TRANSPORT

Transport of substances is important to:
1. Supply cells with oxygen for respiration and raw materials for anabolism.
2. Regulate the pH and solute concentration for maintaining a stable internal environment for enzymes to function optimally.
3. Excrete toxic waste substances.
4. Secrete useful substances for cell activities.

Passive Transport
1. Passive transport is the movement of ions and molecules down their con-centration gradient.
2. The process does not require ATP and energy expenditure.

Simple Diffusion
1. Simple diffusion is the random movement of ions or molecules from a region where they are at higher concentration to a region of lower concentration, that is to move down a concentration gradient until an equilibrium is reached.
2. The phospholipid bilayer is permeable to very small uncharged molecules like oxygen and carbon dioxide. These molecules diffuse freely in and out of the cell through the phospholipid bilayer. Hydrophobic substances, for example, steroids can also diffuse through.

3. The phospholipid bilayer is not permeable to charged ions such as Na+, K+, Cl-, HCO3- and hydrophilic molecules like glucose and macromolecules.
4. Hydrophilic substances like water molecules diffuse across the membranes with the help of transport proteins.
5. The rate of diffusion depends upon: the concentration gradient, surface area, distance over which diffusion takes place, size and nature of the diffusing molecule.

Facilitated Diffusion of Ions and Molecules

Transmembrane proteins form channels or act as transport proteins to facilitate and to increase the rate of diffusion across cell membranes.

Protein Channels

1. Certain transmembrane proteins form different specific water-filled hydro-philic channels to permit diffusion of various charged ions such as Na+, K+, Cl- and HCO3-.

2. The protein channels that can open or close are known as gated-channels. For example, the ligand-gated and voltage-gated channels.

3. There are also specialised channels for water known as aquaporins found in both plant and animal cells. These aquaporins speed up the rate of diffusion of water molecules down its water potential gradient.

Carrier Proteins

1. Some small hydrophilic organic molecules for example glucose and amino acids can pass through cell surface membranes by facilitated diffusion.

2. For example, the binding of glucose to a specific carrier protein causes the molecule to change its shape and the glucose molecule is released into the cell.

Osmosis

Osmosis is the passive movement of water (solvent molecules) from a region of higher concentration of water molecules across a partially permeable membrane to a region of lower concentration.



Water Potential (ψ)

1. In pure water or dilute solution with very few solute molecules, the water molecules have very high kinetic energy and can move very freely. The dilute solution has a high water potential.

2. A concentrated solution has a lower concentration of water molecules. The movement of water is also restricted due to the attraction between solutes and water molecules. There are fewer water molecules with a high kinetic energy to move across the membrane.

3. The concentrated solution has a low water potential. Water moves down a water potential gradient.

4. The water potential of pure water (ψw) at atmospheric pressure is arbitrarily given the value 0 kPa.

5. The addition of solutes lower the water potential. The water potential of solutions therefore is lower than pure water and has a negative value.

ψsolution < 0 kPa

6. The greater the concentration of solutes, the more negative is the water potential.

Solute Potential (ψs)

1. Solute potential is the potential or force of attraction towards water molecules caused by solutes inside the solution.

2. The attraction between solute molecules and and water molecules reduces the random movement of water molecules. The addition of more solute molecules lowers the water potential of a solution.

Pressure Potential (ψp)

1. Pressure potential is the pressure exerted on a fluid by its surrounding.

2. As water diffuses into a plant cell, the cell contents expand causing a pressure (turgor pressure) on the cellulose cell wall. The cell wall develops an inward pressure (pressure potential) to resist the influx of water.

3. The pressure potential has a positive value when the plant cell is turgid and 0 kPa when the cell is flaccid.

4. The water potential of an animal cell depends on its solute potential because the pressure potential generated by the cell membrane is negligible.

Osmosis In Plant Cells

1. When a plant cell is placed in a hypotonic solution, water enters the cell by osmosis. The vacuole expands and the cell contents press against the cell wall.

2. As more water enters, the pressure potential produced by the cell wall increases until the pressure potential equals to the solute potential.

3. There is no net movement of water in either direction. The plant cell is said to be turgid.

4. Turgid cells give support to herbaceous plants. Plant cells do not burst because they are surrounded by their strong cellulose cell walls.

5. In isotonic solutions, there is no net movement of water molecules and no change in the volume of the cell.

6. In hypertonic solutions, there is a net flow of water by osmosis from the cell. The cell vacuole shrinks and the plasma membrane pulls away from the cell wall.

