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Unit 2 - Cell Structure and Function

2.1 - Introduction

  • Cell - basic unit of life, discovered in the 17th century by Robert Hooke.

  • Two major divisions - prokaryotic and eukaryotic.

2.2 - Types of Cells

  • Prokaryotic cell - Simple cell; has no nucleus, no membrane bound organelles. Eg: bacteria. Only found in the kingdom Monera.

  • The genetic material of a prokaryotic cell - found in the nucleoid.

  • Binary fission - process where duplication of genetic material, division into half and production of two identical daughter cells occurs.

  • Eukaryotic cell - Complex cell; contains a nucleus which functions as the control center of the cell, directing DNA replication, transcription and cell growth.

  • May be unicellular or multicellular.

  • Key feature - presence of membrane-bound organelles, each with its own duties.

  • Eg: animal and plant cells.

2.3 - Endosymbiotic Theory

  • The endosymbiotic theory states that eukaryotic cells originated from a symbiotic partnership of prokaryotic cells.

  • This theory focuses on the origin of mitochondria and chloroplasts from aerobic heterotrophic and photosynthetic prokaryotes, respectively.

  • Similarities:

  • Prokaryotes are the same size are eubacteria

  • They reproduce the same way (binary fission)

  • If their ribosomes are sliced open and studied, they are found to more closely resemble those of a prokaryote than of a eukaryote.

  • Archezoa - the eukaryotic organism that scientists believe most closely resembles prokaryotes; does not have mitochondria.

  • One phylum grouped with the archezoa is the diplomonads.

  • Giardia - example of diplomonad - an infectious agent that takes hold in the intestines and denies the body the ability to absorb any fat.

2.4 - Organelles

  • Prokaryotic Organelles

  • Plasma membrane - selective barrier around a cell composed of a double layer of phospholipids.

  • Part of this selectivity is due to the many proteins that either rest on the exterior of the membrane or are embedded in the membrane of the cell.

  • Each membrane has a different combination of lipids, proteins, and carbohydrates that provide it with its unique characteristics.

  • Cell wall - wall or barrier that functions to shape and protect cells.

  • Present in all prokaryotes.

  • Ribosomes - Function as the host organelle for protein synthesis in the cell.

  • Found in the cytoplasm of cells and are composed of a large unit and a small sub unit.

    Eukaryotic Organelles

  • Ribosomes - Serve as the host organelles for protein synthesis.

  • Bound ribosomes - attached to endoplasmic reticula and form proteins that tend to be exported from the cell or sent to the membrane.

  • Free ribosomes - exist freely in the cytoplasm and produce proteins that remain in the cytoplasm of the cell.

  • Eukaryotic ribosomes - built in a structure called the nucleolus.

  • Smooth endoplasmic reticulum - membrane bound organelle involved in lipid synthesis, detoxification, and carbohydrate metabolism.

  • Liver cells - contain a lot of smooth endoplasmic reticulum (SER) as they host a lot of carbohydrate metabolism (glycolysis).

  • Called smooth ER because there are no ribosomes on its cytoplasmic surface.

  • The liver also contains SER as it is the site of alcohol detoxification.

  • Rough endoplasmic reticulum - membrane-bound organelle; termed “rough” because of the presence of ribosomes on the cytoplasmic surface of the cell.

  • The proteins produced by this RER are often secreted by the cell and carried by vesicles to the Golgi apparatus for further modification.

  • Golgi apparatus - Proteins, lipids, and other macromolecules are sent to the Golgi to be modified by the addition of sugars and other molecules to form glycoproteins.

  • The products are then sent in vesicles (escape pods that bud off the edge of the Golgi) to other parts of the cell, directed by the particular changes made by the Golgi.

  • Mitochondria - double-membraned organelles that specialize in the production of ATP.

  • Matrix - the innermost portion of the mitochondrion.

  • Cristae - the folds created by the inner of the two membranes*.*

  • Are the host organelles for the Krebs cycle (matrix) and oxidative phosphorylation (cristae) of respiration.

