Eukaryotic Cell - Cell Membrane PDF

EUKARYOTIC CELL PART 1 CELL MEMBRANE Learning Outcomes At the end of this topic, students should be able to Explain the physical and chemical structure of the eukaryotic cell membranes Explain the functions of the cell membranes Discuss the different methods of transport across cel...

EUKARYOTIC CELL PART 1 CELL MEMBRANE Learning Outcomes At the end of this topic, students should be able to Explain the physical and chemical structure of the eukaryotic cell membranes Explain the functions of the cell membranes Discuss the different methods of transport across cell membranes  Eukaryotic cell is found in protists, fungi, plants & animals. Its size ranges from 10-100 µm diameter It is 1000-10,000 times the volume of prokaryotic cells. 1. Cell membranes The cell membrane has a fluid mosaic model (Singer & Nicolson, 1972 ). The membrane is 7nm thick. Components of the cell membrane a. Phospholipids- form two layers. The hydrophilic phosphate heads of the phospholipids face outwards into the aqueous environment inside & outside the cells. The hydrocarbon tails face inwards and create a hydrophobic interior. Such molecules with distinct hydrophilic & hydrophobic regions are called amphipathic molecules. 2. Membrane Proteins Different types of membrane proteins are present on the phospholipid bilayer. Some protein molecules that penetrate through the membrane are called transmembrane proteins or integral proteins. They provide a passageway for substances and information to cross the membrane. Some proteins penetrate only part of the way into the membrane, which is called peripheral proteins. 3. Network of supporting fibers Proteins that reinforce the membrane shape, control the movement of other proteins in the membrane. 4. Glycoproteins & glycolipids Some proteins and lipids have short branching carbohydrate chains like antennae. 5. Cholesterol Membrane contains cholesterol. Cholesterol increases the flexibility & stability of membranes, without it, membranes break up. Cholesterol disturbs the close packing of the phospholipids and keep them fluid. The two sides of the membrane and membranes of different cell types vary in composition and function. Fluid Mosaic Model The membrane is mosaic Because of the presence of many different types of embedded proteins and cholesterol molecules the term mosaic is used. The membrane is fluid It was observed that the embedded molecules can move sideways or transverse throughout the membrane, meaning the membrane is not solid, but more like a fluid. Lateral movements are more common than transverse movements. Functions of membranes 1. Partially permeable- The phospholipid bilayer restricts the exit and entry of polar molecules and ions. Hence makes the membrane partially permeable. 2. Channel protein & carrier proteins- They help in the selective transport of polar molecules and ions across the membranes. 3. Enzymes-Some proteins in the cell membrane act as enzymes that are the biological catalyst that speeds up a chemical reaction. Ex. Protein in the membrane of microvilli in the gut act as a digestive enzyme. 4. Receptor molecules- Since the proteins have specific shapes they are ideal for chemical signaling between cells. Ex. Hormones recognize receptors in the membrane of target sites. 5. Antigens- They are glycoproteins with carbohydrate side chains in the cell membranes by which cells recognize each other. Helps in identifying foreign molecules. 6. Glycolipids- They are lipids with branching carbohydrate chains, involved in cell-cell recognition; act as receptors for chemical signals & involved in sticking the correct cells together in tissues. Functions of plasma membrane proteins: Transporter Enzyme Cell surface receptor Cell surface identity marker glycolipids glycoprotein Cell adhesion Attachment to the cytoskeleton Functions of plasma membrane proteins: Membrane proteins act as transporters, enzymes, cell surface receptors, and cell surface markers, as well as aiding in cell-to-cell adhesion and securing the cytoskeleton  Transport across the cell surface membrane Transport across membranes should occur To obtain nutrients. To excrete waste. To secrete useful substances. To generate ionic gradient to facilitate nervous & muscular activity. To maintain a suitable pH & ionic concentration within the cell for enzyme activity. 4 basic mechanisms of transport Diffusion Osmosis Active transport Bulk transport (endocytosis, exocytosis) A. Diffusion & facilitated diffusion It is the movement of molecules or ions from a region of their high concentration to a region of their low concentration or down a diffusion gradient. Factors influencing the rate of diffusion 1. Steepness of the diffusion gradient is the difference in concentration gradient. The steeper the gradient the faster the rate of diffusion. 2. The greater the surface area of the membrane through which diffusion occurs, the greater the diffusion rate. This places a limit on cell size. 3. The rateoof diffusion decreases rapidly with distance. Diffusion is effective only over very short distances. This is another factor that limits cell size. 4. Membranes have to be thin for molecules and ions to cross easily through Them. Ex. Oxygen, carbon dioxide, and water are some of the molecules that diffuse across membranes.  