Cell Biology

Template:EventLinksBox Cell Biology is an event in which teams answer questions and/or perform lab tasks relating to cell biology and cellular biochemistry. The event was most recently run nationally in Division C in 2015 and 2016, and will return in 2022.

Overview and Strategy

Cell Biology is an event about eukaryotic and prokaryotic cells. It covers a wide variety of topics in biology, and typically takes the form of several stations with demonstrations or diagrams. Questions may cover subjects such as organelle structure and function, cell membrane structure, the cell cycle and mitosis, the difference between different types of cells (prokaryotic and eukaryotic), and other miscellaneous topics such as enzymes and chromosome structure. Other topics include the field of bioenergetics, homeostasis, and biological polymers. Even more topics relating more heavily to cell communication, cancer and diseases may be tested at the national level.

Building a strong partnership is important for this event, which is typically done through practice and repetition. Avoid focusing on completed stations, and keep moving forward through the test. Mnemonic devices can also be extremely effective, especially in dealing with more complex topics. These methods allow for quick and accurate recollection of information throughout the event.

Finally, if time allows, utilize a technique for writing down brief summaries of unanswered questions and returning to them at a later point. Whether it be a rest station, tiebreaker station, or another regular station that is more quickly finished, it is far better to spend down time answering questions.

Quick Overview of Helpful Ideas

• Start practicing early and efficiently
• Use mnemonic devices to memorize complex concepts
• Answer practice test questions to become familiar with the test format

Macromolecules, Monomers and Polymers

Cells largely consist of a variety of large molecules known as macromolecules. They are made of thousands of atoms, and make up the fundamental building blocks of many biological processes. There are four main groups of macromolecules: carbohydrates, nucleic acids, proteins, and lipids.

These first three groups are polymers, large molecules made up of subunits known as monomers. These monomers are the smallest units that these molecules can be broken down into. While lipids are not polymers, they are important molecules when understanding the structure of the cell.

Carbohydrates

Carbohydrates are biological polymers made up of carbon, hydrogen and oxygen atoms. The smallest carbohydrates are known as monosaccharides, and these are the monomers that make up larger carbohydrates. They serve two main functions in the cell: providing energy through molecules like starch and forming structural components through molecules like cellulose.

Carbohydrates can be categorized based on how polymerized they are, or how many monomers are present in the final molecule. Two monosaccharides link to form a disaccharide. Together the monosaccharides and disaccharides form a group known as sugars, consisting of molecules like glucose and sucrose. The other two main groups of carbohydrates are oligosaccharides and polysaccharides, which are made up of molecules like maltodextrin, amylose, and glycogen.

Name	Image	Description
Monosaccharide	Glucose	Monosaccharides (simple sugars) such as glucose, galactose and fructose are the most basic carbohydrates. These molecules link together to make every other carbohydrate in the body. The most prolific of these is glucose, which is used in processes like glycolysis or photosynthesis in order to provide energy to the cell.
Disaccharide	Lactose	Disaccharides (compound or double sugars) are made up of two monosaccharides linked together. Sucrose is the most abundant disaccharide, consisting of glucose and fructose bonded together. Lactose commonly occurs in milk, and is fermented by certain bacteria in lactic acid fermentation.
Oligosaccharide	Maltodextrin	Oligosaccharides are carbohydrates that typically consist of anywhere from 3 to 9 monosaccharides. In the diagram of maltodextrin, this is shown by placing the monomer (in this case, glucose) within square brackets with an n at the bottom. This shows that maltodextrin is made up of a chain of glucose molecules without having to draw each individual unit. In this case, n can represent anywhere from 2 to 20 units of glucose. Oligosaccharides are often used as food additives, and are used in processes such as cell communication.
Polysaccharides	Amylose	Polysaccharides are carbohydrates made up of more than 9 monosaccharides, typically being made up of hundreds of individual subunits. Amylose and amylopectin are the polysaccharides that make up starch, which is the most common carbohydrate found in foods. Other polysaccharides like glycogen and celluose perform functions like energy storage and structural support.

Nucleic Acids and DNA Structure

See also: Heredity#DNA.

