AQA A level Biology
AQA A level Biology
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Topic 2: Cells
Topic 2: Cells
Stucture of cell membranes
Stucture of cell membranes
Cell membranes are composed of a variety of molecules. One way to understand the cell membrane is through the fluid mosaic model. The primary component of the cell membrane is phospholipids, which consist of a glycerol molecule attached to one phosphate group and two fatty acids. The phosphate group and fatty acids are joined by ester bonds formed through condensation reactions (see the lipids topic for more details).
The phosphate group is negatively charged and attracts water molecules, earning it the name ‘hydrophilic head.’ In contrast, the fatty acids are uncharged and repel water, thus being called the ‘hydrophobic tail.’
One phospholipid.
This characteristic of phospholipids results in membranes being composed of two layers (a bilayer) of phospholipids. One layer has the head groups facing outward towards the tissue fluid and the tails pointing inward towards the tails of the second layer, whose head groups face the cell's interior cytoplasm. This arrangement allows phospholipids to move past one another, explaining the ‘fluid’ part of the fluid mosaic model. The term ‘mosaic’ refers to the many other molecules (proteins, glycolipids and glycoproteins) embedded within the phospholipid bilayer.
Cell membranes are described as partially permeable because some substances can pass through, while others cannot. Some molecules can diffuse directly across the phospholipid bilayer, while others require channel and carrier proteins embedded in the membrane.
Carbohydrates in the membrane can be attached to lipids (glycolipids) or proteins (glycoproteins). These structures typically function as receptors and recognition sites, as well as aiding in cell adhesion.
Cholesterol molecules are also present in the membrane. Since cholesterol is hydrophobic, it interacts with phospholipid tails, adding strength and rigidity to the membrane, particularly at low temperatures. Additionally, cholesterol reduces the movement of other molecules within the membrane and prevents the leakage of water and ions from the cell.
Cross-section of a cell membrane
Definitions
Definitions
Carrier protein: integral protein in a cell or organelle membrane that transport specific particles across the membrane.
Channel protein: integral protein that spans the phospholipid bilayer in a membrane that allows the facilitated diffusion of specific particles across the membrane.
Cholesterol: lipid that acts to reduce the fluidity of cell membranes.
Fluid mosaic model: name given to the structure of the plasma membrane as a result of the appearance brought about by the protein elements of the membrane being studded in the phospholipid bilayer and both the phospholipids and proteins being able to move freely within the membrane.
Glycolipid: lipid with attached carbohydrate region that functions in cell recognition and binding.
Glycoprotein: protein with attached carbohydrate region that functions in cell recognition and binding.
Hydrolysis reaction: breaks a chemical bond between two molecules and involves the use of a water molecule.
Phospholipid: lipid made of glycerol, two fatty acids and a phosphate group; has a hydrophobic tail region and hydrophilic head region; is the main constituent of a cell membrane.
Plasma membrane: partially permeable surface that surrounds the cell and organelles. Composed of phospholipids, proteins and cholesterol.
Protein: functional molecule made up of one or more polypeptides and prosthetic groups.
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