Exchange Across Membranes

Revision cards containing the basic concepts of exchange across membranes needed for AS level biology (OCR exam board)

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Structure Of Cell Membranes


The cell surface (plasma) membrane surrounds all cells and control what enters and leaves the cell. Membranes are visible at TEM microscope magnifications of x100,000. The distance across the membranes is about 7nm.

Membranes are made up of two phospholipid layers, plus protein. The polar heads are hydrophilic and are attracted toward the water and cytoplasm. The hydrocarbon tails are hydrophobic and are held together by weak hydrophobic bonds. Proteins are scattered throughout the membrane, and transmembrane proteins pass through from one side to the other. Carbohydrates are attached to proteins (Glycoproteins) and lipids (Glycolipids) This structure is called the fluid mosaic model.

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Functions Of Cell Surface Membranes


Cell surface membranes:

  • Act as a barrier to many water-soluble substances
  • Keep large molecules, such as enzymes, within the cell
  • Are permeable to  small molecules such as water, oxygen and carbon dioxide
  • Are permeable to selected molecules such as glucose and ions
  • Permit movement of substances by endocytosis and exocytosis
  • permit recognition by other cells, such as those of the immune system
  • Provide receptors for signalling molecules such as hormones

Membranes within cells

  • Isolate potentially harmful enzymes in lysosomes
  • Provide a large surface are for holding the enzymes and co-enzymes for forming ATP in mitochondria and chloroplasts
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Types Of Transport


  • Diffusion occurs passively (No ATP is required)
  • Molecules such as oxygen and carbon dioxide diffuse across membranes
  • Molecules will only diffuse when the concentration gradient exists


  • Osmosis is a form of diffusion
  • Although water molecules are polar, they are small enough to pass through the Phospholipid bi-layer
  • Some cells contain aquaporins to make the cell more permeable to water
  • Osmosis is affected by, the amount of water present in cytoplasm, the concentration of solutes on either side of the membrane, the amount of aquaporins in the membrane and (in plants) the pressure exerted on the cell contents by the cell wall
  • Osmosis relies on the presences of a water potential gradient. The lower the concentration of solutes in the water, the higher the water potential.
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  • Facilitated diffusion moves molecules to large to pass through the phospholipid bi-layer, it can also transport charged molecules
  • Protein molecules exist in membranes to help the diffusion of substances
  • Channel proteins form pores which can transport water soluble and polar molecules through the lipid bi-layer
  • Carrier proteins change shape to move molecules in and out of the cell


  • Used to transport substances possible
  • Carrier proteins use ATP made in respiration to move substances against their concentration gradient.
  • E.G Root hair cells absorb nutrients, such as potassium ions, from water in the soil
  • There are 2 types of bulk transport Endocytosis and Exocytosis
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  • Transports molecules to large or charged to be transported via passive diffusion
  • Protein molecules exist in the membrane to facilitate the diffusion of these substances
  • Channel proteins are transmembrane proteins that form pores for water-soluble molecules. The lining of the pore allows water and  polar substances to pass through
  • Carrier proteins change shape to help move molecules. Molecules bind to the protein, which stimulates it to change its overall shape and allow the molecules to passthrough membrane.


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  • Some substances are in a lower concentration outside the cell than inside it. This means cells can not obtain these substances by diffusion
  • Carrier proteins use energy from ATP made in respiration to move substances against their concentration gradient
  • E.G Root hair cells absorb nutrients, such as potassium and ions, from the water in soil

There are 2 forms of bulk transport:

Exocytosis: Substances packaged by the Golgi apperatus are delivered to the cell cell surface in vesicles, which fuse with the cell surface membrane to push out their contents

Endocytosis: Some cells take up large molecules (such as proteins) and much larger solid objects (such as bacteria) by enclosing them inside vesicles or vacuoles formed by the cell surface membrane

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