  • Mitochondria - the power plants of the cell.

  • Lysosome - membrane-bound organelle that specializes in digestion.

  • Contains enzymes that break down (hydrolyze) proteins, lipids, nucleic acids, and carbohydrates.

  • Is the stomach of the cell.

  • Storage diseases caused by absence of a particular lysosomal hydrolytic enzyme. Eg: Tay-Sachs disease an enzyme used to digest lipids is absent, leading to excessive accumulation of lipids in the brain.

  • Often referred to as “suicide sacs” of the cell; cells that are no longer needed are often destroyed in these sacs. Eg: the cells of the tail of a tadpole are digested as a tadpole changes into a frog.

  • Nucleus - Control center of the cell.

  • In eukaryotic cells, this is the storage site of genetic material (DNA).

  • Site of replication, transcription, and posttranscriptional modification of RNA.

  • Also contains the nucleolus, the site of ribosome synthesis.

  • Vacuole - storage organelle that acts as a vault.

  • Quite large in plant cells but small in animal cells.

  • Peroxisomes - organelles containing enzymes that produce hydrogen peroxide as a by-product while performing various functions, like breakdown of fatty acids and detoxification of alcohol in the liver.

  • Contain an enzyme that converts the toxic hydrogen peroxide by-product of these reactions into cell-friendly water.

  • Chloroplast - site of photosynthesis and energy production in plant cells.

  • Contain many pigments; provide leaves with their color.

  • Divided into an inner portion and an outer portion.

  • Stroma - inner fluid portion, ****surrounded by two outer membranes.

  • Thylakoid membrane system - inner membrane winding through the stroma, where the light-dependent reactions of photosynthesis occur.

  • The light-independent (dark) reactions occur in the stroma.

  • Cytoskeleton - The skeleton of cells consists of three types of fibers that provide support, shape, and mobility to cells: microtubules, microfilaments, and intermediate filaments.

  • Microtubules constructed from tubulin, have a lead role in the separation of cells during cell division; are also important components of cilia and flagella, which are structures that aid the movement of particles.

  • Microfilaments, constructed from actin, play a big part in muscular contraction.

  • Intermediate filaments constructed from a class of proteins called keratins; are thought to function as reinforcement for the shape and position of organelles in the cell.

  • Animal cells - contain all the structures mentioned except cell walls and chloroplasts; their vacuoles are small. Have centrioles (cell division structure).

  • Plant cells - contain all the structures mentioned; their vacuoles are large. Do not have centrioles.

  • As a cell grows in size, its internal volume increases, and its cell membrane (surface area) expands to respond to the growth of the cell.

  • However, as the volume of the cell increases, the cell membrane (surface area) does not keep up.

  • This results in the cell not having enough surface area to pass materials produced by the increasing volume of the cell. So the cell must stop growing in order to survive.

2.5 - Cell Size

https://s3.amazonaws.com/knowt-user-attachments/images%2F1634591881997-1634591881997.png

  • As a cell gets larger, its volume increases at a faster rate than its surface area.

  • If the cell radius increases by 10 times, the surface area increases by 100 times, but the volume increases by 1000 times.

  • A cell’s surface area must be large enough to meet the metabolic needs of its volume.

  • As the surface-area-to-volume ratio of a cell increases, the exchange efficiency of materials with the environment increases as well.

  • The surface-area-to-volume ratio affects the ability of a cell to maintain homeostasis between its internal environment and external environment.

2.6 - Cell Membranes: Fluid Mosaic Model

  • Cell membrane - selective barrier surrounding a cell; has a phospholipid bilayer as its major structural component.

  • Outer portion of bilayer - hydrophilic head; inner portion - hydrophobic tail.

  • Cross section of a cell membrane showing phospholipid bilayer.

  • Fluid mosaic model the most accepted model for the arrangement of membranes.

  • It states that the membrane consists of a phospholipid bilayer with proteins of various lengths and sizes interspersed with cholesterol among the phospholipids.

  • These proteins perform various functions depending on their location within the membrane.

