Cell Biology Lecture 2 Structure of Plasma membrane

Cell Biology Lecture 2

Structure of Plasma membrane �Phospholipidbilayer: protein in embedded in 2 layers of lipid. �All the cell membranes (organelles) have similar structure �In 1925, two Dutch scientists (E. Gorter and R. Grendel) extracted the membrane lipids from a known number of red blood cells and provided the first evidence that biological membranes consist of lipid bilayers by calculating the surface area of lipids. �The principal components of cell membranes, consisting of two hydrocarbon chains (usually fatty acids) joined to a polar head group containing phosphate.

The lipid bilayer of the plasma membrane is less than 5 nm but the individual leaflets of the bilayer can be resolved. Electron microscopic image

Types of phospholipids �phosphatidylcholine, phosphatidyleth anolamine, phosphatidylserine, and sphingomyelin) �A fifth phospholipid, phosphatidylinositol, is also localized to the inner half of the plasma membrane. Although phosphatidylinositol is a quantitatively minor membrane component, it plays an important role in cell signaling.

� The outer leaflet consists predominantly of phosphatidylcholine, sphingomyelin, and glycolipids (their carbohydrate portion exposed on the cell surface), whereas the inner leaflet contains phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol. � Cholesterol is distributed in both leaflets and is present in about the same molar amounts as the phospholipids. The net negative charge of the head groups of phosphatidylserine and phosphatidylinositol is indicated.

Function of Plasma Membrane �Mechanical Support �Cell Signaling �Selective permeability �Active transport �Bulk Transport �Metabolic activity

Mechanical Support � 1. Defines/encloses the cells: maintain the physical integrity by enclosing the cytoplasm and organelles. Forms a barrier between intra and extracellular environment. � 2. Maintains the cytoskeleton: Keeps the organelles and proteins in an appropriate position � 3. Extracellular Matrix: cell membrane of one cell interacts with the cell membrane of adjacent cell. � 4. Protection: protects the cell from harmful chemicals/ infections present in the extracellular environment

Cell Signaling �Cells can not live in an isolated environment. Ø Prokaryotes communicate with each other, other organisms and surrounding environment. Ø Eukaryotes e. g yeasts, slime molds, and protozoans mate, differentiate and respond to the environment by secreting pheromones �Cells are able to receive and process �Individual cells receive many signals. simultaneously, and they then integrate the information they receive into a unified action plan. �They also send out messages to other cells both near and far.

What kind of signals do cells receive? �Most cell signals are chemical in nature. �Prokaryotic organisms have sensors that detect nutrients and help them navigate toward food sources. �In multicellular organisms, growth factors, hormones, neurotransmitters, and extracellular matrix components are some of the many types of chemical signals cells use. �These substances can exert their effects locally, or they might travel over long distances. �Some cells also respond to mechanical stimuli. For example sensory cells in skin

General features of phospholipid bilayer �First, the structure of phospholipids is responsible for the basic function of membranes as barriers between two aqueous compartments. Because the interior of the phospholipid bilayer is occupied by hydrophobic fatty acid chains, the membrane is impermeable to water-soluble molecules, including ions and most biological molecules. �Second, bilayers of the naturally occurring phospholipids are viscous fluids, not solids. The fatty acids of most natural phospholipids have one or more double bonds, which introduce kinks into the hydrocarbon chains and make them difficult to pack together. The long hydrocarbon chains of the fatty acids therefore move freely in the interior of the membrane, so the membrane itself is soft and flexible. In addition, both phospholipids and proteins are free to

Signaling in plants and animals �In plants and animals, extra cellular signaling molecules control �Metabolism �Growth and differentiation of tissues �Synthesis and secretion of proteins �Composition of intracellular and extracellular fluids

General principle signaling 1. 2. 3. 4. 5. 6. Synthesis of signaling molecules by the signaling cells Release of signaling molecules Transport of the signal to the target cell Detection of a signal by a specific receptor protein present on the target cell A change in cellular metabolism, function or development triggered by the receptor-signal complex Removal of the signal, which often terminate the cellular response

1 3 2 4 5 6 Signaling cell

Cellular responses due to cell signaling �Changes in the activity or function of specific enzymes and other proteins present in the cells �Changes in the amount of protein produced by a cell e. g. modification of transcription factors that stimulate or repress gene expression

Types of signaling

Gap Junctions Allow Signaling Information to Be Shared by Neighboring Cells �Signals are passed to the neighboring cells through gap junctions. �These are specialized cell-cell junctions that can form between closely apposed plasma membranes, directly connecting the cytoplasms of the joined cells via narrow water-filled channels. (a) (b) (c) (d) (c 1) (c 2) (c 3) (c 4) (c 5)

Receptor protein exhibit ligand binding effect �Receptor present on Plasma or nuclear membrane has ligand binding sites �Signaling molecules (hormones, pheromones or neurotransmitters) act as ligands �Confirmational change occurs in the receptor that initiate a sequence of chemical reactions �Receptor proteins are specific for each horomone

�Different cells have different sets of receptor for the same ligand each of which induces a different response �Different cells respond in a variety of way to the same ligand (e. g. acetylcholine) �Different ligands can induce the same cellular response in some cells (glucagon/epinephrine) �In most receptor-ligand system, the ligand do not have any function except to bind to receptor �Upon binding it changes the properties of receptor which then produce signals to the cell that a specific product is present �Target cells often degrade or modify the ligand to terminate or modify their response

The same signaling molecule can induce different responses in different target cells

Lecture prepared from �The Cell: A Molecular Approach. 2 nd edition http: //www. ncbi. nlm. nih. gov/books/NBK 9898/ �Molecular Cell Biology, Lodish and co 5 Edition, Chapter 15