Plant & Animal Cells (Leaving Cert Agricultural Science): Revision Notes
Plant & Animal Cells
Introduction to cell structure
Cells are the basic building blocks of all living organisms. Understanding the structure and function of plant and animal cells is essential for agricultural science, as it helps explain how organisms grow, reproduce, and respond to their environment. While plant and animal cells share many similarities, they also have distinct differences that reflect their unique roles and functions.
Both plant and animal cells are eukaryotic cells, meaning they contain a nucleus and various membrane-bound structures called organelles. Each organelle has a specific function that contributes to the cell's overall survival and operation.
Cell organelles and their functions
Cell membrane
The cell membrane acts as a selective barrier that surrounds every cell. This structure is semi-permeable, which means it carefully controls what substances can enter or leave the cell. This selective permeability is crucial for maintaining the cell's internal environment.
The membrane is constructed from two main components: phospholipids and proteins. Each phospholipid molecule has a unique structure with a water-attracting phosphate head and water-repelling lipid tails. This arrangement allows the phosphate groups to face outward towards water-based environments, while the lipid portions remain protected inside the membrane.
The semi-permeable nature of cell membranes is fundamental to all cellular processes. Without this selective barrier, cells would not be able to maintain the specific internal conditions necessary for life.
Nucleus and DNA
The nucleus serves as the cell's control centre and contains the cell's genetic material. It is enclosed by a double membrane that contains nuclear pores, which allow certain materials to pass between the nucleus and the rest of the cell.
Inside the nucleus, you'll find DNA (Deoxyribonucleic Acid), which is organised into structures called chromosomes. Each chromosome consists of DNA combined with proteins. Humans have 46 chromosomes in total. Genes are specific sections located on chromosomes that act as units of inheritance and determine our genetic characteristics.
When chromosomes are not actively dividing, they exist in an extended, interwoven form called chromatin. The nucleus also contains a structure called the nucleolus, which plays a key role in making ribosomes.
The nucleus is often called the "control centre" of the cell because it contains all the genetic instructions (DNA) that determine what the cell will do and how it will function throughout its life.
Mitochondria
Mitochondria are often called the "powerhouses" of the cell because they generate energy for cellular activities. These organelles are surrounded by a double membrane and are particularly abundant in cells that require lots of energy, such as muscle cells and liver cells in animals.
Each mitochondrion contains its own loop of DNA, which is separate from the DNA found in the nucleus. This feature supports the theory that mitochondria were once independent organisms that formed a partnership with early cells.
Ribosomes
Ribosomes are the protein-making factories of the cell. These small structures are composed of RNA (Ribonucleic Acid) combined with proteins. They can be found either floating freely in the cytoplasm or attached to other cellular structures, depending on the type of protein they are producing.
Cytoplasm
Cytoplasm is a jelly-like substance that fills the cell and suspends all the organelles. It provides a medium for chemical reactions to occur and helps maintain the cell's shape. The term cytosol refers specifically to the cytoplasm without the organelles suspended within it.
Cell wall (plants only)
Plant cells have an additional protective structure that animal cells lack: the cell wall. This rigid structure is made primarily of a carbohydrate called cellulose, which provides strength and structural support to the plant.
Unlike the cell membrane, the cell wall is fully permeable, allowing all substances to pass through it. The cell wall serves several important functions: it prevents the plant cell from bursting when water enters through osmosis, and it requires the mineral calcium to maintain a healthy structure.
Remember that only plant cells have cell walls - this is one of the key differences between plant and animal cells. The cell wall provides structural support that allows plants to grow tall and maintain their shape without a skeleton.
Vacuole
The vacuole is a storage organelle that serves multiple functions. It provides strength and shape to the cell while storing water, nutrients (such as sugars and amino acids), ions, and metabolic waste products.
In plant cells, the vacuole is typically much larger than in animal cells. This large central vacuole is essential for maintaining the plant's structure and storing the materials needed for growth and metabolism.
Chloroplasts (plants only)
Chloroplasts are unique to plant cells and are the sites where photosynthesis occurs. These organelles contain a green pigment called chlorophyll, which captures light energy and converts it into chemical energy.
Chloroplasts are found primarily in the leaves of plants, where they have the best access to sunlight. The process of photosynthesis will be examined in greater detail in later chapters on plant metabolism.
Like cell walls, chloroplasts are found only in plant cells. This is why plants can make their own food through photosynthesis while animals must consume other organisms for energy.
Cell classification: prokaryotic vs eukaryotic
All organisms can be classified into two main categories based on their cellular structure and complexity.
Understanding the difference between prokaryotic and eukaryotic cells is fundamental to biology. This classification affects how organisms function, reproduce, and evolve.
