The cell is the smallest unit of life in both plants and animals. A bacterium is an example of a single-cell organism, while an adult human is made up of trillions of cells. Cells are more than important – they are vital for life as we know it. Without cells, no living thing would survive. Without plant cells, there would be no plants. And without plants, all living things would die.
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Plants, which are made up of a variety of cell types organized into tissues, are the Earth's primary producers. Without plant cells, nothing could survive on Earth.
Plant Cell Structure
In general, plant cells are rectangular- or cube-shaped and are larger than animal cells. However, they are similar to animal cells in that they are eukaryotic cells, which means the cell's DNA is enclosed inside the nucleus.
Plant cells contain many cellular structures, which carry out functions essential for the cell to function and survive. A plant cell is made up of a cell wall, cell membrane and many membrane bound structures (organelles), such as plastids and vacuoles. The cell wall, the outermost rigid covering of the cell, is made of cellulose and provides support and facilitates interaction between the cells. It consists of three layers: the primary cell wall, the secondary cell wall and the middle lamella. The cell membrane (sometimes called the plasma membrane) is the outer body of the cell, inside the cell wall. Its main function is to provide strength and protect against infection and stress. It is semi-permeable, meaning only certain substances can pass through it. A gel-like matrix inside the cell membrane is called cytosol or cytoplasm, inside which all the other cell organelles develop.
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Plant Cell Parts
Every organelle within a plant cell has an important role. Plastids store plant products. Vacuoles are water-filled, membrane bound organelles that are also used to store useful materials. Mitochondria carry out cellular respiration and give the cells energy. A chloroplast is an elongated or disc-shaped plastid made up of the green pigment chlorophyll. It traps light energy and converts it to chemical energy via a process called photosynthesis. The golgi body is the part of the plant cell where proteins are sorted and packed. Proteins are assembled inside structures called ribosomes. Endoplasmic reticulum are membrane-covered organelles that transport materials.
The nucleus is a distinctive characteristic of a eukaryotic cell. It is the control center of the cell bound by a double membrane known as the nuclear envelope, and is a porous membrane that allows substances to pass through it. The nucleus plays an important role in protein formation.
Types of Plant Cells
Plant cells come in various types, including phloem, parenchyma, sclerenchyma, collenchyma and xylem cells.
Phloem cells transport sugar produced by the leaves throughout the plant. These cells live past maturity.
The major cells of plants are parenchyma cells, which make up plant leaves and facilitate metabolism and food production. These cells tend to be more flexible than others because they are thinner. Parenchyma cells are found in the leaves, roots and stems of a plant.
Sclerenchyma cells give the plant a great deal of support. The two types of sclerenchyma cells are fiber and sclereid. Fiber cells are long, slender cells that normally form strands or bundles. Sclereid cells may occur individually or in groups and come in various forms. They usually exist in the roots of the plant and do not live past maturity because they have a thick secondary wall containing lignin, the main chemical component of wood. Lignin is extremely hard and waterproof, which makes it impossible for the cells to exchange materials long enough for active metabolism to take place.
The plant also gets support from collenchyma cells, but they are not as rigid as sclerenchyma cells. Collenchyma cells usually give support to the parts of a young plant that are still growing, such as the stem and leaves. These cells stretch along with the developing plant.
Xylem cells are water-conducting cells, which bring water to the plant's leaves. These hard cells, present in the plant's stems, roots and leaves, do not live past maturity, but their cell wall stays to allow the free movement of water throughout the entire the plant.
The different types of plant cells form different types of tissue, which have different functions in certain parts of the plant. Phloem cells and xylem cells form vascular tissue, parenchyma cells form epidermal tissue and parenchyma cells, collenchyma cells and sclerenchyma cells form ground tissue.
Vascular tissue forms the organs that transport food, minerals and water through the plant. Epidermal tissue forms a plant's outer layers, creating a waxy coating that stops a plant from losing too much water. Ground tissue forms the bulk of a plant's structure and performs lots of different functions, including storage, support and photosynthesis.
Plant Cells vs Animal Cells
Plants and animals are both extremely complex multicellular organisms with some parts in common, like the nucleus, cytoplasm, cell membrane, mitochondria and ribosomes. Their cells fulfil the same basic functions: taking nutrients from the environment, using those nutrients to make energy for the organism, and making new cells. Depending on the organism, cells may also transport oxygen through the body, remove waste, send electrical signals to the brain, protect from disease and – in the case of plants – make energy from sunlight.
