Topic 2

Cells

Introducing Cells

• Consist of cytoplasm• Enclosed in a plasma membrane• Usually controlled by a single nucleas

Unicellular Organisms

• Single cell that carries out all the functions of life:– Metabolism (chemical reactions inside the

cell)– Response (reacting to stimuli)– Homeostasis (controlling internal conditions)– Growth (increasing in size)– Reproduction (producing offspring)– Nutrition (obtaining food)

Multicellular Organisms

• Consist of many cells that are specialized for one specific function

• Differentiation: cells become specialized to carry out one process efficiently

• Emergent properties: the whole organism is more than the sum of its parts due to the complex interactions between cells

The Cell Theory

• Living organisms are composed of cells• Cells are the smallest units of life• Cells can only be made from pre-existing

cells

Differentiation

• Differentiation: cells develop in different ways to carry out different functions

• The cells need different genes to develop in different ways – each cell has all of the genes available

• Once a pathway of development has begun in a cell, it is usually fixed

Stem Cells

• Stem cells: cells that have the capacity to self-renew by cell division and to differentiate

• Human embryos consist entirely of stem cells early on

• Interest in stem cells are due to their potential for tissue repair and for treating a variety of degenerative conditions – i.e. Parkinson’s disease

Therapeutic Use of Stem Cells

• Stems Cells can be used therapeuticly – Blood from an umbilical cord contains stem cells– Red blood cells are taken from the cord blood

and the remaining fluid is then tested– Cord blood is matched to the patient’s tissue

type– Cord blood is introduced into the patient’s blood

system

• Used for leukemia patients

Limitations to Cell Size

• Cells have a maximum size– If a cell was too large its surface area to

volume ratio would be too small

• The rate at which materials enter/leave cell depends on the surface area

• The rate at which materials are used/produced depends on the volume

Units for Size Measurements

• 1 meter (m) = 1,000 millimeters (mm)• 1 millimeter (mm) = 1,000 micrometer (µm)• 1 micrometers (µm) = 1,000 nanometers (nm)

Calculating Magnification

• To calculate magnification:1. Choose an obvious length and measure it on

the drawing

2. Measure the same length on the actual specimen

3. Convert the units

4. Divide the drawing length by actual specimen’s length

• Magnification = size of image ÷ size of object

Scale Bars

• Scale bar: Line added to a micrograph or drawing to help show the actual size of the structures

Ultrastructure of Cells

• Two types of cells: prokaryotic and eukaryotic

• Prokaryotic cells have existed longer• Prokaryotic cells reproduce by binary

fission: dividing in two

Functions of Parts of a Prokaryotic Cell

Structure Function

Cell Wall Protective outer layer from external danger and bursting from internal pressures.

Plasma Membrane

Controls entry and exit of substances. Uses pumps for active transport.

Cytoplasm Contains enzymes to catalyze metabolism reactions. Contains DNA in the nucleoid.

Pili Hair-like structures protecting the cell walls that allow bacteria to adhere and share information

Flagella Protein structures that project from the cell wall that rotate and cause locomotion

Ribosomes Synthesize proteins by translating messenger RNA.

Nucleoid Contains naked DNA (the genetic information of the cell)

Comparing Prokaryotic and Eukaryotic Cells

Feature Prokaryotic Cells Eukaryotic Cells

Type of genetic material

Naked loop of DNA Chromosomes consisting of strands of DNA associated with a protein.

Location of genetic material

In the cytoplasm within the nucleoid region

In the nucleus inside the nuclear envelope

Mitochondria Not present Always present

Ribosomes 70s (smaller size) 80s (larger size)

Internal Membranes

Few or none present Many present: endoplasmic reticulum, Golgi apparatus, lysosomes