Prokaryotic cells are simpler in structure and do not contain a nucleus or membrane-bound organelles. Instead, their genetic material exists as a circular loop of DNA that floats freely within the cell. These cells are always unicellular, and mitosis and meiosis do not occur because there is no nucleus to divide. Bacteria are the most common example of prokaryotic organisms.
Eukaryotic cells are more complex and advanced than prokaryotes. They contain a well-defined nucleus and various membrane-bound organelles. Examples of eukaryotic organisms include animals, plants, and fungi.
Organism classification: unicellular vs multicellular
Organisms can also be classified based on their cellular organisation:
- Unicellular organisms consist of just a single cell that carries out all life functions. Examples include bacteria, yeast, and amoeba.
- Multicellular organisms are composed of many cells working together, with different cells often specialising in particular functions. Plants and animals are examples of multicellular organisms.
Movement of substances across membranes
Cells must constantly exchange materials with their environment to survive. Several different mechanisms facilitate this movement.
Diffusion
Diffusion is the movement of substances from areas of high concentration to areas of low concentration. This is a passive process that requires no energy input from the cell. Common examples include carbon dioxide diffusing into leaves and oxygen diffusing out of leaves during gas exchange.
Osmosis
Osmosis is a special type of diffusion that specifically involves the movement of water molecules across a semi-permeable membrane. Water moves from regions with high water concentration to regions with low water concentration. Like diffusion, osmosis is a passive process.
A practical example of osmosis is water entering plant roots from the soil, where the concentration of water is typically higher than inside the root cells.
Active transport
Active transport is the third type of substance movement across cell membranes. Unlike diffusion and osmosis, this process requires energy because it involves moving substances against their natural concentration gradient (from low concentration to high concentration).
Active transport is commonly used for moving glucose and other essential nutrients into cells, even when these substances are more concentrated inside the cell than outside.
The three transport mechanisms work together to maintain cellular function. Passive processes (diffusion and osmosis) move substances down concentration gradients, while active transport can work against gradients when the cell needs to concentrate specific substances.
Osmosis effects on plant and animal cells
The effects of osmosis vary significantly between plant and animal cells due to their structural differences.
Understanding how osmosis affects plant and animal cells differently is crucial because it explains why plant cells don't burst when they absorb water, while animal cells can be damaged or destroyed.
Osmosis in animal cells
When animal cells are placed in solutions of different concentrations, they respond in predictable ways:
- In isotonic solutions (same concentration as the cell's cytoplasm), water moves in and out at equal rates, so the cell volume remains constant.
- In hypotonic solutions (less concentrated than the cytoplasm), water enters the cell through osmosis. The cell swells and may eventually burst and die because animal cells lack the protective cell wall.
- In hypertonic solutions (more concentrated than the cytoplasm), water leaves the cell through osmosis. The cell shrinks and may die from dehydration.
Osmosis in plant cells
Plant cells respond differently to osmotic conditions because of their rigid cell walls:
In hypotonic solutions, water enters the cytoplasm and vacuole, creating turgor pressure. This is the outward pressure exerted by the cell contents against the cell wall. Turgor pressure provides significant strength to the plant and prevents the cell from bursting due to the protective cell wall.
In hypertonic solutions, water leaves the plant cell, causing it to lose turgor pressure and become flaccid (wilted). In extreme cases, the cell membrane may pull away from the cell wall in a process called plasmolysis.
It's important to note that cell walls are fully permeable to water, gases, and many dissolved substances, so they don't interfere with osmotic processes.
Experimental techniques
Scientists use various staining techniques to study different types of cells:
- Methylene blue is commonly used to stain animal cells, causing nuclei to change from blue to dark blue.
- Iodine is typically used to stain plant cells, causing nuclei to change from light brown to dark brown.
These staining methods help make cellular structures more visible under microscopes, allowing for better observation and study of cell components.
Key Points to Remember:
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Plant and animal cells share many organelles but plant cells have additional structures: cell walls, large vacuoles, and chloroplasts that animal cells lack.
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Each organelle has a specific function: nucleus controls cell activities, mitochondria provide energy, ribosomes make proteins, and chloroplasts (in plants) carry out photosynthesis.
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Osmosis affects plant and animal cells differently due to the presence of cell walls in plants, which prevent bursting and create turgor pressure for structural support.
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Prokaryotic cells (bacteria) are simpler than eukaryotic cells (plants and animals) because they lack a nucleus and membrane-bound organelles.
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Three main transport mechanisms move substances across cell membranes: passive diffusion, passive osmosis, and energy-requiring active transport.