7.2 Cell Structure

• Why are cells so small? • How are animal cells different from plant cells?

• What is the cell membrane and what is it constructed of?

Cell Organization● The cytoplasm is the fluid portion of the cell outside the nucleus.● Many cellular structures act as if they are specialized organs. These

structures are known as organelles, literally “little organs.”

The Nucleus • The nucleus is surrounded by a

nuclear envelope composed of two membranes.

• The nuclear envelope is dotted with thousands of nuclear pores, which allow material to move into and out of the nucleus.

• Most nuclei also contain a small, dense region known as the nucleolus.

Vacuoles and Vesicles ● Many cells contain large, saclike, membrane-enclosed structures

called vacuoles that store materials such as water, salts, proteins, and carbohydrates.

Vacuoles and Vesicles ● Nearly all eukaryotic cells contain smaller membrane-enclosed

structures called vesicles. ● Vesicles are used to store and move materials between cell

organelles, as well as to and from the cell surface.

Lysosomes ● Lysosomes are small organelles filled with enzymes that function as

the cell’s cleanup crew. Lysosomes perform the vital function of removing “junk” that might otherwise accumulate and clutter up the cell.

The Cytoskeleton • Eukaryotic cells are given their shape and internal organization by a

network of protein filaments known as the cytoskeleton. • Microfilaments and microtubules are two of the principal protein

filaments that make up the cytoskeleton.

Microfilaments ❏ Microfilaments include actin and intermediate filaments. These are threadlike

structures made up of a protein called actin.❏ Microtubules are hollow structures made up of proteins known as tubulins. ❏ Microtubules build projections from the cell surface. Examples include, cilia and

flagella.

Microtubules

● Centrioles are located near the nucleus and help to organize cell division.

● Centrioles are not found in plant cells.

● They are made up of microtubules.

Ribosomes ● Ribosomes produce proteins by following coded instructions that

come from DNA.

Rough ER● The rough endoplasmic reticulum (rough ER) is the portion of the

ER involved in the synthesis of proteins.

Endoplasmic Reticulum ● The smooth ER contains enzymes that perform specialized tasks,

including the synthesis of membrane lipids and the detoxification of drugs.

Golgi Apparatus ● The Golgi apparatus modifies,

sorts, and packages proteins and other materials from the ER for storage in the cell or release outside the cell.

Chloroplasts

● Chloroplasts are found only in plants. They capture the energy from sunlight and convert it into food in a process called photosynthesis.

Mitochondria Mitochondria are the power plants of the cell. They convert the chemical energy stored in food into compounds that are more convenient for the cell to use.

Mitochondria ● All of our mitochondria come from the cytoplasm of the

ovum, or egg cell. You get your mitochondria from Mom!● Chloroplasts and mitochondria contain their own genetic

information in the form of small DNA molecules. ● The endosymbiotic theory suggests that chloroplasts and

mitochondria may have descended from independent microorganisms.

Three Parents? https://www.youtube.com/watch?v=GT4j7TmOu9Y

Cellular Boundaries● Cells are surrounded by a barrier known as the cell

membrane. ● Many cells, including most prokaryotes, also

produce a strong supporting layer around the membrane known as a cell wall.

Cell Walls ● Prokaryotes, plants, algae, fungi, and many prokaryotes

have cell walls. ● Animal cells do not have cell walls. ● Cell walls lie outside the cell membrane and most are

porous enough to allow water, oxygen, carbon dioxide, and certain other substances to pass through easily.

Cell Membranes The composition of nearly all cell membranes is a

double-layered sheet called a lipid bilayer, which gives cell membranes a flexible structure and forms a strong barrier between the cell and its surroundings.

The Properties of Lipids ● The fatty acid portions of such a lipid are

hydrophobic, or “water-hating.” ● While the opposite end of the molecule is

hydrophilic, or “water-loving.”

The Fluid Mosaic Model

● Most biological membranes are selectively permeable, meaning that some substances can pass across them and others cannot.

● Selectively permeable membranes are also called semipermeable membranes.