- Identify the basic features of eukaryotic and prokaryotic cells.
- Describe the structure and function of organelles and other cell structures.
Living cells must:
- Obtain food and energy
- Convert energy into a form the cell can use
- Construct and maintain molecules that form the cell structure
- Eliminate wastes
- Keep records of how to build structures
Prokaryotes and EukaryotesEdit
Prokaryotes (pro means "before", karyon means "nucleus") are simple cells with their DNA concentrated in an area of the cell called the nucleoid.
Eukaryotes (eu meaning "true", i.e. "true nucleus") are larger more complex cells that keep their DNA enclosed in a double membrane. Eukaryotes also contain a number of specialized structures called organelles. Each organelle has a highly organized structure and specific function in the cell, and most are surrounded by their own membranes. The organelles work together to carry out the life functions of the eukaryotic cell.
The organelles of a eukaryotic cell divide its interior into compartments which allows different chemical reactions to take place at the same time without interfering with each other. Many organelles contain folded membranes to increase the surface area for chemical reactions increasing the reaction rate in the cell. Like the cell membrane, organelle membranes have a fluid-mosaic structure.
Contains DNA in an uncoiled state (chromatin). Each nucleus has at least one area of chromatin called the nucleolus, which is dedicated to producing ribosomes.
Tiny organelles which lack a membrane. They are assembled in the nucleus and consist of a large and small subunit made of ribosomal RNA (rRNA) and proteins. Cytoplasmic ribosomes are attached to the cytoskeleton and help to produce proteins (mostly enzymes) used within the cell. Ribosomes attached to the endoplasmic reticulum (ER) produce proteins that are processed in the ER and sent to the Golgi apparatus.
A folded membrane with a large surface area for chemical reactions. Materials synthesized in the ER are kept separate from the rest of the cell to keep enzymes or proteins from damaging the cell. The surface of the rough ER is covered in ribosomes which synthesize proteins that are processed and modified by enzymes on the inner surface of the ER. The smooth ER has tubules with enzymes embedded on the inside for processing macromolecules like lipids. In testicular cells the smooth ER produces the lipid testosterone. The smooth ER can pinch off vesicles.
A saucer-shaped sac that collects vesicles from the smooth ER to complete processing of macromolecules making them fully function while still keeping the separate from the rest of the cell. It sorts and packages molecules and transports them in vesicles which can leave the cell by exocytosis. The Golgi apparatus also creates lysosomes to transport digestive enzymes.
Transforms energy from macromolecules into a form that the cell can use, ATP. The outer mitochondrial membrane separates the chemical reaction occuring inside the mitochondrion from the rest of the cytoplasm. The folds of the inner membrane, or cristae, provide a large surface area for the enzyme complexes involved in forming ATP. Cells that use a large amount of energy have more mitochondria. For example a liver cell might contain 1000 mitochondria. Mitochondria have their own ribosomes and DNA and replicate by binary fission.
Contain digestive enzymes and break down macromolecules or larger structures such as bacteria or worn-out organelles. This provides raw materials for the cell to build new structures.
Similar to lysosomes but formed in the cytoplasm. Most common in liver, brain, and kidney cells, they break down long fatty acid chains and detoxify alcohol.
Organizes the locations of organelles in the cytoplasm, gives shape to the cell, and allows for movement of parts of the cell. The three components of the cytoskeleton - actin filaments, intermediate filaments, and microtubules - can be assembled and disassembled within seconds or minutes. Each component consists of repeated protein subunits that can be added or removed. When a cell divides the cytoskeleton disassembles and reassembles itself in the form of a spindle (which evenly distributes the genetic material and other cell contents into two daughter cells). Long, thin, flexible cables of actin filaments forma dense web under the cell membrane which allows for the movement of the membrane. Actin is a protein that can contract and forms a key component of muscle cells. Intermediate filaments anchor organelles into regions in the cell, as well as support the nuclear envelope and cell membrane. The rod-like tubes of microtubules act like tracks along which organelles such as vesicles and mitochondria can move, and stabilize the shape of cells with irregular contours, such as nerve cells. Microtubules also form the main structural component of spindle fibres, centrioles, cilia, and flagella.