A diagram of a nucleotide

DNA (deoxyribonucleic acid) is one of two long molecules known as nucleic acids, which code for the genetic information found within cells. It is made up of molecules known as nucleotides, which are made up of a nitrogenous base, a phosphate group, and a deoxyribose sugar. The phosphates of one nucleotide bonds to the carbon of another, forming a long strand. DNA is made of two of these strands, which run in opposite directions (they are antiparallel). The direction of the strand is marked by the orientation of the carbon molecules found in the deoxyribose. The deoxyribose found in a nucleotide has five carbons which are numbered clockwise from the top. The carbon closest to the end of a strand is considered to be prime, denoted with an apostrophe (e.g. 3'). As a result, a DNA strand where the third carbon is closest to the beginning and the fifth carbon is closest to the end runs from 3' to 5'. A DNA strand that is oriented in the opposite direction runs from 5' to 3'. These two strands twist together, forming the characteristic double helix shape.

There are four nitrogenous bases that can be found in DNA: adenine, thymine, guanine, and cytosine. These bases can be divided into two groups based on their structure: purines and pyrimidines. The purines (adenine and guanine) have two rings in their structure, while pyrimidines (thymine and cytosine) only have one ring in their structure. Each base can only pair to one other base. Adenine forms two hydrogen bonds with thymine, and guanine forms three hydrogen bonds with cytosine. Chargaff's rule states that in any given DNA molecule the amount of thymine present will be the same as the amount of adenine present, and the amount of guanine found in the molecule will be the same as the amount of cytosine.

In eukaryotic cells, DNA normally exists in cells in a form known as chromatin. In this form, DNA is packed around proteins known as histones. Because DNA is so long, it has to be wrapped around histones in order to be stored in cells. DNA can also be unwound by enzymes, allowing for it to be replicated. During mitosis the chromatin condenses to form chromosomes, which have four distinct parts. The centromere is the part of the chromosome that separates the short (p) arm from the long (q) arm, and it is also where identical sister chromatids are attached. The telomeres are the very ends of the chromosome, full of the same DNA sequence over and over again. Every time DNA is replicated, a little bit of this region is left off. It exists to prevent the rest of the DNA from breaking down and getting lost during replication.

RNA

RNA (ribonucleic acid) is another nucleic acid which is very similar to DNA. They are both made of nucleotides, though nucleotides in RNA are made of ribose sugar instead of deoxyribose. RNA also only has one strand, instead of the two that DNA has. RNA also has another key difference that distinguishes it from DNA - instead of thymine, it will have uracil. As a result, when reading a sequence of RNA a U will always appear in place of a T. RNA is synthesized in the nucleus, and is used in a variety of places throughout the cell. There are multiple different types as well:

• mRNA (messenger RNA) is the RNA that is used to make proteins
• rRNA (ribosomal RNA) is the RNA that the ribosome is made out of
• tRNA (transfer RNA) is the RNA that brings amino acids to the ribosome in order to make proteins

Proteins

See also: Heredity#Interpreting Genetic Code.

Proteins are long molecules made of building blocks known as amino acids. They are synthesized in the ribosome, based off of the genetic information found in DNA. There are twenty known amino acids, all of which share the same basic structure. Every amino acid has a central carbon with an amino group and a carboxyl group, as shown in the diagram to the right. The only thing that differentiates different amino acids is the R group, also known as the variable group. This group determines the properties of each amino acid, and how they interact to form polypeptides.

A representation of a protein that has both alpha helices (red) and beta sheets (yellow).

Individual amino acids connect from N-terminus to C-terminus, meaning that the amino group of an amino acid is at one end and the carboxyl group of another amino acid is at the opposite end. This long chain of amino acids is known as a polypeptide, and multiple chains can come together to form one protein. However, some proteins are only made of one polypeptide. This is a part of the protein's structure.

Proteins are incredibly complex molecules, and have four different levels of structure. The primary structure of a protein is simply the sequence of amino acids that it is made of. The secondary structures are formed by hydrogen bonds between different amino acids, typically taking the form of alpha helices and beta sheets. Tertiary structures are even more complex, forming the three-dimensional shape of the protein. Tertiary structures form when the R groups of the amino acids interact, often incorporating multiple secondary structures as a part of the final protein shape. Finally, quaternary structures form when multiple protein molecules known as subunits come together to form a final protein.

Lipids

Three types of fatty acids. Note the carboxylic acid as depicted on the right side of the diagram, and the orientation of the hydrogens surrounding the double bonds.

