Bio cell notes

🧬 COMPLETE CELL BIOLOGY STUDY NOTES

PART 1: Introduction to Cells

1. What are cells?

• A cell is the smallest basic unit of life.

• All living organisms are made of one or more cells.

• Cells carry out all the processes necessary for life.

• Cells take in nutrients, produce energy, remove waste, grow, and reproduce.

• Cells are often called the building blocks of life.

The cell theory

The cell theory has three main principles:

1. All living organisms are made of one or more cells.

2. The cell is the basic unit of structure and function in living things.

3. All cells come from pre-existing cells through cell division.

Important: Cells do not spontaneously appear. New cells are produced from existing cells.

2. Two main types of organisms based on cell number

- Unicellular organisms: Organisms made of only one cell.

• Examples: bacteria, amoebas, and many types of yeast.

• A single cell performs all the functions needed to survive.

- Multicellular organisms: Organisms made of many cells.

• Examples: humans, dogs, trees, and insects.

• Different cells have different jobs and work together.

Unicellular

One cell carries out life functions.

Multicellular

Many specialized cells work together.

PART 2: Prokaryotic vs. Eukaryotic Cells

There are two major categories of cells: prokaryotic and eukaryotic.

3. Prokaryotic cells

• Prokaryotic cells are generally smaller and simpler than eukaryotic cells.

• They do NOT have a nucleus.

• Their DNA floats in an area called the nucleoid.

• They do not have membrane-bound organelles.

• They usually have a cell membrane, cytoplasm, ribosomes, and DNA.

• Most prokaryotes are unicellular.

Examples:

• Bacteria

• Archaea

Parts of a prokaryotic cell

• Cell wall: Protects and supports the cell.

• Cell membrane: Controls what enters and leaves.

• Cytoplasm: Jelly-like substance where chemical reactions occur.

• Ribosomes: Make proteins.

• Nucleoid: Region containing the cell's main DNA.

• Plasmids: Small circular pieces of extra DNA found in many bacteria.

• Flagellum: Tail-like structure that helps some bacteria move.

• Capsule: Protective outer covering found in some bacteria.

• Pili: Small hair-like structures that help bacteria attach to surfaces and sometimes exchange DNA.

4. Eukaryotic cells

• Eukaryotic cells are generally larger and more complex.

• They have a true nucleus that contains their DNA.

• They contain membrane-bound organelles.

• They can be unicellular or multicellular.

Examples:

• Animals

• Plants

• Fungi

• Protists

Main characteristics

• Have a nucleus.

• Have mitochondria.

• Have ribosomes.

• Have cytoplasm.

• Have a cell membrane.

• Contain specialized organelles that perform different functions.

5. Prokaryotic vs. eukaryotic comparison

Remember: Prokaryotes = NO nucleus. Eukaryotes = HAVE a nucleus.

PART 3: Plant Cells vs. Animal Cells

6. Plant cells

Plant cells are eukaryotic cells that make up plants.

They have several structures that help them survive and perform photosynthesis.

Important plant cell structures

• Cell wall: A rigid outer layer made mainly of cellulose. Provides protection, support, and shape.

• Cell membrane: Controls what enters and leaves the cell.

• Chloroplasts: Contain chlorophyll and carry out photosynthesis.

• Central vacuole: Large storage space that holds water, nutrients, and waste. Helps maintain pressure inside the cell.

• Nucleus: Stores most of the cell's DNA and controls cell activities.

• Mitochondria: Produce ATP through cellular respiration.

• Ribosomes: Make proteins.

• Cytoplasm: Holds organelles and is where many chemical reactions occur.

• Endoplasmic reticulum: Helps make and transport proteins and lipids.

• Golgi apparatus: Modifies, sorts, and packages materials.

7. Animal cells

Animal cells are eukaryotic cells found in animals and humans.

Important animal cell structures

• Cell membrane: Controls what enters and leaves.

• Nucleus: Contains most of the DNA.

• Mitochondria: Produce ATP.

• Ribosomes: Make proteins.

• Cytoplasm: Holds the organelles.

• Lysosomes: Break down waste and worn-out cell parts.

• Small vacuoles: Store water, nutrients, and waste.

• Centrosome/centrioles: Help organize microtubules during cell division.

• Endoplasmic reticulum: Makes and transports proteins and lipids.

• Golgi apparatus: Packages and distributes proteins and other materials.

8. Plant vs. animal cells

Important things to remember:

• Both plant and animal cells are eukaryotic.

• Both have a nucleus.

• Both have mitochondria.

• Plant cells have a cell wall and usually chloroplasts.

• Animal cells do not have cell walls or chloroplasts.

• Plant cells usually have one large central vacuole.

PART 4: All the Major Organelles

An organelle is a specialized structure inside a cell that performs a particular job.

Think of organelles as different departments in a factory. Each department has its own responsibility.

