3.2.3 Transport Across Cell Membranes Flashcards
What does the cell membrane do?
Separates the living cell from its environment and controls traffic in and out of the cell
Made of a collage of proteins and other molecules embedded in a fluid matrix of the lipid bilayer
What is diffusion?
The net movement of molecules/ions from a region of high concentration to a region of low concentration
Moves down the concentration gradient
Molecules have kinetic energy which makes them move randomly
Molecules reach an equilibrium where there is no net movement
Passive process, no energy is required
What molecules can’t diffuse across the cell membrane?
Not lipid soluble so cannot 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 so are repelled
Polar so have difficulty passing through the non-polar hydrophobic tails in the phospholipid bilayer
The fatty acid tail is small so it mostly doesn’t obstruct water from diffusing through the phospholipid bilayer
What is the rate of diffusion affected by?
Steepness of concentration gradient
Temperature
Surface area
Type of molecule/ion
What is facilitated diffusion?
The passive process of facilitating diffusion through transmembrane channels
Large polar molecules (such as amino acids/glucose/ions) cannot diffuse so they need facilitated diffusion
Molecules go through a protein channel instead of the phospholipids, molecules will randomly move through the pore from an area of high concentration to low concentration
What is a protein channel?
A water-filled hydrophobic channel that allows specific water-soluble ions through
Each channel opens at the presence of a specific molecule
Ions bind with the protein causing it to change shape in a a way that closes it to one side of the membrane and opens it to the other side
What are carrier proteins?
Proteins involved in facilitated diffusion and active transport
Each carrier is very specific about what it carries and how it carries it
What is osmosis?
The diffusion of water molecules from an area of high concentration to an area of low concentration of water
Needs a partially permeable membrane
Water tries to dilute out molecules that can’t move across the membrane until concentration is equal
How does osmosis affect cells?
Cells placed in diluted water will burst because too much water will enter via osmosis
Cells placed in a concentrated solution will shrivel as water will leave via osmosis
A cell wall will stop a plant cell from bursting
What is water potential?
The pressure created by water molecules measured in kiloPascals
The addition of a solute to pure water lowers its water potential, the more that is added, the lower the water potential is
Under standard conditions of temperature and pressure, pure water has a water potential of zero
What is active transport?
The movement of molecules or ions into/out of a cell from a region of lower concentration to a region of higher concentration using ATP and carrier proteins, requires energy so is active
How is ATP used in active transport?
Used to individually move molecules using a concentration gradient which has been set up by direct active transport, known as cotransport
Metabolic energy in the form of ATP is needed
Carrier proteins act as pumps to transport materials
What are the steps of active transport for a single ion?
Carrier protein spans plasma membrane and binds to a molecule/ion to be transported on one side of it
Molecule/ion binds to receptor sites on carrier protein
On inside of organelle, ATP binds to protein, making it split into ADP and a phosphate molecule, so the protein molecule changes shape and opens to the opposite side of the membrane
Molecule/ion is released on the other side of the membrane
Phosphate molecule is released from protein which causes protein to revert to original shape, ready for the process to be repeated
Phosphate molecules combine with ADP to form ATP during respiration
What is water potential?
The tendency of water to move
Represented by 🔱
Water potential of pure water is 0
What are the types of water potential?
🔱s = dissolved substances, solute potential, always a negative number 🔱p = pressure exerted on the membrane/wall, pressure potential
What is a selectively permeable membrane?
A membrane that only allows water molecules across it but not solute molecules
Why does a plant cell not burst?
It has a rigid cell wall which prevents the membrane from bursting because the protoplast of the cell is kept pushed up against the cell wall and doesn’t brake
Does the cellulose cell wall/plasma membrane allow water to move across it?
Cellulose cell wall: freely permeable to water and to solutes
Plasma membrane: freely permeable to water but of limited permeability to solutes
