2.1 Cell Structure: : Subcellular Components

Cell Theory

1) All living organisms are composed of one or more cells.

2) A cell is the basic unit of life

3) All cells come from preexisting cells

- Two types of cells: Prokaryotic and Eurkaryotic

- Archaea and bacteria are prokaryotic & the domains of life

- Eukarya is the third domain including fungi, protists, plants, and animals

Prokaryotic Cells

•  Chromosomes (DNA) are grouped together in a region called the nucleoid,

• There is no nuclear membrane and therefore no true nucleus. 

• No membrane-bounded organelles are found in the cytosol. (Ribosomes are found, but they are not membrane bound.) (not contained by a membrane.)

• Prokaryotes are much smaller than eukaryotes.

Eurkaryotic Cells

• A membrane-enclosed nucleus contains the cell's chromosomes.

• Many membrane-bounded organelles are found in the cytoplasm. 

• Has Internal Structues known as organells

• Eukaryotes are much larger than prokaryotes

Cell Similarities

• Prokaryotes and Eukaryotic cells both include:

  • Plasma membrane : Provides a liquid  for the cell’s contents, separating the cell from its environment 

  • Cytosol: A semifluid gel that fills the cell and is the site of many metabolic chemical reactions

  • DNA/Chromosomes: Lengths of DNA that contain genes

  • Ribosomes: Small cellular parts responsible for protein synthesis, based on the sequence of a strand of messenger RNA (mRNA)

Organelles

Plasma Membrane

• Forms the boundary for a cell. It is selectively permeable and permits the passage of materials into and out of the cell. 

• The plasma membrane is made up of phospholipids, proteins, and a

  ssociated carbohydrates. 

• The head is hydrophilic while the tail is  hydrophobic. The membrane is set up in a way so that the head is pointing towards the inside and outside of the cell (thus touching water) while the tail is nested between the sandwich of the heads. 

Endomembrane System

• The synthesis and transport of proteins within and outside the cell; metabolizes lipid movement and detoxes poisons

• Includes: nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, and plasma membrane.

• Does not include: mitochondria, chloroplasts, or peroxisomes.

Nucleus

• Contains most of the cell's DNA.

•  The genetic information, it is referred to as the control center of the cell.

• The nucleolus is where the ribosomes are assembled within the DNA. 

• Nuclear envelope is the double layer of membranes with pores allowing movement in.out of the nucleus

• Chromatin is the complex of DNA and protein housed in the nucleus 

  that makes up the chromosomes.

Ribosomes

• Carries out protein synthesis which is made from rRNA and protein in nucleolus

  • Free ribosomes are found floating in the cytosol which remains to make protein within the cytosol

  • Bound ribosomes are found on the rough ER which makes proteins for membranes or will be exported out of the cell

Endoplasmic retuculum

• A network of tubules and sacs that function in protein synthesis, lipid metabolism, and detoxification processes.

  • Smooth ER: Has three primary functions: synthesis of lipids, metabolism of carbohydrates, and detoxification of drugs and poisons.

  • Rough ER: Is covered in ribosomes, produces secretory proteins that will be released from the cell, signals molecule insulin and membrane-bound proteins.

Glogi Apperatus

• Modifies, sorts, and packages proteins from the endoplasmic reticulum for storage within the cell or transport out of the cell.

• Has a receiving end known as cis and a shipping end known as trans

• It is in the Golgi that proteins are packaged and distributed to desired locations. These proteins are packaged in little sacs called vesicle

• The Golgi is also involved in the production of lysosomes. 

Lysosomes

• Small organelles filled with enzymes that help break down waste materials and cellular debris inside cells.

• Recycles cell components and digests pathogens (e.g., bacteria) through phagocytosis.

• White blood cells, like macrophages, use lysosomes to destroy invaders.

Vacuoles

• Large central vacuole in plant cells stores water, waste, and toxic materials.

• Similar to lysosomes in function; helps maintain water balance in plant cells.

