Understanding Human Multicellularity: Cell Functions & Reproduction
Humans as Multicellular Organisms
The three primary functions of an organism are interaction, nutrition, and reproduction.
Nutrition: This is the process by which cells and organisms obtain energy from food.
Interaction: This refers to an organism’s ability to detect and react to external or internal stimuli.
Reproduction: This is the biological process by which new individual organisms are produced from their parents.
The Cell
The cell is the functional unit of all organisms, the smallest living part of the body, and the building block of life.
Some organisms, such as bacteria, are unicellular, consisting of a single cell. Other organisms, such as humans, are multicellular.
The largest cells are ostrich eggs.
Types of Cells
There are two main types of cells:
- Eukaryotic cells
- Prokaryotic cells
Prokaryotic cells lack a nucleus and other organelles found in eukaryotic cells. Eukaryotic cells have a nucleus and are generally larger. The main difference is that eukaryotic cells contain membrane-bound compartments.
Cell Components
Key components of the cell include:
- Endoplasmic Reticulum: Exists in two forms: rough endoplasmic reticulum (with ribosomes, producing proteins) and smooth endoplasmic reticulum (lacking ribosomes).
- Cell Nucleus: The most important organelle, controlling the cell. It contains DNA (chromosomes, chromatin). The nucleolus is where ribosome subunits are assembled.
- Plasma Membrane: Envelops the cell, separating its interior from the environment.
- Ribosomes: Complexes made of RNA and proteins that synthesize proteins.
- Mitochondrion: Responsible for cellular respiration and energy generation.
- Vacuoles: Separate waste and other materials.
- Cytoplasm: The internal environment of the cell, containing all organelles.
- Golgi Body: Processes and packages molecules produced by the cell.
- Centriole: Produces microtubules and participates in cell division.
- Lysosomes: Contain digestive enzymes.
All cells carry out three primary functions: nutrition, interaction, and reproduction.
Cell Nutrition
Cell nutrition is the process by which cells exchange substances and energy with their environment. Cells must carry out numerous chemical reactions to maintain proper function, all of which require energy.
Since the beginning of evolution, cells have developed highly efficient metabolic systems to produce energy in the form of ATP molecules.
Cell metabolism is the set of chemical reactions by which cells transform molecules. Metabolism has two categories: anabolism and catabolism.
- Catabolism: The cell breaks down molecules to produce energy.
- Anabolism: The cell uses energy to build complex molecules and carry out biological functions.
Cellular respiration is a catabolic process where the energy of nutrients is released in the form of ATP. Carbohydrates, fats, and proteins can be used as fuel in cellular respiration, which takes place in the mitochondria.
Cell Interaction
Cells receive stimuli from their environment and react to them. The interaction function refers to the cell’s capacity to receive stimuli and respond, known as cell sensitivity.
Cells can respond in two ways: static response and dynamic response.
Cell Reproduction
Through reproduction, a type of cell can increase its population. Sexual reproduction can introduce genetic variation, which can be beneficial.
Cell reproduction, or cell division, consists of a single cell dividing into two daughter cells.
Cells can reproduce in two ways: mitosis and meiosis.
- In mitosis, the daughter cells are identical clones of the original cell.
- In meiosis, cells with two sets of chromosomes (diploid cells) form haploid cells with half the chromosomes. This is a special type of cell division necessary for sexual reproduction.
46 chromosomes (diploid cells) (23 pairs) [2n]
↓
23 chromosomes (haploid cells) [n]
Through cellular differentiation, cells can become specialized, such as nervous cells or blood cells.
Stem Cells
One embryo
Two embryos
Four embryos