Biology Chapter: Cell Theory and Structures

Cell theory states that:

1. All living things are composed of cells.
2. All cells come from preexisting cells.
3. The cell is the basic unit of life.

This theory revolutionized biology, explaining how diverse organisms are all related at a fundamental level. It provided a basis for understanding the structure and function of living organisms, and it paved the way for advancements in fields such as medicine, genetics, and biotechnology.

A typical prokaryotic cell has a cell membrane, cytoplasm, ribosomes, and a single circular DNA molecule located in a region called the nucleoid. It may also have a cell wall, a capsule, flagella, and pili.

<p>Both bacteria and archaea are prokaryotes, meaning they lack a nucleus and other membrane-bound organelles. However, they differ in several key ways: Bacteria have peptidoglycan in their cell walls, while archaea do not. Bacteria are typically found in a wide range of environments, while archaea often inhabit extreme environments like hot springs and salt lakes. The genetic makeup of archaea is distinct from that of bacteria, and they have unique metabolic pathways.</p> Signup and view all the answers

<p>Here are some defining characteristics of various eukaryotic cell types:</p> <ul> <li> <strong>Protists:</strong> A diverse group, often single-celled, with various modes of nutrition (autotrophic, heterotrophic). Some protists exhibit colonial forms.</li> <li> <strong>Plants:</strong> Multicellular, photosynthetic, with cell walls composed of cellulose. They exhibit a large central vacuole and chloroplasts.</li> <li> <strong>Animals:</strong> Multicellular, heterotrophic, lack cell walls, and typically have specialized tissues and organs.</li> <li> <strong>Fungi:</strong> Heterotrophic, often multicellular, with cell walls composed of chitin. They obtain nutrients by absorbing organic matter.</li> </ul> Signup and view all the answers

<p>Plant and animal cells are both eukaryotic cells, but they differ in several key ways. Plant cells have a cell wall, chloroplasts, and a large central vacuole, which are absent in animal cells. These structures contribute to the unique abilities of plants, such as photosynthesis and storage of water and nutrients. Animal cells, on the other hand, have specialized structures like centrioles that help in cell division and movement.</p> Signup and view all the answers

<p>A typical animal cell is characterized by a cell membrane, cytoplasm, ribosomes, a nucleus containing DNA, and a variety of membrane-bound organelles such as the endoplasmic reticulum, Golgi apparatus, mitochondria, lysosomes, centrioles, and vacuoles. Animal cells are typically smaller and more varied in shape than plant cells.</p> Signup and view all the answers

<p>A typical plant cell is characterized by a cell membrane, cytoplasm, ribosomes, a nucleus containing DNA, a large central vacuole, chloroplasts, and a rigid cell wall made primarily of cellulose. Plant cells are typically larger and more rectangular in shape than animal cells and have specialized structures that allow them to carry out photosynthesis and store water and nutrients.</p> Signup and view all the answers

<p>Cell organelles are specialized structures within cells that perform specific functions. Some key organelles include:</p> <ul> <li> <strong>Nucleus:</strong> Contains the cell's DNA and controls cellular activities.</li> <li> <strong>Endoplasmic reticulum (ER):</strong> A network of membranes involved in protein and lipid synthesis.</li> <li> <strong>Golgi apparatus:</strong> Processes and packages proteins and lipids for secretion.</li> <li> <strong>Mitochondria:</strong> The powerhouses of the cell, responsible for ATP production through cellular respiration.</li> <li> <strong>Lysosomes:</strong> Contain enzymes that break down waste materials and cellular debris.</li> <li> <strong>Vacuoles:</strong> Storage compartments for water, nutrients, and waste products.</li> <li> <strong>Chloroplasts (in plants):</strong> Sites of photosynthesis.</li> <li> <strong>Cell wall (in plants):</strong> Provides structural support and protection.</li> <li> <strong>Centrioles (in animals):</strong> Involved in cell division and organization of microtubules.</li> </ul> Signup and view all the answers

