3.2.3 - TRANSPORT ACROSS MEMBRANES

Cell-surface membrane

• Surround cells; act as barrier between cell and environment

• Controls which substances enter/leave cell

• Partially permeable → let some molecules through but not others

Organelle membranes

• Membranes around organelles divide cell into different compartments

• Act as barrier between organelle and cytoplasm

• Partially permeable - control what substances enter/leave organelle

Fluid mosaic structure

• Phospholipid molecules form continuous, double layer (bilayer)

• Bilayer is ‘fluid’ because phospholipids are constantly moving

• Cholesterol molecules are in bilayer

• Proteins scattered through bilayer, like tiles in mosaic

• Contains some glycoproteins

• Some lipids have polysaccharide chain attached - glycolipids

Proteins in cell membranes

• Channel + carrier proteins, allowing large molecules/ions to pass through membrane

• Receptor proteins on cell-surface membrane allow cell to detect chemicals released from other cells

  • Chemicals signal to cell to respond in some way

• Some proteins can move sideways through bilayer, some are fixed in position

• Some proteins have polysaccharide (carbohydrate) chain attached - glycoproteins

Phospholipids in cell membranes

• Phospholipid molecules have ‘head’ and ‘tail’

• Head is hydrophilic - attracts water

  Tail is hydrophobic - repels water

• Molecules automatically arrange themselves into bilayer

  • Heads face out towards water on either side of membrane

• Centre of bilayer hydrophobic → membrane doesn’t allow water-soluble substances (like ions) through it - acts as barrier to these substances

Cholesterol in cell membranes

• Cholesterol is type of lipid, present in all cell membranes (except in bacteria)

• Cholesterol molecules fit between phospholipids

  • They bind to hydrophobic tails, causing them to pack more closely together

  • → restricts movement of phospholipids, making membrane less fluid + more rigid

• Cholesterol helps maintain shape of animal cells (no cell walls)

  • Important for cells that aren’t supported by other cells, e.g. red blood cells (float free in blood)

RP4 method: Effect of temperature on membrane permeability of beetroot

1. Use scalpel to cut five equal-sized pieces of beetroot. Rinse pieces to remove any pigment.

2. Add five pieces to five test tubes, each containing 5cm³ water (measured using measuring cylinder / pipette)

3. Place each tube in water bath at different temp, e.g. 10ᵒC-50ᵒC, for same length of time

4. Remove beetroot pieces from tubes, leaving coloured liquid

5. Use colorimeter - machine that measures absorbance of light
   Higher absorbance = more pigment released = higher permeability of membrane

Effect of temperature on membrane permeability graph

Effect of temperature on membrane permeability (temp below 0ᵒC)

• Phospholipids have little energy, so can’t move much

• They’re packed closely together + membrane is rigid

• Channel + carrier proteins in membrane deform, increasing permeability of membrane

  • Ice crystals may form and pierce membrane, making it highly permeable when it thaws

Effect of temperature on membrane permeability (temp between 0 and 45ᵒC)

• Phospholipids can move around

• They aren’t packed as tightly together - partially permeable

• As temp increases, phospholipids move more because they have more energy → increases permeability of membrane

Effect of temperature on membrane permeability (temp above 45ᵒC)

• Phospholipid bilayer starts to melt → membrane more permeable

• Water in cell expands, putting pressure on membrane

• Channel + carrier proteins deform, so can’t control what enters/leaves cell → increases permeability of membrane

Diffusion definition

Net movement of particles (molecules/ions) from area of higher conc. to area of lower conc.

Concentration gradient

Path from area of higher conc. to area of lower conc.

Particles diffuse down conc. gradient

Diffusion is a ____ process

passive

→ no energy required

Simple diffusion

When molecules diffuse directly through cell membrane

Why is facilitated diffusion needed?

• Some larger molecules (e.g. amino acids, glucose) would diffuse very slowly through phospholipid bilayer because they’re big

• Charged particles, e.g. ions + polar molecules, would diffuse slowly because they’re water soluble, and centre of bilayer is hydrophobic

Facilitated diffusion

• To speed things up, large/charged particles diffuse through carrier/channel proteins in membrane

• Particles move down conc. gradient

Facilitated diffusion is a ____ process

passive - no energy

How do carrier proteins work?

• Move large molecules across membranes

• Diff carrier proteins facilitate diffusion of diff molecules

1. Large molecule attaches to carrier protein in membrane

2. Protein changes shape

3. This releases molecule on opposite side of membrane

How do channel proteins work?

• Form pores in membrane for charged particles to diffuse through

• Diff channel proteins facilitate diffusion of diff charged particles

How does concentration gradient affect rate of diffusion?

