Cell Structure, Function, and Division Fundamentals

Posted on Dec 5, 2025 in Biology

Biology Exam Revision: Chapters 1, 2, 3

Prokaryotes and Eukaryotes

Prokaryote
A single-celled organism made up of prokaryotic cells. They do not contain membrane-bound organelles. Examples: bacteria, archaea.
Eukaryote
A single-celled or multicellular organism made up of eukaryotic cells. They contain membrane-bound organelles. Examples: protists, fungi, plants, animals.

Organelles in Plant and Animal Cells

  • Nucleus: Double membrane-bound organelle; contains genetic material (DNA, RNA).
  • Endoplasmic Reticulum (ER): Membrane-bound.
    • Smooth ER: No ribosomes; synthesizes and transports lipids.
    • Rough ER: Contains ribosomes; transports proteins in vesicles to the Golgi apparatus.
  • Ribosomes: Non-membrane bound; synthesizes protein by reading genetic instructions from RNA and linking amino acids together.
  • Golgi Apparatus: Membrane-bound; modifies and packages proteins.
  • Vesicles: Membrane-bound; transports materials between organelles within the cell.
  • Lysosome: Membrane-bound; contains enzymes that break down foreign matter or materials no longer needed.
  • Vacuole: Membrane-bound storage space; holds water and other substances, maintains plant structure.
  • Mitochondrion: Double membrane; site where stages of aerobic respiration occur, releasing energy (ATP).
  • Chloroplast: Double membrane; site where photosynthesis occurs (found in plants and algae).
  • Cell Wall: Structure that surrounds plant cells; provides structure and protection.

Differences Between Plant and Animal Cells

  • Animal cells do not have chloroplasts, and plant cells typically lack lysosomes.
  • Cell Wall: Plants have a cell wall to provide structural support; animals do not.
  • Vacuole: Plant cells have a large central vacuole for the storage of water and minerals. Animal cells may have smaller, temporary vacuoles.

Movement Across the Plasma Membrane

Hydrophilic and Hydrophobic Substances

Hydrophilic: Dissolves easily in water. Hydrophobic: Does not dissolve in water.

Movement through the plasma membrane happens through two structural elements:

  1. Phospholipid Bilayer: Hydrophobic substances can diffuse directly across the phospholipid bilayer.
  2. Protein Channels/Carriers: Hydrophilic substances pass through channel proteins and carrier proteins.

Cellular Transport Mechanisms

1. Passive Transport (No Energy Required)

Movement occurs down the concentration gradient (high to low concentration).

  • Simple Diffusion: The most basic way for substances to move; does not require energy.
  • Osmosis: Movement of water from high to low concentration; requires no energy.
  • Facilitated Diffusion: High to low concentration; requires no energy, but happens with the assistance of a carrier protein or channel protein.

2. Active Transport (Requires ATP Energy)

Movement occurs against the concentration gradient (low to high concentration).

  • Active Transport: Requires energy in the form of ATP; requires a carrier protein.
  • Bulk Transport: The movement of large particles across the plasma membrane; requires ATP.
Types of Bulk Transport
  1. Endocytosis: Movement of large particles into cells.
  2. Exocytosis: Movement of large particles out of the cell.

Osmosis and Tonicity

Osmosis: The passive movement of free water from a high concentration to a low concentration across a semipermeable membrane.

Tonicity: Describes the effect a solution has on cell volume by influencing the movement of water across the cell membrane through osmosis.

Hypotonic
The solution has a lower solute concentration than inside the cell.
Isotonic
The solution has the same solute concentration as inside the cell.
Hypertonic
The solution has a higher solute concentration than inside the cell.

Surface Area to Volume Ratio (SA:V)

  • A cell’s exchange of material between the internal and external environments needs to be efficient enough to ensure the survival of the cell.
  • The greater the surface area and the smaller the volume, the more efficient the cells are in taking in nutrients and removing waste.
  • Some cells increase the SA:V ratio by having folded plasma membranes.

Cellular Metabolism and Energy

Autotrophs and Heterotrophs

Autotrophs
Self-feeders; they make their own organic material by taking in energy from the environment (e.g., green plants through photosynthesis).
Heterotrophs
Cannot make their own food; they obtain organic materials by feeding on autotrophs or other organisms.

Key Metabolic Processes

Cellular Respiration
Glucose is broken down to produce ATP, releasing CO₂ and water as byproducts. This occurs in the mitochondria of cells.
Formula (Aerobic Respiration):

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP

Photosynthesis
Plants use sunlight to synthesize glucose and oxygen. This occurs in the chloroplasts.
Formula:

6CO₂ + 6H₂O + Light Energy → C₆H₁₂O₆ + 6O₂

ATP (Adenosine Triphosphate)
The primary energy currency used for cell growth and repair, active transport, movement, etc.

Internal Structure of Chloroplasts and Mitochondria

Inside the Chloroplasts
  • Chlorophyll: The green pigment in plants; captures light energy from the sun.
  • Grana: Stacks of thylakoid membranes; site of the light-dependent reactions of photosynthesis.
  • Stroma: Fluid inside the chloroplast; the site of the light-independent stage (Calvin cycle) of photosynthesis.
Inside the Mitochondrion
  • Cristae: The inner membrane of the mitochondrion is folded over and over, forming a layered structure called the cristae.
  • Matrix: Fluid inside the inner membrane; contains enzymes necessary for the Krebs cycle and other reactions.

Stages of Cellular Respiration

Glycolysis
The first stage of cellular respiration; takes place in the cytosol of the cell, involving the breakdown of glucose into two molecules of pyruvate.