Chloroplast

• Organelles found in plant cells and some algae that conduct photosynthesis, where they absorb sunlight and use it in conjunction with water and carbon dioxide gas to produce food for the plant. ( Photosystenesis)

Mitochondria

• Organelles within eukaryotic cells that produce most of the cell’s supply of adenosine triphosphate (ATP), used as a source of chemical energy.

Centrioles

• Cylindrical structures found in most eukaryotic cells, involved in cellular division and the formation of spindle fibers that separate chromosomes during mitosis.

  Peroxisomes

• Contains enzymes that transfer hydrogen to oxygen, producing hydrogen peroxide but contains enzymes that can break hydrogen peroxide down into water

Plant Cells vs. Animal Cells

• Plant have cell walls which is made out of cellulose. It’s a protective outer layer other than the membrane.

Vocabulary

Eukaryotic Cells: These are complex cells with a nucleus and other organelles, all enclosed within membranes. They make up organisms in the Protista, Fungi, Plantae, and Animalia kingdoms.

Prokaryotic Cells: Prokaryotic cells are simple, small cells that lack a nucleus or other membrane-bound organelles. They are typically found in bacteria and archaea.

Flagella: Flagella are long, whip-like appendages that protrude from the cell body and help in movement. They can be found in both prokaryotic and eukaryotic cells.

Plasma Membrane: The plasma membrane is a thin layer that separates the inside of cells from their environment. It controls what enters or leaves the cell.

Hydrophilic: Hydrophilic substances are those that have an affinity for water, meaning they can mix with or dissolve in water.

Hydrophobic: Hydrophobic refers to substances that repel or do not mix with water.

Fluid-Mosaic Model: The fluid-mosaic model describes the structure of cell membranes as a flexible layer made of lipid molecules interspersed with large protein molecules that act as channels through which other molecules enter and leave the cell.

Phospholipid Bilayer: The phospholipid bilayer is a two-layered arrangement of phosphate and lipid molecules that form a cell membrane, the hydrophobic lipid ends facing inward and the hydrophilic phosphate ends face outward.

Nucleolus: The nucleolus is a small, dense region within the nucleus of a cell where ribosomal RNA (rRNA) is produced and assembled with proteins to form ribosomes.

Nucleus: The nucleus is an organelle found in eukaryotic cells that contains most of the cell's genetic material organized as DNA molecules along with proteins forming chromosomes.

Golgi Apparatus: The Golgi apparatus is an organelle in eukaryotic cells that modifies, sorts, and packages proteins from the endoplasmic reticulum for storage within the cell or transport out of the cell.

Trans Face: The trans face is the shipping side of the Golgi apparatus in a cell, where modified proteins and lipids are sorted and packaged into vesicles that transport them to their final destinations.

Vesicles: Vesicles are small membrane-bound sacs that function in moving materials within a cell as well as interactions between cells.

Cis Face: The cis face is the receiving side of the Golgi apparatus where transport vesicles that have budded from the endoplasmic reticulum fuse with the Golgi membrane and empty their cargo into the lumen.

Ribosomes: Ribosomes are tiny structures within cells where proteins are made (protein synthesis).

Ribosomal RNA (rRNA): rRNA is part of ribosomes, serving as structural components and also catalyzing peptide bond formation. It's essential for protein synthesis in all living organisms.

Endoplasmic Reticulum (ER): The endoplasmic reticulum is a network of tubules and sacs that function in protein synthesis, lipid metabolism, and detoxification processes.

Transitional ER: The transitional endoplasmic reticulum (ER) is a region of the ER that specializes in the final steps of protein and lipid synthesis, including packaging these molecules into transport vesicles.

Lysosomes: Lysosomes are small organelles filled with enzymes that help break down waste materials and cellular debris inside cells.

Vacuoles: Vacuoles are membrane-bound sacs within the cytoplasm of a cell that function in several different ways including isolating materials that might be harmful or storing waste products.

Chloroplasts: Chloroplasts are organelles found in plant cells and some algae that conduct photosynthesis, where they absorb sunlight and use it in conjunction with water and carbon dioxide gas to produce food for the plant.

