THE CELL

It is the basic unit that contains the fundamental molecules of life.

CELLS• They were discovered after the invention of the microscope, by a British

naturalist called Robert Hook. He was analyzing a very thin layer of cork tissue under a microscope when he found them out. He called them cells because they seemed as cells in a monastery.

• It is the basic, smallest and simplest unit of all living things.• All organisms are made of one or more cells.• All cells came from pre-existing cells• In multicellular organisms, cells can work independently or work together

in a coordinated way.

TYPES OF CELLS

They can vary according to the shape (that changes relating to the function or if they are animal or plant cells), to the size (though it is usually between 1-100 micrometers) or to their complexity. There are two types according to their complexity:

• PROKARYOTIC: bacteria and cyanobacteria have prokaryotic cells. Prokaryotic cells only have a cell membrane, a cell wall, ribosomes and genetic material, which flows freely in the cytoplasm.

• EUKARYOTIC: protists, fungi, plants and animals have eukaryotic cells. They have several advantages over prokaryotic cells. There are two types: animal and plant cells.

LEVELS OF BIOLOGICAL ORGANISATION

CELL FUNCTIONS

• Interaction: cells can communicate with the environment by adapting to the changes around it and maintaining the activities it needs to live. It consists on to actions: receiving information to detect environmental changes and responding appropriately to them.

CELL FUNCTIONS

• Nutrition: cells take different molecules from the environment and transform them into energy in order to make it function or to renew its cellular structure. These transformation take place in a process called metabolism. It consists on all the chemical reactions that take place inside the cell to produce ATP. Waste products are produced and usually expelled out of the cell.

METABOLISM

• Blosynthesis• Contraction• Chemical activation• Importing metabolites• Active Transport: Na-K Pump• Cytoplasmic transport• Flagellar Movements• Cell crawling• Heat production

CELL FUNCTIONS

• Reproduction: it consists on the formation of two new cells from the existing ones. It is used: for single-celled organisms, to create new individuals, and for multicellular ones to replace dead cells and to increase the number of cells for an organism that is growing. Genetic material of the parent consists on their biological characteristics, which are passed to daughter cells, and thus, they are identical to the parent cell.

THE CELL LIFE CYCLE

• In prokaryotes, cell division can occur at any moment, but eukaryotic cells duplicate their DNA exactly once during a discrete period between cell divisions.

THE CELL LIFE CYCLE• Interphase is the period of time until the cell receives a signal that makes

it replace itself. In G0 (gap 0) , no more synthesis occurs. Some cells such us neurons or muscles cells never leave this stage. In G1 (gap one) RNA and proteins synthesise. In S (synthesis), DNA and chromosomes replicate, protein synthesis, chromosomes duplicate and centromeres duplicate. In G2 (gap 2), centrioles grow, tubulin for spindle apparatus is synthesised, and energy is stored .Mitosis is the process in which the nucleus is divided

• S phase

MITOSIS

• Cell division is a process that cells undergo to repair or grow body tissues. Mitosis is a processs of cell division in which a parent cell produces two daughter cells with identical genetic information. All the cells undergo mitosis except gamates (that are produced through meiosis), neurons and muscle cells. Before mitosis occurs, genetic information has to be replicated (S phase).

MITOSIS

• PROPHASE: centrioles move to each of the sides of the cells and start building up a mitotic spindle made up of microtubules (tubulin protein). The nucleolus and the nuclear membrane disappear. Genetic material thickens and shortens to make chromosomes, that are made up of two chromatids linked by a centrosome.

MITOSIS

• METAPHASE: The chromosomes move to the equatorial plane (centre of the cell) and join to the spindle fibers in the centrosomes. Tubilin shortens and depolymerizes it.

MITOSIS

• ANAPHASE: Spindle fibers break in the centromeres and pull apart chromatids (half of a chromosome) to the opposite sides of a cell.

MITOSIS

• TELOPHASE: when chromatids have reached the centrioles, spindle fiibers disappear. A nuclear membrane surrounds the chromosomes (the chromatids join), forming two daughter nuclei. Chromosomes start to transform into chromatin (the substance in which DNA is found during interphase). The nucleolus reappears too.

CYTOKINESIS• The cytoplasm divides.• In animal cells, cytoplasm stretches, thinnens and separates.• In plant cells, a wall is built in the middle of the cell

(cellplate), which divides the cytoplams in half.

MEIOSIS• It is the process when a

diploid/somatic cells with a complete set of chromosomes (2n) divides into four haploid cells or gamates (n).

• Its purpose is to create gamates with different genetic material creating variability of species to avoid diseases .

• It consists on two consecutive cell divisions:

MEIOSIS• FIRST DIVISION: reduction division. The differences between a normal

mitosis are:• PROPHASE: chromosomes appear in pairs. These pair of chromosomes are

called homologous, and they have the same structure and position. They undergo a process called crossing over. In this process, chromosomes exchange genetic information. However, only the ones that are in the centre are going to exchange particular segments, because bonds are easilier to change. The parts that are farther from the centromere are more likely to cross-over. The physical point where it happpens is called chiasm

• METAPHASE: entire chromosomes (crossed-over chromosomes) go to the equatorial plane and are linked to the mitotic spindle.

