Subject: Biology
Level: IGCSE
Exam Boards: EDEXCEL, AQA, OCR, CIE


1.3

**a) The principal components of the plasma membrane and the fluid mosaic model. Factors affecting permeability of the membrane. **All cells are surrounded by a membrane 7-8nm thick, which separates the living cell from its non-living surroundings. The membrane controls what enters and leaves the cell.

The membrane is made up from proteins combined with a phospholipid bilayer. The phospholipid has a hydrophilic head which is attracted to the water in the cell and the outside, leaving the hydrophobic tails facing each other. It appears under an electron microscope as a double line. Extrinsic proteins lie across some of the bilayer, whereas intrinsic proteins span the whole layer. Cholesterol molecules sit in between the phospholipid molecules, which increases the rigidity and stability of the membrane.

The **fluid mosaic model **was proposed in 1972 by Singer and Nicholson – the membrane is fluid because the molecules are not fixed rigidly in place, and it is made up of a number of different types of molecule – phospholipids, proteins etc.

The cell membrane is selectively permeable and so only allows certain molecules to pass through it. The glycocalyx is used in cell recognition to ensure that pathogens are not able to enter the cell. It also secretes chemicals. Lipid soluble substances move through the membrane much more easily than water soluble substances.

The permeability of the membrane is affected by a number of factors such as temperature and solute. Investigations, such as the red beetroot cores in different temperatures can be carried out to show this. **b) Transport mechanisms: diffusion and factors affecting the rate of diffusion, osmosis and water potential, pinocytosis, facilitated diffusion, phagocytosis, secretion (exocytosis), active transport and influence of cyanide ** **Diffusion **– Diffusion is the net movement of molecules or ions from a region of high concentration to a region of lower concentration down a concentration gradient (until they reach equilibrium). It is a passive process. The rate of diffusion is affected by:

  • The concentration gradient (the bigger the gradient, the faster the rate of diffusion).

  • The diffusion distance (the shorter the distance, the faster the rate of diffusion).

  • Surface area of the membrane (the larger the area, the faster the rate of diffusion).

  • Size of molecule (the smaller the molecule, the faster the rate of diffusion).

  • Lipid solubility (the more soluble the molecule, the faster the rate of diffusion).

  • Temperature (the greater the temperature, the faster the rate of diffusion, as all the particles have more kinetic energy and so move around more.) **Osmosis – **Osmosis is a particular form of diffusion which is defined as the movement of water from an area of high water potential to an area of lower water potential through a partially permeable membrane down the concentration gradient.

Water potential ψ, is the tendency of water molecules to leave a system. Pure water has a water potential of zero (the highest) and all others have a negative value.

In plant cells: ψ = ψs + ψp OR water potential = solute potential + pressure potential

The solute potential is the concentration of dissolved substances. The pressure potential is the pressure exerted by the cell wall, which is usually positive.

A **hypertonic **solution has a lower WP than the cell, so water will move out by osmosis. The vacuole shrinks and the cytoplasm will draw away from the cell wall. This process is called plasmolysis and when a cell is completely plasmolysed, it is said to be flaccid. The moment when the cell membrane begins to move away from the cell wall is the point of incipient plasmolysis.

** hypotonic **solution has a higher WP than the cell, so water will move in by osmosis. It will continue to take in water until the pressure potential is equal to the solute potential and so it cannot take in any more water. At this point the cell is said to be turgid. Turgor is very important in plants as it helps to maintain their shape. In animals, there is no cell wall so if a red blood cell is placed in pure water, it will take in water by osmosis until it bursts – this is called haemolysis.

If the WP of the cell and the surrounding solution are the same, they are said to be isotonic, and there will be no net movement. **Facilitated Diffusion **– Charged particles and large molecules cannot pass through the plasma membrane by simple diffusion, so must go through protein channels. It is a passive process.

Channel Proteins – consists of pores lined with polar groups which allow charged particles to cross the membrane. The channel is hydrophilic and so water soluble substances can pass through easily. Each channel protein is specific for one type of ion, and can open and close dependent on the needs of the cell.

Carrier Proteins – allow the diffusion of larger molecules across the membrane, such as sugars and amino =acids. A particular molecule attaches to the carrier protein at its binding site and causes the carrier protein to change shape, releasing the molecule through the membrane. **Phagocytosis and pinocytosis – **Phagocytosis is where a large molecule may enter the cell, be engulfed in a vesicle and then transported through the cytoplasm. It is a way in which a molecule that is too large to be taken in by diffusion can be taken into the cell. Pinocytosis is the same process, except that it occurs with liquids.

**Exocytosis – **The process by which a substance leaves a cell after being transported through the cytoplasm in a vesicle. The contents is released outside the cell. The cell membrane is continually having portions added and removed through exocytosis and phagocytosis. **Active Transport – **Active transport, unlike diffusion, requires energy, because it is the movement of particles across a membrane against the concentration gradient. This allows molecules to accumulate in a cell. The process occurs through carrier proteins in the membrane. Processes that involve active transport include protein synthesis, muscle contraction, nerve impulse transmission and absorption of mineral salts by plant roots.

Cyanide is a respiratory inhibitor and so prevents cells from respiring, which reduces the amount of ATP available. This means that under the influence of cyanide, active transport cannot take place.

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