Eukaryotic Cells & Their Structures

Eukaryotic Cells

• Eukaryotic cells are 10-100x larger than prokaryotic cells.
• Basic cell structure is very complex:
• Membranes found inside of the cells to hold organelles together: membrane bound organelles.
• Eukaryotic cells are often built into multicellular organisms. Example: Humans, any other animal/plant, fungi and protists.

Composition of an Animal Cell

Components of a Plant Cell

The Nucleus

Structure of the nucleus

• The nucleus is surrounded by a double membrane structure called the nuclear envelope
• Protein nuclear pores are present in the nuclear envelope to allow for the passage for proteins and RNA in and out of the nucleus
• The nucleolus is a region inside the nucleus that is associated with ribosomal RNA (rRNA) production and ribosome assembly
• The nuclear envelope is continuous with the endoplasmic reticulum (cisternae on the diagram below)

Contents of the nucleus

• The cell's genetic information is made up of deoxyribonucleic acid (DNA)
• The cell's DNA is organized into chromosomes
• Each chromosome contains thousands of genes
• Each gene is a segment of DNA that codes for a protein

• The DNA is first "read" and converted to messenger RNA (mRNA)
• The mRNA is then transported out of the nucleus through the nuclear pores
• Once in the cytoplasm, the mRNA is "read" by ribosomes to make the protein

The Cytoplasm

The cytoplasm is the fluid that fills the compartment inside the cell (outside the nucleus)

• The cytoplasm is the site of many biochemical reactions, such as the breakdown of nutrients and construction of cellular building blocks
• These processes generate waste products, which are stored in the cytoplasm until they can be disposed of

The cytoplasm also contains the cytoskeleton

• The cytoskeleton is a network of three kinds of fibres, called actin filaments, intermediate filaments, and microtubules
• The cytoskeleton gives the cell it's shape, organizes the location of organelles, and allows for transport of cellular components throughout the cytoplasm
• The three fibres can be assembled and disassembled by adding or removing the protein building blocks that they're made of

Ribosomes

• Made of one large subunit and one small subunit.
• Composed of proteins and ribosomal RNA (rRNA).
• Not surrounded by membrane.
• Exist in large quantities: thousands to millions of ribosomes per cell!
• Two subcategories of ribosomes:
• Free - they float around the cytosol of the cell;
• Bound - stuck to the side of the ER.

The Endoplasmic Reticulum

The Rough ER

• Appears "rough" or bumpy under the microscope because it has many ribosomes attached to its surface
• The proteins produced by these ribosomes are transported into the ER
• Within the rough and smooth ER, the proteins are modified, processed, and transported to the golgi apparatus

The Smooth ER

• No ribosomes attached to its surface
• Site of phospholipid and fat production

The Golgi Apparatus

• The Golgi Apparatus is a stack of flattened membrane-bound discs
• At the end of the Golgi Apparatus, blobs of membrane "pinch off" to form protein-filled sacs called vesicles
• Vesicles bring their contents to the plasma membrane, where they fuse with it and release their contents outside the cell
• This process is called exocytosis

Mitochondria and Chloroplasts

• The mitochondria
• The chloroplast

Mitochondria

• Found in all eukaryotic cells.
• Surrounded by a double membrane with the intermembrane space between them.
• The inner membrane has a large surface area due to the presence of membrane folds called cristae.
• The inner membrane is the location of the electron transport chain which synthesizes ATP for the cell.
• The inner compartment is called the matrix.

Chloroplast

• Found in photosynthetic eukaryotic cells (e.g. plants).
• Surrounded by a double membrane.
• The inner compartment is called the stroma.
• In the stroma, there are stacks of membrane-bound disks called thylakoids;
• A stack of thylakoids is called a granum (grana is plural);
• Inside the thylakoid is the thylakoid lumen;
• The thylakoid membrane contains the photosynthetic machinery required for harvesting energy from the sun.
• CO2 fixation and sugar production occurs in the stroma.

Lysosomes

• The digestive enzymes are produced by the ribosomes on the rough ER and the lysosomes are then produced by the Golgi Apparatus
• Lysosomes are then pumped full of hydrogen ions to make the interior more acidic, which activates the digestive enzymes

Endocytosis

• Material from outside the cell is engulfed through endocytosis, and is contained within a separate membrane-bound compartment called an endosome
• The lysosome then fuses with the endosome, allowing the digestive enzymes to break down the material
• The materials from the breakdown of these structures are recycled and used as building blocks for the construction of new cellular components and organelles

Cells That Move

Flagella

• Example: the human sperm cell uses a flagellum to move

Cilia

• Example: the cells of the trachea are covered with cilia that help move debris that gets stuck in mucus out of the lungs

Illustrations of Cellular Components: Halloween at the Factory

Plant versus Animal Cells

1. Plant cells have chloroplasts, in addition to mitochondria.
2. They contain a green pigment called chlorophyll that captures light.
3. Plant cells have a cell wall:
4. Provides rigidity, shape and protection;
5. Composed of cellulose: a polysaccharide made up of glucose monomers.
6. Only animal cells have lysosomes.
7. This is where digestive processes occur;
8. The plant cell equivalent is the vacuole.

Vacuoles

1. Central vacuole
2. Food vacuole
3. Contractile vacuole

• Central vacuole: large, occupies most of the volume of mature plant and many fungal cells.
• Maintains proper pressure to support growing plants;
• Performs hydrolytic functions, stores wastes, toxins etc.
• Food vacuoles: contain phagocytosed food.
• Contractile vacuoles: present only in protists/algae, expels water out of cell.

Cell Fractionation

1. Cells are broken open
2. They are placed in a tube and spun in a centrifuge
3. The high rotation of the centrifuge causes heavier structures to move towards the bottom of the tube, while lighter structures remain suspended at the top of the tube
4. After the centrifugation is complete, the cellular components will be separated based on their density
5. The chemical reactions within each layer can then be studied separately