The Cellular Level of Organization

An Introduction to Cells

The cell theory states:

• Cells are the building blocks of all plants and animals

• Cells are produced by the division of preexisting cells

• Cells are the smallest units that perform all vital physiological functions

• Each cell maintains homeostasis at the cellular level

• Homeostasis at higher levels reflects combined, coordinated action of many cells

Cell biology

• Cytology, the study of the structure and function of cells

• The human body contains both somatic and sex cells

A typical cell

• Is surrounded by extracellular fluid, which is the interstitial fluid of the tissue

• Has an outer boundary called the cell membrane or plasma membrane

The Cell Membrane

Cell membrane functions include:

• Physical isolation

• Regulation of exchange with the environment

• Structural support

• The cell membrane is a phospholipid bilayer with proteins, lipids and carbohydrates.

Membrane proteins include:

• Integral proteins

• Peripheral proteins

• Anchoring proteins

• Recognition proteins

• Receptor proteins

• Carrier proteins

• Channels

Membrane carbohydrates form the glycocalyx

• Proteoglycans

• Glycolipids

• Glycoproteins

The Cytoplasm

The cytoplasm contains:

• The fluid (cytosol)

• The organelles the cytosol surrounds

Organelles

• Nonmembranous organelles are not enclosed by a membrane and always in touch with the cytosol

• Cytoskeleton, microvilli, centrioles, cilia, ribosomes, proteasomes

• Membranous organelles are surrounded by lipid membranes

• Endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, mitochondria

Cytoskeleton provides strength and flexibility

• Microfilaments

• Intermediate filaments

• Microtubules

• Thick filaments

Microvilli

· increase surface area

Centrioles

• Direct the movement of chromosomes during cell division

• Organize the cytoskeleton

• Cytoplasm surrounding the centrioles is the centrosome

Cilia

• Is anchored by a basal body

• Beats rhythmically to move fluids across cell surface

Ribosomes

• Are responsible for manufacturing proteins

• Are composed of a large and a small ribosomal subunit

• Contain ribosomal RNA (rRNA)

• Can be free or fixed ribosomes

Proteasomes

• Remove and break down damaged or abnormal proteins

• Require targeted proteins to be tagged with ubiquitin

Endoplasmic reticulum

• Intracellular membranes involved in synthesis, storage, transportation and detoxification

• Forms cisternae

• Rough ER (RER) contains ribosomes

• Forms transport vesicles

• Smooth ER (SER)

• Involved in lipid synthesis

Golgi Apparatus

• Forms secretory vesicles

• Discharged by exocytosis

• Forms new membrane components

• Packages lysosomes

Lysosomes and Peroxisomes

• Lysosomes are

• Filled with digestive enzymes

• Responsible for autolysis of injured cells

• Peroxisomes

• Carry enzymes that neutralize toxins

Membrane flow

• Continuous movement and recycling of membranes

• ER

• Vesicles

• Golgi apparatus

• Cell membrane

Mitochondria

• Responsible for ATP production through aerobic respiration

• Matrix = fluid contents of mitochondria

• Cristae = folds in inner membrane

The Nucleus

The nucleus is the center of cellular operations

• Surrounded by a nuclear envelope

• Perinuclear space

• Communicates with cytoplasm through nuclear pores

Contents of the nucleus

• A supportive nuclear matrix

• One or more nucleoli

• Chromosomes

• DNA bound to histones

• Chromatin

The genetic code

• The cells information storage system

• Triplet code

• A gene contains all the triplets needed to code for a specific polypeptide

Gene activation and protein synthesis

• Gene activation initiates with RNA polymerase binding to the gene

• Transcription is the formation of mRNA from DNA

• mRNA carries instructions from the nucleus to the cytoplasm

Translation is the formation of a protein

• A functional polypeptide is constructed using mRNA codons

• Sequence of codons determines the sequence of amino acids

• Complementary base pairing of anticodons (tRNA) provides the amino acids in sequence

How Things Get Into and Out of Cells

Permeability

• The ease with which substances can cross the cell membrane

• Nothing passes through an impermeable barrier

• Anything can pass through a freely permeable barrier

• Cell membranes are selectively permeable

Diffusion

• Movement of a substance from an area of high concentration to low

• Continues until concentration gradient is eliminated

Osmosis

• Diffusion of water across a semipermeable membrane in response to solute differences

• Osmotic pressure = force of water movement into a solution

• Hydrostatic pressure opposes osmotic pressure

• Water molecules undergo bulk flow

Tonicity

• The effects of osmotic solutions on cells

• Isotonic = no net gain or loss of water

• Hypotonic = net gain of water into cell

• Hemolysis

• Hypertonic = net water flow out of cell

• Crenation

transport

• Carrier mediated transport

• Binding and transporting specific ions by integral proteins

• Cotransport

• Counter-transport

• Facilitated diffusion

• Compounds to be transported bind to a receptor site on a carrier protein

Active transport

• Active transport

• Consumes ATP

• Independent of concentration gradients

• Types of active transport include

• Ion pumps

• Secondary active transport

Vesicular transport: material moves into or out of cells in membranous vesicles

• Endocytosis is movement into the cell

• Receptor mediated endocytosis (coated vesicles)

• Pinocytosis

• Phagocytosis (pseudopodia)

• Exocytosis is ejection of materials from the cell

The transmembrane potential

• Difference in electrical potential between inside and outside a cell

• Undisturbed cell has a resting potential

The Cell Life Cycle

cell division

• Cell division is the reproduction of cells

• Apoptosis is the genetically controlled death of cells

• Mitosis is the nuclear division of somatic cells

• Meiosis produces sex cells

Interphase

• Most somatic cells spend the majority of their lives in this phase

• Interphase includes

• G1

• S

• G2

Mitosis, or nuclear division, has four phases

• Prophase

• Metaphase

• Anaphase

• Telophase

Mitotic rate and cancer

• Generally, the longer the life expectancy of the cell, the slower the mitotic rate

• Stem cells undergo frequent mitoses

• Growth factors can stimulate cell division

• Abnormal cell division produces tumors or neoplasms

• Benign

• Malignant (invasive, and cancerous)

• Spread via metastasis

• Oncogenes

Differentiation

• Process of specialization

• Results from inactivation of particular genes

• Produces populations of cells with limited capabilities

• Differentiated cells form tissues