Module 2 Review: Cellular Basis of Life + Cell Communication

Cellular Basis of Life

All living organisms are composed of one or more cells, which are the fundamental units of life.
Cells carry out essential functions including metabolism, energy conversion, and reproduction.
The two main types of cells are prokaryotic (like bacteria) and eukaryotic (like plant and animal cells).

Cell Structure
- All biological systems are composed of cells.
- Unicellular Organisms
  - Paramecium
  - Amoeba
  - Bacteria
  - Yeast
- Humans are made up of many specialized cells, including:
  - Stem cells
  - Sex cells
  - Muscle cells
  - Fat cells
  - Immune cells
  - Epithelial cells
  - Nervous cells
  - Blood cells
  - Bone cells
Two Kinds of Cells: Prokaryote vs. Eukaryote
- A typical eukaryotic cell is $10$ to $100$ micrometers ( $\mu m$ ) in diameter, making them generally bigger than prokaryotes.
- Prokaryotic cells are typically $1$ to $10$ micrometers ( $\mu m$ ) in diameter.
Discovery of Cells
- Robert Hooke was the first to observe cells in $1665$ .
  - He used cork and saw tiny box-like compartments with a single lens.
  - He gave them the Latin name cellulae, meaning "small rooms".
  - Because cells are so small, they were not discovered until the invention of the microscope in the $17^{th}$ century.
- More than $100$ years later, biologists fully recognized the importance of cells.
  - In $1838$ :
    - Matthias Schleiden stated that the cell is the fundamental unit of plant structure (Botanist).
  - In $1839$ :
    - Theodor Schwann reported that all animal tissues also consist of individual cells.
  - Thus, the Cell Theory was born.
The Principles of Cell Theory
- $1$ . All organisms are composed of one or more cells, and the life processes of metabolism and heredity occur within these cells.
- $2$ . Cells are the smallest living things, of all biological systems (the basic units of structure and function of living organisms).
- $3$ . New cells arise only by division of preexisting cells.
Surface-Area-to-Volume Ratio
- Surface area of a bigger cube is bigger, but the surface-area-to-volume ratio is bigger for the smaller cube.
  - That's why cells are small; they want a higher ratio.
- Cells exchange materials with their surroundings by diffusion.
  - Diffusion: transfer of material from higher concentration to lower concentration (e.g., spraying perfume).
  - The rate of this diffusion is affected by:
    - Higher temperature = high speed of diffusion.
    - Concentration gradient of the diffusing substance: higher = higher diffusion.
    - Distance over which diffusion must occur.
    - The surface area available for diffusion: higher ratio = higher diffusion (why cells prefer a higher ratio).
- Advantage of Small Cell Size:
  - Small cells have more surface area per unit of volume than large ones.
  - Smaller cells are more interactive.
  - Higher surface area to volume ratio speeds up diffusion and makes exchange of materials more efficient.
  - Small cells are like crushed ice; crushed ice melts faster because more surface touches the drink.
- How large cells increase their surface area to volume ratio:
  - Nerve cells are very big, which would decrease their surface area to volume ratio.
  - Cells deal with this issue using Microvilli: tiny, finger-like structures that extend from the surface of these cells, resulting in an increased surface area to volume ratio.
Studying Cells: Microscopes
- Microscopes manipulate light (or electrons) to magnify small objects that are otherwise invisible to the naked eye.
- Our eyes detect light reflection, and our brain processes this information to form images.
- Light microscopes, even compound ones, are not powerful enough to resolve many of the structures within cells.
- Electron microscopes:
  - More efficient and have higher resolved power than light microscopes because electrons can detect smaller structures that light cannot.
  - Have very short wavelengths of $\sim 1 nm$ .
- Light microscope: Visible light wavelength $\sim 400-700 nm$ .
- Eukaryotic ribosomes have a diameter of $\sim 25-30 nm$ .
- Mitochondria have a diameter of around $1 \mu m$ ( $1000 nm$ ).
Fundamental Similarities in Cell Structure
- Despite the diversity of cellular organization, all cells resemble one another in three fundamental ways:
  - Centrally located genetic material (e.g., DNA).
    - Both prokaryote and eukaryote have genetic material.
  - Cytoplasm: A semi-fluid matrix that fills the interior of every cell (not $100\%$ liquid because it's composed of many proteins and other macromolecules).
    - Cytosol: The liquid component of the cytoplasm that surrounds intracellular organelles.
  - The Plasma Membrane:
    - Surrounds the cell and separates it from its surroundings.
    - A thin ( $5-10 nm$ ) phospholipid bilayer with embedded proteins.
    - Under an electron microscope, it appears as two dark lines separated by a lighter area.
    - The proteins of the plasma membrane are generally responsible for a cell’s ability to interact with the environment.
    - Proteins help molecules and ions move across the plasma membrane, either from the environment to the interior of the cell or vice versa.
    - Some molecules cannot pass through freely (it chooses what passes through and what doesn’t).
Prokaryotic Cells
- Examples: Bacteria, Archaea.
- Lack interior organization and membrane-bound organelles ("like a one-room cabin").
