Category: 1. Animal and Plant cell

1. Plant cells have a cell wall.

Plant cells are different from animal cells in a number of ways. Perhaps the most obvious difference is the presence of a cell wall. The cell wall provides strength and support to the plant, much like the exoskeleton of an insect or spider (our skeleton is on the inside of our body, rather than on the outside like insects or spiders).

The plant cell wall is mainly made up of the carbohydrates molecules cellulose and lignin. Cellulose is used extensively by humans for making paper. Cellulose can also be converted into cellulosic ethanol, a type of biofuel. Some animals, such as cows, sheep and goats, can digest cellulose with the help of bacteria in their stomachs. Humans cannot digest cellulose, which passes through our bodies and is better known as dietary fiber, something that we should eat to keep our waste moving as it should! Lignin fills in the spaces between cellulose and other molecules in the cell wall. Lignin also helps water molecules move from one side of the cell wall to the other – an important function in plants.

2. Plant cells contain vacuoles.

Most adult plant cells have one large vacuole that takes up more than 30% of the cell’s volume. At certain times and conditions the vacuole takes up as much as 80% of the cell’s volume! In addition to storing wastes and water, the vacuole also helps to support the cell because the liquid inside the vacuole exerts an outward pressure on the cell, much like the water inside of a water balloon. This is called turgor pressure and keeps the cells from collapsing inward.

3. Plant cells contain chloroplasts.

Unlike animal cells, plant cells can harness the energy of the Sun, store it in the chemical bonds of sugar and later use this energy. The organelle which is responsible for this is the chloroplast. Chloroplasts contain chlorophyll, the green pigment that gives leaves their colour and absorbs light energy. Cyanobacteria, a type of prokaryote capable of photosynthesis, are considered to be the ancestors of chloroplasts!

In spite of the differences in size and complexity, all cells are mostly composed of the same substances and they all carry out similar life functions. These include growth and metabolism and reproduction by cell division.

Cells are made up of subcellular structures that are responsible for different and specific functions. These structures are known as organelles. A number of these organelles are common to both animal and plant cells. This section will focus on those parts which plants have.

Cell Structures (Cell Organelles)

1. Cell Wall: This is the rigid outermost layer of a plant cell. It makes the cell stiff -providing the cell with mechanical support – and giving it protection. Animal cells do not have cell walls.
2. Cell Membrane: This is a protective layer that surrounds every cell and separates it from its external environment. It is found just inside the cell wall and is made up of complex lipids (fats) and proteins.
3. Cytoplasm: The cytoplasm is a thick, aqueous (water-based) solution in which the organelles are found. Substances such as salts, nutrients, minerals and enzymes (molecules involved in metabolism) are dissolved in the cytoplasm.
4. Nucleus: The nucleus is the ‘control center’ of the cell. It contains Deoxyribonucleic acid (DNA), the genetic material that directs all the activities of the cell. Only eukaryotic cells have nuclei (plural for nucleus), prokaryotic cells do not. The nucleus is separated from the cytoplasm by a specialized membrane called the
5. Nuclear membrane.
6. Ribosomes: These are little round structures that produce proteins. They are found in the cytoplasm or attached to the endoplasmic reticulum.
7. Endoplasmic Reticulum (ER): The ER is a membrane system of folded sacs and tunnels. The ER helps move proteins within the cell as well as export them outside of the cell. There are two types of endoplasmic reticulum.
8. Rough endoplasmic reticulum. The rough endoplasmic reticulum is covered with ribosomes. Smooth endoplasmic reticulum (no ribosomes)
9. Golgi body: The Golgi body is a stack of membrane-covered sacs that prepares proteins for export from the cell.
10. Mitochondrion (plural mitochondria): This is the ‘powerhouse’ of the cell. It converts the energy stored in food (sugar and fat) into energy-rich molecules that the cell can use (Adenosine triphosphate – ATP for short).
11. Lysosome: The lysosome is the digestive center of a cell that produces many different types of enzymes which are able to break down food particles and recycle worn out components of the cell.
12. Vacuoles: These are large membrane-enclosed compartments that store toxic wastes as well as useful products such as water. These are mainly found in plants.
13. Chloroplast: Chloroplasts contain a green pigment that traps sunlight and converts it into sugars by a process called photosynthesis. The sugars are a source of energy for the plants and the animals that eat them.

