Tour of the Eukaryotic Cell

All organisms are made of cellsThe cell, as a microcosm, illustrates the following principles:

1. Life at the cellular level arises from interactions among cellular components.

2. Ordered cellular processes (e.g., protein synthesis, respiration, photosynthesis, cell-cell recognition, cellular movement, membrane production and secretion) are based upon ordered structures.

3. Cells are excitable, responding to environmental stimuli. In addition, cells are open systems that exchange materials and energy with their environment.

4. Evolutionary adaptations are the basis for the correlation between structure and function.

Cell Size

Size ranges of cells: Cell Type Diameter Arechaea 0.1 - 1.0 μm Most bacteria 1.0 - 10.0 μm Most eukaryotic cells 10.0 - 100.0 μm

Cell Size

Range of cell size is limited by metabolic requirements.

• The lower limits are probably determined by the smallest size with enough:– DNA to program metabolism.– ribosomes, enzymes and cellular components to

sustain life and reproduce.• The upper limits of size are imposed by the

surface area to volume ratio. – As a cell increases in size, its volume grows

proportionately more than its surface area.

SA/Volume

• The surface area of the plasma membrane must be large enough for the cell volume, in order to provide an adequate exchange surface for oxygen, nutrients and wastes.

Internal membranes compartmentalize the functions of a eukaryotic cell

• The average eukaryotic cell has a thousand times the volume of the average prokaryotic cell, but only a hundred times the surface area.

• Eukaryotic cells compensate for the small surface area to volume ratio by having internal membranes which:– Partition the cell into compartments.– Have unique lipid and protein compositions depending upon

their specific functions.– May participate in metabolic reactions with enzymes

incorporated– directly into the membrane.– Sequester reactions, so they may occur without interference

from incompatible metabolic processes elsewhere in the cell.

The Nucleus and Ribosomes

• The nucleus contains a eukaryotic cell's genetic library– Enclosed by a nuclear envelope.

Nuclear envelope

• A double membrane– Two lipid bilayers separated ~ 20 to 40 nm. Each lipid

bilayer has unique set of proteins.– Attached to proteins on the nuclear side (inner side) is

a network of protein filaments, the nuclear lamina, which stabilizes nuclear shape.

– Is perforated by pores• The envelope's inner and outer membranes are fused at the

lip of each pore.• Pore complex regulates molecular traffic into and out of the

nucleus.

Nuclear Envelope

www.ncbi.nlm.nih.gov/bookshelf/.

DNA

• The nucleus contains most of the cell's DNA which is organized with proteins into a complex called chromatin.– Chromatin = Complex of DNA and proteins, which

makes up chromosomes

• Chromosomes = Long threadlike association of genes

Nucleolus

• The most visible structure within the nondividing nucleus

• Roughly spherical region which consists;– of nucleolar organizers (= genes for ribosomal

RNA synthesis) (thus the nucleolus contains DNA) – ribosomes in various stages of production– Packages of ribosomal subunits• Ribosomal subunits pass through nuclear pores to the

cytoplasm,where their assembly is completed.

Ribosomes

• Is the site for protein synthesis in the cytoplasm.

• Are complexes of RNA (ribo nucleic acid) and protein

• Constructed in the nucleolus

RNA and Protein

www.pbs.org/wgbh/nova/photo51/pict-06.html

Free and Bound Ribosomes

• Bound and free ribosomes are structurally identical and interchangeable.

• Free ribosomes are suspended in the cytosol– Most proteins made by free ribosomes will

function in the cytosol.• Bound ribosomes are attached to the outside

of the endoplasmic reticulum.– Generally make proteins that are destined for

membrane inclusion

Task 1• Draw a model of a nucleus. Include the following

details:• Lipid bilayers – inner and outer layer and inter-membrane space

• Nuclear pores• Nuclear lamina• Chromatin• Nucleolus• Nucleoplasm• Smooth and rough ER

Task 2Arrange the following from largest to smallest:

Place them on scale from .1 ηm to 200 μm