Anatomy of Cells

Overview of the Cellular Basis of Life

• A cell is the basic structural and functional unit of living organisms. When you define the properties of a cell, you are in fact defining the properties of life.

• The activity of an organism is dependent on both the individual and collective activity of the cells.

• According to the principle of complimentarity, the biochemical activities of cells are determined and made possible by the specific subcellular structures of cells.

• The continuity of life has a cellular basis.

Properties of a Cell

Cells are chemically composed chiefly of carbon, hydrogen nitrogen, oxygen, and trace amounts of several other elements.

Size

• Diameters range from about 2 micrometers (1/12,000 of an inch) in to 10cm or more in the largest cell. The typical human cell is about 10 micrometers. The largest, the fertilized egg, is nearly 100 micrometers in diameter.

• Lengths range from a few micrometers to a meter or more. Some skeletal muscle cells are 30cm long, and the nerve cells that cause your foot muscles to contract run from the end of your spinal cord to your foot.

Shape

• A cell’s shape reflects its function. Flat, tile-like epithelial cells that line your cheek fit closely together, forming a living barrier that protests underlying tissues from bacterial invasion.

Examples:• Spherical (fat cells)• Disk-shaped (red blood cells)• Branching (nerve cells)• Cube-like (kidney tubule cells)

3 Major Parts

• Nucleus: Control center

• Cytoplasm: packed with and supports the organelles

• Plasma membrane: forms the external cell boundary

The Plasma MembraneThe plasma membrane is a thin, stable structure composed

of a double layer, or bilayer, of phospholipids molecules with protein molecules dispersed in it. It is called the fluid mosaic model because the molecules are able to slowly float around the membrane.

The lipid bilayer forms the basic “fabric” of the membrane and is relatively impermeable to most water soluble molecules.

Membrane proteins imbedded in the lipid bilayer are responsible for most specialized functions of the plasma

membrane.

Receptor Proteins: react to the presence of hormones or other regulatory chemicals and trigger metabolic changes in the cell

Marker Proteins: (glycoproteins) allows cells to recognize each other for immune or developmental purposes

Transport Proteins: either channel or transport needed chemicals through the membrane that can not otherwise pass through the lipid bilayer

There are 2 distinct protein populations:

• Integral proteins: embedded into the membrane

• Peripheral proteins: attach to integral membrane proteins, or penetrate the peripheral regions of the lipid bilayer

• Branching sugar groups are attached to most of the proteins that abut the extracellular space. The term glycocalyx is used to describe these attachments.

Functions of glycocalyx

1. Determining the ABO and other blood groups.

2. Provide binding sites for other toxins.

3. Recognition of the egg by sperm.

4. Determining cellular life span.

5. Serving in the immune response.

6. Helping to guide and direct embryonic development.

Cytoplasm and OrganellesThe cytoplasm is the gel-like internal substance (cytosol) of

cells that contains many tiny suspended structures called organelles (little organs). Organelles are classified as either membranous or nonmembranous.

Membranous organelles• Endoplasmic reticulum (ER): rough Er

synthesizes proteins; smooth ER synthesizes lipids, steroid hormones, and carbohydrates used in forming glycoproteins

Membranous organelles• Golgi apparatus: synthesizes carbohydrates to

combine with proteins; packages products to be sent out of the cell

Membranous organelles

Lysosomes: bags of digestive enzymes break down worn cell parts and ingested particles; a cell’s digestive system

Membranous organelles

Peroxisomes: contain enzymes that detoxify harmful substances

Membranous organelles

Mitochondria: Catabolism; ATP synthesis; a cell’s power plant

Nonmembranous

• Ribosomes: site of protein synthesis; a cell’s protein factory

Nonmembranous

• Cytoskeleton: acts as a framework to support the cell and its organelles; functions in cell movement; forms cell extensions (microvilli, cilia, flagella) Cell fibers

*Microfilaments: smallest cell fibers; cellular muscles; run parallel to the long axis of the cell; found and function mostly in muscle cells

*Intermediate filaments: slightly thicker fibers; form much of the supporting framework in most cells

*Microtubles: thickest fibers; form tubes; “engines of the cell” because they move things around in the cell or move the cell itself

Cytoskeleton

Nonmembranous

• Centrosome

Nonmembranous

• Cell Extensions

Microvilli: minute, fingerlike extensions of the plasma membrane that project from a free, or exposed, cell surface; they increase the surface area of the membrane

Cilia: hair-like extension of the membrane used in moving substance

Flagella: tail-like extensions used for movement

Cell Nucleus

• Nucleolus: produces ribosomes

• Chromatin: uncoiled DNA

• Chromosomes: coiled DNA

• Chromatids: Duplicated DNA

Specializations of the Plasma Membrane

Membrane junctions help to bind cells closely to form tissues.• Tight junctions: protein molecules in adjacent plasma

membranes fuse together tightly like a zipper, obliterating the intercellular space and forming an impermeable junction; ex. Epithelial cells of the digestive tract

Membrane junctions

• Desmosomes: act as mechanical couplings or adhesion junctions that prevent separation of tissue

layers – membranes do not actually touch

Membrane junctions• Gap junctions: provide for direct passage of chemical

substances between adjacent cells – the cells are connected by hollow cylinders composed of transmembrane proteins, called connexons – embryonic cells prior to development of circulatory system and in adults in electrically excitable tissues such as heart and smooth muscle, and also between some nerve cells