• ANAPHASE: chromosomes are pulled to the sides of the cell.

• TELOPHASE: two daughter hapolid nuclei with n chromosomes are formed.

MEIOSIS• SECOND DIVISION: Before the second mitosis there’s no duplication of

DNA. It is as a normal mitosis. The result is four haploid cells with unique genetic combination.

Production of gamates is random because of:

• Independent assortment (2^23 possibilities to get through independent assortment). Crossing over

• Random fertilization (millions of sperm are ejaculated and you were the one to success).

CELL ORGANELLES

• They are structures consisting on a space usually enclosed by a membrane (or two) that perform specilised functions.

ORGANELLES

• CYTOSKELETON: microscopic network of tubules that give cells their shape and is in charge of the cell’s movility and movement of the cell’s organelles. It is made up of microtubules, microfilaments and intermediate fibers.

ORGANELLE• CELL MEMBRANE: it is a flexible bilayer membrane made up of phospholipids. These

phospholipids have a negative charged heads that are hydrophilic, and a non-polar tails that are hydrophobic. It also contains proteins, that are anchores to the cell membrane thanks to hydrophilic regions. Material inside the membrane is continuously moving around and floating. Its functions are to keep water on either side of the cell as well as toxic substances out of the cell; and proteins can act as receptors can act as receptors, enzymes, surface antigens, transporters and channels that allow to pass specific molecules, such as ions, nutrients or wastes. As lipid’s heads are charged, the only substances that can pass are the uncharged ones. However, oxygen, hydrogen and carbon dioxide don’t need to be charged because they are small and fat-soluble substances that can enter the membrane by diffusion. Despite, negative charged heads repel large, water-soluble molecules, such as sugar, and electrically charged ions, such as calcium. The protein channels let big molecules to pass in or out of the cell, though. When the cell is immersed in a aqueous solution the tails embed deeply and leave the hydrophilic heads exposed, leaving the cell insulated from most substances, plus this is a handy membrane because it can automatically fix itself when it’s torn.

ORGANELLES• CENTROSOME: they are structures

in charge of creating microtubules and take part in cell division. Plant’s centrosomes are simplier than animal’s centrosomes because they are not made up of two centrioles as they are.

• During animal cell division, centrioles replicate and the centrosome divides, resulting of two centrosomes with two pair of centrioles each. The centrosomes move to the opposite side of the nucleus and, from each one, microtubules grow into a “spindle” that separates the chromosomes.

ORGANELLES• ROUGH AND SMOOTH

ENDOPLASMIC RETICULUM: they are a complicated network of tubes (pipes) and vesicles (cisterns) surrounded by a membrane similar to the plasmatic membrane. It makes and transports lipids (smooth) and proteins (rough).

ORGANELLES

• GOLGI APPARATUS: it is an organelle made up of vesicles and flattened sacs called cisternae surrounded by a single membrane. Its function is to take substances or macromolecules from the endoplasmic reticulumand send them through vesicles throughout the cell. Vesicles that transport substances to the outside of the cell are called secretion vesicles. Substances can go to lysosomes, to vacuoles or for secretion. The stack of large vesiclesis surrounded by numerous smaller vesicles containing the packaged macromolecules.

• It is also in charge of builing up lysosomes and of digesting nutrient

ORGANELLES

• LYSOSOMES: tiny vesicles (small bubbles enclosed by a membrane) that contain enzymes that can digest large molecules or bacteria. Uncontrolled release of lysosome content can cause a cell to die.

ORGANNELES• CHLOROPLAST: they have two

smooth membranes. In their interior, there are disc-shaped sacs called thylakoids, who are in charge of taking the electrons released during photosynthesis. It also has chlorophyll inside, that makes plants green and is sensitive to light. Solar radiation is transformed into chemical energy and inorganic substances like carbon dioxide are transformed into organic substances like glucose.

• H

H H2O bonds break.

O

6CO2 + 2protons= C6H12O6C6H12O6 + O2= ATP

ORGANELLES• VACUOLE: they are structures

that store various substances (nutrients, waste substances) and are enclosed by a membrane. In plant cells, they are bigger and more numerous than in animal cells, and they can occupy even th 80% of the total space of the plant cell. They play roles in intracellular digestion and can act as lysosomes in animal cells. In plant cells, they also regulate the pressure of the cell.

ORGANELLES• NUCLEUS: Nucleus: is a membrane-

enclosed organelle found in eukaryotic cells.. It contains most of the cell's genetic material l, made up of DNA molecules. These contain the information necessary to control cellular activity (plus cell growth and reproduction), give the cell its specific characteristics ¡and encode proteins (in nuclei acid). It has pores that open or close if any material (like ribosomes, created in the nucleolus) wants to get out. The nucleus is filled with nucleoplasm and is surrounded by an internal and external membrane. In its interior, it contains the nucleolus and the DNA in form of chromatin (DNA in the interphase).

