3.2.3 Transport Across Membranes

what are plasma membranes?

biological membrane that separates the interior of a cell from its outside environment

• made of a bilayer of phospholipids

• fluid mosaic model

• proteins and cholesterol embedded

• it is impermeable to ions and most water-soluble molecules

describe the structure of a plasma membrane

molecules are arranged within the phospholipid

plasma membrane function

control exit and entry of substances

plasma membrane structure

• phospholipid bilayer,

• molecules embed within and attach onto outside (carbohydrates, protein, cholesterol)

what molecules can pass through a plasma membrane?

• lipid soluble substances - eg: some hormones

• very small molecules - eg: CO2, O2, H2O

what is a partially permeable membrane?

only certain molecules can diffuse through by simple diffusion

lipid soluble (some hormones)

very small molecules - CO2, O2, H2O

NOT: water soluble / polar molecules (sodium ions) and large molecules (glucose)

components of the membrane: phospholipid bilayer

phospholipids align as a bilayer

• due to the hydrophillic heads being attracted to water

• the hydrophobic tails being repelled by water

components of the membrane: proteins

• proteins are embedded across cell surface membrane

• either peripheral or integral

what are carrier proteins?

• bind with other ones

• larger molecules such as glucose and amino acids change shape to transport them to the other side of the membrane

what are carrier proteins in facilliated diffusion?

• carrier proteins will bind with a molecule, such as glucose, which causes a change in the shape of the protein

• this shape change enables the molecule to be releases to the other side of the membrane

what are integral proteins?

span across from one side of the bilayer to the other protein carriers or channel proteins

what are peripheral proteins?

do not extent completely across membrane

provide mechanical support or connected to proteins or lipids to make glycoproteins + glycolipids

function is cell recognition as receptors

what are protein channels in facilliated diffusion?

• form tubes filled with water

• enables water soluble ions to pass through the membrane

• still selective as the channel proteins only open in the presence of certain ions when they bind to the protein

what are protein channels?

form tubes that fill with water to enable water-soluble ions to diffuse

how are carrier proteins specfic?

certain molecules can bind to the receptor site on carrier proteins

what are the properties of a phospholipid head?

phosphate group - negative charge

hydrophilic

what are the properties of a phospholipid tails?

fatty acids - no charge

repel water - hydrophobic

attract other lipids

components of the membrane: cholesterol

present in some membranes

restricts lateral (sideways) movement of other molecules in the membrane

more cholesterol = more rigid

importance of cholesterol

useful as it makes the membrane less fluid at high temperature

• if membranes is too fluid when hot, gaps between phospholipids would be too large

  • water/dissolved ions could leak in or out of cell

  • could cause dehydration and cell could burst

how does cell membrane structure affect movement of substances across a membrane? (9)

1. phospholipid (bilayer) allows movement/diffusion of nonpolar/lipid-soluble substance phospholipid (bilayer)

2. prevents movement/diffusion of polar/ charged/lipid-insoluble substances

3. proteins allow polar/charged substances to cross the membrane/bilayer

4. carrier proteins allow active transport

5. channel/carrier proteins allow facilitated diffusion/co-transport;

6. shape/charge of channel / carrier determines which substances move;

7. number of channels/carriers determines how much movement;

8. membrane surface area determines how much diffusion/movement

9. cholesterol affects fluidity/rigidity/permeability

define simple diffusion

the net movement of molecules from an area of higher concentration to an area of lower concentration until equilibrium is reached

does not require ATP

how do molecules move in simple diffusion?

molecules have kinetic energy enables them to constantly move in fluids (can flow) liquids and gases

define osmosis

the movement of water from an area of higher water potentials to an area of lower water potential across a partially permeable membrane

what is water potential?

the pressure creates by water molecules

measure in kPA

pure water has a water potential of zero

when solutes are dissolved in water, the water potential will become more negative

the more negative the water potential, the more solute must be dissolved in it

what happens as solute is dissolved in water?

water potential becomes more negative

the more negative a water potential, the more solute must be dissolved in it

what is an isotonic solution?

when the water potential is the same in the solution and the cell within the solution

no osmosis

what happens if an animal cell is placed into a hypertonic solution?

lower water potential

water will move out of cell via osmosis

cells will shrink and become shrivelled

what happens if a plant cell is placed into a hypotonic solution?

eg: pure water as hypotonic = higher water potential

a lot of water will move into the cell by osmosis

due to the presence of a cell wall, plant cells will become turgid

what happens if an animal cell is placed in a hypotonic solution?

eg: pure water as hypotonic = high water potential

a lot of water will move into the cell by osmosis

as animal cells have no cell wall, the cell will burst

what is a hypertonic solution?

when the water potential of a solution is more negative than the cell

what is a hypotonic solution?

water potential of a solution is more positive (closer to zero) than the cell

what is active transport?

the movement of molecules and ions from an area of lower concentration to an area of higher concentration (against the concentration gradient) using ATP and carrier proteins

the carrier proteins act as pumps to remove substances across the membrane

very selective as only certain molecules can bind to the carrier proteins to be pumped

where is the ileum?

small intestines

why is active transport and co-transport required in the ileum?

to absorb glucose from the lumen to the gut

their must be a higher concentration of glucose in the lumen compared to the epithelial cell (for facilitated diffusion)

but there is usually a higher concentration in the epithelial cells

steps of active transport

transport is through carrier proteins spanning the cell membrane

1. molecule binds to a receptor complementary in shape on the protein

2. ATP binds to the carrier proteins from the inside of the cell and it is hydrolysed into ADP and Pi

3. this causes the carrier protein to change shape and release the molecule to the other side

4. the phosphate ions is then releases and the protein returns to its original shape

give two similarities between osmosis and diffusion

both are passive processes that do not require ATP

move down a gradient

what is co-transport?

a type of active transport so involves ATP

how glucose and amino acids and absorbed from the ileum into the blood stream

steps of co-transport of glucose and sodium ions in the ileum

1. sodium ions are actively transported out of the epithelial cell into the blood using carrier proteins

2. this reduces the sodium ion concentration in the epithelial cell

3. sodium ions can then diffuse from the lumen down their concentration gradient into the epithelial cell by facilitated diffusion

4. the protein the sodium ions diffuse through is a co-transporter protein so either glucose or amino acids also attach and are transported into the epithelial cell against their concentration gradient

• microvilli on the epithelial cell increases surface area for co-transporter proteins

5. glucose them moves by facilitated diffusion from the epithelial cell to the blood as concentration of glucose in

• the blood is lower than the epithelial cell because the blood flows and carries away absorbed glucose

how is ATP used in active transport?

ATP will bind to the protein on the inside of the membrane and is hydrolysed into ADP and Pi

this causes protein to change shape and open towards the inside of the membrane this causes the molecule to be released on the other side of the membrane

the Pi molecule is then released from the protein, and this results in the protein reverting to its orginal shape

explain how the movement of Na+ out of the cell allows the absorption of glucose into the cell lining the ileum

maintains/generates a concentration gradient for sodium ions from ileum to cell sodium ions can move by facilitated diffusion and co-transport glucose against it's concentration gradient

describe and explain two features you would expect to find in a cell specialised for absorption? (6)

1. Folded membrane/microvilli so large surface area

2. Large number of co-transport/carrier/channel proteins so fast rate (of absorption) OR Large number of co-transport/carrier proteins for active transport

3. Large number of co-transport/carrier/channel proteins for facilitated diffusion

4. Large number of mitochondria so make (more) ATP (by respiration)

5. Large number of mitochondria for aerobic respiration

6. Large number of mitochondria to release energy for active transport