transport across cell membranes ch 12 (quiz 4)

the __ forms the basic structure of the cell membrane

lipid bilayer

the lipid bilayer is __ __ meaning it allows some substances to pass through more easily than others

semi permeable

__ diffuse most readily due to the hydrophobic interior of the lipid bilayer.

small, nonpolar (hydrophobic) molecules

explain ion concentrations inside and outside a typical cell

The distribution of ions across the cell membrane is critical for cellular function. Positive and negative charges must balance.

Membrane Potential

• A ____ across the membrane arises from the unequal distribution of ions.

• This electrical imbalance is known as the membrane potential.

• In an unexcited state, the membrane potential typically ranges from -20 to -70 mV.

charge differential

tomove substances that cannot easily cross the lipid bilayer, ___ are utilized

-integral proteins embedded within the membrane

membrane transport proteins

2 main types of membrane transport (integral) proteins

• Two main types exist:

  • Transporters: Involved in moving small organic molecules and inorganic ions. They are typically multipass proteins.

  • Channel Proteins (e.g., Porins): Primarily involved in the passage of inorganic ions and small, polar molecules.

The movement of molecules across the cell membrane can be classified based on whether it requires ___

energy

The movement of molecules across the cell membrane can be classified based on whether it requires energy. explain the 4 types

• Passive Transport: Does not require cellular energy.

  • Simple Diffusion: Movement directly across the lipid bilayer down the concentration gradient.

  • Facilitated Diffusion: Movement across the membrane with the help of transport proteins (channels or carriers), down the electrochemical gradient.

• Active Transport: Requires cellular energy to move substances against their electrochemical gradient. This typically involves transporter proteins that function as pumps.

Electrochemical gradient

the movement of __ __ across the membrane is influenced by both the concentration gradient and membrane potential

charged solutes

• The movement of charged solutes across the membrane is influenced by both the concentration gradient and the membrane potential.

• The combination of these two forces is called the ____

• This gradient dictates the direction and force of passive transport for charged molecules.

electrochemical gradient

___ specialized channel proteins that facilitate the rapid movement of large volumes of water across membranes.

aquaporins

• is the diffusion of water across a semi-permeable membrane, driven by differences in water concentration (or solute concentration).

osmosis

aquaporins have __ transmembrane helices

six

Preventing Osmotic Swelling

Cells have mechanisms to prevent excessive water uptake and bursting due to osmosis. The ___ ___ in organisms like Paramecium is an example of such a mechanism, actively expelling excess water.

contractile vacuole

Cell membranes contain characteristic ____that move small, water-soluble organic molecules and some ions. These transporters are crucial for bringing essential nutrients into the cell and removing waste products. Examples include transporters for nucleotides, sugars, amino acids, and ions like Na

selective transporter proteins

___ mediate the facilitated diffusion of glucose across the cell membrane.The net movement of glucose occurs down its concentration gradient.

• Traffic can move in either direction (into or out of the cell) depending on the relative glucose concentrations. This process is passive as glucose is uncharged.

glucose transporters

Active transport moves molecules against their electrochemical gradients and requires an energy source. Three primary mechanisms are:

1. Transmembrane Pumps: These proteins directly use energy, often from ATP hydrolysis, to move solutes. An example is bacteriorhodopsin, which uses light energy.

2. Coupled Transporters: Utilize the energy stored in an electrochemical gradient of one solute to drive the transport of another.

3. ATP-Driven Pumps: Directly hydrolyze ATP to power transport.

The Na^+-K^+ Pump (ATPase)

• The Na^+-K^+ pump is a critical ATPase (an enzyme that hydrolyzes ATP) found in the plasma membrane of animal cells.

• It is responsible for approximately 30% of a cell's ATP usage.

• A single cell can contain between 800,000 and 30,000,000 pumps.

• Functions:

  • Maintains high internal K^+ concentration and low internal Na^+ concentration.

  • Helps maintain osmotic equilibrium by controlling ion concentrations.