9. Nucleus

• The nucleus is the control center of a eukaryotic cell.

• Contains most of the cell's DNA.

• Controls gene expression and helps regulate cell activities.

• Surrounded by a double membrane called the nuclear envelope.

• Contains the nucleolus.

Nucleolus:

• Found inside the nucleus.

• Produces ribosomal RNA (rRNA).

• Helps assemble ribosome subunits.

10. Mitochondria

• Known as the powerhouse of the cell.

• Produce most of the ATP used by many eukaryotic cells.

• ATP is the main energy-carrying molecule used by cells.

• Found in both plant and animal cells.

• Have their own DNA and ribosomes.

Mitochondria structure

• Outer membrane: Covers the mitochondrion.

• Inner membrane: Contains proteins involved in ATP production.

• Cristae: Folds of the inner membrane that increase surface area.

• Matrix: Fluid-filled inside space containing enzymes, mitochondrial DNA, and ribosomes.

• Intermembrane space: Space between the two membranes.

Cellular respiration

Cellular respiration is the process that releases energy from glucose and transfers it into ATP.

Overall equation:

C6H12O6+6O2→6CO2+6H2O+ATPC6​H12​O6​+6O2​→6CO2​+6H2​O+ATP

This means:

• Glucose + oxygen are used.

• Carbon dioxide + water are produced.

• Energy is transferred into ATP.

11. Lysosomes

• Lysosomes are membrane-bound sacs containing digestive enzymes.

• Break down waste materials.

• Digest old or damaged organelles.

• Help destroy certain invading microorganisms.

• Recycle useful materials so the cell can reuse them.

Remember: Lysosomes are like the recycling and garbage disposal system of the cell.

What do lysosomes contain?

• Digestive enzymes.

• An acidic internal environment.

• A protective membrane that separates enzymes from the cytoplasm.

12. Ribosomes

• Ribosomes make proteins.

• They are found in both prokaryotic and eukaryotic cells.

• They are made of ribosomal RNA and proteins.

• They can float freely in the cytoplasm or attach to rough ER.

Free ribosomes: Make proteins mainly used inside the cell.

Attached ribosomes: Make proteins that are often secreted, inserted into membranes, or sent to certain organelles.

13. Endoplasmic reticulum (ER)

The endoplasmic reticulum is a network of membranes involved in making and transporting materials.

There are two types:

Rough ER

• Has ribosomes attached to its surface.

• Helps produce proteins.

• Helps fold and modify proteins.

• Sends proteins toward the Golgi apparatus.

Smooth ER

• Does not have ribosomes.

• Makes lipids.

• Helps detoxify certain substances.

• Stores calcium ions in certain cells.

14. Golgi apparatus

• Modifies proteins and lipids.

• Sorts and packages materials.

• Sends materials to their correct destinations.

• Helps form lysosomes.

Remember: The Golgi apparatus is like the post office or shipping center of the cell.

15. Vacuoles

• Store water, nutrients, salts, and waste.

• Plant cells usually have one large central vacuole.

• Animal cells generally have smaller vacuoles or vesicles.

• The central vacuole helps maintain turgor pressure in plants.

16. Chloroplasts

• Found in plants and many algae.

• Carry out photosynthesis.

• Contain chlorophyll, a green pigment that absorbs light.

• Have their own DNA and ribosomes.

Chloroplast structures

• Thylakoids: Flattened membrane sacs where light-dependent reactions occur.

• Grana: Stacks of thylakoids.

• Stroma: Fluid surrounding the thylakoids.

• Chlorophyll: Pigment that absorbs light energy.

Photosynthesis equation

6CO2+6H2O+light→C6H12O6+6O26CO2​+6H2​O+light→C6​H12​O6​+6O2​

Plants use light energy, carbon dioxide, and water to produce sugars and oxygen.

17. Cytoplasm

• Jelly-like material inside the cell membrane.

• Contains the organelles in eukaryotic cells.

• Supports many chemical reactions.

• Cytoplasm includes the cytosol and the structures suspended within it.

18. Cytoskeleton

The cytoskeleton is a network of protein fibers that helps maintain cell shape and organization.

Its functions include:

• Supporting the cell.

• Helping cells move.

• Moving organelles around.

• Helping with cell division.

Three main components:

• Microfilaments: Thin fibers made mainly of actin. Help with movement and shape.

• Intermediate filaments: Provide strength and stability.

• Microtubules: Hollow tubes that help with transport and form the mitotic spindle during cell division.

19. Other important cell structures

PART 5: Cell Membrane and Transport

20. Cell membrane

The cell membrane:

• Surrounds the cell.

• Separates the inside of the cell from its environment.

• Controls what enters and leaves.

• Helps cells communicate with other cells.

• Is selectively permeable.

Selectively permeable means that some substances can cross the membrane more easily than others.