Facilitated diffusion Ions & large polar molecules like amino acids, sugars, fatty acids, and glycerol are repelled by the hydrophobic region of the membrane and diffuse across extremely slowly. So, they diffuse through special channel proteins and carrier proteins. Channel proteins contain water-filled hydrophilic channels or pores whose shape is specific to a particular ion or molecule. Sometimes several proteins combine to form a channel through which diffusion occurs. Since protein or proteins facilitate the movement, this is called facilitated diffusion. Transport proteins that facilitate the movement of ions are called ion channels and could be opened or closed & are called gated channels. Aquaporin is a transmembrane protein, an example of a gated water channel found in human kidney tubules. They respond to signals from hormones.  Channel proteins have fixed shapes. Carrier proteins change shape (upto 100 cycles per second). They exist in two forms called ping & pong state. Protein alternates rapidly between two different shapes, ping and pong  B. Osmosis- Passage of water molecules from a region of their higher concentration to a region of their lower concentration through a partially permeable membrane. The concentration of all solutes in a solution determines the osmotic concentration of the solution. The osmotic potential is defined as the capability of a solution to suck water in if it is separated from another solution by a semi-permeable membrane. If two solutions have unequal osmotic concentrations, the solution with the higher concentration is hyperosmotic (Greek hyper, “more than”). The solution with the lower concentration is hypoosmotic (Greek hypo, “less than”). If the osmotic concentrations of two solutions are equal, the solutions are isosmotic (Greek iso, “the same”). Effects of isotonic, hypotonic, and hypertonic solutions on Human Red Blood Cells If a cell is in a hyperosmotic solution water moves out of the cell toward the higher concentration of solutes, causing the cell to shrivel. In an isosmotic solution, the concentration of solutes on either side of the membrane is the same. Osmosis still occurs, but water diffuses into and out of the cell at the same rate, and the cell does not change size. In a hypoosmotic solution, the concentration of solutes is higher within the cell than without, so the net movement of water is into the cell. The cell swells. Effects of isotonic, hypotonic, and hypertonic solutions on Plant & Animal cells C. Active transport It is the energy–consuming transport of molecules & ions across a membrane against a concentration gradient. It requires a carrier protein to transport materials that require energy from ATP to change shape. Example: Sodium–potassium pump is a typical example of active transport. For every 2 potassium molecules taken in, 3 sodium ions are removed. The pump is a carrier protein that spans the membrane from one side to the other. On the inside, it accepts sodium and ATP, while on the outside, it accepts potassium. Changes in the protein’s shape bring about the transfer of sodium and potassium across the membrane. This pump is essential in controlling the osmotic balance of animal cells. Bacteria, fungi & plants which have cell walls do not have this pump. It is also important for electrical activity in nerve & muscle cells. Active transport in the intestine Products of digestion like glucose, amino acids, salts are absorbed through the epithelial cells lining the gut and into the blood through the wall of the blood capillaries to be taken to the liver. Soon after feeding high concentrations of digested foods are present in the gut & absorption is partly by diffusion. But this is very slow and is supplemented by active transport. Active transport in plants Active transport of sugar synthesized by photosynthesis into the phloem occurs in plants. Uniport (system in which one solute is transported) Cotransport (system in which transport of one solute is coupled to transport of another) Symport (different solutes transported in the same direction) Antiport (different solutes transported in opposite directions)  Examples of cotransport- A carrier protein that transports glucose & sodium, sodium & amino acids, sodium-potassium pump (antiport). This is an active transport. The energy expenditure in these transports is energy used to actively pump out the sodium ions. This is a linked cotransport system. intestinal lumen Plasma membrane of cells lining intestine D. BULK TRANSPORT: It is a mode of transport of large quantities of materials and food particles across the membrane Endocytosis & Exocytosis Endocytosis is movement into the cell. It occurs by infolding or extending the cell surface membrane to form a vesicle or vacuole. (A vacuole is a fluid-filled membrane-bound sac, a vesicle is a small vacuole) Phaogocytosis- The material is taken up in solid form. Such cells are Phaogocytes and said to be Phaogocytic and vacuole is Phaogocytic vacuole. Pinocytosis- The material is taken up in liquid form. When the vesicles formed are extremely small, it is called micropinocytosis & the vesicles are micropinocytic vesicles. Exocytosis- This process removes waste materials or secretes useful products. Secretory product Phagocytosis Pinocytosis Exocytosis Secretory vesicle cytoplasm