The Cell System

The Cell System

Middle School

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Learning Objectives

• Define the three main principles that make up the Cell Theory.

• Compare and contrast the main features of prokaryotic and eukaryotic cells.

• Explain how an organelle’s structure is related to its specific job.

• Describe how cells use passive and active transport to move materials.

Key Vocabulary

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Organelle

A specialized structure within a cell that performs a specific, essential job for the cell.

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Eukaryote

A complex cell type that contains a nucleus and other membrane-bound organelles to function properly.

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Prokaryote

A simple cell type that lacks a nucleus or other membrane-bound organelles, unlike eukaryotes.

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Cell Membrane

The thin, flexible barrier that surrounds a cell and carefully controls what enters and leaves.

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Homeostasis

The process by which a cell or organism maintains a stable and balanced internal environment.

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Passive Transport

The movement of substances across the cell membrane that does not require any cellular energy.

Key Vocabulary

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Active Transport

The movement of substances across the cell membrane against a concentration gradient, requiring energy.

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Mitosis

The stage of the cell cycle where the nucleus divides into two identical nuclei.

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Nucleus

The organelle that acts as the cell's command center, containing its genetic material.

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Mitochondrion

The organelle responsible for breaking down food to release usable energy for the cell.

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The Two Main Types of Cells

Prokaryotic Cells

• These simple cells do not have a nucleus or other membrane-bound organelles.

• Their genetic material, or DNA, is located in a region called the nucleoid.

• Prokaryotic organisms, like bacteria, are typically very small and single-celled.

Eukaryotic Cells

• These complex cells have a true nucleus that encloses the cell's DNA.

• They contain various membrane-bound structures called organelles, each with a specific job.

• Eukaryotic organisms include plants, animals, fungi, and protists, and can be multicellular.

Multiple Choice

What is the main structural difference that distinguishes prokaryotic cells from eukaryotic cells?

• The presence or absence of a nucleus.

• The presence or absence of genetic material.

• Whether they are small or large.

• Whether they belong to plants or animals.

Multiple Choice

How does the location of genetic material differ between prokaryotic and eukaryotic cells?

• A prokaryotic cell has its DNA in the nucleoid, while a eukaryotic cell has its DNA in the nucleus.

• A prokaryotic cell has no DNA, while a eukaryotic cell has DNA.

• A prokaryotic cell's DNA is in its organelles, while a eukaryotic cell's DNA is in the nucleus.

• Both cell types have their DNA contained within a nucleus.

Multiple Choice

A scientist discovers a new single-celled organism. Under a microscope, it is observed to be a simple cell that lacks a nucleus and other membrane-bound organelles. What is the most logical conclusion about this organism?

• It is a prokaryotic organism because it lacks a nucleus and membrane-bound organelles.

• It is a eukaryotic organism because it is single-celled, like a protist.

• It is an animal cell because it does not have a cell wall.

• It is a plant cell because it is a simple, single-celled organism.

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Command Center & Protein Production

• The nucleus acts as the cell's command center, protecting the cell's DNA.

• ​The nucleolus, located inside the nucleus, is where ribosome parts are produced.

• Ribosomes build proteins that are modified by the Endoplasmic Reticulum (ER).

• The Golgi apparatus then sorts, packages, and ships proteins in vesicles.

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Multiple Choice

What is the primary function of the cell's nucleus?

• To produce energy for the cell's activities

• To sort, package, and ship proteins in vesicles

• To act as the cell's command center and protect its DNA

• To build proteins that are modified by the ER

Multiple Choice

Which of the following describes the correct pathway for making and shipping a protein?

• Golgi apparatus, Endoplasmic Reticulum, Ribosome, Nucleolus

• Nucleolus, Ribosome, Endoplasmic Reticulum, Golgi apparatus

• Ribosome, Golgi apparatus, Nucleolus, Endoplasmic Reticulum

• Endoplasmic Reticulum, Nucleolus, Ribosome, Golgi apparatus

Multiple Choice

If a cell's Golgi apparatus were to stop functioning, what would be the most likely outcome?

• The cell's DNA would no longer be protected from damage.

• Ribosome parts would no longer be produced in the nucleolus.

• Proteins would be built, but they would not be modified by the ER.

• Proteins would accumulate in the cell because they could not be packaged and shipped.

Energy, Storage, and Cleanup

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Mitochondria

• ​Often called the "powerhouse" of the cell, they perform cellular respiration.

• ​​They break down food molecules to release energy in the form of ATP.

• ​Its inner folds, called cristae, provide a large surface for energy production.

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Chloroplasts

• ​Found only in plant cells, they are where photosynthesis happens.

• ​​They capture sunlight to convert it into chemical energy or sugars.

• ​They contain a green pigment called chlorophyll, which absorbs the sunlight.

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Storage and Cleanup

• ​Vacuoles are sacs that are used to store water, food, and waste.

• ​​Plant cells have a large central vacuole that provides structural support.

• ​Lysosomes contain digestive enzymes to break down and recycle old cell parts.