Phospholipid bilayer

The cell membrane is mainly made of two layers of phospholipids.

Each phospholipid has:

• A hydrophilic head: Attracted to water.

• Hydrophobic tails: Repelled by water.

Other components include:

• Proteins: Help transport substances and receive signals.

• Cholesterol: Helps regulate membrane fluidity.

• Carbohydrate chains: Help with cell recognition.

21. Passive transport

Passive transport moves substances across a membrane without requiring cellular energy (ATP).

Types:

• Diffusion: Particles move from high concentration to low concentration.

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Facilitated diffusion: Substances move down their concentration gradient through membrane proteins.

Concentration gradient

A concentration gradient is a difference in the concentration of a substance between two areas.

Particles generally move down their concentration gradient until equilibrium is reached.

22. Active transport

• Requires energy, usually from ATP directly or indirectly.

• Moves substances against their concentration gradient.

• Often uses transport proteins.

Examples:

• Sodium-potassium pump.

• Proton pumps.

Bulk transport

• Endocytosis: The cell takes materials in by forming a vesicle.

• Exocytosis: The cell releases materials by moving vesicles to the cell membrane.

PART 6: Cell Size and Surface Area

23. Why are cells so small?

Most cells are microscopic because small cells can exchange materials more efficiently.

Cells need to:

• Take in oxygen and nutrients.

• Remove carbon dioxide and waste.

• Move substances around internally.

• Obtain enough materials to support their activities.

Surface area-to-volume ratio

This is one of the most important ideas in cell biology.

• Surface area = amount of outside membrane available for exchange.

• Volume = amount of space inside the cell.

As a cell grows:

• Its volume increases faster than its surface area.

• It becomes harder to exchange enough materials through the membrane.

• This is one reason cells usually remain small.

Example

Imagine two cube-shaped cells:

Conclusion: Smaller cells have a greater surface area-to-volume ratio, allowing more efficient exchange of substances relative to their volume.

PART 7: DNA AND GENETICS

24. What is DNA?

DNA stands for deoxyribonucleic acid.

• DNA is the molecule that stores hereditary genetic information.

• It contains instructions for making RNA and proteins.

• It influences inherited traits.

• It is passed from parents to offspring.

• In eukaryotic cells, most DNA is found in the nucleus.

• Mitochondria also contain their own DNA.

• Chloroplasts contain their own DNA in plants and algae.

25. Structure of DNA

DNA has a double-helix structure, which looks like a twisted ladder.

DNA is made of repeating units called nucleotides.

Each nucleotide contains three parts:

1. A phosphate group.

2. A deoxyribose sugar.

3. A nitrogenous base.

Four nitrogenous bases

Base-pairing rules:

• A always pairs with T.

• C always pairs with G.

These are called complementary base pairs.

26. DNA strands

• DNA has two strands.

• The strands run in opposite directions, called antiparallel.

• The sugar-phosphate backbone forms the outside.

• The nitrogenous bases face inward.

• Hydrogen bonds hold complementary bases together.

27. Chromosomes

A chromosome is a long DNA molecule associated with proteins.

• DNA wraps around proteins called histones.

• This helps organize and package DNA.

• Chromatin is DNA and its associated proteins in a less condensed form.

• Chromosomes become highly condensed during cell division.

• Genes are sections of DNA that contain instructions for functional products, often proteins or functional RNA.

Human chromosomes

• Most human body cells have 46 chromosomes.

• These are arranged into 23 pairs.

• 22 pairs are autosomes.

• 1 pair is the sex chromosomes.

• Egg and sperm cells normally have 23 chromosomes each.

28. Genes and alleles

Gene:

• A section of DNA that carries information for a functional product.

• Genes can influence characteristics.

Allele:

• A different version of a gene.

• For example, a gene can have different alleles that contribute to different forms of an inherited trait.

Genotype: The genetic makeup or allele combination of an organism.

Phenotype: The observable characteristics of an organism, influenced by genes and the environment.

29. DNA replication

DNA replication is the process of making a copy of DNA before cell division.

Basic steps:

1. The DNA double helix unwinds.

2. The two strands separate.

3. Each original strand acts as a template.

4. Complementary nucleotides are added.

5. Two DNA molecules are produced.

Each new DNA molecule contains:

• One original strand.

• One newly made strand.

This is called semiconservative replication.

30. DNA vs. RNA

31. Protein synthesis

Protein synthesis is how cells use genetic instructions to make proteins.

It involves two main stages.

1. Transcription

• Happens in the nucleus in eukaryotic cells.

• A section of DNA is used as a template.

• Messenger RNA (mRNA) is produced.

• The mRNA carries genetic instructions out of the nucleus.

2. Translation

• Happens at ribosomes.

• The ribosome reads mRNA.

• Transfer RNA (tRNA) brings amino acids.

• Amino acids are connected to form a protein.