  • Consists of integral proteins - implanted within the bilayer and can extend partway or all the way across the membrane; peripheral proteins - not implanted in the bilayer and are often attached to integral proteins of the membrane.

  • A protein that stretches across the membrane can function as a channel to assist the passage of desired molecules into the cell.

  • Proteins on the exterior of a membrane with binding sites can act as receptors that allow the cell to respond to external signals such as hormones.

  • Proteins embedded in the membrane can also function as enzymes, increasing the rate of cellular reactions.

  • The cell membrane is “selectively” permeable - it allows some molecules and other substances through, while others are not permitted to pass.

  • Factors of selectivity - the size of the substance, the charge.

  • Lets small, uncharged polar substances and hydrophobic substances such as lipids through the membrane, but larger uncharged polar substances (such as glucose) and charged ions (such as sodium) cannot pass through.

  • The particular arrangement of proteins in the lipid bilayer is also a factor.

2.7 - Types of Cell Transport

Diffusion

  • The movement of molecules down their concentration gradient without the use of energy.

  • Passive process during which substances move from a region of higher concentration to a region of lower concentration.

  • The rate of diffusion of substances varies from membrane to membrane because of different selective permeabilities.

    Osmosis

  • the passive diffusion of water down its concentration gradient across selectively permeable membranes.

  • Water moves from a region of high water concentration to a region of low water concentration.

  • Water will also flow from a region with a lower solute concentration (hypotonic) to a region with a higher solute concentration (hypertonic).

  • Isotonic indicates there is no net movement of water across the membrane.

  • Does not require the input of energy.

  • For example, visualize two regions—one with 10 particles of sodium per liter of water; the other with 15. Osmosis would drive water from the region with 10 particles of sodium toward the region with 15 particles of sodium.

https://s3.amazonaws.com/knowt-user-attachments/images%2F1634591882281-1634591882280.png

  • Diffusion. If a drop of colored ink is dropped into a beaker of water (a) its molecules dissolve (b) and diffuse (c). Eventually, diffusion results in an even distribution of ink molecules throughout the water (d).

  • How solutes create osmotic pressure. In a hypertonic solution, water moves out of the cell, causing the cell to shrivel.

  • In an isotonic solution, water diffuses into and out of the cell at the same rate, with no change in cell size.

  • In a hypotonic solution, water moves into the cell. Direction and amount of water movement is shown with blue arrows (top).

  • As water enters the cell from a hypotonic solution, pressure is applied to the plasma membrane until the cell ruptures.

  • Water enters the cell due to osmotic pressure from the higher solute concentration in the cell.

  • Osmotic pressure is measured as the force needed to stop osmosis.

  • The strong cell wall of plant cells can withstand the hydrostatic pressure to keep the cell from rupturing.

  • This is not the case with animal cells.

    Facilitated diffusion

  • the diffusion of particles across a selectively permeable membrane with the assistance of the membrane’s transport proteins.

  • These proteins will not bring any old molecule looking for a free pass into the cell; they are specific in what they will carry and have binding sites designed for molecules of interest.

  • Does not require the input of energy.

    Active transport

  • the movement of a particle across a selectively permeable membrane against its concentration gradient (from low concentration to high).

  • Requires the input of energy, which is why it is termed “active” transport.

  • Adenosine triphosphate (ATP) - called on to provide the energy.

  • These active-transport systems are vital to the ability of cells to maintain particular concentrations of substances despite environmental concentrations.

  • Eg: cells have a very high concentration of potassium and a very low concentration of sodium. Diffusion would like to move sodium in and potassium out to equalize the concentrations. Sodium-potassium pump actively moves potassium into the cell and sodium out of the cell against their respective concentration gradients to maintain appropriate levels inside the cell.

    Endocytosis

  • a process in which substances are brought into cells by the enclosure of the substance into a membrane-created vesicle that surrounds the substance and escorts it into the cell.

  • Used by immune cells called phagocytes to engulf and eliminate foreign invaders.

    Exocytosis

  • a process in which substances are exported out of the cell (the reverse of endocytosis).

