Chapters
In this article, we will elucidate and explain the processes of simple diffusion, facilitated diffusion, osmosis, active transport, endocytosis, and exocytosis. Moreover, we will also investigate simple diffusion and osmosis by employing plant tissue. So, let us get started.
Diffusion & Facilitated Diffusion
Diffusion
Diffusion refers to the type of transportation that takes place across the cell membrane. We can define diffusion as:
The net movement that results from the random movement of ions or molecules of a substance from a region of its higher concentration to a region of its lower concentration is known as diffusion.
Remember that the ions and molecules move down the concentration gradient. The natural kinetic energy of molecules and ions causes this random movement. Ions or molecules try to reach a state of equilibrium as a result of diffusion. They do so so that they can be spread evenly within a given volume of space. The rate of diffusion across the membrane is controlled by many factors such as surface area, properties of ions or molecules, temperature, and steepness of concentration gradient.
Facilitated Diffusion
Some substances are unable to diffuse through the phospholipid bilayer of the cell membranes. These substances include:
- Huge polar molecules like amino acids and glucose
- Ions like sodium and chloride ions
These substances require the help of certain proteins to cross the phospholipid bilayer. This creates a diffusion which is referred to as facilitated diffusion. The two types of proteins that enable facilitated diffusion include channel and carrier proteins. These proteins are highly specific, i.e. they allow only a single kind of ion or molecule to pass through.
Osmosis
A partially permeable cell membrane surrounds all the cells.
Osmosis refers to the process through which the water can move in and out of the cell
It is basically the diffusion of water molecules from a high concentration, i.e. a dilute solution to a low concentration of water, i.e. a concentrated solution across the cell membrane. In the entire process, the water moves down the concentration gradient.
The cell membrane is partially permeable which implies that it allows small molecules, for instance, water to pass through it but does not allow the passing of larger molecules like solute.
Active Transport
Active transport refers to the movement of ions or molecules through the partially permeable membrane from a region of lower concentration to a region of higher concentration utilizing energy from respiration.
Carrier proteins are required for active transport. Active transport is different from facilitated diffusion because, besides carrier protein, active transport also needs energy. The energy is needed to allow the carrier protein to alter its shape so that it can transfer ions or molecules across the cell membrane. ATP (adenosine triphosphate) provides the required energy
Importance of Active Transport
Active transport is very important because it helps in:
- Reabsorption of important ions and molecules into the blood after filtration into the kidney tubules
- Absorption of few digestion products from the digestive tract
- Transport around the plant by loading sugar from photosynthesizing cells of the leaves into the phloem tissue
- Passing inorganic ions from the soil into the root hairs
Endocytosis and Exocytosis
The processes of diffusion, active transport, and osmosis are meant for transporting individual ions or molecules across partially permeable membranes. However, it is also possible to transport huge quantities of materials into and out of the cell. Examples of these bulk materials include parts of the cells, entire cells such as bacteria, and huge molecules like polysaccharides and proteins.
Bulk transport into the cells is referred to as endocytosis, whereas the bulk transport out of the cells is known as exocytosis.
Endocytosis
Endocytosis refers to the process by which the cell surface membrane engulfs material, to create a tiny sac or endocytic vacuole around it. Endocytosis is of two types:
- Phagocytosis: It refers to the bulk intake of solid material by the cell. Cells meant for this process are referred to as phagocytes and the vacuole formed are known as phagocytic vacuoles. The example includes engulfing bacteria by phagocytic white blood cells.
- Pinocytosis: It refers to the mass intake of liquids. If the vacuole that is created is very small, then this process is known as micropinocytosis.
Exocytosis
Exocytosis refers to the process by which the materials are eliminated from or move out of the cell. This process is the reverse of endocytosis.
The substances to be released, for instance, hormones or cell wall building materials and enzymes are packaged into secretory vesicles created from the Golgi body. These vesicles then move to the partially permeable membrane where they fuse with the cell membrane and release their substances outside the cell. An example includes the secretion of digestive enzymes from pancreatic cells.
Investigating Diffusion & Osmosis
Investigating diffusion
Environmental factors such as temperature or chemicals can influence the permeability of the cell membranes. We can use beetroot to investigate it:
- Place beetroot pieces into the water at varying temperatures or into various concentrations of alcohol
- An increase in permeability of the cell membrane results in red pigment
- Quantitative or qualitative measurements of variations in the surrounding solution colour can be taken by employing a colourimeter or a set of colour standards
- Through diffusion, the red pigment leaks out from regions of high concentration in the large central vacuoles of the beetroot cells to a region of low concentration in a solution outside the beetroot pieces.
- Diffusion would not normally take place as it is stopped by the cell membrane
- In such cases, sufficiently higher temperatures or alcohol concentrations can damage the cell membrane and its structure, thus enabling the pigment to diffuse out.
Investigating Osmosis
Evidence of osmosis taking place in plant cells can be exhibited when plant cells undergo plasmolysis.
- If we put the plant cell in a solution that has a lower water potential than that of the plant cell, for instance, a concentrated sucrose solution, then water leaves the plant cell through the cell membrane by the process of osmosis.
- The volume of the plant cell decreases when the water leaves the vacuole of the plant cell
- Slowly the protoplast shrinks and does not exert any more pressure on the cell wall.
- When the protoplast shrinks continuously, it starts to pull away from the cell wall. This process is referred to as plasmolysis.
- We can observe the process using epidermal strips which are sections of the extremely thin outer layer of tissue in plants
- The observations become easier when plants with coloured sap like red onion bulbs, red cabbage, and rhubarb petioles are used
- The epidermal strips can be put in a variety of molarities of sucrose or sodium chloride solutions
- Using a light microscope, these strips are observed.
- Plasmolysis may take place after several minutes
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