3.- CELLS

Cell theory, Eukaryotic and Prokaryotic cells, Membranes, Cell division

CELLS

1.- Cell theory

The invention of the microscope in the 17th century quicly led to the discovery of cells.

Robert Hooke was the first to use the term cell in 1665, after examining cork and other plant tissues under the microscope.

Later on the CELL THEORY WAS DEVELOPED.

CELL THEORY:

1.- Cells are the smallest unit of life and nothing smaller can survive independently.

2.- All living things consist of cells, although the smallest organism may consist of one cell only.

3.- All cells come from pre-existing cells, by division and therefore new cells cannot be constructed from non-living chemical substances.

2.- Plant and animal cells

Prokaryotic No nucleus Eukaryotic True nucleus - Animal cells: no chloroplast no cell wall - Plant cells: Chloroplasts and cell wall

There are many similarities in the structure of plant and animal cells.

A nucleus is usually present, with cytoplasm around it.The cytoplasm is enclosed by a plasma membrane.

Animal cell / Plant cell

Main differences between plant and animal cells:

1) Plant cells have cellulose cell wall outside their plasma membrane, which animal cells never have.

2) Plant cells often have large permanent vacuole, which contains a solution of mineral ions such as potassium and other solutes. Animal cells only have small vesicles.

3) Many plant cells in stems and leaves have chloroplasts in their cytoplasm. Thye contain chlorophyll and starch. Animal cells dont have them. Instead of starch they have glycogen.

3.- Unicellular organisms

Some organisms consist of one single cell. Example: Amoebas.

These cells must carry out all the functions of life on their own.

Functions of life:1) Nutrition: obtaining food, to provide energy and the

materials needed for growth2) Metabolism: chemical reactions inside the cell,

including cell respiration to release energy3) Growth: an irreversible increase in size4) Sensitivity: perceiving and responding to changes in

their environment5) Homeostais: keeping conditions inside the organism

within tolerable limits6) Reproduction: producing offspring either sexually or

asexually

4.- Size of cellsCELL TYPE SIZE

Mycoplasma pneumoniae ( a very small bacterium)

200 nm

Minivirus (a giant virus found in amoebae)

400nm

Megakaryocyte (a cell that makes blood platelets)

160 micrometer

Thiomargarita namibiensis ( largest known bacteria)

0.7mm

Diameter of the nerve fiber of a squid giant nerve cell

1mm

Diameter of the yolk cell of an ostrich egg 120mm

Magnification and scale bars

• Biologists often need to do calculations involving the magnification and sizes of images and specimens.

• The size of a specimen is how large something actually is.

• The size of an image is how large the specimen appears in a drawing or a photograph.

• Magnification is how much larger the image is than the actual size.

Magnification= Size image/ actual size sp

Scale bars are sometimes used on drawings or micrographs.

- A micrograph is a photograph of an image under the microscope.

- The scale bar consists of a line, with the actual size that the line represents.

Micrograph

Graphs in Biology

1) Independent variable X- axis2) Dependents Y- axis

5.- Multicellular organisms and cell differentiation

• Some multicellular organisms live together in colonies. In a colony cells act in a co-operating way but they are not fused and do not form a single organism.

• Organisms consisting of a single mass of cells fused together are considered multicellular organisms. They have specialized cells for specific functions.

Algae colony

• The development of cells in different functions is called DIFFERENTIATION. This involves each cell type using some of the genes in its nucleus, and not others.

• When a gene is being used in a cell, we say that the gene is being EXPRESSED.

• In simple terms the gene is switched on and the information in it is used to make a protein or other gene product.

6.- STEM CELLS

• Stem cells are defined as cells that have the capacity of self-renew by cell division and to differentiate.

• At an early stage the whole of a human embryo consists of stem cells. Gradually the cells become committed to differentiating in a particular way. Once committed a cell may still be able to divide about six more times.

• However all the cells produced will differentiate in the same way and will no longer be stem cells.

• In the human body only a few tissues have stem cells: bone marrow, skin and liver.

• There has been great interest in stem cells because of their potential for tissue repair and for treating a variety of degenerative conditions. Example: Parkinson’s disease and strokes are all caused by the loss of neurons or other cells. Stem cells may one day be used to replace these cells.

6.1.- Therapeutic use of stem cells

• The greatest success so far in the therapeutic use of stem cels involves bone marrow transplants.

• The cells needed are hematopoietic stem cells (HS). They give place to white and red blood cells.

• These cells are used in a variety of treatments:- Leukemia- Lymphoma- Myeloma

LYMPHOMA:

Is a cancer of the lymphatic system.Some lymphomas can be treated following this

procedure:1) Cells are removed from the bone marrow of the

patient2) The patient goes through chemotherapy to kill

dividing cells.3) HS cells are retransplanted back into the

patient and begin to produce new healthy cells.

