Notes, summaries, assignments, exams, and exercises for Biology for Secondary education

Plasma Membrane Structure and Function

The plasma membrane is composed of lipids and proteins, chemically arranged in a particular way. According to the Singer-Nicholson fluid mosaic model, there are two layers of phospholipids through which proteins are embedded, extending both outward and inward. These proteins act as channels, allowing substances that cannot cross the lipid layer (due to their hydrophilic character or their size) to pass through. Additionally, peripheral proteins are found on the inside of the membrane and function as enzymes. Finally, we conclude that the structure of the plasma membrane is not rigid due to the constant movement of its molecules.

Cellular Transport: Passive vs. Active Mechanisms

Passive transport is a process...

Structure of the Plasma Membrane

Membrane Amphipathic Properties

The plasma membrane is amphipathic:

• Fatty acid tails are hydrophobic (repel water).
• Phosphate head groups are hydrophilic (attract water).

Transport Through the Membrane

Factors Affecting Transport

• The concentration of substances.
• The type of molecules.

Transport Mechanisms

Based on these factors, transport can occur by three phenomena:

• Passive transport
• Active transport
• Vesicle-mediated transport

Passive Transport

In passive transport, molecules move down their concentration gradient: from regions of higher concentration to regions of lower concentration. The cell does not expend energy; the process occurs spontaneously.

Types of Passive Transport

• Simple diffusion
• Osmosis
• Facilitated diffusion

Simple

Understanding Hemoglobin

Hemoglobin (Hb) is a blood heteroprotein with a molecular weight of 64,000 (64 kDa). This characteristic red pigment transports oxygen from respiratory organs to tissues in mammals, oviparous animals, and other species.

Hemoglobin changes color based on oxygenation: it turns a bright scarlet red when bound to oxygen (arterial blood) and a dark red when oxygen is released (venous blood).

Structure of the Heme Group

Hemoglobin consists of four polypeptide chains (globins), each bound to a heme group containing an iron atom that binds reversibly to oxygen. The heme group is formed through the following process:

1. Synthesis: Succinyl CoA (from the Krebs cycle) joins with glycine to form a pyrrole group.
2. Protoporphyrin Formation:

The Digestive System: Anatomy and Function

The digestive tract processes food through several key structures:

Anatomy of the Digestive Tract

• Mouth: Contains the teeth and tongue.
• Salivary Glands: Parotid, Sublingual, and Submaxillary glands.
• Pharynx: Connects the nasal and oral cavities to the larynx and esophagus.
• Esophagus: Muscular tube leading to the stomach.
• Stomach: Features the cardiac valve and the pyloric valve. It contains gastric glands that produce gastric juice and hydrochloric acid.
• Small Intestine: Composed of the duodenum, jejunum, and ileum. It contains intestinal glands that produce intestinal juice.
• Large Intestine: Includes the cecum, colon, and rectum, ending at the anus.

Accessory Glands and Organs

• Liver: Secretes bile, which aids

Meiosis I

Prophase I

In this stage, DNA is packaged to form chromosomes. Due to the duplication of genetic material during interphase, these chromosomes consist of two arms called sister chromatids, held together by a structure called the centromere.

Homologous chromosomes pair up and swap DNA fragments, a process known as crossing over. This is a vital mechanism for producing genetic variation. After crossing over, the nuclear membrane disappears and spindle fibers begin to form.

Metaphase I

The spindle fibers are fully formed, and homologous chromosomes are randomly located at the equatorial plate of the cell.

Anaphase I

During this phase, each chromosome of a homologous pair is drawn to opposite sides of the cell, independently of the other pairs....

Lamarckism: The First Theory of Evolution

Lamarck published the first theory of evolution. Evolutionary theories are those that explain the diversity of species as a result of successive changes in previous species. Lamarck's theory is based on three main points:

1. Existing species come from other earlier species through successive changes.
2. These changes are produced by an intentional effort to improve certain qualities. These characters are called acquired characters.
3. These acquired characters are passed on to their descendants.

Darwin's Theory of Evolution

Darwin's theory is defined by three fundamental points:

• Variability: Populations of living beings are not uniform; they show variability. In a stable environment, the number of individuals and their

Basic Concepts of Genetics

Genetics studies the transmission of hereditary traits.

• Gene: A fragment of DNA containing the information needed to make proteins or to determine a specific characteristic.
• Genome: The complete set of genes from an organism.
• Genotype: The genetic makeup of an organism (e.g., represented by letters like AA, Aa, aa).
• Phenotype: The observable traits or characteristics resulting from the genotype and its interaction with the environment.

Mendelian Genetics Principles

Mendelian genetics applies the methodology practiced by Gregor Mendel, which involves making crosses between individuals with known phenotypes to infer their genotypes.

Mendel's Laws of Inheritance

Mendel formulated three fundamental laws governing the transmission...

Cell Types

The most primitive cells are called prokaryotes. Bacteria are unicellular prokaryotes. All other living things are composed of eukaryotic cells, which are more complex; in them, the nucleus is well-differentiated.

Eukaryotic organisms, in turn, may include:

• Unicellular organisms such as some fungi, some algae, and protozoa.
• Multicellular organisms such as animals, plants, some fungi, and some algae.

Not all eukaryotic cells are equal. Plant eukaryotic cells differ from animal eukaryotic cells because they present a rigid cellulose wall outside the cell membrane that provides high resistance, and they also contain chloroplasts, where photosynthesis takes place.

The cells of the same organism are not equal. This is because they specialize...

The environment is everything that surrounds us. It can therefore be an urban environment or, more broadly, a natural environment. It is generated by the natural world and influenced by humans. This applies to both transformed and non-transformed environments.

What is Ecology?

The science that studies ecosystems is **ecology** (from Greek: *oikos* 'house' + *logos* 'study'), meaning the study of the 'living house' or ecosystems. Ecologists seek to explain phenomena that occur in the environment.

Earth's Spheres and Ecosystems

On Earth, there are several layers: the atmosphere, hydrosphere, and lithosphere. There are broader terms that encompass the Earth as a whole: the biosphere, ecosphere, and ecosystem.

• Biosphere

  The part of the planet that contains

The Microscopic World of Cells

The Universe Hidden from Our View

The seventeenth-century (17th) invention of the microscope revolutionized our understanding of life.

• In 1674, Anton van Leeuwenhoek invented a powerful microscope.
• He observed a drop of water and what he saw were tiny living creatures, which he called animalcules.

Living Cells: The Building Blocks of Life

We are all formed by living cells.

The Discovery of Cells

• In 1665, Robert Hooke discovered cells while observing cork.

Types of Organisms

• Single-celled organisms:
  • Their size is very variable but they are always microscopic.
  • A single cell is able to perform all the functions of a living organism.
• Multicellular organisms:
  • Can often be seen with the naked eye; they are macroscopic.
  • Their cells