- Simplest organisms in terms of structure, typically oval (pill) shaped.
- Consist of cytoplasm surrounded by a plasma membrane, encased within a rigid cell wall.
- Play a very important role in the ecology of living organisms.
  - Some can photosynthesize (e.g., Cyanobacteria) - their membrane contains bacterial pigments connected with photosynthesis, unlike eukaryotic plant cells with chloroplasts.
  - Some break down dead organisms and recycle components.
  - Others cause disease.
  - Some are used in industrial processes (e.g., cheese, yogurt, bread using Lactobacillus and Streptococcus for fermentation).
- The cell membrane of prokaryotes sometimes takes over the function of some organelles (e.g., photosynthesis in cyanobacteria).
- Kinds of Prokaryote Cells:
  - Archaea: Cell wall does not have peptidoglycan; cell walls are composed of various chemical compounds.
  - Bacteria:
    - Cell wall composition: Encased in a strong cell wall containing peptidoglycan (a complex polymer of sugars and amino acids, cross-linked by short polypeptide units, making it very resistant; found only in bacteria; penicillin can kill these guys by removing the proteins between the carbohydrates).
    - Main functions of the cell wall in bacteria:
      - Protect the cell.
      - Maintain its shape.
      - Prevent excessive uptake or loss of water.
    - The cell wall of bacteria lacks cellulose, whereas plant cell walls are made up of cellulose, hemicellulose, pectin, and lignin.
- Both Archaea and Bacteria belong to the broader category of prokaryotes.
- One of the most important differences is that they are different in size and lack compartmentalization compared to eukaryotes.
Eukaryotic Cells
- Internal membranes organize the interior of eukaryotic cells, making them much more complex than prokaryotes.
- All organelles in plant and animal cells are membrane-bound organelles.
- Specific differences:
  - Chloroplasts are missing in animal cells.
  - Cell wall is present only in plant cells.
  - Centrioles are present only in animal cells.
Key Eukaryotic Organelles:
- Nucleus
  - Acts as the information center; the biggest organelle in cells.
  - Membrane-bound and mostly spherical, typically located in the central region in animal cells.
  - Most cells possess a single nucleus.
  - The nucleus has a dark staining nucleolus, where ribosomal RNA is synthesized.
  - The Nuclear Envelope: Enclosed by a double phospholipid bilayer.
    - Pores allow ions and small molecules to diffuse freely between nucleoplasm and cytoplasm while controlling the passage of proteins and RNA protein complexes.
    - The outer nuclear membrane is continuous with the endoplasmic reticulum.
    - Nuclear basket: Part of the nuclear pore, responsible for regulating the transport of molecules between the nucleus and the cytoplasm.
  - Ribosomes make proteins.
  - Nuclear lamina: Provides structural support, maintaining the shape of the nucleus.
  - Inside the nucleus, DNA is wound tightly around proteins and packaged into compact units named chromatin and chromosomes.
  - The Nucleolus: Before cells can synthesize proteins in large quantity, they must first construct many ribosomes to carry out this synthesis.
    - Ribosomes are the cell's protein synthesis machinery, each composed of two subunits.
    - Once created, it exits the nucleolus to start protein synthesis and go to different destinations through channels ("like a highway").
- Endoplasmic Reticulum (ER)
  - The interior of a eukaryotic cell is channelized with membranes, allowing movement of proteins ("like a highway").
  - The presence of these membranes in eukaryotic cells marks one of the fundamental distinctions between eukaryotic and prokaryotic cells.
  - Two kinds of ER:
    - Rough ER (RER): Disc-shaped, provides a place for ribosomes.
    - Smooth ER (SER): Tube-shaped, no ribosomes, has multiple functions:
      - Lipid synthesis (phospholipids and cholesterol).
      - Metabolism of carbohydrates (contains enzymes that break down glycogen into glucose, which can be released into the bloodstream to maintain sugar levels).
      - Detoxification (responsible for detoxifying various drugs and toxins in liver cells).
      - Storing calcium ions.
- Golgi Apparatus
  - Sorts and packages proteins ("like a post office").
  - Proteins enter in one end (front door) and exit out the other end (back door).
  - Composed of a number of stacked membranes: around $20$ in animal cells and more than $100$ in plants.
  - Also functions in the synthesis of cell wall components.
- Lysosomes
  - Membrane-bounded digestive vesicles that break down molecules into smaller molecules ("like the recycling center").
  - Contain enzymes that break down biomolecules and old organelles, recycling their components for new ones.
- Microbodies
  - Eukaryotic cells contain a variety of enzyme-bearing vesicles called microbodies.
  - Membrane-bound vesicles with selective permeability.
  - Peroxisome: Contains an enzyme called catalase, which breaks down hydrogen peroxide into harmless compounds like water and oxygen.
- Proteasomes
  - Cells recycle their proteins in large, cylindrical complexes called proteasomes.
  - Another microbody responsible for recycling proteins ("one door is enter, one door is exit").
- Vacuoles
  - Membrane-bound structures that store water to maintain tonicity.