Origin of blood cells

7.- Extracellular components

• The plasma membrane of the cell is usually considered to be the barrier between the inside of the cell and the outside.

• The plasma mebrane is composed mainly by phospholipids and glycoproteins.

• These are called EXTRACELLULAR COMPONENTS. Anything inside the plasma membrane is Intracellular.

The matrix glues the cells in animal tissues (adhesion). Helps form the cartilage, tendons,

ligaments, etc.

7.1.- The plant cell wall

• The plant cell wall can be regarded as well as an extracellular component.

• The cell wall is composed by CELLULOSE MICROFIBRILS.

• This wall mantains the shape of the cell and supports the plasma membrane. It also protects the cell againts dehydration and solar radiation.

8.- Life as an emergent property

• Emergent properties are those that arise from the interaction of component parts The whole is greater than the sum of its parts.

• Ex: consciousness is a property that emerges from the interaction of nerve cells in the brain.

• Multicellular organisms show many emergent properties.

• LIFE ITSELF IS AN EMERGENT PROPERTY.

9.- PROKARYOTIC CELLS

• Prokaryotes were the first organisms to evolve on Earth and they still have the simplest structure.

• Bacteria are prokaryotes.• They are mostly small in size, unicellular and

are found almost everywhere: in soil, water, in our skin, hot water, volcanic areas, everywhere.

Prokaryotic cell’s structure1) Cell wall: - Always present- Composed of peptidoglycan- Protects the cell- Mantains its shape- Prevents cell from bursting2) Plasma membrane:- Thin layer mainly composed of

phospholipids - Partially permeable- Controls entry and exit of

substances- Pumps substances by active

transport

3) Ribosomes:- Small granular structures- Smaller tha eukaryotic ribosomes- Synthesizes proteins4) Cytoplasm:- Fluid filling the space inside the

plasma membrane- Water with many dissolved

substances- Contains enzymes and ribosomes- Carries out chemical reactions of

metabolism

5) Nucleoid:- Region of cytoplasm

containing the genetic material (usually one molecule of DNA)

- DNA molecule is circular and naked (no associated proteins)

- Total amount of DNA is much smaller than in eukaryotes

- The nucleoid is stained less densely than the rest of the cytoplasm because there are fewer ribosomes in it and less protein.

6) Flagella:- Structures protruding from the cell

wall- Base is embedded in the cell wall- Using energy they can be rotated to

propel the cell from one area to another

- Unlike aukaryotic flagella they are solid and inflexible

7) Pili:- Protein filaments protruding from

the cell wall- Can be pulled in or pushed out by a

rachet mechanism- Used for cell to cell adhesion- Used when bacteria stick together to

form aggregations of cells- Used in DNA exchanging process:

conjugation

10.- Eukaryotic cells

Eukaryotic cells have a much more complicated internal structure than Prokaryotic cells.- They have a NUCLEUS and

organelles in the cytoplasm with single of double membranes.

- Each organelle has a distinctive structure and function.

Eukaryotic cell micrograph

Eukaryotic cell’s structure

1) Nucleus:-The nuclear membrane is double and has pores through it. -Uncoiled chromosomes are spread through the nucleus and are called chromatin. -The nucleus stores almost all the genetic material of the cell.- It is where DNA is replicated and transcribed into mRNA

2) Rough Endoplasmic reticulum:- Consists of flattened

membrane sacs called cisternae. Attached to its walls there are ribosomes.

- Its function is to produce proteins for secretion from the cell.

3) Golgi apparatus:- Its structure is similiar the RER.- Here the proteins from the

RER are processed and exported out of the cell.

4) Lysosomes:- Spherical organelles with a

single membrane- They contain digestive

enzymes which can be used to break down ingested food, or damaged organelles. In some cases even the entire cell.

5) Mitochondria:- Is a double membrane

organelle.- They produce ATP by aerobic

cell respiration- Fat is digested here if it is

being used as an energy source in the cell.

6) Ribosomes: These appear as dark granules

in the cytoplasm and are surrounded by a membrane.

They synthesize proteins.

11.- MEMBRANES

- Phospholipids are essential components of membranes:1) In the plasma membrane2) Internal cellular membranes (mitochondria, lysosomes,

etc).- They have two regions with very different properties:1) Two hydrocarbon tails which are not attracted to water

hydrophobic2) A phosphate head, that is negatively charged to which

water is attracted hydrophilic

• When phospholipids are mixed with water they become arranged in double layers, with the heads facing outwards and the tails inwards.

• This arrangement called phospholipid bilayer is the basis of cellular membranes.

