Trends in Biomedical Science Review of cells

Cell Theory Cell Theory 1. All organisms are made of cells. 2. Cells are the smallest living things. 3. Cells are only made from parent cells. All cells today come from the first living cells.

Cell size is limited. -As cell size increases, it takes longer for material to diffuse from the cell membrane to the inside of the cell. Surface area-to-volume ratio: as a cell increases in size, the volume increases 10x faster than the surface area

Microscopes are needed to see cells. Light microscopes can resolve structures that are 200nm apart. Electron microscopes can resolve structures that are 0.2nm apart.

All cells have certain structures in common. 1. genetic material – in a nucleoid or nucleus 2. cytoplasm – a semifluid matrix 3. plasma membrane – a phospholipid bilayer

Eukaryotic cells -have a membrane-bound nucleus -separate many cellular functions within organelles and the endomembrane system -have a cytoskeleton for support and to maintain cellular structure and transport within the cell

Cells are not just bags of chemicals. They are crowded with many organelles which interact with each other. Each color represents a structure with a unique function and composition 9

Nucleus -stores the genetic material of the cell in the form of multiple, linear chromosomes -surrounded by a nuclear envelope composed of 2 phospholipid bilayers -in chromosomes – DNA is organized with proteins to form chromatin

The nucleus has a 3 dimensional (3D) structure. Special parts (compartments) of the nucleus are where chromosomes are transcribed and replicated. Chromosomes seem have special areas (chromosome territories). Different parts of chromosomes are next to other chromosomes in different tissues.

Gene-dense human chromosomes are found in the center of the nucleus, while gene-poor chromosomes are found near the boundary of the nucleus. Nuclear organization is highly specific and has functional relevance to the cell. The organization gives proper gene expression, replication and the stability of the genome.

Nuclei are intricately structured, and nuclear metabolism has an elaborate spatial organization. The architecture of the nucleus includes two overlapping and nucleic-acid containing structures - chromatin and a nuclear matrix. The nuclear matrix is a ribonucleoprotein (RNP) network. The ultrastructure of the nuclear matrix is well characterized and consists of a nuclear lamina and an internal nuclear network of subassemblies linked together by highly structured fibers. These complex fibers are built on an underlying scaffolding of branched 10-nm filaments that connect to the nuclear lamina.

The nuclear matrix is a nuclear skeletal structure that is believed to be involved in many nuclear functions including DNA replication, transcription, repair, and pre mRNA processing/transport. The nuclear matrix is now thought to be dynamic. This thinking is partly based on the tracking of the intranuclear movement of proteins tagged with fluorochromes.

Representation of nuclear compartments.

Nuclear pore complexes allow the transport of molecules across the nuclear envelope. This transport includes RNA and ribosomal proteins moving from nucleus to the cytoplasm and proteins (such as DNA polymerase and lamins), carbohydrates, signaling molecules and lipids moving into the nucleus. The nuclear pore complex (NPC) can actively conduct 1000 translocations per complex per second.

Structure and function of the nuclear lamina. http://en. wikipedia Creative Commons Attribution 2.0 Generic license

The nuclear lamina is a dense (~30 to 100 nm thick) fibrillar network inside the nucleus of a eukaryotic cell, on the inner surface of the inner nuclear membrane (INM). It is composed of intermediate filaments and membrane associated proteins. Besides providing mechanical support, the nuclear lamina regulates important cellular events such as DNA replication and cell division. It organizes chromatin and binds nuclear pore complexes (NPCs) and nuclear envelope proteins (purple) and transcription factors (pink).

Ribosomes -the site of protein synthesis in the cell -composed of ribosomal RNA and proteins -found within the cytosol of the cytoplasm and attached to internal membranes

Endomembrane system -a series of membranes throughout the cytoplasm -divides cell into compartments where different cellular functions occur 1. endoplasmic reticulum 2. Golgi apparatus 3. lysosomes

An insulin-producing pancreas cell contains thousands of membrane-enclosed compartments.

Phospholipid membranes form a barrier that most molecules cannot cross Phospholipid membranes form a barrier that most molecules cannot cross. But living cells need to be able to interact with their environment. Membrane proteins that contact the spaces on both sides of the membrane are gates between cellular compartments. Each type of compartment has a specific population of membrane proteins that largely define its function. On average, proteins make up about half the mass of membranes.

Nuclear pore complexes on the nuclear membranes of frog oocytes as seen from the cytoplasm (left) and from inside the nucleus (right)

Rough endoplasmic reticulum (RER) -membranes that create a network of channels throughout the cytoplasm -attachment of ribosomes to the membrane gives a rough appearance -synthesis of proteins to be secreted, sent to lysosomes or plasma membrane

Smooth endoplasmic reticulum (SER) -relatively few ribosomes attached -functions: -synthesis of membrane lipids -calcium storage -detoxification of foreign substances

Golgi apparatus -flattened stacks of interconnected membranes -packaging and distribution of materials to different parts of the cell -synthesis of cell wall components

Lysosomes -membrane bound vesicles containing digestive enzymes to break down macromolecules -destroy cells or foreign matter that the cell has engulfed by phagocytosis

Microbodies -membrane bound vesicles -contain enzymes -not part of the endomembrane system -peroxisomes contain oxidative enzymes and catalase

Vacuoles -membrane-bound structures with various functions depending on the cell type There are different types of vacuoles: -central vacuole in plant cells -contractile vacuole of some protists -vacuoles for storage

Mitochondria -organelles present in all types of eukaryotic cells -contain oxidative metabolism enzymes for transferring the energy within macromolecules to ATP -found in all types of eukaryotic cells

Mitochondria -surrounded by 2 membranes -smooth outer membrane -folded inner membrane with layers called cristae -matrix is within the inner membrane -intermembrane space is located between the two membranes -contain their own DNA

Mitochondria

Cytoskeleton -network of protein fibers found in all eukaryotic cells (probably all cells) -supports the shape of the cell -keeps organelles in fixed locations -helps move materials within the cell

Cytoskeleton fibers include -actin filaments – responsible for cellular contractions, crawling, “pinching” -microtubules – provide organization to the cell and move materials within the cell -intermediate filaments – provide structural stability

Actin filaments – light purple, microtubules – yellow, nuclei – greenish blue

Cell movement takes different forms. -Crawling is done using actin filaments and the protein myosin. (show Active membranes in fish skin http://learn.genetics.utah.edu/content/begin/cells/membranes/)

Cytoskeleton is also connected to proteins in the membranes of the cell. 50

Filaments attached to Membrane Proteins.

Other References Inside the cell - http://learn.genetics.utah.edu/content/begin/cells/insideacell/ MEMBRANES ORGANIZE CELLULAR COMPLEXITY http://learn.genetics.utah.edu/content/begin/cells/membranes/ Inside the Cell NIH Publication No. 05-1051 Revised September 2005 http://www.nigms.nih.gov