BIOLOGY 11 IB 2.4: MEMBRANES

ASSESSMENT STATEMENTS 2.4.1Draw and label a diagram to show the structure of a membrane 2.4.2Explain how the hydrophobic and hydrophilic properties of phospholipids help to maintain the structure of cell membranes 2.4.3List the functions of membrane proteins 2.4.4Define DIFFUSION and OSMOSIS 2.4.5Explain passive transport across membranes by simple diffusion and facilitated diffusion 2.4.6Explain the role of protein pumps and ATP in active transport across membranes 2.4.7Explain how vesicles are used to transport materials within a cell between the rough ER, Golgi apparatus, and plasma membrane 2.4.8Describe how the fluidity of the membrane allows it to change shape, break, and re-form during endocytosis and exocytosis

2.4.2 EXPLAIN HOW THE HYDROPHOBIC AND HYDROPHILIC PROPERTIES OF PHOSPHOLIPIDS HELP TO MAINTAIN THE STRUCTURE OF CELL MEMBRANES. Phospholipid molecules make up the cell membrane Hydrophilic phosphate head (attracted to water) Two hydrophobic hydrocarbon tails (not attracted to water, but attracted to each other) Cell membranes are made up of a double layer (bilayer) of these phospholipid molecules. This makes the membrane very stable but also allows flexibility. The phospholipid in the membrane are in a fluid state which allows the cell to change it’s shape easily.

Cholesterol can be found in the hydrophobic region (fatty acid tails) in animal cells. They determine membrane fluidity, which changes with temperature, allowing effective membrane function at a wider range of temperatures Plant cells, don’t have cholesterol, so they rely on saturated or unsaturated fatty acids to maintain proper membrane fluidity

The membrane also contains proteins: Integral proteins go completely from one side of the membrane to the other (a & b) Peripheral proteins are attached to the membrane but only on one side (c, d, & e)

2.4.3 LIST THE FUNCTIONS OF MEMBRANE PROTEINS. Membrane proteins can act as: 1.hormone binding sites 2.electron carriers 3.pumps for active transport 4.channels for passive transport 5.enzymes In addition they can be used for cell to cell communication as well as cell adhesion.

HORMONE BONDING SITES These proteins may detect hormones arriving at the cells to signal changes in function. They may also be involved in other cell and substance recognition as in the immune system

ELECTRON CARRIERS Help catalyze chemical reaction an important role in photosynthesis and cell respiration.

PROTEINS AS ENZYMES Break down chemicals for use within the cell

PUMPS FOR ACTIVE TRANSPORT  Requires energy, in the form of ATP, or adenosine triphosphate  Molecules are ‘pumped’ across the membrane UP the concentration gradient  Pumps fit specific molecules  The pump changes shape when ATP activates it, this moves the molecule across the membrane

These proteins span the membrane from one side to another. They allow the movement of large molecules across the plasma membrane. CHANNELS FOR PASSIVE TRANSPORT

2.4.4 DEFINE DIFFUSION Diffusion is the passive movement of small particles across a cell membrane. The particle moves from a region of high concentration to a region of low concentration. This process continues until the concentration of the substance is the same on both sides of the membane

Osmosis is the diffusion of water molecules across a semipermeable membrane (allows only some molecules through) from a region of lower solute concentration to a region of higher solute concentration. The movement of water into and out of cells depends on osmosis DEFINE OSMOSIS

2.4.5 EXPLAIN PASSIVE TRANSPORT ACROSS MEMBRANES BY SIMPLE DIFFUSION AND FACILITATED DIFFUSION. Simple DiffusionFacilitated Diffusion Both methods = Passive transport (do not require ATP) Molecules moving from high concentration to low concentration Diffusion of molecules through the phospholipid bilayer Uses channel proteins embedded in the membrane How hydrophobic (lipid soluble) molecules will pass through How hydrophilic molecules and charged particles will pass through.

MEMBRANE TRANSPORT ANIMATIONS

2.4.6 EXPLAIN THE ROLE OF PROTEIN PUMPS AND ATP IN ACTIVE TRANSPORT ACROSS MEMBRANES. Active transport involves the movement of substances through the membrane using energy from ATP. The advantage of active transport is that substances can be moved against the concentration gradient, meaning from a region of low concentration to a region of high concentration. This is possible because the cell membrane has protein pumps embedded it which are used in active transport to move substances across by using ATP. Each protein pump only transports certain substances so the cell can control what comes in and what goes out.

EXAMPLE: SODIUM POTASSIUM PUMP hill.com/sites/ /student_view0/chapter2/animation__how_the_sodi um_potassium_pump_works.html

2.4.7 EXOCYTOSIS Vesicles are used to transport materials within a cell between the rough endoplasmic reticulum, Golgi apparatus and plasma membrane. After proteins have been synthesized by ribosomes they are transported to the rough endoplasmic reticulum where they can be modified. Vesicles carrying the protein then bud off the rough endoplasmic reticulum and are transported to the Golgi apparatus to be further modified. After this the vesicles carrying the protein bud off the Golgi apparatus and carry the protein to the plasma membrane.

2.4.7 EXOCYTOSIS CONTINUED… At the plasma membrane the vesicles fuse with the membrane expelling their content (the modified proteins) outside the cell. The membrane then goes back to its original state

2.4.7 ENDOCYTOSIS Endocytosis is a similar process which involves the pulling of the plasma membrane inwards so that the pinching off of a vesicle from the plasma membrane occurs and then this vesicle can carry its content anywhere in the cell.

2.4.8 DESCRIBE HOW THE FLUIDITY OF THE MEMBRANE ALLOWS IT TO CHANGE SHAPE, BREAK AND RE-FORM DURING ENDOCYTOSIS AND EXOCYTOSIS. The phospholipids in the cell membrane are not solid but are in a fluid state allowing the membrane to change its shape and also vesicles to fuse with it. This means substances can enter the cell via ____________________ and exit the cell via ______________. The membrane then returns to its original state.

EXERCISES Why is the term ‘equilibrium’ used with passive but not active transport? Why are exocytosis and endocytosis know as examples of active transport?