Chapter 4 Cell Structure and Function

Early Scientist

• Robert Hooke: first describe the cell– Small compartments in cells from a cork

• Van Leeuwenhoek: developed the first microscope

• Observed several living cells, including sperms

Cell Theory Scientist

• Schleiden- Botanist proposed that plants are made up of cells

• Schwann- zoologist proposed that animals are composed of cells.

• Virchow- concluded that cells come from other cells

Cell Theory

1- All organisms are composed of one or more cells

2- Smallest unit having the properties of life3- The continuity of life arises directly from the

growth and division of single cells

Cell Size• atoms --> DNA --> virus --> bacteria --

>mitochondria--> Eukaryotic cells• Cells must remain small in size due to the ratio of

surface area and volume• As the cell increases in size, its surface area

becomes to small to support its internal structures. Oxygen and other important substances cannot diffuse fast enough. Cells that get too large, may divide.

Structural Organization of Cells

• Plasma Membrane(“GATEKEEPER”) separates the cell from the environment, permits the flow of molecules across the membrane, and contains receptors that can affect the cell’s activities.– Composed of lipid bilayer– Very controlled

Cytoplasm

• It is everything between the plasma membrane and nucleus– This includes organelles, semifluid substance

(cytosol), and filaments (cytoskeleton)

Genetic Material

• 1. provides cellular "blueprint" that controls the functions of the cell2. In the form of DNA (Deoxyribonucleic acid)3. DNA is universal for all cells, and all living things - evidence of common ancestry4. Chromatin is the complex of proteins and DNA, it condenses into chromosomes before cell division

Eukaryotic versus Prokaryotic• Nucleus• Membrane bound

organelles• Examples: fungi, protist,

animal, and plants

• No Nucleus: Chromosomes grouped together known as a nucleoid

• Smaller than eukaryotes• Bacteria

The Nucleus• --nuclear envelope contains pores for some things to

enter and exit-- chromatin is DNA and proteins, when the cell begins to divide, chromatin condenses and forms chromosomes--DNA remains in the nucleus, it sends instructions to the cytoplasm via messenger RNA--RNA directs the synthesis of proteins on ribosome in the cytoplasm--Nucleolus assembles ribosome within the nucleus, ribosome contain the "tools" to construct proteins

Endoplasmic reticulum (ER)

• Transport System (endoplasm means "within cytoplasm", reticulum means "little net"

• --divides cell into compartments--channels molecules through the cell's interior, like a little highway

Smooth ER

• Smooth ER• --mostly contains enzymes that unction in lipid

synthesis (such as hormones like estrogen and testosterone)

Rough ER

• --has ribosomes which give it its "rough" appearance

• --functions in protein synthesis• --ER transports newly assembled proteins to

the Golgi Apparatus

Golgi Appratus • Delivery System• --flattened stacks of membranes• --functions in collection, packaging and distribution of

molecules made in the cell and used elsewhere• -- front end (cis) faces the ER, and the back end (trans)

faces the cell membrane• --unprocessed proteins enter the Golgi apparatus, are

processed and exit near the cell membrane for export• --the folded stacks are called cisternae

Jobs of Golgi Appratus

• 1. separates proteins according to their destinations2. modifies proteins (adds sugar and makes glycoproteins)3. packages materials into vesicles which are exported outside the cell

Lysosome (Lip smacking good)• --vesicles that are used to digest• --contain high levels of degrading enzymes (to "lyse"

means to dissolve)• --recycle old and worn out cell parts• --"suicide sac“• --digest other particles taken in by phagocytosis• --this "food" is stored in food vacuoles, the

lysosomes fuse with the vacuoles and release digestive enzymes

• --found in animal cells

Ribosomes

• --each is composed of two subunits, one large and one small

• --mRNA is "read" by the ribosomes and amino acids are assembled into proteins

• --ribosomes are manufactured by the nuceolus inside the nucleus

Mitochondria • Powerhouse: cell furance• --contains its own DNA, support for Endosymbiosis Theory• --singular is "mitochondrion“• --2 membranes, one smooth outer membrane, and an

inner membrane folded into layers called cristae• --Cristae has two compartments: the matrix and the

intermembrane space• --mitochondria divide before cell division, they are not

synthesized like other cell parts• --function to store energy for cell use. Energy is stored in

the form of ATP - adenosine triphosphate

Mitochondria

Chloroplast

• --only found in plant cells• --has its own DNA, like mitochondrion• --functions to convert light energy to ATP• --consists of grana, closed compartments that

are stacked• --thylakoids are the individual disk shaped

compartments that make up the grana• --stroma is the fluid surrounded the thylakoids

Cytoskeleton

• Cell is not "just a bag in a bubble". Lots of internal fibers = internal "skeleton". Not rigid like bone; capable of being assembled, broken down in minutes. Allows cell movement, cell division, internal motion of compartments.

• Composed of microfilaments and microtubules

Centrioles

• --usually occur in pairs arranged at right angles

• --assemble microtubules which influence the cell shape and movement - part of the cytoskeleton

• --also function in cell division, mitosis• --only found in animal cells

Vacuoles

• --in plants the vacuoles are large and centralized, storage of water makes the cell turgid

• --in animals, they store food, water and other substances (much smaller)

How Do Cells Move?• Motor Proteins:– Through controlled assembly and disassemby of

their subunits, microtubules, and microfilaments grow or diminish in length, thereby the structures attached to them are thereby pushed or dragged through the cytoplasm

– Parallel arrays of microfilaments or microtubules actively slide past one another to bring about contraction, as in muscle

– Microtubules or microfilaments shunt organelles from one location to another as in cytoplasmic streaming

Cilia, Flagella, and False Feet• Flagella are quite long, not usually numerous,

and found on one-celled protistians, and animal sperm cells

• Cilia are shorter and more numerous and can provide locomotion for free-living cells may move surrounding water and particles if the cilated cells is anchored

• Pseudopods “false foot” temporary lobes that project from the cell, used in locomotion and food capture

Cell Surface Specialization• Eukaryotic Cell Walls– Cell walls are carbohydrate frameworks for

mechanical support in bacteria, protistans, fungi, and plants

– In growing plant parts, bundles of cellulose strands form the primary cell wall (pliable enough to allow for enlargement)

– Secondary walls are formed inside the primary walls• 25% is composed of ligin

Matrixes between animal cells• The matrix between animal cells includes cell

secretions and materials drawn from the surroundings between cells.

• Example: Cartilage consists of scattered cells – Collagen embedded in a “ground substance” of

modified polysaccharide

Cell Junctions

• In plants tiny channels called plasmodesmata cross the adjacent primary walls and connected the cytoplasm

Continue…• Animal cells display three types of junction:– Tight Junctions: occur between cells of epithelial

tissues in which cytoskeletal strands of one cells fuse with strands of neighboring cells causing an effective seals

– Adhering junctions are like spot welds at the plasma membranes of two adjacent cells that need to be held together during stretching as in the skin and heart.

– Gap Junctions: are small, open channels that directly link the cytoplasm of adjacent cells

Cell Communication

• Signals and receptors allow cells to change their activities – Example: cell division– Hormones- stimulators of cell activity

Prokaryotic Cells

• “Before nucleus”• Indicates existence of bacteria before

evolution of cells with a nucleus; bacterial DNA is clustered in a distinct region of the cytoplasm (nucleoid)

• Bacteria are some of the smallest and simplest cell – Bacterial flagella project from the membrane and

permit rapid movement– Rigid cell wall supports cell and surrounds the

plasma membrane which regulates transport into and out of the cell

– Ribosomes, protein assembly sites, are dispersed throughout the cytoplasm