Chlorophyll: Chlorophyll is a green pigment found in plants, algae, and cyanobacteria that absorbs light energy (specifically blue and red wavelengths) to carry out photosynthesis.

Stroma: The stroma is the fluid-filled space surrounding the grana (stacks of thylakoids) within the chloroplast where the "dark" reactions of photosynthesis (Calvin cycle) occur.

Thylakoids: Thylakoids are flattened sac-like membranes arranged in stacks (grana) inside the chloroplasts. They contain chlorophyll and play a crucial role in capturing light energy and converting it into chemical energy during photosynthesis.

Mitochondria: Mitochondria are organelles within eukaryotic cells that produce most of the cell’s supply of adenosine triphosphate (ATP), used as a source of chemical energy.

Cristae: Cristae are internal compartments formed by the inner membrane folding over itself within mitochondria. They provide an increased surface area for chemical reactions that produce ATP.

Mitochondrial Matrix: The mitochondrial matrix is a gel-like substance inside mitochondria where certain metabolic reactions occur.

Krebs Cycle: The Krebs Cycle, also known as the citric acid cycle, is a series of chemical reactions used by all aerobic organisms to generate energy through the oxidation of acetyl-CoA derived from carbohydrates, fats and proteins into ATP.

Centrioles: Centrioles are cylindrical structures found in most eukaryotic cells, involved in cellular division and the formation of spindle fibers that separate chromosomes during mitosis.

Cell Walls: Cell walls are rigid layers surrounding some types of cells providing structural support and protection. They're found outside the cell membrane in plant cells, bacteria, fungi, and some protists.

Cellulose: Cellulose is a complex carbohydrate, or polysaccharide, that is composed of glucose units and forms the main component of plant cell walls.

2.2 Cell Structure and Function

Mitochondria

• Has a double membrane and the convolutions of the inner membrane (cristae) which allows efficiency on cellular respiration

• The folded membrane provides more space for more membrane-bound molecules that perform ATP production 

• This is essential to the increased surface area of the inner membrane. This allows more space for Electron Transport Chain (ETC), an essential part of cellular respiration.

• The matrix, used as the site of the Krebs cycle, is right next door and can therefore transfer its products easily to the ETC. With the ETC, ATP synthesis happens.

• Cellular respiration transfers chemical energy of organic compounds into the usable energy for the cell (Adenosine Triphosphate or ATP).

• The mitochondrial membranes sequester metabolic reactions into compartments, improving efficiency

Chloroplast

• Contain stacks of membranous sacs, called thylakoids

  • Allows for an increase in surface area necessary for the ETC of the light-dependent reactions of photosynthesis.

  • These membranes are also lined with photosystems and chlorophyll, increasing the amount of energy that the plant can get from light.

• Stacks of thylakoids are referred to as grana

  • The ATP synthesis (photosynthesis) happens here

• Stroma is the fluid the fills the chloroplast

  • The proximity of the stroma to the thylakoids allows for the sharing of products between the Calvin Cycle (which happens in the stroma) and the light-dependent reactions.

Lysosomes

• Contains enzymes that are responsible for digesting material inside of vacuoles

• Recycle materials and old organelles inside the cells

• Can digest foods by using phagocytosis or engulfing nutrients to digest them.

  • The hydrolytic enzymes inside of the lysosome work to break down anything that comes into contact with it.

  • If the hydrolytic enzymes inside of the lysosome were to burst, all of the cell's organelles may burst as well, and the cell would die.

  • In order to prevent this, the membrane is specially designed to keep these enzymes in.

• In charge of apoptosis, which is programmed cell death. Essentially, the lysosome bursts, causing the acid to kill the cell. 

• Apoptosis is programmed cellular death - needed when cells are old or damaged

Vocabulary:

Mitochondria: Organelles within eukaryotic cells that produce adenosine triphosphate (ATP), used as a source of chemical energy.

Cristae: The convoluted inner membrane of mitochondria that increases the surface area for ATP production.

