Sec 6- Membranes Flashcards
Unit II- Life of a Cell
What are plasma membranes made of?
Phospholipids and proteins
Membrane Proteins
- Membrane proteins that interact with molecules outside of the cell are called receptors.
- They are improtant for cell-to-cell signaling.
- Viruses can take advantage of these receptors and bind to (and enter) our cells.
What is the function of membrane proteins?
- Anchor the cell to the ECM
- Transport substances across the membrane
- Catalyze enzymatic reactions near the cell surface
- Receive information from other cells in the form of chemical or electrical signals
- Serve as identification tag
What is the movement of molecules across the Plasma Membrane?
For metabolism to work, a cell must keep its internal composition stable- even when conditions outside are greatly different.
What is selective permeability?
Membrane property that allows some substances, but not others, to cross.
- Nonpolar, small molecules easily cross
- Ions and/or large molecules can’t cross
What is concentration?
The number of molecules or per unit volume of a fluid.
What is the concentration gradient?
Difference in concentration of a substance between adjoining regions of fluid.
What is passive transport?
Substance moving down its concentration gradient drives it across a cell membrane.
- Requires no direct energy input
- Diffusion (including osmosis)
- can be through the phospholipids directly or through a transport protein (pore)
- Molecules will move down it’s own concentration gradient until equilibrium is reached.
- This movement is independent of other molecules and their concentration gradients.
What is active transport?
A transport protein use energy, usually from ATP, to pump a substance across a cell membrane against its concentration gradient (pump).
What is diffusion?
Molecules and ions tend to follow their own concentration gradient and fdiffuse into an adjoing region in which they are less concentrated.
- net movement of molecules from a region of higher concentration to a region of lower concentration
What is diffusion rate?
How quickly a particular solute diffuses through a particular solution depends on various factors:
1. size
2. temp
3. steepness of the conc. gradient
4. solvent density
5. pressure
6. distance travelling
- Diffusion rate across a membrane is also influenced by:
1. solubility (nonpolar vs polar/ion)
2. surface area of membrane
3. thickness of membrane
Osmosis
Net diffusion of water molecules across a selectively permeable membrane between two fluids with different water concentrations.
- Passive transport
- water molecules tend to diffuse in response to its own concentration gradient
What is tonicity?
Describes relative concentrations of solutes in fluids separated by a selectively permeable membrane.
- Hypertonic solution
- Hypotonic solution
- Isotonic solution
What is a hypertonic solution?
Has a higher concentration of solute/stuff.
Has a low concentration of water.
What is a hypotonic solution?
Has a lower concentration of solute.
Has a high concentration of water.
What is an isotonic solution?
Has an equal concentration of solute/stuff.
Has an equal concentration of water.
What is osmosis?
Osmosis is the diffusion of water across a membrane from the region with a higher water concentration (low solute concentration = hypotonic) toward the region with a lower water concentration (high solute concentration = hypertonic).
Osmosis will continue until the two fluids are isotonic.
What happens when a cell is not in an isotonic solution?
Cells burst or shrink.
Ex: RBCs
Maintain the tonicity of extracellular fluids is an improtant part of homeostasis.
What is paramecia?
Paramecia are single-celled organisms that live in freshwater environments (ponds).
- They are surrounded by a hypotonic environment that they can’t control.
- They have contractile vacuoles, which actively pump water tot he outside of the cell.
- Otherwise they would burst because water continuously rushes in through osmosis!
How does osmosis affect plant cells?
Cells lose turgor pressure in hypertonic solutions (when not being watered).
- Hypertonic- water goes out of cell
- Isotonic- plant cell is healthy
- Hypotonic- vacuole is full of water
What are the different membrane crossing mechanisms?
- Directly- Gases and small nonpolar molecules can diffuse across a lipid bilayer.
- Transport proteins- Most other moluecules and ions cross only with the help of transport proteins, which gives a cell or membrane-enclosed organelle control over which substances enter and exit.
- Water crosses membranes both ways
What are the two types of transport proteins?
- Channel proteins
- Carrier proteins
* Each type of transport protein moves one (or sometimes two) specific molecule(s) across a membrane.
What are channel proteins?
- Barrel shaped proteins
- Makes a tunnel through the plasma membrane
- Always passive transport
What are carrier proteins?
- Carrier proteins undergo conformational changes
- Passive or active depending on concentration gradient
What is passive transport: Facilitated Diffusion?
- Passive transport is when no energy is needed to transport.
- Glucose from the blood enters RBCs through a carrier protein.
- No energy is required because glucose moves down its concentration gradient.
- The RBC adds a phosphate group to the glucose once it enters the cell to maintain the concentration gradient.
What is passive transport in osmosis?
Aquaporins are channel proteins specific for water.
- Water molecules move down their concentration gradient.
What is active transport: Calcium Pump?
Requires a direct input of ATP.
- Moves against the concentration gradient.
- Muscle cells have calcium pumps to keep the calcium levels low in the cytosol.
What is active transport: Sodium-Potassium Pump?
Uneven exchange: Two K^+ enter for every three N^+ exiting.
- In all animal cells
- Creates an electrical potential; more positive outside than inside of the cell.
- Membrane potential
- Electrochemical gradient
WHat is active transport: Cotransport?
The sodium concentration gradient that is established by the sodium-potassium pump can be used to transport glucose into a cell (hitches a ride) against its concentration gradient.
What are the different types of transport proteins?
- Uniporter: one molecules or ion
- Symporter: two different molecules/ions in same direction
- Antiporter: two different molecules/ions in different direction
How does the membrane trafficking of larger molecules?
Patches of membrane constantly move to and from the cell surface as vesicles that fuse with or pinch off from the plasma membrane.
- The lipid bilayer reseals itself when the membrane is disrupted.
- Molecules move across the lipid bilayer by exocytosis and endocytosis.
- Exocytosis and endocytosis require energy (active transport).
What is active transport: Exocytosis?
Ex:
* Expelling waste from the cell
* Releasing ECM proteins
* Neurotransmitter release into synaptic cleft
What is active transport: Endocytosis?
Two types:
1. Phagocytosis (eating)
2. Pinocytosis (drinking)
What is phagocytosis?
- Macrophages and other white blood cells engulf particles such as microbes or cellular debris
- Amoebas engulf food, like algae
What are receptor-mediated endocytosis?
- Example is the removal of “bad” cholestrerol from the blood (LDL)
- Viruses take advantage of cell receptors to enter cells
Diffusion
- Active
- Phospholipids
- Small molecules
Osmosis
- Passive
- Phospholipids
- Water
Facilitated Diffusion
- Passive
- Proteins
- Glucose
(Facilitated) Osmosis
- Passive
- Proteins
- Water
Active Uniporter
- Active
- Proteins
- Calcium
Symporter
- Active/Passive
- Proteins
- Glucose and sodium
Antiporter
- Active
- Proteins
- Sodium-potassium
Exocytosis
- Active
- Vesicle
- Waste disposal
Endocytosis
- Active
- Vesicle
- “Bad” cholesterol into cell, Macrophages eating bacteria, Fluid intake
- Phagocytosis
- Pinocytosis