While lipids are not polymers, they are important macromolecules to understand in the context of the cell. Lipids are non-polar, meaning that they will not dissolve in water (which is polar). Lipids store energy throughout the body in the form of fat. Fats are a type of lipid specifically known as triglycerides, which are made up of one glycerol molecule and three fatty acids. A fatty acid is a carboxylic acid with an aliphatic chain consisting of a carbon backbone with hydrogen atoms attached to it. In a saturated fatty acid, all of these carbon atoms are linked by single bonds. In an unsaturated fatty acid, some of these carbon atoms are linked by double bonds. Unsaturated fatty acids can be either cis or trans, which refers to the orientation of the hydrogen atoms on the molecule. In cis fatty acids, the hydrogens surrounding the double bond are on the same side, which results in a bend or kink in the molecule. In trans fatty acids, the hydrogens surrounding the double bond are on opposite sides, which results in a straight chain and no bend in the molecule. This means that the straight saturated fats can pack together more closely, and tend to be solid at room temperature. Unsaturated fats tend to be liquid at room temperature since they do not interact as strongly with each other.

Lipids also have important roles in cell signaling and structure, with a type of lipid known as a phospholipid making up a majority of the cell membrane. These phospholipids have a hydrophilic (water-loving) "head" which contains a phosphate group and a glycerol as well as two hydrophobic (water-hating) "tails" which are made up of fatty acids. Because of the structure of these molecules, the fatty acids align themselves together to form the interior of the cell membrane. The hydrophilic head faces outwards and the tails face inwards. These types of membranes are found almost everywhere in the body surrounding the cell, its nucleus, and other organisms such as bacteria.

Cell Membrane

The cell membrane (also known as the plasma membrane) is a barrier that surrounds a cell, separating its interior from the outside environment. A membrane is a selective barrier which allows certain things to pass through but not others—for this reason, the cell membrane is said to be selectively permeable. This selective permeability is essential to the functioning of the cell, allowing for the maintenance of homeostasis. Large molecules like carbohydrates or amino acids are not able to move passively across the cell membrane, and require energy to be actively transported. This prevents foreign substances from penetrating the membrane and causing damage to a cell or an organelle. Cell membranes also play a role in cellular communication, as well as the detection of external signals sent to the cell.

While the cell membrane is essential to controlling the movement of substances in and out of the cell, it also plays an important role in compartmentalizing the cell. Both prokaryotic and eukaryotic cells are surrounded by cell membranes, but eukaryotes also possess membrane-bound organelles like mitochondria or lysosomes. In these cases, the cell membrane defines a space separate from the rest of the cell where specific processes such as cellular respiration take place.

Membrane Structure

A depiction of the phospholipid bilayer, with the white heads oriented to the outside and the yellow tails oriented to the inside.

The cell membrane is made up of proteins, lipids, and carbohydrates, with a majority of the cell membrane being made of a phospholipid bilayer (two layers of phospholipids). A phospholipid is a type of lipid which has both hydrophobic and hydrophilic regions, making it a amphipathic compound. The outside of the cell is surrounded by polar fluids such as the cytosol, so the hydrophilic "head" orients itself to face outward. As a result, the hydrophobic "tails" orient themselves towards the inside of the membrane. The phospholipids are held together by weak interactions between the tails, letting individual phospholipids to move within the membrane and allowing for the membrane to be fluid and flexible.

The cell membrane follows the fluid mosaic model or the Singer-Nicholson model, with the phospholipid bilayer behaving more like a fluid than a solid. Lipids and proteins can move laterally within the bilayer, and the pattern ("mosaic") of lipids and proteins constantly changes. However, other models for the cell membrane were initially developed before being disproven. One former model known as the sandwich model or Davson-Danielli model proposed that the phospholipid bilayer was surrounded by proteins, as opposed to having proteins which are integrated within the phospholipid bilayer. However, there were several limitations to this model—it did not account for the selective permeability of the membrane, and it assumed that all membranes were of a uniform thickness. Further experimentation disproved the model, and led to the development of the fluid mosaic model.

The fluidity of the cell membrane is determined by a variety of factors:

• Single and double bonds in fatty acid tails, where saturated lipids composed of solely single bonds can pack more tightly, and where unsaturated lipids composed of chains with double bonds have kinks, resulting in a bent chain. The kinks introduce space, and thus a membrane filled with only saturated phospholipids would be solid rather than fluid-like at physiological temperature.
• Presence of cholesterol (a sterol - steroid alcohol), which packs between phospholipids, reducing permeability (prevents water-soluble molecules from diffusing across) and increasing rigidity (four linked hydrocarbon rings + hydrocarbon tail + hydroxyl)

Lipids

The structure of a phospholipid. The phosphate group is shaded in purple, the glycerol in red, and the fatty acids in yellow.

