Cell Membranes & Transport (College Board AP® Biology): Flashcards
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Define phospholipid.
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Define phospholipid.
Phospholipid is a molecule consisting of a phosphate head and two fatty acid tails that forms the basic structural framework of cell membranes.
Which region of a phospholipid is hydrophilic, and which is hydrophobic?
The phosphate head is hydrophilic because it is polar.
The fatty acid tails are hydrophobic because they are nonpolar.
The polar hydrophilic phosphate regions of phospholipids are oriented toward the environment.
The polar hydrophilic phosphate regions of phospholipids are oriented toward the aqueous environment.
How are the hydrophobic fatty acid tails arranged within a phospholipid bilayer?
They face each other within the interior of the membrane, oriented away from water.
True or False?
Embedded membrane proteins can be hydrophilic, hydrophobic, or both.
True.
Proteins can have hydrophilic regions (charged and polar side groups), hydrophobic regions (nonpolar side groups), or both.
Where are the hydrophilic regions of an embedded membrane protein located?
Either folded within the interior of the protein, or exposed to the cytosol (cytoplasm).
The hydrophobic regions of an embedded protein interact with the in the interior of the membrane.
The hydrophobic regions of an embedded protein interact with the fatty acids in the interior of the membrane.
Define glycoprotein.
Glycoprotein is a protein with an attached carbohydrate chain, involved in cell recognition and cell signaling.
Define glycolipid.
Glycolipid is a lipid with an attached carbohydrate chain, involved in cell recognition and cell signaling.
What is the role of cholesterol in the cell membrane?
Cholesterol is a steroid lipid that regulates membrane fluidity.
Define fluid mosaic model.
Fluid mosaic model is a model describing the plasma membrane as a structural framework of phospholipids with embedded components (proteins, steroids, glycoproteins, and glycolipids) that can move around within the membrane.
According to the fluid mosaic model, embedded components can around the surface of the cell within their membrane layer.
According to the fluid mosaic model, embedded components can move around the surface of the cell within their membrane layer.
True or False?
In the fluid mosaic model, only the phospholipids are able to move within the membrane.
False.
Embedded components such as proteins, steroids, glycoproteins, and glycolipids can also move around within their membrane layer.
Define selective permeability.
Selective permeability is the ability of the plasma membrane to control which substances can cross between the internal and external environment of the cell, resulting from its hydrophobic interior.
What feature of the plasma membrane makes it selectively permeable?
Its hydrophobic interior, formed by the nonpolar fatty acid tails of the phospholipids.
Small, molecules, including N₂, O₂ and CO₂, freely pass across the membrane.
Small, nonpolar molecules, including N₂, O₂ and CO₂, freely pass across the membrane.
Why can small, nonpolar molecules such as O₂ and CO₂ pass freely across the membrane?
They can interact with the nonpolar fatty acid tails of the phospholipids and are small enough to pass between them.
True or False?
Large, polar molecules such as glucose can pass freely between the phospholipids of the membrane.
False.
Large, polar molecules and ions cannot pass between the phospholipids, because they cannot interact with the hydrophobic interior; they cross only via transport proteins.
How do large, polar molecules and ions move across the plasma membrane?
Through embedded channels and transport proteins.
The substances that can cross are determined by the number and type of transport proteins present.
Small, polar, uncharged molecules, like H₂O and NH₃, pass through the membrane in amounts.
Small, polar, uncharged molecules, like H₂O and NH₃, pass through the membrane in small amounts.
True or False?
Water relies mainly on aquaporins to cross cell membranes.
True.
Although small amounts of water can slip between the phospholipids, it relies mainly on aquaporins (water channel proteins) to cross.
Define cell wall.
Cell wall is a structural boundary outside the cell membrane that provides rigidity, acts as a permeability barrier for some substances, and protects against osmotic lysis.
Which groups of organisms have cell walls in addition to a cell membrane?
Bacteria
Archaea
Fungi
Plants
How does the cell wall help prevent osmotic lysis?
