Campbell Biology ch 7 Membrane Structure and Function
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Questions and Answers

What type of reactions primarily hold the plasma membrane together?

Hydrophobic reactions primarily hold the plasma membrane together.

Describe the movement capabilities of phospholipids in the plasma membrane.

Phospholipids can rapidly change position, switching places up to $10^7$ times per second.

How does cholesterol function as a fluidity buffer in the plasma membrane?

Cholesterol decreases fluidity at human body temperature while preventing phospholipids from packing closely together.

What impact does temperature have on membrane fluidity?

<p>Membranes stay fluid at low temperatures if rich in unsaturated hydrocarbon tails, while cold temperatures can solidify membranes.</p> Signup and view all the answers

Explain the difference between integral and peripheral proteins in the membrane.

<p>Integral proteins penetrate the hydrophobic layer, while peripheral proteins are loosely bound to the surface.</p> Signup and view all the answers

Why do some fish have higher proportions of unsaturated hydrocarbon tails?

<p>Fish in cold environments have more unsaturated tails to keep their membranes fluid despite low temperatures.</p> Signup and view all the answers

What role do membrane proteins play in the functionality of the plasma membrane?

<p>Membrane proteins determine the functions of the membrane, providing various roles like transport and signaling.</p> Signup and view all the answers

How do immune cells resist HIV infection related to membrane proteins?

<p>Certain individuals have genes coding for CCR5, a co-receptor HIV must bind to, offering resistance to infection.</p> Signup and view all the answers

What adaptations do bacteria and archaea have for high-temperature environments?

<p>They include unusual lipids in their membranes to prevent excessive fluidity in extreme heat.</p> Signup and view all the answers

How do seasonal changes affect membrane composition in winter wheat?

<p>Winter wheat increases its unsaturated phospholipids in autumn to adapt to colder winter temperatures.</p> Signup and view all the answers

What are amphipathic molecules, and why are they important in cell membranes?

<p>Amphipathic molecules have both hydrophilic and hydrophobic regions, which allows them to form the bilayer structure of cell membranes.</p> Signup and view all the answers

How does the Fluid Mosaic Model describe the structure and behavior of the plasma membrane?

<p>The Fluid Mosaic Model depicts the plasma membrane as a fluid structure with proteins embedded in it, allowing for movement and various functions.</p> Signup and view all the answers

What are lipid rafts, and what significance do they hold in the plasma membrane?

<p>Lipid rafts are patches within the plasma membrane enriched with specific lipids and proteins that facilitate particular cellular functions.</p> Signup and view all the answers

Explain how cholesterol affects the fluidity of cell membranes at different temperatures.

<p>Cholesterol reduces fluidity at human body temperature by restraining phospholipid movement while preventing solidification at lower temperatures.</p> Signup and view all the answers

Describe the movement of phospholipids within the plasma membrane.

<p>Phospholipids can rapidly change positions, swapping places with adjacent phospholipids up to $10^7$ times per second.</p> Signup and view all the answers

What is the role of unsaturated hydrocarbon tails in maintaining membrane fluidity at lower temperatures?

<p>Unsaturated hydrocarbon tails prevent tight packing of lipids, allowing the membrane to remain fluid even at lower temperatures.</p> Signup and view all the answers

How does the attachment of membrane proteins to the cytoskeleton affect their movement?

<p>Membrane proteins attached to the cytoskeleton are typically immobile, limiting their movement within the membrane.</p> Signup and view all the answers

What is the significance of the weaker hydrophobic interactions in the plasma membrane compared to covalent bonds?

<p>Weaker hydrophobic interactions allow for flexibility and fluidity in the membrane structure, essential for cellular functions.</p> Signup and view all the answers

How does cholesterol function as a fluidity buffer in membranes?

<p>Cholesterol helps maintain membrane fluidity by preventing it from becoming too rigid in cold temperatures and too fluid in warm temperatures.</p> Signup and view all the answers

What is the significance of unsaturated hydrocarbon tails in cell membranes of fish living in cold environments?

<p>Unsaturated hydrocarbon tails keep membranes fluid in cold temperatures by preventing solidification due to double bonds.</p> Signup and view all the answers

Describe the role of integral proteins in cell membranes.

<p>Integral proteins penetrate the hydrophobic layer of membranes and often span the entire membrane, facilitating various functions.</p> Signup and view all the answers

What are peripheral proteins, and how do they differ from integral proteins?

<p>Peripheral proteins are loosely bound to the membrane surface and are not embedded, unlike integral proteins that penetrate the membrane.</p> Signup and view all the answers

Explain how carbohydrates on the extracellular surface of plasma membranes contribute to cell recognition.