Electron Transport Chain (ETC): A series of protein complexes and other molecules that transfer electrons from electron donors to electron acceptors via redox reactions, crucial for ATP synthesis.

Krebs Cycle: A series of reactions in the mitochondria that generate energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins.

Cellular Respiration: The metabolic process through which cells convert biochemical energy from nutrients into ATP, using oxygen, while releasing waste products.

Chloroplast: Organelles found in plant cells and some algae that conduct photosynthesis, converting light energy into chemical energy.

Thylakoids: Membranous sacs inside chloroplasts where the light-dependent reactions of photosynthesis occur.

Grana: Stacks of thylakoids within chloroplasts that increase surface area for light absorption.

Stroma: The fluid-filled space in chloroplasts where the Calvin Cycle takes place.

Lysosomes: Organelles that contain enzymes responsible for digesting waste materials and cellular debris.

Phagocytosis: The process by which a cell engulfs solid particles, forming a vesicle to digest them.

Hydrolytic Enzymes: Enzymes that break down macromolecules into their subunits, functioning within lysosomes.

Apoptosis: A programmed cell death process necessary for the removal of old or damaged cells.

2.3 Cell Size

Surface Area to Volume Ratios

• The amount of nutrients a cell needs depends on the volume of the cell, while the surface area/volume ratio determines how well nutrients are absorbed. 

• Cells need to main a small size to have the maximum amount of space to trance nutrients.

• Metabolic requirement set limit on the size of cells

• As the volume increases, the cell will need more and more nutrients to enter,

• The higher the surface area, the more nutrients can enter. 

  • This includes exchanging materials with the environment and waste removal

• As the surface area increases by a factor of n², the volume increases by a factor of n³

• The greater the surface area to volume (SA/V) ratio is, the more efficient the cell becomes.

• Some cells possess highly convoluted membranes to increase surface area while minimally increasing volume

  • Root hair cells on plant root tissue increase surface area for water absorption

  • Microvilli on intestinal epithelial cells increase surface area for nutrient absorption

  • As organisms and/or cells increase in size, metabolic efficiency goes down, including efficiency in heat loss to the environment

• Organisms have beneficial adaptation to maximize exchange of material with the environment

  • Absorption of nutrient and elimination of wastes

  • Gas exchange is the process by which gaseous molecules from the environment are absorbed by a cell, while waste gases from the cell are released into the environment

  •  Stomata in plants to help them more efficiently exchange CO2 and O2. 

Vocabulary:

2.4 Plasma Membrane

Plasma Membrane

• The plasma membrane is made up of a phospholipid bilayer.

• Has 2 parts:

  • A hydrophilic (water-loving) part

    • The hydrophilic heads, comprised of a phosphate group, face the outside and inside of the cell, where water is present.

    • Hydrophilic substances are polar and attract water, moving towards aqueous solutions like extracellular fluid and cytosol. I

  • A hydrophobic (water-hating) part

    • The hydrophobic tails, comprised of fatty acids, face inward and do not interact with water.

    • Hydrophobic substances are nonpolar and repel water, moving away from aqueous solutions and clustering together.

• Proteins are scattered throughout the membrane

  • Peripheral proteins are on the membrane’s exterior or interior surface

  • Integral proteins penetrate the membrane 

    • Transmembrane proteins pass completely through the bilayer

    • adhesion proteins - form junction between cells

    • receptor proteins - receive messages such as hormones (act as docking site)

    • transport proteins - pumps that actively transport stuff using ATP

    • channel proteins - form channel that passively transport stuff

    • cell surface markers - act as ID card for the cell

• Proteins may be hydrophilic, possessing polar and charged R-groups, or hydrophobic, with nonpolar R-groups

• Other membrane components include steroids, glycoproteins and glycolipids

  • Steroids contribute to membrane fluidity - increased numbers of steroids increase fluidity

    • Some organisms can alter membrane fluidity to survive in changing temperatures

• Glycoproteins and glycolipids are useful in cellular identification

  • This helps immune cells recognize the difference between body cells and foreign tissue (bacteria or viruses).  

