Biology of Cell. Nucleus Nuclear envelope. Ribosomes Rough endoplasmic reticulum. Nucleolus. Smooth endoplasmic reticulum.

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1 Nucleus Nuclear envelope Nucleolus Nuclear pore Intermediate filament The fluid mosaic model Ribosomes Rough endoplasmic reticulum Smooth endoplasmic reticulum Microvilli Cytoskeleton Actin filament (microfilament) Microtubule Intermediate filament Ribosomes Biology of Cell Centriole Lysosome Exocytosis Vesicle Golgi apparatus Plasma membrane Peroxisome Mitochondrion

2 The cell membrane The fluid mosaic model shows proteins embedded in a fluid lipid bilayer. All eukaryotic cell membranes are formed by Phospholipid stratus arranged in a bilayer and Membrane proteins inserted in the lipid bilayer Biology Extracellular of Cell matrix protein Glycoprotein Integral proteins Glycolipid Glycoprotein Cholesterol Actin filaments of cytoskeleton Peripheral protein Intermediate filaments of cytoskeleton

3 Phospholipid bilayer Every cell membrane is composed of phospholipids in a bilayer. The other components of the membrane are embedded within the bilayer, which provides a flexible matrix and, at the same time, imposes a barrier to permeability. Animal cell membranes also contain cholesterol, a steroid (lipid) with a polar hydroxyl group ( OH). The partially hydrophilic, partially hydrophobic phospholipid spontaneously forms a bilayer: fatty acids are on the inside, phosphate groups are on both surfaces of the bilayer Polar Hydrophilic Heads Nonpolar Hydrophobic Tails H 2 C O C CH 3 O H C O O C O CH CH CH 3 N ; (CH 3 ) 3 O P O O :

4 Membrane proteins Membrane proteins have various functions: 1.Transporters. Membranes are very selective, allowing only certain chemicals to enter or leave the cell, either through channels or carriers composed of proteins. 2.Enzymes. Cells carry out many chemical reactions on the interior surface of the plasma membrane, using enzymes attached to the membrane. 3.Cell-surface receptors. Membranes are sensitive to chemical messages, which are detected by receptor proteins on their surfaces. The message is then sent to the interior of the cell. Outside cell Inside cell Transporters Enzymes Cell-surface receptors

5 Membrane proteins Membrane proteins have various functions: 4.Cell-surface identity markers. Membranes carry cell-surface markers that identify them to other cells. Most cell types carry their own ID tags, specific combinations of protein complexes such as glycoproteins (proteins + carbohydrates) that are characteristic of that cell type. 5.Cell-to-cell adhesion proteins. Cells use specific proteins to glue themselves to one another. Some act by forming temporary interactions, and others form a more permanent bond. 6.Attachments to the cytoskeleton. Some surface proteins are anchored to the microfilaments of the cytoskeleton by linking proteins. Cell-surface identity markers Cell-to-cell adhesion proteins Attachments to the cytoskeleton

6 Membrane proteins Membrane proteins also have common structural features related to their role. They can be classified considering their position. Peripheral membrane proteins Anchored to a phospholipid in one layer of the membrane, they possess nonpolar regions that are inserted in the lipid bilayer and are free to move throughout one layer of the bilayer Protein anchored to phospholipid Transmembrane proteins (or Integral membrane proteins) Permanently attached to the membrane and span across the membrane at least once. They differ in the way they traverse the lipid bilayer. Retinal chromophore b-pleated sheets

7 Passive Transport Across Membranes - Simple diffusion Many substances can move in and out of the cell without the cell s having to expend energy. This type of movement is termed passive transport. Passive transport can occur by simple diffusion Some ions and molecules can freely pass through the membrane and do so because of a concentration gradient (a difference between the concentration on the inside of the membrane and that on the outside). This random motion causes a net movement from regions of high concentration to regions of lower concentration, this process is called diffusion. The movement will continue until the concentration is the same in all regions. Diffusion If a drop of colored ink is dropped into a beaker of water its molecules diffuse. Eventually, diffusion results in an even distribution of ink molecules throughout the water

8 Passive Transport Across Membranes - Simple diffusion The major barrier to crossing a membrane is the hydrophobic interior that repels polar molecules but not nonpolar molecules. If a concentration difference exists for a nonpolar molecule (e.g. O2) it will move across the membrane until the concentration is equal on both sides Nonpolar region outer environment

9 Passive Transport Across Membranes - Selective diffusion Many important molecules (macro-molecules, polar molecules etc) required by the cell cannot easily cross the membrane. These molecules can still enter the cell by selective diffusion through specific channel proteins or carrier proteins embedded in the plasma membrane. We call this process of diffusion mediated by a membrane protein facilitated diffusion. Channel proteins have a hydrophilic interior that provides an aqueous channel through which polar molecules can pass when the channel is open. Extracellular fluid Extracellular fluid Extracellular fluid Carrier proteins, bind specifically to the molecule they assist, much like an enzyme binds to its substrate

10 Passive Transport Across Membranes - Selective diffusion Aquaporins (proteins): an example of water channels Aquaporins are integral membrane proteins that form pores in the membrane of biological cells. Genetic defects involving aquaporin genes have been associated with several human diseases. The plasma membranes contain aquaporins through which water can flow more rapidly inside the cell than by diffusing through the phospholipid bilayer.

