The Structure of Cell: Part II

Plastids Plastids Chromoplasts Chloroplasts responsible for photosynthesis. Leukoplasts responsible for storage of starch. Plastids are organelles that are found in certain plant cells. They arise from unspecialized plastids called proplastids. There are 2 different types of plastids- chromoplasts that contain pigments and leukoplasts that store starch. Chromoplasts contain pigments such as xanthophylls and carotenes. Chloroplasts are a type of chromoplast that contain chlorophyll and other pigments. They convert light energy into chemical energy and are found in leaves and stems.

Structure of Chloroplast Chloroplasts have three membranes. The outer two are smooth and the inner one makes stacks of thylakoids or grana. The chlorophyll and other pigments are found in the thylakoid membrane where light energy is converted into chemical energy.

Thylakoid A stack of thylakoids is a granum. The matrix found around the thylakoid is called the stroma. It contains enzymes for carbohydrate synthesis. An electron transport chain is embedded in the thylakoid membrane. In addition, there are collections of chlorophyll and other pigments which collectively are called photosystems. The photosystems are able to donate light-energized electrons to the electron transport chain.

Ribosomes Ribosomes are workbenches for protein synthesis. They are made from rRNA and proteins to form two subunits. They do not contain any membranes. Cells can have thousands. All cells contain ribosomes because all cells must synthesize proteins.

Evolution of Organelles The endomembrane system includes all the organelles that are derived from one another, or are continuous with one another. Graphic http://faculty.college-prep.org/~bernie/sciproject/project/Kingdoms/Protista4/Protists/Relationship%20to%20Evolution.htm They have evolved from the folding in of the plasma membrane over time. The membrane system includes the nucleus, the E.R., the Golgi apparatus and the lysosomes.

The Endomembrane System

The mitochondria, chloroplasts and the flagella evolved from prokaryotic cells that engulfed other prokaryotic cells. Instead of being destroyed, the prokaryotic cell took up residence and began replicating and dividing inside the host cell. Over time some of the genes of the engulfed cell were transferred to the host cell making them dependent on the host and becoming organelles. This is the endosymbiosis hypothesis. Evidence for endosymbiosis Chloroplasts and mitochondria Have circular DNA like prokaryotic cells Have ribosomes similar to prokaryotic cells Do protein synthesis similar to prokaryotic cells

The Cytoskeleton The cytoskeleton found throughout the cytosol in eukaryotic cells. Functions of the cytoskeleton Move the cell. Move organelles within the cell Provide support and structure Cytoplasmic streaming (not prokaryotic cells) Types of structures- microtubules and microfilaments intermediate filaments Gives the cell shape Movement of organelles Cytoplasmic streaming in plant cells

Microtubules Microtubules- Hollow tubes made of protein spheres. Used for cell shape, cilia, flagella & centrioles. Are used to move organelles (like a monorail) and chromosomes. Microtubules are made in two regions called the centrosomes which are found on the outside of the nucleus. Centrosomes in animals contain centrioles but are not found in higher plant cells.

Cilia and Flagella Flagella and cillia differ in length and the number found on a cell. Usually cilia are shorter coat the cell, whereas flagella are longer and usually bundle 1-8 on the end of a cell. Flagella typically propel a cell through a medium, whereas cilia may either move a cell or move materials past a stationary cell, as in the cilia found lining the respiratory pathway. Dynein is the motor protein that bends the flagellum and cilium by interacting with the cross-link proteins. The dynein spokes alternately grab the microtubules and release the other microtubules causing the cilium or flagellum to bend. Flagella and cilia are composed of ( 9 double + 2 single ) microtubules .

Movement of Flagella and Cilia Both flagella and cilia are covered with the plasma membrane.

Difference Between Flagella and Cillia

Centrioles and Microtubules Centrioles are microtubule organizing centers for animal cells. They are constructed of nine triplet microtubules. Basal bodies anchor cilia and flagella. Centrioles are made of nine triplets of microtubules. This structure is also identical to the interior of a basal body which anchors flagella and cilia in the cell. Cilia and flagella are identical in structure but differ in length, movement, and number found on a cell.

Microfilaments Microfilaments are made from two intertwined strands of actin subunits. They are thinner than microtubules. Microfilaments are two strands of protein made of actin subunits and are used for supporting the cell shape and changing the shape of the cell. They are also involved in muscle contraction, movement of pseudopods, cytoplasmic streaming and microvilli of the small intestines. Microfilaments are used to bear tension, resisting pulling forces within the cell.

Muscle Contractions and Cytoplasmic Streaming

Intermediate Filaments

Cell Walls Most cells have materials external to the plasma membrane. Cell walls are found in prokaryotes, plants, fungi and some protists. Cell walls of plants are made of cellulose; in fungi they are made of chitin; in prokaryotes they are murein (or muramic acid) and in protists they vary. Cell walls give shape and prevent turgid cells from bursting. Plant cells may have a primary cell wall (outer most walls) and a secondary cell wall (inside the primary). The middle lamella is a substance holds the walls together.

Plasmodesmata Plasmodesmata are channels between plant cells that allow direct flow from one cell’s cytoplasm to the cytoplasm in adjacent cells.

Glycocalyx or Extracellular Matrix Animal cells lack cell walls but have an extra cellular matrix (ECM) or glycocalyx. The ECM is made of glycoproteins such as collagen, proteoglycan, and fibronectins. These glycoproteins are connected to receptor proteins in the cell membrane called integrins. Used for support, adhesion, movement and identity.

Animal Cell Junctions There are several types of intercellular junctions in animal cells Tight junctions- membranes of neighboring cells are pressed together Desmosomes-(anchoring junctions) fasten cells together into strong sheets Gap junctions-(communicating junctions) provide cytoplasmic channels between adjacent cells like plasmodesmata in plant cells