
Water loss
Water loss
0.88% NaCl
2.06
0.53
2.15
0.56
0.09
0.03
4.37
5.66
Water gain
Water gain
Distilled water
2.00
0.56
2.21
0.64
0.21
0.08
10.50
14.29
Water gain
Water gain

Experiment 3

Blood cell in distilled water

Fig 3.1
Blood cell in 4% (w/v) NaCl

Fig 3.2

Blood cell in 0.85% (w/v) NaCl

Fig 3.3

DISCUSSIONS

Experiment 1

The epidermal layer of an onion is placed on a liquid in different concentrations. The structure of an onion cell vary depends on the concentration as we examined it through microscope. In distilled water, the onion cells become turgid. This is due to water from extracellular fluid moves into the cell by osmosis causing the cell to swell without bursting due to the presence of the cell wall. The increased in pressure pushes the cytoplasm against the cell wall and the cells become turgid as shown in figure 1.1. Distiiled water is said to be hypotonic solution. In 5% (w/v) sucrose solution, the onion cells retains in normal shape. This is due to water from extracellular fluid moves in and out of the cell at the same rate hence there is no net movement of water. The cell retains its normal shape as shown in figure 1.2. Hence, 5% (w/v) sucrose solution is said to be isotonic solution. In 30% (w/v) sucrose solution, the onion cells become plasmolysed as water moves out from the cell by osmosis, the cytoplasm pulls away from the cell wall and the cell becomes flaccid. This is said to be plasmolysed as shown in figure 1.3. Therefore, 30% (w/v) sucrose solution is said to be hypertonic solution.

Experiment 2

Potato cells also show the same effects as the epidermal onion cells when placed in hypotonic, isotonic and hypertonic solutions.
In 10% (w/v) NaCl, the weight of potato sticks decreasing in large percentage around 16% – 13%. This shows the water in the cells moves out by osmosis to the extracellular fluid which hypertonic. The cell is said to be flaccid as water loss.

In 3.5% (w/v) NaCl , the weight of potato sticks is decreased around 9% -8 %. This shows the water in the cells moves out by osmosis to the extracellular fluid which hypertonic. The cell is said to be flaccid as water loss.

In 0.88% (w/v) NaCl , the weight of potato sticks is slightly increased around 4% -5 %. This shows the water in the extracellular fluid moves into the cells by osmosis. The cell can be said to be no change in shape as minimal changing in weight . Therefore no change in shape of the cells and the solution is said to be isotonic.

In distilled water , the weight of potato sticks is greatly increased around 10% to 14%. This shows the water in the extracellular fluid moves into the cells by osmosis. The cell can be said become turgid and distilled water is said to be hypotonic.

Experiment 3

Test tube A, B and C is filled with different concentration solution and a few drops of blood was added to each test tube. The structure of blood cell vary depends on the concentration as we examined it through microscope. In distilled water, the blood cells bursts. This is because water from extracellular fluid enters the cell, the cell increases in size and finally bursts. The cell membrane cannot withstand the high pressure inside the cell and this is shown in figure 3.1. This process called as haemolysis. Distilled water is said to be hypotonic solution. In 4.0% (w/v) NaCl, the
blood cells becomes plasmolysed. Plasmolysis occurs where water from the cell moves out. The cells shrinks and becomes crenated. This is shown in figure 3.2. Hence, 4.0% (w/v) NaCl is said to be hypertonic solution. In 0.85% (w/v) NaCl, the blood cells retains in normal shape. This is due to water moves in and out of the cell at the same rate hence there is no net movement of water. The cell retains its normal shape as shown in figure 3.3. Hence 0.85% (w/v) NaCl is said to be isotonic.

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