The outer portion of the cell membrane is composed of the hydrophilic heads of the phospholipids. The heads consist of a glycerol and a negatively charged phosphate group, which may have a variable R group bound to it similarly to amino acids. This R group is typically a simple organic molecule such as choline, which can associate with some proteins and allow cells to recruit certain proteins to the cell membrane. This is important for cell communication. Conversely, the inner portion of the phospholipid bilayer is composed of the hydrophobic tails of the phospholipid molecules. The tight packing of these phospholipid molecules controls the fluidity of the membrane and (specifically these hydrophobic fatty acid tails) prevents larger molecules and ions from passively diffusing into the cell.

Sphingolipids are another type of lipid found in the cell membrane. Their metabolism produces bioactive signaling molecules that modulate fundamental cellular processes. The core of a sphingolipid is an amino alcohol called sphingosine.

Proteins

Function of the proteins is to form channels in membranes that allow the passage of specific molecules or ions; act as enzymes to increase the rate of cellular reactions (and modify proteins in blood or extracellular space); act as receptors that detect the presence of specific molecules or ions in the external environment; and interact with proteins in other membranes, generating sites of attachment between membranes and cells

Integral membrane proteins - exposed to interior AND exterior

• Form channels (or pores or pumps), receptors (that recognize & respond to hormones), or adhesion points
• Also can be cell-surface markers, such as glycoproteins which have carbohydrates that act as labels attached to the external side (these labels allow cells to recognize each other and viruses use the labels as “docks” to enter and infect cells)
• Can span membrane at least once and cross it several times
• They are permanently embedded and can only be removed through expenditure of large amounts of energy or digestions

Peripheral membrane proteins - exposed to one side (interior OR exterior) provide structural support to membranes

• Participate in transmitting cell signaling events
• Alter the topology of membranes in the secretory pathway
• Can be enzymes
• Associate with the head groups of specific phospholipids or portions of integral membrane proteins (hence the name “peripheral”)

Unlike integral proteins, the association is impermanent - they can be easily removed by changing the composition of the membrane or the morphology or charge of the protein

Some proteins in the outer leaflet form covalent links (through the amino acids in their C-terminuses) with the head groups of phospholipids; these are the proteins that act as enzymes.

Carbohydrates

Membrane carbohydrates account for approximately 2-10 % of the mass of the cell membrane. They are confined mainly to the non-cytosolic surface on the extracellular surface of the cells. They are covalently bonded to proteins and lipids, forming glycoproteins/proteoglycans and glycolipids, respectively.

Cytoskeleton

Structural support (maintaining shape & preventing damage) for the cell membrane is provided by cytoskeleton.

• Sits directly under the cell membrane and is composed of a “mesh” of actin filaments
• Interacts with integral membrane proteins by limiting the diffusion of membrane proteins and providing a stable framework to which membrane proteins attach
• Prevents damage to membranes when external forces pull or push on integral membrane proteins
• Microtubules that form unique structures (ex: 9 + 2 arrangement for cilia)

Movement Across Membranes

An important ability of cells is to transport materials in and out of the cell via the cell membrane. Such movements generally fall under the categories of passive transport or active transport.

Passive Transport

Passive transport is the movement of molecules that does not require the energy of the cell. There are three primary forms of passive transport.

Simple diffusion is the movement of molecules down their concentration gradient (region of high concentration to region of low concentration) without the use of energy. The rrate of diffusion varies from membrane to membrane because of different selective permeabilities. Examples of items that pass easily include small, uncharged substances, such as the following:

• Water
• Lipids (due to nonpolarity)
• Oxygen (due to nonpolarity)
• Carbon dioxide
• Some waste
• Some amino acids

Osmosis is the passive diffusion of water down its concentration gradient (region of high concentration to region of low concentration) across selectively permeable membranes. Water will flow from a region with a lower solute concentration (hypotonic) to a region with a higher concentration (hypertonic). Water “dilutes” area with more solute and makes the area with less solute less watery until both areas have equal concentration of solute.