ORGANELLES

• NUCLEOLUS: it is a spherical structure with no membrane which is involved in the synthesis of ribosomes. To get outside the nucleus, ribosomes have o break in two halves. Then, they go to the rough endoplasmic reticulum.

• RIBOSOMES: they are found in the cytoplasm or attached to the rough endoplasmic reticulum. They have no membrane, and are responsible for the synthesising of proteins. They have two subunits with three nucleoitides each one.

• They are also in charge of the transference of RNA (tRNA). mRNA codifies two proteins that are built in the ribosomes.

RIBOSOME

ORGANELLES• MITOCHONDRIA: It appears both in

animal and plant cells. It is a double membrane organelle. The external membrane is smooth while the other is folded inside in multiple folds called cristae. In its interior, it contains mitochondrial matrix, a gel-like substance with ribosomes, DNA and enzymes.

• It produces ATP: from the rupture of glucouse into two piruvic acids (a process called glycosis that happens in the matrix) plus oxygen we have carbon dioxide , water and ATP. This process is called cellular respiration or aerobic respiration.

• C6H12O6+ 6 O2 = 6 CO2 +6 H2O+ ENERGY.

Glucouse

2 pyruvic acids

THE BRAKE OF THE CARBON BONDS FROM THE PYRUVIC ACIDS CREATE ENERGY THAT IT’S STURED IN FORM OF ATP (ADENINE TRIPHOSPHATE) A EXTREMELY UNSTABLE COMPOUND.

ORGANELLESCells that move:Some eukaryotic cells can move

using cilia and flagella or using the changes in the viscosity (resistance to flow of a liquid) of its cytoplasm to produce temporary proyections called pseudopods.

Cilia and flagella are organelles formed from the same protein fibers of the cytoskeleton (tubulin). They are located outside the cell. Its movement is controlled by the centriole.

GENETICS

VOCABULARY

• Gene: segments of DNA that codify particular proteins. They transmit genetic information

• Allele: any of two or more variants of a gene that have the same relative position on homologous chromosomes and are responsible for alternative characteristics, such as smooth or wrinkled seeds in peas .

• Genotype: All the genes that make up an organism• Phenotype: particular genes that are “active”. They become

active according to the internal and external environment. They are observable traits in an individual.

• Phenotype: The observable characteristics of a genotype of an organism.

GENES• A gene can have two variants

(because the genetic material has two strands), that are called alleles. The individual can have two alleles for each trait: in homozygous individuals both alleles are the same, and in heterozygous individuals alleles are different. When the alleles are different, one allele can be dominant. This means that, if the allele N is dominant, the trait of Nn will be the same as if it was NN. The other allele is called recessive. There is a co-dominance when there is no dominant allele, so the traits are mixed.

GREGOR MENDEL• Mendel was the first person to

study transmission of biological traits. He was an Austrian monk, mathemathisian and gardener that lived during the 19th century. He made an experiment using different species of peas and cultivated them until they were purebred (homozygous). Then, he crossed. His discoveries form the basis of modern genetics, and can be divided into three principles.

MENDEL’S EXPERIMENT

MENDEL’S LAWS• Principle of uniformity:When two homozygous individuals are crossed,

their offspring (F1) will have identical phenotypes and genotypes. If there is dominance between the alleles, the phenotype of the offspring is the same as one of the parents. If there is co-dominance their phenotype is an intermediate between the phenotype of both parents.

Example: We have the Parents, PP and pp. The particles (P or p) divide in meiosis and separate: P-P p-p gamates. Then they combine :

There are four combinations Pp- Pp - Pp - Pp. The First Generation will have the same phenotypes and genotypes.

If one allele is dominant (let’s suppose it is P like in the fourth picture), the offspring will have identical phenotypes as its parent PP.

MENDEL’S LAWS• Principle of segregation: it is the first law

of inheritance. When two heterozygous individuals are crossed (F1) their offsprings will have different genotypes and phenotypes composed of the combination of the different gamates.

Example: We take the previous example. We have the First Generation: Pp- Pp - Pp - Pp .

Then, the particles separate in meiosis creating gametes. They are: P-p P-p

Now we combine it with itself, producing: PP- Pp- Pp- pp. As we can see, the Second Generation has a 25% of possibilities of being homozygous PP, 25% of possibilities of beign homozygous pp and 50% of possibilities of beign heterozygous Pp.

MENDEL’S LAWS

• Principle of independent assortment: It is Mendel’s second law of inheritance. When two individuals with two or more traits are crossed, each trait will transmit independently.

• This Mendel law isn’t universal, it only applies for non-linked traits.

OSMOSIS

OSMOSIS• It is a type of diffusion. Osmosis

is the passage of water or any other solvent through a semipermeable membrane. The semipermeable membrane allows the movement of water but doesn’ allow the movement of the solute molecules. The water flows from an area of high water concentration to an area of low water concentration. Sugar molecules can’t cross that semipermeable membrane.

Rosalía Martín Muñoz

Biology, 4th A