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 a 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
What happens when a plant cell is placed in a solution of higher water potential?
It becomes turgid
Water enters the cell
The protoplast swells
Protoplast is pushed against the cell wall
What happens when a plant cell is placed in a solution of equal water potential?
Water neither enters nor leaves
The protoplast doesn’t change
Called incipient plasmolysis
Protoplast begins to pull away from the cell wall
What happens when a plant cell is placed in a solution with a lower water potential?
Water leaves the cell
The protoplast shrinks
The cell is plasmolysis
Protoplast is completely pulled away from the cell wall
How do roots collect mineral ions?
Active transport
What is cytosis?
The term for transport mechanisms for moving large quantities of cells
What is endocytosis?
When a molecule causes the cell to bulge inward, forming a vesicles
What is exocytosis?
Movement of material out of a cell
What is phagocytosis?
Endocytosis where an entire cell is engulfed
How are small intestines adapted?
Long + large surface area to aid digestion
What are villi?
Finger like projections
Contain blood capillaries
What are microvilli?
Finger-like projections of cell-surface membrane
Called a brush order as they look like a brush
Provide more surface area for the insertion of protein channels and carrier proteins where diffusion/active transport/facilitated diffusion takes place
How does glucose move into the blood?
Facilitated diffusion
What happens to remaining glucose/amino acids at the end of the small intestine?
They’re moved by active transport
How much of glucose and amino acids are absorbed by the small intestine?
Normally, all of it
What is the ileum?
Another name for the small intestine
What are the steps for the co-transport of sodium/potassium ions?
Sodium ions = actively transported out of epithelial cells via the sodium-potassium pump
Concentration of sodium inside epithelial cells is lowered
Concentration of sodium inside the lumen is higher
Sodium diffuses down this concentration gradient using a co-transport protein in the cell surface membrane
Sodium carries glucose or amino acids into the cells with them
The glucose or amino acids pass into the blood plasma via facilitated diffusion of a different carrier
What are plasma membranes?
The membranes around and within all cells
What is the cell surface membrane?
The membrane that surrounds cells and forms the boundary between the cytoplasm and the environment
Controls movement of substances into and out of the cell
What is the role of phospholipids in the cell-surface membrane?
Two layers of phospholipids make up the membrane
Hydrophilic phosphate heads point to the outside of the membrane, attracted by water on both sides
Hydrophilic tails point towards the centre, repelled by water on both sides
Allows lipid-soluble substances to enter and leave the cell
Prevents water soluble substances from entering and leaving the cell
Makes the membrane flexible and self healing
What is the role of proteins in the cell-surface membrane?
Provide structural support
Act as channels transporting water soluble substances across the membrane
Allows active transport across the membrane through carrier proteins
Forms cell-surface receptors for identifying cells
Helps cells to adhere together
Acts as receptors
What is the role of cholesterol in the cell-surface membrane?
Reduce movement of other molecules
Makes the membrane less fluid at high temperatures
Prevents leakage of water and dissolved ions from the cell
Pulls together fatty acid tails to limit movement without creating a rigid structure
Adds strength
What is the role of glycolipids in the cell-surface membrane?
Acts as a recognition site
Helps to maintain the stability of the membrane
Helps cells to attach to one another and so form tissues
What is the role of glycoproteins in the cell-surface membrane?
Act as recognition sites
Help cells to attach to one another and so form tissues
Allows cells to recognise one another
Cell-surface receptors
What are the functions of membranes within cells?
Control entry and exit of material
Separates organelles from the cytoplasm
Provides an internal transport system
Isolates enzymes that might damage the cell
Provides surfaces on which reactions can occur
Why is the cell-surface membrane known as a fluid mosaic model?
Fluid: individual molecules can move relative to each other, flexible structure, constantly changing shape
Mosaic: proteins vary in shape, size and pattern just like the tiles of a mosaic
Why is the co-transport of glucose into the blood considered indirect active transport?
It is the sodium ion concentration gradient, rather than the ATP directly which powers the movement of glucose
What is the process of active transport involving carrier proteins?
Molecules bind to the carrier protein and ATP attaches to the membrane protein on the inside of the cell/organelle
Binding of phosphate ion to protein causes the protein to change shape so that access for the molecules is open to the inside of the membrane but closed to the outside