Topic 2.3: Eukaryotic Cells

IB Biology HL

E. McIntyre

Cytoplasm (cytosol): a cell's inner space

cytosol

Cytosol: large fluid-filled space (mostly water + proteins that control most of the cell metabolisms): glycolysis, transcription

factors, intracellular receptors etc.

Plasma membrane

Organelles

• Nucleus• Protein production machine: the ribosomes• Mitochondria and Chloroplasts• Endoplasmic Reticulum and the Golgi

apparatus (we don’t say ‘Golgi body’ in IB)

• Lysosomes• Adapted and/or specialized for carrying out

one or more vital functions

Reminder! Membrane-bound organelles are only found in eukaryotes.

Nucleus: a cell's centerFunction of the nucleus:• Stores the chromosomes

(genetic material) Note: Two kinds of genetic material:– deoxyribonucleic acid

(DNA)– ribonucleic acid (RNA)

• Keeps the DNA molecules separated from metabolic machinery of cytoplasm.

• Makes it easier to organize DNA and to copy it before parent cells divide into daughter cells

• DNA replication and synthesis of messenger RNA (mRNA). The mRNA is allowed to leave the nucleus. It is read by the ribosomes during protein synthesis.

...Nucleus: a cell's center

Nucleolus

• Dense mass of material in nucleus

• Cluster of DNA and proteins

• Subunits must pass through nuclear pores to reach cytoplasm

• production of ribosomes in nucleolus

• Two outer membranes (lipid bilayers)

• Pores span bilayer and controls what enters/leaves nucleus.

• Much more detail to follow!

Nuclear Membrane (Envelope)

Chromatin

• Cell’s collection of DNA and associated proteins• A chromosome is one DNA molecule and its

associated proteins• Appearance changes as cell divides

FUNCTION:

• With the help of tRNA, ribosomes use information in mRNA to assemble proteins

• small semi-spherical, black dots

• some are free in cytoplasm (cytoplasmic ribosomes), others are attached to the ER (ER ribosomes)

Ribosomes

STRUCTURE

• ER is a continuation of the outer nuclear membrane.

• Two types: Smooth and Rough. (Rough ER has ribosomes on it)

• Network of membrane that connects nuclear envelope to the cell membrane

Endoplasmic Reticulum (ER)

FUNCTION:

• Smooth ER (no ribrosomes): responsible for lipid and membrane protein modifications). In some cells, it regulates carbohydrate metabolism and breaks down toxins and drugs

• E.g., in liver, metabolizes alcohol and barbiturates; in testes and ovaries, synthesizes sex steroids; etc.

• Rough ER (has ribosomes on it) responsible for the synthesis of other proteins). The ribosomes assemble the proteins within the rough ER. Once assembled, the proteins pinch off the ER and are released in a vesicle.

Endoplasmic Reticulum

Endoplasmic Reticulum (ER)

…Endoplasmic Reticulum (ER)

FUNCTION:

• processes, packages, and, secretes cell products (lipids & proteins)

• Put finishing touches on proteins and lipids that arrive from ER

• Package finished material for shipment to final destinations. (Exocytosis)

• Material arrives and leaves in vesicles

Golgi Body (Golgi Apparatus)

Golgi Apparatus

…Golgi Body (Golgi Apparatus)

STRUCTURE

• Double membrane. Inner membrane is folded for greater surface area. Folds are called cristae.

• Interior is called the matrix.

• Contain their own genome (distinct from nuclear genome)

• Self-replicating

FUNCTION

• ‘burns’ food, i.e. glucose to release energy. Energy is used to make ATP. ATP is a high-energy molecule that can be directly used by the cell.

Mitochondria

The “powerhouse” of the cell

…Mitochondria

TEM of Mitochondria

STRUCTURE• Formed by Golgi bodies• Bound by a single membrane

FUNCTIONS• Digests the food particles inside the cell with help of enzymes. These enzymes

work at low pH.• Involved in autolysis (suicide of cell)• Digest foreign bacteria that invade a cell• Recycling of membrane components• Repair damage to plasma membrane

Lysosomes

lysosomes McGraw Hill

Lysosomes

• Made by Golgi

• Surrounded by a single membrane

• Get rid of toxic substances

• Many in the liver/ brain

lysosome

Peroxisomes

FUNCTION:

• maintains the shape of a plant cell

• found in plants, some fungi, some protists

• In plants, cell walls are made of a polysaccharide called cellulose.

• Structural component that wraps around the plasma membrane

Cell Wall

…Cell Wall

FUNCTION:

• Photosynthesis: converts sunlight, carbon dioxide, and water into sugar (makes food for plants)

Chloroplasts

• A large membrane bound sac

FUNCTION:• storage area for proteins and

water• Maintain structure in plant cells by means of turgor pressure

Vacuole

1.3 Eukaryotic Cells2.3.5 Describe three differences between plant and animal cells.

Carbohydrates stored as starch.

Carbohydrates stored as glycogen.

Stores large amounts of liquid (juice). Larger size of cell.

Central Vacuole

XDoes not store large amounts of liquid. Smaller size of cell.

Rigid, cannot easily change shape.

Cell WallXFlexible, can easily change shape.

Can produce its own food.

ChloroplastXCannot produce its own food

Plant CellsStructureAnimal Cells

• very complex and dynamic cell component!

• organizes and maintains the cell's shape (e.g. epithelial cells of the gut)

• anchors the organelles in place

• helps during the uptake of external material (endocytosis)

• involvement in cell movement

• growth

Cytoskeleton: a cell's scaffold

microtubules microfilamentsintermediate filaments

tubulin actin keratin family

All help maintain cell shape

Cilia, flagellaChromosome movementOrganelle movement

Cytoplasmic streamingPseudopodiaCell division

Nuclear laminaAnchorage of nucleus and some other organelles

...Cytoskeleton

...Cytoskeleton

...Cytoskeleton

Molecular motors

...Cytoskeleton

Macrophage attacks bacteriaMicrofilaments (actin) allow formation of pseudopods

...Cytoskeleton

Striated Muscle

...Cytoskeleton

Centrosome: Made up of centriole pairs

...Cytoskeleton

2.3.4 Compare prokaryotic and eukaryotic cells.

• Prokaryotic cells have naked DNA which is found in the cytoplasm in a region named the nucleoid. On the other hand, eukaryotes have chromosomes that are made up of DNA and protein. These chromosomes are found in the nucleus enclosed in a nuclear envelope.

• Prokaryotes do not have any mitochondria whereas eukaryotes do. Prokaryotes make use of their cell surface membrane and mesosomes instead.

• Prokaryotes have small ribosomes (70S) compared to eukaryotes which have large ribosomes (80S).

• In prokaryotes there are either no organelles bounded by a single membrane in comparison to eukaryotes which have many of them including the Golgi apparatus and the endoplasmic reticulum.