• The cytoskeleton is a structural framework made of proteins that helps the cell keep its shape, holds organelles in place, and provides a network for intracellular trafficking

Vocabulary:

2.5 Plasma Membrane

• Due to the structure of the membrane the cellular membrane has selective permeability.

  • Selective permeability is the membrane’s ability to regulate the molecules or ions that are able to pass into and out of the intracellular environment

• This allows some substances to cross easily, while others may not be able to cross or may require a special transport protein to do so.

• The membrane acts like a barrier separating the inside of the cell from the external environment of the cell.

• The hydrophobic interior of the lipid bilayer makes it very unlikely that polar/large/charged molecules can cross

• Small, non-polar molecules are able to freely cross the cell membrane,

  • Small, nonpolar molecules cross with ease

    • Such as O₂ and CO₂

  • Small, polar molecules may cross, but very slowly 

    • For example, H₂O

• Polar or charged molecules require transport proteins to cross. 

  • The transport protein has a specific shape and polarity to accommodate a specific polar/charged/large substance

Vocabulary:

2.6 Membrane Transport

• The membrane allows for various types of transport

Passive Transport

• Molecules move across the membrane from a region of high concentration to a region of low concentration 

• High to low = with the concentration gradient 

• This happens naturally so does not need to use ATP energy.

• Diffusion - Small nonpolar molecules can easily diffuse through the membrane. Like oxygen and Carbon Dioxide. 

• Simple diffusion is a passive movement of a substance from an area of higher to lower concentration. With no energy needed, a substance will diffuse down its concentration gradient, until equilibrium occurs

• Facilitated diffusion is a a passive movement of a substance from an area of higher to lower concentration, like simple diffusion. No energy is required for this movement, but a transport protein is needed in order for the substance to get across the membrane. 

• Osmosis- the net movement of water molecules across the cell membrane. Water is polar and pass through the membrane in small amounts.

• Gradient is the difference in concentrations between two different areas - a larger difference indicates a steeper gradient and a faster rate of diffusion

Active Transport

• Active transport uses energy, in the form of ATP, to transport molecules against their concentration gradient

• Molecules move across the membrane from a region of low concentration to a region of high concentration. 

• Low to high = against the gradient 

• This requires the use of ATP energy!

• Pumps - proteins act as pumps using ATP to pump molecules against the concentration gradient.

• Vesicles are used to pull in or push out large molecules and need energy to do so

• Active transport may also be used for large or bulky molecules or to transport large quantities of small molecules.

• These processes are referred to as exocytosis and endocytosis. 

• When a large amount of molecules are entering a cell it is called endocytosis.

• In this there are 3 kinds of endocytosis

  • Phagocytosis, the cell engulfs a large molecule, and brings it into the cell. These are generally food vacuoles.

  •  Pinocytosis, the cell “gulps” surrounding solutes into small vesicles that are covered in a layer of protein. (liquids)

  •  Receptor-mediated endocytosis, a receptor binds to a cell. When solute binds to the receptor, the plasmid pulls away creating a vesicle with the solutes.

• Exocytosis, when molecules are secreted from the cell

• A transport vesicle from the Golgi apparatus moves along the microtubules in the cell until it reaches the plasma membrane. Then the vesicle fuses with the Plasma membrane, releasing the contents out of the cell.

  

Vocabulary:

2.7 Facilitated Diffusion

• Facilitated diffusion speeds diffusion of large/polar/charged molecules by utilizing transport proteins

  • A form of passive transport, when molecules go down the concentration gradient. 

• A concentration gradient is when particles or solutes move from a highly concentrated area of particles to a less concentrated area of particles.

• This process is aided by proteins located on the plasma membrane (membrane proteins) such as transport proteins: Channel proteins and carrier proteins. 