<p>Carbohydrates on the extracellular surface facilitate cell recognition by binding to molecules, often forming glycoproteins and glycolipids.</p> Signup and view all the answers

How do glycoproteins vary between different blood types?

<p>Glycoproteins vary in structure and composition among different blood types, affecting blood type compatibility in transfusions.</p> Signup and view all the answers

What is the Fluid Mosaic Model in the context of cell membranes?

<p>The Fluid Mosaic Model depicts membranes as fluid structures with various proteins embedded, allowing for flexibility and movement.</p> Signup and view all the answers

What is meant by 'amphipathic' in relation to phospholipids?

<p>Amphipathic refers to phospholipids having both hydrophilic (water-attracting) and hydrophobic (water-repelling) parts.</p> Signup and view all the answers

Discuss the role of lipid rafts in membrane structure.

<p>Lipid rafts are patches within the membrane that contain concentrated lipids and proteins associated with specific functions.</p> Signup and view all the answers

How does the binding of HIV to CD4 and CCR5 illustrate the importance of membrane proteins?

<p>The binding of HIV to CD4 and the co-receptor CCR5 demonstrates how specific proteins on cell membranes can facilitate viral entry.</p> Signup and view all the answers

What role does the absence of CCR5 play in HIV resistance?

<p>The absence of CCR5 allows patients to be resistant to HIV by preventing the virus from entering their cells.</p> Signup and view all the answers

What are glycolipids and glycoproteins, and how do they differ from each other?

<p>Glycolipids are carbohydrates covalently bonded to lipids, while glycoproteins are carbohydrates bonded to proteins.</p> Signup and view all the answers

How does selective permeability affect the movement of hydrophilic molecules through cell membranes?

<p>Selective permeability limits the movement of hydrophilic molecules, making it harder for them to pass through the hydrophobic lipid bilayer.</p> Signup and view all the answers

What is the function of aquaporins in cellular transport?

<p>Aquaporins are channel proteins that facilitate the rapid movement of water molecules across the cell membrane.</p> Signup and view all the answers

Define passive transport and provide an example.

<p>Passive transport is the movement of substances across a membrane without the use of energy, such as the diffusion of oxygen into cells.</p> Signup and view all the answers

Explain the concept of a concentration gradient.

<p>A concentration gradient is a difference in the concentration of a substance across a space, driving the movement from high to low concentration.</p> Signup and view all the answers

What happens during osmosis when a semi-permeable membrane is involved?

<p>Osmosis occurs when water molecules pass through a semi-permeable membrane, moving towards areas of higher solute concentration.</p> Signup and view all the answers

Describe how nonpolar molecules cross the cell membrane.

<p>Nonpolar molecules can easily pass through the cell membrane because they dissolve in the hydrophobic lipid bilayer.</p> Signup and view all the answers

Why are carrier proteins specific in their transport function?

<p>Carrier proteins are specific because they bind to particular molecules, changing shape to transport them across the membrane.</p> Signup and view all the answers

What is the importance of glycoproteins in blood types?

<p>Glycoproteins determine blood type by varying in structure between individuals, which impacts blood compatibility.</p> Signup and view all the answers

How do hydration shells impact the penetration of polar ions through the lipid bilayer?

<p>Hydration shells make it difficult for polar ions to penetrate the lipid bilayer, as the water molecules surrounding them hinder their movement.</p> Signup and view all the answers

Explain the mechanism by which glucose transport proteins operate in red blood cells.

<p>Glucose transport proteins facilitate the rapid transport of glucose by binding to it and then changing shape to move it across the membrane.</p> Signup and view all the answers

Discuss the significance of thermal energy in the process of diffusion.

<p>Thermal energy causes particles to move randomly, driving the process of diffusion until equilibrium is reached.</p> Signup and view all the answers

What is osmosis and how does it relate to solute concentration?

<p>Osmosis is the diffusion of free water across a selectively permeable membrane, occurring from a higher concentration of free water to a lower concentration, until equilibrium is reached.</p> Signup and view all the answers

Define tonicity and its significance to cell function.

<p>Tonicity is the ability of a solution to cause a cell to gain or lose water, significantly affecting the cell's shape and function depending on solute concentration.</p> Signup and view all the answers

What happens to a cell in a hypertonic environment?

<p>In a hypertonic environment, a cell loses water, causing it to shrivel and potentially die due to the high concentration of non-penetrating solutes outside the cell.</p> Signup and view all the answers

Explain the term isotonic with respect to cell dynamics.

<p>Isotonic refers to an environment where the solute concentration is equal inside and outside the cell, resulting in no net movement of water and a stable cell volume.</p> Signup and view all the answers

What is the role of aquaporins in cellular function?