7. Plasmolysis of the cell occurs and the cell becomes flaccid. When cells become flaccid, they cause the plant to wilt.

Water Potential In A Plant

The water potential of a plant cell is the sum of its solute potential and pressure potential.

Osmosis In Animal Cells

1. When an animal cell, for example, a red blood cell, is placed in a hypotonic solution, water enters the cell by osmosis. There is a net movement of water into the cell.

2. The cell expands and the thin plasma membrane bursts, releasing the cell contents. The red blood cell is said to be haemolysed.

3. In isotonic solutions, there is no net movement of water molecules and no change in the shape or volume of the cell.

4. In hypertonic solutions, there is a net outflow of water by osmosis form the cell. The cell shrinks and the plasma membrane has a wrinkled appearance. The cell is said to be crenated.

5. Unicellular protoctists such as Amoeba and Paramecium have contractile vacuoles to regulate the water content in the cell.

Active Transport

1. Active transport is the movement of ions or molecules across a cell membrane (aided by a protein pump with a specific binding site) against their concentration gradient. This process requires energy expenditure provided by ATP.

2. Cells which carry out active transport have a high respiratory rate and a large number of mitochondria to generate a high concentration of ATP.

3. Active transport can be slowed down or inhibited by respiratory poisons such as cyanide or a lack of oxygen. The energy from ATP may be used directly or indirectly in active transport.



Direct Active Transport

1. An example is the Na+ - K+ protein pump. ATP is hydrolysed and the binding of the phosphate group to the protein pump changes the protein conformation.

2. It actively transports 3Na+ ions out of the cell for every 2K+ ions pumped against their concentration gradient into the cell. This generates a difference in ionic charge on the two sides of the membrane which is important for the transformation of nerve impulses.

3. The Na+ ion gradient is also used in the coupled uptake of solutes such as glucose into the cells against its concentration.

Indirect Active Transport (Secondary Active Transport)

1. An example is the coupled uptake of glucose into the cells lining the ileum in mammals. The precess is also known as cotransport.

2. ATP is used by Na+ protein pump to pump Na+ ions out of the cells. This creates a Na+ concentration gradient. The Na+ ion and glucose molecule bind to the same transmembrane protein called cotransport proteins. They are then moved by the proteins inside the cells.

3. The Na+ moves down its concentration gradient while the glucose moves against its concentration gradient.

Several types of membrane proteins are involved in active transport:

(a) Uniport carriers which carry a single ion or molecule in a single direction. 

(b) Symport carriers which carry two substances in the same direction.

(c) Antiport carriers which carry two substances in opposite directions.

1. Examples of processes that involve active transport are muscle contraction, transmission of nerve impulses, absorption of amino acids and glucose in the ileum and out of the proximal convoluted nephron tubules back into the blood capillaries and absorption of mineral salts by plant roots.

2. Some human diseases are caused by inherited mutation in genes encoding formation of transmembrane protein channels. For example, cystic fibrosis is a fatal disease due to the failure of the chloride ion channel to regulate the movement of chloride ions. Thick mucus is secreted which clogs the lungs, liver or pancreas.

Exocytosis

1. Exocytosis and endocytosis are active transport processes that move materials in bulk across membranes.

2. A vesicle membrane becomes incorporated into the plasma membrane. This helps to restore the membranes which are used in endocytosis. The contents in the vesicles are released outside the cell. Examples are enzymes, hormones and excretory products.

Endocytosis

Phagocytosis

1. Cytoplasmic projections are formed that extend around solid particles. The projections formed then fuse together to trap the solid particles within a vacuole. The surface of the vacuole is derived from the cell surface membrane.

2. Examples of phagocytosis are:

(a) Unicellular organism Amoeba capturing its prey by extending pseudopodia around it.

(b) The leucocytes, neutrophils and monocytes can engulf foreign substances like bacteria by phagocytosis.

Pinocytosis

1. Plasma membranes invaginate inwards forming a flask-shaped vesicle which is then pinched off to form a pinocytic vesicle in the cytoplasm.

2. Vesicles formed by pinocytosis are smaller than phagocytic vacuoles and contain fluid and dissolved solutes.

Receptor-mediated Endocytosis

1. It is a process for the selective uptake of specific macromolecules.

2. For example, the binding of cholesterol molecules to specific receptor proteins on the plasma membrane triggers the inward folding of the cell membrane. A vesicle is formed that carries the cholesterol molecule into the cell.

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