MOVEMENT OF SUBSTANCES
Cell Size & Surface Area:Volume
Optimum Temperature - The temperature at which cells can maximize their performance. If the temperature increases beyond this point cells will become permanently damaged.
Rate of Material Exchange Depends on Cell size
Volume = Area x Height
Surface Area (S.A.) = Area x No. of Sides
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Surface Area to Volume Ratio (S.A. : V)
Surface Area is the amount of nutrients and material a cell can take in.
Volume is the amount of nutrients and material a cell needs to function properly.
As long as surface area is larger than volume in the S.A. : V ratio, the cell will continue to work properly, but if the volume of the cell increases, the cell will not be able to function and will divide into two or more cells to regain an optimum size.
As can be seen on the right the greater the ratio between the surface area to the volume is, the better the cell functions. Thus it can deduced that the smaller the cell it is the faster it will be able to carry out three vital functions - get rid of waste products, lose heat and take in nutrients.
SIZE OF CELLS (Optimum Size)
Allows for
- Waste to leave from cell faster
- Heat to be lost faster
- Nutrients to enter cell faster
Figure 1. Surface Area to Volume Ratio in Cells
Transport of Substances
Diffusion - The net movement of particles from an area of higher concentration to an area of lower concentration across a concentration gradient.
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Osmosis - The net movement of free water molecules from an area of higher water potential to an area of lower water potential across a concentration gradient/ selectively permeable membrane.
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Active Transport - The net movement of particles from an area of lower concentration to an area of higher concentration against a concentration gradient (active transport requires energy).
Q. If a bottle of perfume is spilled by mistake in one room, how is it that we can smell it in another room in a few minutes time?
A. Perfume is a volatile liquid (it evaporates easily; has a low boiling point) and is made up of many particles. These particles are constantly moving, and in gaseous state, spread outwards throughout the house until the particles are equally spread across the entire house. This is the process of diffusion - the spill being the point of higher concentration and the rest of the house being the area of lower concentration (since at first there were a lot of particles near the perfume spill, but they move outwards to areas where there are fewer or no other perfume particles).
Dynamic Equilibrium - Particles are always moving about, and do not stop moving even when there is the same number of them everywhere in a system. Once these particles are evenly spread about said system, there is no net change in this system.
Brownian Motion - It is the unpredictable movement of particles in a system where they have no path and continue to move even after equilibrium has been reached.
Figure 2. Osmosis Diagram
Figure 3. Diffusion Diagram
Figure 4. Active Transport Diagram
As can be seen through the three diagrams above all three modes of transport work in very different ways according to the situation. Osmosis is the movement of water molecules when a semi permeable membrane is in place. As can be seen through Figure 2.2, the sugar molecules cannot move through the semi-permeable membrane whereas water molecules can. Since one sugar solution is more concentrated (contains more sugar molecules) than the other, the water distributes itself in such a way that the concentration of the sugar solution on both sides of the membrane is equal. This movement of water results in the decrease of water on one side and an increase on the other.
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In diffusion it is assumed that the molecules do not have to move through any selectively permeable membrane - as is the case in osmosis - and both water molecules and dye molecules (shown in Figure 2.3) can equally distribute throughout the solution until dynamic equilibrium is achieved. Diffusion takes place in solids, liquids, and gases - although it cannot be seen in solids due to how long it takes.
In active transport (refer to Figure 2.4) molecules move from an area where they are present in lower concentration and move to an area where they are present in higher concentration. This process takes energy and happens around the selectively permeable membrane of cells, where the membrane uses energy to selectively take in the particular molecule (even if it is lower in concentration outside the cell).
These three modes of transport can be visualized in the concentration gradients shown below:
Diffusion is a process which can be observed in our lungs, as there is a constant gradient created between carbon dioxide and oxygen. Since there is an oxygen deficit in the blood going into the lungs as well as a surplus of carbon dioxide, and more oxygen in the atmosphere, a constant gradient is created between carbon dioxide and oxygen. This way oxygen enters blood in the lungs, and carbon dioxide leaves into the atmosphere when we breathe.
Active transport occurs only in living things because living cells respire. The energy created during respiration is used in the process. Active transport is used by root hair cells to take in minerals from the soil when the soil is mineral-deficit (because there will be a greater amount of minerals in the plant than in the surroundings and no concenetration gradient will be set up.
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Osmosis helps regulate the amount of water a cell takes in, and can be best observed in plants. Before understanding the effects of osmosis the three types of solutions must be established:
1. Isotonic Solution - Equal water potential and concentration
2. Hypertonic Solution - Lower water potential, higher concentration
3. Hypotonic Solution - Higher water potential, lower concentration
Figure 5. Concentration Gradients
ISOTONIC SOLUTION
Animal Cell - In an isotonic solution the water potential inside and outside the cell is the same, hence there is no effect on the cell.
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Plant Cell - In an isotonic solution the water potential inside and outside the cell is the same, hence there is no effect on the cell.
HYPERTONIC SOLUTION
Animal Cell - In a hypertonic solution, the water potential inside the cell is higher than that outside the cell. Due to this reason water leaves the cell into the solution, and the cell undergoes crenation (spikes appear on the cell, and cell decreases in size). If water is not provided soon, the cell will die.
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Plant Cell - In a hypertonic solution, the water potential inside the cell is higher than that outside the cell. The cell undergoes plasmolysis as water leaves the vacuole into the solution. Due to the cell wall, there is no effect on the structure of the cell but the cell appears to be flaccid. If water is not provided soon, the cell will die.
HYPOTONIC SOLUTION
Animal Cell - In a hypotonic solution, the water potential inside the cell is lower than that outside the cell. Due to this reason water enters the cell from the solution, and the cell undergoes cellysis (cell increases in size until it eventually bursts).
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Plant Cell - In a hypotonic solution, the water potential inside the cell is lower than that outside the cell. The cell becomes turgid as water enters the vacuole. Due to the cell wall, the cell will not burst as is the case in animal cells but will appear to be larger in size. The pressure exerted by the vacuole onto the cell wall is called turgor pressure.
Figure 6. The effects of osmosis on cells
** When plants wilt it is due to lack of water. The water in the plant leaves through osmosis (since the soil is hypertonic), leaving the plant cells plasmolysized and flaccid.
*** Plants which do not have skeletons use the principle of osmosis to stand upright. When watered, the soil becomes hypotonic, resulting in water to enter the plant. This results in the plant cells to become turgid, and it is this same turgor pressure which allows such plants to stand upright.