Transport across membranes
Diffusion
Simple diffusion
Facilitated diffusion
Movement of molecules from an area of high concentration of that molecule to an area of low concentration
Molecules move down the concentration gradient
ATP is not needed
Small molecules can pass through the cell membrane via simple diffusion
Eg oxygen and carbon dioxide
Movement of molecules from an area of high concentration of that molecule to an area of low concentration
Across a partially permeable membrane
Molecules move via protein carriers or channels
Does not involve ATP
Fat soluble molecules can diffuse through the cell membrane if they dissolve in the lipid bilayer
Eg steroid hormones
Water soluble molecules diffuse via water channel proteins called aquaporins
Happens in membranes where a high rate of water movement is required
Simple diffusion is also possible
Water is present in high concentration in the cell membrane
Maintenance of the concentration gradient
Oxygen
Diffuses into the cytoplasm of respiring cells
Then diffuses into mitochondria
Used for aerobic respiration
Carbon dioxide
Carbon dioxide
Diffuses into the palisade mesophyll cells
Diffuses into chloroplasts
Used for photosynthesis
Factors affecting rate of simple diffusion
Temperature
Diffusion distance
Surface area
Size of molecule
Concentration gradient
Temperature increases
Molecules have more kinetic energy
Rate of diffusion increases
Temperature decreases
Molecules have less kinetic energy
Rate of diffusion decreases
Thick membrane
Greater diffusion distance
Rate of diffusion decreases
Thin membrane
Shorter diffusion distance
Rate of diffusion increases
Larger surface area
Large volume of molecules can pass through at one time
Rate of diffusion increase
Small surface area
Small volume of molecules pass through at one time
Rate of diffusion decreases
Small molecule
More rapid diffusion
Large molecule
Slower diffusion
Steep gradient
Rate of diffusion increases
Shallow gradient
Rate of diffusion decreases
Small molecules with polarity are insoluble in the lipid bilayer
Cannot interact with the hydrophobic tails of the lipid bilayer
Cholesterol
Reduce permeability of the membrane to small water molecules
Osmosis
Passage of water molecules
Water molecules move down the concentration gradient
A partially-permeable membrane is present
Water potential
Measure of the tendency of water molecules to diffuse from one region to another
Pure water has the highest water potential
The addition of solute molecules lowers the water potential
Area of higher water potential
Area of lower water potential
0 kilopascals (kPa)
The numerical value will be negative in kPa
Water potential in cells
Water potential is lower due to the presence of solutes
When cells are placed in a solution of higher water potential, the water molecules move by osmosis into the cell
Animal cells
Plant cells
A large volume of water molecules enter the cell
Cell swells and bursts as plasma membrane breaks
Cytolysis
Rigid and strong cellulose wall prevents cell from bursting
Cell swell up to a certain size
Contents push against the cell wall
Cell is turgid
When cells are placed in a solution of lower potential, water leaves the cell via osmosis
Animal cells
Plant cells
Shrivel and described as crenated
Cytoplasm shrinks
Membrane pulls away from cellulose cell wall
Cell is described as plasmolysed
Plant tissue with plasmolysed cells is described as flaccid
Suffer a degree of dehydration
Metabolism cannot proceed as enzyme-catalysed reactions need to be in the solution
Active transport
The movement of substances against their concentration gradient
ATP and protein carriers are used
Energy is provided by the hydrolysis of ATP
Cells sometimes need to accumulate more of a particular ion than they could by simple or facilitated diffusion
Root hair cells
Active transport is used to absorb ions from soil
Carrier proteins
Have specific regions or sites combine reversibly with only certain solute molecules or ions
Have a region that binds the hydrolysis of ATP to release energy
Act as enzymes
Energy enables carrier proteins change its conformation
Ion is carried from one side of the cell to the other
Guard cells
ATP made by chloroplasts
Provides energy to actively transport potassium ions from surrounding cells in the guard cells
Influx of ions lowers the water potential in cell
Water enters surrounding cells via osmosis
Guard cells swell
Tips bulge
Stoma opens
Bulk transport
Endocytosis
Exocytosis
Large particles cannot be brought into the cell
Instead, a segment of the plasma membrane surrounds and encloses the particle
Particle is brought into the cell in a vesicle
Phagocytosis
When a phagocytic cell ingests a bacterium
Refers to the type of intake of solids matter
Pinocytosis
Ingestion of liquids
Exportation of large molecules out of the cell
ATP is required
Molecules are enclosed in a vesicle
Vesicle fuses into the membrane
Moved using molecular motor proteins along cytoskeleton threads into the cell interior
A molecule of ATP is hydrolysed for every step a motor protein takes along the cytoskeleton thread as it moves the vesicle