Multiple Choice

What is the primary role of mitochondria in a cell?

• To break down food molecules to release energy.

• To capture sunlight to produce sugars.

• To store water, food, and waste materials.

• To provide structural support to the cell.

Multiple Choice

Which statement best compares the primary functions of chloroplasts and mitochondria?

• Chloroplasts capture energy to make food, while mitochondria release energy from food.

• Chloroplasts provide structural support, while mitochondria store water.

• Chloroplasts are only in animal cells, while mitochondria are only in plant cells.

• Chloroplasts break down waste, while mitochondria perform photosynthesis.

Multiple Choice

A scientist observes a plant cell that is wilting due to water loss and is also accumulating old, worn-out parts. Which two organelles are most likely malfunctioning?

• The large central vacuole and the lysosomes.

• The mitochondria and the chloroplasts.

• The cristae and the chlorophyll.

• The lysosomes and the mitochondria.

Cell Boundary and Support

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Cell Membrane

• ​Controls what enters and leaves the cell to maintain balance.

• ​​It is a flexible barrier made of a phospholipid bilayer.

• ​Proteins act as channels and pumps for moving materials.

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Cytoskeleton

• ​A network of protein fibers acts as the cell’s internal framework.

• ​​Provides structural support and helps the cell to maintain its shape.

• ​Forms tracks for moving materials and anchoring cell organelles.

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Cell Wall

• ​A rigid outer layer found in plant cells, fungi, and bacteria.

• ​​Gives additional support, protection, and helps maintain a fixed shape.

• ​In plants, this strong wall allows them to grow upright.

Multiple Choice

What is the main purpose of the cell's boundary and support structures as a whole?

• To provide structural shape and control what passes into and out of the cell.

• To create energy and store the cell's genetic information.

• To help the cell move and digest food particles.

• To produce proteins and transport them throughout the cell.

Multiple Choice

How does the structure and function of a cell wall differ from that of a cell membrane?

• The cell wall is a rigid layer for support, while the cell membrane is a flexible barrier that controls transport.

• The cell wall helps the cell move, while the cell membrane is the cell's internal framework.

• The cell wall is found inside the cell membrane, while the cytoskeleton is on the outside.

• The cell wall allows plants to grow upright, while the cytoskeleton is only found in animal cells.

Multiple Choice

If a plant cell lost its ability to stand upright and maintain a fixed, rigid shape, which structure would a scientist predict has been damaged?

• The cell wall, because it provides the rigid structure needed for a fixed shape.

• The cell membrane, because it is responsible for moving materials in and out.

• The cytoskeleton, because it forms tracks for moving organelles.

• The protein channels, because they are responsible for maintaining balance.

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Plant vs. Animal Cells

Plant Cells

• Plant cells have a rigid cell wall outside the cell membrane for structural support.

• They contain chloroplasts, which are the sites of photosynthesis for making their own food.

• A large central vacuole stores water and helps the cell to maintain its firmness.

Animal Cells

• Animal cells lack a cell wall, allowing them to have much more varied shapes.

• They do not have chloroplasts because animals get energy by eating other organisms.

• Animal cells may have small vacuoles and lysosomes, which help break down cellular waste.

Multiple Choice

Which structure provides rigid support to a plant cell but is absent in an animal cell?

• A cell wall

• A cell membrane

• Small vacuoles

• Lysosomes

Multiple Choice

What is the primary reason that plant cells have chloroplasts and animal cells do not?

• Plants make their own food, while animals get energy by eating other organisms.

• Plants have rigid shapes, while animals have more varied shapes.

• Plants store water in a large vacuole, while animals use small vacuoles.

• Plants require structural support, while animals need to break down waste.

Multiple Choice

A scientist observes a cell that has a rigid cell wall and a large central vacuole. What is the most logical conclusion about how this cell obtains energy?

• It most likely makes its own food using chloroplasts.

• It most likely has a varied shape to move around.

• It most likely gets energy by eating other organisms.

• It most likely uses lysosomes to break down waste.

Passive Transport: No Energy Needed

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Simple Diffusion

• Small molecules like O₂ and CO₂ move directly across the membrane.

• This movement occurs from a high to a low concentration area.

• It does not require the cell to use any form of energy.

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Osmosis

• Osmosis is the specific diffusion of water molecules across a membrane.

• Water moves to an area with a higher concentration of solutes.

• This process is crucial for maintaining the cell's water balance.

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Facilitated Diffusion

• Larger molecules like glucose get help from special transport proteins.

• These proteins form a channel or carry the molecules across.

• No cellular energy is used in this form of passive transport.

Multiple Choice

What is the main characteristic of all forms of passive transport?

• They do not require the cell to use energy.

• They only move water molecules across the membrane.

• They require help from special transport proteins.

• They move molecules from low to high concentration.

Multiple Choice

How does the movement of molecules in simple diffusion differ from their movement in facilitated diffusion?

• Facilitated diffusion uses transport proteins for larger molecules, while simple diffusion allows small molecules to pass directly through the membrane.

• Simple diffusion moves molecules from high to low concentration, while facilitated diffusion moves them from low to high.