  - Plants have one large central vacuole:
    - Tonoplast: A kind of phospholipid bilayer that contains channels for water used to help the cell maintain its tonicity or osmotic balance.
    - Semipermeable, allowing some molecules to pass while blocking others.
    - Regulates the balance of ions (potassium and sodium, $K^+$ and $Na^+$ ) inside and outside the vacuole, using this strategy to maintain proper turgor pressure inside the cell.
    - ATP converted into ADP in relation to ion pumps.
  - Animals have multiple small vacuoles.
- Mitochondria and Chloroplasts
  - Membrane-bound, phospholipid bilayer organelles that produce ATP (energy-processing organelles).
  - Responsible for respiration (Mitochondria) and photosynthesis (Chloroplasts).
  - Mitochondria:
    - Singular, mitochondrion.
    - Around the same size as bacteria; the "power battery" of our body.
    - Tube-shaped, found in eukaryotic cells.
    - Metabolize sugar to generate ATP.
    - Double membrane bound.
    - The cristae (singular, crista) partition the mitochondrion into two compartments:
      - $1$ . A matrix, located inside the inner membrane.
      - $2$ . An outer compartment, or intermembrane space, located between the two mitochondrial membranes.
    - Smooth outer membrane.
    - Inner folded membrane with numerous contiguous layers called cristae that play a key role in ATP generation.
    - Contain their own ribosomes (because they create proteins that generate enzymes, increasing reaction time for energy/protein processes).
    - Proteins are embedded on the surface of the inner membrane that carry out oxidative metabolism.
    - Have their own DNA.
  - Chloroplasts:
    - Use light to generate ATP and sugars; the "glucose synthesizers" of the cell.
    - Found in plant cells only (from one to several hundred), tube-like shape.
    - Equation: $\text{Carbon dioxide} + \text{water} \longrightarrow \text{glucose (sugar)} + \text{oxygen}$ .
    - Larger and more complex than mitochondria.
    - Double membrane bound.
    - Have closed compartments of stacked membrane grana, which lie inside the inner membrane.
    - May contain a hundred or more grana, and each granum may contain from a few to several dozen disk-shaped structures called thylakoids.
    - Light-capturing photosynthetic pigments are located on the surface of the thylakoids.
    - Created from proplastids which convert into other pigments (like chloroplasts, etioplasts, and leucoplasts); chloroplasts can also be converted to chromoplasts.
    - Leucoplasts can be converted into amyloplasts, elaioplasts, and proteinoplasts.
Cytoskeleton
- A network of protein filaments in the cytoplasm of all cells.
Importance of Blood Cell Shape
- Blood cells are circular (biconcave) because it increases their surface area, allowing more oxygen and carbon dioxide to be carried throughout the body.
- The biconcave shape is an example of cooperation between embedded proteins and the phospholipid bilayer of the cytoskeleton.
- The protein spectrin forms a scaffold that connects proteins in the plasma membrane to actin filaments in the cytoskeleton, contracting the cell membrane and resulting in the biconcave shape.
Molecules Dissolving in Water: Diffusion and Osmosis
- Diffusion (Osmosis) occurs when molecules dissolve in water (molecules dispersed).
- Concentration is the most important cause of diffusion (high concentration to low concentration).
- When molecules dissolve, they are always moving, dynamically.
  - This random movement of molecules and ions is called Brownian movement.
- Osmosis in non-biological systems involves a selective permeable membrane.
Impact of Solute Concentrations on Cells
- We can characterize the relative concentrations of solutes in solutions and their impact on cells:
  - $1$ . Hypertonic Solution:
    - Has a higher concentration of solutes (dissolved substances) compared to another solution.
    - Effect on cells: When a cell is placed in a hypertonic solution, water molecules move out of the cell (osmosis), leading to cell shrinkage.
  - $2$ . Hypotonic Solution:
    - Outside solute concentration is lower than inside the cell.
    - Effect: Swelling occurs as water moves into the cell.
  - $3$ . Isotonic Solution:
    - The concentration of solutes inside the cell and outside is the same.
    - Effect: Size of cell stays the same.
Transmembrane Domains and Membrane Transport
- One of the most important functions of the cell membrane is to control the entry and exit of water and molecules.
- Nonpolar molecules, based on diffusion, travel from higher concentration to lower.
- Active Transport:
  - Needed to move molecules against their concentration gradient (from lower to higher concentration).
  - Requires energy (ATP converted to ADP).
- Passive Transport:
  - Two types of Diffusion from high to low concentration:
    - Simple Diffusion: Nonpolar molecules pass through directly (like "people just walking across the street with no sidewalk").
    - Facilitated Diffusion: Polar molecules and ions must go through specific channels ("like lines or tunnels").
      - Specific channels for water are called Aquaporin channels.
  - Ions have a charge and are repelled by nonpolar molecules of the plasma membrane's lipid bilayer. Therefore, ions cannot move between the cytoplasm of a cell and the extracellular fluid without the assistance of membrane transport proteins.