What happens after glycolysis depends on whether oxygen is present:

Aerobic Cellular Respiration

The second and third stages occur in the mitochondria (the powerhouse of the cells).

  • The Krebs Cycle: Occurs in the fluid matrix of the mitochondria and produces 2 ATP molecules.
  • The Electron Transport Chain: Occurs on the inner membrane of the mitochondria (cristae); most ATP is produced in this stage.
Anaerobic Cellular Respiration

Begins with the breakdown of glucose to pyruvate. The next stage continues in the cytosol and depends on the organism (e.g., fermentation).

Haploidy, Diploidy, and Embryonic Layers

Haploid
Contains one set of chromosomes (n).
Diploid
Contains two sets of chromosomes (2n).
Zygote
A diploid cell, formed when the nuclei of an ovum and a sperm fuse.
Gastrulation
The process that results in the formation of three primary embryonic germ layers:
  1. Ectoderm: Outer layer
  2. Mesoderm: Middle layer
  3. Endoderm: Inner layer

Stem Cells and Differentiation

Stem Cell: A type of cell that is capable of differentiating into a range of specialized cells within an organism. Stem cells are also capable of self-renewal, meaning they can replace themselves, giving rise to more of the same type of stem cell.

Types of Stem Cells (By Source)

  1. Embryonic Stem Cells: Found in an embryo, in the developmental stage prior to uterine implantation.
  2. Adult Stem Cells: Undifferentiated stem cells found in certain tissues throughout the life of an individual. Locations include the brain, skin, heart, liver, etc.

Advantages and Disadvantages

Embryonic Stem Cells
  • Advantages: Can differentiate into all types of specialized cells (pluripotent); easy to source.
  • Disadvantages: Involve the destruction of life; can be unstable.
Adult Stem Cells
  • Advantages: Does not involve the destruction of life; research is more advanced.
  • Disadvantages: Difficult to obtain; cells have limited ability to divide.

Types of Stem Cells (By Potency)

  • Totipotent: Can differentiate into all possible cell types (including placental tissue).
  • Pluripotent: Can differentiate into almost any type of cell (but not placental tissue).
  • Multipotent: Can differentiate into a variety of closely related types of cells.
  • Unipotent: Can only produce one type of cell (their own).

Stem Cell Therapy: The treatment and prevention of disease through the use of stem cells.

DNA Structure and Packaging

DNA: Deoxyribonucleic acid.

DNA is made up of nucleotides, each having three main components:

  1. Phosphate group
  2. Deoxyribose sugar
  3. Nitrogenous bases: Adenine (A), Cytosine (C), Guanine (G), and Thymine (T).

DNA is packaged into the nucleus in the form of chromatin.

Chromosomes: The highly compact form of DNA that is visible in eukaryotic cells during division.

Cell Division and Replication

The fundamental purpose of replication is the passing on of genetic material to the next generation. The two main reasons for cellular replication are growth and repair.

Eukaryotic Cells:
Mitosis
Prokaryotic Cells:
Binary Fission

Eukaryotic Cell Cycle Stages

  1. Interphase: Consists of the G1, S, and G2 phases. Cells undergo several changes in preparation for division, focusing on cell growth and DNA synthesis.
    • G1 Phase: Cell grows larger and almost doubles in size.
    • G0 Phase: Some cells exit the G1 phase and enter G0 phase (a resting state).
    • S Phase: DNA within the nucleus of a cell is replicated.
    • G2 Phase: Final stage of interphase.
  2. Mitosis: Separation of sister chromatids and formation of two nuclei.
The Four Subphases of Mitosis
  • Prophase: Breakdown of the nuclear membrane and appearance of distinct chromosomes.
  • Metaphase: Lining up of chromosomes along the middle of the cell (metaphase plate).
  • Anaphase: Double chromosomes separate, and sister chromatids are pulled apart to the opposite sides of the cell.
  • Telophase: Two nuclei are formed, and the cell prepares to divide.
  1. Cytokinesis: The final stage of the cell cycle; the splitting of the cytoplasm.

Prokaryotic Division: Binary Fission

Binary fission is an asexual process that does not include spindle formation or sister chromatids, making the process very quick. The three steps are:

  • Replication
  • Elongation
  • Division

Cell Cycle Checkpoints

Checkpoints are control factors that process internal and external signals before determining whether a cell can proceed to the next stage of division.

  • G1 Checkpoint: Checks for cell size, nutrients, DNA damage, and growth factors.
  • G2 Checkpoint: Checks for DNA damage, DNA replication accuracy, and cell size.
  • M Checkpoint: Checks for spindle attachment.

Cell Death: Apoptosis and Necrosis

Apoptosis: Programmed Cell Death

A highly controlled process that results in a eukaryotic cell committing cellular suicide. It performs three key functions:

  1. Protection: Ensures unhealthy or damaged cells do not divide.
  2. Development: Essential for the development of an organism (e.g., the formation of fingers and toes).
  3. Balance: Assists an organism in maintaining balance (homeostasis).

For a cell to undergo apoptosis, it must receive the correct signals:

  • Positive Signals: Necessary for the continued survival of the cell.
  • Negative Signals: Indicate the need to activate the apoptosis pathway.
Necrosis: Unprogrammed Cell Death
Cell death that occurs as a result of trauma or injury. It is not controlled. Results in the swelling of the cell before it bursts, causing inflammation.

Cancer and Tumor Classification

Cancer: A disease that occurs as a result of uncontrolled cell division.

Tumor Classification

  • Benign: Not cancerous.
  • Malignant: Cancerous.

Characteristics of Cancer Cells

  • Random arrangement of cell layers.
  • Increased division rate.
  • Do not require growth factor signals.