Bigger gradient = faster diffusion

→ as diffusion occurs, difference in conc. between two sides of membrane decreases until it reaches equilibrium

→ diffusion slows down over time

How does thickness of exchange surface affect rate of diffusion?

Thinner exchange surface = faster diffusion

→ shorter distance for particles to travel

How does surface area affect rate of diffusion?

Larger SA = faster diffusion

Example of adaptation to increase rate of simple diffusion

Some cells (e.g. epithelial cells in small intestine) have microvilli - projections formed by cell-surface membrane folding up on itself

Microvilli give cell larger SA

Larger SA → more particles exchanged in same time → faster diffusion

How does number of carrier/channel proteins affect rate of facilitated diffusion?

Once all proteins are in use, facilitated diffusion can’t happen any faster

→ greater number of channel/carrier proteins = faster diffusion

Example of adaptation to increase rate of facilitated diffusion

Aquaporins are special channel proteins that allow facilitated diffusion of water through cell membranes

Some kidney cells are adapted to have lots of aquaporins

Aquaporins allow cells to reabsorb a lot of water that would otherwise be excreted by body

Osmosis definition

Diffusion of water molecules across partially permeable membrane, from area of higher water potential to area of lower water potential

Water potential

Potential (likelihood) of water molecules to diffuse out of or into a solution

Pure water has the ____ water potential

highest (0), all other solutions are negative

Isotonic

Two solutions with same water potential

RP3 method: Production of dilution series of a solute to identify water potential of plant tissue

1. Line up five test tubes in a rack

2. Add 10cm³ of 2M sucrose solution to first test tube and 5cm³ of distilled water to the other four test tubes

3. Using pipette, draw 5cm³ of solution from first tube, add it to distilled water in second tube and mix thoroughly

   You now have 10cm³ of solution that’s half the concentration of solution in first test tube (1M)

4. Repeat process three more times to create solutions of 0.5M, 0.25M, 0.125M

RP3 calibration curve: Production of dilution series of a solute to identify water potential of plant tissue

Can use solutions to find water potential of potato cells

1. Use cork borer to cut potatoes into identically sized chips, around 1cm diameter

2. Divide chips into groups of three and measure mass of each group using mass balance

3. Place one group into each sucrose solution

4. Leave chips in solutions for 20 mins

5. Remove chips and dry with paper towel

6. Weigh each group again and record results

7. Calculate % change in mass for each group

8. Use results to make calibration curve, showing % change in mass against sucrose conc.

RP3 analysis: Production of dilution series of a solute to identify water potential of plant tissue

• Potato chips gain water (and therefore mass) in solutions with higher water potential than the chips, and lose water in solutions with lower water potential

• The point where the curve crosses x-axis (% change in mass is 0) is the point where water potential of sucrose solution is same as water potential of potato cells

  → find the conc. at this point, then look up the water potential for that conc. of sucrose solution in a textbook

How does active transport work?

• Molecule attaches to carrier protein

• Protein changes shape → moves molecule across membrane

• Molecule released on other side

Active transport involves ____ proteins

carrier

Differences between active transport and facilitated diffusion

1. Active transport usually moves solutes from low to high conc.
   Facilitated diffusion always moves them from high to low conc.

2. Active transport requires energy - facilitated diffusion doesn’t

How is ATP used in active transport?

ATP undergoes hydrolysis reaction, splitting into ADP + Pᵢ

→ releases energy so solutes can be transported

Active transport of calcium

Co-transporters

• Type of carrier protein

• Bind two molecules a time

• Conc. gradient of one of the molecules is used to move other molecule against its own conc. gradient

  • Diagram shows sodium ions moving into cell down conc. gradient

    → this moves glucose into cell, against its conc. gradient

How does speed of individual carrier proteins affect rate of active transport?

The faster they work, the faster the rate of active transport

How does number of carrier proteins affect rate of active transport?

More proteins = faster rate of active transport

How does rate of respiration affect rate of active transport?

If respiration is inhibited, won’t be any ATP → active transport can’t take place

Why is co-transport needed in the ileum?

In ileum, glucose conc. is too low for glucose to diffuse out of blood

→ glucose is absorbed from lumen of ileum by co-transport

Co-transport of glucose in ileum

1. Sodium ions are actively transported out of ileum epithelial cells, into blood, by Na-K pump

   → creates conc. gradient - now there’s higher conc. of Na⁺ in lumen of ileum than inside cell

2. Causes Na⁺ to diffuse from lumen of ileum into epithelial cell, down conc. gradient, via sodium-glucose co-transporter proteins

3. Co-transporter carries glucose into cell with sodium

   → glucose conc. inside cell increases

4. Glucose diffuses out of cell, into blood, down conc. gradient, through protein channel by facilitated diffusion