The Fluid-Mosiac Structure of the Cell (Plasma) Membrane “FLUID” – the molecules can move within the membrane “MOSAIC” – other molecules eg proteins are embedded within the phospholipid bilayer

Transport across membranes (Ref: p54-62 NT)

1) Diffusion DEFINITION: The net movement of molecules or ions from a region of higher concentration to a region of lower concentration, until they are equally distributed. See figure 1 p54.

Oxygen, carbon dioxide and small uncharged molecules diffuse through phospholipid bilayer Glucose, large water-soluble molecules and charged ions cannot diffuse through the phospholipid bilayer

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Small molecules can squeeze through the phospholipid bilayer. They must be small, uncharged or lipid soluble Examples include water, CO 2 and O 2

Factors affecting the rate of diffusion Steepness of concentration gradient. The greater the difference in conc., the faster the diffusion. Distance over which diffusion occurs. The thinner the distance, the quicker the diffusion. Temperature : Increasing temperature increases the kinetic energy of the molecules and diffusion is faster. Surface area between the two regions. Increasing the surface area increases the rate of diffusion.

2) Facilitated Diffusion DEFINITION: The transport of molecules from higher to lower concentration across a membrane using channel proteins or carrier proteins and no energy is needed (it is a passive process) Important for: –Lipid insoluble molecules / water soluble eg glucose. –Large molecules –Charged ions eg K+,Na+, Cl-

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cell membrane – phospholipid bilayer Ion channel protein Ions to be transported Outside cell Inside cell Gate closed, Ions cannot pass through

cell membrane – phospholipid bilayer Outside cell Inside cell

cell membrane – phospholipid bilayer Outside cell Inside cell

cell membrane – phospholipid bilayer Outside cell Inside cell

cell membrane – phospholipid bilayer Outside cell Inside cell

cell membrane – phospholipid bilayer Outside cell Inside cell

cell membrane – phospholipid bilayer Outside cell Inside cell

cell membrane – phospholipid bilayer Outside cell Inside cell

cell membrane – phospholipid bilayer Outside cell Inside cell

cell membrane – phospholipid bilayer Outside cell Inside cell

cell membrane – phospholipid bilayer Outside cell Inside cell

cell membrane – phospholipid bilayer Outside cell Inside cell

cell membrane – phospholipid bilayer Outside cell Inside cell

Channel protein Molecules travel from high conc. to low conc. No energy required Molecule diffuses into channel protein and crosses membrane Molecule or ion cannot diffuse into channel Shape of channel is specific to molecule or ion transported

cell membrane – phospholipid bilayer carrier protein substance to be transported Outside cell – high concentration of diffusing substance Inside cell – low concentration of diffusing molecule

cell membrane – phospholipid bilayer Outside cell Inside cell

cell membrane – phospholipid bilayer Outside cell Inside cell

cell membrane – phospholipid bilayer Outside cell Inside cell

cell membrane – phospholipid bilayer Outside cell Inside cell

cell membrane – phospholipid bilayer Outside cell Inside cell

Carrier proteins Molecule or ion diffuses into carrier protein Carrier protein changes shape and allows molecule or ion through From high conc to low conc (down the concentration gradient). No energy required Shape of carrier is specific to molecule or ion transported

3) Active transport DEFINITION: The transport of molecules or ions across a membrane from lower concentration to higher concentration. Active Transport involves specific carrier proteins and uses energy as ATP. Eg Glucose, Amino acids, Na+,K+,Cl-

cell membrane – phospholipid bilayer carrier protein substance to be transported, eg. sodium Outside cell – low concenration of molecules, eg. sodium Inside cell – high concenration of molecules, eg. sodium

cell membrane – phospholipid bilayer Outside cell Inside cell

cell membrane – phospholipid bilayer Outside cell Inside cell ATP

cell membrane – phospholipid bilayer Outside cell Inside cell ATP energy

cell membrane – phospholipid bilayer Outside cell Inside cell ADP

cell membrane – phospholipid bilayer Outside cell Inside cell

Molecule binds to specific site on carrier protein ATP is broken down Energy released is used to change the shape of the carrier protein to allow the molecule through the membrane against it’s conc. Gradient. ATP molecule

Don’t forget these points about Active Transport! Any factor that reduces the rate of respiration will slow down the rate of active transport.

Active transport will take place when a substance needs to be accumulated (collected) against a concentration gradient Examples include (1) absorption of glucose and amino acids in the small intestine of mammals, (2) absorption of mineral ions from the soil by plant roots.

Cells carrying out a lot of active transport will have a lot of carrier proteins in their membrane and a lot of mitochondria (WHY?)

4) Osmosis GCSE definition of Osmosis: Osmosis is the movement of water molecules by diffusion, from a region with a higher concentration of water molecules to a region with a lower concentration of water molecules, through a semi-permeable membrane.

4) Osmosis A-Level definition of Osmosis: Osmosis is the net movement of water molecules by diffusion, from a higher (less negative) water potential to a lower (more negative) water potential through a partially permeable membrane.

See HB pages on Osmosis p22-23 JA show: (1) Scan of HB Osmosis page called ‘Osmosis Notes’, (2) pdf of Osmosis and plant and animal cell diagrams, to annotate/and ones already annotated. (3) ‘Box shaped cell diags’ task to add on arrows to show water movement direction by osmosis.

HW: Complete the Osmosis tasks on the handout sheet for next lesson.

The Water Potential Scale. 0 kPa pure water. A less negative (or higher) water potential Ψ -100kPa -200kPa As the solution has more solutes dissolved in it, the water -300kPa potential becomes more negative. -400kPa The direction of diffusion of -500kPa water by osmosis. -600kPa A solution with a more negative (or lower ) water potential Ψ. Remember: A solution has a negative water potential – The more negative it’s Ψ, the more concentrated the solution is. is.

The effects of osmosis on plant cells Plant cell in solution with same water potential as the cell cytoplasm Cell in solution with more negative water potential Cell in solution with less negative water potential Water leaves cell and cytoplasm shrinks away from cell wall - plasmolysed Cell wall pushes on expanding cell and prevents bursting - turgid

The effects of osmosis on human red blood cells Red blood cell in solution with same water potential as the cell cytoplasm Cell in solution with less negative water potential Cell in solution with more negative water potential Cell smaller and appears ‘crinkled’ Cell swells and burst

small water molecule large protein molecule partially permeable membrane Solution B : more concentrated protein solution Solution A : less concentrated protein solution

Solution BSolution A :

Solution BSolution A

Solution BSolution A

Solution BSolution A

Solution BSolution A

Solution BSolution A

Solution BSolution A

Solution BSolution A