Facilitated diffusion is the diffusion of particles across a selectively permeable membrane with the assistance of the membrane’s transport proteins. Transport channels are specific in what they can carry and have binding sites designed for molecules of interest. These binding sites are designed in such a way that using these to transport particles requires no energy.

Active Transport

Active transport is the movement of particles across a selectively permeable membrane against its concentration gradient (from low concentration to high), requiring an input of energy (ATP). Active transport is vital to the ability of cells to maintain particular concentrations of substances despite environmental concentrations.

Some processes associated with active transport include the following:

• Endocytosis - a process in which substances are brought into cells by the enclosure of the substances into a membrane-created vesicle
• Pinocytosis - involves the transport of solutes or fluids
• Phagocytosis - the movement of large particles or whole cells (ex: phagocytes are immune cells which engulf bacteria and viruses and eliminate them with lysosomal enzymes)
• Exocytosis - a process in which a vesicle functions like a trash chute by escorting the (packaged) substance to the plasma membrane, fusing with the membrane, and ejecting the substance outside the cell

The sodium-potassium pump is a major pump in animal cells. Through this transport, 2 potassiums are moved in for every 3 sodium out against their respective concentration gradients. This makes sure that cells have a very high concentration of potassium and a very low concentration of sodium at all times (diffusion wants to move sodium in and potassium out to equalize)

Cellular Homeostasis

Tonicity

• Hypertonic - when the concentration of solute molecules outside the cell is higher than the concentration in the cytosol, the solution outside is hypertonic to the cytosol (and cytosol is hypotonic to outside solution), so water diffuses out of the cell until equilibrium is established
• Hypotonic - when the concentration of solute molecules outside the cell is lower than the concentration in the cytosol, the solution outside is hypotonic to the cytosol (and cytosol is hypertonic to outside solution), so water diffuses into the cell until equilibrium is established
• Isotonic - when the concentrations of solutes outside and inside the cell are equal, the outside solution is said to be isotonic to the cytosol, so water diffuses in and out of the cell at equal rates and there is no net movement of water

Tonicity can be understood in different ways for animal cells and plant cells. In an animal cell:

• In a hypertonic environment, water rushes out of the cell to establish equilibrium and cell shrivels
• In a hypotonic environment, water rushes into the cell to establish equilibrium and cell lyses (bursts)
• Isotonic environment is IDEAL

By contrast, in a plant cell:

• In a hypertonic environment, water rushes out of the cell to establish equilibrium and cell becomes plasmolyzed (shrinking of cell’s cytoplasm away from the cell wall)
• In an isotonic environment, water diffuses in and out at equal rates but cell is flaccid
• A hypotonic environment is IDEAL because water rushes into the cell to establish equilibrium and fills the central vacuole, causing it to press against the cell wall and create turgor pressure (a turgid plant cell is best)

Some cells prefer a hypotonic environment, so as cells accumulate water, they must pump excess water out in order to maintain a lower concentration of water in the cytosol (maintain osmotic pressure). A contractile vacuole is an organelle that uses energy to collect excess water and then contract, pumping water out of the cell (found in paramecium).

Cell Structure

Prokaryotic Cells

See also: Microbe Mission#Prokaryotes

File:Prokaryotediagram.jpg

An example diagram of a prokaryotic organism

Prokaryotic organisms (bacteria and archaea) are thought to be the ancestors of all life on earth. As a result, they are much more simplistic than eukaryotic organisms. Prokaryotes do not have the same membrane-bound organelles, and their cells also tend to be much smaller. However, as they share common ancestry, prokaryotes and eukaryotes have a lot in common. While they may not have organelles present in the same way, they have a variety of similar structures to perform vital functions. Prokaryotic organisms also typically store their genetic information in the form of DNA. This DNA is not found in a nucleus, and is instead just found in a region of the cell known as a nucleoid. Prokaryotic DNA also takes the form of one circular DNA molecule, as opposed to in eukaryotes where DNA can be found across multiple linear molecules. Prokaryotes also reproduce in a different fashion, because they lack many of the same structures as eukaryotes. As opposed to undergoing mitosis, prokaryotes go through a process known as binary fission where they reproduce asexually.