  • Drives other transport processes by establishing ion gradients.

• Mechanism: The pump actively transports 2 K^+ ions inward and 3 Na^+ ions outward per molecule of ATP hydrolyzed. This creates a net outward movement of positive charge.

The Na^+/K^+ pump is essential for maintaining the characteristic ion distributions across the plasma membrane, which are vital for cell function.

Ca^{2+} Pump

• The Ca^{2+} pump actively transports calcium ions out of the cytosol and into the extracellular space or organelles like the endoplasmic reticulum.

• This is crucial because the cytosolic Ca^{2+} concentration is kept very low (10^{-4} mM) compared to the extracellular concentration (1-2 mM).

• Function: By maintaining low cytosolic Ca^{2+} levels, the cell can use transient increases in Ca^{2+} as signaling mechanisms. For example, Ca^{2+} influx triggers the release of cortical granules to modify the zona pellucida, preventing polyspermy during fertilization 16.

Coupled Transport Systems

• Transporters can move one or two solutes simultaneously.

• The energy stored in the electrochemical gradient of one ion can be exploited to transport another solute.

Glucose-Na^+ Symport Protein

• A symport protein simultaneously transports two different solutes in the same direction.

• The glucose-Na^+ symport protein uses the inward movement of Na^+ (down its steep electrochemical gradient) to drive the inward transport of glucose, even against glucose's own concentration gradient.

• A ___ simultaneously transports two different solutes in the same direction.

symport protein

H + Gradients in Transport

• primarily use H^+ gradients to transport solutes into the cell, as they often lack Na^+/K^+ pumps.

• Plants, fungi, and bacteria

H^+ Gradients in Transport

• plants, fungi, bacteria primarily use H+ gradients

• n some cases, like the vacuolar membrane in plant and animal cells, H^+ pumps maintain an acidic internal environment (e.g., in lysosomes or plant vacuoles) while keeping the cytosol neutral 20.

• Mitochondria and prokaryotes also use H^+ pumps as part of their electron transport chains (ETCs) to generate energy 21.

Glucose Transport from Gut Lumen

In the gut lining, __ glucose transporters work in as series. what are they and what do they do?

• In the gut lining, two glucose transporters work in series.

• First, a glucose-Na^+ symporter on the apical membrane brings glucose into the epithelial cells from the gut lumen, driven by the Na^+ gradient.

• Then, a facilitated diffusion glucose transporter on the basolateral membrane releases glucose into the extracellular fluid, moving it down its concentration gradient.

__ __ is a charge differential across the plasma membrane, arising from the uneven distribution of ions

This potential difference can be detected as an electrical current and is measured in millivolts (mV).

membrane potential

• Ion selectivity is based on:

• The presence of charged amino acids lining the channel pore.

• The size of the channel's inner diameter.

• ___ allows K^+ ions to pass through without a significant conformational change in the protein itself.

K+ channel protein

• The selective filter within the pore ensures that only ions with the appropriate charge and size can pass.

Resting Membrane Potential

• The resting membrane potential in a non-excited cell is established when the membrane potential counterbalances the flow of K^+ down its concentration gradient.

• In animal cells, K^+ concentration gradients and K^+ leak channels (channels that are generally open) play a major role in generating the resting membrane potential across the plasma membrane 28. This typically results in the inside of the cell being negative relative to the outside.

Ion channels can be gated, meaning their opening and closing are controlled by specific stimuli. 4 types of gating, explain

• Mechanical Gating: Channels that open or close in response to physical deformation of the membrane (e.g., touch receptors). Plants like the "Sensitive Plant" exhibit responses involving mechanically gated channels 31.

• Voltage-Gated Channels: Respond to changes in the membrane potential 32. These are crucial for nerve impulse transmission.

• Ligand-Gated Channels: Open or close when a specific signaling molecule (ligand, such as a neurotransmitter) binds to them.

