Biology - Chapter 4: Transport Across Cell Membranes

What does ethanol do to phospholipds?

Phospholipids dissolve in ethanol

How can you create colour standards to compare the effect of a factor on the permeability the CSM (without using a colorimeter)? (3)

1. Use a known concentration of the coloured solution (e.g. blueberry juice)
2. Prepare dilution series
3. Compare results with colour standards to give concentration

Plasma membranes

All membranes around and within all cells and cell organelles, which have the same basic structure

Cell-surface membrane

The plasma membrane that surrounds cells and forms the boundary between the cell cytoplasm and the environment.

2 functions of cell-surface membranes

1. Allows different conditions to be established inside and outside a cell.
2. Controls the movement of substances in and out of the cell

5 molecules that make up the CSM

1. Phospholipids
2. Proteins
3. Cholesterol
4. Glycoproteins
5. Glycolipids

Why are phospholipids important for the CSM? (structural)

Hydrophilic heads are attracted by water on both sides, point to outside of CSM while hydrophobic tails are repelled by water on both sides, point into centre of CSM.

What type of material moves through the membrane via the phospholipid portion?

Lipid-soluble

3 functions of phospholipids in the CSM

1. Allow lipid-soluble substances to enter/leave cell
2. Prevent water-soluble substances from entering/leaving the cell (as middle is hydrophobic so aqueous molecules can't diffuse through)
3. Make the membrane flexible and self-sealing

2 types of proteins embedded in the phospholipid bilayer

1. Peripheral membrane proteins: embedded in surface of the bilayer, never extending completely across it
2. Integral membrane proteins: completely span the phospholipid bilayer

2 functions of peripheral membrane proteins

- Give mechanical support to membrane
- Act as receptors for molecules (e.g. hormones) in conjunction with glycolipids

2 functions of integral membrane proteins

1. Protein channels: water-filled tubes to allow water-soluble ions to diffuse across membrane
2. Carrier proteins: bind to ions/molecules then change shape to move them across the membrane

6 functions of proteins in the membrane (peripheral, integral, glycoprotein)

- Provide structural support
- Act as receptors for hormones to identify molecules

- Act as channels to transport water-soluble substances across the membrane
- Enable active transport as carrier proteins

- Act as cell-surface receptors to identify cells
- Help cells adhere together

How many phosphate heads and fatty acid tails are there in a phospholipid?

1 phosphate group head and 2 fatty acid tails

3 functions of cholesterol in the CSM

STRENGTH!!!
- Makes membrane more rigid: reduces lateral movement of other molecules (by pulling together the fatty acid tails of phospholipids without making membrane too rigid)
- Makes membrane less fluid: at high temperatures, pulls together fatty acid tails)
- Makes membrane less permeable: prevents leakage of water and dissolved ions from the cell (very hydrophobic)

4 functions of glycolipids in CSM

- Act as cell-surface receptors
- Act as recognition sites
- Help cells attach to one another to form tissues
- Help maintain stability of the membrane

3 functions of glycoproteins

- Act as cell-surface receptors
- Act as recognition sites, more specifically for hormones and neurotransmitters
- Help cells attach to one another to form tissues

5 functions of membranes within cells

1. Controls entry and exit of materials in discrete organelles
2. Separates organelles from cytoplasm so specific metabolic reactions can take place within them
3. Provides an internal transport system
4. Isolates enzymes that might damage the cell
5. Provides surfaces on which reactions can occur

4 reasons why most molecules do not freely diffuse across the cell-surface membrane

1. Not soluble in lipids, can't pass through phospholipid bilayer
2. Too large to pass through channels in membrane
3. Of same charge as charge in protein channels so are repelled
4. Polar (electrically charged) so have difficulty passing through non-polar hydrophobic tails in phospholipid bilayer

What is the model of the CSM called and why?

Fluid-mosaic model:
- Fluid: individual phospholipid molecules move relative to one another so has a flexible structure that constantly changes shape
- Mosaic: proteins embedded in phospholipid bilayer vary in shape, size and pattern (like tiles in a mosaic)

3 facts that enable diffusion to occur

- All particles are constantly in motion due to their kinetic energy
- This motion is random
- Particles are constantly bouncing off one another and other objects

2 types of transport

1. Passive: does not require metabolic energy - relies on kinetic energy from natural motion of particles
2. Active: requires metabolic energy from ATP

Diffusion

The net movement of molecules or ions from a region where they are more highly concentrated to one where their concentration is lower until evenly distributed.

What molecules can diffuse across membranes?

Small, non-polar ones

Why can't charged ions and polar molecules diffuse easily?

The fatty-acid tails in phospholipids are hydrophobic.

Facilitated diffusion

The movement of polar molecules and ions into and out of a cell using carrier and channel proteins.

Where does facilitated diffusion occur?

At specific points on the plasma membrane where there are special protein molecules - protein channels and carrier proteins.

Describe protein channels and how they work (5).