  • vesicle - escorts the substance to the plasma membrane, causes it to fuse with the membrane, and ejects the contents of the substance outside the cell; functions as the trash chute of the cell.

https://s3.amazonaws.com/knowt-user-attachments/images%2F1634591880663-1634591880663.png

Pinocytosis

  • process of bringing in droplets of extracellular fluid via tiny vesicles.

    Receptor-mediated endocytosis

  • a specialized type of pinocytosis that moves specific molecules into a cell due to the budding of specific molecules with receptor sites on the cell membrane.

2.8 - Water Potential

  • Water potential (Ψ = Ψp + Ψs) indicates how freely water molecules can move in a particular environment or system.

  • Determined by the solute potential and pressure potential of each environment.

  • Solute potential (Ψs)/osmotic potential depends on the amount of solute in a solution; decreases as the concentration of solute increases. Is negative in a plant cell and zero in distilled water.

  • Pressure potential (Ψp)/turgor potential - the physical pressure exerted by objects or cell membranes on water molecules and increases with increasing pressure. Plant cells maintain a positive pressure to hold their shape, allowing them to stay rigid.

https://s3.amazonaws.com/knowt-user-attachments/images%2F1634591881821-1634591881821.png

https://s3.amazonaws.com/knowt-user-attachments/images%2F1634591882501-1634591882500.png

https://s3.amazonaws.com/knowt-user-attachments/images%2F1634591882121-1634591882120.png

Determining water potential

  • a. Cell walls exert pressure in the opposite direction of cell turgor pressure.

  • b. Using the given solute potentials, predict the direction of water movement based only on solute potential.

  • c. Total water potential is the sum of ψs and ψp. Because the water potential inside the cell equals that of the solution, there is no net movement of water.

  • In an open container, water potential is equal to the solute potential due to the pressure potential being zero in an open container.

  • Osmoregulation - the ability to maintain water balance; allows organisms to control their internal environments by maintaining the right concentrations of solutes and the amount of water in their body fluids.

2.1 - Introduction

  • Cell - basic unit of life, discovered in the 17th century by Robert Hooke.

  • Two major divisions - prokaryotic and eukaryotic.

2.2 - Types of Cells

  • Prokaryotic cell - Simple cell; has no nucleus, no membrane bound organelles. Eg: bacteria. Only found in the kingdom Monera.

  • The genetic material of a prokaryotic cell - found in the nucleoid.

  • Binary fission - process where duplication of genetic material, division into half and production of two identical daughter cells occurs.

  • Eukaryotic cell - Complex cell; contains a nucleus which functions as the control center of the cell, directing DNA replication, transcription and cell growth.

  • May be unicellular or multicellular.

  • Key feature - presence of membrane-bound organelles, each with its own duties.

  • Eg: animal and plant cells.

2.3 - Endosymbiotic Theory

  • The endosymbiotic theory states that eukaryotic cells originated from a symbiotic partnership of prokaryotic cells.

  • This theory focuses on the origin of mitochondria and chloroplasts from aerobic heterotrophic and photosynthetic prokaryotes, respectively.

  • Similarities:

  • Prokaryotes are the same size are eubacteria

  • They reproduce the same way (binary fission)

  • If their ribosomes are sliced open and studied, they are found to more closely resemble those of a prokaryote than of a eukaryote.

  • Archezoa - the eukaryotic organism that scientists believe most closely resembles prokaryotes; does not have mitochondria.

  • One phylum grouped with the archezoa is the diplomonads.

  • Giardia - example of diplomonad - an infectious agent that takes hold in the intestines and denies the body the ability to absorb any fat.

2.4 - Organelles

  • Prokaryotic Organelles

  • Plasma membrane - selective barrier around a cell composed of a double layer of phospholipids.

  • Part of this selectivity is due to the many proteins that either rest on the exterior of the membrane or are embedded in the membrane of the cell.

  • Each membrane has a different combination of lipids, proteins, and carbohydrates that provide it with its unique characteristics.