Three types of RNA

• mRNA: Carries genetic instructions from DNA to ribosomes.

• tRNA: Brings amino acids to the ribosome.

• rRNA: Forms part of the ribosome and helps it function.

Important vocabulary

• Codon: A group of three mRNA bases that specifies an amino acid or stop signal.

• Anticodon: A three-base sequence on tRNA that pairs with an mRNA codon.

• Amino acid: Building block of proteins.

• Protein: Molecule that performs many functions, including structure, transport, signaling, and chemical reactions.

PART 8: CELL DIFFERENTIATION

32. What is cell differentiation?

Cell differentiation is the process in which an unspecialized cell develops into a specialized cell with a particular structure and function.

For example:

• Stem cells can develop into nerve cells.

• Stem cells can develop into muscle cells.

• Stem cells can develop into blood cells.

• Stem cells can develop into skin cells.

Why does differentiation happen?

• Multicellular organisms need different types of cells.

• Different cells perform different jobs.

• Cells become specialized to perform their functions efficiently.

• Different genes are switched on or off in different cell types.

Important: Most specialized cells in one person's body contain essentially the same DNA, but they use different genes.

Examples of specialized cells

PART 9: AUTOLYSIS AND CELL DEATH

33. What is autolysis?

Autolysis is the process in which a cell breaks down through the action of its own enzymes.

The word can be broken down into:

• Auto = self.

• Lysis = breaking down.

How does autolysis happen?

1. A cell becomes damaged or dies.

2. Cellular compartments may lose their integrity.

3. Digestive enzymes can reach cellular materials.

4. The enzymes break down proteins, membranes, and other cell components.

5. The cell's structures begin to deteriorate.

What role do lysosomes play?

• Lysosomes contain digestive enzymes.

• These enzymes break down biological materials.

• Lysosomal enzymes can contribute to cellular breakdown during autolysis.

• Autolysis can also involve enzymes from other cellular compartments.

Autolysis vs. apoptosis

These are NOT the same thing.

Apoptosis: Programmed cell death that removes unwanted or damaged cells in a controlled way.

PART 10: CELL DIVISION

34. Why do cells divide?

Cells divide to:

• Allow organisms to grow.

• Replace old or damaged cells.

• Repair tissues.

• Reproduce in certain organisms.

There are two major types of cell division: mitosis and meiosis.

35. The cell cycle

The cell cycle is the series of stages a cell goes through as it grows and divides.

Interphase

Interphase is the longest part of the cell cycle for many cells.

It has three stages:

• G1 phase: Cell grows and performs normal functions.

• S phase: DNA is replicated.

• G2 phase: Cell continues growing and prepares for division.

M phase

• Mitosis: The nucleus divides.

• Cytokinesis: The cytoplasm divides, producing two daughter cells.

36. Mitosis

Mitosis produces two genetically similar daughter cells with the same chromosome number as the original cell.

Four major stages:

1. Prophase: Chromosomes condense and the spindle begins forming.

2. Metaphase: Chromosomes line up in the middle of the cell.

3. Anaphase: Sister chromatids separate and move to opposite sides.

4. Telophase: New nuclear envelopes form around the chromosome sets.

Cytokinesis follows or overlaps with the final stages.

37. Meiosis

Meiosis produces sex cells (gametes) in organisms that reproduce sexually.

• Involves two rounds of division.

• Produces four haploid cells in the typical textbook model.

• Reduces chromosome number by half.

• Creates genetic variation.

Mitosis vs. meiosis:

PART 11: IMPORTANT BIOLOGY VOCABULARY

Here are more terms you should know.

PART 12: THINGS YOU SHOULD ABSOLUTELY MEMORIZE

These are some of the most important facts to remember for a cell biology test.

The essential cell biology cheat sheet

• Nucleus = control center; contains most DNA.

• Mitochondria = ATP production.

• Lysosomes = digestion and recycling.

• Ribosomes = protein production.

• Golgi apparatus = modifies, sorts, and packages.

• Rough ER = protein production and processing.

• Smooth ER = lipid production and detoxification.

• Chloroplasts = photosynthesis.

• Vacuole = storage.

• Cell membrane = controls entry and exit.

• Cell wall = support and protection.

• Cytoplasm = site of many chemical reactions.

• Cytoskeleton = structure and movement.

• Nucleolus = ribosome subunit assembly.

• DNA = genetic instructions.

• RNA = helps use genetic instructions.

• ATP = usable cellular energy.

• Prokaryotic = no nucleus.

• Eukaryotic = has a nucleus.

• Differentiation = cells become specialized.

• Autolysis = self-digestion by enzymes.

• Apoptosis = programmed cell death.

PART 13: Check Your Understanding

Here are some questions to help you see how well you understand the notes.

Try answering these after studying. You can use your notes to check yourself.

