Transport Across the Plasma Membrane

3.1 The movement of substances across the plasma membrane

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1. Plasma membrane is the boundary that separates the living cell from its surrounding.
2. In 1972, S.J.Singer and G.Nicolson proposed a membrane model called the fluid mosaic model to describe the structure of a plasma membrane.
3. Based on this model, the structure of the plasma membrane is comprised of the phospholipid bilayer and various types of protein molecules.
4. A membrane is pictured as a mosaic because it has various protein molecules embedded in the phospholipid bilayer.
5. Since the membrane is fluid, most of the individual proteins and phospholipid molecules can drift laterally in the membrane.
6. A phospholipidis an amphipathic molecule. This means that it has both a hydrophilic region and a hydrophobic region.

1. The polar head region is hydrophilic region (attracted to water) while the non-polar tail is hydrophobic(repelled by water).[pic 3]
2. The interaction of the tails of the phospholipid bilayer prevents adjacent phospholipids from packing too tightly together. This makes the membrane more fluid.
3. Small molecules and neutral molecules such as water, oxygen, carbon dioxide and lipid-soluble substances can easily cross the plasma membrane through the phspholipid bilayer.
4. Slightly bigger polar molecules such as glucose and amino acids as well as charged ions cannot cross the phospholipid bilayer. These substances can cross the plasma membrane with the help of protein molecules called transport proteins.
5. These transport proteins in the plasma membrane function as :

• Carrier protein – a protein molecule that has a shape that fits the shape of a specific molecule so that it can only carry specific molecules across the membrane.
• Protein pore – a pore made of protein that provides a passage for a particular solute to pass through.

1. These types of substances that can be carried across the plasma membrane depends on the characteristics of the phospholipid bilayer and the types of protein molecules in it.
2. Hence, the plasma membrane only allows certain substances to move across it. This property makes the membrane semi-permeable.
3. As the plasma membrane is semi-permeable it has selective permeability and only allows small molecules such as water and glucose to cross it easily. Big molecules such as sucrose, protein and starch cannot move across the membrane.

Passive Transport of Substances Across the Plasma Membrane.

1. Passive transport is the movement of substances across the plasma membrane from a region of high concentration to a region of  lower concentration.
2. In passive transport, the substances move down their concentration gradient and therefore do not require energy.
3. Substances move across the plasma membrane down the concentration gradient through three different ways :

• Phospholipid bilayer
• Pore protein
• Carrier protein

4.Substances can move freely without using energy across the membrane through simple  diffusion or with the help of transport proteins in the membrane through facilitated diffusion.

Simple Diffusion

1. In simple diffusion, molecules diffuse across the plasma membrane down the concentration gradient through the phospholipid bilayer until equilibrium is reached.

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1. At equilibrium, molecules continue to move back and forth, but there is no net change in concentration on either side of the membrane. The molecules are evenly distributes with uniform concentration.
2. Soluble substances that move across the plasma membrane by simple diffusion are:

• Small uncharged polar(water soluble) molecules such as oxygen, carbon dioxide and water.
• Substances that dissolve in fats such as fatty acids, glycerol and also vitamins A,D,E and K.

1. As long as there is a concentration gradient, these substances will diffuse down the concentration gradient through the phospholipid bilayer of the plasma membrane. The bigger the concentration gradient, the faster the rate of diffusion,
2. Passive transport by simple diffusion occurs at the alveolus in the lungs during gaseous exchange.

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1. The concentration of oxygen is higher in the alveolus than in the blood capillary around it. Hence, oxygen diffuses out of the alveolus into the blood capillary along the concentration gradient.
2. The concentration of carbon dioxide in the blood capillary is higher than that in the alveolus. Hence, carbon dioxide diffuses out of the blood capillary into the alveolus to be exhaled.
3. Simple diffusion  also occurs during gaseous exchange between the body cells and blood capillaries.

Osmosis

1. Osmosis is the passive transport of water. Osmosis is the diffusion of water molecules (solvent molecules) down their concentration gradient through a semi-permeable membrane.
2. In osmosis, water molecules move from a region of higher water concentration(dilute solution) to a region of lower water concentration(concentrated solution) intul the concentration on both sides are the same. Then, the water molecules will move at the same rate in both directions.

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1. An example of osmosis is the absorption of water by root hairs of a plant.
2. Normally the soil has a higher water concentration than the cytoplasm in the root hairs. As a result, the water molecules from the soil diffuse across the semi-permeable membrane of the root hairs into the root hairs of the plant.
3. The movement of water molecules across the plasma membrane by osmosis can be shown by using a simple osmometer in the following experiment.

Facilitated Diffusion

1. Some substances cannot cross the plasma membrane by simple diffusion.
2. These substances need the help of transport protein in the plasma membrane to move across the membrane.
3. The passive transport of substances across the plasma membrane with the help of transport proteins is called facilitated diffusion.
4. In facilitated diffusion, the substances move down the concentration gradient without expending energy.
5. The rate of facilitated diffusion depends on the number of transport protein molecules in the membrane and how fast they can move their specific solute.
6. The solutes are transported across the plasma membrane by transport proteins such as carrier proteins and protein pores.
7. Facilitated diffusion that occurs through protein pores only allows small charged molecules such as mineral ions to pass through the protein molecules.

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1. Facilitated diffusion that occurs with the help of carrier proteins only allows larger uncharged polar molecules (molecules insoluble in fats) such as glucose and amino acids to cross the membrane.
2. Each carrier protein moves a specific substance across the membrane.
3. The mechanism of carrier proteins in facilitated diffusion:

• The solute moves to the binding site of the specific carrier protein.
• The solute binds to the carrier protein at the binding site and triggers the carrier protein to change its shape.
• The carrier protein changes its shape and moves the solute across the membrane.
• The carrier protein returns back to its original shape.

1. The solutes can be transported by carrier proteins in either direction but the net movement is always down the concentration gradient.
2. Examples of facilitated diffusion are the transportation of glucose, amino acids and mineral ions across the membrane of the villus at the ileum and also the body cells.
3. Transportation of glucose and amino acids is by the carrier proteins while that of mineral ions is through the pore protein molecules.
4. Digested food such as glucose and amino acids are absorbed into the villus by facilitated diffusion, through the carrier proteins. The concentration of this digested food is higher in the ileum than in the blood capillaries of the villus. Hence, the digested food diffusses across the membrane of the villus through the carrier protein.
5. The absorption of mineral ions in the villus is through the pore protein molecules in the plasma membrane.

Active Transport of Substances Across The Plasma Membrane

1. Active transport is the movement of substances across the plasma membrane form a region of low concentration to a region of high concentration.
2. In active transport, the substances move across the membrane against the concentration gradient. This transport requires work, therefore the cell must expend its own metabolic energy.
3. The active transport of substances against the concentration gradient is performed by specific protein molecules embedded in the plasma membrane.
4. These transport proteins which function as carrier proteins require energy to change the shape of the protein and to pump or transport the substance across the membrane.
5. The energy required for active transport is suppled by ATP (adenosine Triphosphate).
6. All living cells can carry out active transport.

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1. An example of active transport is the intake of mineral ions by the root hairs of a plant.
2. In the soil, the mineral salts dissolve in water to form mineral ions.
3. The concentration of ions in the root hairs is higher than its concentration in the soil.
4.  As the plant needs mineral ions, it has to pump the ions across the membrane of the root against the concentration gradient. Energy is expended in the active transport of mineral ions into the toot hairs of the plant.

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