The plant cell and its organelles
The parts of a plant cell and what each does, how plant and animal cells differ, plastids, the symplast and apoplast, and microscope size sums.
On this page
Every crop, from a blade of grass to a mango tree, is built from cells. What a plant can do, capture light, take up nutrients, resist disease, store sugar or oil, happens inside these cells and depends on their structure. A grasp of the plant cell is therefore the starting point for plant physiology, pathology, biotechnology and breeding.
Cell structure is one of the most frequently examined biology topics at every level, especially the functions of organelles and the differences between plant and animal cells.
The cell theory
In 1665 Robert Hooke looked at thin slices of cork under an early microscope and named the box-like compartments he saw "cells". In 1838–39 Matthias Schleiden (for plants) and Theodor Schwann (for animals) proposed that all living things are made of cells, and in 1855 Rudolf Virchow added that all cells come from pre-existing cells. Together these make the cell theory.
Plant cells are eukaryotic: they have a nucleus and membrane-bound organelles, unlike the prokaryotic cells of bacteria, which have no nucleus.
The parts of a plant cell
Cell wall
A rigid layer outside the plasma membrane, made mainly of cellulose microfibrils embedded in hemicelluloses and pectins.
- The primary wall is thin and flexible, allowing the cell to grow.
- Many cells add a thick secondary wall inside it, often strengthened with lignin, as in wood, fibres and xylem vessels.
- Neighbouring cells are glued together by the pectin-rich middle lamella; when it breaks down, fruit softens.
The wall gives shape and strength, resists turgor pressure, and is a first barrier against pathogens. It is also the raw material of cotton, jute, paper and much of the world's dietary fibre.
Plasma membrane
A phospholipid bilayer with embedded proteins, described by S. J. Singer and G. L. Nicolson's fluid mosaic model (1972). It controls what enters and leaves the cell through channels, carriers and pumps, and senses signals.
Nucleus
Contains the chromosomes (DNA) within a double-membrane nuclear envelope with pores. The nucleolus inside it makes ribosomal RNA. The nucleus directs the cell's activities through gene expression.
Ribosomes
Sites of protein synthesis, free in the cytoplasm or attached to the endoplasmic reticulum. Cytoplasmic ribosomes are 80S; chloroplasts and mitochondria have smaller 70S ribosomes, like those of bacteria.
Endoplasmic reticulum (ER)
A network of membranes. Rough ER, studded with ribosomes, makes proteins for membranes and secretion; smooth ER makes lipids.
Golgi apparatus
Stacks of flattened sacs (in plants often called dictyosomes) that modify, sort and package proteins, and make the hemicelluloses and pectins of the cell wall.
Vacuole
Mature plant cells usually have a large central vacuole, often occupying 80 to 90 per cent of the cell volume, bounded by a membrane called the tonoplast. It stores water, ions, sugars, pigments (such as anthocyanins), and defensive compounds; maintains turgor; and breaks down unwanted materials.
Plastids
A family of organelles with their own DNA, all developing from small proplastids:
- Chloroplasts: photosynthesis. A double envelope surrounds the stroma (where the Calvin cycle runs) and stacks of thylakoids called grana (where light reactions happen).
- Chromoplasts: carotenoid pigments that colour many fruits, flowers and roots, such as tomato, chilli and carrot.
- Leucoplasts: colourless storage plastids, including amyloplasts (starch, as in potato tubers and cereal grains), elaioplasts (oil) and proteinoplasts (protein).
Mitochondria
Sites of aerobic respiration, with a folded inner membrane (cristae) carrying the electron transport chain.
Peroxisomes and glyoxysomes
Small organelles with oxidative enzymes. Peroxisomes in leaves take part in photorespiration; glyoxysomes in germinating oilseeds convert stored fats into sugars through the glyoxylate cycle.
Cytoskeleton
Microtubules and actin filaments organise the cell, guide cellulose deposition and move organelles; microtubules form the spindle at cell division.
Plasmodesmata
Channels through the cell walls that connect the cytoplasm of neighbouring cells. They allow sugars, hormones, signals and, unfortunately, many plant viruses to move between cells.
Symplast and apoplast
Plasmodesmata link the cytoplasm of cells into one continuous system, the symplast. The cell walls and the spaces between cells form the apoplast. Water and nutrients travel through both routes in roots, until the Casparian strip of the endodermis blocks the apoplast and forces them into the symplast, letting the root control what enters the xylem.