1. What are the three main principles of cell theory?

2. What is the biggest difference between prokaryotic and eukaryotic cells?

3. What are three structures found in plant cells but not animal cells?

4. What is the function of mitochondria?

5. Why are lysosomes important?

6. What is the difference between rough ER and smooth ER?

7. What are the four nitrogenous bases in DNA?

8. What does A pair with in DNA?

9. What is the difference between DNA and RNA?

10. What is cell differentiation?

11. What is autolysis?

12. What is the difference between apoptosis and autolysis?

13. Why are cells generally small?

14. What happens during DNA replication?

15. What are the four stages of mitosis?

16. What is the function of chloroplasts?

17. What is ATP?

18. What are the three types of RNA?

PART 14: Differences Between Plant and Animal Cells

Both plant and animal cells are eukaryotic cells, meaning they have a nucleus and membrane-bound organelles. However, they have some important differences in structure and function.

1. Plant cells

Plant cells:

• Have a cell wall made primarily of cellulose.

• Have a cell membrane underneath the cell wall.

• Usually contain chloroplasts for photosynthesis.

• Have one large central vacuole.

• Often have a more rectangular or rigid shape.

• Store carbohydrates, often as starch.

• Use chloroplasts to convert light energy into chemical energy.

Important plant cell structures

Cell wall

• Provides support and protection.

• Helps maintain the cell's shape.

• Prevents the cell from bursting when it takes in too much water.

Chloroplasts

• Contain chlorophyll, which absorbs light.

• Carry out photosynthesis.

• Convert light energy into chemical energy stored in sugars.

Central vacuole

• Stores water, nutrients, and waste.

• Helps maintain turgor pressure.

• Keeps the plant firm and upright.

2. Animal cells

Animal cells:

• Do not have a cell wall.

• Have a cell membrane.

• Do not have chloroplasts.

• Usually have smaller vacuoles or vesicles.

• Often have a more rounded or irregular shape.

• Have lysosomes that help digest and recycle materials.

• Commonly have centrosomes containing centrioles.

Important animal cell structures

Lysosomes

• Contain digestive enzymes.

• Break down waste and damaged cell parts.

• Recycle materials.

Centrioles

• Help organize microtubules.

• Assist in organizing the spindle during cell division in many animal cells.

Cell membrane

• Controls what enters and leaves.

• Provides a flexible boundary around the cell.

3. Plant vs. animal cell comparison chart

Important to memorize

• Plant cells have cell walls, chloroplasts, and usually a large central vacuole.

• Animal cells do not have cell walls or chloroplasts.

• Both have mitochondria, ribosomes, DNA, a nucleus, and a cell membrane.

Common test question: Do plant cells have mitochondria?

Answer: YES! Plant cells need mitochondria for cellular respiration, even though they also have chloroplasts for photosynthesis.

PART 15: Microfilaments and Microtubules

Both microfilaments and microtubules are parts of the cytoskeleton.

The cytoskeleton is a network of protein fibers inside the cell that provides support, shape, organization, and movement.

There are three main components of the cytoskeleton:

1. Microfilaments

2. Intermediate filaments

3. Microtubules

4. Microfilaments

Microfilaments are the thinnest fibers of the cytoskeleton.

• Made primarily of a protein called actin.

• Help maintain cell shape.

• Help cells move.

• Help muscles contract.

• Assist in cytokinesis.

Functions of microfilaments

1. Cell movement

• Help cells change shape and move.

• Important in cells that crawl or move across surfaces.

2. Muscle contraction

• Actin interacts with another protein called myosin.

• This interaction helps muscles contract.

3. Cell shape

• Help support the cell's outer shape.

• Especially important just underneath the cell membrane.

4. Cytokinesis

- Form a contractile ring that pinches an animal cell into two daughter cells.

Remember: Microfilaments = ACTIN = movement and contraction.

5. Microtubules

Microtubules are the largest fibers of the cytoskeleton.

• Made of proteins called tubulin.

• Have a hollow tube-like shape.

• Help maintain cell structure.

• Help transport materials inside cells.

• Form the mitotic spindle during cell division.

• Form the internal structure of cilia and flagella.

Functions of microtubules

1. Cell structure

• Help maintain the cell's shape.

• Provide internal support.

2. Transportation

- Act like tracks that motor proteins use to move vesicles and organelles around the cell.

3. Cell division

• Form spindle fibers.

• Help separate chromosomes during mitosis and meiosis.

4. Movement

• Form the internal framework of cilia and flagella.

• Help certain cells move or move substances across cell surfaces.

Remember: Microtubules = TUBULIN = transport and cell division.

6. Microfilaments vs. microtubules

What about intermediate filaments?

Don't forget the third type!

• Made of different fibrous proteins, such as keratins.

• Provide mechanical strength.

• Help cells resist stretching.

• Help hold organelles in place.

• Help maintain cell structure.

PART 16: Chromatin vs. Chromosomes

This is another VERY important comparison because both involve DNA.

