Biology cells membranes

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Role of membranes

Membranes control what passes through them

  • Cells and many organelles inside them are surrounded by membranes

 Membranes at the surface of cells (PLASMA MEMBRANE):

  • Controll which substances enter and leabe cell
  • Partially permeable
  • Substances can move across plasma membrane by diffusion, osmosis or active transport
  • Allow recognition by other cells (e.g. immune system)
  • Allow cell communication

Membranes within cells:

  • Membranes around organelles divide cell into different compartments - making different functions more efficient (e.g. substances needed for respiration are kept togethere inside mitochondria
  • Membranes of some organelles are folded, increase sa and making chemical reactions more efficient
  • Can form vesicles to tansport substances between different areas of cell
  • Control which substances enter and leabe organelle (e.g. RNA leaves nucleus via nuclear membrane)
  • Partially permeable
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Membranes are very thin

Membranes are visible in the TEM magnification of x100000 as two dark lines, separated by clear space.

Distance across membrane about 7nm

Membranes are made of phospholipids bilayer and proteins

The polar heads are hydrophilic – attracted to water – why they face towards cytoplasm and towards exterior of the cell – dominated by water

Hydrocarbon tails of the two layers are hydrophobic – held together by weak hydrophobic bonds

Proteins scattered about the membranes

Transmembrane proteins pass right through one side to the other

Carbohydrates are attached to protein and lipid, and face outside of cell

Membrane structure is called fluid mosaic

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Components of cell membrane


  • Form bilayer that acts as a barrier between cytoplasm and cell exterior
  • Fluid – components can move within membrane
  • Permeable to non-polar molecules e.g. oxygen
  • Permeable to small polar molecules e.g. ethanol, water
  • Impermeable to ions and large polar molecules e.g. sugars, amino acids

Cholesterol (only in eukaryotic):

  • Stabilises phospholipid blayer by binding to polar heads and non-polar tails of phospholipids
  • Controls fluidity by preventing phospholipids solidifying at low temperatures and becoming too fluid at high temperatures
  • Reduces permeability to water, ions and polar molecules


  • Transmembrane proteins acting as channels and carriers
  • Receptors for chemicals made by other cells e.g. hormones

Glycolipids and glycoproteins (carbohydrate chains attached):

  • Carbohydrate chains only found on the exterior surface of cell membranes
  • Act as receptors for signalling molecules (hormones) and drugs
  • Act as cell surface markers that identify the cells to others (cell surface antigens)
  • Involved in ‘sticking’ cells to one another (cell adhesion) 
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Functions of cell surface membranes

Membranes are partially permeable because some substances pass through but others don’t

Membranes Permeability is determined by phospholipids and proteins

Act as a barrier to many water-soluble molecules

Keep many large molecules, within cell (enzymes)

Permeable to small molecules e.g. water, o2 and co2

Permeable to selected molecules e.g. glucose and ions

Permit movement of substances by endocytosis and exocytosis

Permit recognition by other cells e.g. those of immune system

Provide receptors for signalling molecules e.g. hormones

Often extended into microvilli to provide a large surface area for the absorption of substances by animal cells

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Membranes within cells

Divide cell into compartments where functions can occur more efficiently

Isolate potentially harmful enzymes in lysosomes

Provide large surface for holding the enzymes and coenzymes for forming ATP in mitochondria and chloroplasts

Surround vesicles that transport molecules between parts of the cell e.g. that transport proteins from RER to golgi apparatus. 

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Exchanges across membranes

Substances are exchanged between cells and their surroundings across cell surface membranes

Cells obtain all their requirements in this way and their waste substances and some of their products pass in the opposite direction

Molecules move across by diffusion, active transport, bulk transport etc

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Net movement of particles from area of high conc. to low conc. 

  • Molecules wil diffuse both ways 
  • Net movement will be to area of lower conc. until particles are evenly distributes throughout
  • Conc. gradient is the path from an area of high conc. to low conc. 
  • Particles diffuse down a conc. gradient
  • Diffusion is passive process - no energy needed
  • Particles can diffuse across plasma membranes if they can move freely through membrane (e.g o2 and co2 molecules are small enough to pass easily through spaces between phospholipids)

Rate of diffusion depends on:

  • Conc gradient - higher it is faster the rate of diffusion
  • Thickness of exchange surface - thinner the exchange surface (ie. shorter distance particles have to travel) the faster the rate of diffusion
  • Surface area - larger sa (e.g. of cell membrane) faster the rate of diffusion
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The movement of water from a high conc to a low conc across a partially permeable membrane

Water molecules polar and small enough to pass through phospholipid bilayer.

Membranes of some cells (e.g. RBC) are permeable to water as they have channel proteins for water - aquaporins. 

