Cell Structure and Function

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Lecture Outline

3.1 Cellular Organization

o 3 main parts of a cell• Plasma membrane• Nucleus• Cytoplasm

o Organelles are scattered throughout the cytoplasm and have various functions

o The cytoskeleton maintains cell shape and allows the cell and its content to move

Cellular Organization

o Plasma Membrane• Separates the inside of the cell (cytoplasm)

from the outside• Phospholipid bilayer• Attached peripheral and integral proteins

Receptors Channels Carriers

• Cholesterol molecules stabilize the membrane

• Glycoproteins and glycolipids attached to outer surface of some protein and lipid molecules mark cells as belonging to a particular individual

Cellular Organization

o The Nucleus• Stores genetic information• Chromatin

Contains DNA, protein, and some RNA Coils into rodlike structures called

chromosomes before the cell divides

• Nucleoli Contains rRNA Site where ribosomes are formed

• Nuclear envelope separates nucleus from cytoplasm

Cellular Organization

o Ribosomes• Composed of subunits containing

proteins and rRNA• Can be found free within the

cytoplasm sometimes in groups called polysomes

• Also found attached to the endoplasmic reticulum

• Produce proteins that carry out various functions within the cell

Cellular Organization

o Endomembrane System• Nuclear envelope• Endoplasmic reticulum (ER)

Continuous with the outer membrane of the nuclear envelope

Rough ER Has attached ribosomes Processes proteins produced by attached

ribosomes Smooth ER synthesizes phospholipids

Cellular Organization

o Endomembrane System• Golgi apparatus

Processes, packages, and secretes various substances

Receives protein and/or lipid-filled vesicles from ER

Contains enzymes that modify proteins and lipids

Produces lysosomes

Cellular Organization

o Endomembrane System• Lysosomes

Contain hydrolytic digestive enzymes Autodigestion responsible for cell

rejuvenation and development

• Vesicles – tiny membranous sacs

Cellular Organization

o Peroxisomes and Vacuoles• Peroxisomes

Enzyme-containing vesicles, similar to lysosomes

Detoxify drugs, alcohol, and other toxins Large numbers found in liver and kidney Break down fatty acids from fats

• Vacuoles isolate substances captured inside the cell

Cellular Organization

o Mitochondria• Bound by a double membrane• Site of ATP production• Undergo cellular respiration (use up

oxygen and release carbon dioxide)o The Cytoskeleton

• Helps maintain the cell’s shape and anchors or assists the movement of organelles

• Includes microtubules, intermediate filaments, and actin filaments

Cellular Organization

o Centrioles• Composed of microtubules• A pair of centrioles are found near the

nucleus of every cell• Involved in cell division• Form basal body (anchor point) for

each cilium or flagellum Cilia and flagella are projections of cells Allow for movement of cell or movement

of material along the cell surface

3.2 Crossing the Plasma Membrane

o Simple Diffusion• Movement of atoms or molecules

from an area of higher concentration to an area of lower concentration

• Movement of atoms or molecules occur until they are equally distributed

• Only certain types of molecules can enter and exit a cell by simple diffusion

• No cellular energy is required

Crossing the Plasma Membrane

o Osmosis• Diffusion of water across a plasma

membrane• Occurs when there is an unequal

distribution of water on either side of a selectively permeable membrane

• Osmotic pressure – force exerted on a selectively permeable membrane

Crossing the Plasma Membrane

• Tonicity – concentration of solute versus the concentration of water

Isotonic – equal concentration of solutes (dissolved substances) and solvent (water) inside and outside cell; cell shape is maintained

Hypotonic – higher concentration of water (lower concentration of solutes) outside cell; water moves into cell causing it to swell and eventually lyse

Hypertonic – lower concentration of water (higher concentration of solutes) outside cell; water moves out of cell causing it to shrink or crenate

Crossing the Plasma Membrane

o Filtration is the movement of liquid from high pressure to low pressure

o Transport by Carriers• Facilitated diffusion

Solutes are transported by means of a protein carrier

Movement from area of higher concentration to area of lower concentration (no energy required)

Crossing the Plasma Membrane

o Transport by Carriers• Active transport

Solutes move up their concentration gradient

Requires a protein carrier (often called pumps)

Requires the use of cellular energy

Crossing the Plasma Membrane

o Endocytosis and Exocytosis• In endocytosis the plasma membrane

envelopes a substance and forms an intracellular vesicle

Phagocytosis (“cell eating”) – cell ingests solid particles

Pinocytosis (“cell drinking”) – cell consumes solutions

• In exocytosis a vesicle fuses with the plasma membrane as secretion occurs

3.3 The Cell Cycle

o Set of stages that take place between the time a cell divides and the time the daughter cell divides

o Apoptosis (cell death) occurs at the restriction checkpoint if the cell did not complete mitosis and is abnormal

o Some specialized cells no longer go through the cell cycle

• Muscle cells• Nerve cells

The Cell Cycle

o Cell Cycle Stages• Interphase

Cell is not dividing, but is preparing to divide

The cell carries on regular activities Three phases

G1 phase – cell doubles number of organelles and accumulates materials used for DNA synthesis

S phase – “synthesis” phase; DNA replication occurs

G2 phase – cell synthesizes proteins that will assist cell division

The Cell Cycle

• Major events during interphase Replication of DNA

Before replication, the two strands of DNA are hydrogen bonded together

Parental DNA strands unwind (hydrogen bonds are broken)

New complimentary nucleotides pair with nucleotides in the parental DNA strands and DNA polymerase joins the new nucleotides

When replication is complete, two identical double helix molecules have been formed

Each strand of this double helix is equivalent to a chromatid

The Cell Cycle

Protein synthesis DNA also serves as a template for RNA

formation and protein construction Two steps involved in protein synthesis are:

Transcription – formation of mRNA Translation – involves mRNA, tRNA, and

rRNA; specifies the order of amino acids in a polypeptide

The Cell Cycle

• Mitotic stage Mitosis – cell division stage divided into 4

phases Prophase

The centrioles near nucleus begin moving towards opposite ends of nucleus

Spindle fibers appear Nuclear envelope begins to fragment Nucleolus begins to disappear

Metaphase Spindle is fully formed Chromosomes are aligned at the equator

The Cell Cycle

Anaphase Sister chromatids separate (now called

chromosomes) Chromosomes move toward opposite poles

of the spindle Telophase

Chromosomes become chromatin Spindle disappears and nucleoli appear Nuclear envelope reassembles and two

daughter cell nuclei can be observed Cytokinesis – division of the cytoplasm

and organelles

The Cell Cycle

Importance of Mitosis Each cell in our body is genetically identical Important to the growth and repair of

multicellular organisms Meiosis: Reduction-Division

Produces gametes (sex cells) Stages of mitoses are repeated twice Gametes (sperm or ova) that results have half

the normal number of chromosomes