U2 - Transport Across Cell Membranes

What are the 6 roles of membranes within cells?

1. Controlling entry and exit of materials into / out of organelles

2. Separating cell components from the cytoplasm so specific reactions can take place in specific organelles - mitochondria (aerobic respiration), chloroplasts (photosynthesis)

3. Holding the components of some reactions in place - e.g. ribosomes on rough endoplasmic reticulum

4. Regulating the transport of materials into or out of cells - providing a transport medium - - endocytosis (phagocytosis), exocytosis (secretary vesicles)

5. Provide an internal support system - endomembrane system - important for protein synthesis

6. Isolate potentially damaging enzymes form other cellular content - e.g. hydrolytic enzymes in lysosomes (required for photosynthesis)

Why does a plasma membrane have a “fluid mosaic model”?

• FLUID - The individual phospholipids can move relative to one another

  • This gives the membrane a flexible, self-sealing structure

  • Constantly changes shape

  • Helps phagocytosis and vesicle formation

• MOSAIC - The proteins that are embedded in the bilayer. These vary in shape, size and pattern

Where in the membrane is cholesterol found?

Within the phospholipid bilayer (amongst the lipid tails)

What is the function of cholesterol in the cell membrane?

• Reduces lateral movement of other molecules including phospholipids

• Makes the membrane less fluid at high temperatures

• Prevent leakage of water and dissolved ions from the cell

Where are the intrinsic (channel and carrier) proteins found in the membrane?

Span the whole width of the bilayer

What is the function of channel proteins?

To transport water soluble substances across the membrane

What is the function of carrier proteins?

To allow active transport / facilitated diffusion - e.g. ions, glucose, amino acids

Where is the membrane are extrinsic proteins found?

On the surface or partly spanning the bilayer

What are extrinsic proteins’ functions?

• Provide structural support

• Help join cells together = tissue

• Receptors - e.g. hormones

• Cell-surface receptors for indentifying cells - e.g. immune system

Where are glycolipids found in the membrane?

Partly spanning the bilayer

What is the function of glycolipids?

• Act as cell recognition sites

• Help cells attach to one another to form tissues

Where are glycoproteins found in the membrane?

Partly spanning the bilayer

What is the function of glycoproteins?

• Allows cells to recognise one another - e.g. lymphocytes

• Help cells attach to one another to form tissues

• Act as cell recognition sites

Where in the membrane are phospholipids found?

• Hydrophilic phosphate head outside

• Hydrophobic fatty acid tail inside

What is the function of phospholipids in the bilayer?

• Allows entry and exit of lipid soluble molecules

• Prevents water soluble substances entering / exiting

• Make the membrane flexible and self-sealing

Why do most molecules other than O2 and CO2 not freely diffuse across cell surface membranes?

• Not soluble in lipids, therefore can’t pass through the phospholipid bilayer

• Too large to pass through the channels in the membrane

• Of the same charge as the charge on the protein channels and so, even if they are small enough, they are repelled

• Electrically charged (polar) therefore have difficulty passing through the non-polar hydrophobic tails in the phospholipid bilayer

What 2 things effect the permeability of the membrane?

• Ethanol (alcohol)

• Temperature

How does ethanol affect the permeability of the membrane?

The presence of ethanol will damage cell membranes because they dissolve lipids = increased membrane permeability

How does temperature affect the permeability of the membrane?

Increased temperature =

• Increased kinetic energy

• Increased movement of phospholipids and other components (channel / carrier proteins)

• Increased fluidity and permeability of the membrane

This ultimately affectd the way the membrane proteins are positioned and may function (temp too high, proteins will denature)

What is (simple) diffusion?

The net movement of particles, from a region of higher concentration to a region on lower concentration, down a concentration gradient

What is Fick’s law?

Rate of diffusion is proportional to:

(surface area x concentration difference) / diffusion distance

Rate of diffusion is directly proportional to the surface area of the membrane and concentration difference across the membrane

Rate of diffusion is inversely proportional to the diffusion difference

What is facilitated diffusion?

Difusion that occurs down a concentration gradient that requres membrane-bound proteins

How do channel proteins aid facilitated diffusion?

They form water-filled channels across the membrane. They allow water soluble ions (charged particles) and molecules such as glucose and amino acids to pass through. Channels are selective and specific; each opens only in the presence of a specific molecule

How do carrier proteins aid facilitated diffusion?

They change shape when a particular molecule (e.g. glucose) binds to it, allowing the molecule to move across the membrane from a higher to lower concentration. No external energy (ATP) is needed for this, simply the kinetic energy of the molecules themselves

What is the definition of a solute?

A solid that dissolves in a liquid (e.g. salt in water)

What is the definition of a solvent?

A liquid that dissolves solids (e.g.water)

What is the definition of a solution?

A solute and a solvent mixed together (e.g. salt-water solution)

What is water potential?

The pressure created by water molecules

What is the highest water potential possible?

0KPa - pure water

What is the water potential of any water with solutes in?

Will have a lower water potential. The more solute, the more negative the water potential

What is the definition of osmosis?

The movement of water from an area of higher water potential to an area of lower water potential, down a water potential gradient through a partially permeable membrane. - - water passes through the bilayer through aquaporins (water channel)

What does “hypo” mean?

Lower concentration of solute, therefore a higher water potential

What happens when a cell is put into a hypotonic solution?

Water will move into the cell, from a higher water potential (outside of the cell) to a lower water potential (inside the cell) through a partially permeable membrane via osmosis

What happens when an animal cell is put into a hypotonic solution?

The cell will burst (osmotic lysis)

What happens when a plant cell is put into a hypotonic solution?

The cell will become turgid

What does “hyper” mean?

There is a higher concentration of solute, therefore a lower water potential.

What happens when a cell is put into a hypertonic solution?

Water will move out of the cell, from a higher water potential (inside the cell) to a lower water potential (outside the cell), through a partially permeable membrane via osmosis

What happens when an animal cell is put into a hypertonicn solution?

The cell shrivels

What happens when a pant cell is put into a hypertonic solution?

The cell becomes plasmolysed (cell membrane pulls away from the cell wall)

What does “iso” mean?

There is the same concentration of solute, therefore the same water potential

What happens when a cell is put into an isotonic solution?

There will be no net osmosis as the water potential is the same inside the cell as it is on the outside

What happens when an animal cell is put into an isotonic solution?

The cell remains normal

What happens when a plant cell is put into an isotonic solution?

The cell becomes flaccid (protoplast is beginning to pull away from the cell wall)

What is the definition of active transport?

The movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration using ATP (energy) and carrier proteins.

The movement occurs against a concentration gradient

What is the process of active transport in a cell membrane?

1. Molecule or ion binds to receptor sites on carrier protein

2. On the inside of the cell / organelle, ATP binds to the protein, causing it to split by hydrolysis into ADP and a Pi molecule

3. As a result, the protein changes shape and opens to the opposite side of the membrane

4. The molecule or ion is then released to the other side of the membrane

5. The phosphate ion is released (recombines with ADP → ATP) from the protein, which causes the protein to revert to its original shape, ready for the process to be repeated