Transport Across Cell Membranes Flashcards
Function of the plasma membrane
Selectively permeable which means the membrane allows some materials to freely enter and others to not
Phosholipids
Form a bilayer with hydrophobic tails facing inwards that repel water - fatty acid chains and hydrophilic heads face outwards - phosphate groups
Acts as a barrier to most water soluble substances so they cant leak out of the cell or enter
Cholesterol
Fit in between phospholipid molecules with hydrophobic tails and hydrophilic heads but are absent in prokaryotic membranes
Increase the fluidity of the membrane because it stops the phospholipid tails packing too closely together
Glycolipids and importance
Lipids with carbohydrate chains found on the outer phospholipid mono layer
Act as receptor molecules which allows them to bind with certain substances:
Signalling receptors for hormones and neurotransmitters
Receptors in cell adhesion
Proteins and importance
Intrinsic proteins - transmembrane proteins e.g transport proteins
Create hydrophilic channels to allow ions and polar molecules to travel through the membrane
When do membranes become less/more fluid
More: an increased proportion of saturated fatty acid chains as the chains pack together tightly so there is a high number of intermolecular forces between the chains
Less: unsaturated fatty acids chains are bent which means they are less likely to be packed closely together
What Is diffusion
The net movement as a result of random motion of its molecules of a substance from a region of its higher concentration to a region of its lower concentration
What factors control the rate of diffusion
Steepness of the concentration gradient: a greater difference in concentration means a greater difference in the number of molecules passing in two directions
Temperature : molecules and ions have more kinetic energy at higher temperatures
Surface area : greater number of molecules or ions that can cross at any one moment
Properties of molecules : large molecules diffuse more slowly as they require more energy to move
What is facilitated diffusion
When large polar molecules and ions can’t cross the phospholipid bilayer without the help of certain proteins
What’s a channel protein
Water filled pores which allow charged substances to diffuse through
What’s a carrier protein
Binding site of the carrier proteins is open to one side of the membrane and then open to other side when its changes shape
Osmosis
The net movement of water molecules from a region of high water potential to a region of lower water potential through a partially permeable membrane
What is water potential
The tendency of water to move out of a solution so a dilute solution has a high water potential and a concentrated solution has a lower water potential
High water potential in plants
As water enters the vacuole which expands the protoplasm and pressure builds up
This stops too much water entering and is described as turgid which is important for support and strength
Low water potential in plants
As water leaves the vacuole the volume of the plant cell decreases - the protoplast gradually shrinks and no longer exerts pressure
The protoplast continues to shrink and pull away from the cell wall which known as plasmolysis
What is active transport
Movement of molecules through a cell Membrane from a region of low concentration to a region of higher concentration using energy from respiration
Why does active transport require energy
Energy is required to make carrier proteins change shape which allows it to transfer molecule
What is active transport important in
Reabsorption of useful molecules in the blood after filtration
Absorption of some products of digestion
What is co transport
Coupled movement of substances across a cell membrane via a carrier protein
Absorption of glucose
Sodium ions are actively transported out of the ileum epithelial cells, into the blood by the sodium potassium pump which creates a concentration gradient which causes sodium ions to diffuse from lumen of the ileum into the epithelial cell by sodium-glucose co transport proteins which carries glucose with it and as a result the concentration of glucose inside the cell increases so it then diffuses out the cell into the blood
How is a root hair cell specialised
Adapted for the absorption of water and mineral ions
That have a specialised shape that increases the cell’s surface area so the rate of osmosis is increased
Thinner walls so a short diffusion distance
How is the epithelial cell of the small intestine specialised
- have microvilli which increases the surface area
Each villus has a constant blood supply which continually transports the products of digestion which maintains a high concentration gradient
How are kidney cells specialised for facilitated diffusion
High number of aqua porins which allow kidney cells to reabsorb water
How are neurones and muscle cells specialised for facilitated diffusion
Channel proteins for sodium , potassium and calcium ions which can open and close
They play a role in the speed of electrical transmission
