Cell Theory and Structure

BiologyCorsicana High School

Development of the Cell Theory

• Robert Hooke (1665)---first to observe cells. Looked at thin slices of cork. Saw “little boxes.” Named them cells.

• Anton von Leeuenhoek (1675)---first to observe living cells: “wee beasties” in drop of pond water

Definitions

• cell---the smallest unit that can carry on all the processes of life

• unicellular---a complete living thing that consists of only one cell

• multicellular---a living thing consisting of more than one cell

Development of the Cell Theory (continued)

• Matthias Schleiden (1838)---concluded that all plants are composed of cells

• Theodor Schwann (1839)---concluded that all animals are composed of cells

• Rudolph Virchow (1855)---determined that cells come only from other cells

The Cell Theory• All living things are composed

of one or more cells.• Cells are organisms’ basic units

of structure and function.• Cells come only from existing

cells.

Two Main Types of Cells

• eukaryotes---contain a definite nucleus and membrane-bound organelles– examples: animal cells, plant cells– “eu” = true; “karyo” = nucleus

• prokaryotes---cells that do not have a definite nucleus or organelles– example: bacteria– “pro” = before

Parts of the Cell

• organelles---cell components that perform specific functions in the cell (“little organs”)

Three Main Components of Cells

• cell membrane---forms the outer boundary of the cell

• cytoplasm---inside the membrane. Contains water, salts, organelles

• nucleus---contains DNA and directs the activities of the cell

Cytoplasmic Organelles

• ribosome---tiny granules, site of protein synthesis

• endoplasmic reticulum---system of membranes throughout the cytoplasm. Acts as a transportation system– smooth ER---does not have ribosomes

attached to it– rough ER---has ribosomes attached

Cytoplasmic Organelles

• Golgi apparatus---stack of fluid-filled membranes or sacs near the nucleus. The cell’s processing, packaging and secreting organelle

• mitochondrion---respiration centers of the cell– release energy from molecules– produce ATP– have their own DNA

Cytoplasmic Organelles

• lysosomes---organelles that contain digestive enzymes– only in animal cells– “suicide sacs”

• microtubules---long, slender protein tubes which shape and support the cells– spindle fibers---specialized microtubules

that aid in movement of chromosomes during cell division

Cytoplasmic Organelles

• microfilaments---fine protein threads which contract to move cellular materials– cytoskeleton---microfilaments and

microtubules collectively forming the framework of the cell

Organelles for Movement• cilia---large numbers of short, hairlike

extensions out from the surface of the cell; for movement

• flagellum---long, whiplike structure for movement. Occur singly or in pairs

• “9 + 2” construction of cilia and flagella---composed of 9 pairs of microtubules around the circumference and 2 in the center

Plant Cell Organelles• cell wall---surrounds the cell

membrane. Supports and protects the plant cell– structure is long chains of cellulose

embedded in pectin and lignin

• vacuole---large, fluid-filled cavity which stores enzymes, wastes, etc. In a mature plant cell, the vacuole may take up 90% of the volume of the cell.

Plastids

• make and/or store food or pigments• chloroplast---contains chlorophyll;

the site of photosynthesis• chromoplast---synthesizes and

stores other pigments• leucoplast---stores food such as

starches, proteins, lipids

Nucleus

• contains DNA and directs the activities of the cell

• nuclear membrane (nuclear envelope)---double membrane that surrounds the nucleus. Has pores for substances to enter and leave

• nucleolus---spherical body in nucleus; stains dark; synthesizes ribosomes

Nucleus (continued)

• chromatin---fine strands in nucleus made of DNA and protein– chromosome----when the cell gets ready

to divide, the chromatin coils and condenses and can be seen with a microscope as these rod-shaped bodies in the nucleus

• nucleoplasm---dense, protein-rich substance inside the nuclear membrane

Cell Membrane

• selectively permeable---allows some molecules to pass through, but not others

Composition of Cell Membrane

• 2 layers of lipid molecules– carboxyl group (hydrophilic) is turned

to outside of membrane– hydrocarbon end (hydrophobic) is

turned to inside of membrane

• protein molecules embedded in the lipid layer

Outside the cell

Inside the cell

Fluid Mosaic Model

• the lipid molecules in the membrane can move around (flow---->fluid)

• proteins also move• the pattern (“mosaic”) of proteins

and lipids is constantly changing

Cell Types

• size ranges from 2 meters long (nerve cell in giraffe’s leg) down to 0.2 micrometers (bacteria)– usually 10--50 micrometers– size is limited by ratio of surface area to

volume

• shape is mostly cube- or sphere-shaped. The shape of a cell depends on its functions

Differences Between Plant Cells and Animal Cells

• lysosomes found only in animal cells• vacuoles are large, central

structures in plant cells. If present in animal cells, they are small and scattered

• cell wall found only in plant cells• plastids (chloroplasts, chromoplasts,

leucoplasts) found only in plant cells

Multicellular Organization• In true multicellular organisms:

– cell specialization---each cell does only one particular job for the organisms

– division of labor---each cell depends on other cells to perform one or more functions to keep the entire organism alive

• colonial organism---a group of more or less similar cells that live together in closely-connected groups, but in which there is no cell specialization

Levels of Structure

• cell---the basic unit of life• tissue---a group of similar cells that

carry out a common function• organ---several types of tissues that

interact to perform a specific function• organ system---a group of organs

that work together to perform a set of related tasks