Biological cell membranes

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1 Unit 14: Cell biology Biological cell membranes The cell surface membrane surrounds the cell and acts as a barrier between the cell s contents and the environment. The cell membrane has multiple functions that include controlling materials moving in and out of the cell, separating cell structures from the cytoplasm and cell recognition and signalling. On successful completion of this topic you will: understand the structural features of eukaryotic and prokaryotic cells (LO1). To achieve a Pass in this unit you will need to show that you can: relate the structure of membranes to their function (1.3) explain the interdependence of subcellular organelles (1.4). 1

2 1 Cell membranes Cell membranes are formed from a double layer (bilayer) of phospholipids usually 7 10 nm thick (see Figure ). Each phospholipid has a water soluble (hydrophilic) head and an oil soluble (hydrophobic) tail. This means that the molecules naturally form bilayers with the heads pointing outwards, exposed to the aqueous environment, and the tails inside. Bilayers also have proteins, cholesterol and polysaccharides embedded into the membrane. In 1972 scientists Singer and Nicolson introduced the Fluid Mosaic Model to describe the structure of biological membranes. The cell membrane bilayer is very fluid-like and the phospholipids are constantly moving. Phospholipid: A phosphate head and two fatty acid tails. Fluid Mosaic Model: A model in which biological membranes are considered as a two-dimensional liquid, allowing other embedded molecules such as proteins and carbohydrates to diffuse freely. Diffusion: The movement of molecules from a high concentration to a low concentration down a concentration gradient. Biological membranes are selectively permeable. Although it is the role of the membrane to separate the contents from the environment, some substances do need to get into and out of cells. Some molecules can diffuse directly across membranes (e.g. oxygen), whereas others move through specific channels or are transported across by proteins. Proteins embedded in the membranes act as channels that allow specific water-soluble molecules to pass through and they span the width of the membrane. Also, carrier proteins, present in membranes actively move molecules across the membrane but they require ATP. Some biological molecules only span half of the membrane; cholesterol present in membranes fits between fatty acid tails and provides stability. Some proteins have small polysaccharide branches attached to them these are called glycoproteins. Phospholipids with polysaccharide branches attached are called glycolipids. Both glycoproteins and glycolipids help in cell recognition and can identify the difference between foreign cells and body cells. Carbohydrate chains Protein molecule Lipids (bilayer) Protein channel Figure : Mosaic model of phospholipid membrane. 2

3 Osmosis: The movement of water from a high water potential to a low water potential down a water potential gradient across a biological membrane. Active transport: The movement of molecules across a biological membrane against a concentration gradient using energy. 2 Membrane transport Diffusion Diffusion is the movement of molecules down a concentration gradient, from a high concentration to a low concentration, until they are spread evenly. There are many factors that affect the rate of diffusion. These include: the distance that the molecules are going to move; the concentration gradient; the size of the particles; and the temperature. The rate of diffusion will be slower if the molecules have to travel a greater distance. The higher the concentration gradient, the faster the rate of diffusion. The smaller the particles, the quicker the rate of diffusion. The higher the temperature, the more kinetic energy the molecules will have to diffuse, so this will speed up the rate of diffusion. Facilitated diffusion Facilitated diffusion uses carrier proteins and ion channels that facilitate the movement of molecules across a membrane. Carrier proteins bind with the molecule, and the protein changes shape so that the molecule makes its way to the other side of the membrane. Ion channels facilitate the diffusion of charged particles such as Na + and K +. Facilitated diffusion occurs down a concentration gradient and does not require any energy. Osmosis Osmosis is the diffusion of water across a partially permeable membrane, from a high concentration to a low concentration down a concentration gradient. Active transport Active transport is the movement of molecules against a concentration gradient. Molecules can move from a low concentration to a high concentration but it requires energy in the form of ATP. Carrier proteins present in the cell membrane require energy from respiration to move molecules against a concentration gradient. Link The structure of phospholipids is discussed in Unit 1: Biochemistry of macromolecules and metabolic pathways. Activity Use the Internet to research and source molecular diagrams to illustrate diffusion, facilitated diffusion, osmosis and active transport. Checklist In this topic you should now be familiar with the following ideas about cell membranes: cell membranes consist of phospholipids, proteins, cholesterol and polysaccharides cell membranes allow the movement of molecules because they are partially permeable the Fluid Mosaic Model refers to the fluidity and mosaic appearance of cell membranes. 3

4 3 Interdependence of organelles Protein sorting Protein sorting initially starts at the ribosomes where the amino acid sequence is formed from the triplet codons on the mrna. The newly synthesised protein and the ribosome are transported to the endoplasmic reticulum (ER) where protein synthesis is completed. The polypeptide chain is then transported to the Golgi body where it may need slight modification before being packaged into vesicles. This is known as the secretory pathway. Some of these proteins are secreted out of the cell and some are used in the cell. Vesicular transport Transport vesicles are membrane-bound sacs found in eukaryotic cells. They move many different substances around the cell and to the cell s surface as part of the secretory pathway. Vesicles are very important for the organisation and functioning of cells; for example, lysosomal enzymes must be transported from the Golgi body to the lysosomes. It would be detrimental if these digestive enzymes were transported to the membrane or mitochondria. Endocytosis: The process of bringing materials into the cell. Exocytosis: The process of transporting molecules out of the cell. Link Protein synthesis, mrna and triplet codons are discussed in Unit 7: Molecular biology and genetics. Ingestion pathways Endocytosis is the process of bringing materials into the cell in small vesicles and exocytosis is the process of transporting molecules out of the cell the vesicle fuses with the cellular membrane and expels the material. Phagocytosis is the transport of solid material (bacteria or dead red blood cells) into a cell through a vesicle, for example, in order to destroy them. When the cell membrane fuses around the molecule it produces a food vacuole and lysosomes use their enzymes to digest the material inside. Pinocytosis is similar to phagocytosis but it is the transport of fluids into the cell. Checklist In this topic you should now be familiar with the following ideas about cells: newly synthesised proteins and ribosomes are transported to the endoplasmic reticulum (ER) polypeptide chains are transported to the Golgi body where they are modified and packaged into vesicles transport vesicles move many different substances around the cell and to the cell s surface endocytosis is the process of bringing materials into the cell exocytosis is the process of transporting molecules out of the cell. Further reading Kennedy, P., Sochacki, F. and Hocking, S. (2008) OCR Biology AS, Heinemann (Pearson Education Ltd.) 4

5 Acknowledgements The publisher would like to thank the following for their kind permission to reproduce their photographs: Corbis: Steve Gschmeissner / Science Photo Library All other images Pearson Education Every effort has been made to trace the copyright holders and we apologise in advance for any unintentional omissions. We would be pleased to insert the appropriate acknowledgement in any subsequent edition of this publication. 5

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