Transport across cell membranes

Describe the fluid mosaic model

• Consists of a phospholipid bilayer with a hydrophobic core - which controls passage of substances across it

• Phospholipids can move sideways, making the membrane fluid

• Proteins are embedded in the bilayer (extrinsic proteins act as receptors whilst intrinsic proteins act as channel and carrier proteins)

• Cholesterol molecules maintain membrane stability and flexibility

How does cholesterol regulate the membrane at high temperatures?

• At high temperatures, the phospholipids gain kinetic energy and drift apart more (laterally)

• So the membrane becomes too fluid and there are spaces in the membrane

• So cholesterol binds to hydrophobic tails, restricting movement

• This makes the membrane more rigid and less permeable

How does cholesterol regulate the membrane at low temperatures?

• At low temperatures the phospholipids pack tightly together

• So the membrane could become rigid

• Cholesterol prevents packing together by sitting between tails and preventing them from packing together

• This disrupts the regular arrangement of the bilayer and maintains fluidity (as there are spaces for the phospholipids to move)

Define simple diffusion

Simple diffusion is the movement of small, non-polar, lipid-soluble substances directly across the cell surface membrane from a high concentration to a low concentration down the concentration gradient

Define facilitated diffusion

Facilitated diffusion is the movement of large polar or water-soluble substances from an area of high concentration to low concentration down the concentration gradient using channel or carrier proteins

What factors affect diffusion

• Concentration gradient (affects both)

• Thickness of exchange material [affects simple only]

• Surface area (affects both)

• Temperature (affects both however rate decreases once channel proteins/carrier proteins denature)

• Number of channel and carrier proteins [affects facilitated diffusion only]

Describe Fick’s law

Rate of diffusion ∝ surface area

Rate of diffusion ∝ concentration gradient

Rate of diffusion ∝ 1/Thickness of exchange material

Define osmosis

The net movement of water molecules from an region of high water potential (dilute solution) to a region of low water potential (concentrated solution) across a partially permeable membrane

What factors affect osmosis

• Water potential gradient

• Thickness of exchange material

• Surface are of exchange material

• Temperature

Define water potential

Water potential is the likelihood of water molecules to move into or out of a solution

Describe the movement of water in a hypertonic solution

• Water potential gradient is higher inside the cell than in the solution outside the cell

• So water moves out of the cell by osmosis across the cell membrane down the water potential gradient

• This causes the cell to shrink and the cytoplasm pulls away from the cell wall (plasmolysis)

Describe the movement of water in a hypotonic solution

• Water potential gradient is lower inside the cell than in the solution outside the cell

• So water moves into of the cell by osmosis across the cell membrane down the water potential gradient

• This causes the cell to swell and the cytoplasm to push against the cell wall (turgidity)

• However plants have a cell wall which prevent them from bursting

Describe the movement of water in an isotonic solution

• The water potential inside the cell is equal to the outside solution.

• No net movement of water occurs across the cell membrane

What would happen to animal cells in hypotonic solutions?

• Water moves into cell by osmosis down the water potential gradient

• This causes the cell to swell and expand

• No cell wall to resist high water pressures against cell membrane so above a certain point, cell bursts and cell dies by osmolysis

Define active transport

The movement of ions from a low concentration to a high concentration against the concentration gradient using ATP to change the shape of a carrier protein to carry the molecules across the cell membrane

What affects the rate of active transport?

• ATP availability and rate of respiration

• Number and speed of carrier proteins

• Concentration gradient

Define co-transport

• The simultaneous movement of two different substances across a membrane by a single protein

• One substance usually moves down its concentration gradient, helping the other move against its gradient