Animal and Plant Cell Structure: A Detailed Comparison
Animal Cells
Animal cells are surrounded by an outer membrane, called the cell or plasma membrane. This membrane surrounds the protoplasm. The protoplasm of a cell is made up of the nucleus and the surrounding cytoplasm. Many of the reactions in a cell take place in the cytoplasm.
Plant Cells
Plant cells are enclosed by a rigid cell wall made of cellulose. Cellulose is a strong structural carbohydrate (or polysaccharide) and is the main component in paper and cotton wool. The cell wall gives the cell strength and makes it less flexible. The cell membrane is usually found just inside the cell wall. The wall and membrane are often so close that the membrane is not seen clearly. Vacuoles contain a fluid called cell sap. This is a solution of salts, sugars, and pigments. Animal cells have no (or very few) vacuoles.
Plant cells also have a nucleus and cytoplasm. Plant cells that are green contain structures called chloroplasts. These are the structures in which photosynthesis takes place.
Key Terms
- Protoplasm: All the living parts of a cell.
- Cytoplasm: The living material in a cell outside the nucleus.
- Ultrastructure: The detail of a structure as seen using an electron microscope.
Electron Microscopes
- Transmission Electron Microscope (TEM): Sends a beam of electrons through a thin section of the specimen. This shows the internal structure of the specimen in great detail.
- Scanning Electron Microscope (SEM): Uses a beam of electrons to provide a surface view of the specimen.
Parts of a Cell
Cell (or Plasma) Membrane
All membranes have the same structure: phospholipids and proteins. Phospholipids, which have a water-loving phosphate group and a water-hating lipid group, are arranged into double layers (bilayers). The phosphates are on the exposed outer surfaces with the lipids in the middle. In a membrane, protein molecules are completely or partially embedded in the phospholipid bilayer. Some of these proteins are attached to the bilayer; others are detachable and can move throughout the bilayer.
Functions of Membranes
The functions or roles of membranes include:
- Retain the cell contents.
- Recognize molecules that touch them.
- Control what enters and leaves the cell.
Membranes can allow the free passage of some molecules and prevent the passage of others; in this way, they are said to be selectively (or semi-) permeable. For example, water and oxygen can pass freely across a membrane, but sodium ions and large proteins have to be moved across using energy.
- Give some support to the cell.
Nucleus
A nucleus is surrounded by a double membrane with numerous nuclear pores. These allow the controlled entry and exit of molecules into and out of the nucleus. The nucleus contains strands of deoxyribonucleic acid (DNA). DNA is arranged into structures called chromosomes.
Every organism has a definite number of chromosomes in each nucleus (e.g., humans have 46). Genes are located randomly along chromosomes. Genes are the structures that inform the cell how to make certain proteins. Genes control features such as the number of fingers, color of eyes, etc. When a cell is not dividing (i.e., most of the time), chromosomes are very elongated and interwoven. In this form, they are called chromatin.
- Chromatin: The name given to chromosomes when they are elongated and not dividing.
- Nuclear Pores: Allow a type of RNA (ribonucleic acid) called mRNA (messenger RNA) to pass in and out of the nucleus.
- Nucleolus: An area in the nucleus that stains very darkly.
Cytoplasm
The cytoplasm is the jelly-like liquid in a cell that surrounds the nucleus. A number of small bodies called organelles (such as mitochondria, chloroplasts, and ribosomes) are suspended in the cytoplasm.
Mitochondria
Mitochondria (singular is mitochondrion) are the sites of respiration.
- Cells with many mitochondria (e.g., muscle and liver in animals, meristems in plants) produce lots of energy.
- Cells with few mitochondria (e.g., fat in humans, ground tissue in plants) produce less energy.
Mitochondria are surrounded by a double membrane. It is on the inner membrane, especially the infoldings, that energy is released. The more infoldings that are present, the greater the surface area for cellular respiration, which results in the production of greater quantities of energy. Each mitochondrion has its own loop of DNA.
Chloroplasts (Plants Only)
Chloroplasts are surrounded by double membranes. They have membrane stacks, which contain the green pigment chlorophyll. They also have a loop of DNA.
Cell Wall (Plants Only)
Plant cell walls are made of cellulose (which is a structural polysaccharide). Cell walls are fully permeable. This means that all molecules (big or small) can pass in or out through cell walls.
Ribosomes
Ribosomes are very tiny, bead-like structures. They are made of RNA and protein. They work by combining a sequence of amino acids to form a protein.
Prokaryotic vs. Eukaryotic Cells
Prokaryotic Cells
- Do not have a nucleus or membrane-enclosed organelles.
- Are single-celled.
- Have a circular loop of DNA (not surrounded by a membrane, i.e., do not have a nucleus).
- Have small cells.
- Do not have membrane-enclosed structures such as mitochondria and chloroplasts.
- Include bacteria.
Eukaryotic Cells
- Have a nucleus and cell organelles, all of which are enclosed by membranes.
- Have a nucleus (i.e., DNA enclosed by a membrane).
- May have membrane-enclosed organelles such as mitochondria and chloroplasts.
- Have large cells.
- Include animals, plants, and fungi.
- Are more advanced than prokaryotes (i.e., life originated with prokaryotic cells and has evolved into eukaryotic cells).