Biological Membranes


Plasma Membrane

Is a partially permeable barrier between the cell and it's environment. It keeps the contents of the cell separate from the environment, and controls what enters and leaves the cell. It acts as the site for certain chemical reactions and enables the cell to communicate with other cells through cell signalling due to the receptors on the membrane. It also has antigens so that the body's immune system can recognise the cell as 'self' and not attack it,

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Organelle Membranes

These membranes seperate organelles from the cytoplasm. They compartmentalise the cell, separating processes so that each process can occur in a specialised area of the cell. Concentration gradients can be formed across the membranes. They may act as a site of specific chemical reactions, such as oxidative phosphorylation in aerobic respiration.

Mitochondria have folded inner membranes called cristae. They give a large SA for some reactions of aerobic respiration,

Chloroplasts have inner membranes called thylakoid membranes, which house chlorophyll. On these membranes some of the reactions of photosynthesis occur.

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Fluid Mosaic Model

Consists of:

  • Bilayer of phospholipid molecules
  • Cholesterol which regulates the fluidity of the membrane; stabilising it
  • Glycolipids and glycoproteins that function in cell signalling or cell attachment
  • Proteins embedded in the bilayer. Can be intrinsic (embedded) or extrinsic (surface)
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Phospholipids form a barrier that limits movement of some substances into and out of the cell, or organelles, so the membrane is partially permeable. Small, fat-soluble molecules dissolve into the phospholipid bilayer and diffuse across the membrane. 

They contain a fatty acid tail which is hydrophobic and a phosphate head which is hydrophilic and has a charge

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Cholesterol fits between the tails of trhe phospholipid molecules. It inhibits movement of the phospoholipids, reducing the fluidity of the membrane. It also holds he phospholipid tails together, for mechanical stability. Cholesterol makes the membrane less permeable to water and ions.

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Glycolipids and Glycoproteins

The carbohydrate group on the protein or lipid molecule always has a specific shape and is used to recognise the cell and identify as 'self'. Antigens on cell surfaces are usually glycolipids or glycoproteins.

Drugs and hormones can bind to these membrane-bound receptors

Cells communicate by cell signalling to coordinate activities of the organism. The shape of the glycoprotein or glycolipid may be complementary to the shape of a signalling molecule in the body. Binding sites are also used for cell attachment - the cells of a tissue bind together to hold the tissue together.

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Some proteins may form:

  • pores that allow the movement of molecules that cannot dissolve in the phospholipid bilayer
  • Carrier molecules that allow facilitated diffusion
  • Active pumps

Some proteins may be attached to carrier proteins and function as enzymes, antigens or receptor sites for complementary-shaped signalling chemicals eg, hormones

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Affect of Temperature

If temperature increases, the molecules gain kinetic energy and move about more, This increases the permeability of the membranes to certain molecules. Molecules that diffuse through the bilayer will do so more quickly. This is because as phospholipids move more they leave gaps between them.

If temperature increases further, the bilayer may lose its mechanical stability and the membrane becomes even more permeable. Eventually, the proteins in the membrane will denature. This will further damage the structure of the membrane and it will become completely permeable. An increase of membrane fluidity will also affect cell signalling.

When temperature drops saturated fatty acids become compressed. The kinks in phospholipid tails push adjacent molecules away, maintaining the fluidity. Cholesterol also buffers the effects of lower temperature by preventing the phospholipids from packing together too closely

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Affect of Solvents

Solvents such as alcohol dissolve fatty substances and lipids. As the concentration of alcohol increases, the membrane is more likely to dissolve.

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Beetroot investigations


Inside beetroot cells, within the large vacuole that are nitrogenous, water soluble pigments called betacyanins, a type of betalain. If you heat pieces of beetroot tissue, the plasma membrane and tonoplast membrane will be disrupted and pigment will leak out. The amount of leakage of red pigment is proportional to the degree of damage to beetroot plasma and tonoplast membranes, and can be measured using a colorimeter, by measuring the absorbance of green light


Betalain pigments do not change as pH changes

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Passive Transport

The movement of molecules that does not require ATP. Instead, it uses kinetic energy. It only occurs when molecules move down a concentration gradient. Occurs in 3 forms:

  • Diffusion - the net movement of molecules away from a concentrated source, This may occur across a membrane if the molecules are fat-soluble or small enough to fit between phospholipids in a membrane
  • Facilitated Diffusion - diffusion across a membrane with the help of a transport portein. THis could be a channel protein or a carrier protein
  • Osmosis - the net movement of water molecules across a partially permeable membrane. Water molecules move down their water potential gradient.
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Rate of Diffusion

Rate of diffusion is affected by:

  • Temperature - a higher temperature gives molecules more kinetic energy. At higher temperatures the molecules move faster, so diffusion rate increases.
  • Concentration gradient - more molecules on one side of the membrane increases the concentration gradient, increasing the rate of diffusion
  • Size of molecule - small molecules or ions can move more quickly than larger ones. Therefore, they diffuse more quickly
  • Thickkness of membrane - a thick barrier creates a longer pathway for diffusion, so diffusion is slowed down by a thick membrane
  • SA - diffusion occurs more rapidly with a greater SA
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Active Transport

Involves the movement of molecules using ATP. It can move molecules against their concentration gradient and uses membrane-bound proteins that change shape to move the molecules across the membrane.

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Bulk Transport

The movement of molecules through a membrane by the action of vesicles.

  • Endocytosis - a segment of the plasma membrane surrounds and encloses the particle and brings it into the cell, enclosed in a vesicle. ATP provides energy to move vesicles, using motor proteins, along cytoskeleton threads. Phagocytosis - intake of solid matter. Pino(endo)cytosis - intake of liquids.
  • Exocytosis - vesicle moves along cytoskeleton threads, with the help of motor proteins, and fuses with the plasma membrane, where it is emptied out of the cell.
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Water Potential

Pure water has a water potential of 0kPa. As solutes are added to a solution, the water potential gets lower. Therefore, a salt solution has a water potential below 0 - negative potential

Water molecules will move from a solution with a higher water potential to a solution with a more negative water potential. Therefore, water always moves down their water potential gradient. 

The water potential inside cells is lower than that of pure water.

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A cell placed in water has a lower (more negative) water potential than the surrounding water. There is a water potential gradient from high outside the cell to lower inside the cell. As a result, water molecules enter the cell.

A cell placed in a strong salt solution has higher (less negative) water potential than the surrounding solution There is a water potentialn gradient from higher inside the cell to lower outside the cell, so water molecules leave the cell

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Water Potential in Cells

If many water molecules enters an animal cell, it will swell and burst as the plasma membrane breaks. This is cytolysis

In plant cells, the cell wall will prevent bursting and will swell up to a certain size, this is described as turgid

When plant cells are placed in a strong salty solution it will become flaccid. Eventually the vacuole and cytoplasm will shrink and the plasma membrane will pull away from the cell wall. This is plasmolysis

If an animal cell is placed in a strong salty solution the cytoplasm will shrink and cell will shrivel. The cell becomes crenated

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