Chapter 5: biological membranes

1. To act as partially (and sometimes selectively) permeable membranes between:

a) A cell and its environment

b) An organelle and the cytoplasm which surrounds it

c) Within organelles (e.g. mitochondria/chloroplasts)

2. Sites of chemical reactions — e.g. respiratory enzymes on the inner mitochondrial membrane

3. Sites of cell communication and signalling

• Phospholipids, which form the bilayer and the bulk of the membrane surface area.

• Cholesterol, which regulates membrane fluidity and flexibility.

• Proteins, which are involved in more complex functions such as carrying out chemical reactions or regulating cross-membrane transport.

• Can be intrinsic/integral (span the entire membrane)

• Can be extrinsic (embedded in one half of the membrane)

• Can form cross-membrane channels

• Glycoproteins and glycolipids — proteins and lipids with sugars attached

• Can be antigens or receptor molecules.

• Can bind with extracellular signalling molecules such as hormones

• An increased temperature has several effects on a membrane which increase its fluidity and permeability.

• Phospholipids acquire more kinetic energy so move around more

• Fatty acids become less compressed

• Proteins can denature at high temperatures, reducing their ability to control transport across the membrane

• Changes in pH away from the optimum can also affect the function of proteins in the membrane.

• Organic solvents such as ethanol will damage a membrane and increase its permeability by dissolving lipids held in it.

• Movement of molecules or ions from an area of high concentration to an area of lower concentration until the concentrations of the two regions are equal -> establishing a dynamic equilibrium

The rate of diffusion depends on:

• surface area of the cell

• thickness of the membrane

• strength of the diffusion gradient (i.e. the difference in concentration)

• temperature

• Small, non-polar molecules can diffuse through a membrane

• Oxygen

• Carbon dioxide

• Steroids

• Fat-soluble vitamins (e.g. A, D, E, K)

• Specialised form of diffusion

• Diffusion of water molecules from an area of high water potential to an area of low water potential (low concentration)

• Through a partially permeable membrane

• Therefore, cells are affected by the water potential outside a cell

Facilitated diffusion

• Faster than normal diffusion

• Used to transport large and polar molecules:

• Glucose

• Fructose

• Non-fat-soluble vitamins

• urea

• ions

• There are three steps involved in the facilitated diffusion of large molecules through a carrier protein:

1. Molecule binds with carrier protein molecules on the cell surface

2. The protein changes shape (known as a conformational change), in doing so the molecule is transported through to the inside of the cell

3. The molecule detaches from the transporter protein and the protein reverts to its original shape

• Alternatively, small polar molecules can diffuse through the pores in the membrane created by channel proteins

• The rate of facilitated diffusion is proportional to the concentration gradient and to the number of channels or transporter proteins that are available

• Moves substances against a concentration gradient

• Carrier proteins (such as Na+/K+ pumps) span the membrane and use ATP to transport molecules across

• Upon binding, a carrier protein undergoes a conformational change that pushes the molecule into or out of the cell/organelle

• Bulk transport involves the active transport of large molecules that cannot fit through the plasma membrane

• Moves substances against a concentration gradient

• Carrier proteins (such as Na+/K+ pumps) span the membrane and use ATP to transport molecules across

• Upon binding, a carrier protein undergoes a conformational change that pushes the molecule into or out of the cell/organelle

• Bulk transport involves the active transport of large molecules that cannot fit through the plasma membrane

Bulk transport involves the active transport of large molecules that cannot fit through the plasma membrane

• Endocytosis is the bulk transport of substances into the cell

• The cell membrane dimples to engulf the substances

• The membrane surrounds the molecules forming a vesicle inside the cell that contains them

• Endocytosis of liquids is called pinocytosis

• Exocytosis is the bulk transport of substances out of the cell

• A membrane bound vesicle moves to the plasma membrane

• The membranes fuse together, releasing the contents of the vesicle to the extracellular environment

Hypotonic:

• water moves in by osmosis

• cell swells and may burst

Isotonic:

• water flows in and out at equal rate

Hypertonic

• Water moves out of the cell by osmosis

• Cell shrinks, called crenation

Hypotonic: 

Solution has higher water potential than the vacuole • Water flows in by osmosis • Cell swells, becomes turgid • Cannot burst because of cell wall

Isotonic: 

- Neither turgid nor plasmolysed

- Water moving in and out at same rate

Hypertonic:

• Solution has lower water potential than vacuole • Water flows out by osmosis • Cell shrinks and becomes plasmolysed • Cell wall rigid and does not collapse