11 Passive Transport Across Membranes - Osmosis Osmosis is the movement of water across membranes The cytoplasm of a cell contains ions and molecules (sugars, amino acids) dissolved in water. The mixture of these substances and water is called an aqueous solution. Water is termed the solvent, and the substances dissolved in the water are solutes. Both water and solutes tend to diffuse from regions of high concentration to ones of low concentration. H2O molecules move down their concentration gradient, toward the higher solute concentration. This net diffusion of water across a membrane toward a higher solute concentration is called osmosis A cell in any environment can be thought of as a plasma membrane separating two solutions: the cytoplasm and the extracellular fluid

12 Passive Transport Across Membranes - Osmosis Hypertonic, hypotonic and isotonic solutions Hypertonic Solution Isotonic Solution Hypotonic Solution If two solutions have unequal osmotic concentrations, the solution with the higher concentration is hypertonic (Greek hyper, more than ), and the solution with the lower concentration is hypotonic (Greek hypo, less than ). When two solutions have the same osmotic concentration, the solutions are isotonic (Greek iso, equal ). Human Red Blood Cells Shriveled cells Normal cells Cells swell and eventually burst 0.55 µm 0.55 µm 0.55 µm Plant Cells Cell body shrinks from cell wall Flaccid cell Normal turgid cell The cell s cytoplasm is hypertonic relative to the extracellular fluid, that is, water diffuses into the cell from the environment, causing the cell to swell. The amount of water that enters the cell is measured as osmotic pressure, defined as the force needed to stop osmotic flow. Prokaryotes, fungi, plants, and many protists are surrounded by strong cell walls, which can contrast high internal pressures without bursting

13 Active Transport Across Membranes Diffusion, facilitated diffusion, and osmosis are passive transport processes. However, cells can also actively move substances across a cell membrane up their concentration gradients. This process requires the expenditure of energy (from ATP), and is therefore called active transport. Active transport involves highly selective carrier proteins that bind to the substance (an ion or a simple molecule), such as a sugar, an amino acid, or a nucleotide. These carrier proteins are: Outside of cell Na ; Glucose Uniporters: transport a single type of molecule. Symporters: transport two molecules in the same direction Antiporters: transport two molecules in opposite directions Na ; / K ; pump ATP ADP+P i Coupled transport protein Inside of cell K ;

14 Bulk Transport: Endocytosis Some substances are large or polar molecules that cannot cross the hydrophobic barrier. This bulk material enters the cell in vesicles. Two processes are involved in this bulk transport: endocytosis and exocytosis. In endocytosis, the plasma membrane envelops food particles and fluids. Cells use three major types of endocytosis: phagocytosis, pinocytosis, and receptor-mediated endocytosis. Like active transport, these processes also require energy. If the material the cell takes in is particulate (particles), such as an organism or some other fragment of organic matter, the process is called phagocytosis (Greek phagein, to eat, + cytos, cell ). If the material the cell takes in is liquid, the process is called pinocytosis (Greek pinein, to drink ). Plasma membrane a. Bacterial cells Solute Plasma membrane Molecules are often transported into eukaryotic cells through receptor-mediated endocytosis. These molecules first bind to specific receptors in the plasma membrane. Different cell types contain a characteristic set of receptors, each for a different kind of molecule in their membranes. Target molecule Receptor protein Coated pit Coated vesicle Clathrin c.

15 Bulk Transport: Material can leave the cell by exocytosis The reverse of endocytosis is exocytosis, the discharge of material from vesicles at the cell surface. In plant cells, exocytosis is an important means of exporting the materials needed to construct the cell wall through the plasma membrane. Among protists, contractile vacuole discharge is considered a form of exocytosis. In animal cells, exocytosis provides a mechanism for secreting many hormones, neurotransmitters, digestive enzymes, and other substances. Plasma membrane Secretory product Secretory vesicle Proteins and other molecules are secreted from cells in small packets called vesicles, whose membranes fuse with the plasma membrane, releasing their contents outside the cell.