Transmembrane Domains and Membrane Transport

One of the most important functions of the cell membrane is to control the entry and exit of water and molecules.
Nonpolar molecules, based on diffusion, travel from higher concentration to lower.
Active Transport:
- Needed to move molecules against their concentration gradient (from lower to higher concentration).
- Requires energy ( $ATP$ converted to $ADP$ ).
- Sodium-Potassium Pump: Uses energy to move ions against their concentration gradient.
  - $3$ sodium ions exit for every $2$ potassium ions enter.
  - Results in the outside becoming more positive than the inside.
  - This is not diffusion because the direction is against the concentration gradient.
  - Maintaining cell potential and regulating cellular volume are performed by the sodium-potassium pump.
  - Steps:
    - $1$ . Three sodium ions bind with the protein pump inside the cell.
    - $2$ . The carrier protein then gets energy from $ATP$ and changes shape.
    - $3$ . It pumps the three sodium ions out of the cell.
    - $4$ . Two potassium ions from outside the cell bind to the protein pump.
    - $5$ . The potassium ions are then transported into the cell.
Passive Transport:
- Two types of diffusion from high to low concentration:
  - Simple Diffusion: Nonpolar molecules pass through directly (like "people just walking across the street with no sidewalk").
  - Facilitated Diffusion: Polar molecules and ions must go through specific channels ("like lines or tunnels").
    - Specific channels for water are called Aquaporin channels.
- Ions have a charge and are repelled by nonpolar molecules of the plasma membrane's lipid bilayer. Therefore, ions cannot move between the cytoplasm of a cell and the extracellular fluid without the assistance of membrane transport proteins.
- Gated channels are specific for ions.
- Transfer depends on concentration and the status of those transport proteins.

