AS OCR Biology - Chapter 2 Cell Membranes

Phospholipid Bilayer -

• Phospholipid heads have tiny charges making them attracted to water molecules.
• Phospholipid tails have no charge so do not interact with water.
• Can get glycolipids; phospholipids have short chains of sugars on them.
• Has many molecules in it including; extrinsic/intrinsic proteins, channel/carrier proteins, glycolipids, glycoproteins and cholesterol.
• Diagram showing all these known as fluid mosaic model.

Cholesterol -

• A lipid containing a hydroxyl (-OH) at end of chain.
• Lies along side the phospholipids.
• Helps to keep the membrane fluid.
• Stops it getting too stiff or too fluid when temperatures are cold or high.

Proteins -

• Can be on outside layer, inside layer, or all the way through.
• Can be glycoproteins; short chains of sugar molecules attached to them.

Channel Proteins -

- Proteins that form a channel through the phospholipid bilayer.

- Allow large molecules and charged ions to pass through by facilitated diffusion.

Carrier Proteins -

- Move small molecules or ions across the membrane.

- Move them by facilitated diffusion or active transport.

Each protein specific to certain molecules/ions.

Cell Signalling -

• Cells need to communicate so do it with cell signalling.
• Signal molecules arrive at plasma membrane, the recepter picks up signals and brings about change in the cell.

•  
  • Receptor acting as ion channel - signal molecule activates receptor which changes shape to allow the ions entry.
  • Receptor activates a G-protein - signal molecule activates receptor which activates G-protein, which in turn activates the enzyme.
  • Receptor acts as enzyme - signal molecule arrives and connects the two parts of the receptor to activate the enzyme.

Diffusion -

• Diffusion is when particles spread about evenly in the area they are in. There is a concentration gradient when there is more particles in one area.

• Diffusion is the movement of molecules from a high C.G. to a low one.

• Small, uncharged molecules like oxygen and carbon dioxide can diffuse across the cell membrane into the cell.

• Because they are moving from a high C.G. to a low one, some molecules may travel the other way as well.

Facilitated Diffusion -

• Across cell membranes this is done by the channel proteins.

• It allows large or highly charged molecules to travel into the cell.

• The channels are hydrophilic and each proteins only lets a specific ion or molecule pass through.

• The proteins are able to open or close to allow or deny entry to the cell.

Osmosis -

• Water moves through the cell membrane by diffusion, even though the particles are charged.
• Osmosis - the movement of water molecules down their diffusion gradient through a partially permeable membrane.
• Water potential - how much water a solution contains in relation to other substances.
• More water and more pressure means high water potential.
• Highest water potential, that of pure water is 0.
• Water moves down water potential gradient.

Osmosis and Animal Cells -

Putting an animal cell in a solution with a high water potential -

The water will enter the animal cell (as the animal cell has a lower water potential) and so much water will enter that the animal cell bursts.

Putting an animal cell in a solution with an equal water potential -

Not much will change as there is no high water potential and low water potential; so the water doesn't move. The cell won't change.

Putting an animal cell in a solution with a low water potential -

The water will move down the water potential gradient out of the cell and into the solution; the cell will shrink significantly. The shape of the cell is described as being crenated.

Osmosis and Plant Cells -

A plant cell in a solution with a higher water potential.

• Water moves into the cell making the pressure on the cell wall higher as the contents are pushing against it.
• Wall pushes back on contents - turgid cell.
• The water potential finds equilibrium and the cell becomes fully turgid.

A plant cell in a solution with a lower water potential.

• Water diffuses out of cell down water potential gradient.
• Contents of cell don't push on cell wall - flaccid cell. 
• Plasma cell eventually pulled away from cell wall in places, solution fills gap between wall and plasma membrane.
• There are still a few places where the plasma membrane is attached to the cell wall, but if these are torn away (plasmolysed) then the cell dies (miserably).

Active Transport -

• Requires energy.
• Moving substances against concentration gradient.
• Done by carrier/transporter proteins.
• Breakdown of ATP provides energy for proteins to change shape and to allow the ions to move.

Exocytosis -

• Moving substances in bulk out of the cell.
• Substance released in sac called vesicle.

Endocytosis -

• Moving substances in bulk into the cell.
• Cell membrane spreads around/'hugs' substance, and the area becomes a vesicle.
• The contents of vesicle are absorbed into the cytoplasm and the vesicle membrane is recycled.

Membrane Permeability -

• Put cut and washed beetroot into water and nothing happens.
• But heat the beetroot in water, and the pigment stains the water.
• Heating the cells make the membrane more permeable, as the phospholipids vibrate leaving temporary holes in the cell membrane, this means the pigment can move into the water out of the cell.
• Cooling the water and beetroot cells makes the phospholipids vibrate much, much less and they pack more closely together so no pigment can escape into the surrounding water.