  • Cell wall - wall or barrier that functions to shape and protect cells.

  • Present in all prokaryotes.

  • Ribosomes - Function as the host organelle for protein synthesis in the cell.

  • Found in the cytoplasm of cells and are composed of a large unit and a small sub unit.

    Eukaryotic Organelles

  • Ribosomes - Serve as the host organelles for protein synthesis.

  • Bound ribosomes - attached to endoplasmic reticula and form proteins that tend to be exported from the cell or sent to the membrane.

  • Free ribosomes - exist freely in the cytoplasm and produce proteins that remain in the cytoplasm of the cell.

  • Eukaryotic ribosomes - built in a structure called the nucleolus.

  • Smooth endoplasmic reticulum - membrane bound organelle involved in lipid synthesis, detoxification, and carbohydrate metabolism.

  • Liver cells - contain a lot of smooth endoplasmic reticulum (SER) as they host a lot of carbohydrate metabolism (glycolysis).

  • Called smooth ER because there are no ribosomes on its cytoplasmic surface.

  • The liver also contains SER as it is the site of alcohol detoxification.

  • Rough endoplasmic reticulum - membrane-bound organelle; termed “rough” because of the presence of ribosomes on the cytoplasmic surface of the cell.

  • The proteins produced by this RER are often secreted by the cell and carried by vesicles to the Golgi apparatus for further modification.

  • Golgi apparatus - Proteins, lipids, and other macromolecules are sent to the Golgi to be modified by the addition of sugars and other molecules to form glycoproteins.

  • The products are then sent in vesicles (escape pods that bud off the edge of the Golgi) to other parts of the cell, directed by the particular changes made by the Golgi.

  • Mitochondria - double-membraned organelles that specialize in the production of ATP.

  • Matrix - the innermost portion of the mitochondrion.

  • Cristae - the folds created by the inner of the two membranes*.*

  • Are the host organelles for the Krebs cycle (matrix) and oxidative phosphorylation (cristae) of respiration.

  • Mitochondria - the power plants of the cell.

  • Lysosome - membrane-bound organelle that specializes in digestion.

  • Contains enzymes that break down (hydrolyze) proteins, lipids, nucleic acids, and carbohydrates.

  • Is the stomach of the cell.

  • Storage diseases caused by absence of a particular lysosomal hydrolytic enzyme. Eg: Tay-Sachs disease an enzyme used to digest lipids is absent, leading to excessive accumulation of lipids in the brain.

  • Often referred to as “suicide sacs” of the cell; cells that are no longer needed are often destroyed in these sacs. Eg: the cells of the tail of a tadpole are digested as a tadpole changes into a frog.

  • Nucleus - Control center of the cell.

  • In eukaryotic cells, this is the storage site of genetic material (DNA).

  • Site of replication, transcription, and posttranscriptional modification of RNA.

  • Also contains the nucleolus, the site of ribosome synthesis.

  • Vacuole - storage organelle that acts as a vault.

  • Quite large in plant cells but small in animal cells.

  • Peroxisomes - organelles containing enzymes that produce hydrogen peroxide as a by-product while performing various functions, like breakdown of fatty acids and detoxification of alcohol in the liver.

  • Contain an enzyme that converts the toxic hydrogen peroxide by-product of these reactions into cell-friendly water.

  • Chloroplast - site of photosynthesis and energy production in plant cells.

  • Contain many pigments; provide leaves with their color.

  • Divided into an inner portion and an outer portion.

  • Stroma - inner fluid portion, ****surrounded by two outer membranes.

  • Thylakoid membrane system - inner membrane winding through the stroma, where the light-dependent reactions of photosynthesis occur.

  • The light-independent (dark) reactions occur in the stroma.

  • Cytoskeleton - The skeleton of cells consists of three types of fibers that provide support, shape, and mobility to cells: microtubules, microfilaments, and intermediate filaments.

  • Microtubules constructed from tubulin, have a lead role in the separation of cells during cell division; are also important components of cilia and flagella, which are structures that aid the movement of particles.

