The Cell Membrane and Transport

Transcription

1 The Cell Membrane and Transport

2 Membrane Structure Fluid Mosaic Structure: The Fluid Part Phospholipids: main lipid in the cell membrane; in a bilayer Polar head = attracted to water - hydrophilic Non-polar tails = not attracted to water - hydrophobic Orients itself so that the hydrophilic heads are bathed in the aqueous solutions and the hydrophobic tails are protected by turning inward Cholesterol: type of lipid hormone that helps to stabilize the membrane Reduce phospholipid movement at moderate temperatures making it firmer. Prevents the membrane from freezing at low temperatures by preventing the membrane from packing

3 Membrane Structure Con t Fluid Mosaic Structure: The Mosaic Part Proteins: Play an important role in transporting molecules through the cell membrane Two major groups of proteins: Peripheral proteins: embedded on the interior and exterior surfaces Integral proteins: Span the entire membrane Transport materials Act as channels or pores Bind to a molecule and transport it. Some proteins have carbohydrates attached Used as a docking station Labels to help cells recognize and attach to each other.

4 The Fluid Mosaic Model Plasma membrane dynamic. We describe the cell membrane as a fluid mosaic. Fluid mosaic model states that the phospholipid bilayer behaves like a fluid more than it behaves like a solid. Lipids and proteins can move laterally within the bilayer As a result of such lateral movement, the pattern, or mosaic, of lipids and proteins in the cell membrane constantly changes.

5 Some other functions of the cell membrane

6 Transport into the cell The cell membrane is selectively permeable Some substances can easily cross, some cannot cross at all. This is why proteins are needed; they can act as channels, or gates, to let ions, and larger molecules through the cell membrane

7 Diffusion The simplest form of transport Substances move from an area of high concentration to areas of low concentration (down the concentration gradient) until there is an equal distribution. Driven entirely by kinetic energy: molecules move in a straight line until they hit something and rebound. If they do not hit anything, they will just keep going When the concentration of the substance is the same throughout the entire space we say that it has reached equilibrium Even at equilibrium there is still molecular movement!!!

8 Diffusion Across a Membrane Selectively permeable: cell membranes allow some molecules to pass through, but not others. If a molecule can pass through a cell membrane, it will diffuse from an area of higher concentration on one side of the membrane to an area of lower concentration on the other side. Diffusion across a membrane is also called simple diffusion Only allows certain molecules to pass through the membrane. Depends on the size and type of molecule and on the chemical nature of the membrane. Molecules that can dissolve in lipids may pass directly through the membrane by diffusion. Molecules that are very small but not soluble in lipids may diffuse across the membrane by moving through the pores in the membrane.

9 Osmosis Diffusion of water across a membrane Direction depends on the concentrations of solutes on either side of the membrane Hypertonic: higher concentration of solutes Hypotonic: lower concentration of solutes Isotonic: equal concentrations of solutes Notice that the prefixes refer to RELATIVE solute concentrations!!

10 How Cells Deal With Osmosis: Isotonic Environment Usually have no difficulty keeping the movement of water across the cell membrane in balance. Cells of vertebrate animals on land and of most other organisms living in the sea.

11 How Cells Deal With Osmosis: Hypotonic Environment Unicellular freshwater organisms Water constantly diffuses into these organisms. Because they require a relatively lower concentration of water in the cytosol to function normally they must get rid of the excess water. Contractile vacuoles collect the excess water and then contract, pumping the water out of the cell. Unlike diffusion and osmosis, this pumping action is not a form of passive transport because it requires the cell to expend energy.

12 Hypotonic Environment Con t In many multicellular organisms: Pump solutes out of the cytosol. This lowers the solute concentration in the cytosol, bringing it closer to the solute concentration in the environment. Water molecules are less likely to diffuse into the cell. Most plant cells live in a hypotonic environment. Water moves into plant cells by osmosis Cells swell as they fill with water until the cell membrane is pressed against the inside of the cell wall Strong enough to resist the pressure exerted by the water inside the expanding cell. The pressure that water molecules exert against the cell wall is called turgor pressure.

13 How Cells Deal With Osmosis: Hypertonic Environment Water leaves the cells through osmosis. Plant cells shrink away from the cell walls, and turgor pressure is lost: plasmolysis Animal cells shrivel: crenate

14 Facilitated Diffusion Molecules that cannot diffuse through the cell membrane, even when there is a concentration gradient: They may be too large or not soluble in lipids Channel proteins (ion channels) Acts as open doors Specific to one type of ion. Assisted by carrier proteins Binds to the molecule it transports Protein changes shape Altered shape protects the molecule from the hydrophobic interior of the membrane. Molecule can then be transported across.

15 Active transport Until now, the processes we have discussed do not require the input of energy. Passive transport Active transport: move UP a concentration gradient with the input of energy. What form?

16 Sodium-Potassium Pump Important in sending nerve impulses Na-K Pump One Na-K Pump Two

17 Endocytosis The process by which cells ingest external fluid, macromolecules, and large particles, including other cells. External materials are enclosed by a portion of the membrane which folds itself into a pouch. The pouch pinches off and becomes a membrane-bound organelle called a vesicle Pinocytosis: fluids or solutes Phagocytosis: Large particles or whole cells.

18 Exocytosis Essentially the reverse of endocytosis Vesicles in the cytoplasm fuse with the cell membrane, releasing their contents into the cell s external environment.