<p>Aquaporins are specialized transport proteins that facilitate the rapid diffusion of water across cell membranes, essential for maintaining water balance in cells.</p> Signup and view all the answers

Describe the concept of plasmolysis in plant cells.

<p>Plasmolysis occurs when plant cells lose water to a hypertonic environment, causing the plasma membrane to pull away from the cell wall, leading to cell shrinkage.</p> Signup and view all the answers

How does the study of osmoregulation relate to cellular adaptations?

<p>Osmoregulation involves the adaptations organisms make to deal with osmotic pressure changes, such as maintaining solute concentrations and water balance despite external conditions.</p> Signup and view all the answers

Explain the function and significance of gated ion channels.

<p>Gated ion channels open and close in response to stimuli, allowing specific ions to flow across the membrane, which is vital for processes like nerve signal transmission.</p> Signup and view all the answers

What distinguishes passive transport from active transport in cells?

<p>Passive transport occurs without energy input as substances move down their concentration gradient, while active transport requires energy to move substances against their gradient.</p> Signup and view all the answers

Define the effects of hypotonic solutions on animal and plant cells.

<p>In a hypotonic solution, water enters both animal and plant cells, which may cause animal cells to burst while plant cells become turgid, a healthy state.</p> Signup and view all the answers

How does diffusion contribute to cellular respiration?

<p>Diffusion allows oxygen to move into cells and carbon dioxide to exit, facilitating the process of cellular respiration by maintaining the necessary gas concentrations.</p> Signup and view all the answers

What is the significance of the concentration gradient in passive transport?

<p>The concentration gradient represents potential energy that drives passive transport, allowing substances to diffuse from areas of high concentration to low concentration without energy expenditure.</p> Signup and view all the answers

How do contractile vacuoles function in protists like paramecium?

<p>Contractile vacuoles in protists like paramecium pump out excess water that enters the cell through osmosis, helping to maintain osmotic balance.</p> Signup and view all the answers

What is osmosis and how does it differ from diffusion?

<p>Osmosis is the diffusion of free water across a selectively permeable membrane, while diffusion refers to the movement of solutes from higher to lower concentration without involving a membrane.</p> Signup and view all the answers

Define tonicity and explain its importance in cell behavior.

<p>Tonicity refers to the ability of a surrounding solution to cause a cell to gain or lose water, critically influencing whether a cell swells, shrinks, or remains stable.</p> Signup and view all the answers

What happens to a cell in a hypertonic solution?

<p>In a hypertonic solution, a cell will lose water, causing it to shrivel and potentially die.</p> Signup and view all the answers

Explain the concept of plasmolysis in plant cells.

<p>Plasmolysis occurs when water leaves a plant cell in a hypertonic environment, causing the plasma membrane to pull away from the cell wall.</p> Signup and view all the answers

What role do aquaporins play in facilitated diffusion?

<p>Aquaporins are specialized proteins that facilitate the rapid diffusion of water across cell membranes.</p> Signup and view all the answers

How does facilitated diffusion differ from active transport?

<p>Facilitated diffusion occurs passively and relies on concentration gradients, whereas active transport requires energy to move substances against their concentration gradients.</p> Signup and view all the answers

Describe the significance of the sodium-potassium pump in animal cells.

<p>The sodium-potassium pump is essential for maintaining high potassium (K+) and low sodium (Na+) concentrations inside animal cells, crucial for cellular function.</p> Signup and view all the answers

What is meant by turgor pressure, and why is it essential for plant cells?

<p>Turgor pressure is the pressure exerted by water inside the plant cell against the cell wall, which helps maintain the cell's shape and structural integrity.</p> Signup and view all the answers

What can occur in a hypotonic solution for cells without cell walls?

<p>In a hypotonic solution, cells without walls can swell rapidly and possibly burst due to excess water intake.</p> Signup and view all the answers

How does a gated channel function in response to stimuli?

<p>A gated channel opens or closes in response to a specific stimulus, such as a voltage change or the binding of a chemical signal.</p> Signup and view all the answers

What happens to a plant cell when it is in an isotonic environment?

<p>In an isotonic environment, there is no net movement of water, and the plant cell remains flaccid.</p> Signup and view all the answers

What adaptations might unicellular organisms have for osmoregulation?

<p>Unicellular organisms, like paramecium, have a contractile vacuole that helps pump excess water out of the cell to maintain homeostasis.</p> Signup and view all the answers

How does the concept of osmoregulation differ between organisms with and without cell walls?

<p>Organisms without cell walls, such as animals, actively regulate water and solute levels, while those with cell walls, like plants, mainly rely on cell walls for structural support during osmotic changes.</p> Signup and view all the answers

Illustrate the relationship between concentration gradients and facilitated diffusion.

