Cell Structure and Function

Chapter 3

Basic Characteristics of Cells

• Smallest living subdivision of the human body

• Diverse in structure and function

• Small

Basic Characteristics of Cells

• Divided into organelles – Structures and

compartments in the cell– Isolation of chemical

reactions – Each organelle performs a

specific function

Each Cell Has Three Primary Regions

• Plasma Membrane• Cytoplasm• Nucleus

The Plasma Membrane

• Plasma membrane = outer casing of the cell– separates intracellular environment from the

extracellular environment.

• Regulates movement of materials into and out of cell– a selective mechanical barrier

The Plasma Membrane

• Composed of Lipids, Proteins and Carbohydrates– Arranged in a lipid

bilayer– “Fluid mosaic model”

Cell Membrane Components:Phospholipids

• one end is hydrophilic• other end is

hydrophobic• tend to aggregate in a

lipid bilayer – hydrophilic ends face

outward, hydrophobic ends in the middle

Cell Membrane Components:Proteins

• Suspended in lipid bilayer

• May span layer or attached only to one surface

• Different functions

Types of Membrane Proteins

• Structural Proteins– Maintain membrane shape

and integrity, movement

• Channel Proteins– Pore-like proteins– Enable small ions to pass

• Carrier proteins– Shuttle specific substances

across membrane

Types of Membrane Proteins

• Enzymes– Catalyze specific

reactions

• Receptor proteins– Bind cellular signals

• Signaling proteins / Recognition proteins– Enable cells to identify

and interact with each other

Cell Membrane Components:Carbohydrates

• Branched polymers attached to extracellular surface

• Forms “sugar coating” (glycocalyx) of cells

• Functions– Lubricated surface– Protect underlying

proteins– Cell recognition

The Cytoplasm

Cytosol

• semi-liquid, gel-like• contains various

dissolved materials, enzymes, etc.

• cytoskeleton = network of protein fibers throughout cytosol– structure– movement

Cytoplasmic Organelles: Ribosomes

• Large protein-RNA complexes

• Found free floating in cytosol or attached to endoplasmic reticulum

• synthesize proteins based on RNA codes

Cytoplasmic Organelles: Endoplasmic Reticulum (ER)

• Rough Endoplasmic Reticulum (RER)– Coated with ribosomes– Synthesis of proteins for:

• Cell membranes• Interior (lumen) of

membrane-bound organelles

• Export out of the cell

Cytoplasmic Organelles: Endoplasmic Reticulum (ER)

• Smooth Endoplasmic Reticulum (SER)– No ribosomes– Functions

• Lipid synthesis• Ca2+ storage• Vesicular release

Cytoplasmic Organelles: Golgi Apparatus

• Stacks of flattened sacs • Functions:

– processes ER products into final form

– sort and direct finished products to final destinations (intracellular or extracellular)

– vesicles - small membrane coated chambers used to transport materials

Cytoplasmic Organelles: Lysosome

• Sac-like organelles which contain enzymes

• Break down large molecules – cell’s “digestive system”

• Destroy bacteria, old organelles, etc.

Cytoplasmic Organelles: Mitochondria

• Surrounded by two membranes

• Possess own DNA• Power generators of the

cell• Convert food energy (e.g.,

glucose) to usable form  – ATP

• high energy compound.• major source of metabolic

energy for the cell

The Nucleus

The Nucleus

• Cell's "control center" • Largest organelle• Surrounded by

nuclear envelope• Contains DNA

– genetic information in form of nucleic acid polymers

– instructs synthesis of proteins

DNA Structure

• Deoxyribonucleic acid• Polymer of nucleotides

– base + sugar + phosphate

• Four different bases – adenine (A), guanine (G),

cytosine (C), thymine (T)– alternate in order– “Four-letter alphabet”

• Double helix– Two strands linked

together by hydrogen bonds

– Complementary sequences

Genes and Chromosomes

• Genes – sequences of DNA

containing information needed to make proteins with the proper amino acid sequence

• DNA is wrapped around proteins to form chromosomes

Chromosomes

• 46 chromosomes per human cell

• 23 different types – pairs of homologous

chromosomes– one of each pair

inherited from each parent

DNA function

• Information in DNA used to make proteins

1. Transcription: • sequence of DNA copied into

an RNA (ribonucleic acid) molecule

2. Translation• RNA sequence read by

ribosomes to generate a protein with a specific amino acid sequence

Cell Reproduction

• Reproduction of cells (cell division) requires :– copying of the DNA (chromosomes) – distribution of chromosomes to daughter cells

• Two types of cell division– mitosis (most cells)– meiosis (gametes)

Mitosis

• Cells duplicate all their chromosomes

• One copy of each of the 46 chromosomes is distributed to each of the daughter cells.

Meiosis• Occurs only in gametes• Cells duplicate all

chromosomes• Cells undergo two separate

divisions– each daughter cell has 23 total

chromosomes

• Gametes combine to form a new cell (zygote) with 46 chromosomes