Prokaryotic Organelles

Organelle	Function
Cell membrane	Functions in transport, the movement of substances in and out of the cell, and in energy production (breakdown of large molecules, photosynthesis).
Cell wall	Gives structural strength (rigidity) to the cell.
Capsule or slime layer	Jelly-like substance which protects the cell wall from environmental damage.
Nucleoid	Contains a single circular molecule of DNA. This DNA is not packaged around histones like eukaryotic DNA, and is not bound within a nucleus.
Cytoplasm	Region surrounding the nucleoid and within the cell membrane. Contains ribosomes and RNA (site of protein synthesis).
Flagellum	Protein fiber that functions in movement.

Eukaryotic Cells

Eukaryotic Cells contain:

Cell Wall: Found in plant cells, provides protection and support. Prevents the cell from bursting when turgid.

Plasma Membrane: Control substances coming in and out of the cell. Selectively permeable. Consists of a phospholipid bilayer and various embedded proteins.

Cilia: Sweeps materials across the cell surface.

Flagellum: Enables a cell to propel and move in different directions (uncommon).

Cytoplasm: Contains the organelles of the cell (see table below).

Plant Cell	Animal Cell
Diagram of a typical Eukaryotic Plant Cell	Diagram of a typical Eukaryotic Animal Cell

Cell Structure and Organelles

Cell Structure and Organelles

Organelles	Functions	Images
Nucleus	- The "brain" of the cell - Found in most eukaryotic cells - Enclosed in double membrane - Communicates with surrounding cytosol via nuclear pores - Filled with nuclear chromatin-DNA and surrounding protein
Nucleolus	- Inside the nucleus - Produces ribosomes which then leave the nucleus and go into the rough endoplasmic reticulum where they’re critical to protein synthesis
Cytosol	- “soup” where all organelles reside - Where most of cellular metabolism occurs - Mostly water, but full of proteins that control cell metabolism including glycolysis, transcription factors, intracellular receptors, etc.
Cytoplasm	- Collective term for cytosol and organelles in it
Centrosome	- AKA microtubule organizing center (MTOC) - Area where microtubules are produced - Animal cell centrosome is a small pair of organelles called centrioles
Centriole	- Ring of nine groups of fused microtubules - 3 microtubules in each group - Microtubules and centrioles are part of the cytoskeleton - The centrioles in a complete animal centrosome are perpendicular
Golgi Apparatus	- Membrane-Bound Structure with a single membrane - Actually a stack of membrane-bound vesicles that are important in packaging macromolecules for transport elsewhere in the cell - Stack of larger vesicles surrounded by smaller vesicles containing packaged macromolecules
Lysosomes	- Membrane-Bound - Common in animal cells - Contain hydrolytic enzymes necessary for cell digestion - In white blood cells that eat bacteria, lysosomes are released into the vacuole around the bacteria and kill them - Necrosis: an uncontrolled mass release of lysosomal contents into a cell that causes cell death
Peroxisome	- Protect the cell from its own production of toxic hydrogen peroxide - Ex. white blood cells produce hydrogen peroxide to kill bacteria - Oxidative enzymes in peroxisomes break down hydrogen peroxide into water and oxygen
Secretory Vesicles	- Cell secretions (hormones, neurotransmitters) are packaged in secretory vesicles at Golgi apparatus - Secretory vesicles then transported to cell surface for release
Cell Membrane	- Cell enclosed in membrane, a double layer of phospholipids-lipid - Exposed heads are hydrophilic, hidden tails are hydrophobic - This allows membrane to act as protective barrier to uncontrolled water flow - Proteins on membrane: Receptors for odors, tastes, and hormones Responsible for controlled entry and exit of ions like sodium potassium, calcium, and chloride
Mitochondria	- Provide energy-power centers - About the size of bacteria - Membrane bound-double membrane - Outer membrane smooth, inner forms folds(cristae) which greatly increases the inner membrane’s surface area. - Food (sugar) is combined with oxygen to make ATP on these cristae
Vacuole	- Membrane-bound sac - Plays roles in intracellular digestion and release of cellular waste products - Generally small in animal cells - Large in plant cells - Storing nutrients and waste products - Helps increase cell size during growth - Acts like lysosomes of animal cells - Regulates turgor pressure in cell-water in vacuoles produce rigidity in plant
Cell Wall	- Only found in plant cells - Rigid protective cell wall made of polysaccharides (usually cellulose) - Provides and maintains shape of cells and serves as protective barrier - Fluid collects in vacuole and pushes against cell wall - This turgor pressure is responsible for crispness of fresh vegetables
Chloroplasts	- Specialized organelles found in higher plant cells - Contain chlorophyll responsible for plants’ green color and absorbing energy from sunlight - Energy used to convert water and carbon dioxide into sugars through photosynthesis - Have a double outer membrane - Within stroma are thylakoids appearing in stacks called grana
Smooth Endoplasmic Reticulum	- Vast network of membrane-bound vesicles and tubules called endoplasmic reticulum - A continuation of the outer nuclear membrane and its varied functions suggest the complexity of the eukaryotic cell - Named because of its smooth appearance in electron microscopy - Plays different functions depending on cell type including: Lipid and steroid hormone synthesis Breakdown of lipid soluble toxins in liver cells Control of calcium release in muscle cells
Rough Endoplasmic Reticulum	- Appears pebbled by ribosomes on its surface - Proteins synthesized by ribosomes collect in rough ER for transport throughout the cell
Ribosomes	- Membrane-bound - Packets of RNA and protein - The site of protein synthesis - Comprised of two parts, large and small subunit
Cytoskeleton	- Maintains cell shape - Primary importance is cell motility-internal movement of organelles - Organized network of three primary protein filaments - Microtubules - Largest filaments. They mainly provide mechanical support, as well as organize the cytoplasm, move the cell, and separate chromosomes during telophase. 25 nm diameter. - Intermediate filaments - "Middle-sized filaments." This also has a main job of providing support for the cell when it comes into contact with other cells. 10 nm diameter. - Actin filaments (Microfilaments) - Smallest filaments. Most abundant protein in eukaryotic cells. This is used in muscle contraction, cell movement, cytokinesis, cell signaling, movement of organelles. 7 nm diameter.

