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Cell Membrane

Transcript: cell structure Holding it all together The plasma membrane The plasma membrane encloses the cell, making it distinct from other cells and from the outside environment. The plasma membrane is vital to the process of homeostasis, or the maintenance of a stable, steady internal environment. The plasma membrane enables this stability because it is selectively permeable. This simply means that the membrane controls what can enter or exit the cell. Useful nutrients are selectively allowed entrance while waste materials are selectively allowed to leave. Phospholipid membranes are fluid structures, similar in consistency to cooking oil. As you can see in the pictures, the plasma membrane is largely made up of molecules called phospholipids. Each phospholipid has a phosphate head and lipid tails. The head portion is hydrophilic, meaning that it interacts well with water. The tails are hydrophobic, avoiding contact with water. As a result, the phospholipids organize themselves into a phospholipid bilayer, with heads pointing outward and tails hidden away from the watery cytosol or extracellular (outside-the-cell) environment. Cholesterol molecules embedded in the bilayer help to maintain this fluidity. Also embedded within the bilayer are membrane proteins that act as gatekeepers, enforcing the selective permeability of the membrane. This description of the plasma membrane, with membrane proteins moving freely along the membrane, is called the fluid mosaic model The membrane that encloses and define all cells as seperate from the enviroment is called the plasma membrane or cell membrane The job of the plasma membrane is to seperate the chemical reactions occuring inside the cell from the chemicals outside The fluid inside the cell, called the cytoplasm, contains all the organelles and is very different from (cyto = cell) and (plasm = shape) cytoplasm literally means cell shape - fitting as the plasma membrane is what defines the cell shape plasma membrane are made of several different components, much like a mosaic work of work because membranes are a mosaic, and because they are flexible and fluid, scientists call them a 'fluid mosaic model'. TIP: Remember the plasma membrane as an international border controlling what enters or leaves the a particular country is a good way of remembering the plasma membrane in each cell the hydrophilic heads point towards watery environments (outside and inside the cell) sandwitching the hydrophobic bilayers Because cells reside in watery solutions (extra cellular matrix) and because they contain a watery solution inside of them (cytoplasm), the plasma membrane forms a sphere around each cell - so the water attracting heads are in contact with fluid and water repelling ttails are protected on the inside Other Components In addition to phospholipids, proteins play a major component of plasma membranes The proteins are embedded in the phospholipid layer, but they can drift laterally - like ships sailing in an oily ocean Cholesterol and carbohydrates are minor components of plasma membrane but they play fairly significant roles Cholesterol makes your membrane stable and prevents it from solidifying when your body temperature is low Carbohydrate chains attach to the outer-surface of the plasma membrane on each cell When they attach to the to phospholipids they form glycolipids - when they attach to the proteins they form glycoproteins Your DNA determines which specific carbohydrates attach to your cells, affecting characteristics such as your blood type Nuclear membrane The nucleus is limited by its nuclear membrane, which is a phospholipid bilayer interrupted at intervals known as nuclear pores The gaps provide direct communication between the nucleus and the cytoplasm It is difficult to overstate the importance of membranes to living cells; without them life could not exist. The plasma membrane, also known as the cell surface membrane or plasmalemma, defines the boundary of the cell. It regulates the movement of materials into and out of the cell and facilitates electrical signaling between cells. Straight Through the Membrane: Diffusion Through the Bilayer Molecules of oxygen are uncharged. Although they dissolve readily enough in water, they are also able to dissolve in the hydrophobic interior of lipid bilayers. Oxygen molecules can therefore pass from the extracellular fluid into the interior of the plasma membrane, and from there pass on into the cytoplasm, in a simple diffusion process Three other small molecules with important roles in biology—carbon dioxide, nitric oxide, and water itself—also pass across the plasma membrane by simple diffusion, as do the uncharged hormones of the steroid family. In contrast, charged ions cannot dissolve in hydrophobic regions and therefore cannot cross membranes by simple diffusion. Passively moving along Importance TRANSPORT Actively helping molecules across Osmosis: Osmosis is simply the

Cell Membrane

Transcript: Cell Membrane Acts as a gatekeeper, provides structural support to the cytoplasm, recognizes foreign materials, and communicates with other cells. Turn to page 175 in your text book. Please label your image of the cell membrane, proteins, and enzymes. Transport Proteins Forms a doughnut-shaped channel. Allows for passage of sugars, ions, etc. Channels are particular. Different types of cells have different types of channels. Receptor Proteins Transmit information to inside cell. Protrudes out from cell: has particular shape: substances such as hormones bind to it and send messages inside cell. Cell Surface Markers Long, exterior arms: often with carboydrates attached. Identifies the type of cell. Immune defenses rely on markers to tell one cell from another. Enzymes - proteins in the membrane that help with biochemical reactions inside the cell. Cells must move substances that vary in size, electrical charge, and composition into and out of cell. Why must cells transport substances in and out of cell? A constant internal state that is maintained in a changing environment by continually making adjustments to the internal and external environment. Cell Surface Proteins Facilitated Diffusion What is it? Transport proteins help substances that cannot pass through lipid bilayer diffuse into cell. 2 types of Transport Proteins 1. Channel Proteins: Ions, sugars, and amino acids use Channel Proteins. Each channel is specific to substance with correct size and charge. Example: Only sodium ions can pass through sodium ion channel. 2. Carrier Proteins: binds to a specific substance on one side of membrane. The binding causes protein to change shape. As protein's shape changes , the substance passes through the membrane and is released on other side. Cell Membrane Some substances diffuse through lipid bilayer. Others diffuse through transport (channel) proteins. Simple Diffusion Small, nonpolar molecules such as oxygen pass directly through bilayer. Concetration of oxygen higher outside than inside. So oxygen molecules move down concentration gradient into cell. Cell Transport Homeostasis Passive Transport Cell does not use energy to move substances in/out of cell. In solution, molecules move randomly to fill up space. Equilibrium is reached when space is evenly filled. Concentration gradient forms when one area has a higher concentration until equilibrium is reached. This movement is known as diffusion. Movement Through the Cell Regulates what enters and leaves cell: helps maintain homeostasis. Is selectivily permeable. Provides support and protection Composed of a "phospholipid bilayer" (lipids with phosphate group attached). Has polar head and nonpolar tail. Heads are hydrophillic/tails hydrophobic. Water soluble molecules are stopped by water insoluble tailes (sugars, proteins, ions) Facilitated Diffusion

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