7. What is chromatin?

Chromatin is DNA combined with proteins, mainly histones.

• Found in the nucleus of eukaryotic cells.

• Usually exists in a less condensed form during interphase.

• Allows DNA to be accessible for processes such as transcription and replication.

• Can condense to form visible chromosomes during cell division.

How is chromatin organized?

1. DNA wraps around proteins called histones.

2. This creates structures called nucleosomes.

3. Nucleosomes help organize and package DNA.

4. Chromatin can become more tightly condensed into chromosomes.

Important vocabulary:

• Histones = proteins that DNA wraps around.

• Nucleosomes = DNA wrapped around a group of histone proteins.

• Chromatin = DNA and associated proteins.

• Chromosomes = organized DNA-protein structures that become highly condensed during cell division.

8. What are chromosomes?

Chromosomes are organized structures made of DNA and proteins that carry genetic information.

• Contain many genes.

• Are made from chromatin.

• Become highly condensed during cell division.

• Help ensure DNA is accurately distributed into daughter cells.

Parts of a replicated chromosome

• Sister chromatids: Two identical copies of a replicated chromosome.

• Centromere: Region where sister chromatids are joined.

• Telomeres: Protective DNA-protein structures at the ends of linear chromosomes.

• Genes: DNA sequences located along chromosomes.

9. Chromatin vs. chromosomes comparison

Remember this!

Chromatin and chromosomes are NOT two completely different substances.

They are different organizational states of the same DNA and associated proteins.

Think of it like this:

• Chromatin = loosely packed thread.

• Chromosome = tightly packed thread.

10. How does chromatin become chromosomes?

1. DNA wraps around histones.

2. Nucleosomes form.

3. The chromatin becomes further organized and condensed.

4. During cell division, the chromosomes become highly condensed and visible under a microscope.

PART 17: Quick comparison and memorization sheet

These are the facts I would put on flashcards.

1. What different cell walls are made of

Different organisms have different cell wall materials. Here is how to tell them apart:

Plant cells

• Cell walls are made of cellulose.

• Provides strength and support.

• Helps the cell keep its shape.

Fungal cells

• Cell walls are made of chitin.

• Provides strength and protection.

Bacterial cells

• Cell walls are made of peptidoglycan (murein).

• Protects the cell and maintains its shape.

Algal cells

• Most have cell walls made mainly of cellulose.

• Some algae have other materials too.

Easy way to remember:

Note: Animal cells do not have cell walls!

2. Chromosomes vs Chromatin

Both chromosomes and chromatin are made of DNA wrapped around proteins. The main difference is how tightly the DNA is packed.

Chromatin

• DNA is loosely packed.

• Looks like thin, tangled threads.

• Found when the cell is not dividing.

• Allows DNA to be read and used.

Chromosomes

• DNA is tightly coiled and condensed.

• Can be seen clearly during cell division.

• Helps DNA separate correctly into new cells.

• Contains genes.

Main differences

Remember: Chromatin and chromosomes are made of the same material. They are just different forms of DNA packaging!

Think of it like this:

• Chromatin = a loosely wound ball of wool.

• Chromosome = the wool wound up tightly into a neat bundle.

🧬 Biology Study Guide: Cells and Life

1. The nucleus and its parts

Nucleus

• The control center of the cell.

• Stores the cell's DNA.

• Controls cell activities by regulating gene expression.

Nuclear envelope

• A double membrane surrounding the nucleus.

• Separates the contents of the nucleus from the cytoplasm.

• Protects the DNA.

Nuclear pores

• Small openings in the nuclear envelope.

• Control what enters and leaves the nucleus.

• Allow materials such as RNA and proteins to move through.

Nucleolus

• A dense area inside the nucleus.

• Makes ribosomal RNA (rRNA).

• Helps assemble ribosome subunits.

How many nucleoli can a cell have?

• A cell can have one or several nucleoli.

• The number depends on the cell type and its activity.

• Some cells have multiple nucleoli, and some may have none visible at a particular time.

2. Cell size and microns

Cell size

• Most cells are microscopic, meaning they cannot be seen clearly with the naked eye.

• Cells need to stay small enough to exchange materials efficiently.

• Smaller cells generally have a higher surface-area-to-volume ratio.

Why does cell size matter?

Cells need to take in nutrients and oxygen and remove waste.

As a cell gets bigger:

• Its volume increases faster than its surface area.

• It becomes harder to move materials in and out quickly enough.

Microns (micrometers)

• A micron is another name for a micrometer.

• Symbol: μm.

• One micrometer = one-millionth of a meter.

• Most animal and plant cells are measured in micrometers.

Important conversions:

• 1 mm = 1,000 μm.

• 1 μm = 1,000 nm.

3. Cilia and flagella

Both are structures that help cells move or move substances around them.