Comparing Plant and Animal Eukaryotic Cells

Feature Animal Plant

Cell wall No cell wall, only plasma membrane

Both cell wall and plasma membrane

Chloroplasts Not present Present in cells that photosynthesize

Polysaccharides Glycogen is used as a storage compound

Starch is used as a storage compound

Vacuole Not usually present; few and temporary

Large vacuole often present

Shape Able to change shape; usually rounded

Fixed shape; usually rectangular

Fluid Mosaic Model of a Biological Membrane

Phospholipids

• Phosphate heads are hydrophilic– They are attracted to water

• Lipid tails are hydrophobic– They are not attracted to water, but are

attracted to each other

• Phospholipids form double layers with hydrophilic heads facing outward and lipid tails facing inward

• Very stable, yet fluid

Fluidity of Membranes

• Phospholipids in membranes are in a fluid state

• Allows membranes to change shape• Allows vesicles to be pinched off from or

fuse with the membrane

Functions of Membrane Proteins

• Hormone bonding sites• Protein pumps for Active Transport• Channels for Passive Transport• Cell-to-cell communication and cell

adhesion• Enzymes

Diffusion

• Diffusion: the passive movement of particles from a region of higher concentration to a region of lower concentration, as a result of the random motion of particles

• Can occur across semi-permeable membranes if there is a concentration graident

Simple and Facilitated Diffusion

• Partially permeable membranes: allow some substances to diffuse through, but not others

• Simple diffusion: when substances move between the phospholipid molecules in the membrane

• Facilitated diffusion: substances that are unable to pass between phospholipids are moved through using specific channel proteins

• Both processes do not need to use energy

Osmosis

• Solvent: liquid in which particles dissolve• Solutes: dissolved particles in solvent• Osmosis: passive movement of water

molecules from a region of lower solute concentration to a region of higher solute concentration, across a partially permeable membrane

Pump Proteins and Active Transport

1. Particle enters pump from the side of lower concentration

2. Particle binds to a specific site on the protein pump

3. Energy from ATP changes the shape of the pump

4. The change in shape allows the particle to be released on the side of higher concentration

5. Pump returns to its original shape

Endocytosis and Exocytosis

• Endocytosis: enters the cell1. Part of membrane is pulled inwards

2. Fluid becomes enclosed when a vesicle is pinched off

3. Vesicles can then move inside the cell

• Exocytosis: exits the cell1. Vesicles fuse with the plasma membrane

2. Contents of vesicle are expelled

3. Membrane flattens out again

Extracellular Components

• Extracellular components: components produced by cells that are placed outside the plasma membrane using exocytosis

• Examples:– Plant cell wall– Glycoproteins

• Carbohydrate attached to a protein in plasma membrane

Interphase

• Interphase is the longest phase of cell cycle of Eukaryotes

• Interphase has 3 stages:1. G1: period of growth, DNA transcription,

protein synthesis

2. S phase: period during which all DNA in the nucleas is replicated

3. G2: period in which the cell prepares for division

Mitosis

• Mitosis: process that divides nucleus into two genetically identical nuclei

• Mitosis is used during growth, embryonic development, repairing tissues, produce asexually

• Mitosis has four phases:1. Prophase2. Metaphase3. Anaphase4. Telophase

• Ends with cytokinesis

Prophase

• Spindle microtubules grow• Move to extend from each pole to the

equator• Chromsomes supercoil to become shorter

and fatter• Each chromosome are made up of

identical chromatids held together by a centromere

Metaphase

• The nuclear membrane brakes down containing the chromosomes

• Chromosomes move to the equator• Spindle microtubules attach to each

centromere (on opposite sides of the centromeres)

Anaphase

• The centromeres divide and the two cromatids have become chromosomes

• Spindle microtubules pull the genetically identical chromosomes to opposite poles

Telophase

• Chromsomes reach the poles and nuclear membranes form around them

• Spindle microtubules break down within the membranes

• Chromosomes uncoil and are no longer individually visible

• Two cells are formed with genetically identical nuclei

Cytokinesis

• The process of dividing the cytoplasm to form two cells at the end of mitosis

Tumors

• The genes in a cell change and so the normal control of mitosis fails

• Repeated uncontrolled divisions produce a mass of cells

• These cells are called a tumor– Can happen in any organ or tissue– Can spread to other parts of body– Cancer: diseases caused by tumors