<p>Facilitated diffusion relies on concentration gradients to allow materials to move passively through specific channels or transport proteins.</p> Signup and view all the answers

What is the main function of the proton pump in plant cells?

<p>The proton pump transports hydrogen ions (H+) outside the cell, generating voltage and storing energy for cellular functions.</p> Signup and view all the answers

How do cotransport proteins function in plant cells to transport sucrose?

<p>Cotransport proteins allow H+ ions to enter the cell only when combined with a sucrose molecule, utilizing the H+ gradient created by proton pumps.</p> Signup and view all the answers

What is the role of exocytosis in cellular functions?

<p>Exocytosis is the process where a cell secretes molecules by fusing vesicles with the plasma membrane, releasing their contents outside the cell.</p> Signup and view all the answers

Describe how receptor-mediated endocytosis works.

<p>Receptor-mediated endocytosis involves specific receptors on the plasma membrane binding to solutes, clustering in coated pits, and forming vesicles to bring substances into the cell.</p> Signup and view all the answers

What distinguishes phagocytosis from pinocytosis?

<p>Phagocytosis involves the engulfing of large particles by the cell, while pinocytosis is the process of taking in extracellular fluid in small vesicles.</p> Signup and view all the answers

Explain the significance of sodium-glucose cotransport in treating diarrhea.

<p>Sodium-glucose cotransport allows for the absorption of sodium and glucose in the intestines, which helps replenish lost electrolytes during diarrhea.</p> Signup and view all the answers

What is the role of vesicles formed from the Golgi apparatus in cellular transport?

<p>Vesicles from the Golgi apparatus transport molecules to the plasma membrane for secretion or integration into the membrane during exocytosis.</p> Signup and view all the answers

What types of molecules are typically taken up by cells through endocytosis?

<p>Cells commonly take up large molecules, like proteins and polysaccharides, and various nutrients through endocytosis.</p> Signup and view all the answers

In animals, what are low-density lipoproteins (LDL) used for during endocytosis?

<p>LDL particles transport cholesterol to cells, where they bind to LDL receptors and are taken up via receptor-mediated endocytosis.</p> Signup and view all the answers

How do coated pits function in pinocytosis?

<p>Coated pits are specialized regions of the plasma membrane lined with coat proteins that form vesicles during pinocytosis to engulf extracellular fluid.</p> Signup and view all the answers

What is the electrogenic function of the potassium-sodium pump in animal cells?

<p>The potassium-sodium pump creates a net negative charge inside the cell by moving 3 sodium ions out while bringing 2 potassium ions in, maintaining the membrane potential.</p> Signup and view all the answers

What happens to the receptors after receptor-mediated endocytosis?

<p>After the vesicle is formed and the substances are taken in, the receptors are recycled back to the plasma membrane for further use.</p> Signup and view all the answers

How does the proton pump contribute to the energy needs of plant cells?

<p>The proton pump establishes a proton gradient across the membrane, which can be used to drive various cellular processes requiring energy.</p> Signup and view all the answers

What is the difference in the mechanism of endocytosis and exocytosis?

<p>Endocytosis involves the formation of vesicles to take in substances, while exocytosis entails the fusion of vesicles with the membrane to release contents outside the cell.</p> Signup and view all the answers

What role does ATP play in the function of the sodium-potassium pump?

<p>ATP provides the energy necessary for the sodium-potassium pump to transport Na+ out of the cell and K+ into the cell by transferring a phosphate group to the pump, causing it to change shape.</p> Signup and view all the answers

Explain the concept of membrane potential.

<p>Membrane potential is the voltage across a cell's plasma membrane, typically ranging from -50 to -200 millivolts, with the inside of the cell being more negative relative to the outside.</p> Signup and view all the answers

What is the electrochemical gradient and how does it influence ion diffusion?

<p>The electrochemical gradient combines chemical forces (concentration differences) and electrical forces (charge differences), influencing ions to move down their gradient, not just their concentration gradient.</p> Signup and view all the answers

Describe the electrogenic nature of the sodium-potassium pump.

<p>The sodium-potassium pump is electrogenic because it exports more positively charged Na+ ions than it imports K+ ions, creating a net negative charge inside the cell.</p> Signup and view all the answers

How do proton pumps function in plants, fungi, and bacteria?

<p>Proton pumps transport hydrogen ions (H+) out of the cell, generating a voltage across membranes that facilitates energy storage for cellular functions.</p> Signup and view all the answers

What is the function of cotransporters in cells?

