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Cell Membrane & Cellular Transport Biology 1 HOMEOSTASIS AND TRANSPORT Cell membranes help organisms maintain homeostasis by controlling what substances may enter or leave cells Homeostasis – the internal balance in your body Some substances can cross the cell membrane without any input of energy by the cell The movement of such substances across the membrane is known as passive transport To stay alive, a cell must exchange materials such as food, water, & wastes with its environment These materials must cross the cell or plasma membrane Plasma Membrane Boundary that separates the living cell from it’s non-living surroundings. Phospholipid bilayer (2 layers) hydrophilic heads Phospholipid hydrophobic tails Phospholipid Bilayer Plasma Membrane - cont. Controls what comes into and out of the cell with phospholipids and transport proteins. Selectively permeable (allows some molecules to move across cell membrane) Small molecules like water, oxygen, & carbon dioxide can move in and out freely Large molecules like proteins & carbohydrates cannot move easily across the plasma membrane The Cell Membrane is semi permeable or selectively permeable only allowing certain molecules to pass through Passive Transport NO ENERGY required. When substances move from a high concentration to a low concentration. Three types: Simple Diffusion Facilitated Diffusion Osmosis Hypertonic Hypotonic Isotonic Diffusion Passive transport: NO energy is required. Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration Substances continue to move until the molecules are even throughout the space- it is called EQUILIBRIUM Diffusion The net movement of a substance (molecules) down a concentration gradient from an area of high concentration to an area of low concentration. Concentration gradient = happens anytime there is a difference in concentration in one place compared to another Small molecules can pass through the cell membrane by a process called diffusion Examples: oxygen and carbon dioxide CELL EXAMPLE: DIFFUSION automatically moves oxygen from HIGHER concentration (in lungs) to a LOWER concentration (in blood) CO2 automatically moves from where there is a HIGHER concentration (in blood) to where there is a lower concentration (in lungs) http://www.le.ac.uk/pa/teach/va/anatomy/case2/2_2.html Facilitated Diffusion A type of passive transport that requires the use of transport(carrier) proteins. Facilitate = to help Transport proteins help the molecules across the membrane that can’t pass through simple diffusion. Still requires the use of NO ENERGY. (Passive transport)! Facilitated Diffusion Uses two methods: Transport (carrier) proteins: proteins that are already embedded in the cell membrane that grabs molecules, changes shape to help move molecule, then moves the molecule to the other side of the membrane. Protein channel: proteins already embedded in the cell membrane create a tunnel which the molecule can pass through. Video Osmosis The diffusion of water across a semi~permeable membrane is called osmosis The water moves from a high concentration to low concentration. 3 kinds of Osmosis in cells: Hypotonic, Hypertonic, Isotonic Hypotonic, Hypertonic, & Isotonic OSMOSIS This diagram shows water molecules moving across a selectively permeable membrane. Water molecules are the small blue shapes, and the solute is the green. What's happening? The solute (green blobs) is more concentrated on the right side, which pulls the water molecules toward that side. The green blobs would move to the left to spread out evenly, but the membrane won't let those pass Solutions Solutions have two parts -- Solute: the substance being dissolved Solvent: the substance doing the dissolving Water serves as the main solvent in living things Hypertonic Solution Solute concentration outside the cell is higher (less water) Water diffuses out of the cell until equilibrium is reached Cells will shrink & die if too much water is lost Plant cells become flaccid (wilt); called plasmolysis Hypotonic Solution Solute concentration is less inside the cell (more water) Water moves into the cell until equilibrium is reached Animal cells swell & burst (lysis) if they take in too much water Cytolysis is the bursting of cells Plant cells become turgid due to water pressing outward against cell wall Turgor pressure in plant cells helps them keep their shape http://faculty.etsu.edu/currie/images/osmosis1.jpg Isotonic Solutions Concentration of solutes same inside & outside the cell Water moves into & out of cell at an equal rate so there is no net movement of water Animal Cells Animal cells placed into a hypotonic solution will CYTOLYSIS (BURST). Animal cells placed into a hypertonic solution will CRENATE (SHRIVEL). Hemolysis Crenation Red Blood Cells Plant Cells Firmness or tension (vacuole full) that is found in plant cells (cell wall) that are in a hypotonic environment is called TURGID. This process is called TURGOR PRESSURE. Water Water Central Vacuole Cell Wall Water Plant Cells When the plasma membrane pulls away from the cell wall (vacuole empty) in a hypertonic environment (loss of water) is called PLASMOLYSIS. Water Water plasma membrane Cell Wall Water Effect of Solutions on Cells Active Transport The movement of molecules (small or large) across the plasma membrane in which energy (ATP) is required. Moves materials against their concentration gradient from an area of low concentration to a high concentration. Kinds of ________Transport ACTIVE ________________________________________ PUMPS •Sodium-Potassium __________________________________ •Proton ________________________________ •Vesicles ___________________________________ •Endocytosis ________________________________ •Exocytosis ________________________________ Sodium-Potassium Pump The mechanism that uses energy (active transport) released from splitting ATP to transport Sodium (Na+) out of and Potassium (K+) into cells. extracellular fluid intracellular fluid K+ K+ Na+ Na+ Na+ and K + Animation from: http://www.lionden.com/cell_animations.htm PUMP Bulk Transport Moves large, complex molecules such as proteins across the cell membrane Large molecules, food, or fluid droplets are packaged in membrane-bound sacs called vesicles Endocytosis moves large particles into a cell Exocytosis moves large particles out of a cell. There are two forms of endocytosis: a) Pinocytosis - a form of endocytosis that transports liquids into the cell. b) Phagocytosis - a form of endocytosis that transports solids into the cell Animation of endocytosis & exocytosis Video
