P2- Plasma Membrane Structure and Function Movement of Substances

Key Knowledge The structure and function of the plasma membrane and the movement of substances across it  The fluid-mosaic model  The packaging, transport, import and export of bio macromolecules  The role of ribosomes, endoplasmic reticulum, Golgi apparatus and associated vesicles in the export of proteins

Plasma membrane This is the insoluble boundary of the living cell that maintains the contents of the cell and regulates the movement of substances in and out of the cell. ALL cells have a plasma membrane. The plasma membrane is differentially or selectively permeable. The plasma membrane is composed of two layers of phospholipids.

Fluid mosaic model The lipid component of the membrane allows flexibility and the membrane has the ability to repair itself.

Glycoprotein  Sugar attached to a protein  Often a marker or receptor molecule that identifies ‘self’ from ‘non self’  Important in cell recognition and communication

Glycolipid  Sugar attached to a lipid  Their role is to provide energy for the cell and also as a marker.

Membrane Proteins Important for:  Regulation of cell behaviour  Organisation of cells in tissues  Cellular communication : receptor sites detect hormone and other chemical molecules to control transmission of messages within and between cells.

Plasma membrane – the Sieve  Small particles can pass through openings or pores between the phospholipids easily – eg: H 2 0, CO 2, O 2 and other small, non polar particles.  Within the membrane are channels made by proteins – these allow a variety of larger or polar substances in and out eg: glucose.  Large proteins cannot pass through the membrane.

Plasma membrane – the Sieve Copy diagram into your workbook

Diffusion The net or overall movement of particles (molecules or ions) from a region where they are at a relatively high concentration to a region where they are at a relatively low concentration. Concentration gradient = difference between the two regions. Diffusion continues until an equilibrium is reached and there is the same concentration of particles on either side of the membrane. Passive process – no energy is required. Diffusion is quicker if greater concentration gradient. hill.com/sites/ /student_view0/chapter2/ani mation__how_diffusion_works.html hill.com/sites/ /student_view0/chapter2/ani mation__how_diffusion_works.html

Diffusion

What determines the rate of diffusion? There 4 factors: 1. The steepness of the concentration gradient. The bigger the difference between the two sides of the membrane the quicker the rate of diffusion. 2. Temperature. Higher temperatures give molecules or ions more kinetic energy. Molecules move around faster, so diffusion is faster. 3. The surface area. The greater the surface area the faster the diffusion can take place. This is because the more molecules or ions can cross the membrane at any one moment. 4. The type of molecule or ion diffusing. Large molecules need more energy to get them to move so they tend to diffuse more slowly. Non-polar molecules diffuse more easily than polar molecules because they are soluble in the non polar phospholipid tails.

Osmosis Terms  Solute: a substance dissolved into another substance (the solvent) to create the solution.  Solvent: a substance in which other substances (solutes) can be dissolved to create a solution.  E.g. Water is a solvent because it is polar and many other polar solutes can dissolve in it.

Osmosis High concentration solution: High solute concentration and low solvent concentration. Low concentration solution: Low solute concentration and high solvent concentration. Isotonic Same concentration of solvent and solute Hypotonic Lower concentration (low solute and high solvent) Hypertonic Higher concentration (high solute and low solvent)

Osmosis continued...  If a vacuole swells and pushes the plasma membrane against the cell wall it is called turgid  The pressure on the cell is called turgor pressure  keeps cell firm & maintains shape.  Opposite of turgid is a process called plasmolysis. Water diffuses out of the cell and the plasma membrane is withdrawn from the cell wall making the cell limp or flaccid.

Transport across the membrane Crenated

Osmosis continued...  Osmosis Animation:  hill.com/sites/ /student_view0/chapter2 /animation__how_osmosis_works.html hill.com/sites/ /student_view0/chapter2 /animation__how_osmosis_works.html  Osmosis: establishing an equilibrium:  at/transport/osmosis.swf at/transport/osmosis.swf

Facilitated Diffusion  Substances are helped across the cell membrane by carrier molecules. Their diffusion is facilitated (helped).

Facilitated Diffusion  Transport proteins in membrane assist in movement of simple sugars, amino acids, nucleotides, charged ions and other nutrients. hill.com/sites /student_view0/chapter3/animation_how_facilitated _diffusion_works.html

Carrier Proteins  Assist the passage of other molecules across the membrane in facilitated and active transport.  Carry some molecules to other side of membrane. Channel Proteins  Form pores in membrane that fill with water.  Lining of protein = hydrophilic  Particles that are able to dissolve in water move through membrane this way.  Selective  Open only when correct signal received.

Active Transport  Requires input of energy  Cells that undergo lots of active transport of substances have lots of mitochondria to provide energy.  Involves a carrier protein attached to a source of energy.  Function in one direction

Bulk Transport  Large molecules also need to move in and out of cells. E.g. hormones, mucus, enzymes etc.  Sometimes this is not as easy as facilitated diffusion.  Endocytosis and exocytosis is then required.

Endocytosis  Inward movement of particles into a cell. The membrane engulfs the substance and forms a endocytic vesicle which stores and transports the substance throughout the cytoplasm.  Phagocytosis – encloses solid material e.g. food  Pinocytosis – encloses droplets of liquid.

Exocytosis  Movement of particles out of a cell via the plasma membrane.  Small membrane bound vesicles move throughout the cytoplasm to the plasma membrane where it fuses and releases its contents to the external environment.   hill.com/olc.dl bio02.swf hill.com/olc.dl bio02.swf

Synthesis and transport Involves the coordinated activity of a number of organelles, including nucleus, ribosomes, endoplasmic reticulum, Golgi apparatus and vesicles.

STEP 2- Polypeptide produced on ribosome STEP2- Polypeptide enters ER tubule STEP 3- Polypeptide modified in ER tubule environment STEP 4- Polypeptide enclosed in transport vesicles STEP 5- Further modification of polypeptide in Golgi complex. Finally ready for export STEP 6- Protein placed in protein transport vesicle STEP 7- Protein transport vesicle fuses with plasma membrane. Exocytosis: Protein product secreted STEP 1- mRNA made in nucleus enters cytoplasm STEP 2- The mRNA Codes for protein Being made at ribosome

Questions Quiz  membrane.htm membrane.htm  Complete the following Questions Heinemann Biology 2 Textbook: Chapter 1 Review Question Key Questions page – 21 page 44