Bulky Materials Cross Membranes Within Vesicles

Most large polar molecules needed for cell growth cannot pass through the hydrophobic barrier of the lipid bilayer.
These substances get into cells via processes involving vesicles.
Two processes are involved in this bulk transport:
- Endocytosis:
  - The process by which cells take in substances from outside by engulfing them in a vesicle.
  - Cells use three major types of endocytosis:
    - Phagocytosis: "Cell eating"; uptake of large particles or whole cells (e.g., immune cells engulfing bacteria).
    - Pinocytosis: "Cell drinking"; uptake of fluid and dissolved solutes.
    - Receptor-mediated endocytosis: The cell uses surface receptors to capture and take in specific molecules from outside.
  - How cells take in cholesterol: Low-density lipoproteins ( $LDL$ ), which carry cholesterol in the blood (from the liver to the cells), bind to $LDL$ receptors on the cell surface.
  - The liver is the central part of cholesterol metabolism.
  - High-density lipoproteins ( $HDL$ ) carry cholesterol back to the liver.
  - Note: $HDL$ can cause stroke (Generally inaccurate; $HDL$ is associated with reduced risk of stroke and heart disease, while $LDL$ is associated with increased risk).
- Exocytosis:
  - The process by which cells release substances from inside to outside by fusing vesicles with the plasma membrane.
  - Examples of exocytosis in plants and animals:
    - In plant cells, exocytosis exports material necessary for building the cell wall.
    - In animal cells, exocytosis releases hormones, digestive enzymes, and other substances.
    - Release of insulin from the pancreas: When blood sugar levels rise, insulin is packaged into vesicles and released via exocytosis to help cells absorb glucose from the bloodstream.

Cell-to-Cell Communication: Intracellular Signaling

Communication between cells is common in nature; cell signaling happens in all life forms, allowing cells to interact.
Allergy: One of the most important examples of cell signaling.
- What happens when allergens (pollen or pet dander) enter the body?
  - They bind to specific receptors on the surface of immune cells called mast cells.
  - This binding activates the mast cells to release histamines.
  - The results: Symptoms like itching, swelling, and inflammation.
- We treat allergies using antihistamines, which block the receptor for the histamine signal.
An effective signaling process requires a signaling molecule, called a ligand.
A cellular protein that binds this ligand (or signal) is called a receptor protein (embedded proteins).
The interaction of these two components (ligand and receptor) initiates the process of signal transduction.
- Signal transduction converts the information in the external signal into a cellular response (e.g., itching, sneezing, swelling).
Steps in cell signaling:
- $1$ . Ligand (signaling molecule) goes to the receptor.
- $2$ . Receptor (embedded protein) accepts the ligand.
- $3$ . A series of molecular events happens (signal transduction).
- $4$ . Then the signal transduction causes a cellular response.
The cells in all biological systems use a variety of molecules as signals:
- $1$ . Peptides
- $2$ . Large proteins
- $3$ . Individual amino acids
- $4$ . Nucleotides (phosphate group, nitrogenous base, pentose sugar)
- $5$ . Steroids
- $6$ . Even dissolved gases such as nitric oxide ( $NO$ ) (inorganic) are used as signals.

A signal molecule called a ligand, a receptor, and a signal transduction pathway are core components of cell signaling, which leads to the production of a cellular response.

Production of Cellular Response

All cells in biological systems are exposed to a constant stream of signals; at any time, hundreds of different chemical signals may be present in the surrounding area.
A cell responds to only certain signals.
The number and kind of receptor molecules determine which signals a cell will respond to.
When a ligand approaches a receptor protein that has a complementary shape, they can form a complex.
This is analogous to a person following the conversation of one or two individuals in a noisy, crowded room.

Four Kinds of Cell Signaling

Direct Contact: Occurs between directly contacting cells.
- The surface of a eukaryotic cell is full of proteins, carbohydrates, and lipids.
- Nearby cells recognize each other’s membrane molecules.
- The molecules on one cell act as a signal and the molecule of another cell acts as a receptor.
- Signaling is performed through adjacent plasma membranes.
Paracrine Signaling: Signal molecules are released by cells and diffuse through the fluid to nearby target cells in the same area.
- Example: In the stomach, the hormone gastrin is released by certain cells. These signals then diffuse to neighboring cells in the stomach and stimulate them to secrete gastric acid.
- In this process, signaling molecules affect nearby target cells within the same tissue or region.
Autocrine Signaling: Cells signal themselves by binding to their own receptors.
- This kind of signaling is an important component of signaling in the immune system.
- The self-sustaining growth of cancer cells is attributed to autocrine signaling.
- An autocrine cell signaling-mediated proliferation is responsible for tumor formation, increased cell differentiation, and cell division.
Endocrine Signaling: A released signal molecule that remains in the extracellular fluid can enter the organism’s circulatory system and travel widely throughout the body.
- Example: Hormone secretion into the blood by an endocrine gland travels via a blood vessel to distant target cells.
Synaptic Signaling: In this signaling system, the cells of the nervous system provide rapid communication with distant cells.
- Signals don’t travel through the bloodstream; nerve fibers release neurotransmitters near target cells across a synaptic gap.

