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Questions and Answers
What is the main function of the rough endoplasmic reticulum?
What modification occurs to proteins within the endoplasmic reticulum?
Which of the following statements is FALSE regarding endocytosis?
What is a unique characteristic of the smooth endoplasmic reticulum?
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Which type of junction prevents leakage of substances between cells?
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How does the mitochondria contribute to cellular function?
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What is the primary reason that mature red blood cells lack endoplasmic reticulum?
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What role do gap junctions play in cardiac muscle cells?
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What is the main function of the sodium-potassium pump?
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Which mechanism primarily uses ATP directly to move molecules against their concentration gradient?
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How does the nervous system differ from the endocrine system in terms of communication?
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What creates the membrane potential in living cells?
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What is the typical membrane potential maintained by a typical cell?
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Which statement about secondary active transport is true?
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Which example best illustrates tertiary active transport?
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What defines passive transport across a cell membrane?
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What happens to potassium ions when they move out of the cell?
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Which types of molecules can freely diffuse across the lipid bilayer?
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What is a characteristic of facilitated diffusion?
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How does the sodium-potassium pump primarily function?
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What is the concentration gradient responsible for in passive transport?
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Which statement best describes secondary active transport?
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In what manner do gated channels regulate transport across membranes?
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What role do intrinsic membrane proteins play in passive transport?
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Study Notes
Active Transport
- Movement of molecules against the concentration gradient (low to high concentration).
- Requires energy (ATP) and involves transport proteins called pumps.
- Example: sodium-potassium pump
Primary Active Transport
- Directly uses the energy of ATP to move sodium and potassium.
- Example: sodium-potassium pump (present in all cells).
Secondary Active Transport
- Indirectly uses the energy of ATP to transport substances.
- Examples:
- Sodium/Hydrogen Antiporter (in kidney cells).
- Sodium/Glucose Symporter (in the small intestine and kidney cells).
Tertiary Active Transport
- Example: Hydrogen/Peptide Symporter (in the small intestine).
Membrane Potential
- All living cells have a membrane potential (electrically polarized).
- This potential is the separation of opposite charges across the plasma membrane.
- The separation of charges across the membrane is called "potential" because it has the potential to do work.
- This work can be used for communication.
How Sodium and Potassium ATPases influence Membrane Potential
- They maintain the sodium and potassium gradients across the cell membrane.
- A large population of potassium leak channels allows a lot of potassium to leak out of the cell.
- A small population of sodium leak channels allows some sodium to leak into the cell.
- Net Effect: A typical cell maintains a membrane potential of about -70mV, relative to the extracellular space.
Potassium
- The concentration gradient for potassium tends to move this ion out of the cell.
- The outside of the cell becomes more positive as potassium ions move to the outside down their concentration gradient.
- The membrane is impermeable to the large intracellular protein anion.
Osmosis
- Movement of solvent (water) across a semipermeable membrane from a high to low solvent concentration.
- Only the solvent moves.
Transport Across a Plasma Membrane
- One main function of the plasma membrane is to serve as a permeability barrier.
- It is a selective membrane.
- Lipid-soluble substances are free to diffuse, other substances must be transported across the membrane.
Fick’s Equation for Diffusion Across a Lipid Membrane
- Q = the speed at which particles or molecules move from areas of high concentration to areas of low concentration.
- Triangle C: Concentration Gradient of that Substance.
- P = Permeability of that substance (P = lipid partition coefficient).
- A = Surface Area of the barrier (membrane).
- MW = molecular weight of the substance (square root of MW approximates molecular radius).
- Triangle X: Membrane Thickness.
What is the lipid bilayer permeable to?
- Hydrophobic Molecules (oxygen, carbon dioxide, nitrogen, steroids).
- Small Uncharged Polar Molecules (water, glycerol, urea, ethanol).
- These molecules move by diffusion.
What is the lipid bilayer not permeable to?
- Large uncharged polar molecules (glucose and sucrose).
- Ions (sodium, potassium, calcium, hydrogen, chloride).
Passive Transport
- Movement of molecules across the cell membrane from an area of high concentration to low concentration (down the concentration gradient).
- This kind of movement does not require the use of energy.
- Examples: diffusion, osmosis, and facilitated diffusion.
Passive Transport Examples
- Non-lipid-soluble substances cross cell membranes via intrinsic membrane proteins (channels and carriers).
- Channels select what gets through based on size and charge.
- In gated channels, there is a "gate" that opens static pores.
- There are temperature-gated, pH-gated, phosphorylation, and mechanically-gated channels.
- Carriers select what gets through based on binding specificity.
- Facilitated diffusion via carriers relies on specific recognition of the transported substance by the carrier protein AND a concentration gradient.
- Ionic transport via channels relies on specific recognition of the transported ion and a concentration and/or electrical gradient.
Active Transport (in the context of protein synthesis)
- The rough ER, in association with its ribosomes, synthesizes and releases new proteins into the ER lumen.
- Within the ER lumen, a newly synthesized protein is folded into a final conformation and may be modified.
- A new protein is not able to traverse the ER membrane.
- Transport vesicles that fuse with the Golgi complex are formed.
- As the vesicles travel through the ER and Golgi complex they are modified to their final form by post-translational modifications. Contents are sorted, packaged, and directed to final destinations.
- Secretory vesicles receive signals that initiate their fusion and release of vesicle contents by exocytosis.
- The smooth ER does not contain ribosomes but packages the new proteins into transport vesicles.
Endocytosis
- Recycling of Content.
- Fusion with Lysosome for Degradation.
- Reuse and Redistribution.
Endoplasmic Reticulum in Different Cell Types
- Muscle Cell: Sarcoplasmic Reticulum stores calcium.
- Plasma Cell: Secretes huge amounts of antibodies. ER is very developed and occupies the whole cytoplasm.
- Mature Red Blood Cell: It has no nucleus and no ER. After maturation, it becomes a packet of circulating hemoglobin.
Mitochondria
- The site of MOST ATP production.
- Source of energy of the cell and recharges an ADP with phosphate.
The ATP Cycle
- When a cell needs energy, ATP is broken down, or hydrolyzed, into ADP and phosphate, which releases energy.
- The ADP produced is recycled back into ATP by adding a phosphate group.
Specialized Cell Junctions
- Desmosomes: Anchor adjacent cells together, providing mechanical stability and strength to tissues. They involve cytoskeletal components and other proteins.
-
Tight Junctions: Create a barrier to prevent the passage of molecules and ions between cells.
- Intestinal Epithelium: Tight junctions prevent leakage of nutrients and pathogens from the gut lumen into the bloodstream.
- Blood-Brain Barrier: Restricts the movement of harmful substances.
-
Gap Junctions: Intercellular connections that allow direct communication between adjacent cells. They contain protein channels called connexons, which enable the transfer of ions, small molecules, and signaling compounds.
- Cardiac Muscle: Gap junctions allow for synchronized contraction of heart cells by enabling the rapid spread of electrical impulses.
Diffusion
- Movement of molecules from high concentration to low concentration.
- Both the solute and solvent move.
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Description
Explore the various types of active transport mechanisms, including primary, secondary, and tertiary active transport. This quiz covers key examples like the sodium-potassium pump and the significance of membrane potential in cellular functions.