<p>Cotransporters utilize the gradient of one ion (like H+ or Na+) to facilitate the transport of another solute (like sucrose or glucose) across the plasma membrane.</p> Signup and view all the answers

Explain why the interior of the cell is typically negatively charged relative to the extracellular matrix.

<p>The unequal distribution of charged ions, particularly the higher concentration of K+ inside the cell relative to Na+ outside, creates a negative charge on the cytoplasmic side of the plasma membrane.</p> Signup and view all the answers

What is the significance of facilitated diffusion?

<p>Facilitated diffusion allows substances to cross membranes passively via protein channels, enhancing the efficiency of transport without needing energy.</p> Signup and view all the answers

Why are carrier proteins essential for active transport?

<p>Carrier proteins are essential for active transport because they can change shape to move substances against their concentration gradient, a process that requires energy.</p> Signup and view all the answers

Describe the effect of sodium levels in the colon during diarrhea.

<p>During diarrhea, rapid expulsion of waste prevents the reabsorption of sodium, potentially leading to dangerously low sodium levels in the body.</p> Signup and view all the answers

What happens to sodium ions when a nerve cell is stimulated?

<p>When a nerve cell is stimulated, protein channels open, allowing sodium ions (Na+) to rapidly enter the cell, driven both by the concentration gradient and electrical forces.</p> Signup and view all the answers

How do sodium-potassium pumps contribute to the resting membrane potential?

<p>Sodium-potassium pumps help maintain the resting membrane potential by constantly exchanging Na+ for K+ across the plasma membrane, preserving the negative internal charge.</p> Signup and view all the answers

What is a primary characteristic of facilitated diffusion compared to active transport?

<p>A primary characteristic of facilitated diffusion is that it does not require energy, whereas active transport relies on ATP to move substances against their concentration gradient.</p> Signup and view all the answers

What role do carrier membrane proteins play in establishing membrane potential?

<p>Carrier membrane proteins regulate the entry and exit of ions, directly influencing the ion distribution that establishes the membrane potential.</p> Signup and view all the answers

Flashcards

Amphipathic molecule

A molecule with both hydrophilic (water-loving) and hydrophobic (water-fearing) regions.

Fluid Mosaic Model

Model describing the plasma membrane as a fluid, flexible structure, with embedded proteins like a mosaic.

Lipid rafts

Patches of lipids in the plasma membrane, which are debated in their function, if they exist.

Membrane fluidity

The ability of lipids and proteins to move within the membrane.

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Unsaturated hydrocarbon tails

Hydrocarbon tails with double bonds, preventing tight packing of phospholipids.

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Cholesterol's role in membrane

A steroid that wedged between phospholipids, impacting membrane fluidity at different temperatures.

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Membrane protein movement

Proteins move much slower than lipids, and can be anchored and immobile.

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Effect of temperature on membrane fluidity

Lower temperatures reduce membrane fluidity, as phospholipids can't move easily, much like how grease solidifies.

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What's special about membranes in cold environments?

Membranes in cold environments have a higher proportion of unsaturated hydrocarbon tails in their phospholipids. These tails have double bonds, preventing tight packing and keeping the membrane fluid even in low temperatures.

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What's the role of cholesterol in membranes?

Cholesterol acts as a 'fluidity buffer' in membranes. It helps maintain fluidity at different temperatures, preventing the membrane from becoming too rigid or too fluid.

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Integral protein

A protein embedded within the cell membrane, spanning the entire hydrophobic layer or only partially extending into it.

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Peripheral protein

A protein that is loosely attached to the surface of the cell membrane, often interacting with integral proteins.

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Glycoprotein

A protein on the cell surface with attached carbohydrates, acting as a 'recognition tag' for other cells.

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Why are membrane proteins amphipathic?

Membrane proteins are amphipathic because they have both hydrophilic (water-loving) and hydrophobic (water-fearing) regions. This allows them to interact with both the watery environment and the lipid bilayer of the membrane.

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What makes cell membranes fluid?

Cell membranes are fluid because their phospholipids can move freely within the membrane, and proteins are also able to move, though slower than lipids.

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What is a 'lipid raft'?

A hypothetical patch of lipids enriched in certain types of lipids, potentially serving as a signaling platform for cell activities. Their existence and function are still under debate.

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How do cells recognize each other?

Cells recognize each other by binding to molecules on the extracellular surface of the plasma membrane, often glycoproteins and glycolipids, which act as 'recognition tags'.

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What is the Fluid Mosaic Model?

The model describes the plasma membrane as a fluid, flexible structure with embedded proteins, like a mosaic, illustrating the dynamic and diverse nature of the cell membrane.

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What holds the plasma membrane together?