Its rigid structure limits the volume of water a cell can take up, preventing the cell from bursting.
True or False?
Cell walls are freely permeable to all substances.
False.
Cell walls are freely permeable to small molecules, but can act as a permeability barrier for some substances, such as large molecules.
Define passive transport.
Passive transport is the net movement of molecules from a region of high concentration to a region of low concentration, without the direct input of metabolic energy.
Define active transport.
Active transport is the movement of molecules across a membrane that requires the direct input of energy, often moving substances from a region of low concentration to a region of high concentration.
Define concentration gradient.
Concentration gradient is a difference in the concentration of a substance across a membrane, from a region of higher concentration to a region of lower concentration.
How does the selective permeability of a membrane allow concentration gradients to form?
A selectively permeable membrane controls which substances cross it, so it can prevent the passage of a substance or actively pump it to one side. This allows a higher concentration to build up on one side of the membrane.
What factors determine which mechanism is needed to move a substance across a membrane?
The nature of the substance, e.g. whether it is small, large, polar or nonpolar
The concentration gradient across the membrane
In passive transport, molecules move down their gradient.
In passive transport, molecules move down their concentration gradient.
Name three examples of passive transport.
Simple diffusion
Facilitated diffusion
Osmosis
Define simple diffusion.
Simple diffusion is the movement of molecules down a concentration gradient, directly across the phospholipid bilayer, without the input of metabolic energy.
True or False?
Active transport can move molecules from a region of low concentration to a region of high concentration.
True.
Active transport uses energy from ATP to move molecules against their concentration gradient, from low to high concentration.
True or False?
Passive transport requires ATP.
False.
Passive transport occurs without the direct input of metabolic energy; only active transport requires ATP.
What is the source of energy for active transport?
ATP, produced by respiration, provides the energy required for active transport.
What type of membrane protein does active transport use to move substances across a membrane?
Carrier proteins, which use energy from ATP to transport substances across the membrane.
What is the role of the Na+/K+ ATPase pump in nerve cells?
It uses energy from ATP to pump sodium ions out of the axon and potassium ions in. This establishes and maintains ion gradients, contributing to the membrane potential of neurons.
The Na+/K+ ATPase pumps sodium ions of the axon and potassium ions in.
The Na+/K+ ATPase pumps sodium ions out of the axon and potassium ions in.
What do exocytosis and endocytosis move across the membrane, and why do they require energy?
They move large substances, or large amounts of substances, into and out of cells.
They require energy (from ATP), so they are forms of active transport.
Define exocytosis.
Exocytosis is the process by which materials are transported out of a cell, using vesicles that fuse with the plasma membrane.
Define endocytosis.
Endocytosis is the process by which a cell takes in large molecules and particles by folding the plasma membrane inward to form vesicles.
Describe the steps of exocytosis.
The substance is packaged into an internal vesicle.
The vesicle travels to the plasma membrane, fuses with it, and secretes its contents outside the cell.
Describe the steps of endocytosis.
The plasma membrane folds inward to form a vesicle.
Material from outside is engulfed by the vesicle, which then carries it into the cell.
Because they require metabolic energy, exocytosis and endocytosis are both forms of transport.
Because they require metabolic energy, exocytosis and endocytosis are both forms of active transport.
Give two examples of exocytosis in the body.
Any two of:
Release of neurotransmitters at a nerve synapse
Secretion of insulin during blood glucose regulation
Secretion of digestive enzymes
Give an example of endocytosis.
Phagocytosis — immune cells engulf and destroy pathogens.
(Other examples: taking in cholesterol to build membranes, or taking in water.)
True or False?
Exocytosis moves substances out of the cell, while endocytosis moves substances into the cell.
True.
Exo- (out) secretes materials from the cell; endo- (in) brings materials into the cell.
True or False?
Endocytosis and exocytosis can occur without any input of energy.
False.
Both processes require energy and are forms of active transport.
Define facilitated diffusion.
Facilitated diffusion is the movement of large, polar molecules and ions across membranes via transport proteins, occurring down a concentration gradient and requiring no energy input.
