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ALevel

Biology-9700

Movement Of Substances Into And Out Of Cells

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Movement Of Substances Into And Out Of Cells

Movement Of Substances Into And Out Of Cells


Objective

This unit aims to make the student familiar with how substances are transported across the cell membrane.

Cell Membrane

The cell membrane is semi-permeable. Thus, it allows some substances to pass through while blocking others.

•   Hydrophobic molecules like Oxygen, Co2, etc can diffuse easily through the membrane. This is because the core of the membrane is made up of lipids.

•   Small charged particles like H20 can move through cell membrane but it needs a channel to do so.

•   Large uncharged polar molecules and ions cannot move through cell membranes on their own. As they have charged they are repelled by the cell membrane.

Passive Transport

Movement of substances from high concentration to low concentration without using energy is called passive transport.

•   Diffusion
•   Osmosis
•   Facilitated diffusion

Diffusion:

Molecules can move freely in gases and liquid. They occupy the space available.

Diffusion is:

The net movement of molecules
From the region of high concentration to low
Thus, it does not need energy as the transport is down the concentration gradient.

For Example:

Co2 is at high concentration outside the cell. As it has no charge it can easily diffuse through the hydrophobic cell membrane. The movement of Co2 will be from outside to the inside of the cell across the concentration gradient.

Facilitated Diffusion:

Movement of molecules from high concentration to low concentration via a channel protein is called facilitated diffusion.

The movement takes place across a concentration gradient thus does not need energy.

Ions or charged particles cannot diffuse through the cell membrane. This is because of the hydrophobic core of the cell membrane. Thus, these particles need a channel through which they can move. This is provided by proteins in the cell membrane.

Each channel protein transports only a specific molecule. Thus, there are different proteins for the transport of different substances.

Example:

Amino acids are transported by facilitated diffusion.

Osmosis:

Movement of water molecules from high concentration to low concentration through a semi-permeable membrane is called osmosis.

Water molecules are polar but have small size thus they can easily move through the cell membrane.

Solution with greater water concentration haveà more energyà thus more molecules can move and diffuseà the solution is said to have greater potential.

That is why solutions with more water concentration are said to have greater water potential.

Thus, osmosis can be defined as the movement of water from high water potential to low water potential.

Water Potential

Pure water = 0 kPa
Solutions= -ve kPa
The more solute is added in water the negative the water potential will be,
Thus, dilute solutions have less negative potential than concentrated solutions.

Active Transport

Sometimes molecules have to be transported against the concentration gradient in that case energy has to be used by cells.
Transport which takes place against the concentration gradient by using energy is called active transport.

For Example

Minerals and ions are pumped actively from the soil into the root hair cells. The concentration of ions is greater in the root hairs. Thus, the concentration gradient is from root hairs to soil. In order to absorb more minerals, the ions have to be pumped actively into the root hairs. This needs energy.
Active transport needs carrier proteins. These carrier proteins break down ATP to release energy. This energy is used to pump ions across the cell membrane.

Endocytosis And Exocytosis (Bulk Transport)

Transport of macromolecules across the cell membrane cannot take place through proteins. It takes place through vesicles.

Exocytosis

Molecules that needs to be transported out area surrounded by vesicle inside the cell.

These vesicles are then moved towards the cell membrane.

The membrane of vesicle fuses with the cell membrane.

The molecules are then transported out of the cell.

Endocytosis

Movement of molecules inside the cell takes place in the same way as exocytosis.

The cell sends out the projection of the cell membrane around the molecules to be transported in.

The membrane then fuses around the molecule to form a vesicle.

This vesicle than pushes the molecule inside the cell membrane.

Calculation Of Surface Area And Volume Of Differrent Shapes:          

      

a)   The radius of the sphere increases to 1X
        The volume of sphere 1X
        The surface area of sphere 1X

b) The radius of sphere b increases to 2X
        Volumes of sphere 8X
        Surface areas of sphere 4X

c) The radius of sphere c increases to 3X
        Volumes of sphere 27X
        Surface area of sphere 9X

The figure shows that when the radius of a sphere increases the surface area and volume increase as well.

But the increase in volume is greater than the surface area. Thus, when a cell grows in size its volume becomes more than the surface area available for diffusion. This means less cytoplasm has access to the surface of the cell for diffusion. That is why cell stops growing after a certain time.

Conclusion:

As cell increases in size à  Volume increase, 3X à surface area increases 2X à  Surface area/volume ratio à  decreased.

Experiments On Diffusion And Osmosis


Experiment


•   Cubes of agar placed in the methylene blue solution will absorb it by diffusion. The cubes are left for different periods of time. After some time the cubes are cut open. The distance between the edge of the cube and edge of colored agar shows diffusion.

•   A drop of KmO4 is dropped in a solution of water to note diffusion.

•   Visking tube can be used for demonstrating osmosis. One end of tube is tied and filled with sucrose solution. The other end is attached to a capillary tube. The concentration of sucrose solution is noted both before and after placing the tubing in water.

•   Onion epidermis can be used to know the effect of osmosis. It is cut into small pieces and then mounted on a slide to observe it in isotonic, hypotonic and hypertonic saline.

•   RBCs obtained from blood can be observed on a slide in different concentrations of solutions ( isotonic, hypotonic and hypertonic).

•   Potato cubes are used to observe the effect of surface area on absorption. Cubes of different sizes are used. Mass of each cube is measured and then the cubes are immersed in water. After one hour the cubes are dried and mass is recorded again. The percentage increase in mass for cubes of different surface area to mass ratio can be compared.

Movement Of Water Between Cell And Solution Of Different Water Potential


Hypertonic Solution

A hypertonic solution has low water potential than the cell. Thus, the potential gradient is outside the cell. This results in osmosis of water out of the cell in the solution which will result in plasmolysis or shrinking of cell cytoplasm.

Hypotonic Solution

A hypotonic solution has greater water potential than the cell. The potential gradient is towards the inside of the cell. Thus, water moves inside the cell through osmosis.

Isotonic Solution

An isotonic solution has the same water potential than the cell. Thus, there will no net movement of water.

Difference Between Animal And Plant Cells

Plant cells have a cell wall. Thus, they generate pressure potential which resists further movement of water inside or out of the cell. This prevents the bursting of the cell.

An animal cell does not have a cell wall. Thus, osmosis of a large amount of water inside the cell can burst the cell. That is why animals have to maintain an isotonic body environment.

Investigating Effect Of Immersing Plant Tissues In Solution Of Different Water Potential

There are different methods of how this can be done.

Method #1

Take potato or any other solid part of the plant. Cut it into pieces. Measure the mass or length of the pieces before immersing them in a solution. Leave the parts in the solution for a long time so that they come to equilibrium. Then measure the length or mass of pieces again. Calculate the percentage change.

Method # 2

Cut pieces of onion skin. Mount them on a slide and add a drop of sugar solution. Observe the no of cells plasmolyzed.


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