Module 2, Section 5, Biological Membranes

Membranes at surface of cells

• barrier between cell and environment
• controls what enters and leaves with a partially permeable membrane
• allow recognition by other cells
• allow cell signalling

Membranes within cells

• barrier between oragnelles and cytoplasm
• makes functions more efficient
• forms vesicles to transport substances
• membranes within cells can be site of chemical reactions
• also partially permeable.

Composed of lipids, proteins and carbohydrates

• phospholipids form a bilayer
• it is fluid because phospholipids are constantly moving
• cholestrol present within bilayer
• protein molecules scattered through bilayer to make a mosaic
• protein with polysaccharide chain attached - GLYCOPROTEIN
• lipid with polysaccharide chain attached - GLYCOLIPID

Phospholipids form a barrier to dissolved substances

• head is hydrophilic (phospholipid)
• tail is hydrophobic (fatty acid)
• molecules automatically arrange themselves into a bilayer with heads facing out
• centre of bilayer is hydrophobic (water soluble molecules -example: ions - can not pass through) (fat soluble molecules - example: vitamins - can pass through)

Cholestrol gives the membrane stability

• type of lipid
• present in all cell membranes
• bind to hydrophobic tails causing phospholipids to pack more closely, less fluid, more rigid.

Proteins control what enters and leaves the cell

• channel proteins allow small or charged particles through
• carrier proteins transport molecules and ions by active transport and facillitated diffusion
• act as receptors for molecules in cell signalling

Glycolipids and glycoproteins act as receptors for messenger molecules

• stabalise membrane by forming hydrogen bonds with water
• sites where drugs, hormones and antibodies bind
• act as receptors for cell signalling
• also antigens

They do this to: communicate and respond to changes in environment

They do this by: releasing a messenger molecule which travels to another cell and is detected and then binds to a receptor on the membrane.

Cell membrane receptors

• a cell that responds to a particular messenger molecule is called a target cell
• receptor proteins have to have a complimentary shape to the messenger molecule for binding to occur

Examples

Glucagon = hormone released which binds to liver cells causing liver cell to break down glycogen to glucose

Antihistamines = blocking histamine receptors. This prevents histamine from binding to the cell and stops inflammation.

Affected by 1)Temperature 2)Solvent [type and concentration]

at 0 degrees or less

permeability is very high due to deformity in carrier and channel proteins

between 0 and 45 degrees

partially permeable and as temp increases so does permeability due to more movement because more energy

at 45 degrees or above

phospholipid bilayer melts, proteins deform, water inside cell expands and puts pressure on membrane, permeability increases

• Solvent itself will increase permeability such as ethanol.
• different solvents increase permeability more than others
• increasing concentration also increases permeability
• Why do solvents increase permeability? Because solvents dissolve lipids in the membrane so it looses its structure.

Diffusion is the net movement of particles, from an area of higher concentration to an area of lower concentration.

• down a concentration gradient
• molecules diffuse both ways
• does not include energy - passive

Diffusion in cell membranes - small, non-polar molecules such as oxygen, carbon dioxide and water are able to pass through spaces between phospholipids

Rate of diffusion is affected by:

• the concentration gradient - the higher it is, the faster it is
• the thickness of the exchange surface - the thinner it is, the faster it is
• the surface area - the larger it is, the faster it is
• the temperature - the warmer it is, the faster it is

Larger molecules, ions and polar molecules don't diffuse through, so they use carrier and channel proteins - called facillitated diffusion - also passive and moves down concentration gradient

Carrier Proteins (moves large molecules/ions)

• 1st a larger molecule attaches to a carrier protein in the membrane
• Then, the protein changes shape
• This releases the molecule on the opposite side of the membrane

Channel Proteins (moves charged particles)

• This is a pore in the membrane that charged particles can diffuse through
• Different channel proteins facillitate different charged particles

• This uses energy to move molecules and ions against a concentration gradient.
• This also involved carrier proteins.
• The process is very similar to facillitated diffusion through a carrier protein.
• The only difference is that energy is used to move the solute against its concentration gradient.

Endocytosis

• some substances are too large to be taken in
• instead the cell surrounds the substance with a section of the cell membrane
• the membrane then pinches off to form a vesicle inside the cell containing the ingested substance - this is ENDOCYTOSIS
• like active transport this process also uses ATP for energy

Exocytosis

• some substances produced by the cell need to be released from the cell - this is done by EXOCYTOSIS
• vesicles containing these substances pinch off and move towards the plasma membrane
• the vesicles fuse with the plasma membrae and release their contents outside the cell
• some substances aren't released outside the cell - instead thay are inserted straight into the plasma membrane
• uses ATP as an energy source

Osmosis is the diffusion of water molecules across a partially permeable membrane down a water potential gradient. Water moves from less negative (higher water potential) to more negative (lower water potential)

Water potential = the potential of water molecules to diffuse out of or into a solution

PURE WATER HAS THE HIGHEST WATER POTENTIAL OF 0

Cells are affected by water potential

Animal Cell

• when water moves in (hypertonic) the cell bursts
• solution same potential (isotonic) cell stays the same
• solution with lower water potential (hypertonic) water moves out, cell shrinks

Plant Cell

• hypotonic solution water moves in, cell now turgid
• isotonic solution, cell stays the same
• hypertonic solution, water moves out, cell becomes flaccid, eventually become plasmolysed (when membrane pulls away from the cell wall)