The centrosome assembles and co-ordinates the activity of spindle fibres when the cell divides. Since the centrosome lacks a membrane, its most distinctive feature in an animal cell is a pair of short, cylindrical centrioles. Centriols are a ring of nine sets of microtubule triplets with no microtubules in the middle (called a 9 + 0 pattern). The centrioles may be involved in the formation of cilia and flagella.
Cilia and FlagellaEdit
Flexible projections enclosed in cell membrane that extend outward from the cell. Short, cylindrical projections are called cilia (singular cilium) and produce a wave-like motion. The long projections are called flagella (singular flagellum) and produce a whip-like motion. Both have the same internal construction and can move a cell through its environment. Some organisms also use cilia to move food inside of them and the human trachea has cilia which sweep out foreign particles. The movement of the cilia and flagella requires energy from the cell.
Structures of Plant CellsEdit
The cell wall around plant and fungus cells, as well as some single-celled eukaryotes, consists mostly of cellulose fibres and adds strength and rigidity to the cell. Spaces between the cellulose fibres allow molecules to pass to and from the cell. Plant cells that form part of a supporting structure in plants have a secondary, stronger, cell wall (which usually contains a substance called lignin) inside the primary cell wall. Unlike the cell membrane, the cell wall does not control the materials that can pass through it. However, the cell wall does help the cell deal with hypotonic or hypertonic environments. It prevents the cell from bursting in hypotonic environments and provides structural support for when the cell contents shrink in hypertonic conditions.
Plant cells usually have a large fluid-filled central vacuole. This presses outward on the cell wall to help support it. It also provides storage space for water and other substances. These may include toxins that make the cell, and thus the plant, taste bad to animals.
The green chloroplast is one of a group of organelles called plastids. All plastids contain stacked internal membrane sacs. These sacs are enclosed in a double membrane and have the ability to perform photosynthesis. Only plastids exposed to light develop pigments and participate in collecting energy from light. Plastids also act as storage containers for starches, lipids, and proteins. These organelles contain their own DNA and ribosomes, and new plastids are produced through the division of existing plastids. The green pigment chlorophyll gives chloroplasts their colour and also absorbs solar energy, allowing photosynthesis to occur. The chlorophyll, other pigments, and enzymes necessary for photosynthesis are contained within a special membrane system.
Prokaryotic cells lack any membrane-bound organelles. They do contain thousands of ribosomes. The nucleoid of a prokaryote contains a single loop of double stranded DNA. Hoever, some prokaryotes also contain self-replicating circular loops of DNA called plasmids. Plasmids often carry information that gives the cell resistance to certain antibiotics and heavy metals or the ability to synthesize or break down certain compounds.
Almost all prokaryotes have a cell wall consisting of a unique macromolecule called peptidoglycan. Some prokaryotes have a capsule or slime layer around the cell wall. Bacteria that can carry out photosynthesis have thylakoids that develop from invaginations of the cell membrane.
Some prokaryotes have flagella. unlike eukaryotic flagella, these rotate like propellers. A prokaryotic cell may also have hollow appengages called pili. These allow the cells to stick to other cells or surfaces and to exchange plasmids with other prokaryotic cells. Without the many folded membranes of eukaryotic organelles, prokaryotic cells have limited surface area available for reaction sites - although some species have areas of folded cell membrane. Still, prokaryotes work efficiently to meet their needs using a smaller number of reactions than eukaryotes do. Small eukaryotic cells benefit from rapid internal diffusion of materials. Prokaryotic intracellular fluid contains areas of different concentrations of materials, which provide some segregation of chemicals and reactions.