<p>The Fluid Mosaic Model describes the cell membrane as a fluid structure composed of a phospholipid bilayer embedded with proteins.</p> <p>The phospholipid bilayer is composed of two layers of phospholipid molecules, each with a hydrophilic phosphate head and a hydrophobic fatty acid tail. The hydrophilic heads face the watery environment inside and outside the cell, while the hydrophobic tails are tucked inside the bilayer, forming a barrier to water.</p> <p>Proteins are embedded within the phospholipid bilayer, some extending across the entire membrane (integral proteins) and others attached to only one side (peripheral proteins). These proteins play vital roles in transporting molecules across the membrane, receiving signals from the environment, and anchoring the cell to other structures. The fluid nature of the membrane allows the phospholipid molecules and proteins to move laterally within the bilayer, providing flexibility and dynamism to the cell membrane.</p> Signup and view all the answers

<p>Passive transport refers to the movement of substances across a cell membrane without requiring the cell to expend energy. This movement is driven by the concentration gradient, the difference in concentration of the substance across the membrane. Substances move from an area of high concentration to an area of low concentration until equilibrium is reached.</p> <p>Several types of passive transport include:</p> <ul> <li> <strong>Diffusion:</strong> The movement of molecules from an area of high concentration to an area of low concentration.</li> <li> <strong>Facilitated diffusion:</strong> Diffusion of molecules assisted by membrane proteins that act as channels or carriers.</li> <li> <strong>Osmosis:</strong> The movement of water molecules across a semi-permeable membrane from a region of high water concentration to a region of low water concentration.</li> </ul> Signup and view all the answers

<p>Diffusion is the movement of molecules from an area of high concentration to an area of low concentration. This movement is driven by random thermal motion of molecules, causing them to spread out until they are evenly distributed. This process does not require energy and is considered a form of passive transport.</p> <p>A classic example of diffusion is the spreading of perfume in a room. If you spray perfume in one corner of the room, the fragrance molecules will gradually move from the area of high concentration towards areas of lower concentration, eventually becoming evenly distributed allowing you to smell the perfume throughout the room.</p> Signup and view all the answers

<p>Facilitated diffusion is a type of passive transport where molecules move across a membrane with the help of specific transport proteins. These proteins act as channels or carriers, providing pathways for molecules to cross the membrane, even if the molecules are too large or polar to pass through directly. Unlike simple diffusion, facilitated diffusion is saturable, meaning that the rate of transport can be influenced by the concentration of the transported molecule and the number of transport proteins available.</p> <p>For example, glucose molecules are too large and polar to pass through the cell membrane directly by diffusion. However, glucose transport proteins (also known as glucose transporters) are embedded in the membrane, providing a path for glucose to enter the cell. When glucose binds to the transport protein, it causes a conformational change in the protein, opening a channel for glucose to move across the membrane.</p> Signup and view all the answers

<p>Osmosis is the movement of water molecules across a semi-permeable membrane from a region of high water concentration to a region of low water concentration. This movement is driven by the difference in water potential between the two regions, which is influenced by the concentration of solutes in the solutions.</p> <p>The effects of osmosis on cells depend on the tonicity of the solution, which refers to the relative concentration of solutes inside and outside the cell.</p> <ul> <li> <p><strong>Hypertonic Solution:</strong> A hypertonic solution has a higher concentration of solutes outside the cell than inside. Water will move out of the cell, causing it to shrink or shrivel.</p> </li> <li> <p><strong>Hypotonic Solution:</strong> A hypotonic solution has a lower concentration of solutes outside the cell than inside. Water will move into the cell, causing it to swell. If too much water moves into the cell, it could burst.</p> </li> <li> <p><strong>Isotonic Solution:</strong> An isotonic solution has an equal concentration of solutes inside and outside the cell. There is no net movement of water, and the cell maintains its normal shape.</p> </li> </ul> Signup and view all the answers