• Simple diffusion is for water only, while facilitated diffusion is for gases like O₂ and CO₂.

• Facilitated diffusion requires cellular energy, while simple diffusion does not.

Multiple Choice

If a cell is placed in a solution that has a much higher concentration of solutes than the inside of the cell, what is the most likely outcome due to osmosis?

• Water will move out of the cell toward the higher concentration of solutes.

• Water will move into the cell, away from the higher concentration of solutes.

• The solutes will move into the cell using transport proteins.

• The cell's water balance will not be affected by the solution.

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Active and Bulk Transport

Active Transport

• ​Cells move materials from an area of low concentration to an area of high concentration.

• ​​This process requires the cell to use energy, which is supplied in the form of ATP.

• ​Special protein pumps in the cell membrane bind to molecules and push them across the membrane.

Bulk Transport

• ​This is how cells move very large particles or large amounts of small particles across the membrane.

• ​​Endocytosis is the process of bringing materials into the cell by pinching the membrane to form a vesicle.

• ​Exocytosis is the opposite, where a vesicle fuses with the membrane to release its contents out of the cell.

Multiple Choice

What is a key characteristic of active transport?

• It requires the cell to use energy.

• It moves materials from high to low concentration.

• It only moves very large particles.

• It involves the cell membrane pinching to form a vesicle.

Multiple Choice

What is the primary difference between endocytosis and exocytosis?

• Endocytosis brings materials in, while exocytosis releases them out.

• Endocytosis releases materials out, while exocytosis brings them in.

• Endocytosis uses protein pumps, while exocytosis uses vesicles.

• Endocytosis moves small particles, while exocytosis moves large ones.

Multiple Choice

A cell produces a large protein that needs to be exported. Which transport mechanism is the most likely to be used and why?

• Exocytosis, because it is used to release large particles from the cell.

• Endocytosis, because the cell needs to bring the particle in first.

• Active transport, because removing waste requires energy.

• A protein pump, because it can push the particle across the membrane.

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The Cell Cycle and Division

• The cell cycle is the process of a cell growing and dividing.

• In Interphase, the cell grows, functions, and duplicates its DNA for division.

• Mitosis divides the nucleus in four phases: Prophase, Metaphase, Anaphase, and Telophase.

• Cytokinesis splits the cell, creating two new identical daughter cells.

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Multiple Choice

What is the cell cycle?

• The process of a cell growing and dividing.

• The process of a cell shrinking and combining.

• The process of a cell creating energy.

• The process of a cell moving to a new location.

Multiple Choice

What is the role of Interphase in the cell cycle?

• It divides the cell's nucleus into four parts.

• It splits the cell into two identical daughter cells.

• It prepares the cell for division by duplicating its DNA.

• It causes the cell to stop growing permanently.

Multiple Choice

What would be the most likely result if a cell completed mitosis but failed to undergo cytokinesis?

• Two new cells with no nuclei.

• A single cell with two nuclei.

• Four new cells with half the DNA.

• A single cell that is identical to the parent.

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Specialized Cells & Organization

Specialized Cells

• In multicellular organisms, different cells are specialized to perform very specific jobs.

• A cell's structure is always adapted to its function, helping it perform its job well.

• Neurons have long axons to send signals and red blood cells are disc-shaped to carry oxygen.

Levels of Organization

• Similar cells that work together to perform a specific job form a tissue, like muscle tissue.

• Different tissues combine to form an organ, such as the heart, which performs a complex function.

• Organs work in organ systems, and all systems make up a complete, functioning organism.

Multiple Choice

What is the main characteristic of cells within a multicellular organism?

• They are all identical in structure and function.

• They are specialized to perform different, specific jobs.

• They can each survive independently outside the organism.

• They are larger than the cells of single-celled organisms.

Multiple Choice

How is a tissue different from an organ in the levels of organization?

• A tissue is a complete organism, while an organ is only one part.

• A tissue is made of similar cells, while an organ is made of different tissues working together.

• A tissue's job is more complex than an organ's job.

• An organ is made of one type of cell, while a tissue is made of many.

Multiple Choice

A neuron has long, thin axons to send signals, while a red blood cell is disc-shaped to carry oxygen. What conclusion do these examples support?

• All cells in an organ system have the exact same structure.

• The structure of a cell is designed to help it perform its specific function.

• A cell's function is determined by the organ system it is in, not its structure.

• The main purpose of a cell's structure is to store nutrients.

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Common Misconceptions

Misconception	Correction
All cells are the same.	Cells are diverse with specialized structures for different functions.
Mitosis is the entire life of a cell.	Mitosis is just the short nucleus-division phase of the cell cycle.
Materials can freely pass through the cell membrane.	The membrane is selectively permeable, controlling what enters and exits the cell.

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Summary

• All living things are made of prokaryotic or eukaryotic cells.

• An organelle’s structure determines its function, like making proteins.

• The cell membrane controls what enters and exits the cell.

• Cells divide for growth and specialize to form tissues and organs.

Poll

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