AP Biology Unit 2: Cell Structure and Function

Prokaryotic Cells

Singled-cell organisms belonging to the domains of Archaea and Bacteria, they have no nucleus, and lack organelles.

Nucleoid Region

In prokaryotes, the DNA is a circular chromosome located in the center of the cell in this area.

Plasmids

In prokaryotes, bacteria may contain extra genetic material outside of the chromosome, which is contained in these small circular pieces of DNA.

Eukaryotic Cells

Have a nucleus, linear DNA, highly structured cell membranes, and organelles, and they reproduce via mitosis and meiosis. Animals, plants, protists, and fungi are all composed of these cells.

Membrane-Bound Nucleus

In eukaryotes, DNA is packaged into linear chromosomes that are contained in this.

Ribosomes

Functions in protein synthesis and is found in both prokaryotic and eukaryotic cells. They are made up of proteins and ribosomal RNA (rRNA). Both the prokaryotic and eukaryotic versions have a large and a small subunit, but the sizes of these subunits differ slightly. During translation, they assemble amino acids into polypeptide chains according to the mRNA sequence.

Free Ribosomes

Found in the cytosol in both prokaryotes and eukaryotes.

Bound Ribosomes

In eukaryotes, they are found on the membrane of the rough endoplasmic reticulum.

Endoplasmic Reticulum

A series of membrane channels in eukaryotes that forms a series of flattened sacs within the cytoplasm and serves multiple functions, being important in the synthesis, folding, modification, and transportation of proteins.

Rough ER

An organelle that has ribosomes bound to its membranes, functions in protein synthesis, manufactures steroid molecules, cholesterol, and other lipids.

Smooth ER

An organelle found in both animal and plant cells. It does not contain ribosomes and functions in the synthesis of lipids and the detoxification of harmful substances in the cell.

Golgi Complex

A stack of flattened membrane sacs (cisternae). It controls the modification and packaging of proteins for transport. Proteins made on the free ribosomes of the rough ER are sent here, which modifies the proteins into their final confirmation and packages the finished proteins into vesicles for transport throughout the cell. It is often found near the rough ER.

Lumen

What the interior of each cisterna is called; it contains the enzymes necessary for the Golgi complex to function.

Lysosomes

Membrane-bound sacs, containing hydrolytic enzymes, that function in a variety of cell processes. They can help digest macromolecules, break down worn-out cell parts, function in apoptosis, or destroy bacteria and viruses that have entered the cell.

Vacuoles

Membrane-bound sacs in eukaryotes, they function in food or water storage, water regulation in a cell, or waste storage until the waste can be eliminated from the cell. In well-hydrated plant cells, they often occupy the majority of the volume of the cell. By filling up space within the cell, they provide the plant cell with turgor pressure and support.

Mitochondria

Produce energy for the cell. It has double membranes, with a smooth outer membrane and a folded inner membrane. These folds increase the surface area available for energy production during cellular respiration. The double-membrane structure of the mitochondria also allows them to create the proton gradients that are necessary for ATP production. They also contain their own mitochondrial DNA (mtDNA).

Matrix

An enzyme-containing fluid in the center of the mitochondria. The reactions of the Krebs cycle (citric acid cycle) occur here.

Chloroplasts

Organelles in plants and algae that conduct photosynthesis; they have a double-membrane structure and a smooth outer membrane. They contain their own chloroplast DNA (cpDNA).

Thylakoids

Pancake-shaped membranous sacs. The membranes of these function in the light-dependent reactions of photosynthesis.

Grana

The structures that thylakoids are stacked into.

Stroma

The liquid inside the chloroplast that surrounds the grana. The enzymes in it function in the light-independent reactions of photosynthesis.

Centrosome

Found in animal cells and helps the microtubules assemble into the spindle fibers needed in cell division. Defects in its function have been associated with dysregulation of the cell cycle in some cancers.

Amyloplasts

Plant cells may contain them. Excess glucose produced by photosynthesis is stored as starch molecules in these. They are frequently found in the root and tubers of starchy vegetables, such as potatoes.

Peroxisome

Helps oxidize molecules and break down toxins in the cell.

Nucleolus

Not a membrane-bound organelle; it is a condensed region of chromatin where ribosome synthesis occurs.

Cytoskeleton

A network of protein filaments that extends throughout the cytoplasm. Its fibers help give cells their shape and can be used to move items in the cell.

Endosymbiosis Hypothesis

States that membrane-bound organelles, like mitochondria and chloroplasts, were once free-living prokaryotes that were absorbed into larger prokaryotes; these prokaryotes became interdependent on each other. The smaller prokaryotes that were engulfed by the larger prokaryotes evolved to become membrane-bound organelles.

Endosymbiosis Hypothesis Evidence

Mitochondria and chloroplasts have their own DNA. Mitochondrial and chloroplast DNA is circular, similar to that of prokaryotic DNA.

Mitochondria and chloroplasts have their own ribosomes, which are similar in structure to prokaryotic ribosomes.

Mitochondria and chloroplasts reproduce by binary fission, similar to how bacteria reproduce.

