Describe the similarities and differences between plant and animal cells.

Faizah M 30/04/2007

Essay Question

Describe the similarities and differences between plant and animal cells

(20 marks)

There are various differences and similarities between the plant and animal cells in terms of their organelles.

A main overall similarity between both cells is that they are the two mail types of the eukaryotic cell. A eukaryotic cell possesses nuclei and their DNA lies in a nucleus. Eukaryotic includes animals, plants, fungi and protoctists. In contrast, organisms that lack nuclei are called prokaryotes and are referred to as bacterium.

Other similarities of a plant cell and an animal cell are as follows.

The Nucleus

This is the most important organelle in the cell. It is bounded by a double membrane, the nuclear envelope. It possess many large pores, 40-60nm in diameter, which permits the passage of large molecules, such as RNA, between it and the cytoplasm. Chromatin is found in the nucleoplasm and is made up of coils of DNA bound to proteins. During division the chromatin aggregates from the chromosomes but these are rarely if ever visible in a non-dividing cell. Also within the nucleus are one or two small round bodies each called a nucleus. They cannot be seen very well as they are not bounded by a membrane. They manufacture RNA using the information in its own DNA. The functions of the nucleus are:

To contain the genetic material of a cell is the form of chromosomes.
To act as a control centre for the activities of a cell.
The nuclear DNA carries the instructions for the synthesis of proteins.
It is involved in the production of ribosomes and RNA.
It is essential for cell division.

Mitochondria

Mitochondria are found within the cytoplasm of all eukarotic cells. They range in shape from spherical to highly elongated. In length they lay form 1 to 10μm and in width form 0.25 to 1.0μm. They are bounded together by a double membrane, the outer of which controls the entry and exit of chemicals. The inner membrane is folded inwards, giving rise to extensions called cristae, some of which extend across the entire organelle. They function to increase the surface area on which respiratory process takes place. The surface of these cristae has stalled granules along its length. The remainder of the mitochondrion is in the matrix. It is a semi-rigid material containing protein, lipids and traces of DNA. Electron dense granules of 25nm diameter also occur. The main function of the mitrochondrian is sites for certain sites of respiration. They make ATP – the universal energy carrier in cells. They are also involved in the synthesis of lipids.

Endoplasmic Reticulm (ER)

The endoplasmic reticulm (ER) is an elaborate system of membranes found throughout the cell, forming a cytoplasmic skeleton. It is an extension of outer nuclear membrane with which it is continuous. The membranes form a series of sheets, which enclose flattened sacs called cisternae. Where the membranes are linked with ribosomes they are called rough endoplasmic reticulm. The rough ER are involved with the synthesis of protein. Where the membranes lack ribosomes they are called smooth endoplasmic reticulm. The smooth ER makes lipids and steroids (e.g. cholesterol and reproductive hormones). Overall, the functions of the ER ...

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Endoplasmic Reticulm (ER)

Provides a large surface area for chemical reactions.
Provides a pathway for the transport of materials through the cell.
Provides lipids and steroids (smooth ER).
Collects and stores synthesized material.
Provides a structural skeleton to maintain cellular shape. (e.g. the smooth ER of a rod cell form the retina of the eye).

Cell Membrane

The cells main function is to serve as a boundary between the cell and its environment. It is a functional organelle, which allows some substances to pass freely in and out of the membrane. Yet others may be excluded at one moment only to pass freely across the membrane on another occasion. On account of the membrane’s ability to permit different substances to pass across it at different rates, it is said to be differentially permeable. The cell membrane is made up of almost two chemical groups – proteins and phospholipids. The precise arrangement of these chemicals is less certain. They are either in a protein-phospholipid sandwich or the fluid-mosaic model. The proteins in the membrane have a number of functions. Apart from giving structural support they have very specific, varying from cell to cell. It is this specifically, which allows cells to be recognised by other agents in the body, e.g. enzymes, hormones and antibodies. In the fluid-mosaic model it is thought probable that the proteins also assist the active transport of materials across the membrane.

Ribosomes

Ribosomes are small cytoplasmic granules found in all cells. They are around 20nm in diameter in eukaryotic cells (80S type) but slightly smaller in prokaryotic ones (70S). They occur either singly or in groups called polysomes. They may be associated with endoplasmic reticulm or occur freely within the cytoplasm. Despite their small size, their enormous numbers mean that they can account for up to 20% of the mass of a cell. Ribosomes are made up of relatively small RNA molecules and protein. They are important in protein synthesis.

The Golgi Body

The Golgi apparatus has a similar structure to the smooth ER but is more compact. It is composed of stacks of flattened sacs made of membranes. The sacs are fluid-filled and pinch off smaller membranous sacs, called vesicles, at their ends. There is normally only one Golgi apparatus in each animal cell but in plant cells there may be a large number of stacks known as dictyosomes. Its position and size varies form cell to cell but it is well developed in secretory cells and neurones and is small in muscle cells. This suggests that the Golgi apparatus plays some role in the production of secretary material. In particular, it is thought to perform the following functions.

Produces glyco-proteins such as mucin required in secretions by adding the carbohydrate part to the protein.
Produces secretary enzymes, e.g. the digestive enzymes of the pancreas.
Secretes carbohydrates such as those involved in the production of new cell walls.
Transports and stores lipids. Also, forms lysosomes.