During animal cell division centrioles replicate and centrosome divides. Two resulting centrosomes and centrioles move to opposite ends of the nucleus. From each centrosome, microtubules grow into a spindle which is responsible for separating replicated chromosomes in the daughter cells.

Cell Cycle

In animals, autosomal cells are diploid (2n), with two copies of each chromosome. Germ cells, on the other hand, are haploid (n), containing only one copy of each chromosome. Eukaryotic cells replicate through the cell cycle, a specific series of phases during which a cell grows, synthesizes DNA, and divides. Derangements of the cell cycle can lead to unchecked cell division and may be responsible for the formation of cancer.

Preparation

• Interphase - DNA is chromatin (chromosomes not visible), cell spends most time here (90%)
• G1 phase (presynthetic gap) - Cell grows to mature size and makes sure it has all material necessary for DNA synthesis, also obtains nutrients and begins metabolism
• G1 checkpoint (aka restriction point)- Cell irreversibly commits to the cell division process and goes into S phase (if all conditions favorable) or advances into G0. Growth factors (and other external influences) play a role in carrying the cell past the G1 checkpoint. The cell (1) must be of appropriate size, (2) have adequate energy reserves, (3) no damage to DNA.
• G0 phase (inactive phase) - Cell makes the decision to exit cycle after G1 and does not replicate DNA or divide (ex: fully developed cells in the central nervous system)
• S phase - DNA is replicated (synthesized) so that each daughter cell will have a complete set of chromosomes after the parent cell divides; transition to S phase is signaled by cyclins and CDKs. Following S phase, each chromosome consists of two identical chromatids that are bound together at a specialized region known as the centromere.
• G2 phase (postsynthetic gap) - Cell grows more and prepares for division by making sure that it has the material (ex: doubles of organelles) necessary for the physical separation and formation of daughter cell
• G2 checkpoint - Bars entry into mitotic phase if conditions not met. The cell checks for DNA integrity and DNA replication, and if errors or damage are detected, the cell will pause to allow for repairs; if the damage is irreparable, the cell may undergo apoptosis. If no problems are found, CDKs signal beginning of mitotic cell division.

Depending on the type of cell, either meiosis or mitosis can proceed.

Mitosis

See also: Heredity#Mitosis & Cell Cycle

Mitosis, taking up 10% of the cell cycle, divides an autosomal cell into 2 diploid (2n) daughter cells.