The plasma membrane is primarily held together by hydrophobic interactions between the nonpolar tails of phospholipids. These interactions are weaker than covalent bonds, allowing for membrane fluidity.

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How do lipids and proteins move in the membrane?

Lipids and proteins can move laterally within the plane of the membrane, like floating in a sea of other molecules. This movement is called lateral diffusion. While it's rare, lipids and proteins can also flip to the other side of the membrane.

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What makes a membrane more fluid?

Membranes are more fluid when they contain a higher proportion of unsaturated hydrocarbon tails. These tails have double bonds that introduce kinks, preventing tight packing and making the membrane more flexible.

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How does cholesterol affect membrane fluidity?

Cholesterol acts as a fluidity buffer in membranes. At high temperatures, it reduces fluidity by restraining phospholipid movement. At low temperatures, it prevents solidification by hindering close packing.

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Why are membranes fluid?

Membrane fluidity is essential for proper function. It allows for permeability (molecules pass through), and enables membrane protein movement when needed.

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What is the function of the membrane?

Membranes are functional and structural matrices, providing a framework for the cell and mediating various processes. Individual proteins can perform specific functions, while some proteins can have multiple roles.

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How does HIV infect cells?

HIV infects cells by binding to CD4 and CCR5, proteins on the surface of human immune cells. This interaction allows the virus to enter and replicate within the cell.

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What is the importance of CCR5?

CCR5 is a co-receptor that HIV needs to bind to in order to successfully infect a cell. Individuals who are more resistant to HIV often have variations in their CCR5 gene that make it difficult for the virus to bind.

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CCR5 absence

The lack of the CCR5 protein on the surface of immune cells, making individuals resistant to HIV infection.

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Cell recognition

Cells identify each other by binding to molecules, often carbohydrate-based, on the cell surface.

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Glycolipids

Carbohydrates attached to lipids on the cell membrane.

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Membrane asymmetry

The two sides of a cell membrane have different lipid compositions.

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Selective permeability

The ability of a cell membrane to allow some substances to pass through easier than others.

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How nonpolar molecules cross membranes

Nonpolar molecules, like lipids, can easily pass through the hydrophobic interior of the cell membrane.

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Hydrophilic molecule transport

Hydrophilic molecules, like sugars, have difficulty passing through the hydrophobic interior of the membrane.

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Hydration shells

Water molecules surrounding polar ions, making it difficult for them to enter a hydrophobic environment.

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Transport proteins

Proteins creating hydrophilic channels for hydrophilic molecules to pass through the membrane.

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Aquaporins

Channel proteins allowing water to enter the cell.

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Carrier proteins

Proteins that transport materials by binding them and changing shape.

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Specificity of transport proteins

Transport proteins are specific for the molecules they carry.

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Diffusion

The random movement of particles from a high concentration area to a low concentration area.

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Concentration gradient

The difference in concentration of a substance across a membrane.

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Osmosis

The movement of water across a selectively permeable membrane from a region of higher free water concentration to a region of lower free water concentration.

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Tonicity

The ability of a surrounding solution to cause a cell to gain or lose water. It depends on the concentration of solutes that cannot cross the cell membrane.

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Isotonic

When the solute concentration is the same inside and outside the cell, resulting in no net movement of water.

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Hypertonic

When the solute concentration is higher outside the cell, causing water to move out of the cell and the cell to shrink.

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Hypotonic

When the solute concentration is lower outside the cell, causing water to move into the cell and the cell to swell.

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Osmoregulation

The regulation of solute concentrations and water balance in organisms, especially important for those lacking cell walls.

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Turgor Pressure

The pressure exerted by water inside a plant cell against its cell wall, crucial for plant rigidity and support.

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Turgid

The state of a plant cell when it is full of water and firm due to turgor pressure.

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Flaccid

The state of a plant cell when it lacks water and becomes limp due to low turgor pressure.

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Plasmolysis

The shrinking of a plant cell's cytoplasm due to water loss in a hypertonic environment, causing the plasma membrane to pull away from the cell wall.

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Facilitated Diffusion

The passive movement of molecules across a membrane with the help of transport proteins, still driven by the concentration gradient.

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Ion Channels

Membrane proteins that form channels allowing the passage of specific ions across the membrane.

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Gated Channels

Ion channels that open and close in response to stimuli, such as electrical or chemical signals.

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Active Transport

Movement of molecules across a membrane against their concentration gradient, requiring energy expenditure.

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Sodium-Potassium Pump

A protein pump found in animal cells that actively transports sodium ions out of the cell and potassium ions into the cell, maintaining a concentration gradient.

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Electrogenic Pump

A carrier protein that generates a voltage across the membrane by transporting more positively or negatively charged ions in one direction than in the other.

