Cellular Biology Essentials: Structure, Function, and Transport

Cell Theory: Fundamental Principles

Cell Theory is built upon three main ideas:

• All living things are composed of cells and their products. Therefore, cells are the morphological units of living beings.
• The fundamental activities that characterize life occur within cells. Therefore, cells are the physiological or functional units of living beings.
• New cells are formed from other preexisting cells through cell division.

Comparing Plant and Animal Cells

Plant Cell Characteristics

• Possesses large vacuoles.
• Features a rigid cell wall.
• Contains plastids (e.g., chloroplasts, which perform photosynthesis, making the cell autotrophic).
• Absence of centrioles.
• During cell division, cytokinesis occurs outwards.

Animal Cell Characteristics

• Contains centrioles.
• Has small vacuoles with various functions, differing from plant cell vacuoles (e.g., residual, contractile).
• Lacks a cell wall.
• Does not possess plastids (making the cell heterotrophic).
• During cell division, cytokinesis occurs inwards (with spindle fibers not found in plant cells).

Key Cellular Organelles and Their Functions

Endoplasmic Reticulum

A membrane system involved in communication, storage, transport, and manufacturing of substances. The rough endoplasmic reticulum (also known as granular ergastoplasm) is responsible for protein synthesis.

Golgi Complex

Similar to the endoplasmic reticulum in structure but with a more orderly arrangement; primarily related to cellular secretion.

Mitochondrion

The structure responsible for cellular respiration, a process that releases energy.

Chloroplast

An organelle that performs photosynthesis, thanks to chlorophyll, absorbing light and producing food for cells. It is present in some plant cells and tissues.

Nucleoplasm (or Nuclear Sap)

The substance in which chromatin is immersed within the nucleus.

Nucleoli

Structures present in the nucleus, primarily related to ribosome production.

Vacuoles

Spaces within the cell where water and various other substances are stored.

Cellular Boundaries: Wall and Membrane

Cell Wall

A very rigid structure, primarily made of cellulose, found in plant cells.

Plasma Membrane

The boundary between the cell's interior and exterior.

Functions of the Plasma Membrane

• Restricts and defines intra- and extracellular space.
• Enables cellular recognition.
• Regulates the passage of substances.

Fluid Mosaic Model

Describes the plasma membrane as a double layer of lipids with hydrophobic extremities facing inwards and hydrophilic extremities facing outwards. Its composition includes proteins (integral or spherical) and glycans linked to proteins (glycoproteins) or lipids (glycolipids).

Functions of Membrane Proteins

Membrane proteins perform diverse roles:

• Porins: Responsible for the transport of small metabolites.
• Glycoproteins: Involved in cell recognition (protein + carbohydrate).
• Enzymes: Catalyze chemical reactions within or on the membrane.
• Membrane Receptors: Proteins that receive stimulation, leading to specific cellular responses.

Cellular Transport Mechanisms

Passive Transport

This process does not require energy expenditure and always occurs down the concentration gradient. Transport can happen with or without the activity of specific proteins.

• Simple Diffusion: The movement of gases or small particulate matter directly across the membrane.
• Osmosis: The diffusion of water across a selectively permeable membrane. Key terms include:
  • Crenation: Cell shrinks due to water loss.
  • Turgor: Cell swells due to water intake (especially in plant cells).
  • Plasmolysis: In plant cells, the cell membrane detaches from the cell wall due to significant water loss.
  • Lysis: Cell bursts due to excessive water intake (especially in animal cells).
• Facilitated Diffusion: Involves transport proteins but still follows the concentration gradient without energy input.

Active Transport

This process requires energy expenditure and occurs against the concentration gradient, carried out by specific transport proteins.

Na+/K+ Pump Functions

• Electrogenic Nature: The pump can generate electronegativity on the inner face of the membrane.
• Maintenance of Cell Volume: Regulates the volume of liquid entering or leaving the cell by balancing Na+ and K+ movement.

Phospholipids: The Membrane's Foundation

Phospholipids constitute the double layer of membrane molecules.

• Hydrophilic Region: The part of the molecule containing phosphorus, which is attracted to water.
• Hydrophobic Region: The lipid tail part that is repelled by water.

The plasma membrane, primarily composed of phospholipids, plays a crucial role in selective substance transport, acting as a dynamic and selective barrier, not merely a mechanical one.

Specialized Organelles: Peroxisomes

Peroxisomes are cellular organelles present in all eukaryotic cells (which possess a karyotheca or nuclear membrane), separating the cytoplasm from the genetic material.

They contain catalase, an enzyme that decomposes hydrogen peroxide into water and oxygen. This process is vital in the liver for alcohol detoxification in the body.