Structure and Function of the Cell Biology Chapter 7

Cells A cell is the smallest unit that can carry on all the processes of life. A complete living thing that consists of only one cell is called unicellular. A living thing that consists of more than one cell is called multicellular. Cells were discovered in the 1600’s soon after microscopes were first developed.

Organisms In a unicellular organism, one cell carries out all the functions of life. Some life forms, called colonial organisms, appear to be neither unicellular nor multicellular. A colonial organism is a group of more or less similar cells that live together in closely formed groups. In a multicellular organism, cells are so specialized that they are dependent on the function of other cells in the organism.

Multicellular Organization In most multicellular organisms, cells are organized into tissues. A tissue is a group of similar cells that carry out a common function. Several types of tissue that interact to perform a specific task form an organ. An organ system is made up of a group of organs that work together to perform a set of related tasks.

Cell Theory The observations of early scientists were compiled into cell theory. Cell theory states, –All living things are composed of one or more cells. –Cells are organisms' basic units of structure and function. –Cells can only come from existing cells.

Cell Diversity All though all living things are composed of cells, not all cells are alike. Cell within the same living thing may differ in size, structure and shape. Most animal cells range from 10 to 50 micrometers long and are only visible through a microscope.

Cell Size and Shape Cell size is limited by the ratio of volume to outer surface area. The larger the cell, the larger the surface area that must be maintained. Cell shape is typically related to the cell’s function. Most cell shapes are roughly cuboidal or spherical. Some cells have long extensions to reach out in various directions. –Ex. Nerve cells

Internal Organization Cells also differ in internal organization. Two major types of cells exists –Prokaryotes: cells that do not have a nucleus and membrane bound organelles –Eukaryotes: cells that do have a nucleus and membrane bound organelles The nucleus is the large structure in the center of the cell that directs the activities of the cell. It also contains DNA Organelles are cell components that perform specific functions for the cell.

Parts of a Cell Each living cell carries out the tasks of taking in food, transforming food into energy, getting rid of wastes and reproducing. Most eukaryotic cells have the following components: –Cell membrane –Cytoplasm –Nucleus

Cell Membrane The cell membrane is the structure that separates the cell from its external environment. It gives shape and flexibility to the cell. The cell membrane is selectively permeable which means it allows some molecules to pass through, but keeps others out. It is composed of two layers of lipids and is embedded with proteins. The lipids and proteins that form the membrane are fluid and can move about, therefore it is sometimes called a fluid mosaic model.

Cytoplasm and Organelles The cytoplasm is a jelly- like material found inside the cell membrane. It contains water, salts and organic molecules. The constant motion of particles and nutrients moving around in the cytoplasm is called cytoplasmic streaming. The cytoplasm also surrounds the organelles.

Ribosomes Each organelle performs a specific function for the cell. The most numerous of the cell’s organelles is the ribosome. Each ribosome is a spherical structure composed of nucleic acid molecules and proteins. Ribosomes are the sites of protein synthesis. Proteins that are used inside the cell float freely in the cytoplasm, those used outside the cell are attached to the endoplasmic reticulum.

Endoplasmic Reticulum The endoplasmic reticulum or ER is a membrane system of folded sacs and tunnels. ER that is covered with ribosomes is called the rough ER. ER that does not have ribosomes is called the smooth ER. ER functions primarily as an intracellular highway to move molecules from one part of the cell to another. The smooth ER also serves as a storage area for proteins that will be exported from the cell.

Golgi Apparatus The Golgi apparatus is the processing, packaging and secreting organelle of the cell that consists of a stack of membranes or sacs filled with fluid and dissolved substances. The Golgi apparatus operates like an assembly line running from the ribosome to the Golgi apparatus which packages the protein and attaches it to the cell membrane to be discharged.

Mitochondria The mitochondria are the respiration centers of the cell and appears as large organelles scattered throughout the cytoplasm that can grow, divide and fuse with one another. They are responsible for releasing energy from nutrients taken into the cell. Mitochondria have two membranes: a smooth outer membrane and a many folded inner membrane where the chemical reactions take place. Mitochondrion is where ATP is formed during respiration. Mitochondria also have their own DNA.

Lysosomes Lysosomes are small, spherical organelles surrounded by a single membrane that contain digestive enzymes. They digest food particles, disease-causing bacteria and worn out broken parts of the cell. Lysosomes also play a role in early development.

Microtubules and Microfilaments Long, slender protein tubes called microtubules and fine protein threads called microfilaments help shape and support cells. They form the cytoskeleton, or framework of the cell. They are assembled as needed then disassembled. Specialized microtubules called spindle fibers aid in movement of chromosomes during cell division.

Cilia and Flagella Cilia and flagella are two kinds of structures that extend out from the surface of the cell and function in movement. Cilia are short, hair like extensions that occur in large numbers on the surfaces of some cells. Cilia move the cell like oars on a boat. Flagella are whip-like, longer than cilia and occur singly or in pairs. By whipping back and forth, flagella propel the cell forward.

Cell Nucleus The nucleus directs the activities of the cell and is the site where nucleic acids are synthesized. It is surrounded by a double membrane called the nuclear envelop which uses pores to control what enters and leaves the nucleus. Inside the nuclear envelope, is a dense, protein rich substance called the nucleoplasm which contains chromatin. Chromatin is a combination of DNA and proteins that condenses into chromosomes during cell division. Most nuclei also contain a nucleolus which is a spherical body where ribosomes are manufactured.

Plant Cells Plant cells possess some structures in addition to those found in animal cells. In plants, the cell wall surrounds the cell membrane and helps support and protect the cell. Plant cells also have vacuoles and plastids.

Cell Wall A cell wall is the rigid covering of a plant cell. It is made primarily of cellulose embedded in pectin and lignin. Pores in the cell wall allow ions and molecules to pass to and from the cell membrane. When a plant cell divides, a middle lamella is first formed then the primary wall, finally after growth ceases the secondary wall is added. The wood of a tree is actually its secondary cell wall.

Vacuoles Vacuoles are organelles found in plant cells that store enzymes and waste products. In a mature cell, vacuoles may take up to 90% of the cell’s volume. Some of the waste products stored by vacuoles are toxic materials that need to be kept away from the rest of the cell.

Plastids Plants depend on light as a source of energy. Solar energy is then converted into chemical energy by plant plastids. Chloroplasts are plastids that contain the pigment chlorophyll that absorbs sunlight. Chromoplast are plastids that synthesize and store pigments such as carotene and various red and yellow pigments. Leucoplasts store food such as starches, proteins and lipids.