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Membrane Potential

The voltage difference across a cell membrane, typically with the inside of the cell negative relative to the outside.

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Electrochemical Gradient

The combined influence of concentration difference (chemical) and electrical charge difference across a membrane.

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Proton Pump

A major electrogenic pump in plants, fungi, and bacteria that actively transports hydrogen ions (H+) out of the cell.

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Cotransport

A type of active transport where the movement of one molecule down its concentration gradient provides energy for the movement of another molecule against its gradient.

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H+/Sucrose Cotransporter

A protein in plants that helps transport sucrose into cells using the energy from the movement of protons down their concentration gradient.

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Na+/Glucose Cotransporter

A protein in animal cells that helps transport glucose into cells by using the energy from the movement of sodium ions down their concentration gradient.

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Channel Protein

A protein that provides a passageway for molecules to cross a membrane, without binding to them.

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How does ATP power active transport?

ATP transfers a phosphate group to the carrier protein, causing a conformational change that moves the molecule across the membrane.

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What is the importance of the membrane potential?

It creates an electrical gradient across the membrane, influencing the movement of charged molecules and acting as a battery for the cell.

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What are the two forces driving ion movement?

The electrochemical gradient, consisting of the concentration difference (chemical force) and the charge difference (electrical force) across the membrane.

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Exocytosis

The process where a cell releases molecules by fusing vesicles with the plasma membrane.

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Endocytosis

The process by which a cell takes in molecules by forming new vesicles from the plasma membrane.

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Receptor-Mediated Endocytosis

A specialized type of endocytosis where specific molecules bind to receptors on the cell surface, triggering the formation of vesicles.

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Phagocytosis

A type of endocytosis where a cell engulfs large particles, like bacteria, and digests them within a vesicle.

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Pinocytosis

A type of endocytosis where a cell takes in small droplets of extracellular fluid and their dissolved contents.

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Coated Pits

Vesicles formed during pinocytosis, typically coated with proteins on their cytoplasmic surface.

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LDL (Low-Density Lipoprotein)

A particle carrying cholesterol in the bloodstream that binds to receptors on cells for uptake.

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What is the role of the proton pump in plants?

The proton pump is a major electrogenic pump in plants that transports H+ ions out of the cell, generating a voltage gradient crucial for nutrient uptake and other cellular processes.

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How does cotransport work?

Cotransport uses the energy of a solute moving down its concentration gradient to drive the movement of another solute against its gradient.

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What is receptor-mediated endocytosis?

A specific type of endocytosis where a cell takes in large amounts of a particular substance by binding to receptors on its surface.

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How are exocytosis and endocytosis different?

Exocytosis releases materials from the cell by fusing vesicles with the plasma membrane, while endocytosis brings materials into the cell by forming new vesicles from the plasma membrane. These processes are essentially opposites.

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Study Notes

Cell Membrane Structure and Function

  • Amphipathic Molecules: Cell membranes primarily consist of phospholipids. These are amphipathic, meaning one part is hydrophilic (water-loving) and the other is hydrophobic (water-fearing). Membrane proteins are also amphipathic.

  • Fluid Mosaic Model: The plasma membrane is described as fluid because its components can move, and mosaic because various proteins are embedded within it. These embedded proteins are often grouped in stable patches for specific functions. Lipid rafts are patches of lipids within these areas, though their existence is debated. Membrane components are held together mainly by hydrophobic interactions, which are weaker than covalent bonds.

  • Membrane Fluidity: Phospholipids frequently change position, swapping places with neighbors up to 107 times per second, and traveling roughly 2 nanometers per second (the length of a typical bacterium). Proteins, larger than lipids, move slower but can be immobile if attached to the cytoskeleton or extracellular matrix. Some proteins move in a directed manner, likely driven by cytoskeleton fibers and motor proteins. Membranes remain fluid if they have unsaturated hydrocarbon tails or cholesterol.

  • Cholesterol's Role: Cholesterol is a steroid that wedges between phospholipids, affecting membrane fluidity at different temperatures. At 37°C, it restricts phospholipid movement, reducing membrane fluidity. This means that cholesterol is described as a "fluidity buffer." Plants have less cholesterol than animals.

  • Evolutionary Adaptations: Membrane composition is an evolutionary adaptation. For example, fish in cold environments have more unsaturated hydrocarbon tails (which remain fluid at lower temperatures). Organisms in extreme heat have unusual lipids that prevent excessive fluidity.

  • Membrane Composition Summary: Phospholipids form the membrane's fabric, while proteins give it its function.