Movement is influenced by:

  • amount of water in cytoplasm and exterior environment
  • conc. of solutes (ions and sugars) on either side of CSM
  • presence of aqauporins in membranes
  • pressure exerted on cell contents by cell wall which is rigid and resists expansion and thus the uptake of water

(diagram p16)

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Water potential

The tendency for water to move from one place to another

Determined by osmosis and diffusion

Solutions with high water potential (WP) have few solute molecules

Solutions with low water potential have many dissolved solute molecules

Water moves from a solution with high WP to lower WP.

The diffusion of water through a partially permeable membrane down a WP gradient is osmosis

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Water potentials on plant and animal cells

Hypertonic - solution with a higher water potential than the cell

Isotonic - solution with the same water potential as the cell

Hypertonic - solution with a lower water potential than the cell

Plant cells:

  • Hypotonic solution:
    • Net movement of water is into cell
    • Vacuole swells
    • Vacuole and cytoplasm push against cell wall
    • Cell becomes turgid (swollen) 
  • Isotonic solution:
    • Water molecules move into and out of cell in equal amounts
    • Cell stays the same 
  • Hypertonic solution:
    • Net movement of water is out of cell
    • Cell becomes flaccid
    • Cytoplasm and membrane pull away from the cell
    • Called plasmolysis

Animal cells:

  • Hypotonic solution:
    • Net movement of water molecules is into cell
    • Cell bursts
  • Isotonic solution:
    • Water molecules pass into and out of cell in equal amounts
    • Cell stays the same
  • Hypertonic solution:
    • Net movement of water molecules is out of cell
    • Cell shrinks
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Facilitated diffusion

Many molecules cells need are too large to pass between phospholipid molecules

May be charged and therefore unable to pass through hydrophobic region in the centre of the bilayer

Protein molecules exist in membranes to help (facilitate) the diffusion of these substances

Channel proteins:

  • transmembrane proteins that form pores through the bilayer for water-soluble molecules
  • some channels are open all the time others open when triggered by signalling molecules such as hormones
  • lining of the pore allows water and polar substances to pass through

Carrier proteins:

  • change shape to help move molecules into or out of the cell
  • molecules bind to the protein which stimulates the protein to change its overall shape so allows the molecule to pass through the membrane
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Active transport

Some substances required by cells are in lower conc outside than inside the cell. Cells cannot obtain these substances by diffusion 

Carrier proteins use energy from ATP made in respiration to move substances agains conc gradient

Root hair cells absorb nutrients (e.g. potassium ions) from water in soil

Active transport used to pump molecules and ions out of cells

Bulk transport forms:

  • Exocytosis - substances packaged by golgi apparatus are delivered to cell surgace in vesicles, which fust with CSM to push out their contents
  • Endocytosis - some cells take up large molecules (e.g. protens) and much larger solid objects (e.g. bacteria) by enclosing them inside vesicles or vacuoles formed by CSM
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Cell signalling

Cell signalling - the way cells communicate with one another 

  • Using messenger molecules:
    • One cell releases a messenger molecule (hormone)
    • This molecules travels to another cell (e.g, in blood)
    • The messenger molecule is detected by the cell because it binds to a receptor on its cell membrane

Cell membrane receptors play important roll in cell signalling:

  • Membrane bound proteins act as recptors for messenger molecules
  • Receptor proteins have specific shapes - only messenger molecules with a complementary shape can bind to them
  • Different cells have different types of receptors - they respond to different messenger molecules
  • Target cell is a cell that responds to a particular messenger molecule 
  • (diagram p14)

Drugs also bind to cell membrane receptors:

  • Many drugs work by binding to receptors in cell membranes
  • They either to trigger a response in the cell, or block the receptor and prevent it from working
  • E.g. antihistamines - cell damage causes the release of histamine, which binds to receptors on surface of other cells and causes inflammation. Antihistimes block histamine receptors on CS, preventing histamine from binding to cell and stops inflammation. 
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Membranes affected by temperature

Temperature below 0degrees:

  • Phospholipids dont have much energy so cant move much
  • Packed closely together and membrane rigid
  • Channel and carrier proteins in membrane denature increasing permeability of membrane
  • Ice crystals may form and pierce membrane making highly permeable

Temperature between 0+45degrees:

  • Phospholipids can move around and not too tightly packed
  • Membrane partially permeable
  • As temp increases phospholipids move more as more energy increase permeability of membrane

Temperatre above 45degrees:

  • Phospholipid bilayer starts to melt
  • Membrane becomes more permeable
  • Water inside cell expands putting pressure on membrane
  • Channel + carrier proteins in membrane denature so can control what enters or leaves cell - increases permeability of membrane
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