Microtubules

Microtubules occur widely throughout living cells. They are slender, unbranched tubes about 20nm in diameter and up to several micros in length. Their functions are as follows. To provide an internal skeleton (Cytoskeleton). for cells and so help determine their shape. They may aid transport within cells by providing routes along which materials move. They form a framework along which the cellulose cell wall of plants is laid down. They are the major components of cilia and flagella where they contribute to their movement. They are found in the spindle during cell division and within the centrioles from which the spindle is formed.

The following organelles are all featured in an animal cell but not in a plant cell.

Lysosomes

Lysosomes are similar in size in size to spherical mitochondria, being 0.2-0.5μm in diameter. Lysosomes are bounded by a single membrane, but unlike mitochondria they have no cristae or internal structure. They contain a large number of enzymes, mostly hydrolases, in acid solution. They isolate these enzymes from the remainder of the cell. By so doing they prevent them from acting upon other chemicals and organelles within the cell. The functions of lysosomes are as follows.

They digest material, which the cell consumes from the environment.
They digest parts of the cell, such as worn-out organelles, in a similar way to that described above.
They release their enzymes outside the cell (exocytosis) in order to break down other cells.

Centrioles

Centrioles have the same basic structure as the basal bodies of cilia. They are hollow cylinders about 0.2μm long. They are in the centrosome, part of the cytoplasm. It contains two Centrioles. At cell division they migrate to opposite poles of the cell where they synthesize the microtubules of the spindle. Despite the absence of Centrioles, the cells of higher plants do form spindles.

Glycogen granules

Every cell contains a limited store of food energy. This store may be in the form of soluble material such as the sugar found in vacuoles of plant cells. It may also occur in insoluble form, as grains or granules, with cells or organelles. Glycogen granules occur throughout the cytoplasm of animal cells. They store animal starch or glycogen. Oil or lipid droplets are found within the cytoplasm of both plant and animal cells.

Cillium

Cilia and flagella are almost identical, except that cilia are usually shorter and more numerous. Both are around 0.2μm in diameter; cilia are about 10μm long whereas flagella may be 100μm long. They are found in a limited number of cells but are nevertheless of great importance. They function to either move an entire organism or to move material within an organism.

Pinocytosis

Pinocytosis or ‘cell drinking’ is very similar to phagocytosis except that the vesicles produced, called pinocytic vesicles, are smaller. The process is used for the intake of liquids rather than solids. Even smaller vesicles, called micropinocytic vesicles, may be pinched off in the same way. Both Pinocytosis and phagocytosis are methods by which materials are taken into the cell in bulk. This process is called endocytosis. By contrast, the reverse process, in which materials are removed from cells in bulk, is called exocytosis.

The following organelles are all featured in a Plant cell but not in an animal cell.

Chloroplasts

Chloroplasts are found only in plant cells. Their shape and size vary with species but there is always a double unit membrane surrounding a matrix (stoma) in which are stacks of lamellae (grana). The lamellae hold the chlorophyll in the most suitable position for photosynthesis. The stroma contains numerous starch granules and enzymes for the reduction of carbon dioxide.

Vacuoles

Vacuoles are a fluid-filled sac bounded by a single membrane. Within mature plant cells there is usually one large central vacuole. The single membrane around it is called the tonoplast. It contains a solution of mineral salts, sugars, amino acids, wastes and sometimes also pigments such as anthocyanins. Plant vacuoles serve a variety of functions.

The sugars and amino acids may serve as a temporary food store.
The anthocyanins are of various colours and so may colour petals to attract pollinating insects, or fruits to attract animals for dispersal.
They act as temporary stores for organic wastes, such as tannins. They may accumulate in the vacuoles of leaf cells and are removed when the leaves fall.
They occasionally contain hydrolytic enzymes and so perform functions similar to those of lysosomes.
They support herbaceous plants, and herbaceous parts of woody plants by providing an osmotic system, which creates a turgor pressure.

Cell wall

A cell wall is a characteristic feature of plant cells. It consists of cellulose fibres embedded in an amorphous polysaccharide matrix. The matrix is usually composed of polysaccharides, e.g. pectin or lignin. The fibrils may be regular or irregular in arrangement. The main functions of the cell wall are as follows.

To provide support in herbaceous plants. As water enters the cell osmotically the cell wall resists expansion and an internal pressure is created which provides turgidity for the plant.
To give direct support to the cell and the plant as a whole by providing mechanical strength. The strength may be increased by the presence of lignin in the matrix between the cellulose fibres.
To permit the movement of water through and along it and so contribute to the movement of water in the plant as a whole, in particular in the cortex of the root.
In some cell walls the presence of cutin, suberin or lignin in the matrix makes the cell less permeable to substances. Lignin helps to keep the water within the xylem; cutin in the epidermis of leaves prevents water being lost from the plant and suberin in root endodermal cells prevents movement of water across them, thus concentrating its movement through special passage cells.
The arrangement of the cellulose fibrils in the cell wall can determine the pattern of growth and hence the overall shape of a cell.
Occasionally cell walls act as food reserves.

Starch grains

Starch grains occur within chloroplasts and the cytoplasm of plant cells. Starch may also be stored in specialized leucoplasts called amyloplasts. More information on starch grains can be found in the section ‘glycogen granules’ in this essay.

The next two pages, shows diagrams of a plant cell and an animal cell.