• Light-Gated Channels: Open in response to light, as seen in optogenetics

• Nerve signals are transmitted through ___ which are rapid, transient changes in membrane potential.

action potentials (APs)

__ refers to a shift in the membrane potential towards zero, making the inside of the cell less negative.

depolarization

an action potential is __ along the length of an axon

propagated

• The inactivated state of Na^+ channels in the region just passed ensures that the AP travels in one direction and prevents immediate re-firing. This makes the AP self-propagating.

Synaptic Transmission

• When an action potential reaches the nerve terminal, it triggers a conversion from an electrical signal to a chemical signal.

• This involves the opening of voltage-gated Ca^{2+} channels in the presynaptic membrane.

• Ca^{2+} influx causes synaptic vesicles, containing neurotransmitters, to fuse with the presynaptic membrane and release their contents into the synapse (the small gap between neurons, approx. 20 nm).

• Neurotransmitters diffuse across the synapse and bind to receptors on the postsynaptic cell (another neuron or muscle cell).

• This binding opens ligand-gated ion channels in the postsynaptic membrane, generating a new electrical signal (either excitatory or inhibitory) in the postsynaptic cell.

__ is a neurotransmitter crucial in the peripheral nervous system (PNS), particularly at neuromuscular junctions.

• It binds to acetylcholine receptors (composed of five transmembrane proteins) on skeletal muscle tissue.

• Binding causes a conformational change, opening a channel that allows Na^+ influx, leading to depolarization and muscle contraction.

• These receptors have two binding sites for ACh and negatively charged amino acids at the channel ends, which likely play a role in ion selectivity.

acetylcholine (ACh)

Neurotransmitters determine the response of the __ __

postsynaptic cell.

• Inhibitory Neurotransmitters

• (e.g., GABA, glycine):

  • Bind to ligand-gated Cl^- channels.

  • Cause Cl^- influx into the postsynaptic neuron.

  • This hyperpolarizes the membrane or stabilizes it near resting potential, suppressing the generation of an action potential.

• Excitatory Neurotransmitters (e.g., acetylcholine, glutamate):

• Bind to ligand-gated cation channels (often permeable to Na^+).

• Cause Na^+ influx.

• This leads to depolarization, triggering an action potential in the postsynaptic neuron.

Neuromuscular Toxins and Drugs

• Curare: A neuromuscular blocking toxin that blocks excitatory acetylcholine receptors at the neuromuscular junction, preventing muscle contraction and causing paralysis

• Strychnine: Blocks inhibitory glycine receptors, leading to uncontrolled muscle spasms

• Botulism: Caused by Clostridium botulinum, produces toxins that interfere with neurotransmitter release

• Tetanus: Caused by Clostridium tetani, its toxin (tetanospasmin) blocks the release of inhibitory neurotransmitters (GABA and glycine), resulting in severe muscle spasms

• Dopamine and Cocaine: Cocaine blocks the reuptake of dopamine, leading to increased dopamine levels in the synapse and potentially affecting mood and behavior

__ is a revolutionary technique that uses light to control the activity of genetically modified cells.

Optogenetics

Explain the mechanisms and application of optogenetics

Optogenetics is a revolutionary technique that uses light to control the activity of genetically modified cells.

• Mechanism:

  • Cells are genetically engineered to express light-sensitive proteins, typically ion channels like channelrhodopsins 45. These are often derived from microorganisms like Chlamydomonas.

  • When these light-gated channels are activated by specific wavelengths of light (e.g., blue light), they open, allowing ion influx (e.g., Na^+ influx) and causing depolarization.

• Application:

  • By targeting specific cell populations (e.g., neurons) with these light-sensitive proteins, researchers can precisely control their activity using light.

  • This allows for the study of neural circuits and the modulation of cellular functions in various tissues, organs, or even whole organisms 46.

  • The photoreceptor gene is cloned under the regulation of control elements that allow for targeting specific cell types.

  • The genetic material is delivered to the target tissue using vectors like viruses.

• Example: In Chlamydomonas, channelrhodopsin in the eyespot is a light-gated channel that modulates flagellar movement in response to blue light 45.