• Its a very stable structure.

Membrane structure• Membrane’s are not composed entirely by

phospholipids, there are also proteins.-Integral proteins: embedded in the bilayer-Peripheral proteins: attached to the surface.

Membrane proteinsPROTEIN FUNCTION

Insuline receptor Hormone receptor

Enzymes Proteins with enzymatic activity. Ex: In the small intestine

Cell adhesion proteins Cell adhesion to form tight junctions between groups of cells in tissues

Transport proteins Channels for passive transport to allow hydrophilic particles across by facilitated difussion

Cell signaling proteins Cell.to-cell communication. Ex: receptors for neurotransmitters at synapses.

Pumps Pumps for active transport which use ATP to move particles across the membrane.

a) Diffusion• Liquids and gases are fluids. Within fluids,

individual particles move independently and the direction of their movement is random.

• These movements of particles results in a process called DIFFUSION.

• More particles move randomly from an area with a high concentration to an area of a low concentration.

• There is a net movement from the higher to the lower concentration, in order to reach BALANCE.

• For example oxygen and carbon dioxide move into and out of cells by diffusion.

• It is an effective method of moving particles a few micrometres but it is too slow over greater distances.

DIFFUSION: is the passive movement of particles from a region of high concentration to a region of lower concentration.

b) Simple difussion across membranes

• Simple diffusion across membranes involves particles passing between the phospholipids in the membrane.

• It can only happen if the phospholipids bilayer is permeable to the particles.

• Small particles can pass through more easily than other large particles or charged ones (positive or negatively charged molecules cannot easily go through).

• Simple diffussion also happens if the concentration of the particle is higher on one side of the membrane than the other.

c) Facilitated diffusion

• Ions and other particles, which cannot diffuse between phospholipids, can pass into or out of cells if there are channels for them, through the plasma membrane.

• The diameter and chemical properties of the channel ensure that only one type of particles passes through.

• Because these channels help particles to pass through the membrane, from a higher to a lower concentration, this process is called FACILITATED DIFUSSION.

d) Osmosis• Osmosis is the diffusion of water across the

plasma membrane.• Osmosis only occurs when there are

substances dissolved in water solutes.• Regions with a higher solute concentration

have a lower water concentration, than regions with a lower solute concentration.

• Water tends to move from regions of lower to higher solute concentration.

ACTIVE TRANSPORT

• When substances move across the plasma membrane against the cincentration gradient, energy must be spent for the process to take place this type of transport is called ACTIVE TRANSPORT.

• The energy supplied for this process comes from ATP (cellular respiration).

• Transporter proteins, carry out active transport.

ENDOCYTOSIS AND EXOCYTOSIS

• A vesicle is a small sac of membrane with fluid inside.

• To form a vesicle, a small region of a membrane is pulled from the rest of the membrane and is pinched off.

• This vesicles are used to absorb fluid from the outside ENDOCYTOSIS

• Or also to secrete certain fluids towards the outside EXOCYTOSIS

12.- CELL DIVISION

In prokaryotic: Cells its called BINARY FISSION and it is used for asexual reproduction.-It involves single circular chromosome replication. -The two copies os the chromosome move to opposite ends of the cell. -Finally the cytoplasm is divided by a process called cytokinesis.

Binary fission

In Eukaryotic cells:-First step is the nucleus division MITOSIS-DNA is replicated in order to obtain two identical DNA molecules-Once the DNA has been replicated, the cytoplasm of the cell divides by cytokinesis and two identical daughter cells are obtained. Mitosis is a 4 phases process: Prophase, Metaphase, Anaphase and Telophase

The cell cycle• The cell cycle refers to the events between

one cell division and the next one, in eukaryotic cells.

• It can be divided in 2 stages:1).- Interphase: cellular growth + metabolic

reactions + DNA replication. It has 3 stages: G1 phase, S phase and G2 phase.2).- Cell division: MITOSIS

Mitosis1) Prophase:- The chromosomes become shorter. - The nuclear membrane breaks down.- Microtubules grow from the poles of the cell from a

structure MTOC (microtubules organizing center). These microtubules conform the mitotic spindle.

2) Metaphase:- Chromosomes are moved to the equator of the cell, with a spindle microtubule attached to one of the sister chromatids from one pole, and another s.microtubule attached to the opposite sister chromatid from the other pole.

3) Anaphase:- The pairs of sister chromatids separate and the

spindle microtubules pull the towards the poles of the cell.

- Identical chromatides go to both poles of the cell in order to obtain two indentical daughter cells.

4) Telophase:Nuclear membranes reform around the chromatids, now called chromosomes, and uncoil into chromatine.- The cell divides and the two daughter cells enter interphase again.