• Channel proteins are laid throughout the membrane to provide a Hydrophilic passage through for the molecules to avoid the Hydrophobic core)

• Aquaporins are transport proteins specialized for the movement of water

• Ion channels are specialized for the movement of particular ions, such as Na⁺ or Cl^-

  • The movement of ions in one direction can create an electrochemical gradient across a cell membrane

  • This creates a membrane potential, polarizing the membrane

    Active transport also requires a transport protein, known as a pump, to shuttle molecules through the membrane against the gradient

  • Pumps require energy to 

    • move molecules against the concentration gradient

    • maintain concentration gradients, preventing the cell from reaching equilibrium

    • The energy is usually supplied in the form of ATP

• Secondary active transport, something is actively transported by using energy from another substance going through the membrane through simple diffusion.

Vocabulary:

2.8 Tonicity and Osmoregulation

Tonicity

• Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water

• The movement of water inside and out of the cell is essential for it's survival.

• Water travels from a higher concentration of itself to a lower concentration of itself. This movement can have large impacts on the cell.

• Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water

• Depending on the amount of material outside of a cell compared to inside, the environment outside of a cell can be hypotonic, hypertonic or isotonic to the internal environment of a cell. 

• HYPO=Water goes in

• Hypotonic solutions have a solute concentration less than that inside the cell.

  • less solute, more water

  • Water will move to where there is more solute (therefore the cell has less water)

  • Water will move into the cell, causing the cell to expand. (Cells could burst)

• HYPER= Water comes out

• Hypertonic solutions have a solute concentration greater than that inside the cell

  • More solute, less water

  • Water will still move to where there is more solute. (therefore moving out of the cell)

  • This will cause the cell to shrink.

• ISO=EQUAL

• Isotonic solution is where solute concentration is the same as that inside the cell

  • No net water movement

  • The cell does not change shape

Osmosis

• Osmosis is the diffusion of water through a selectively permeable membrane

• Also defined as the passive transport of water from areas of high water concentration to low water concentration

• Allows organisms to control their internal solute composition and water potential.

• Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration until the solute concentration is equal on both sides

• Water will move from a more watery side to the more concentrated side, meaning water will move from where there's less solute to more solute. 

• Water can move across the membrane through the help of aquaporins. However substances like sugar can’t cross. But the cell is not happy with uneven concentrations, causing water to move instead of the "stuff" moving.

• Plant have a easier way to protect themselves from osmotic changes due to it cell wall

  • Plant cells do not burst when in a hypotonic solution due to rigid cell walls, instead it causes a potential pressure to be exerted. (pressure increases the water potential) 

• Osmoregulation is the ability of organisms to maintain water balance with their environment and control their internal solute concentration

  • A contractile vacuole is an adaptation possessed by freshwater protists, Paramecia, to osmoregulate and maintain homeostasis

Human Kidney and Osmoregulation

• Osmoregulation is the process the body does to regulate the amount of water in the body. 

• The kidneys can regulate water levels in the body; they conserve water if you are dehydrated, and they can make urine more dilute to expel excess water if necessary.

  • Plasma osmolality is the ratio of solutes to water in blood plasma. A person’s plasma osmolality value reflects his or her state of hydration.

  • The water that leaves the body is extracted from blood plasma. As the blood becomes more concentrated, the thirst response is triggered.

  • The “thirst center” is found in the hypothalamus which measures the concentration of solutes in the blood. 

ADH and Hydration

• Antidiuretic hormone (ADH), also known as vasopressin, controls the amount of water reabsorbed from the collecting ducts and tubules in the kidney.

• If you are dehydrated the hypothalamus will signal for the release of a ADH. 

• ADH signals the kidneys to recover water from urine, effectively diluting the blood plasma.

• ADH also causes the epithelial cells that line the renal collecting tubules to move aquaporins, from the interior of the cells to the surface. The result is a large increase in water passage from the urine through the walls of the collecting tubules, leading to more reabsorption of water into the bloodstream.

• When the blood plasma becomes less concentrated and the level of ADH decreases, aquaporins are removed from collecting tubule cell membranes, and the passage of water out of urine and into the blood decreases.
  

Water Potential

• Describes osmosis and the direction of the flow of water in mathematical terms.

