Cell Structure and Function Chapter Notes | Preparation for EmSAT Grade 6 PDF Download

Introduction

Cells are the tiny building blocks of all living things. They are so small that we need a microscope to see them, but they do very important jobs to keep organisms alive. In this chapter, we will learn about the different parts of a cell, what they do, and how they work together to help the cell function. We will explore what surrounds a cell, what powers it, what controls it, and how plant and animal cells are different. By understanding cells, we can see how life works at its smallest level!

What surrounds a cell?

All cells have some structures that protect them and help them work properly. These structures keep the inside of the cell safe and control what goes in and out.

Cell Membrane

• All cells have a protective layer called the cell membrane.
• The cell membrane is a flexible covering that keeps the inside of the cell safe from the outside environment.
• The cell membrane is semipermeable, which means it only lets certain things, like nutrients and waste, go in and out of the cell.
• The cell membrane surrounds the cytoplasm, which is a fluid inside the cell that contains salts and other molecules.

Cell Wall

• Some cells, like plant cells, fungal cells, bacteria, and some protists, have a cell wall.
• The cell wall is a stiff structure outside the cell membrane that protects the cell from viruses and harmful organisms.
• In plant and fungal cells, the cell wall helps keep the cell’s shape and gives it support.

How does cell size affect the transport of materials?

• Nutrients, oxygen, and other materials move in and out of a cell through the cell membrane.
• The size of a cell affects how well it can move these materials.

Did You Know?

• A ratio compares two numbers, like surface area and volume.
• For a cube-shaped cell, surface area is calculated by: Surface Area = length (ℓ) × width (w) × 6 (because a cube has 6 sides).
• The volume of the cell is found by: Volume = length (ℓ) × width (w) × height (h).
• To get the surface-area-to-volume ratio, divide surface area by volume: Ratio = Surface area ÷ Volume.

Surface Area and Volume 

• The movement of nutrients, waste materials, and other substances into and out of a cell is essential for the cell’s survival.
• For this movement to be effective, the cell membrane’s surface area must be large in comparison to the cell’s volume.
• The surface area refers to the total area of the cell membrane, while the volume is the space inside the cell.
• As the cell grows, both its volume and surface area increase.
• However, the volume increases faster than the surface area as the cell grows.
• A larger cell would require more nutrients and would generate more waste.
• But, the cell membrane’s surface area would be too small to move all the necessary nutrients in and waste out efficiently.
• Therefore, if a cell continues to grow too large, it may not be able to move enough substances across the membrane to stay alive.

What organelles are involved in the transport of materials?

The cell membrane helps materials enter and leave the cell.

• Ribosomes are small structures that make proteins, which are made from amino acids (molecules with carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur).
  • Proteins help cells communicate with each other and move substances inside the cell.
  • Ribosomes are not surrounded by a membrane, unlike other organelles.
• The endoplasmic reticulum (ER) is a web-like organelle that spreads from the nucleus through the cytoplasm.
  • Rough ER has ribosomes on its surface and is where proteins are made.
  • Smooth ER does not have ribosomes and helps remove harmful substances from the cell.
• Vacuoles are sac-like structures that store food, water, and waste.
  • Plant cells usually have one large vacuole, while animal cells have many small ones.
  • A plant cell’s large vacuole stores water and helps the plant stay rigid and supported when filled with water.
• The Golgi apparatus prepares proteins for their specific jobs and packages them into vesicles.
  • Vesicles are tiny, membrane-bound structures that transport substances within the cell.
  • Lysosomes are vesicles in animal cells that contain substances to break down and recycle cell parts.

What powers cellular activity?

Cells need energy to do their jobs, like transporting materials.

• Mitochondria are bean-shaped organelles that power the cell through chemical reactions.
  • Mitochondria have two membranes to increase the surface area for these reactions.
  • Mitochondria are found in both plant and animal cells and are important for cellular respiration.
• Cellular respiration is a process that converts energy from food molecules into a usable form called ATP.
  • Plant cells also have chloroplasts, which use light energy to make food (a sugar called glucose) from water and carbon dioxide.

What controls all of this activity?

• The nucleus is the largest organelle in most eukaryotic cells and controls all cell activities.
• The nucleus contains DNA, which is organized into structures called chromosomes.
• DNA has instructions for making all the proteins a cell needs.
• The nucleus also contains proteins and an organelle called the nucleolus.
• The nucleolus makes ribosomes, which are needed to produce proteins.
• The nucleus directs protein synthesis, which is important for almost every function in the body.

What is the difference between plant and animal cells?

• Plant cells have a cell wall, but animal cells do not.
• Plant cells usually have one large vacuole, while animal cells have many small vacuoles.
• Plant cells contain chloroplasts for making food using light energy, but animal cells do not have chloroplasts.
• Both plant and animal cells have a cell membrane, cytoplasm, nucleus, mitochondria, ribosomes, endoplasmic reticulum, Golgi apparatus, and vesicles.

What can different cells do?

• All living things are made of one or more cells.
• In multicellular organisms, different cells have different jobs to help the organism survive.
• A cell’s shape is related to its function.

Types of Cells and Structures 

• Cells in the body are very diverse in their structure and function.
• Red blood cells are disk-shaped, which allows them to move easily through blood vessels and carry oxygen throughout the body.
• Xylem cells are tube-shaped and help transport water from a plant’s roots to its leaves.
• Neurons are cells found in many animals that transmit impulses between different parts of the body.
• Although each type of cell is unique, all cells work together to help carry out the functions of the body.

A Closer Look: Vivacious Vorticella

• Vorticella is a unicellular organism (protozoan) found in freshwater ponds and lakes.
• It has everything it needs to survive in one cell.
• Vorticella has hair-like structures called cilia that create water currents to bring food toward the cell.
• It has a spiral stalk that helps it attach to surfaces.
• Vorticella was first discovered by a Dutch scientist named Anton van Leeuwenhoek, who thought its mouth parts were horns.
• Scientists have found that Vorticella can bind to mosquito larvae and stop them from growing, helping control pests.
• Vorticella is small but has all the structures it needs to survive and thrive.