Why do large polar molecules and charged ions require proteins to cross the plasma membrane?
They cannot pass directly through the hydrophobic interior of the phospholipid bilayer, so they need transport or channel proteins to move across.
True or False?
Facilitated diffusion requires an input of energy from ATP.
False.
Facilitated diffusion moves substances down a concentration gradient, so it does not require energy from ATP. Only active transport (against the gradient) requires ATP.
What are the two types of transport protein involved in facilitated diffusion?
Channel proteins
Carrier proteins
Define channel protein.
Channel protein is a transport protein that forms a pore, allowing specific substances, such as charged ions, to diffuse across the membrane.
Define carrier protein.
Carrier protein is a transport protein with a binding site on one side of the membrane that changes shape to release the transported substance on the other side.
Transport proteins are highly , allowing the passage of only one type of substance.
Transport proteins are highly specific, allowing the passage of only one type of substance.
Which two ions are given as examples of charged particles that require channel proteins to cross the membrane?
Na+ (sodium) and K+ (potassium) ions.
are transport proteins that move large quantities of water across membranes.
Aquaporins are transport proteins that move large quantities of water across membranes.
True or False?
The movement of ions across a membrane can cause it to become polarized.
True.
When charged ions move across the membrane, a difference in charge builds up between the two sides, causing the membrane to become polarized.
How does facilitated diffusion differ from active transport?
Facilitated diffusion moves substances down a concentration gradient with no energy input
Active transport moves substances against a concentration gradient and requires energy from ATP
True or False?
Some channel proteins are gated, meaning they can open or close depending on conditions.
True.
Gated channel proteins can switch between open and closed states, controlling when a substance is able to diffuse across the membrane.
Define hypotonic solution.
A hypotonic solution has a lower solute concentration than the cell contents and a higher water potential, so water moves into the cell by osmosis.
Define hypertonic solution.
A hypertonic solution has a higher solute concentration than the cell contents and a lower water potential, so water moves out of the cell by osmosis.
Define isotonic solution.
An isotonic solution has an equal solute concentration to the cell contents, so there is no net movement of water into or out of the cell.
Define osmosis.
Osmosis is the movement of water molecules from a region of high water potential (a hypotonic solution) to a region of low water potential (a hypertonic solution).
True or False?
Tonicity is a relative term, so a solution's tonicity is always described in relation to the cell's contents.
True.
A solution is described as hypotonic, hypertonic, or isotonic compared to the solute concentration of the cell contents.
Water moves into a cell that is placed in a solution, because this solution has a higher water potential than the cell.
Water moves into a cell that is placed in a hypotonic solution, because this solution has a higher water potential than the cell.
What happens to the water in a cell that is placed in a hypertonic solution?
Water moves out of the cell by osmosis, because a hypertonic solution has a lower water potential and a higher solute concentration than the cell.
Define water potential.
Water potential is the tendency of water molecules to move from one place to another; water moves from a region of high water potential to a region of low water potential.
Which two factors determine the water potential (Ψ) of a solution?
Solute potential (Ψs) and pressure potential (Ψp), related by the equation:
Ψ = Ψp + Ψs
Define solute potential.
Solute potential is the effect that dissolved solutes have on water potential; pure water has a solute potential of zero, which becomes more negative as solutes are added.
As solutes are added to a solution, its solute potential and becomes more negative.
As solutes are added to a solution, its solute potential decreases and becomes more negative.
Define osmoregulation.
Osmoregulation is the control of an organism's internal water balance; an example of homeostasis that regulates internal solute concentration and water potential.
How does the contractile vacuole help a Paramecium maintain water balance in freshwater?
Paramecium live in a hypotonic environment and take in water by osmosis; the contractile vacuole collects this excess water and pumps it out of the cell.
What role does the central vacuole play in plant cell homeostasis?
Water storage: a full vacuole maintains pressure potential and provides structural support
Regulation of ion concentration: channels in the vacuole membrane control the movement of ions into and out of the cytoplasm
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