Chapter 6 “The Cell”

Chapter 6“The Cell”

Life is Cellular • Cell Theory: • All living things are made up of cells • Cells are the basic units of structure & function in living things • New cells are produced from existing cells

All cells share 2 characteristics • 1. A barrier called a cell membrane that surrounds the cell, and • 2. At some point in their lives they contain DNA. DNA is the molecule that carries biological information.

Cells Fall Into 2 Broad Groups, Based on Whether They Have a Nucleus 1. Prokaryotes: do not have nuclei. They have genetic material that is not contained in a nucleus. Bacteria are prokaryotes.

2. Eukaryotes are cells that have nuclei. Eukaryotes have a nucleus in which their genetic material is separated from the rest of the cell. • Plants, animals, fungi, and protists are eukaryotes.

Eukaryotic Cell Structure • The eukaryotic cell is divided into 2 main parts: the nucleus and the cytoplasm. • The cytoplasm is the part of the cell outside the nucleus.

In the Nucleus • The nucleus contains most of a cell’s DNA. The DNA contains the coded instructions for making proteins and other important molecules. • The nucleus is surrounded by a double membrane called a nuclear envelope • Inside the nucleus is chromatin. Chromatin is made up of DNA bound to proteins. When the cell divides, this chromatin condenses into chromosomes. • Most nuclei also have a small, dense region known as the nucleolus where ribosomes are made.

In the CytoplasmEukaryotic cells have structures called organelles within the cytoplasm. • Ribosomes are small particles of RNA and protein spread throughout the cytoplasm. Proteins are made on ribosomes. • The endoplasmic reticulum (ER) is an internal membrane system. The ER is where lipids are assembled, along with proteins and other materials that are exported from the cell. • 2 types of ER: (1) Rough ER has ribosomes on its surface and is the site of protein synthesis; and, (2) Smooth ER which has no ribosomes and helps make lipids.

golgi • Golgi Apparatus: job is to change, sort, and package proteins and other materials from the ER for storage in the cell or secretion outside of the cell.

lysosomes • Lysosomes are small organelles filled with enzymes. • They help break down lipids, carbohydrates, and proteins into smaller molecules that can be used by the rest of the cell. • Lysosomes can also play an important role in destroying harmful substances or bacteria that enter the cell.

vacuoles • Vacuoles are saclike structures that are used to store materials.

Mitochondria • Almost all eukaryotic cells contain mitochondria. • Mitochondria convert the chemical energy stored in food into compounds that are more convenient for the cell to use.

chloroplasts • Plants and some other organisms contain chloroplasts. • Chloroplasts capture the energy in sunlight and convert it into chemical energy.

Cytoskeleton • The structure that helps support the cell is called the cytoskeleton. • The cytoskeleton is a network of protein filaments that helps the cell maintain its shape. • It is also involved in movement.

Endomembrane System for the AP Student • The ER is continuous with the nuclear envelope • It encloses a network of interconnected tubules called cisternae.

Smooth ER • Smooth ER serves diverse functions in different cells • Its enzymes are involved in phospholipid and steroid synthesis, carbohydrate metabolism, and detoxification of drugs & poisons. • Alcohol and drugs increase a liver cell’s production of smooth ER, leading to an increased tolerance • Smooth ER also functions in storage and release of calcium ions during muscle contraction

Rough ER • Rough ER manufactures membranes for the cell. • Enzymes built into the membrane assemble phospholipids, and membrane proteins formed by bound ribosomes are inserted into the ER membrane • Transport vesicles transfer ER membrane to other parts of the endomembrane system.

Golgi: shipping and receiving center • The Golgi apparatus consists of a stack of flattened sacs. • Vesicles that bud from the er join to the cisface of a Golgi stack, adding to their contents and membrane • Golgi products are processed and tagged from the cisto the trans face • Glycoproteins often have their attached carbohydrates modified • Golgi products are sorted into vesicles, which pinch off from the trans face • These vesicles may have surface molecules that help direct them to the plasma membrane or to other organelles.

Lysosomes: Digestive Compartments • Lysosomes are membrane-enclosed sacs of hydrolytic enzymes used by animal cells to digest macromolecules • Lysosomes provide an acidic pH for these enzymes • In some protists, lysosomes fuse with food vacuoles to digest material ingested by phagocytosis. • Macrophages, a type of wbc, use lysosomes to destroy ingested bacteria • Lysosomes also recycle a cell’s own macromolecules by engulfing damaged organelles or small bits of cytosol, a process known as autophagy

The Endomembrane System: A Review • As membranes move from the ER to the Golgi and then to other organelles, their compositions, functions, and contents are modified.