- Water-filled, hydrophilic channels
- Allow water-soluble ions to pass through
- Selective: channels open in presence of specific ion, remain closed if ion is not present
- Ions bind with protein
- Protein changes shape so it closes it to one side of membrane and opens it to other side

Describe carrier proteins and how they work (3).

- Transport polar molecules (e.g. glucose)
- Polar molecule specific to protein binds with it
- Protein changes shape so molecule is released to inside of membrane

Osmosis

The movement of water from a region where it has a higher water potential to a region where it has a lower water potential through a semi-permeable membrane

What is meant by semi-permeable?

They are permeable to water molecules and few other small molecules but not to larger molecules.

Symbol and unit of water potential

Psi - Ψ, measured in kilopascals (kPa)

Water potential

The pressure created by water molecules

Water potential of pure water at standard conditions (100kPa, 25°C)?

0 kPa

What is the effect of adding solute on water potential?

Adding solute lowers the water potential (becomes more negative)

Hypotonic solution

A solution with a higher water potential compared to bodily fluids (lower solute concentration, higher water concentration)

Hypertonic solution

A solution with a lower water potential compared to bodily fluids (higher solute concentration, lower water concentration)

Isotonic solution

A solution with an equal water potential compared to bodily fluids

Effect of hypotonic solution on cells

Water enters cells
Plant: protoplast swells, cell becomes turgid
Animal: Swells and bursts

Effect of isotonic solution on cells

No net movement of water
No change

Effect of hypertonic solution on cells

Water leaves cells
Plant: Shrinks, plasmolysed
Animal: Shrinks

Active transport

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

2 uses of ATP in active transport

1. Directly moves molecules
2. Co-transport: Individually moves molecules using a concentration gradient that has already been set up by active transport

Active transport vs Passive transport (4):

Active:
- Needs metabolic energy in the form of ATP
- Substances are moved against the concentration gradient
- Involves carrier proteins, act as 'pumps' (ONLY carrier proteins, not both like in facilitated diffusion)
- Selective process, specific substances are transported

Describe the process of active transport (5)

1. Carrier proteins that span plasma membrane bind to the molecule/ion to be transported on one side of it
2. Molecule/ion binds to receptor sites on carrier protein
3. On the inside of the carrier protein, ATP binds to it - ATP is hydrolysed into ADP and Pi, protein molecule changes shape, opens to the opposite side of the membrane
4. Molecule/ion is released to the other side of the membrane
5. Phosphate molecule is released from protein: protein returns to original shape so process can repeat + phosphate molecule recombines with ADP to form ATP in respiration

How does the sodium potassium pump work?

3 sodium ions are actively removed from cell, 2 potassium ions are actively taken in

3 ways to increase the rate of movement across cell membranes

1. Increase the surface area (e.g. villi and microvilli) - larger surface area
2. Increase the number of protein channels and carrier proteins in membranes for facilitated diffusion
3. Increase the number of carrier proteins for active transport

In the ileum, why can't all the available glucose and amino acid molecules enter the blood via diffusion?

At best, diffusion results in the concentration in the intestines and in the blood becoming equal

Describe the process of co-transport (5)

1. Sodium-potassium pump: Na+ ions are actively transported out of epithelial cells into the blood using a carrier molecule on the cell membrane (and K+ ions are actively transported into epithelial cells)
2. This generates a concentration gradient of Na+ ions - concentration of Na+ ions in the epithelial cells is lower than in the lumen
2. So Na+ ions move from lumen into epithelial cells via facilitated diffusion down their concentration gradient (using a co-transport protein)
3. The Na+ ions carry glucose/amino acid molecules from the lumen into epithelial cells as they diffuse, moving glucose/amino acid molecules against concentration gradient
4. Glucose/amino acid molecules pass into the blood via facilitated diffusion with another carrier protein

What powers the co-transport of glucose and amino acids?

The concentration gradient of Na+ ions

When carrying out osmosis practical, why should you blot dry the outside of each potato cylinder before reweighing them?

Water will affect the mass of each cylinder - only want to measure mass of the water taken up / lost

How does the cell membrane structure affect the movement of substances across it? (8)

1. Phospholipid bilayer allows movement of non-polar/lipid-soluble substances
2. It prevents movement of polar/lipid-insoluble substances
3. Carrier proteins allow active transport
4. Channel and carrier proteins allow facilitated diffusion/co-transport
5. Shape and charge of the channels determine which substances move
6. Number of channels/carriers determines how much movement
7. Membrane surface area determines how much movement/diffusion
8. Cholesterol affects fluidity/rigidity/permeability

Why do Na+ ions move out of the cells during co-transport?

To maintain a concentration gradient of Na+ ions so they bring glucose/amino acids in with them when they move back into the cell by facilitated diffusion

How can you find the water potential of an object using a dilution series and the ratio of final : initial mass?

1. Plot calibration curve / graph of ratio against concentration of solution
2. Interpolate from ratio of 1
3. Change concentration into water potential

Co-transport

The movement of 2 different substances using a carrier protein (called a co-transport protein)