Chapter 3: Cell Structure and Function Prof.Tamboli Alija Z. Dept of Zoology, S.M.Joshi Collge , Hadapsar. F.Y.B.Sc

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.

Eukaryotic Cells Eukaryotic cells have a nucleus that controls the workings of the cell. All cells are surrounded by a plasma membrane made of phospholipids and proteins.

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.

Nucleus and 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 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 endoplasmic reticulum

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.

The Golgi apparatus

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.

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.

Mitochondrion structure

Microtubles are small hollow cylinders made of the globular protein tubulin. Microtubule assembly is controlled by the microtubule organizing center, called the centrosome. Microtubules help maintain the shape of the cell and act as tracks along which organelles can move. Motor molecules kinesin and dynein move along microtubules. One type of kinesin moves vesicles arising from the ER along microtubules. There are different types of kinesin proteins, each specialized to move one kind of vesicle or organelle. A second kind of cytoplasmic motor molecule is called cytoplasmic dynein because it is closely related to the dynein found in flagella.

Microtubule structure

Intermediate filaments are ropelike assemblies of fibrous polypeptides that support the plasma membrane and nuclear envelope.

Structure of intermediate filaments

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.