• The more solute in a solution, the greater the interactions between the solutes and the polar water molecules (water moves towards solutes!)

  • Unit for water potential = bars 

  • Pure water = 0 bars

  • Pressure potential is represented by Ѱ_p and is 0 bars in an open container at STP

    • Exerting pressure can raise the water potential. 

    • Always a positive value. 

• Water in the context of water potential flows from high potential to low potential.

• The solute potential is the solute concentration in consideration of the water flow. If you add more solute, the water potential of the solution will be lowered(There is more solute than water)

• Likely that more water flows into the solution to counter it. 

• Solute potential is represented by Ѱ_s (the effect of solutes on the movement of water)

  • As solutes are added the solute potential becomes more negative and the water potential lowers.

  • Solute potential is always negative in value.

• Ѱ_s can be calculated using Ѱ_s = -iCRT

  • i= the ionization constant which equals 1 for a substance that does not ionize in water 

  • C= the solute concentration (molarity/molar concentration)

  • R= the pressure constant ((0.831) L · bar∕ mol · K)

  • T= the temperature in Kelvin 

Vocabulary:

2.9 Mechanisms of Transport

• The cell membrane, also known as the plasma membrane, is a thin and flexible barrier that surrounds the cell and separates the interior of the cell from the external environment.

• Contains proteins that perform various functions, including transport, signaling, and recognition. T

• These proteins can be integral (meaning they are embedded within the phospholipid bilayer) or peripheral (meaning they are attached to the surface of the membrane)

• Helps the cell to maintain a stable internal environment, exchange materials with the external environment, and interact with other cells.

Active Transport

• The movement of molecules across a cell membrane against a concentration gradient, which requires energy.

• Necessary when the concentration of a substance is higher inside the cell than outside, or when the substance to be transported is too large or polar to pass through the cell membrane by diffusion.

  1) Primary active transport 

  • The direct transfer of molecules across the membrane using energy from ATP (adenosine triphosphate)

  • Examples: the sodium-potassium pump and the calcium pump.

  2) Secondary active transport 

  • This process moves molecules across a membrane by using the energy from the concentration gradient of another substance, typically through proteins known as cotransporters or exchangers.

  • Example: The facilitated diffusion of glucose into cells using the glucose transporter (GLUT) protein.

• Play a crucial role in the absorption of nutrients, the elimination of waste products, and the maintenance of ionic balance and membrane potential.

  
  

  

Passive Transport

• Passive transport is the movement of molecules across a cell membrane down a concentration gradient, without the expenditure of energy.

  1) Diffusion

  • The movement of molecules from an area of high concentration to an area of low concentration.

  • It occurs due to the random thermal motion of the molecules, and no energy is required.

  2) Osmosis

  • The movement of water molecules across a semi-permeable membrane from an area of high water concentration (high osmotic pressure) to an area of low water concentration (low osmotic pressure).

• Occurs due to the random thermal motion of the water molecules and does not require energy.

  3) Facilitated diffusion

  • The movement of molecules across a membrane down a concentration gradient with the help of transport proteins.

  • These proteins form channels or carriers through which the molecules can pass, but no energy is required.

  • Passive transport plays a vital role in the exchange of materials between cells and their environment, as well as in the maintenance of homeostasis within the cell.

Endocytosis

• A a cellular process where a cell takes in materials from outside by engulfing them in a small vesicle.

  1) Phagocytosis

  • A cell takes in solid particles, such as bacteria or cell debris, by enclosing them in a vesicle called a phagosome.

  • Phagocytosis is carried out by specialized cells called phagocytes, which are found in tissues such as the skin and the immune system.

  2) Pinocytosis:

  • A cell takes in liquids, such as extracellular fluid or dissolved substances, by enclosing them in small vesicles called pinocytotic vesicles.

  • Pinocytosis is also known as "cell drinking" and is often referred to as a type of "non-specific" endocytosis.

  3) Receptor-mediated endocytosis

  • A specific type of endocytosis where cells take in specific molecules by binding them to receptors on their surface.