Cilia

• Short, hair-like structures.

• Usually found in large numbers.

• Move substances across the cell surface.

• Example: Cilia in the human respiratory tract move mucus and trapped dust out of the airways.

Flagella

• Longer, whip-like structures.

• Usually one or a few per cell.

• Help the entire cell move.

• Example: The tail of a sperm cell helps it swim.

4. Contractile vacuoles

• Found in some freshwater single-celled organisms.

• Collect excess water inside the cell.

• Pump the extra water out of the cell.

• Prevent the cell from swelling and bursting.

Remember: Contractile vacuoles help with water balance (osmoregulation).

5. Chloroplasts: thylakoids, grana and stroma

Chloroplasts are organelles found in plants and algae. They carry out photosynthesis.

Thylakoids

• Flattened, membrane-bound sacs inside chloroplasts.

• Contain chlorophyll, the green pigment that absorbs light.

• Where the light-dependent reactions of photosynthesis occur.

Grana (singular: granum)

• Stacks of thylakoids.

• Increase the surface area available for light-dependent reactions.

Stroma

• Fluid-filled space surrounding the thylakoids.

• Where the Calvin cycle takes place.

• Contains enzymes, DNA and ribosomes.

Easy way to remember:

• Thylakoids = individual discs.

• Grana = stacks of discs.

• Stroma = fluid surrounding the stacks.

6. Golgi body: cis and trans sides

The Golgi apparatus modifies, sorts and packages proteins and lipids so they can be transported where needed.

Cis side

• The receiving side of the Golgi.

• Receives transport vesicles from the endoplasmic reticulum (ER).

• Think: CIS = Comes In Side.

Trans side

• The shipping side of the Golgi.

• Packages materials into vesicles.

• Sends them to their final destinations.

7. The endomembrane system

The endomembrane system is a group of membranes and organelles that work together to make, modify, package and transport materials.

Main parts include:

• Nuclear envelope

• Endoplasmic reticulum (ER)

• Golgi apparatus

• Lysosomes

• Vesicles

• Plasma membrane

How it works:

1. Ribosomes on the rough ER make proteins.

2. The rough ER helps fold and transport proteins.

3. Vesicles carry proteins to the Golgi apparatus.

4. The Golgi modifies and sorts the proteins.

5. New vesicles transport them to their destinations.

Remember: The endomembrane system is like a factory and shipping center inside the cell.

8. The mitotic spindle

The mitotic spindle is a structure made of microtubules that helps separate chromosomes during cell division.

Its functions:

• Attaches to chromosomes at their kinetochores.

• Helps line chromosomes up in the middle of the cell.

• Pulls sister chromatids toward opposite sides of the cell.

• Makes sure each new cell receives the correct set of chromosomes.

Remember: The mitotic spindle helps divide genetic material equally between two daughter cells.

9. Centrioles

• Centrioles are small cylindrical structures.

• Usually found in pairs in animal cells.

• Located inside an area called the centrosome.

• Help organize microtubules.

• Help organize the mitotic spindle during cell division.

Important facts:

• Each centriole is made of 9 groups of microtubule triplets.

• The two centrioles are usually arranged at right angles to each other.

• Most higher plant cells do not have centrioles.

Centrioles vs. centrosomes:

• Centrioles = the small cylindrical structures.

• Centrosome = the main microtubule-organizing center containing the centriole pair in typical animal cells.

10. Microfilaments and microtubules

These are both parts of the cytoskeleton.

The cytoskeleton is a network of protein fibers that gives the cell shape, support and movement.

Microfilaments

• Made of a protein called actin.

• The thinnest cytoskeleton fibers.

• Help cells move and change shape.

• Help muscles contract.

• Help the cell pinch in two during cytokinesis.

Microtubules

• Made of a protein called tubulin.

• Hollow, tube-shaped structures.

• Help maintain cell shape.

• Form the mitotic spindle.

• Make up the internal structures of cilia and flagella.

• Help transport materials around the cell.

11. Genes

• A gene is a section of DNA.

• Genes contain instructions for making functional products, usually proteins or RNA.

• Genes help determine inherited characteristics.

• Genes are found on chromosomes.

Examples of characteristics influenced by genes:

• Eye color

• Hair characteristics

• Blood type

Remember: DNA makes up genes, and genes are located on chromosomes.

12. Chromatin vs. chromosomes

Both are made of DNA wrapped around proteins called histones.

Chromatin

• DNA in a loose, uncondensed form.

• Looks like thin, tangled threads.

• Allows the cell to access DNA to read genes.

• Most common when the cell is not dividing.

Chromosomes

• DNA that has become tightly condensed.

• Easier to see during cell division.

• Helps genetic material separate correctly into new cells.

Important: Chromatin and chromosomes are not two different substances. They are two forms of the same DNA-containing material.

13. Glycoproteins

• Glycoproteins are proteins with carbohydrate (sugar) chains attached.