  • Microfilaments, constructed from actin, play a big part in muscular contraction.

  • Intermediate filaments constructed from a class of proteins called keratins; are thought to function as reinforcement for the shape and position of organelles in the cell.

  • Animal cells - contain all the structures mentioned except cell walls and chloroplasts; their vacuoles are small. Have centrioles (cell division structure).

  • Plant cells - contain all the structures mentioned; their vacuoles are large. Do not have centrioles.

  • As a cell grows in size, its internal volume increases, and its cell membrane (surface area) expands to respond to the growth of the cell.

  • However, as the volume of the cell increases, the cell membrane (surface area) does not keep up.

  • This results in the cell not having enough surface area to pass materials produced by the increasing volume of the cell. So the cell must stop growing in order to survive.

2.5 - Cell Size

https://s3.amazonaws.com/knowt-user-attachments/images%2F1634591881997-1634591881997.png

  • As a cell gets larger, its volume increases at a faster rate than its surface area.

  • If the cell radius increases by 10 times, the surface area increases by 100 times, but the volume increases by 1000 times.

  • A cell’s surface area must be large enough to meet the metabolic needs of its volume.

  • As the surface-area-to-volume ratio of a cell increases, the exchange efficiency of materials with the environment increases as well.

  • The surface-area-to-volume ratio affects the ability of a cell to maintain homeostasis between its internal environment and external environment.

2.6 - Cell Membranes: Fluid Mosaic Model

  • Cell membrane - selective barrier surrounding a cell; has a phospholipid bilayer as its major structural component.

  • Outer portion of bilayer - hydrophilic head; inner portion - hydrophobic tail.

  • Cross section of a cell membrane showing phospholipid bilayer.

  • Fluid mosaic model the most accepted model for the arrangement of membranes.

  • It states that the membrane consists of a phospholipid bilayer with proteins of various lengths and sizes interspersed with cholesterol among the phospholipids.

  • These proteins perform various functions depending on their location within the membrane.

  • Consists of integral proteins - implanted within the bilayer and can extend partway or all the way across the membrane; peripheral proteins - not implanted in the bilayer and are often attached to integral proteins of the membrane.

  • A protein that stretches across the membrane can function as a channel to assist the passage of desired molecules into the cell.

  • Proteins on the exterior of a membrane with binding sites can act as receptors that allow the cell to respond to external signals such as hormones.

  • Proteins embedded in the membrane can also function as enzymes, increasing the rate of cellular reactions.

  • The cell membrane is “selectively” permeable - it allows some molecules and other substances through, while others are not permitted to pass.

  • Factors of selectivity - the size of the substance, the charge.

  • Lets small, uncharged polar substances and hydrophobic substances such as lipids through the membrane, but larger uncharged polar substances (such as glucose) and charged ions (such as sodium) cannot pass through.

  • The particular arrangement of proteins in the lipid bilayer is also a factor.

2.7 - Types of Cell Transport

Diffusion

  • The movement of molecules down their concentration gradient without the use of energy.

  • Passive process during which substances move from a region of higher concentration to a region of lower concentration.

  • The rate of diffusion of substances varies from membrane to membrane because of different selective permeabilities.

    Osmosis

  • the passive diffusion of water down its concentration gradient across selectively permeable membranes.

  • Water moves from a region of high water concentration to a region of low water concentration.

  • Water will also flow from a region with a lower solute concentration (hypotonic) to a region with a higher solute concentration (hypertonic).

  • Isotonic indicates there is no net movement of water across the membrane.

  • Does not require the input of energy.

  • For example, visualize two regions—one with 10 particles of sodium per liter of water; the other with 15. Osmosis would drive water from the region with 10 particles of sodium toward the region with 15 particles of sodium.

https://s3.amazonaws.com/knowt-user-attachments/images%2F1634591882281-1634591882280.png

  • Diffusion. If a drop of colored ink is dropped into a beaker of water (a) its molecules dissolve (b) and diffuse (c). Eventually, diffusion results in an even distribution of ink molecules throughout the water (d).

