The Structure of

Cell:

Part II

PlastidsPlastids

• Chromoplasts–Chloroplasts

responsible for photosynthesis.

• Leukoplasts responsible for storage of starch.

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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.

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Thylakoid

A stack of thylakoids is a granum. The matrix found around the thylakoid is called the stroma. It contains enzymes for carbohydrate synthesis.

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RibosomesRibosomes 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.

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Evolution of Organelles

The endomembrane system includes all the organelles that are derived from one another, or are continuous with one another.

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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

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The Cytoskeleton

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Gives the cell shapeMovement of organellesCytoplasmic streaming in plant cells

Microtubules

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Cilia and Flagella

Flagella and cilia are composed of ( 9 double + 2 single ) microtubules .

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Movement of Flagella and Cilia

Both flagella and cilia are covered with the plasma membrane.

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Difference Between Flagella and Cillia

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Centrioles and Microtubules

Centrioles are microtubule organizing centers for animal cells.They are constructed of nine triplet microtubules. Basal bodies anchor cilia and flagella.

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Microfilaments

Microfilaments are made from two intertwined strands of actin subunits. They are thinner than microtubules.

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Muscle Contractions and Cytoplasmic Streaming

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Intermediate Filaments

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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.

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Plasmodesmata

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

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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.

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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

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