2. Transport Across Cell Membranes Flashcards
What is passive transport?
No energy required (no ATP expenditure)
High to low concentration gradient
What is active transport?
Against concentration gradient (low to high)
Requires energy in the form of ATP
Need carrier proteins found in plasma membrane
How do large particles pass into a cell? What is formed?
Endocytosis - active (needs ATP).
Plasma membrane surrounds the particle, forming a membrane-bound vesicle that pinches off the membrane.
How do large proteins pass out of a cell?
Exocytosis. Energy required.
Diffusion
Net movement of molecules or ions from a region of high concentration to a region of low concentration
3 features of simple diffusion
- Relies on random movement
- Continues until equilibrium
- Passive
What types of particles undergo simple diffusion?
Small, uncharged, hydrophobic & non-polar molecules (oxygen & CO2)
What types of particles undergo facilitated diffusion?
Hydrophilic molecules
Ions larger than CO2
Polar/charged molecules
Features of facilitated diffusion
- Water-filled channel proteins
- Carrier proteins that change shape
- Passive
- Ion/molecule specific
Facilitated diffusion: either channel or carrier, or both?
Either channel or carrier!
What is the rate of facilitated diffusion limited by?
Number of channel/carrier proteins
3 features of channel proteins
- Span the membrane
- Some are gated (e.g. open/close when hormone is present/absent)
- Voltage-gated channels open & close depending on potential difference - this occurs in nerve impulses!
3 features of carrier proteins
- Molecule or ion binds to specific site
- Protein changes shape to allow molecule across the membrane - but the process is STILL PASSIVE!!
- Movement in either direction but ALWAYS DOWN THE CONCENTRATION GRADIENT!!
Osmosis
Diffusion of water from high to low water potential across a partially permeable membrane
4 features of osmosis
- Passive
- Requires a partially permeable membrane
- Continues until equilibrium
- Only FREE water molecules can move - not those associated with solute molecules
Hypertonic solutions
Solution has a lower water potential
Isotonic solutions
Both have equal water potentials
Hypotonic solutions
Solution has a higher water potential
Active transport
Movement of substances against the concentration gradient
3 features of active transport
- ALWAYS involves a carrier protein (specific for molecules/ions)
- Requires energy in the form of ATP
- Rate-limited
What is ATP used for in active transport?
To change the shape of the carrier proteins to move the molecule to the other side of the membrane
Why is active transport rate-limited?
Depends on number of carrier proteins present in the membrane
Where does active transport occur?
In ALL CELLS!!
The two example of bulk transport are…
Exocytosis and endocytosis
What is exocytosis?
Release of substances - usually proteins and polysaccharides
Examples of exocytosis
Insulin & neurotransmitters
What is endocytosis?
Substances taken into cells by the creation of a vesicle
Examples of endocytosis
Phagocytosis
Endocytosis special case
Substances may first attach to receptors on membrane (e.g. cholesterol)
Difference between epithelial cell and cilia
Cilia have hairs; epithelial cells have folds
How is ileum adapted to increase rate of absorption? (4 things to mention)
- Villi - increases surface area
- Microvilli - increases surface area
- Length of small intestine - increases surface area
- Large surface area (provided by the 3 things above) for the insertion of channel/carrier proteins for facilitated diffusion, and carrier proteins for active transport. High density of channel/carrier proteins.
What are villi?
Folds of the wall of the small intestine
What are microvilli?
Extensions of the cell membrane of individual epithelial cells
Co-transport of glucose is a type of…
Active transport!
4 steps of co-transport of glucose
- Na ions are actively transported using ATP from the epithelial cells into the blood by a carrier protein (sodium-potassium pump)
- This maintains a higher concentration of Na ions in the lumen of the ileum than inside the epithelial cell
- Na ions diffuse down the conc. gradient through a carrier protein called a co-transport protein, carrying glucose with it.
- Glucose moves via facilitated diffusion into the blood
Why is the process called co-transport (in the case of glucose)?
It relies on Na ions being actively transported into the bloodstream!
The only place where active transport takes place is step 1!
Co-transport of glucose diagram
See Notion
Co-transport is what type of process?
Active!
Even though some individual steps (i.e. facilitated diffusion) are passive