• Found on the surface of cell membranes.

• Help cells recognize each other.

• Help with cell-to-cell communication.

• Can act as receptors for chemical signals.

Example: Glycoproteins help your immune system recognize cells as belonging to your body.

14. Phospholipids

Phospholipids are the main building blocks of cell membranes.

Each phospholipid has two parts:

• Hydrophilic head: Attracted to water.

• Hydrophobic tails: Repelled by water.

They form a phospholipid bilayer (two layers).

Why are they important?

• Form the cell membrane.

• Separate the inside of the cell from the outside.

• Help control what enters and leaves the cell.

Remember:

• HydroPHILIC = loves water.

• HydroPHOBIC = fears or avoids water.

15. Living and nonliving things in a cell

This is an important distinction!

Not everything inside a cell is considered alive.

Important clarification: Organelles such as mitochondria and chloroplasts are not considered independent living organisms in modern cells. They are parts of the cell, even though they have some features associated with independent life.

Viruses are also generally not considered living because they cannot reproduce independently.

16. Specialized plant cells

Specialized cells are cells that have adapted to perform a particular job.

Root hair cells

• Absorb water and minerals from soil.

• Have a long extension to increase surface area.

Palisade cells

• Found in leaves.

• Contain many chloroplasts.

• Carry out photosynthesis.

Xylem cells

• Transport water and minerals from roots to the rest of the plant.

• Have thick, lignified walls.

• Mature vessel elements are dead.

Phloem cells

• Transport sugars around the plant.

• Sieve tube elements work with companion cells.

Guard cells

• Control the opening and closing of stomata.

• Help regulate gas exchange and water loss.

17. Specialized animal cells

Red blood cells

• Carry oxygen around the body.

• Contain hemoglobin.

• Have a biconcave shape to increase surface area.

• Mature human red blood cells have no nucleus.

Nerve cells (neurons)

• Carry electrical signals.

• Have long extensions called axons.

• Communicate with other cells.

Sperm cells

• Fertilize the egg cell.

• Have a flagellum for movement.

• Have many mitochondria in the middle piece for energy.

Egg cells (ova)

• Female reproductive cells.

• Contain nutrients for early development.

• Have a haploid set of chromosomes.

Muscle cells

• Contract and relax to produce movement.

• Contain actin and myosin proteins.

• Need lots of energy from ATP.

18. Endosymbiotic theory

The endosymbiotic theory explains how mitochondria and chloroplasts may have originated from ancient bacteria.

The theory says:

1. An ancient larger cell engulfed a smaller bacterium.

2. Instead of digesting it, the larger cell kept it inside.

3. The smaller bacterium provided useful benefits to the larger cell.

4. Over millions of years, they became permanent parts of the cell.

Evidence supporting the theory:

Mitochondria and chloroplasts:

• Have their own circular DNA.

• Have their own ribosomes.

• Have double membranes.

• Reproduce by a process similar to bacterial binary fission.

• Are similar in size to some bacteria.

Remember: Mitochondria are thought to have originated from aerobic bacteria, while chloroplasts are thought to have originated from photosynthetic bacteria called cyanobacteria.

19. The beginning of cell theory

Cell theory explains the basic principles of cells and living organisms.

Important scientists and discoveries:

Who first found cells?

Robert Hooke in 1665. He looked at a thin piece of cork through a microscope and saw tiny compartments that reminded him of small rooms, which he called cells.

Who first saw cells dividing?

Walther Flemming is particularly known for the first detailed description of mitosis and chromosome behavior during cell division. Earlier scientists had also studied cell division.

20. The three main parts of cell theory

1. All living organisms are made of one or more cells.

2. The cell is the basic unit of structure and function in living things.

3. All cells come from pre-existing cells.

Memorize these three statements! They are very commonly tested.

21. Levels of organization

Living things are organized from the smallest level to the largest.

Learn more

Here is the order:

1. Cell: The smallest living unit.

2. Tissue: A group of similar cells working together.

3. Organ: Different tissues working together.

4. Organ system: Different organs working together.

5. Organism: A complete living thing.

Example:

- Muscle cell → muscle tissue → heart → circulatory system → human.

Other levels you might learn:

• Atom

• Molecule

• Organelle

• Cell

• Tissue

• Organ

• Organ system

• Organism

• Population

• Community

• Ecosystem

• Biosphere

22. Main characteristics of life

Living things share several important characteristics.

A common way to remember them is MRS GREN:

Other important characteristics include:

• Homeostasis: Maintaining stable internal conditions.

• Organization: Having an organized structure.

• Adaptation: Having inherited features that help organisms survive and reproduce over generations.

Important: Respiration does NOT just mean breathing. Cellular respiration is the chemical process of releasing energy from nutrients.

23. Super important facts to memorize

These are some of the most testable facts from all your topics.