Signal Transduction Pathways Lead to Cellular Responses

After generating a ligand-receptor complex, a signal transduction pathway is initiated, which results in a cellular response.
Phosphorylation = activating proteins (addition of phosphate groups).
Dephosphorylation = deactivating proteins (removal of phosphate groups).
Many proteins are inactive or nonfunctional as they are initially synthesized, then are later activated by modification.
Another modification is deactivation of previously active proteins.
Deactivating and activating of proteins is performed by phosphorylation or dephosphorylation, which is the addition or removal of phosphate groups.
This often involves $ATP$ . Protein kinase adds phosphate groups from $ATP$ to proteins, leading to protein activation.
Always, activation of a protein is associated with protein conformation.
A protein activated by a kinase will be deactivated by a phosphatase, and a protein deactivated by a phosphatase will be activated by a kinase.

Receptor Types

Receptors can be categorized based on their structure and function. They can be Intracellular receptors or Membrane receptors.

Intracellular Receptors

Located inside the cell (typically in the cytoplasm or nucleus); bind to hydrophobic ligands.
The ligands of intracellular receptors are small, hydrophobic molecules because they must be able to cross the plasma membrane to reach the receptors.
In these cases, an extracellular signaling molecule must diffuse through the plasma membrane to gain access to its receptor.
Example: Estrogen is a group of steroid hormones that are responsible for the development and regulation of the female reproductive system. Because estrogen is hydrophobic, it can diffuse through the plasma membrane of a target cell and bind to a receptor inside the cell.

Membrane Receptors

Embedded in the cell membrane; bind to hydrophilic ligands.
Types include:
1. Channel-linked receptors (Example: gated proteins).
  - Gated proteins are responsible for the passage of ions.
  - When the ligand binds to this protein as a signal, the gated protein is opened, and the ion can enter or leave the cell.
2. Enzymatic receptors.
  - Signal molecule binds to the receptor.
  - It activates the enzyme, leading to cellular responses.
3. G protein-coupled (linked) receptors ( $GPCR$ s).
  - Are membrane proteins that transmit signals inside the cell by activating G proteins in response to external molecules like hormones or neurotransmitters.
  - $GTP$ (guanosine triphosphate) acts as an energy-carrying molecule that activates G proteins, enabling signal transmission in cellular pathways.
  - $GTP$ is a molecule that serves as a crucial energy carrier, similar in structure to adenosine triphosphate ( $ATP$ ), which is another important energy-carrying molecule in cells.
    - $ATP$ : contains the adenine base – universal energy currency.
    - $GTP$ : contains the guanine base – specialized voucher.

Cell Signaling: A Three-Stage Process

Cells usually respond to signals via a three-stage process:

Stage 1: Receptor Activation
- A signaling molecule binds to a receptor in the target cell, causing a conformational change in the receptor that activates its function.
Stage 2: Signal Transduction
Stage 3: Cellular Response

Apoptosis: Programmed Cell Death

Apoptosis is a key process of programmed cell death in eukaryotic cells; the cell, a living system, concludes its existence through death.
Programmed cell death, or apoptosis, involves a series of steps:
1. Initiation: Signals trigger the cell to begin the death process.
2. Execution: The cell’s internal machinery breaks down its parts.
  - The cell shrinks and rounds as its nucleus and cytoskeleton break down.
3. Removal: The cell breaks into small parts, which are cleared away by other cells.
  - The membrane forms extensions that break into small bubbles as the cell breaks down (membrane blebbing).
So: Normal cell ---> shrinkage ---> membrane blebbing ---> cells break into apoptotic bodies ---> phagocytosis.
Cell biologists have discovered that apoptosis plays many important roles:
- Human fingers are webbed but separate as cells between them die during development.
- Programmed cell death helps control cancer.
- Apoptosis is also necessary in adult organisms to maintain the proper cell number in tissues and organs.
- Programmed cell death also eliminates cells that have become infected by viruses or have the potential to cause cancer.