• Prophase - Nucleus and nucleolus disappear; chromosomes appear as two identical, connected sister chromatids; mitotic spindle (made of microtubules) begins to form; centrioles move to opposite poles of the cell (plant cells do not have centrioles)
• Metaphase - the sister chromatids line up along the middle of the cell, ready to split apart
• M checkpoint (spindle checkpoint) - occurs near the end of the metaphase stage of mitosis and determines whether all the sister chromatids are correctly attached to the spindle microtubules; mitosis will not proceed until the kinetochores of each chromatid pair are firmly anchored to at least 2 spindle fibers arising from the opposite poles of the cell
• Anaphase - The sister chromatids split and move via the microtubules to opposite poles of the cell (pulled by the spindle apparatus so that each pole of the cell has a complete set of chromosomes
• Telophase - the nuclei for the newly split cells form; the nucleoli reappear, and the chromatin uncoils
• Cytokinesis - Newly formed daughter cells split apart. Animal cells are split by the formation of a cleavage furrow, plant cells by the formation of a cell plate

Meiosis

See also: Heredity#Meiosis

Meiosis, divided into two stages, divides one diploid (2n) cell into 4 haploid (n) daughter cells. It occurs in cells of gonads to produce gametes (part of process of sexual reproduction).

Meiosis I

• Prophase I - Each chromosome pairs with its homolog. Crossover (synapsis) occurs in this phase. The nuclear envelope breaks apart and spindle apparatus begins to form.
• Metaphase I - Chromosomes align along the metaphase plate matched with their homologous partner. This stage ends with the separation of the homologous pairs.
• Anaphase I - Separated homologous pairs move to opposite poles of the cell.
• Telophase I - Nuclear membrane reforms; process of division begins.
• Cytokinesis - After the daughter cells split, the two newly formed cells are haploid (n).

Following meiosis I, cells enter a period of rest called interkinesis or prophase II. No DNA replication occurs during this stage.

Meiosis II

• Prophase II - Nuclear envelope breaks apart and spindle apparatus begins to form.
• Metaphase II - Sister chromatids line up along the equator of the cell.
• Anaphase II - Sister chromatids split apart and are called chromosomes as they are pulled to the poles.
• Telophase II - The nuclei and the nucleoli for the newly split cells return.
• Cytokinesis - Newly formed daughter cells physically divide.

Enzymes and Inhibition

Enzymes are special proteins that regulate nearly every biochemical reaction in the cell. Proteins are built from amino acids, which have a carboxyl end (C-terminus), an amino end (N-terminus), an “R” group and a hydrogen, with carbon as the center.

Enzymes function to:

• Provide energy to cells
• Build new cells
• Aid in digestion
• Break down complex molecules (“substrate” = reactant)
• Catalyze reactions (speed up chemical reactions without being used up or altered)

The function of an enzyme is determined by its structure. The structure of an enzyme, especially its primary structure, is determined during protein synthesis. Other factors such as pH, temperature, and quantity also affect an enzyme's effectiveness.

The rate of enzymatic reactions is determined by the Michaelis-Menten equation, given by

[math] v = \frac{d [P]}{d t} = \frac{ V_\max {[S]}}{K_\mathrm{M} + [S]} .[/math]

Here, [math]V_\max[/math] represents the maximum rate achieved by the system, at saturating substrate concentration. The Michaelis constant [math]K_\mathrm{M}[/math] is the substrate concentration at which the reaction rate is half of [math]V_\max[/math].

Enzyme activity can be regulated through allosteric regulation, encompassing negative regulation (feedback inhibition) and positive regulation (subtract activation), as well as enzyme inhibitors, including reversible and irreversible inhibitors.

Resources and Study Materials

• Albert's Molecular Biology of the Cell (6th Edition). Most higher-level tests will draw from this textbook.
• Campbell Biology (11 Edition); alternatively, any college-level biology textbook.
• Any high school biology teacher, and especially the AP/IB Biology teacher (if available). Seek out these teachers as they can be important and at times invaluable resources for learning difficult or confusing material. Teachers also can aid in answering questions that may arise during study.
• AP Biology CD. Most AP Biology teachers have an interactive CD that comes with the textbook that they use. If possible, borrowing the CD can provide another portable resource.
• AP Biology review books, inlcuding Cliff's AP Biology (less detailed) and Barron's AP Biology (more detailed).
• Old tests. Event members should ask the coach for old tests from previous invitationals as most invitationals provide the test and answers after the competition is over. The Test Exchange on this site is also an excellent source for additional practice tests.
• Review, study, and practice often. Trust, respect, and understanding communication is key to an effective partnership.

Links

CELLS alive!

UW Department of Pathology Cytogenetics

The Biology Project