<p>Hypertonic, hypotonic, and isotonic solutions are terms used to describe the relative concentration of solutes in two solutions, typically when comparing a solution to the interior of a cell.</p> <ul> <li> <p><strong>Hypertonic Solution:</strong> A hypertonic solution has a higher concentration of solutes than the solution it's being compared to. In the context of a cell, this means the solution outside the cell has a higher concentration of solutes than the inside of the cell.</p> </li> <li> <p><strong>Hypotonic Solution:</strong> A hypotonic solution has a lower concentration of solutes than the solution it's being compared to. In the context of a cell, this means the solution outside the cell has a lower concentration of solutes than the inside of the cell.</p> </li> <li> <p><strong>Isotonic Solution:</strong> An isotonic solution has an equal concentration of solutes as the solution it's being compared to. In the context of a cell, this means the solution outside the cell has an equal concentration of solutes as the inside of the cell.</p> </li> </ul> Signup and view all the answers

<p>Endocytosis and exocytosis are two important processes that allow cells to transport large molecules, particles, and even other cells across the cell membrane. They are essentially opposite processes:</p> <ul> <li> <p><strong>Endocytosis:</strong> The process of taking in material from the outside of the cell. The cell membrane invaginates, forming a pocket around the substance, ultimately pinching off to form a vesicle that encloses the material. This vesicle then moves inside the cell, where its contents can be processed or released.</p> </li> <li> <p><strong>Exocytosis:</strong> The process of releasing material from the inside of the cell. Vesicles containing the material fuse with the cell membrane, releasing their contents outside the cell. This process is vital for secreting hormones, enzymes, signaling molecules, and waste products.</p> </li> </ul> Signup and view all the answers

<p>Unlike passive transport, which relies on the concentration gradient, active transport requires the cell to expend energy to move substances across the membrane against their concentration gradient. The primary source of energy for active transport is the molecule adenosine triphosphate (ATP). ATP is produced in the cell's mitochondria through cellular respiration. Active transport proteins use the energy from ATP to change shape and move substances across the membrane, even if the substance is moving from an area of low concentration to an area of high concentration.</p> Signup and view all the answers

<p>Passive transport and active transport are two distinct mechanisms for moving substances across the cell membrane.</p> <ul> <li> <p><strong>Passive Transport:</strong> This process does not require the cell to expend energy. Substance movement is driven by the concentration gradient, moving from a high concentration to a low concentration area. Examples include diffusion, facilitated diffusion, and osmosis.</p> </li> <li> <p><strong>Active Transport:</strong> Requires the cell to expend energy, typically in the form of ATP, to move substances across the membrane against their concentration gradient. This is essential for maintaining gradients of ions and nutrients vital for cellular function. Examples include the sodium-potassium pump and other protein pumps.</p> </li> </ul> Signup and view all the answers

<p>Aquaporins are specialized channel proteins embedded in the cell membrane, providing a pathway for water molecules to cross the membrane. These channels are highly selective for water, allowing it to move across the membrane much faster than simple diffusion. Aquaporins play a crucial role in facilitating water movement across the membrane, particularly in cells that need to regulate their water content rapidly, such as kidney cells and plant cells. They are vital for maintaining cell volume, transporting water, and regulating the osmotic balance of cells.</p> Signup and view all the answers

<p>Neurons, also known as nerve cells, are specialized cells that transmit electrical signals throughout the nervous system. They are composed of three primary parts:</p> <ul> <li> <strong>Cell Body (Soma):</strong> Contains the nucleus and other organelles, carrying out the neuron's basic functions.</li> <li> <strong>Dendrites:</strong> Branching extensions that receive incoming signals from other neurons or sensory receptors.</li> <li> <strong>Axon:</strong> A long, single extension that transmits outgoing signals to other neurons, muscles, or glands.</li> </ul> <p>The axon is typically covered with a myelin sheath, which acts as an insulator, increasing the speed of signal transmission. The gaps between segments of the myelin sheath are called nodes of Ranvier. At the nodes, electrical signals jump between segments, further speeding up transmission. Neurons are crucial for communication and control throughout the nervous system, allowing us to sense our environment, process information, and respond appropriately.</p> Signup and view all the answers