  • How solutes create osmotic pressure. In a hypertonic solution, water moves out of the cell, causing the cell to shrivel.

  • In an isotonic solution, water diffuses into and out of the cell at the same rate, with no change in cell size.

  • In a hypotonic solution, water moves into the cell. Direction and amount of water movement is shown with blue arrows (top).

  • As water enters the cell from a hypotonic solution, pressure is applied to the plasma membrane until the cell ruptures.

  • Water enters the cell due to osmotic pressure from the higher solute concentration in the cell.

  • Osmotic pressure is measured as the force needed to stop osmosis.

  • The strong cell wall of plant cells can withstand the hydrostatic pressure to keep the cell from rupturing.

  • This is not the case with animal cells.

    Facilitated diffusion

  • the diffusion of particles across a selectively permeable membrane with the assistance of the membrane’s transport proteins.

  • These proteins will not bring any old molecule looking for a free pass into the cell; they are specific in what they will carry and have binding sites designed for molecules of interest.

  • Does not require the input of energy.

    Active transport

  • the movement of a particle across a selectively permeable membrane against its concentration gradient (from low concentration to high).

  • Requires the input of energy, which is why it is termed “active” transport.

  • Adenosine triphosphate (ATP) - called on to provide the energy.

  • These active-transport systems are vital to the ability of cells to maintain particular concentrations of substances despite environmental concentrations.

  • Eg: cells have a very high concentration of potassium and a very low concentration of sodium. Diffusion would like to move sodium in and potassium out to equalize the concentrations. Sodium-potassium pump actively moves potassium into the cell and sodium out of the cell against their respective concentration gradients to maintain appropriate levels inside the cell.

    Endocytosis

  • a process in which substances are brought into cells by the enclosure of the substance into a membrane-created vesicle that surrounds the substance and escorts it into the cell.

  • Used by immune cells called phagocytes to engulf and eliminate foreign invaders.

    Exocytosis

  • a process in which substances are exported out of the cell (the reverse of endocytosis).

  • vesicle - escorts the substance to the plasma membrane, causes it to fuse with the membrane, and ejects the contents of the substance outside the cell; functions as the trash chute of the cell.

https://s3.amazonaws.com/knowt-user-attachments/images%2F1634591880663-1634591880663.png

Pinocytosis

  • process of bringing in droplets of extracellular fluid via tiny vesicles.

    Receptor-mediated endocytosis

  • a specialized type of pinocytosis that moves specific molecules into a cell due to the budding of specific molecules with receptor sites on the cell membrane.

2.8 - Water Potential

  • Water potential (Ψ = Ψp + Ψs) indicates how freely water molecules can move in a particular environment or system.

  • Determined by the solute potential and pressure potential of each environment.

  • Solute potential (Ψs)/osmotic potential depends on the amount of solute in a solution; decreases as the concentration of solute increases. Is negative in a plant cell and zero in distilled water.

  • Pressure potential (Ψp)/turgor potential - the physical pressure exerted by objects or cell membranes on water molecules and increases with increasing pressure. Plant cells maintain a positive pressure to hold their shape, allowing them to stay rigid.

https://s3.amazonaws.com/knowt-user-attachments/images%2F1634591881821-1634591881821.png

https://s3.amazonaws.com/knowt-user-attachments/images%2F1634591882501-1634591882500.png

https://s3.amazonaws.com/knowt-user-attachments/images%2F1634591882121-1634591882120.png

Determining water potential

  • a. Cell walls exert pressure in the opposite direction of cell turgor pressure.

  • b. Using the given solute potentials, predict the direction of water movement based only on solute potential.

  • c. Total water potential is the sum of ψs and ψp. Because the water potential inside the cell equals that of the solution, there is no net movement of water.

  • In an open container, water potential is equal to the solute potential due to the pressure potential being zero in an open container.

  • Osmoregulation - the ability to maintain water balance; allows organisms to control their internal environments by maintaining the right concentrations of solutes and the amount of water in their body fluids.