  • The receptors and the bound substances are then internalized in a vesicle called an endosome.

  • Receptor-mediated endocytosis plays a key role in the uptake of large molecules, such as hormones and growth factors, as well as in the immune system.

  • Endocytosis is an important process for cells because it allows them to take in nutrients, eliminate waste, and interact with their environment.

  • It is also involved in many physiological processes, including the immune response, signal transduction, and the uptake of drugs and toxins

Exocytosis

• Exocytosis is a type of cellular process by which a cell releases substances by pushing them out through the cell membrane

  1. Substances like proteins or lipids made in the cell are transported to a storage organelle called a vesicle and then released outside the cell.

  2. The vesicle fuses with the cell membrane, and the substance is released into the extracellular space. This process is regulated by signaling pathways and specific proteins called SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors).

     • Exocytosis plays a vital role in many physiological processes, including the secretion of hormones, enzymes, and immune mediators; the elimination of waste products; and the communication between cells. It is also involved in the release of neurotransmitters from neurons and the exfoliation of cells in tissues such as the skin.

Vocabulary:

2.10 Cell Compartmentalization

• The main difference between Eukaryotes and Prokaryotes is that eukaryotes are compartmentalized in membrane-bound organelles

• Eukaryotes: After RNA is made from DNA in the process of transcription, it moves to the ribosome to go through the process of translation. The RNA has to move out of the nucleus to either a free-standing ribosome or to the rough endoplasmic reticulum. In addition, ATP is made from mitochondria, which has its own internal membrane. 

• Prokaryotes: RNA is converted to proteins right after being made from DNA, as they do not have a nucleus or endoplasmic reticulum.

• By reducing the amount of competing space and surface area, and also reducing the amount of competing reactions

• Eukaryotic cells are able to be more efficient than prokaryotic cells.

Internal Membranes

• Internal membranes within the cell, such as mitochondria or chloroplasts, make cellular processes easier by minimizing competing interactions and by increasing the surface area to volume ratio.

• Mitochondria: The inner membranes are highly convoluted, and the increased surface area of the cristae allow for increased numbers of ETC proteins and ATP synthases, maximizing oxidative phosphorylation

• Chloroplast: Grana in chloroplasts also increase surface area for Photosystems and ETC proteins in the thylakoid membranes, increasing photophosphorylation and NADPH in the light reactions.

Vocabulary:

2.11 Origins of Cell Compartmentalization

• Prokaryotic cells are believed to the earliest forms of life found on Earth. Even 4 billion years ago, prokaryotes existed on earth

• Eukaryotic cells only appeared 1.8 billion years ago

• Most biologists think the ancestor of eukaryotic cells are prokaryotic cells. But eukaryotic cells are much more complex than prokaryotic cells.

• Mitochondria and chloroplasts contain shared, derived characters with prokaryotes, providing evidence of common ancestry

  • Shared, derived characters are features possessed by the descendants of a single common ancestor and define a clade

  • A clade is a group of different organisms that share a common ancestor

• Both chloroplasts and mitochondria:

  • Contain circular DNA

  • Possess ribosomes

  • Have a double membrane

  • Are self-replicating

• These characteristics are typical of prokaryotes, and provide evidence that mitochondria and chloroplasts were likely once independent prokaryotic cells

• The endosymbiont theory serves as the explanation for the origins of mitochondria and chloroplasts

• Symbiosis describes a close, long-term, physical interaction between two different organisms

• Endosymbiont theory states that an ancestral eukaryotic cell engulfed an ancestral mitochondrion, establishing a mutualistic relationship

• Mutualism is a type of symbiosis where both parties benefit

• The ancestral eukaryote survived and reproduced often, establishing a lineage of eukaryotes that too possessed mitochondria

• A descendant of this ancestral lineage later engulfed a photosynthetic prokaryote (an ancestral chloroplast) and also survived and reproduced often

• The lineage produced by this second endosymbiosis lead to the plant cells extant on Earth

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