Compartmentalization

Membrane-bound organelles in eukaryotes allow different parts of the cells to specialize their functions for greater efficiency within the cell, leading to a formation of compartments. This allows the cell to separate the enzymes involved in different metabolic processes, minimizing the risks of enzymes and molecules from different processes cross-reacting, making the processes less efficient and more difficult to regulate.

Surface Area to Volume Ratio

The concept that as cells or organisms increase in size, their surface area-to-volume ratio decreases, leading to less efficient exchange of materials with the environment, limiting how much the cell can grow; making it advantageous that an organism be made up of many smaller cells rather than a few large ones. Small organisms lose heat at much higher rates than larger organisms due to their efficient exchange of heat.

Plasma Membrane

Critical in allowing cells to maintain an internal environment that is favorable to life, they only let some materials cross the membrane while others cannot, allowing the cell to maintain its internal environment.

Selective Permeability

The membranes ability to differentiate between different molecules, only allowing some through while blocking others.

Phospholipid Bilayer

What the plasma membrane is made of and what makes it selectively permeable. When it is formed in an aqueous environment, the hydrophobic lipid tails orient themselves away from the water toward the interior of the membrane, while the hydrophilic phosphate heads are oriented towards the water on the exterior of the membrane. The hydrophobic lipids are much larger than the hydrophilic phosphates, so small hydrophobic molecules can easily pass between the phospholipids into and out of the cell while large polar molecules and ions cannot cross the cell membrane unassisted and must use embedded membrane channels or transport proteins to enter or exit the cell.

Aquaporins

Specialized proteins that allow for most of the passage of water in and out of the cell.

Membrane Proteins

Function in transporting materials, participating in cell signaling processes, anchoring the cell to its surroundings, and catalyzing chemical reactions.

Glycoproteins and Glycolipids

Modified proteins and lipids, they function in cell recognition.

Steroids

Can adjust membrane fluidity in response to changing environmental conditions and the needs of the cell.

Fluid Mosiac Model

What the structure of the membrane is often referred to as because the components of the plasma membrane have mobility.

Cell Walls

What plants, fungi, and prokaryotes are surrounded by outside of the cell membrane. They are composed of large carbohydrates (cellulose in plants, glucans in fungi, and peptidoglycan in prokaryotes). They provide rigidity to the cell and are an additional barrier to substances entering or exiting the cell.

Passive Transport

The movement of molecules from areas of higher concentration to areas of lower concentration. It moves molecules down its concentration gradient and does not require energy (diffusion).

Facilitated Diffusion

The process of passive transport that uses a membrane protein because molecules that are polar or charged may require a membrane protein in order to diffuse across the cell membrane. Because molecules that use this require a membrane protein in order to cross the membrane, the rate of it is limited by the number of membrane proteins available and is saturable.

Channel Proteins

Can allow the passive transport of ions, down their concentration gradients.

Active Transport

The movement of molecules from areas of lower concentration to higher concentration. It moves molecules against their concentration gradient so it requires energy.

Endocytosis

Requires an input of energy and is a form of active transport. It is used by the cell to take in water and macromolecules by enfolding them into vesicles formed from the plasma membrane.

Exocytosis

Requires an input of energy and is a form of active transport. The reverse of endocytosis, vesicles are fused with the plasma membrane, which then allows these molecules to be expelled from the cell.

Hypotonic Solution

Has a lower concentration of solute.

Hypertonic Solution

Has a higher concentration of solute.

Isotonic Solution

Has the same concentration of solute as that of another solution.

Water Potential

The potential energy of water in a solution, or the ability of water to do work. The more water there is in a solution, the higher its water potential will be and vice versa. Water flows from areas of higher water potential to areas of lower water potential, so it will flow from hypotonic to hypertonic solutions. It focuses on the concentration of water in a solution, so if there is more solute in a solution, the water is less concentrated in that solution. It is calculated from the equation: Ψ = Ψp + Ψs.

Pressure Potential (Ψp)

The water potential due to the pressure on the system. Most biological systems are open to the atmosphere and are in pressure equilibrium with their environment so in most cases it is 0.

Solute Potential (Ψs)

The water potential due to the solute concentration. It depends on the concentration of the solute, how many particles the solute forms when in solution, and the temp of the solution. It is calculated from the equation:

Ψs = -iCRT

Ionization Constant (i)

A function of how many particles or ions a solute will form in solution. For covalent compounds, which do not separate into ions in solution, this is one since it will form one particle in solution. For ionic compounds, which do separate into ions in solution, this depends upon how many ions the compound will form in solution.

Concentration of Solute (C)

Since there is a negative sign in the solute potential formula, as this increases, the solute potential decreases. Solutions with more solute will have lower water potentials if all other conditions are equal.

Pressure Constant (R)

0\.0831 liters-bars/mole-K.

Temperature of the Solution (S)

It is in Kelvin, not Celsius, because the pressure constant uses units of Kelvin, so consistent units are required.

Osmolarity

The total concentration of solutes in a solution. Living organisms need to closely regulate their internal solute concentration and their water potential. If an organism became too dehydrated, it might die. If too much water moved into a cell, the pressure potential from the water moving into the cell could cause the cell to burst.