Mitochondria & Chloroplasts change energy from one form to another • Cellular respiration, the metabolic processing of fuels to produce ATP, occurs within the mitochondria • Photosynthesis occurs in the chloroplasts of plants and algae, which produce organic compounds from CO2 & H2O by absorbing solar energy • Each contain a small amount of DNA that direct the synthesis of some of their proteins

Mitochondria: chemical Energy Conversion • The folds of the inner membrane, called cristae, create a large surface area and enclose the mitochondiral matrix. • Many respiratory enzymes, mitochondrial DNA, and ribosomes are housed in the matrix

Chloroplasts: Capture of Light Energy • Plastids are plant organelles that include amyloplasts, which store starch • Chromoplasts, which contain green pigments, and • Chloroplasts which contain the green pigment chlorophyll and function in photosynthesis

Chloroplasts are bounded by 2 membranes separated by a thin intermembrane space • Inside the inner membrane is a fluid called the stroma surrounding a membranous system of flattened sacs called thylakoids • Photosynthetic enzymes are embedded in the thylakoids, which may be stacked together to form structures called grana. • Chloroplast DNA, ribosomes, and many enzymes are contained in the stroma.

Peroxisomes: oxidation • Peroxisomes are oxidative organelles filled with enzymes that function in a variety of metabolic pathways • They break down fatty acids for energy or detoxifying alcohol and other toxins • An enzyme that converts hydrogen peroxide (H2O2), a toxic by-product of these pathways, to water is also packaged within

Cytoskeleton: Support, Motility, and Regulation • The cytoskeleton is a network of protein fibers that give mechanical support, function in cell motility, & transmit mechanical signals from the cell’s surface to its interior. • The cytoskeleton interacts with special proteins called motor proteins to produce cellular movements

Components of the cytoskeleton • Three main types of fibers: microtubules, microfilaments, and intermediate filaments • ALL eukaryotic cells have microtubules, which are hollow rods constructed of columns of globular proteins called tubulins • Microtubules change length through the addition or subtraction of tubulin dimers • In addition to providing a supportive framework, they also serve as tracks along which organelles move with the aid of motor molecules

In many animal cells, microtubules grow out from a region near the nucleus called a centrosome • A pair of centrioles, each composed of nine sets of triplet microtubules arranged in a ring, is associated with the centrosome and replicates before cell division • Yeast and plant cells lack centrosomes

Cilia and flagella are locomotor extensions of some eukaryotic cells • Cilia are numerous and short • Flagella occur one or two to a cell and are longer • Many protists use cilia or flagella to move through aqueous media • Cilia or flagella attached to stationary cells of a tissue move fluid past the cell

6.7 Extracellular components and connections between cells help coordinate cellular activities • Plant cell walls are composed of microfibrils of cellulose embedded in a matrix of polysaccharides and protein • The primary cell wall secreted by a young plant cell is relatively thin and flexible • Adjacent cells are glued together by the middle lamella, a thin layer of polysaccharides (pectin). • When they stop growing, some cells secrete a thicker and stronger secondary cell wall between the plasma membrane & primary cell wall.

the secondary wall, often deposited in several laminated layers, has a strong and durable matrix that affords the cell protection and support • Wood, for ex., consists mainly of secondary walls. • Plant cell walls are commonly perforated by channels between adjacent cells called plasmodesmata

Extracellular Matrix of Animal Cells (ECM) • Animal cells lack cell walls but do have an elaborate ECM. • The ecm is made of glycoproteins. The most abundant glycoprotein is collagen, which forms strong fibers outside the cells • In fact, collagen accounts for about 40% of the total protein in the human body. • Cell surface receptor proteins called integrins that are built into the plasma membrane • Integrins are in a position to transmit signals between the ecm and cytoskeleton and thus to integrate changes occurring outside and inside the cell

Intercellular junctions • Plasmodesmata are channels in plant cell walls through which the plasma membranes of bordering cells connect • Water, small solutes, and even some proteins and RNA molecules can move through these channels

There are 3 main types of intercellular junctions btw animal cells: 1. Tight junctions: proteins hold adjacent cell membranes tightly together, creating an impermeable seal across a layer of epithelial cells.

2. Desmosomes (aka anchoring junctions): are reinforced by intermediate filaments and rivet cells into strong sheets

3. Gap junctions aka communicating junctions: are cytoplasmic connections that allow for the exchange of ions and small molecules between cells through protein-lined pores

Chapter 6 “The Cell”