Plant and animal cells compared
| Feature | Plant cell | Animal cell |
|---|---|---|
| Cell wall | present (cellulose) | absent |
| Plastids | present | absent |
| Vacuole | large central vacuole | small, temporary vacuoles |
| Centrioles | absent in most flowering plants | present |
| Storage carbohydrate | starch | glycogen |
| Cell division | cell plate forms from the centre | cleavage furrow from outside |
| Shape | usually fixed, often box-like | variable |
The endosymbiotic origin of organelles
Chloroplasts and mitochondria have their own circular DNA, 70S ribosomes and double membranes, and divide by a process like bacterial fission. These and other features support the endosymbiotic theory, championed by Lynn Margulis from 1967: mitochondria descend from bacteria, and chloroplasts from cyanobacteria, that were engulfed by ancestral cells and became permanent partners.
Cell size and microscopy
Working out real sizes from a micrograph
Magnification = image size ÷ actual size, so actual size = image size ÷ magnification. Keep units consistent: 1 mm = 1,000 µm.
A chloroplast measures 25 mm long on an electron micrograph taken at × 5,000.
- Actual length = 25 mm ÷ 5,000 = 0.005 mm = 5 µm.
A leaf cell is 60 µm long. To show it as 12 cm (120,000 µm) long in a drawing, the magnification is 120,000 ÷ 60 = × 2,000.
A typical plant cell is about 10 to 100 µm across, visible with a light microscope, whose resolution is about 0.2 µm; ribosomes (about 25 nm) and membrane detail need an electron microscope.
Key points
- Cell theory: all organisms consist of cells, and all cells come from cells (Schleiden, Schwann, Virchow).
- Plant cells have a cellulose wall, plastids and a large central vacuole, which animal cells lack.
- Organelles: nucleus (DNA), ribosomes (80S, 70S in organelles), ER, Golgi, vacuole and tonoplast, plastids, mitochondria, peroxisomes, cytoskeleton, plasmodesmata.
- Plastids include chloroplasts, chromoplasts and leucoplasts (amyloplasts, elaioplasts, proteinoplasts).
- The symplast and apoplast are the two transport routes; the Casparian strip blocks the apoplast in roots.
- Mitochondria and chloroplasts arose by endosymbiosis.
- Magnification = image size ÷ actual size.
What the exams ask
- School-leaving and entrance papers: labelled diagrams of plant cells, functions of organelles, and plant versus animal cells.
- Degree and research-fellowship entrances: wall structure and composition, types of plastids, ribosome types, the fluid mosaic model, endosymbiotic theory, symplast and apoplast.
- Professional and research examinations: cell walls in fibre and food quality, organelle genetics (cytoplasmic male sterility), and cell biology in biotechnology.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why do plant cells need a cell wall?
It lets them build up turgor pressure without bursting, which keeps non-woody plants upright, and gives shape, strength and a barrier against pathogens.
Do all plant cells have chloroplasts?
No. Only cells exposed to light and involved in photosynthesis, mainly in leaves and green stems, have many. Root cells, storage cells and most inner tissues have other plastids instead.
Why are mitochondria and chloroplasts important in breeding?
They have their own genes, usually inherited only from the mother. Cytoplasmic male sterility, used in hybrid seed production, is often caused by mitochondrial genes.
Sources
- Raven, P. H., Evert, R. F. and Eichhorn, S. E. Biology of Plants, 8th edition. W. H. Freeman, 2013.
- Buchanan, B. B., Gruissem, W. and Jones, R. L. (eds) Biochemistry and Molecular Biology of Plants, 2nd edition. Wiley-Blackwell, 2015.
- Singer, S. J. and Nicolson, G. L. "The fluid mosaic model of the structure of cell membranes." Science 175, 720–731 (1972).
- Sagan (Margulis), L. "On the origin of mitosing cells." Journal of Theoretical Biology 14, 225–274 (1967).
- Taiz, L., Zeiger, E., Møller, I. M. and Murphy, A. Plant Physiology and Development, 6th edition. Sinauer Associates, 2015. Chapter 1.
More in plant physiology and botany
Discussion
Questions, corrections and notes from readers on this article. Every comment is checked before it appears: keep it on the topic, and do not share phone numbers, links or contact details.