Cell Membrane Proteins

  • Integral Proteins: These proteins penetrate the hydrophobic core of the membrane. They are often transmembrane proteins (spanning the entire membrane), though some extend only partially. Hydrophobic regions of integral proteins are nonpolar amino acids (typically 20–30 amino acids long), which interact with the hydrophobic interior of the membrane. Hydrophilic regions face the aqueous environment on either side. Some integral proteins have channels for materials to pass through.

  • Peripheral Proteins: These proteins are not embedded in the membrane, but loosely bound to its surface, often attached to exposed parts of integral proteins. On the cytoplasmic side, peripheral proteins may be attached to the cytoskeleton; on the ECM side, they may be attached to extracellular matrix (ECM) materials. In animal cells these proteins contribute to a stronger membrane framework. They can provide single or multiple functions.

  • CD4 and HIV: CD4 is a protein on human immune cells. HIV uses CD4 and a co-receptor (CCR5) to infect the cells. Genetic variations in CCR5 can make some individuals resistant to HIV.

Cell Membrane Carbohydrates

  • Cell Recognition: Cells recognize each other by binding to molecules containing carbohydrates on their extracellular surface. These carbohydrates are short, branched chains of fewer than 15 sugar units.

  • Glycolipids and Glycoproteins: Carbohydrates, covalently bonded to lipids (glycolipids) or proteins (glycoproteins), are found on membrane surfaces. Different glycoproteins distinguish blood types and cells of different species.

Selective Permeability

  • Hydrophobic vs. Hydrophilic: Nonpolar molecules (hydrophobic) dissolve easily through the lipid bilayer, as do small enough hydrophilic molecules. Hydrophilic molecules (like glucose, sugars) pass through the membrane slower than nonpolar molecules. Hydrophilic ions are also hindered by their hydration shells.

  • Transport Proteins: Channel proteins create hydrophilic channels that allow hydrophilic substances to pass through the hydrophobic interior.

  • Aquaporins: These channel proteins specifically facilitate water movement.

  • Carrier Proteins: These proteins translocate substances by changing shape. They are extremely specific for the materials they transport, such as glucose transport proteins which allow glucose to pass far faster.

Active and Passive Transport

  • Diffusion: Particles randomly move to distribute evenly. Diffusion proceeds down the concentration gradient without energy input.

  • Passive Transport: Diffusion across the membrane, needing no extra energy input

  • Osmosis: Water diffusion across a selectively permeable membrane; it moves from a high to a low concentration of free water.

  • Tonicity: The environment's ability to cause a cell to gain or lose water. It depends on the concentration of non-penetrating solutes.

  • Isotonic: The same solute concentration inside and outside the cell, thus there is no net water movement.

  • Hypertonic: Higher solute concentration outside the cell than inside; water leaves the cell and the cell shrinks.

  • Hypotonic: Lower solute concentration outside the cell than inside; water enters the cell and the cell swells.

  • Osmoregulation: Organisms without cell walls regulate water balance.

  • Turgor Pressure: Cell walls prevent excessive water uptake in plant cells. A turgid cell is firm; a flaccid cell is limp; and plasmolysis is when a cell loses water and the plasma membrane shrinks away from the cell wall.

  • Facilitated Diffusion: Transport proteins speed up passive diffusion for polar molecules.

  • Active Transport: Moves substances against the concentration gradient which requires energy input in the form of ATP. Carrier proteins are necessary for active transport.

  • Sodium-Potassium Pump: Animal cells maintain higher K+ and lower Na+ concentrations inside than outside using this pump, which uses ATP to move these ions against their concentration gradients.

  • Membrane Potential: Difference in electrical charge across a membrane, typically negative inside (∼−50 to −200mV). This favors the entrance of positive ions and exit of negative ions.

  • Electrochemical Gradient: Drives diffusion of ions, including forces from both concentration difference and electrical charge difference.

  • Electrogenic Pump: Carrier proteins create a voltage across the membrane by moving ions in a way that generates more positive charge on one side.

  • Proton Pump: In plant, fungal, and bacterial cells, the main electrogenic pump is the proton pump, which transports H⁺ out of the cell, building a positive charge outside.

  • Cotransport: A solute's movement down its concentration gradient can power the movement of another substance against its concentration gradient. This often involves specific transport proteins ("cotransporters").

Vesicular Transport

  • Exocytosis: Secretion of materials by vesicles fusing with the plasma membrane.

  • Endocytosis: Uptake of molecules by forming vesicles from the plasma membrane.

  • Phagocytosis, Pinocytosis, Receptor-mediated endocytosis

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Explore the intricate details of cell membrane structure and function in this informative quiz. Focus on the roles of amphipathic molecules, the fluid mosaic model, and membrane fluidity. Test your understanding of how these elements contribute to cellular activity.

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