The Cellular Level of Organization Living things are constructed of cells. Living things may be unicellular or multicellular. Cell structure is diverse but all cells share common characteristics.

The cell theory states: All organisms are composed of one or more cells. Cells are the basic unit of structure and function in organisms. All cells come only from other cells. Cell Theory came about through the work of seventeenth century scientists who had invented fairly primitive microscopes. Antonie van Leeuwenhoek made his own microscopes and observed many types of tiny things. Englishman Robert Hooke was the first to use the term “cell” and confirmed Leeuwenhoek’s findings. During the 1830s, German microscopist Matthias Scheiden found that plants were composed of cells, and Theodore Schwann found that animals were composed of cells. Rudulf Virchow of Germany came to the conclusion that cells come only from preexisting cells. All of these scientists contributed to the Cell Theory. Modern scanning and transmission electron microscopes have allowed scientists to determine the structure of cells at the level of the organelle.

Cells are small so they can exchange materials with their surroundings. Surface area relative to the volume decreases as size of cell increases. - limits the size of cells

Sizes of living things

Eukaryotic Cells All cells are surrounded by a plasma membrane made of phospholipids and proteins. Eukaryotic cells have membrane bound intracellular organelles. The most prominent is the nucleus that controls the workings of the cell.

The plasma membrane regulates what enters and exits the cell. Inside the plasma membrane, the nucleus is surrounded by cytoplasm. Plant cells have a cell wall in addition to the plasma membrane. Cytoplasm is a semi-fluid medium. Primary cell walls are made from cellulose. The secondary cell wall, which forms inside the primary wall, is formed from lignin. Its purpose is to give strength to the cell.

Animal and plant cells have organelles. Organelles compartmentalize functions within the cell. The organelles of animal and plant cells are similar to each other except that centrioles are present only in animal cells, and chloroplasts are present only in plant cells.

Animal cell anatomy

Plant cell anatomy

Structure of the Nucleus Chromatin: DNA and proteins Nucleolus: Chromatin and ribosomal subunits Nuclear envelope: Double membrane with pores Nucleoplasm: semifluid medium inside the nucleus.

The nucleus and the nuclear envelope

Ribosomes Protein synthesis occurs at tiny organelles called ribosomes. Ribosomes are composed of a large subunit and a small subunit. Ribosomes can be found alone in the cytoplasm, in groups called polyribosomes, or attached to the endoplasmic reticulum.

The endomembrane system consists of: Nuclear envelope Endoplasmic reticulum Golgi apparatus Vesicles The membranes that make up the endomembrane system are connected by direct physical contact and/or by the transfer of vesicles from one part to the other.

The endoplasmic reticulum The endoplasmic reticulum (ER) is a system of membranous channels and saccules. Rough ER is studded with ribosomes and is the site of protein synthesis and processing. Smooth ER lacks ribosomes and is the site of synthesis of phospholipids and the packaging of proteins into vesicles, among other functions. Saccules are flattened vesicles.

The Golgi apparatus The Golgi apparatus consists of a stack of curved saccules. The Golgi apparatus receives protein and also lipid-filled vesicles from the ER, packages, processes, and distributes them within the cell. This organelle may also be involved in secretion.

Lysosomes and vacuoles Lysosomes are vesicles produced by the Golgi apparatus. Lysosomes contain hydrolytic enzymes and are involved in intracellular digestion. Vacuoles (large) and vesicles (small) are membranous sacs in the cell that store substances. Digestion within the cell disposes of worn-out cell parts and is necessary for cell rejuvenation. Vacuoles and vesicles can store a variety of substances for the cell. Anything from water to plant pigments and toxic substances are stored in these membranous sacs.

Peroxisomes Peroxisomes are vesicles than contain enzymes. The enzymes in these organelles use up oxygen and produce hydrogen peroxide. Peroxisomes are abundant in the liver where they produce bile salts and cholesterol and break down fats.

Energy-Related Organelles The two energy-related organelles of eukaryotes are chloroplasts and mitochondria. Both organelles house energy in the form of ATP.

Chloroplasts A chloroplast is bounded by two membranes enclosing a fluid-filled stroma that contains enzymes. Membranes inside the stroma are organized into thylakoids that house chlorophyll. Chlorophyll absorbs solar energy and carbohydrates are made in the stroma.

Mitochondria Mitochondria are found in plant and animal cells. Mitochondria are bounded by a double membrane surrounding fluid-filled matrix. The inner membranes of mitochondria are cristae. The matrix contains enzymes that break down carbohydrates and the cristae house protein complexes that produce ATP.

The Cytoskeleton The eukaryotic cytoskeleton is a network of filaments and tubules that extends from the nucleus to the plasma membrane. The cytoskeleton contains three types of elements responsible for cell shape, movement within the cell, and movement of the cell: Actin filaments Microtubules Intermediate filaments

Actin filaments occur in bundles or mesh-like networks. Actin filaments play a structural role in intestinal microvilli and also interact with motor molecules, such as myosin.

Microtubles are small hollow cylinders made of the globular protein tubulin. Microtubules help maintain the shape of the cell and act as tracks along which organelles can move.

Centrioles Centrioles are short cylinders with a 9 + 0 pattern of microtubule triplets. Centrioles may be involved in microtubule formation and disassembly during cell division and in the organization of cilia and flagella.

Cilia and flagella Cilia (small and numerous) and flagella (large and single) have a 9 + 2 pattern of microtubules and are involved in cell movement. Cilia and flagella move when the microtubule doublets slide past one another. Each cilium and flagellum has a basal body at its base. Basal bodies have the same microtubule arrangement as centrioles and are believed to be derived from them. The basal body initiates polymerization of the nine outer doublets of a cilium or flagellum.

Prokaryotic Cells Prokaryotic cells include the bacteria and archaea. Bacterial cells have these constant features: Outer Boundary: Cell wall Plasma membrane Cytoplasm: Ribosomes Thylakoids (Cyanobacteria) Innumerable enzymes Nucleoid: Chromosome (DNA only)

Bacterial cells may have plasmids, small accessory rings of DNA. Some bacteria have a capsule or a slime layer. Most bacteria have flagella. Some also have fimbriae that help cells attach to surfaces. Bacteria have a great metabolic diversity.

Evolution of the Eukaryotic Cell Endosymbiotic hypothesis. Eukaryotes arose from a symbiotic relationship between various prokaryotes. Heterotrophic bacteria became mitochondria. Cyanobacteria became chloroplasts. Host cell was a large eukaryotic cell. Evidence for the endosymbiotic hypothesis is the following: Mitochondria and chloroplasts are similar to bacteria in both size and structure. Both organelles are bounded by a double membrane – the outer membrane may represent the engulfing vesicle, and the inner membrane from the prokaryote. Mitochondria and chloroplasts each contain a limited amount of genetic material and divide by splitting. The DNA is a circular loop like that of prokaryotes. Most proteins of mitochondria and chloroplasts are now produced by the host cell, but these organelles do have their own ribosomes and do produce some proteins. Their ribosomes resemble those of prokaryotes. The RNA base sequence of the ribosomes in chloroplasts and mitochondria suggests a prokaryotic origin.

Evolution of the eukaryotic cell