The three main parts of the cell theory are:

1. All living things are made of cells.
2. The cell is the basic unit of structure and function in all living things.
3. Cells only come from other pre-existing cells by cell division.

While some organisms are single-celled, others are made up of many cells. These organisms are called multicellular (having many cells). Cells differ in their size and complexity.

Eukaryotes are organisms which are made up of large and complex cells, whereas prokaryotes are organisms which are made up of small and simple cells. Animals and plants are examples of eukaryotes (have eukaryotic cells) while bacteria are examples of prokaryotes (have prokaryotic cells).

The cell membrane surrounds the entire cell and separates its components from the outer environment. The cell membrane is a double layer made up of phospholipids (called the phospholipid bilayer). Phospholipids are molecules with a phosphate group head attached to glycerol and two fatty acid tails. They spontaneously form double membranes in water due to the hydrophilic properties of the head and hydrophobic properties of the tails.

The cell membrane is selectively permeable, meaning it only allows certain molecules to enter and exit. Oxygen and carbon dioxide pass through easily, while larger or charged molecules must go through special channels, bind to receptors, or be engulfed.

The cytoskeleton is a network of filaments and tubules found throughout the cytoplasm of the cell. It has many functions: it gives the cell shape, provides strength, stabilizes tissues, anchors organelles within the cell, and has a role in cell signaling. It also provides mechanical support to allow cells to move and divide. There are three types of cytoskeletal filaments: microfilaments, microtubules, and intermediate filaments.

The cytosol is the gel-like liquid contained within cells. The cytosol and all the organelles within it – except for the nucleus – are collectively referred to as the cell’s cytoplasm. This cytosol consists primarily of water, but also contains ions, proteins, and small molecules. The pH is generally neutral, around 7.

Mitochondria are the energy-producing organelles, commonly known as “the powerhouse of the cell.” The process of cellular respiration occurs in the mitochondria. During this process, sugars and fats are broken down through a series of chemical reactions, releasing energy in the form of adenosine triphosphate (ATP).

ATP is like the energy currency of the cell. Think of each molecule like a rechargeable battery that can be used to power various cellular processes.

Lysosomes are a type of vesicle. Vesicles are spheres surrounded by a membrane that excludes their contents from the rest of the cytoplasm. Vesicles are used extensively within the cell for metabolism and transport of large molecules that cannot cross membrane unaided.

Lysosomes are specialized vesicles that contain digestive enzymes. These enzymes can break down large molecules like organelles, carbohydrates, lipids, and proteins into smaller units so that the cell can reuse them. Therefore, they are like the waste disposal/recycling department of the cell.

The Golgi apparatus, also called the Golgi complex or Golgi body, receives proteins from the ER and folds, sorts, and packages these proteins into vesicles. The Golgi apparatus is like the shipping department of the cell, as it packages proteins up for delivery to their destinations.

Like the ER, the Golgi apparatus also consists of a series of membrane-bound sacs. These sacs originate from vesicles that have budded off from the ER. Unlike the system of membranes in the ER, which are interconnected, the pouches of the Golgi apparatus are discontinuous.

The endoplasmic reticulum (ER) is a network of flattened, membrane-bound sacs that are involved in the production, processing, and transport of proteins that have been synthesized by ribosomes. The endoplasmic reticulum is like the assembly line of the cell, where the products produced by the ribosomes are processed and assembled.

There are two kinds of endoplasmic reticulum: smooth and rough. The rough ER has ribosomes attached to the surface of the sacs. Smooth ER does not have ribosomes attached and has functions in storage, synthesizing lipids, removing toxic substances.