However, there are some differences between plant cells and animal cells. Unlike plant cells, animal cells don't contain a cell wall, chloroplast or prominent vacuole. If you view both types of cell under a microscope, you can see large, prominent vacuoles at the center of a plant cell, whereas an animal cell has only a small, inconspicuous vacuole.
Animal cells are typically smaller than plant cells and have a flexible membrane around them. This lets molecules, nutrients and gases pass into the cell. The differences between plant cells and animals cells allow them to fulfill different functions. For example, animals have specialized cells to allow rapid movement because animals are mobile, whereas plants are not mobile and have rigid cells walls for extra strength.
Animal cells come in various sizes and tend to have irregular shapes, but plant cells are more similar in size and are typically rectangular or cube-shaped.
Bacterial and yeast cells are quite different to plant and animal cells. For starters, they are single-celled organisms. Both bacterial cells and yeast cells have cytoplasm and a membrane surrounded by a cell wall. Yeast cells also have a nucleus, but bacterial cells do not have a distinct nucleus for their genetic material.
Importance of Plants
Plants provide habitat, shelter and protection for animals, help to make and preserve soil, and are used to make many useful products, such as fibers and medicines. In some parts of the world, wood from plants is the primary fuel used to cook people's meals and heat their homes.
Possibly the most important function of a plant is to convert light energy from the sun into food. In fact, a plant is the only organism that can do this. Plants are autotrophic, meaning they produce their own food. Plants also produce all the food animals and people eat – even meat, because the animals that provide meat eat plants like grass, corn and oats.
When plants make food, they produce oxygen gas. This gas forms a crucial part of the air for the survival of plants, animals and humans. When you breathe, you take oxygen gas out of the air to keep your cells and body alive. In other words, all of the oxygen needed by living organisms is produced by plants.
Plants and Photosynthesis
Plants produce oxygen as a waste product of a chemical process called photosynthesis, which, as the University of Nebraska-Lincoln Extension notes, literally means, "to put together with light." During photosynthesis, plants take energy from sunlight to convert carbon dioxide and water into molecules necessary for growth, such as enzymes, chlorophyll and sugars.
The chlorophyll in plants absorbs energy from the sun. This enables the production of glucose, made up of carbon, hydrogen and oxygen atoms, thanks to the chemical reaction between carbon dioxide and water.
Glucose made during photosynthesis may be transformed into chemicals the plant cells need to grow. It may also be converted into the storage molecule starch, which can later be converted back into glucose when needed by the plant. It may also be broken down during a process called respiration, which releases energy stored within the glucose molecules.
Many structures inside the plant cells are required for photosynthesis to take place. The chlorophyll and enzymes are contained within the chloroplasts. The nucleus houses the DNA necessary for carrying the genetic code for the proteins used in photosynthesis. The plant's cell membrane facilitates the movement of water and gas in and out of the cell, and also controls the passage of other molecules.
Dissolved substances move in and out of the cell through the cell membrane, through different processes. One of these processes is called diffusion. This involves the free movement of oxygen and carbon dioxide particles. A high concentration of carbon dioxide moves into the leaf, while a high concentration of oxygen moves out of the leaf into the air.
Water moves across cell membranes via a process called osmosis. This is what gives plants water via their roots. Osmosis requires two solutions with different concentrations as well as a semi-permeable membrane separating them. Water moves from a less-concentrated solution to a more-concentrated solution until the level on the more-concentrated side of the membrane rises and the level on the less-concentrated side of the membrane falls, until the concentration is the same on both sides of the membrane. At this point, the movement of water molecules is the same in both directions and the net exchange of water is zero.
Light and Dark Reactions
The two parts of photosynthesis are known as the light (light-dependent) reactions and the dark or carbon (light-independent) reactions. The light reactions need energy from sunlight, so they can only take place during the day. During a light reaction, water is split and oxygen is released. A light reaction also provides the chemical energy (in the form of the organic energy molecules ATP and NADPH) needed during a dark reaction to transform carbon dioxide into carbohydrate.
A dark reaction doesn't require sunlight and takes place in the part of the chloroplast called the stroma. Several enzymes are involved, mainly rubisco, which is the most plentiful of all plant proteins and consumes the most nitrogen. A dark reaction uses the ATP and NADPH produced during a light reaction to produce energy molecules. The reaction cycle is known as the Calvin Cycle or the Calvin-Benson Cycle. ATP and NADPH combine with carbon dioxide and water to make the end product, glucose.