<p>Active transport would be the best type of cell transport to move substances into or out of the cell quickly, as shown in the graph. Active transport is energy-dependent and can overcome the concentration gradient to move substances rapidly, unlike diffusion and facilitated diffusion, which rely on the concentration gradient and may be slower, especially with increasing concentration gradients.</p> Signup and view all the answers

<p>Active transport is the best option for a cell that needs to respond quickly to a sudden change in concentration gradient. Active transport is not limited by the concentration gradient, unlike diffusion and facilitated diffusion, which rely on concentration gradients. While diffusion and facilitated diffusion are effective for maintaining equilibrium in the long term, active transport is crucial for rapidly responding to changes in the cell's environment.</p> Signup and view all the answers

<p>The line representing facilitated diffusion levels off at higher concentrations because the transport proteins responsible for facilitated diffusion become saturated. While diffusion is non-saturable and its rate increases linearly with increasing concentration, facilitated diffusion has capacity limitations. Once all the available transport proteins are bound to the transported molecules, the rate of facilitated diffusion plateaus, even if the concentration continues to increase.</p> Signup and view all the answers

<p>Active transport initially starts with a high rate compared to diffusion and facilitated diffusion because it requires energy input. Since active transport is not limited by concentration gradients, it can begin transporting substances rapidly, even when the concentration gradient is low. As the concentration gradient increases, the rate of diffusion and facilitated diffusion increases, but active transport maintains its high rate because it is coupled with the continuous input of energy.</p> Signup and view all the answers

<p>Spraying produce with water in a grocery store is a practice that takes advantage of osmosis. Osmosis is the diffusion of water across a selectively permeable membrane from a region of high water concentration to a region of low water concentration. The solution on the surface of the produce is hypotonic to the solution inside the produce cells, drawing water into the cells due to osmosis. This water movement would cause the produce to become more plump and firm, improving its visual appeal and perceived freshness.<br /> This change is considered desirable because it enhances the produce's attractiveness to consumers, suggesting it is fresh and high-quality, making it more appealing for purchase and consumption.</p> Signup and view all the answers

<p>The wilting of the lettuce and the presence of water in the bowl after being in the refrigerator can be explained using the principles of osmosis. When salt and seasonings were added to the lettuce, the solution surrounding the lettuce cells became hypertonic—meaning it had a higher solute concentration than the inside of the lettuce cells. Because of this hypertonic environment, water moved out of the lettuce cells and into the surrounding solution due to osmosis. This water loss caused the lettuce cells to shrink and the lettuce to wilt. The water that moved out of the lettuce cells collected in the bottom of the bowl.</p> Signup and view all the answers

<p>Drinking an excessive amount of water in a short period can lead to a dangerous condition called water intoxication or hyponatremia. This occurs when the concentration of sodium in the blood becomes dangerously low. When a person drinks too much water, the solution surrounding their brain cells becomes hypotonic, meaning it has a lower concentration of solutes (including sodium) than the inside of the brain cells. Due to osmosis, water moves into the brain cells, causing them to swell. This swelling of brain cells, also known as cerebral edema, can lead to increased pressure in the skull, known as intracranial pressure, which can put pressure on the brain, leading to a range of neurological symptoms. In severe cases, excessive water consumption can be fatal as the swelling of brain cells disrupts normal brain function.</p> Signup and view all the answers

<p>Facilitated diffusion is a type of passive transport where molecules move across a membrane with the help of specific transport proteins. These proteins act as channels or carriers, providing pathways for molecules to cross the membrane, even if the molecules are too large or polar to pass through directly. Unlike simple diffusion, facilitated diffusion is saturable, meaning that the rate of transport can be influenced by the concentration of the transported molecule and the number of transport proteins available. For example, glucose molecules are too large and polar to pass through the cell membrane directly by diffusion. However, glucose transport proteins (also known as glucose transporters) are embedded in the membrane, providing a path for glucose to enter the cell. When glucose binds to the transport protein, it causes a conformational change in the protein, opening a channel for glucose to move across the membrane.</p> Signup and view all the answers